Dissertations / Theses on the topic 'Laser melting'

Selective laser melting (SLM) is a powder-based additive manufacturing technique consisting of the exact reproduction of a three dimensional computer model (generally a computer-aided design CAD file or a computer tomography CT scan) through an additive layer-by-layer strategy. Because of the high degree of freedom offered by the additive manufacturing, parts having almost any possible geometry can be produced by SLM. More specifically, with this process it is possible to build parts with extremely complex shapes and geometries that would otherwise be difficult or impossible to produce using conventional subtractive manufacturing processes. Another major advantage of SLM compared to conventional techniques is the fast cooling rate during the process. This permits the production of bulk materials with very fine microstructures and improved mechanical properties or even bulk metallic glasses. In addition, this technology gives the opportunity to produce ready-to-use parts with minimized need for post-processing (only surface polishing might be required). Recently, significant research activity has been focused on SLM processing of different metallic materials, including steels, Ti-, Ni- and Al-based alloys. However, most of the research is devoted to the parameters optimization or to feasibility studies on the production of complex structures with no detailed investigations of the structure-property correlation. Accordingly, this thesis focuses on the production and structure-property correlation of Al-12Si samples produced by SLM from gas atomized powders. The microstructure of the as-prepared SLM samples consists of supersaturated primary Al with an extremely fine cellular structure along with the residual free Si situated at the cellular boundaries. This microstructure leads to a remarkable mechanical behavior: the yield and tensile strengths of the SLM samples are respectively four and two times higher than their cast counterparts. However, the ductility is significantly reduced compared with the cast samples. The effect of annealing at different temperatures on the microstructure and resulting mechanical properties of the SLM parts has been systematically studied by analyzing the size, morphology and distribution of the phases. In addition, the mechanical properties of the SLM samples have been modeled using micro- structural features, such as the crystallite and matrix ligament sizes. The results demonstrate that the mechanical behavior of the Al-12Si SLM samples can be tuned within a wide range of strength and ductility through the use of the proper annealing treatment. The Al-Si alloys are generally used as pistons or cylinder liners in automotive applications. This requires good wear resistance and sufficient strength at the operating temperature, which ranges between 373 – 473 K. Accordingly, the tensile properties of the SLM samples were also tested at these temperatures. Changing the hatch style during SLM processing vary the texture in the material. Hence, samples with different hatch styles were produced and the effect of texture on their mechanical behavior was evaluated. The results show that the hatch style strongly influences both the mechanical properties and the texture of the samples; however no direct correlation was observed between texture and mechanical properties. The wear properties of the Al-12Si material was evaluated using pin-on-disc and fretting wear experiments. These experiments show that the as-prepared SLM samples exhibit better wear resistance than their cast counterparts and the SLM heat-treated samples. Finally, the corrosion investigations reveal that the SLM samples have similar corrosion behavior as the cast specimens under acidic conditions. A major drawback for the wide application of SLM as an industrial processing route is the limited size of the products. This is a direct consequence of the limited dimensions of the available building chambers, which allow for the production of samples with volumes of about 0.02 m3. A possible way to overcome this problem would be the use of the welding processes to join the small SLM objects to form parts with no dimensional limitations. In order to verify this possibility, friction welding was employed to join Al-12Si SLM parts. The results indicate that friction welding not only successfully permits the join materials manufactured by SLM, but also helps to significantly improve their ductility. This work clearly demonstrates that SLM can be successfully used for the production of Al-12Si parts with an overall superior performance of the mechanical and physical properties with respect to the conventional cast samples. Moreover, the mechanical properties of the SLM samples can be widely tuned in-situ by employing suitable hatch styles or ex-situ by the proper heat treatment. This might help the development of SLM for the production of innovative high-performance Al-based materials and structures with controlled properties for automotive and aerospace applications.

Selective laser melting is a promising metallic additive manufacturing process with many potential applications in a variety of industries. Through a gracious donation made by Lawrence Livermore National Laboratory, California Polytechnic State University received and installed an SLM 125 HL selective laser melting machine in February 2017. As part of the initial setup effort, a preliminary machine verification study was conducted to evaluate the general print quality of the machine with default parameter settings. Coincidentally, the as-printed microstructure of SLM components was evaluated through nil strength fracture surface examination, an alternative to conventional polish-and-etch metallography. A diverse set of components were printed on the SLM 125 HL to determine the procedural best practices and inherent constraints. Additionally, the mode and mechanism of failure for a defective Lawrence Livermore National Laboratory component fabricated at their facility was investigated. From these studies, extensive documentation in the form of standard operating procedures, guidelines, templates, and summary reports was generated with the intent of facilitating future selective laser melting research at Cal Poly and strengthening the learning of students interfacing with the novel technology.

Additively manufactured parts produced using selective laser melting (SLM) are prone to defects created during the build process due to part shrinkage while cooling. Currently defects are found only after the part is removed from the printer. To determine whether cracks can be detected before a print is completed, this project developed print parameters to print a test coupon with inherent defects – warpage and cracking. Data recorded during the build was then characterized to determine when the defects occurred. The test coupon was printed using two sets of print parameters developed to control the severity of warpage and cracking. The builds were monitored using an accelerometer recording at 12500 samples per second, an iphone recording audio at 48000 samples a second, and a camera taking a photo every build layer. Data was analyzed using image comparison, signal amplitude, Fourier Transform, and Wavelet Decomposition. The developed print parameters reduced warpage in the part by better distributing heat throughout the build envelope. Reducing warpage enabled the lower portion of the part to be printed intact, preserving it to experience cracking later in the build. From physical evidence on the part as well as time stamps from the machine script, several high energy impulse events in the accelerometer data were determined to be when cracking occurred in the build. This project’s preliminary investigation of accelerometers to detect defects in selective laser melting will be used in future work to create machine learning algorithms that would control the machine in real time and address defects as they arise.

Selective Laser Melting (SLM) is a laser based Solid Freeform Fabrication (SFF process which uses laser energy to melt a thin layer of metal powder. This process is repeated to produce a 3-dimensional metallic part. SLM is capable of producing intricate parts which are otherwise difficult to produce with conventional manufacturing techniques. As compared to traditional manufacturing processes, SLM can also produce parts with higher density. Before a material is processed using SLM, suitable processing parameters are first identified. Over the years, different materials have been processed using the SLM process. However, very little work has been done on SLM of bio-compatible precious metals such as gold and its alloys. Gold and its alloys have been used for manufacturing of dental crowns for centuries. The SLM process could be used to produce intricate metallic substructures for porcelain fused to metal dental restorations. This research work was focused on understanding the processing parameters for SLM of 24 carat gold powder. The gold powder was analyzed for Particle Size Distribution (PSD), apparent density and tap density before identifying suitable processing parameters for SLM. The gold powder particles were found to be spherical in nature but smaller particles stuck to each other and formed larger powder agglomerates. From the apparentdensity experiments, the gold powder was found to be cohesive and non-flowing in nature which hindered powder flowability during the powder deposition process with the existing system. This issue was resolved by designing a new powder deposition system which could allow the gold powder to flow evenly over the substrate. The tap density of the gold powder was found by Constant Weight Tap Density (CWTD) and Constant Volume Tap Density (CVTD) techniques. The difference in results from these two techniques was negligible. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) of gold powder showed it to be more than twice as reflective as other commonly processed metal powders such as stainless steel and H13 tool steel. This analysis proved useful in understanding the laser processing of gold powder. Due to the high cost and small quantity of material available for this work, a very small build platform was designed to optimise material utilisation and reduce wastage. Single scans were performed on a single layer of gold powder to identify the good melting region. Five different regions i.e. balling, good melting, unstable melt, weak sintering and very little sintering were observed in the processing window. The balling phenomenon was observed at low and high scan speeds, which was due to the melt pool instability at these parameter settings. The size of droplets (balling) also increased with decreasing scan speed and increasing laser power which was due to an increase in the break up time of the molten metal. In the good melting region, the gold powder was found to be completely melted and continuous beads were successfully produced. The unstable melt region showed the melt pool spreading unevenly in different directions whereas in the weak sintering and very little sintering regions the gold powder did not melt completely. Single layers were produced on a layer of gold powder, which showed the parameters in the good melting regions to be suitable for multiple layer parts manufacturing. Gold cubes were produced using the suitable processing parameters identified from single scan and single layer experiments and then analyzed for their internal porosity. The porosity in the gold cubes was found to be at a minimum for parameters obtained in the good melting region. The internal porosity was found to be mostly inter-layer porosity; this indicated less heat transferred to the region between the two layers which could be associated with the high reflectivity of gold. The inter-layer porosity in gold cubes was further reduced by reducing the layer thickness. This could be due to the thinner layers requiring less energy to melt and be fused to the previous layers. The hatch distance had a negligible effect on the inter-layer porosity of gold cubes. The reduction in hatch distance increased the energy delivered but it was still not enough to completely melt the gold powder and fuse it to the previous layer. A pre-scan technique was also tested to be used for pre-heating the powder bed. However, due to the rapid drop in temperature, this technique was not found suitable to be used as a powder bed pre-heating technique. The gold cubes were checked for their mechanical properties i.e. hardness and modulus. The hardness of gold cubes was found to be higher than expected for 24 carat gold. The modulus was found to be less than 24 carat gold. This variation in the mechanical properties of gold cubes could be due to the rapid heating and cooling of material during the laser processing or presence of internal porosity in these gold cubes. After single scans and single layers manufacturing, gold dental parts (premolar and molar) were also manufactured using the optimum processing parameters. These gold dental parts were also analyzed for their internal porosity, which was found to be less than that observed in gold cubes. This difference in porosity could be due to the difference in structure of gold cubes and premolar part, where the latter was a thin wall structure.

Selective Laser Melting (SLM), a layer-based Solid Freeform Fabrication (SFF) process, was used to fabricate micro cross-flow heat exchangers from 316L stainless steel, bronze (Cu 90%, Sn 10%) and Inconel 718 powder. Their mechanical and thermal properties were determined using solid blocks of SLM material prior to the fabrication of the micro cross flow heat exchangers. Initially the process parameters for the fabrication of high density (>97%) parts for the different materials were defined. The mechanical and thermal properties of SLM parts were then measured. The tensile test results exhibited yield strength values superior to the parent metals, but also showed low tensile strength and ductility as a result of the inherent residual porosity (2-4%). Results obtained from the thermal conductivity of the stainless steel material system were in good agreement with the bulk material values. The heat transfer performance of the heat exchangers with either micro channels or lattice structures as heat exchange surfaces was investigated experimentally and the results were evaluated in terms of geometry and materials. The performance of the micro heat exchangers was found to be dependent not only on the choice of material but also on the heat exchanger media geometry.

Selective laser melting is an attractive technology, enabling the manufacture of customised, complex metallic designs, with minimal wastage. However, uptake by industry is currently impeded by several technical barriers, such as the control of residual stress, which have a detrimental effect on the manufacturability and integrity of a component. Indirectly, these impose severe design restrictions and reduce the reliability of components, driving up costs. This thesis documents work on investigating the generation of residual stresses created in the selective laser melting process by the use of a finite element thermo-mechanical model. The thermo-mechanical model incorporated an adaptive meshing strategy which was used in conjunction with the use of high performance computing facilities. These together significantly increased the computational throughput for simulating selective laser melting of a single layer. Additionally, a volumetric hatching method was created to generate the laser scan vectors used in the process, with the ability to both simulate and manufacture on selective laser melting machines. A number of studies were performed to better understand the effect of laser scan strategy on the generation of residual stress in selective laser melting. Using this model, a series of investigations were performed to understand the effect of scan strategy and scan area size on the generation of residual stress in this process. Further studies were also performed to investigate the role of laser parameters, geometry, and support structures in selective laser melting and their effect on the generation of residual stress. The studies showed a complex interaction between transient thermal history and the build-up of residual stress has been observed in two conventional laser scan strategies (unidirectional and alternating) investigated. The temperature gradient mechanism was discovered for the creation of residual stress and the scan area size had an effect on the temperature sustained within the region. The parametric study of the laser parameters showed that an increase in laser scan speed increased the melt pool aspect ratio, and increase in laser power increased the melt pool width. The parametric thermo-mechanical analysis revealed that the laser scan speed had the most influence on the magnitude and anisotropy of the residual stresses generated. Varying the hatch distance had little effect on the maximum magnitude of residual stresses generated, but decreasing the hatch distance significantly increased the level of yielding that occurred. A study of the geometrical effect on scan strategy revealed the importance of the thermal history on the transverse stresses generated, influenced by the arrangement of scan vectors. The higher magnitude longitudinal stresses had predictable behaviour; only dependent on the scan vector length and not the thermal history generated by the choice of laser scan geometry. It was shown that the laser scan strategy becomes less important for scan vector length beyond the typical 5 mm island sizes. From the study of the support structures, it was found the insulating properties of the metal powder used in selective laser melting provide a significant thermal resistance for the dissipation of heat, and caused uniform overheating across the scanned region. In particular, the analysis showed localised overheating using support structures, which affected the melt pool geometry, and the residual stresses generated due to resistance against dissipating heat. Additionally, lattice structures such as the double gyroid showed localised overheating occurs using repeated exposures of short scan vectors. Suitable scan strategies therefore need to be developed to account for support structures. A multi-scale methodology was developed by combining information from the meso -scale obtained from the thermo-mechanical model. This model was used to predict the mechanical response of amacro -scale part. This approach used the assumption that meso -scale regions in island scan strategies behave independently from each other. This assumption was verified by comparing with a thermo-mechanical analysis. This multi-scale method was applied to a 3D structure and also to a complex 2D geometrical shape. Performing the multi-scale analyses has verified that the proposed technique of superposition of meso-scale stress fields at the macro -scale is a valid technique. The main strengths of the proposed multi-scale method is the decoupling of the meso and macro scale analyses. This has the benefit of reducing computational cost of the macro -scale analysis because it is independent of the complexity of the meso -scale analysis, and only requires performing once. These strengths translate into large computational time savings and also great flexibility in the physics incorporated at each scale.

This diploma thesis deals with finding and verification of appropriate technological parameters of SLM technology for the processing of aluminum alloy 2618. In the theoretical part, an introduction to additive manufacturing of aluminum alloys and general description of processes occurring during SLM production is given. Based on general knowledge were designed different types of testing samples produced by sintering the metallurgical powder using 400 W ytterbium fiber laser, which so far in the literature for aluminum alloy 2618 were not described. As the result, the technological parameters dependence on relative density and the detailed overview of the 2618 alloy processing by SLM technology is determined.

This diploma thesis deals with finding copper alloy suitable for processing SLM technology and determining the process parameters leading to a relative density close to the full material. The theoretical part provides an insight into additive technology and the processing of new alloys in SLM. Work also contains a search report of processed copper alloys used in SLM. Based on the theoretical part were designed test samples and method of evaluation. Samples were produced by melting metallurgical powder using ytterbium laser with an output power 400 W. The testing is divided into three stages; Determination of the parameters of the SLM process, Debug strategies for larger parts, Geometric precision and mechanical testing. Based on the results was determined dependence of relative density on the input parameters. For the best parameters were determined geometric precision correction and mechanical properties.

Method selective laser melting can produce metal parts by using 3D printing. This diploma thesis deals with the influence of process parameters on the workability of AlSi9Cu3 high-strength aluminum alloy using selective laser melting. The theoretical part deals with relations between process parameters and identifies phenomena occurring during the processing of metals by this technology. It also deals with conventionally manufactured aluminum alloy AlSi9Cu3. In the work, material research is performed from single tracks tests, porosity tests with different process parameters and mechanical testing. Here are showing the trends of porosity change at scanning speed, laser power, individual laser stop distance, bulk energy, and powder quality. The workability of the material can be judged by the degree of relative density achieved. Simultaneously the values of the achieved mechanical properties of the selected process parameters are presented. The data obtained are analyzed and compared with literature.

The research reported in this thesis focuses on assisting the design process in respect of end use metallic products produced using the Selective Laser Melting (SLM) technology. The advancements in layer additive manufacturing technologies such as SLM have enabled the manufacture of end use products directly from Computer Aided Design data. Many companies and researchers are exploring the application of SLM in industry for specific applications, such as the mass customisation of biomedical implants and novel lattice structures. However, bridging SLM from research into mainstream manufacturing is not straightforward, as demanding industry standards and compliance need to be fulfilled. Additive Manufacturing (AM) technologies are often perceived by designers as being able to generate all conceivable geometries. However, SLM is not completely freeform as the inherent process difficulties can distort many part geometries, and designers often lack an understanding of these process issues and their effect on the final SLM product. The aim of this research is to address this lack of design knowledge, by developing a set of design rules to allow for more predictable and reliable results when manufacturing parts with SLM. This thesis documents how the design rules were created. Firstly, the geometric limitations of SLM were evaluated through a quantitative cyclic experimental methodology. Part orientation, fundamental geometries and compound design features were explored until self-supporting parts with the optimum part accuracy were achieved. Design rules were then created and evaluated through a series of interviews with industrial and academic design professionals.

This research focuses on reducing the limitations imposed on the repeating topologies in lattice structures that restrict what can be created using the RealiZer Selective Laser Melting (SLM) machine. The creation of regular, randomly perturbed, polar mapped, and random metallic lattice structures using SLM apparatus is reported and discussed in this thesis. It was observed that a new technique was required to generate the slice data files used to control the SLM equipment in order to create structures that measured significantly more than 10 cells in each axis. The research details the motivations behind the development of the computational methods utilised to develop lattice parts and how the iterations of these methods enabled different areas of research to progress. The limits of the angles from the horizontal that elements could be built are reviewed and scanning techniques are developed that create elements below these values. In order to create horizontal links significant proportions of the machine control software were replaced with software developed during the course of the research. This is discussed at length along with how the limitations on the number of processing parameters available could be removed and how pauses which let sections of the melt on horizontal links freeze before processing the next section could be used. It is suggested that systems or experimental set ups are developed that allow greater control over the duration of these pauses. This would enable further research into the processing of horizontal links, developing them to the point where they are mechanically consistent and comparable to other links in the structures

Selective laser melting (SLM) has been widely used to manufacture customised metallic parts because it provides an integrated way to manufacture three-dimensional (3D) parts from computer-aided design models after several sub-processes. On the other hand, aluminium-based nanocomposites are widely used in the aerospace and automotive industries due to their light weight, high specific strength, excellent wear resistance, but their manufacturability and mechanical properties are not well understood when these new materials are employed in SLM. This is an important consideration because, compared with traditional manufacturing technologies, SLM offers the ability to manufacture engineering parts with very complex geometries by employing a layer-by-layer manufacturing principle. Hence, this thesis systematically studies the SLM of an advanced Al-Al2O3 nanocomposite that is synthesised using high-energy ball-milling (HEBM) process. The aim of this study is to use SLM to fabricate a nearly full dense Al-Al2O3 nanocomposite composed of 96 vol.% Al and 4 vol.% Al2O3 powder. The synthesis and characterisation of ball-milled powder is the first contribution of this study, which also investigates the influence of milling and pause duration on the fabrication of ball-milled composite powder. The second contribution of this work is the development of a 3D finite element model to predict the thermal behaviour of the first layer’s composite powder. Both the transient temperature distribution and molten pool dimensions are predicted within the laser scanning, which VI enables a more efficient selection of the process parameters (e.g. hatch spacing and scanning speed). The third contribution of this study is the optimisation of the SLM process parameters and microstructure investigation of the fabricated samples. The optimum laser energy density and scanning speed that are used to fabricate nearly full dense Al-Al2O3 nanocomposites are found to be 317.5 J/mm3 and 300 mm/s, respectively. The relative density is evaluated by quantifying the porosity on both the horizontal and vertical sections. The fabricated composite parts were observed to exhibit a very fine granular-dendrite microstructure due to the rapid cooling, while the thermal gradient at the molten pool region along the building direction was found to facilitate the formation of columnar grains. The final contribution of this study is the investigation of mechanical properties such as tensile strength, microhardness and macro and nanoscale wear behaviour. Compared to pure Al, the addition of 4 vol.% Al2O3 nanoparticulates was found to contribute to a 36.3% and 17.5% increase in the yield strength and microhardness of the composite samples, respectively. Cold working was found to contribute to a 39% increase in microhardness due to grain deformation. The pin-on-disc wear testing and atomic force microscopy (AFM) nanoscratching were performed to study the macro and nanoscale wear behaviour of the fabricated samples, respectively.

Selective Laser Melting (SLM) is an additive manufacturing technology that consolidates layers of metal powder using a high power laser. The laser's small spot size and relative accuracy facilitates the production of high resolution parts with great complexity that would be otherwise difficult to manufacture using conventional manufacturing techniques (e.g. casting, machining etc.). The possibility to build thin wall high resolution parts complements the technology's main advantage and extends its manufacturing capabilities. The high heat input delivered by the laser and complex melt pool dynamics, requires that laser process parameters are carefully controlled in order to prevent solidified parts from exhibiting poor properties such as a high surface roughness and poor resolution.

Quel que soit le secteur d’activité, les procédés de fabrication additive pour les matériaux métalliques ont un fort potentiel industriel, spécifiquement pour la production de pièces à haute valeur ajoutée. Le secteur de l’outillage est l’un des utilisateurs de ces procédés, et plus particulièrement du Selective Laser Melting (SLM). Ce procédé permet de diminuer les coûts et les temps de production des outillages, tout en augmentant la complexité des pièces fabriquées. Cependant, pour améliorer la qualité des pièces fabriquées, une meilleure compréhension des mécanismes physiques qui le régissent est nécessaire. Dans ce travail de thèse, consacré à la modélisation du procédé SLM, les approches suivies sont multiphysiques à deux échelles. La première échelle de modélisation, utilisant la méthode Volume Of Fluid, correspond à la fusion d’un lit de poudre par un laser puis sa solidification. Le lit de poudre numérique est produit à partir d’un générateur spécifique basé sur la granulométrie identifiée expérimentalement. Après certaines hypothèses simplificatrices posées sur les phénomènes physiques à modéliser, la tension superficielle a été implémentée et a nécessité l’utilisation de la méthode des « heights functions ». La seconde échelle de modélisation correspond à la construction d’une succession de cordons à l’aide de la méthode des éléments finis. Le modèle thermomécanique utilise la méthode « element birth » pour se rapprocher au plus près des conditions réelles du procédé. Après leur validation par des essais expérimentaux, les simulations ont permis de prédire le champ de température, la largeur de la zone fondue, ainsi que la formation du « keyhole »
Regardless the industry, additive manufacturing processes for metallic materials have a great industrial potential, especially to product high added value parts. One of the main users of these processes, and more specifically the Selective Laser Melting (SLM), is the tooling industry for plastics processing. It make possible to reduce production costs and manufacturing times while increasing the complexity of manufactured parts. However, in order to improve the quality of the latter and ensure their certifications, a better insight into the related physical phenomena undergone by the material during the process is still needed. In this PhD thesis, the SLM process modeling is multiphysic and concerns two different scales. The first modeling scale uses the Volume Of Fluid method to model the powder bed melting and its ensuing solidification. The numerical powder bed is computed thanks to a specific generator enabling to take account for the experimental granulometry. Once some simplifying assumptions on the physical phenomena stated, the surface tension has been implemented requiring the "heights functions" method use. The second modeling scale corresponds to the building of laser tracks series through the finite element method. The thermomechanical approach uses the element birth method in order to meet as far as possible the experimental conditions. Following its assessment through experiment/simulation face off, model have enable to predict the temperature field and the melted zone width as well as the keyhole formation

Heat removal from electronic systems relies on forced convection heat transfer from extended surfaces. The favoured extended surface for avionics enclosures is the cast pin fm array. However, due to the increasing heat dissipation of electronic components, the cooling offered by cylindrical arrays will be insufficient in the near future and higher performance extended surfaces are required. The current study introduces a novel manufacturing technique, Selective Laser Melting (SLM), and' demonstrates its ability to fabricate new designs of extended surface that have not previously been considered, primarily due to their geometric complexity. The SLM processing parameters for aluminium 6061, a new material to the technique, were developed to allow the fabrication ofparts that achieved a density of 90% of the parent metal. Investigations on the SLM-fabricated aluminium 6061 revealed that its thermal conductivity was 75 W/mK, 48% less than the thermal conductivity predicted by standard correlations. 65% of the reduction in thermal conductivity was found to be caused by interparticle thermal resistances, the remainder a result ofporosities. The heat transfer and pressure loss performance of several types of heat sink were characterised by experimental investigations. The extended surfaces manufactured and tested included cylindrical arrays, for validation purposes, elliptical, offset strip and diamond arrays, and various types of lattice. In the acceptable flow rate and pressure drop range, a tightly packed diamond array was found to offer the best performance with a 62% improvement in heat transfer over a typical cylindrical pin arrangement and only 60% ofthe pressure loss. Computational Fluids Dynamic (CFD) studies of the flow in three pin arrangements were conducted to aid visualisation of the fluid flow and to assess the predictive capabilities of Fluent, a commercial CFD code. The CFD pressure loss predictions were within 8 % ofthe experimental results and the heat transfer results were found to be qualitatively meaningful. Results from the experimental and CFD studies indicate that laminar flow in streamlined fm arrays with tight pin spacings offers improved heat transfer performance benefits over turbulent flow in cylindrical arrays at a similar flow rate and pressure drop thereby offering the required improvement in performance for high-power electronic components.

Research focused on the selective laser melting (SLM) of stainless steels and aluminium alloys. For steels, the possibility of creating a magnetically graded material was demonstrated as well as the ability to improve consolidation with austenitic and martensitic stainless steel powder mixtures. Stainless Steel/CoCr hybrid samples were also manufactured and tested to investigate the advantages of functionally graded materials (FGMs). Al alloy research began with examining the requirements for successful Al alloy consolidation in SLM and through experimentation it was found that Al alloys with good welding properties were the best choice: pure Al was found to be completely unsuitable. 6061 Al alloy was then used as a base material to manufacture Al-Cu alloy samples. Single layer SLM samples were produced first, which resulted in recognised Al-Cu microstructures forming. Multilayer Al alloy SLM research resulted in the discovery of the theorised ability to manufacture Al-Cu alloy parts with a nanocrystalline Al matrix with dispersed Al2Cu quasicrystals, resulting in a material comparable to a metal matrix composite that showed excellent corrosion resistance and compressive strength. Finally, a demonstration part was made to test the capability of the SLM process producing an aerospace type geometry using a customised Al alloy. Observations during manufacture and post process analysis showed that Al alloys were susceptible to changes in mechanical properties due to the geometry of the manufactured part.

Selective Laser Melting (SLM) as a method of netshape manufacture is of growing interest within the aerospace industry. There is currently a lack of understanding of the influence of process variables on the integrity and properties of the as fabricated material. The research presented investigates the SLM fabrication of three nickel superalloys: Primarily CM247LC and CMSX486/IN625 as secondary alloys. CM247LC is Ni-base superalloy hardenable by the precipitation of the coherent \(\gamma\)'phase. It presents a particular challenge due to weld-crack susceptibility. This research aims to establish a processing route for CM247LC components via SLM: Parametric studies are presented to quantitatively assess the cracking behaviour based on microstructural observations; Hot Isostatic Pressing HIPping) has been investigated as a retro-fix solution to cracking; Electron BackScatter Diffraction (EBSD), MicroCT Tomography and microscopy have been used to characterise the SLM microstructure. The \(\gamma\)' evolution through the manufacturing stages (SLM & Heat Treatment) has been examined. Mechanical testing creep/tensile) was performed for comparison against cast material. Research was extended to two additional alloys: CMSX486 and IN625. Statistical design of experiments methodology was used to rapidly establish process parameters for these two alloys and assess them by mechanical testing. In conclusion a processing route capable of yielding fully dense material with a satisfactory \(\gamma\)' structure is presented; however, it involves significant post-fabrication processing which reduces the attractiveness of SLM. Further research is suggested, specifically into modelling and thermal measurement of SLM

The presented diploma thesis deals with the control of the geometric accuracy of the parts produced by additive manufacturing technology selective laser melting. The paper first analyzed the work of the other authors dealing with this issue. Based on obtained informations from this analysis were developed benchmark test parts for quality control of production on a commercial machine SLM 280 HL supplied by SLM Solutions GmbH. The work was carried out several tests to determine the appropriate parameters of construction parts. These tests, their results and conclusions are fully described in this papper.

Metal additive technology allows to create objects with complex shape that are very difficult to produce by conventional technologies. An example of such component is a porous structure which is composed of periodical truss cells. This diploma thesis deals with the prediction of the mechanical properties of very small lattice structures made of additive manufacturing technology Selective Laser Melting. Using the proposed test specimens it was found that real dimensions of the trusses varies with size and orientation to the base platform. It was proposed and tested samples for rod tensile test made of SLM. Based on the real information about dimensions and mechanical properties of rods were predicted mechanical properties of lattice structures. A lot of mechanical tests were carried out to obtain the real mechanical properties. Test results and conclusions are described in the thesis.

Oxide dispersion strengthened (ODS) steels contain a fine dispersion of nano-sized, typically Y based, oxide particles which result in the material displaying significantly better creep, irradiation and oxidation resistance when compared to conventional alloys. Thus, such materials are considered as candidate structural materials for a number of applications in the fossil and nuclear energy sectors and in other high-temperature applications. ODS steels are currently produced by powder metallurgy which includes mechanical alloying (MA) of master alloys or elemental powder, hot extrusion or hot isostatic pressing (HIP) followed by a final heat treatment. Recent studies revealed that Y added during MA in the form of yttria (Y2O3) breaks down and the elements go into supersaturated solution in the Fe matrix; and Y based dispersoids form during fabrication of the alloy. In this work, an additive manufacturing method, selective laser melting (SLM), was applied to as-MA ODS-PM2000 (Fe-19.0wt.%Cr-5.5Al-0.5Ti-0.5Y2O3) powder. SLM produces almost fully dense solid freeform components by successively melting thin layers of metal powder. In order to investigate the feasibility of SLM in an ODS alloy environment, a number of builds were fabricated. These included a complex thin walled structure, coatings on Inconel IN939 (Ni-22.5wt.%Cr-19.0Co-3.7Ti-2W-1.9Al-1.0Nb-1.4Ta-0.15C), a nickel based superalloy, and optimised wall and solid builds. A wide range of microstructural and mechanical characterisation techniques were carried out on these builds with the focus to study the fundamentals of SLM in ODS environment. The most important finding of this work was that a fine homogeneous dispersion of globular shaped nanoscopic particulates could be retained in the SLM build configurations investigated. Indications were found that there is a very low number of dispersoids in the deposited layer after it was put down. Repeated heating cycles during SLM deposition of further slices resulted in coarsening and growth of existing precipitates, but probably also in nucleation and growth of new dispersoids in the α-Fe matrix. Such heating cycles and post-build annealing trials resulted in modification of initially multiphased dispersoid compounds including originally a number of elements, such as O, Al, Si, Ti, Cr, Fe and Y, into structures having significantly increased concentrations of Al and Y. After post-build annealing, the particles were most frequently of the compound type yttrium aluminium monoclinic, Y4Al2O9. SLM processing parameters were developed leading to a relative density of >99.5 % for wall builds having different thicknesses and of >98.5 % for solid builds. Electron backscatter diffraction (EBSD) was conducted and revealed a strong [001] fibre texture along the growth direction of a wall build. For annealed walls, values of the 0.2% offset yield strength YS0.2 up to those of recrystallized conventional produced PM2000 could be achieved. Fracture behaviour and the individual key parameters determined, YS0.2 and Young's modulus were anisotropic due to this texture. In coatings, Y-rich dispersoids could be retained. When oxidised isothermally in laboratory air at 1100°, the SLM deposit PM2000 formed a mainly α-Al2O3 (alumina) scale, which was similar to conventionally fabricated PM2000. Oxidation at 870°C, however, resulted in different scale morphologies between both variants. Those were Al-rich equiaxed structures and nodules and Ti-rich needles for conventional PM2000. On the other hand, the SLM material exhibited Al-rich platelet structures and Al-rich equiaxed crystals in pores. The observations of this work confirmed the feasibility of SLM in ODS alloy environment, which may motivate further studies in this field.

Laser Melting (LM) is an Additive Layer Manufacturing (ALM) process used to produce three-dimensional parts from metal powders by fusing the material in a layerby- layer manner controlled by a CAD model. During LM, rapid temperature cycles and steep temperature gradients occur in the scanned layers. Temperature gradients induce thermal stresses which remain in the part upon completion of the process (i.e. residual stresses). These residual stresses can be detrimental to the functionality and structural integrity of the built parts. The work presented in this thesis developed a finite element model for the purpose of investigating the development of the thermal and residual stresses in the laser melting of metal powders. ANSYS Mechanical software was utilised in performing coupled thermal-structural field analyses. The temperature history was predicted by modelling the interaction of the moving laser heat source with the metal powders and base platform. An innovative ‘element birth and death’ technique was employed to simulate the addition of layers with time. Temperature dependent material properties and strain hardening effects were also considered. The temperature field results were then used for the structural field analysis to predict the residual stresses and displacements. Experiments involving laser melting Ti-6Al-4V powder on a steel platform were performed. Surface topography analyses using a laser scanning confocal microscope were carried out to validate the numerically predicted displacements against surface measurements. The results showed that the material strain hardening model had a direct effect on the accuracy of the predicted displacement results. Using the numerical model, parametric studies were carried out to investigate the effects of a number of process variables on the magnitude of the residual stresses in the built layers. The studies showed that: (i) the average residual stresses increased with the number of melted powder layers, (ii) increasing the chamber temperature to 300°C halved the longitudinal stresses. At 300°C, compressive stresses appeared on the Ti64 surface layer, (iii) reducing the raster length from 1 mm to 0.5 mm reduced the average longitudinal stress in the top layer by 51 MPa (0.04σy), (iv) reducing the laser scan speed from 1200 mm/s to 800 mm/s increased the longitudinal stress by 57 MPa (0.05σy) but reduced the transverse stress by 46 MPa (0.04σy).

Selective Laser Melting (SLM) is an Additive Manufacturing (AM) process used to create 3D objects by laser melting pre-deposited powdered feedstock. During SLM, powdered material is fused layer upon layer, the scanning laser melts regions of the powder bed that corresponds to the geometry of the final component. During SLM the component undergoes rapid temperature cycles and steep temperature gradients. These processing conditions generate a specific microstructure for SLM components. Understanding the mechanism by which these generated microstructures evolve can assist in controlling and optimising the process. The present research develops a two dimensional Cellular Automata – Finite Element (CA-FE) coupled model in order to predict the microstructure formed during the melting process of a powdered AA-2024 feedstock using the AM process SLM. The presented CA model is coupled with a detailed thermal FE model which computes the heat flow characteristics of the SLM process. The developed model takes into account the powder-to-liquid-to-solid transformation, tracks the interaction between several melt pools within a melted track, and several tracks within various layers. It was found that the simulated temperature profiles as well as the predicted microstructures bared a close resemblance with manufactured AA-2024 SLM samples. The developed model predicts the final microstructure obtained from components manufactured via SLM, as well as is capable of predicting melt pool cooling and solidification rates, the type of microstructure obtained, the size of the melt pool and heat affected zone, level of porosity and the growth competition present in microstructures of components manufactured via SLM. The developed models are an important part in understanding the SLM process, and can be used as a tool to further improve consistency of part properties and further enhance their properties.

Novel aspects of solid state laser spot melting of aluminium using a pulsed solid state laser were investigated. After a thorough characterisation of the performance of the solid state laser, an initial series of ranging trials were performed to identify parameters which produced cosmetically satisfactory consistent melt spots on the surface of a commercially available aluminium alloy. These melt spots demonstrated a number of features of interest, including symmetrical concentric ring structures on the surface of the spots. A review of published literature on the use of laser beams as an intense radiation source for pulsed laser surface melting was carried out which confirmed that these phenomena have not been researched or reported in any depth. Experimental work identified the conditions under which they could be reliably reproduced, and these conditions are very close to laser parameters used commercially for pulsed laser welding. Further investigations to understand their origin involved using modified aluminium surfaces and temporally shaped laser pulses. Experimental details are included which will allow reliable reproduction of this effect in the future. Specific thresholds were identified for these phenomena and this has led to an improved understanding of solid state laser spot melting on aluminium. It appears that these rings are part of a continuum of irradiance which leads to melt expulsion due to reactive vapour pressure.

The aim of this project is developing a near net-shape processing route for Al-alloy compressor components with Selective Laser Melting (SLM). Design of experiments (DOE) techniques such as the Response Surface Method, and statistical analysis using the analysis of variance (ANOVA) were applied for processing parameter optimisation in order to minimise the defects (pores or cracks) and studying the influence of powder, such as chemical composition, particle shape/ morphology and particle size and SLM parameters, such as laser power, laser scan speed, hatch spacing, laser scan strategy, island size. The tensile, fatigue and creep properties of the samples built using the optimum parameters were assessed. The microstructures were assessed using optical, scanning and electron transmission microscopy. The influence of building directions (0°, 45°, and 90°) and post processing (T6 heat treatment and Hot Isostatic Pressing (HIPping)) on microstructure and mechanical properties were also investigated.

Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2018.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 111-117).
Selective Laser Melting (SLM) is an industrially viable means of additively manufacturing metal components with complex geometries from a wide variety of alloys. In this process, a metal powder is spread onto a build surface in a thin layer, and then the powder is selectively melted to form a cross-sectional slice of the part. This process is repeated until the part is complete. The packing density and uniformity of the powder layer are key to creating robust SLM parts. In commercial SLM equipment, the layer is spread using a moving blade or roller mechanism (the "recoater"). There is still opportunity to optimize the process and understand how powder mechanics influence the layer quality. This thesis focuses on experimental and computational methods to study powder recoating in SLM. An instrumented recoater was built with the capability to measure forces and vary important recoating parameters, such as recoating velocity, blade height and blade geometry. The instrumented recoater was then manufactured, assembled, tested and incorporated into a custom built SLM testbed at MIT. The recoater demonstrated the ability to vary the blade height with a 70 pm stroke and measure force in the milinewton range. Furthermore, angle of repose measurements were performed on powders of various size distributions and used to calibrate a model (developed by collaborators), which demonstrates the influence of cohesion on these powders. In addition, preliminary single-particle adhesion tests were performed. Together, these capabilities allow the rational development of powder spreading parameters to achieve uniform layers in SLM.
by Johannes Weinberg.
S.M.

Thesis: S.M., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2017.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 167-179).
Additive manufacturing (AM) is a rapidly advancing manufacturing paradigm that involves selective placement of material, layer-by-layer, as determined by a three-dimensional digital model. AM allows for freeform geometries and optimized structures that are impractical or impossible to create with traditional manufacturing practices. Among several mainstream AM methods, powder bed fusion is compatible with both plastics and metals, and thereby allows construction of a wide spectrum of end-use parts. A significant challenge in exploring this process from a research perspective is the predominance of commercial systems which are costly and offer limited flexibility to the user. To address this challenging lack of access, this thesis develops a low-cost and open-architecture laser powder bed fusion metal printer to enable the exploration of new materials and process concepts. Starting with a broad review of additive manufacturing, this thesis then explains the powder bed fusion process and reviews the design considerations for powder bed fusion equipment. This understanding is then applied to design an open-architecture galvanometer-driven optical scanning system. In addition, two deposition chambers are constructed, including a high-pressure vessel with a novel multi-layer recoating build platform that allows for the study of pressure in the powder bed fusion process. The operational performance is then evaluated, and the capability to achieve programmed scanning of the laser is demonstrated through point-wise and raster scan melting.
by Stuart Polak Baker.
S.M.

The Selective Laser Melting (SLM) process generates large thermal gradients during rapid melting, and during solidification certain nickel superalloys suffer from thermally induced micro-cracking which cannot be eliminated by process optimisation. This investigation sought to investigate and understand the root cause of micro-cracking in nickel superalloys when processed by SLM, with the aim of ultimately being able to predict the crack susceptibility of an alloy from composition alone. Microstructural analysis as well as implementation of Rapid Solidification Processing (RSP) theory and solute redistribution theory was used to establish SLM as a rapid solidification process. As a consequence, secondary dendrite arm formation and solute redistribution is largely inhibited, resulting in a bulk material which is near to a super saturated solid solution. The establishment of SLM as an RSP along with morphological and chemical analysis of micro-cracks support Elevated Temperature Solid State (ETSS) cracking as the primary cracking mechanism in SLM processed nickel superalloys. The crack susceptibility of a nickel superalloy, χ, was defined as the ratio between the solid solution strengthening contribution from alloying elements and apparent thermal stress generated by the process. Minor increases in the wt% of solid solution strengthening elements in Hastelloy X, a high crack susceptibility alloy, resulted in average reductions of crack density of 65%. Thereby supporting solid solution strength as a key factor in the crack susceptibility of a nickel superalloy. The addition of the apparent thermal stress component, further supported the crack susceptibility model, with the modified Hastelloy X being predicted to have a lower crack susceptibility. Additional validation of the crack susceptibility predictor was determined by taking compositions and material properties from published SLM investigations and calculating the crack susceptibility of the respective alloy. The results were found to be in good agreement with the reported observations.

This diploma thesis deals with development and design modifications of tire mould segment which will be batch produced by additive technology Selective Laser Melting. Material for its production is maraging steel 1.2709. Lattice structure was used inside the segment construction. The geometry of the lattice cell was chose based on two main factors – eliminating production costs and providing sufficient stiffness. Strength of the segment was calculated by FEM. The functional sample was made and its distortion was analyzed by optical digitalization.

The initiative of a sustainable material system needs to lower the environmental and economic impact of production processes and adopt new ways of synthesizing and re-using materials. Even though the current conventional manufacturing processes, such as powder metallurgy, casting, forging, and rolling, have already shown their excellent ability to manufacture a large variety of parts and efficiently yield high volume products. Nevertheless, there are still many obstacles in manufacturing metallic components, such as complicated process procedures, high-energy consumption, large material waste, and high machinery cost for reasons that the excess materials need to be removed and extra post-processing time needs to be taken to acquire desired shapes during the machining stage. Thus, finding innovative solutions for producing complex structures is becoming increasingly desirable in the industry. Innovative additive manufacturing (AM, also known as 3D printing) techniques have proved their capacity to manufacture metallic materials with designed complex shapes and tailored properties. The selective laser melting (SLM) is one of the most popular AM techniques, which has the ability to manufacture a wide range of metallic powders in a layer-wise manner and fabricate complex shapes without compromising dimensional accuracy. The toxicity, biocompatibility, corrosion resistance, and stress shielding effect are the key challenges for developing titanium biomaterials for orthopedic applications. Adding nontoxic alloying elements into titanium can solve the issues of toxicity and biocompatibility. One of the best solutions for minimizing the stress-shielding effect and prolonging implant lifetime is to tailor the modulus of implant materials closer to that of bones. Nb is a nontoxic alloying element and an excellent β phase stabilizer, which plays a significant role in reducing the elastic modulus and in improving the corrosion resistance of Ti-based alloys. Accordingly, obtaining a highperformance simple alloy by reducing the alloying elements and substituting toxic elements can facilitate the improvement of sustainability. Thus, the β-metastable Ti-Nb alloys with relatively low elastic modulus have been studied for orthopedic implants due to their high strength to weight ratio, excellent corrosion resistance, and high biocompatibility in the human body. In addition, the high reactivity of titanium with hydrogen and oxygen as well as the high melting points of titanium alloys make conventional manufacturing difficult and cost intensive. As such, the SLM provides an innovative solution to manufacture shape-complicated products in a building chamber under the flow of high purity argon gas to minimize oxidation. However, the availability, printability, and high cost of high-quality raw metallic alloy powder are the limits for the SLM process. The individual elemental powder is relatively cheap and easy to manufacture. Thus, the use of elemental powder mixture results in greater alloy choices as well as lower cost and wider commercial availability. The issues of resultant microstructural and chemical inhomogeneity of the produced parts using the powder mixture have been the major concerns and challenges in the field. Since the mechanical behaviors and chemical properties directly depend on the microstructural homogeneity and phase composition, an in-depth understanding of the effect of inhomogeneity is required. It is necessary to have further advances in manufacturing optimization to extend the benefit of low production costs. In particular, in-situ alloying prospects make SLM a potential route to use a powder mixture with near infinite chemical compositions to synthesize desired titanium alloys for broad applications. As such, synthesizing the proper titanium alloys using the SLM technique, minimizing defect formation, controlling phase composition, evaluating their properties, and investigating the performances of SLM-processed products could significantly advance the applications in various industries and academia. The aim is to apply the SLM technique to process titanium alloys for biomedical and industrial applications. The results help to improve the scientific understandings of the interrelation among alloy compositions, processes, microstructures, defects, properties, and deformation behaviors of 3D-printed parts. Chapter 1 introduces additive manufacturing (AM) has huge potential to realize new alloys with flexible design and easy manufacturing. Especially for the customized healthcare products and services, such as biomedical implants, prosthetics, and hip replacement. Titanium alloys have desirable properties for various applications. Combining additive manufacturing with affordable and biocompatible titanium alloys can further advance and benefit the healthcare industry. Accordingly, the objectives are to fabricate titanium alloys by SLM and to investigate the microstructure, mechanical performance, and corrosion properties. Chapter 2 overviews the type, utilization, and advantage of AM techniques, biomaterials, and titanium alloys. The SLM process can manufacture parts with high precision and superb asbuilt surface quality but relatively high residual stress due to the rapid cooling rate. The raw powder properties and processing parameters play important roles in the densification and mechanical property of built products. The physical factors in the melting process and simulation are shown to understand the melt pool characteristics and stability, which is the critical factor to a successful and desired part. The microstructure, mechanical properties, and corrosion performance of different titanium alloys are also reviewed in order to design the powder, understand the mechanism, and improve the properties. Chapter 3 shows insight into the manufacturing of a Ti-35Nb composite using SLM and post heat treatment. The results emphasize the capability of SLM to fabricate alloys from elemental powder mixtures, even suitable for those with a significant difference in melting point. It provides a significant advance in the understanding of the effect of microstructural inhomogeneity on the resultant mechanical and chemical properties. Heat treatment can further enhance the corrosion resistance of SLM-produced Ti-35Nb samples because the improved chemical homogeneity can facilitate the homogeneous formation of titanium oxides and niobium oxides. It presents a different method of synthesizing novel β-type composites at a relatively lower cost and in easy manufacture. Chapter 4 shows the microstructure, phase response, and mechanical properties of the SLM-fabricated Ti-35Nb using an elemental powder mixture with reduced Nb particle size and its heat-treated counterpart. The results provide significant advances in the understanding of the role of undissolved Nb particles, Nb-rich interfaces, and Ti-Nb-based β phases on the mechanical performance. The nanoindentation mappings provide direct evidence of the contribution of the different phase responses to overall mechanical properties. The Nb particle segregation zones have lower hardness and higher deformation compared to the Ti-Nb matrix. The as-SLMed Ti- 35Nb exhibits relatively high tensile yield strength (648 ± 13 MPa) due to the formation of dendritic β grains. However, the ductility is relatively low (3.9 ± 1.1%) as a result of the weak bonding of undissolved Nb particles within the matrix. The heat-treated counterpart shows a slightly lower yield strength (602 ± 14 MPa) but a nearly 43% increase in ductility (5.6 ± 1.9 %) due to the improved homogeneous Ti-Nb β phase. Chapter 5 shows the microstructure, phase composition, melt pool morphology, and mechanical properties of a prealloyed Ti-35Nb alloy manufactured using SLM and compares it to one produced using an elemental powder mixture. The SLM-processed Ti-35Nb from both feedstocks retained a high volume fraction of β phase due to adequate β stabilization by the Nb and the fast cooling of the SLM process; however, other phase compositions were quite different. The chemical heterogeneity and inhomogeneous microstructure of the SLM-produced sample from powder mixture are results of the fast cooling rate of the melt pool and the high difference of melting temperature and density between elemental powders. However, a uniform microstructure and chemical composition can be achieved in the SLMed prealloyed Ti-35Nb. The variances of powder morphology, density, and melting point between mixed powder and prealloyed powder induce different melt pool status, where the stability of the melt pool plays a critical role in the homogeneity and microstructure. The SLMed Ti-35Nb prealloyed powder samples present a slightly lower yield strength (485 ± 28 MPa) but higher plastic strain (23.5 ± 2.2 %). The excellent ductility has been attributed to the high homogeneity, strong interface bonding, and the existence of a large amount of β phase. Chapter 6 shows the understanding of the homogeneity effect on the coexistence of the acicular α″, β grains, and melt pool boundary for a homogeneous microstructure. It provides some new insight into the phase response and the effect of homogeneity on the SLMed Ti-35Nb alloy using prealloyed powder. The reduced elastic modulus of β phase (89.6 ± 2.1 GPa) is close to that of α″ phase (86.3 ± 2.0 GPa) from the indentation measurement, which is in favor of orthopedic implants application. It also reveals that the nanoindentation test can provide a fast mapping and considerable potential to evaluate the homogeneity, microstructural features, individual phase strength, and deformation behavior in a fine microstructure of SLM-fabricated metallic alloys. Chapter 7 shows the preliminary design in porous structures and compressive behavior of different prealloyed Ti-35Nb sandwich composite porous structures manufactured using SLM. The simulation results were in good agreement with the compression tests. The compression tests show that the sandwich composites with different layers have different deformation behavior and mechanical properties. The rhombic dodecahedron porous structure with added layers could achieve balanced compressive strength and ductility. The preliminary sandwich design with the verified finite element method (FEM) models can be employed in other metallic porous structures to improve the strength and ductility without affecting the porosity. Chapter 8 concludes the present findings in this thesis and suggests the future challenges and development using SLM to tailor titanium alloys for specific applications. As such, the SLM technique is a promising route to develop titanium alloys from powder mixture with wider alloy choices at a cheaper cost and in easier availability. Even though a uniform microstructure and chemical composition can be achieved in the SLM-produced Ti-35Nb using prealloyed powder, there are still challenges on how to achieve full melting of elemental powder particles and obtain a homogeneous β phase microstructure. With the investigation of β- type Ti-Nb alloys, this thesis aims to further understand the effect of the unmelted Nb particles in the synthesized Ti-Nb alloys and melt pool stability as well as improve the Nb melting, microstructure, and mechanical properties for industrial and biomedical applications. Understanding the effect of powder feedstock type and phase features of the SLM-produced Ti- 35Nb using prealloyed powder further provides insights into the homogeneity, microstructure, and resultant properties. The novel design in Ti-35Nb sandwich composite cellular structures can benefit biomedical and industrial applications. By taking advantage of the commercial availability and lower cost of elemental powder, finding solutions to achieve full melting and homogeneous microstructure for nontoxic and biocompatible β-type Ti-Nb alloys with promising mechanical and corrosion properties is significant in future research and development.

Recently, the additive manufacturing techniques (e.g., selective laser melting and electron beam melting) have attracted great attention in manufacturing titanium and titanium alloys because the additive manufacturing techniques could produce parts with almost no geometric constraints. Many researchers have studied the mechanical properties of additive manufactured titanium and titanium alloys, and the results indicated that the additive manufactured titanium and titanium alloys have different degrees on enhanced mechanical properties compared with the alloy manufactured by traditional methods. However, the titanium alloys in applications usually encounter the environment of corrosion, such as biomedical, aerospace, chemical implants, etc. Unfortunately, rare studies have addressed the corrosion behavior of the additive manufactured titanium products. The present corrosion studies on additive manufactured titanium alloys show that the α’ phase presented in the selective laser melted samples has a detrimental effect on the corrosion resistance in simulated seawater and simulated human body fluids. Even the α’ phase in the selective laser melted titanium alloys was eliminated by heat treatment, the selective laser melted titanium alloys still show inferior corrosion resistance in simulated seawater and human body fluids compared to the counterparts produced by traditional methods. By contrast, the electron beam melted titanium alloy have a slightly better corrosion resistance than the wrought sample as the microstructure of electron beam melted titanium alloy only contain α phase and β phase. Therefore, the current issue is arising and to whether the unique phase produced by selective laser melted titanium alloys affect the corrosion resistance? By overviewing the current literature, the corrosion study of the additive manufactured titanium alloys are mainly focused on the Ti-6Al- 4V, CP-Ti, and Ti-Al alloys but rare research on the other titanium alloys. In this thesis, the corrosion behavior of Ti-5Cu and Ti-24Nb-4Zr-8Sn produced by the selective laser melting has been systematically investigated and discussed. The electrochemical testing methods, such as open circulate potential, electrochemical impedance spectroscopy, and potentiodynamic polarization curve has been employed. Transmission electron microscopy, scanning electron microscopy, optical microscopy, and X-ray diffraction has been jointly used to characterize the microstructure and surface morphology of the samples. The inductively coupled plasma has been applied to examine the released ions in the electrolyte after electrochemical corrosion tests. The selective laser melted Ti-5Cu has a severe pitting phenomenon which can be alleviated by heat treatment. The heat-treated Ti-5Cu samples produced by selective laser melting exhibit a significant passivation behavior instead of pitting phenomenon in potentiodynamic polarization test. Such a phenomenon is attributed to that the present α’ phase in melting pool boundary forms a galvanic couple, causing severe pitting corrosion. In another study, the selective laser melted Ti-24Nb-4Zr-8Sn and wrought Ti-24Nb-4Zr-8Sn only contain single β phase and they display similar corrosion results. The above results suggest that the selective laser melted titanium alloys have a great potential to replace the conventionally manufactured titanium alloys in the simulated seawater environment and simulated body fluid environment.

Uno dei temi chiave della ricerca sull’Additive Manufacturing (AM) è l’accrescimento del campo di materiali processabili. La tecnologia garantisce diversi vantaggi rispetto alle normali tecniche di lavorazione ma richiede esperienza nella lavorazione e conoscenza del materiale. Nello studio oggetto di questa dissertazione si mostrano i risultati di un primo approccio alla lavorazione di WC-Co (carburo di tungsteno in matrice di cobalto) attraverso Selective Laser Melting (SLM). Lo scopo è di gettare le basi per un processo che permetta la produzione di utensili da taglio in WC-Co. Lo studio si è incentrato su un’attenta analisi microstrutturale di diversi gruppi di campioni processati attraverso SLM e prodotti a partire da due polveri a diverso contenuto di Co (9% e 17%). Le densità, le durezze e le microstrutture ottenute sono state confrontate con quelle relative a prodotti sinterizzati industriali. Lo studio ha portato a nuove conoscenze sulla microstruttura del materiale derivante dalla lavorazione ed ha permesso di stabilire relazioni tra parametri di processo e densità del prodotto finale. Per l’ultimo gruppo di campioni prodotto (17% di Co) si è ottenuto un miglioramento in termini di qualità del materiale prodotto, nel particolare si è incrementata la densità e ridotta l’estensione di difetti microstrutturali quali cricche e porosità.

This study attempts to develop a set of parameters controlling the bead profile of deposits in powder melting, based on the spatial energy distribution of laser. Four parameters, identified as the laser material interaction parameters were used to study the bead profile formation in powder melting. The focus is put on control of the dimensional accuracy of powder deposits independently of the optical set-up and laser system. In the initial stage to understand the effect of welding parameters on the development of the fusion zone, a solid metal with homogenous and known thermal properties was used. The results indicate that for large beam diameters, typically used in cladding, power density and interaction time control the depth of penetration and beam diameter and interaction time controls the weld width. However, for small beam diameters, typically used in powder bed additive manufacturing, it was found that it is more difficult to achieve steady state conduction welds due to high conduction losses to the bulk material and rapid transition to keyhole regime. Therefore, with small beam diameters it is challenging to achieve pure conduction welds, which should guarantee good quality of deposits and low spatter. In the second part, the melting behaviour of solid material and powder for the same material type was compared. The build height in powder melting depends on layer thickness of the deposited powder and energy density, which needs to be provided to fuse the powder to the workpiece, which is equivalent to penetration in laser welding of solids. Similar to solid melting, the build width in powder melting is controlled by beam diameter and the interaction time. It was also found that with small beam diameters and large particle sizes it is more difficult to generate keyhole in the base plate, as compared to solid material. Therefore, despite the presence of spatter in the process, a full keyhole is often not generated. A set of parameters to describe the conduction welding process based on spatial distribution of laser energy has been developed. This enables achievement of a particular weld profile with various optical set-ups and potentially transfers of results between machines. However, more complex melting characteristics of powder requires some additional factors to be included to develop a similar model for powders.

During the Selective Laser Melting (SLM) process large temperature gradients can form, generating a mismatch in elastic deformation that can lead to high levels of residual stress within the additively manufactured metallic structure. Rapid melt pool solidification causes SLM processed Ti6Al4V to form a martensitic microstructure with a ductility generally lower than a hot working equivalent. Post-process heat treatments can be applied to SLM components to remove in-built residual stress and improve ductility. This investigation sought to investigate and understand the root cause of residual stress formation and lower ductility in Ti6Al4V components when processed by SLM, with the aim of ultimately being able to reduce the residual stress and enhance ductility by SLM parameter adjustment. The effect of individual SLM parameters on residual stress was studied by using hole drilling method. Microstructural analysis, tensile, and Vickers hardness testing was used to understand the effect of SLM parameters on mechanical properties of Ti6Al4V components. Experimental study was carried out using a commercial Renishaw AM250 SLM machine and a modified Renishaw SLM125 machine. FEA modelling in ABAQUS with user subroutines DFLUX and USDFLD was used to predict the correlation between SLM parameters, cooling rates and temperature gradients. The experimental investigation included studying the effect of scanning strategy, layer thickness, rescanning, power and exposure variation keeping energy density constant, and bed pre-heating temperature on residual stress and mechanical properties of SLM Ti6Al4V parts. Finally an I-Beam geometry was created to identify the geometrical dependence of residual stress in SLM Ti6Al4V components. Stress reduction strategies from the study of individual SLM parameters were strategically applied to high stress regions of the I-Beam geometry to devise techniques for stress reduction across the cross section of a complex geometry.

By using camera systems – suitable for industrial applications – in combination with a large number of different measurement sensors, it is possible to monitor laser welding processes and their results in real-time. However, a low signal to noise ratio at framerates up to 2,400 fps allows only limited statements about the process behavior; especially concerning the analysis of new welding parameters and their impact on the melting bath. This article strives towards research of kinetic and geometric dependencies of the melting zone induced by different laser parameters through usage of a camera system with a high frame rate (1280x800 by 3,140 fps) in combination with model-driven image and data processing.

The effect of annealing on the tribological and corrosion properties of Al–12Si samples produced by selective laser melting (SLM) is evaluated via sliding and fretting wear tests and weight loss experiments and compared to the corresponding material processed by conventional casting. Sliding wear shows that the as-prepared SLM material has the least wear rate compared to the cast and heat-treated SLM samples with abrasive wear as the major wear mechanism along with oxidation. Similar trend has also been observed for the fretting wear experiments, where the as-prepared SLM sample displays the minimum wear loss. On the other hand, the acidic corrosion behavior of the as-prepared SLM material as well as of the cast samples is similar and the corrosion rate is accelerated by increasing the heat treatment temperature. This behavior is due to the microstructural changes induced by the heat treatment, where the continuous network of Si characterizing the as-prepared SLM sample transforms to isolated Si particles in the heat-treated SLM specimens. This shows that both the wear and corrosion behaviors are strongly associated with the change in microstructure of the SLM samples due to the heat-treatment process, where the size of the hard Si particles increases, and their density decreases with increasing annealing temperature.

The accomplished doctoral study concerns the interaction of the powerful laser radiation with powder metallic materials. The problem is of a great scientific interest, since it is a multi-disciplinary subject integrating powder metallurgy, thermo-physics, radiation and heat tranfer, phase transformations. Along with this, the subject has a considerable practical interest because Laser-assisted Direct Manufacturing based on Selective Laser Melting (SLM) is an emerging technology for manufacturing 3D functional objects with great added value, and also complex customized parts. Systematic study is accomplished for the powder materials currently employed in laser-assisted direct manufacturing : stainless steel 316L (-25 µm), tool steel H13 (-25 µm), Inconel 718 (-25 µm), CuNi10 (-25 µm), titanium grade 2 (-25 µm) and NiTi (-45 µm) ; stainless steel 904L (-16 µm et -7 µm), Inconel 625 (-16 µm), Co212F (CoCr, -31 µm). The above mentioned powders were employed in the experimental study for fabrication of 2D planar objects, 3D models and functional components. Comprehensive experimental research on laser-matter interaction are carried out for interaction of a powerful (0. 3-1. 3x106 W/cm²) moving laser beam with a complex system « metallic powder on solid metallic substrate ». Manufacturing strategies allowing 100% density on the fabricated objects are found. Optimal parameters for stable SLM process are determined
L'objectif principal de la thèse de doctorat présentée dans ce mémoire est l'étude de l'interaction d'un faisceau laser de puissance avec des poudres métalliques. Le sujet est d'un grand intérêt scientifique par sa multidisciplinarité intégrant la métallurgie de poudres, la physique thermique, le transfert de chaleur et radiatif, la transformation de phases. En même temps, le sujet a une signification pratique considérable car la Fabrication Directe par fusion laser sélective des poudres (SLM) est une technologie émergente de fabrication d'objets 3D avec une grande valeur ajoutée et de pièces fonctionnelles complexes sur mesure. Une étude systématique a été réalisée sur les poudres actuellement utilisées dans la Fabrication Directe assistée par laser : Inox 316L (-25 µm), acier d'outillage H13 (-25 µm), Inconel 718 (-25 µm), CuNi10 (-25 µm), Ti grade 2 (-25 µm) et NiTi (-45 µm) ; Inox 904L (-16 µm et -7 µm), Inconel 625 (-16 µm), Co212F (CoCr, -31 µm). A partir de ces poudres, des objets plats 2D, des modèles 3D et des pièces fonctionnelles ont été fabriqués. Des recherches expérimentales approfondies sur l'interaction laser/matière sont effectuées, plus particulièrement sur l'interaction d'un faisceau laser de haute puissance mobile (0. 3-1. 3x106 W/cm²) avec un système complexe de poudres métalliques sur substrat métallique solide. Les stratégies de fabrication permettant d'obtenir la densité 100% de pièces résultantes sont identifiées. Les paramètres optimaux pour assurer la stabilité du procédé SLM sont définis

The diploma thesis deals with finding suitable process parameters for the production of molding segment by SLM technology. It consists of a combination of structures, shells, thin slats and bulk parts. The tested material is maraging steel 300. The research part deals with the problem of choice of suitable process parameters, such as laser power and speed, hatch distance and thickness of the built layer. The achievable mechanical properties of the parts and the choice of the suitable structure were also examined. In the thesis were found suitable process parameters for printing of bulk parts and structures.

The diploma thesis deals with the study of the influence of process parameters of AlSi7Mg0.6 aluminum alloy processing using the additive technology Selective Laser Melting. The main objective is to clarify the influence of the individual process parameters on the resulting porosity of the material and its mechanical properties. The thesis deals with the current state of aluminum alloy processing in this way. The actual material research of the work is carried out in successive experiments from the welding test to the volume test with subsequent verification of the mechanical properties of the material. Material evaluation in the whole work is material porosity, stability of individual welds, hardness of the material and its mechanical properties. The results are compared with the literature.