Dissertations / Theses on the topic 'Hot metal forming'

Oxide scale can affect many aspects of hot metal forming operations, such as heat transfer, friction and the final surface finish of the rolled product. Surface oxide scale is always present on the steel slab and sometimes on cold rolls. Therefore, the study of the thermo-mechanical behaviour of oxide scale, particularly under conditions as close as possible to the steel manufacturing process is very important. In order to undertake a detailed study of oxide scale behaviour, several hightemperature testing techniques were applied. First, high-temperature tensile tests were carried out to investigate the brittle fracture and cracking of the surface oxide scales. Second, high-temperature compression tests were developed and the results obtained revealed many different failure mechanisms that are present in the compressed oxide scale. Finally, a tensile-compressive test was developed and performed under thermal conditions which were as close as possible to hot rolling. The best results in the understanding of oxide scale failure were achieved by closely linked combination of laboratory testing and measurements coupled with detailed finite element analysis. A close microstructural examination of the morphology of oxides was carried out after each experiment and finite element modelling was performed. The three-dimensional finite element simulations helped to improve the interpretation of the thermo-mechanical testing and to obtain more accurate heat transfer and stress-strain relationships. In this work the following thermo-mechanical failure modes of the oxide scales were observed and investigated: brittle fracture (through-thickness cracking, blister failure), indications of plastic behaviour (arrested cracks, unbroken top layers of the oxide scale) and a sticking effect (equivalent to the mill pick-up).

A decoupled thermo-mechanical model which includes elastic behaviour is cast into a finite element formulation, which is numerically unified for simulation of both cold and hot metal forming processes. Computational attention has been focused on the mechanical aspects, where the elasto-plastic constitutive law is utilized for cold (practically rate-independent) regimes, while metalurgically sound high temperature interpolation function are employed in the computational framework of Perzyna type elasto-viscoplasticity for hot (rate-dependent) processes. Introduction of a logarithmic strain based finite strain model within the context of a geometrically nonlinear assumed strain method characterizes the numerical treatment of incompressibility at large elastic-inelastic deformations. The plastic theory of quasi-static friction with a non-associated slip rule is employed for general interface frictional contact condition. The contact constraints are imposed pointwise at the specimen (slave) boundary nodes. The assumption of rigid tools is made and nonlinear tool (master) segment geometry describing the contact kinematics is introduced. Consistent linearization in all aspects of algorithmic development provides robust and quadratically convergent solutions. After the capabilities of the model in representing physically realistic behaviours are rigorously tested in plane strain localization and axisymmetric necking benchmark tests, several numerical examples are presented, where simulations of both cold and hot metal forming processes including spike forming, industrial forging, flat rolling, tube expansion and thin sheet forming are performed. The robust and consistent numerical treatment of the thermo-mechanical theoretical formulations ensures generality and future upgradability of the model.

In the last two decades the international community has been looking for solutions to preserve the environment, and in particular the atmosphere, from the CO2 emissions through the car exhausts, considered one of the main responsible of the greenhouse effect and, therefore, of the Earth temperature increase. Rules and limits were fixed in the 1997 with the Kyoto Protocol that entered in force in 2005, by which the international community signed the legal responsibility for producing vehicles with CO2 emission limited to 95g/km to be reached in 2020. The production of cars using lightweight materials can represent an optimal solution because the lower weight means lower energy consumption. Therefore, the automotive companies are now investigating the feasibility of producing parts made of lightweight materials to replace conventional steels for the car chassis and body-in-white components, but without decreasing the passenger safety. High resistance steels and aluminium alloys have demonstrated to be the best solution thanks to their low density, high corrosion resistance and excellent stiffness-to-weight ratio. In case of use of aluminium alloy sheets and tubes, it is possible to reduce the car weight of about 15–20 % with also a consequent weight reduction of all the connected vehicle parts and therefore a substantial reduction of the pollutant exhausts. The main limit of light alloys is the poor formability and the high springback exhibited during room temperature deformation. Temperature assisted processes have proven to increase material formability: Superplastic and Quick Plastic Forming, already used for shaping aluminium sheets, have shown a relevant increase in the material formability allowing to form very complex parts but are extremely expensive due to the very long process times, therefore not applicable for mass production. On the other hand, cold and warm hydroforming processes, nowadays at the state-of-the-art for shaping hollow components, exhibit very high initial investment cost due to the high pressure of the fluid used as deformable mean and to the high tons presses needed for keeping the dies closed during the process. Moreover, a strict forming temperature limit is fixed by the fluid boil and burst temperatures, which may limit the material formability. In this research work, innovative forming processes were investigated to prove the feasibility of shaping aluminium sheets and tubes at high temperature, exceeding the limits of the already available process technologies. In particular, the Hot Stamping (HS) technology was applied to form 5xxx and 6xxx series aluminium alloys proving the capability of stamping an automotive component on a hot stamping industrial plant, and thus validating the laboratory tests results. An experimental apparatus able to work with the innovative technology of the Hot Metal Gas Forming (HMGF) process was designed and developed to form aluminium alloy tubes. In doing so, resistance heating was used as heating system and cold air in pressure was used to bulge-up the tubes during the process. The formability of different 6xxx series aluminium alloys tubes was investigated by means of free bulging tests and, afterwards, shaping component inside a die, evaluating the influence of the most important process parameters. Finally, in collaboration with an industrial company, the shaping of an aesthetic component with also the evaluation of the surface appearance was carried out demonstrating the applicability of the new process to form an industrial part.
Negli ultimi decenni, la comunità internazionale è alla continua ricerca di provvedimenti per salvaguardare l’atmosfera e l’ambiente terrestre. In campo automobilistico e dei trasporti la produzione di biossido di carbonio dai gas di scarico delle autovetture, meglio conosciuto come CO2, è ritenuto tra i maggiori responsabili del rafforzamento dell’effetto serra e dunque dell’innalzamento del clima terrestre. Per porre un concreto rimedio e regolamentare l’efficienza sul consumo medio di un autoveicolo, con il protocollo di Kyoto stipulato nel 1997 ed entrato in vigore nel 2005, la comunità internazionale si è impegnata legalmente alla produzioni di veicoli in grado di rispettare il limite di emissione di 95 g di CO2 per kilometro entro l’anno 2020. L’alleggerimento complessivo di un automobile è sicuramente tra le soluzioni più immediate per la riduzione delle particelle inquinanti, in quanto veicoli più leggeri richiedono minore forza motrice e di conseguenza minore consumo di energia. Per questo motivo le compagnie automobilistiche negli ultimi anni sono alla ricerca di materiali innovativi per sostituire l’acciaio che comunemente è impiegato per la realizzazione di telai e parti di carrozzeria, senza pregiudicare la sicurezza dei passeggeri. Gli acciai alto resistenziali ma soprattutto le leghe leggere, hanno dimostrato essere delle ottime alternative grazie alle loro proprietà di bassa densità, resistenza alla corrosione, ed ottimo rapporto rigidezza-peso. Con l’utilizzo di parti stampate ma anche di elementi tubolari in lega di alluminio il peso medio della sola scocca di una vettura può essere ridotto del 15 – 20 %, portando ad un conseguente ridimensionamento di tutte gli organi connessi ed ad una sostanziale riduzione delle emissioni dannose. La principale limitazione nella lavorazione delle leghe di alluminio è la loro scarsa attitudine a subire deformazione plastica a temperatura ambiente collegata oltretutto ad un elevato ritorno elastico. Per far fronte a questa problematica, numerosi processi innovativi utilizzanti alta temperatura sono stati o sono tuttora in fase di studio con l’obiettivo principale di incrementare la formabilità del materiale. I confermati processi di deformazione di lamiera di alluminio quali Superplastic Forming e Quick Plastic Forming, hanno dimostrato sicuramente un vantaggio in termini di formabilità riuscendo oltretutto a generare parti complesse, ma sono d’altro canto estremamente costosi e soggetti a tempi molto lunghi di processo, per cui non applicabili per produzioni in larga scala. L’idroformatura a freddo e a tiepido, invece, che rappresenta l’attuale tecnologia all’avanguardia per la sagomatura di parti cave, oltre a necessitare di elevati costi iniziali connessi alle elevate pressioni del fluido necessarie per la deformazione e alle presse ad alto tonnellaggio richieste per la chiusura degli stampi durante l’iniezione del liquido stesso, presenta severi limiti nella temperatura massima di processo. Infatti le emulsioni acqua olio generalmente impiegate come mezzo deformante risultano infiammabili al di sopra del campo tiepido per l’alluminio, limitando dunque il range termico utilizzabile per il processo e di conseguenza la formabilità del materiale. In questo lavoro di ricerca sono stati studiati processi innovativi per la produzione di componenti di alluminio in lamiera e tubolari che superassero i limiti di processo delle attuali tecnologie produttive. In particolare la tecnologia dello stampaggio a caldo (Hot Stamping), oggigiorno applicata agli acciai alto resistenziali, è stata applicata con successo su lamiere di alluminio serie 5xxx e 6xxx, e validata con test industriali eseguiti su una vera linea di stampaggio producendo un componente automobilistico. Inoltre è stato realizzato e sviluppato un prototipo in grado di operare con la tecnologia innovativa del Hot Metal Gas Forming, che utilizza gas in pressione invece di fluidi per deformare componenti tubolari al alta temperatura. Prove di formabilità su tubi di alluminio serie 6xxx, ma anche la realizzazione di componenti in stampo, hanno permesso inoltre lo studio di numerosi aspetti critici per il processo. In fine, la sagomatura di un componente industriale in collaborazione con una azienda, curando oltretutto la qualità estetica del formato, ha permesso di verificare l’applicabilità e l’efficacia di questo processo anche a livello industriale.

WEAR ANALYSIS OF HOT FORGING DIES ABACHI, Siamak M. S., Department of Mechanical Engineering Supervisor: Prof. Dr. Metin AKKÖ
K Co-Supervisor: Prof. Dr. Mustafa lhan GÖ
KLER December 2004, 94 pages The service lives of dies in forging processes are to a large extent limited by wear, fatigue fracture and plastic deformation, etc. In hot forging processes, wear is the predominant factor in the operating lives of dies. In this study, the wear analysis of a closed die at the final stage of a hot forging process has been realized. The preform geometry of the part to be forged was measured by Coordinate Measuring Machine (CMM), and the CAD model of the die and the worn die were provided by the particular forging company. The hot forging operation was carried out at a workpiece temperature of 1100°
C and die temperature of 300°
C for a batch of 678 on a 1600-ton mechanical press. The die and the workpiece materials were AISI L6 tool steel and DIN 1.4021, respectively. The simulation of forging process for the die and the workpiece was carried out by Finite Volume Method using MSC.SuperForge. The flow of the material in the die, die filling, contact pressure distribution, sliding velocities and temperature distribution of the die have been investigated. In a single stroke, the depth of wear was evaluated using Archard&rsquo
s wear equation with a constant wear coefficient of 1¥
10-12 Pa-1 as an initial value. The depth of wear on the die surface in every step has been evaluated using the Finite Volume simulation results and then the total depth of wear was determined. To be able to compare the wear analysis results with the experimental worn die, the surface measurement of the worn die has been done on CMM. By comparing the numerical results of the die wear analysis with the worn die measurement, the dimensional wear coefficient has been evaluated for different points of the die surface and finally a value of dimensional wear coefficient is suggested. As a result, the wear coefficient was evaluated as 6.5¥
10-13 Pa-1 and considered as a good approximation to obtain the wear depth and the die life in hot forging processes under similar conditions.

Semi-hot or warm forging is an economical alternative to the conventional forging processes by combining advantages of hot and cold forging processes. In this study, a new forging process sequence and design of the preform die for a part which has been produced by hot forging are proposed to be produced by semi-hot forging. Thermo-mechanical finite element analyses are performed over the stages of forging process. The billet and the dies are modeled as elastic-plastic bodies. Effects of preform die geometry on the die stresses and the forging load are investigated using finite element method. Comparison of the results obtained by using two different commercial finite element analysis programs is done for semi-hot and hot forging temperature ranges. The forging temperatures are determined for the particular part and the experiments are conducted by using the 1000 ton forging press. The parts are produced without any defects and material wastage in the form of flash is reduced. The numerical results are also compared with the experimental results and a good agreement is achieved.

Sheet metal working operations at elevated temperature have gained in the last years even more importance due to the possibility of producing parts characterized by high strength-to-mass ratio. In particular, the hot stamping of ultra high strength quenchenable steels is nowadays widely used in the automotive industry to produce body-in-white structural components with enhanced crash resistance and geometrical accuracy. The optimization of the process, where deformation takes place simultaneously with cooling, and of the final component performances requires the utilization of FE-based codes where the forming and quenching phases have to be represented by fully thermo-mechanical-metallurgical models. The accurate calibration of such models, in terms of material behaviour, tribology, heat transfer, phase transformation kinetics and formability, is therefore a strong requirement to gain reliable results from the numerical simulations and offer noticeable time and cost savings to product and process engineers. The main target of this PhD thesis is the development of an innovative approach based on the design of integrated experimental procedures and modelling tools in order to accurately investigate and describe both the mechanical and microstructural material properties and the interface phenomena due to the thermal and mechanical events that occur during the industrial press hardening process. To this aim, a new testing apparatus was developed to evaluate the influence of temperature and strain rate on the sheet metal elasto-plastic properties and to study the influence of applied stress and strain of the material phase transformation kinetics. Furthermore, an innovative experimental setup, based on the Nakazima concept, was designed and developed to evaluate sheet formability at elevated temperature by controlling the thermo-mechanical parameters of the test and reproducing the conditions that govern the microstructural evolution during press hardening. This equipment was utilized both to determine isothermal forming limit curves at high temperature and to perform a physical simulation of hot forming operations. Finally, a thermo-mechanical-metallurgical model was implemented in a commercial FE-code and accurately calibrated to perform fully coupled numerical simulations of the reference process. The material investigated in this work is the Al-Si pre-coated quenchenable steel 22MnB5, well known with the commercial name of USIBOR 1500P’®, and the developed approach proves to be suitable to proper evaluate high strength steels behaviour in terms of mechanical, thermal and microstructural properties, and to precisely calibrate coupled numerical models when they are applied to this innovative manufacturing technology. The work presented in this thesis has been carried out at DIMEG labs, University of Padova, Italy, from January 2005 to December 2007 under the supervision of Prof. Paolo F. Bariani.

Les objectifs de cette thèse de doctorat étaient de caractériser l’efficacité des revêtements de surface développé pour lutter contre les problèmes de transfert de matière rencontrés lors de la mise en forme de l’aluminium à chaud, et d'étudier ces mécanismes de transfert. Le matériau utilisé était un alliage d'aluminium AA 6082-T6, largement employé dans la fabrication de composants automobiles.Le test de Compression-Translation à chaud (WHUST) a été retenu comme tribomètre principal pour cette étude. Afin de contrôler précisément les températures des essais, un dispositif miniaturisé du WHUST a été conçu afin être intégré dans la chambre chauffante de la plateforme Bruker UMT TriboLab. Les tests préliminaires avec ce nouvel appareil ont montré un empilement significatif de matière devant le contacteur. De nouvelles équations analytiques ont donc été développées pour identifier le coefficient de frottement de Coulomb (COF) et le facteur de frottement (loi de Tresca) en tenant compte de cet empilement de matière.Le WHUST a ensuite été utilisé pour évaluer les performances tribologiques de trois revêtements PVD commerciaux : un AlCrN, un TiAlN et un Arc-DLC. Les expériences ont été menées sans lubrifiant, à des températures variant de 300 °C à 500 °C, sous des pressions de contact comprises entre 40 et 100 MPa, avec une vitesse de glissement égale à 0,5 mm/s. Les résultats ont montré que le revêtement Arc-DLC était plus efficace que les revêtements AlCrN et TiAlN pour atténuer les problèmes de transfert d’aluminium. En particulier, le revêtement Arc-DLC provoquait moins d'adhésion et moins de transfert d'aluminium, notamment lors du début du glissement. Ces résultats ont été confirmés par des essais sous des pressions de contact plus élevées, réalisés à l’aide l’essai de forgeage en T à chaud (HVGCT).Dans la deuxième partie de cette thèse, le revêtement Arc-DLC a été sélectionné pour étudier en détail les mécanismes de transfert d’aluminium sur les outils de mise en forme. Des essais ont été réalisés avec une courte distance de glissement (2 mm) pour examiner les premières étapes du transfert d’aluminium, tandis que des tests avec une distance de glissement de 38 mm ont permis d’étudier l’évolution du transfert. Les expériences ont été conduites aux mêmes températures d’essai (300-500°C), avec deux vitesses de glissement différentes, 0,5 mm/s et 5,0 mm/s, et toujours sans lubrifiant. Les topographies de surface et les images SEM prises le long de la piste de frottement ont montré que le transfert d'aluminium se produit en deux étapes principales : une phase initiale principalement due au labourage mécanique, suivie d'une phase de croissance dominée, en fonction des températures et des vitesses de glissement, par du labourage mécanique ou par de l'adhésion.Dans la dernière partie de cette thèse, l'apprentissage automatique (ML) a été utilisé pour étudier les mécanismes de transfert d’aluminium. Les topographies de surface et les images SEM prises le long de la piste de frottement ont été analysées. Elles ont été classifiées à l’aide de cinq algorithmes d’apprentissage automatique simples et d'une architecture de réseau neuronal convolutif (CNN) personnalisée. Il a été démontré que le ML appliqué aux données topographiques et le CNN appliqué aux images SEM permettaient tous deux d’identifier les modes d’usure avec précision
The aims of this PhD thesis were to find effective surface coatings to prevent the material transfer issue and to study the mechanisms of material transfer in the hot forming of aluminum alloy. The workpiece material was AA 6082-T6 aluminum alloy, which is widely used to produce automotive components.The warm and hot upsetting sliding test (WHUST) was selected as the main tribometer in this study. To control the testing temperatures precisely, a scaled-down apparatus of the WHUST was designed to integrate into the heating chamber of the Bruker UMT TriboLab platform. The preliminary experiments of the new apparatus found that the pile-up material significantly occurred in front of the contactor due to the high friction at the interface and the deformation characteristic of the aluminum alloy at high temperatures. From this point, the pile-up material was considered as a new parameter in analytical equations used to identify the Coulomb coefficient of friction (COF) and the shear friction factor.The new apparatus of the WHUST was then used to evaluate the tribological performance of three commercial PVD coatings: AlCrN, TiAlN, and Arc-DLC. The experiments were performed at temperatures between 300˚C and 500˚C, at 0.5 mm/s of sliding speed under non-lubrication contact conditions. Those conditions led to the mean contact pressure between 40 MPa and 100 MPa. The results showed that the Arc-DLC coating had better efficiency in alleviating the aluminum transfer issue than the AlCrN and TiAlN coatings. The Arc-DLC coating caused less adhesive to the aluminum alloy and less transferred aluminum, especially in the initial period. Moreover, these findings were consolidated under higher contact pressure by using the hot V-groove compression test (HVGCT).Following that, the Arc-DLC coating was selected to study the mechanisms of aluminum transfer on the forming tool in detail. The WHUST was performed with the specific short sliding distance (2 mm) to investigate the initial stage of aluminum transfer, while the full sliding distance (38 mm) was used to examine the evolution of aluminum transfer. The experiments were conducted at the same testing temperatures with two different sliding speeds, 0.5 mm/s and 5.0 mm/s, under non-lubrication contact conditions. It was found that the aluminum transfer in the initial stage was mainly caused by mechanical plowing. Then, during the grow-up stage, the aluminum transfer was dominated by mechanical plowing and/or adhesive bonding, depending on the testing temperatures and the sliding velocities. Additionally, the different transfer mechanisms caused dissimilar COFs, surface characteristics along the friction track of the specimen, as well as transferred aluminum.In the last part of this PhD thesis, Machine Learning (ML) was involved to study the mechanisms of aluminum transfer. The previous part found that the wear characteristics along the friction track could be a significant indicator to differentiate the transfer mechanisms. Thus, the surface topographies and the SEM images along the friction track were used to classify by five simple ML algorithms and a custom Convolutional Neural Network (CNN) architecture, respectively. It was proved that the ML with topographic data and the CNN with SEM image data had the potential to identify the wear mode accurately

Working at elevated temperature has its challenges due to the high level of complexity whenthe tribosystems operate under harsh conditions, commonly resulting in an increase on thefriction and thermal softening that goes into severe adhesion, severe abrasion and materialtransfer. Despite considerable research, there is a lack of research on tribology applied to hightemperature processes.  The aim of this project is to understand the tribological behavior of tool steel sliding againstaluminum under lubrication conditions working at high temperature. Salt-based, graphite-based, and polymer-based lubricants were evaluated as they are commonly used for aluminum forming. The cleanability of the lubricants after being subjected to elevated temperatures is also studied. High temperature tribological tests were carried out in a reciprocating sliding flat-on-flat configuration for 15 seconds. Optical microscope, SEM and EDS were performed to analyze the specimens after the tribological tests. Then a cleanability study was done to evaluate the cleanability of the lubricants and the effect of temperature on the cleanability of the lubricant. The concentration of the lubricants played an important role in the lubricant’s friction stability and dispersion, particularly for the polymer-based and graphite-based lubricants. Under the tested conditions the salt-based lubricant was ineffective as it showed high and unstable friction. The 10 wt.% polymer-based lubricant concentration presented severe adhesion and material transfer from the aluminum onto the tool steel.  The effect of temperature on the cleanability of the lubricants was correlated to the temperature in which the lubricants start to degrade. Nevertheless, the best cleanability was achieved when using ethanol as a cleaning agent in combination with high pressure spraying, and wirebrush techniques. Mild and high alkaline agents had poor cleanability abilities resulting insurface damage and corrosion on the tool steel.

Producing axi-symmetrical parts with holes from tubular stock by tube upsetting is a frequently used technique in industry. There are basically four types of tube upsetting process
external, internal, simultaneous internal and external upsetting, and expanding of tube. In general, tubular parts require more than one upsetting stage. In industry, generally trial-error methods, which require lots of time and effort depending on experience, are used for the design of stages. Wrong design causes failures during production. On the other hand, the problems, which are likely to be encountered in manufacturing, can be observed and solved in the design stage by using finite element analysis. In this study, the finite element analyses of external, internal, simultaneous internal and external tube upsetting, and tube expanding processes have been realized. During the analyses, the part and the die geometries at the intermediate stages, which have been designed according to the proposed procedures, have been used. The stress and strain distributions and die filling actions have been observed during the process. The process design and die geometries have been evaluated according to the finite element results. It has been seen that the recommended procedures generally generate acceptable designs. In some cases, it has been noted that minor modifications may be required on the design.

Forging is a metal forming process commonly used in industry. Forging process is strongly affected by the process temperature. In hot forging process, a wide range of materials can be used and even complex geometries can be formed. However in cold forging, only low carbon steels as ferrous material with simple geometries can be forged and high capacity forging machinery is required. Warm forging compromise the advantages and disadvantages of hot and cold forging processes. In warm forging process, a product having better tolerances can be produced compared to hot forging process and a large range of materials can be forged compared to cold forging process. In this study, forging of a particular part which is being produced by hot forging at 1200°
C for automotive industry and have been made of 1020 carbon steel, is analyzed by the finite volume analysis software for a temperature range of 850-1200°
C. Experimental study has been conducted for the same temperature range in a forging company. A good agreement for the results has been observed.

Induction Assisted Single Point Incremental Forming (IASPIF) is a die-less hot sheet metal forming. The IASPIF does not apply characteristic complex tooling like those applied in deep drawing and bending. In this thesis, induction heating was used to heat up the sheet while simultaneously forming with a tool. The research goal is to improve the formability of high strength steels by heating. The IASPIF consists of non-complicated set up that allows induction heating to be utilized through the coil inductor moved under the sheet and synchronized with the forming tool that moves on the upper side of the sheet. The advanced high strength steel alloys, DP980, DP600 and 22MnB5 steels, were investigated. The influence of induction heating on formability was evaluated by the maximum wall angle that can be achieved in a single pass. Additionally, tool diameter and tool feed rate was also varied. The most influencing parameters were tool feed rate, induction power, and the profile depth. A new forming strategy was also developed by control the heating temperature through coupling the formed profile depth with a successively increased tool feed rate. The forming forces of DP980 steel sheet, were reduced from 7 kN to 2.5 kN when forming process was performed at room and elevated temperature, respectively. Stretching stresses were developed during forming process causing a high reduction in the resulting wall part thickness. New findings in this investigation were the reverse relationship between the step-down depth and the thickness reduction percentage. The smaller the tool diameter, the better was the formability. The finite element simulation of the investigated forming process showed that the increase in heating temperature has a direct effect on rising the plastic strain from 0.2 at room temperature to 1.02 at 800 ◦ C. The maximum true strain achieved in the resulting wall part thickness was determined by FEM simulations and validated with experimental trials. The part shape accuracy was measured and the highest deflection was founded when the part was formed by the highest step-down depth. Moreover, the minimum deflection in the part shape was achieved by utilizing a high induction power in the experiments. Finally, the resulting mechanical properties of the 22MnB5 alloy sheet material were tailored during IASPIF. For this purpose, the sheets were locally heated by induction during the forming process and subsequently quenched at different rates. As a result, the produced tailored parts consist of three different regions, which consist of a ductile, transitional and hardened region. The proposed procedure allows forming and quenching at the same time without transfer and thus, process time was reduced.
Die induktionsgestützte, inkrementelle Blechumformung (englisch: Induction Assisted Single-Point Incremental Forming IASPIF) ist Warmumformprozess, bei dem keine komplexen Werkzeuge wie beim Tiefziehen und Biegen benötigt werden. Inhalt dieser Arbeit ist die inkrementelle Umformung eines Bleches mit gleichzeitig ablaufender induktiver Erwärmung. Das Forschungsziel bestand in der Verbesserung der Umformbarkeit von hochfesten Stahlwerkstoffen wie DP600, DP980 und 22MnB5 durch eine gezielte partielle Erwärmung. Der prinzipielle Aufbau des Versuchsstandes besteht aus einem Spuleninduktor, der unterhalb des umzuformenden Blechs platziert ist, und der synchron mit dem Werkzeug – einem Drückdorn – während des Umformvorganges verfährt. Ein wesentlicher Untersuchungsschwerpunkt bestand in der Ermittlung der Einflussgrößen auf den untersuchten IASPIF-Prozess. Für die Bewertung der Umformbarkeit wurden hierbei der maximal erreichbare Teilwandwinkel und die Profiltiefe, die in einem Umformdurchgang herstellbar waren, ermittelt und ausgewertet. Darüber hinaus konnten im Rahmen der Arbeit die Induktionsleistung des Generators, der Werkzeugdurchmesser und die Werkzeugvorschubgeschwindigkeit als relevante Prozessparameter identifiziert werden. Im Ergebnis der durchgeführten Untersuchungen zeigten die Werkzeugvorschubgeschwindigkeit und die Induktionsleistung einen wesentlichen Einfluss auf die erreichbare Profiltiefe. Aufbauend auf den erzielten Ergebnissen konnte eine prozessangepasste Umformstrategie entwickelt werden, bei der eine konstante Erwärmungstemperatur durch das Koppeln der momentanen Profiltiefe mit einer sukzessiv steigenden Werkzeugvorschubgeschwindigkeit erreicht wird. Weiterhin ließen sich die Kräfte bei der Umformung eines Stahlbleches aus DP980 von 7 kN (bei Raumtemperatur) auf 2,5 kN (bei erhöhter Temperatur) reduzieren. Aufgrund des mit einem Streckziehvorgang vergleichbaren Spannungszustandes während des Umformprozesses war eine starke Verringerung der resultierenden Wanddicke zu beobachten. Als neue Erkenntnis in dieser Untersuchung konnte die umgekehrte Beziehung zwischen der Zustelltiefe und dem Dickenreduktionsprozentsatz abgleitet werden. Aus der Finite - Elemente - Simulation des vorgestellten Umformprozesses wurde erkennbar, dass die Erhöhung der Erwärmungstemperatur einen direkten Einfluss auf die plastische Dehnung von 0,2 (bei Raumtemperatur) auf 1,02 (bei 800 °C) hat. Mittels der numerischen Simulation und der nachfolgenden experimentellen Validierung erfolgte darüber hinaus die Bestimmung der maximalen wahren Dehnung, die in der resultierenden Wanddicke erreicht wurde. Bei den Versuchen mit der größten Zustellung ließ sich durch die Bestimmung der Teileformgenauigkeit die höchste Abweichung von der Sollgeometrie CAD Modell feststellen. Abschließend wurde nachgewiesen, dass der IASPIF Prozess auch zur Einstellung maßgeschneiderter Bauteileigenschaften wie der resultierenden mechanischen Eigenschaften des Blechmaterials aus 22MnB5 einsetzbar ist. Zu diesem Zweck wurden die Bleche während des Umformprozesses lokal induktiv erwärmt und anschließend zur Einstellung des gewünschten Gefüges bei unterschiedlichen Abkühlgeschwindigkeiten abgeschreckt.

In automotive industry, forgings are widely used especially in safety related applications, typically suspension, brake and steering systems. In this study, forging process of a steering joint used in heavy vehicles has been examined. This particular part has a non-planar parting surface and requires a series of operations, which includes fullering, bending and piercing on a forging press. Forging companies generally use trial-and-error methods during the design stage. Also to ensure complete die filling at the final stage, extra material is added to the billet geometry. However, the forging industry is becoming more competitive finding a way to improve the quality of the product while reducing the production costs. For this purpose, a method is proposed for the design of the preform dies to reduce the material wastage, number of applied strokes and production costs. The designed operations were examined by using a commercially available finite volume analysis software. The necessary dies have been manufactured in METU-BILTIR CAD/CAM Center. The designed process has been verified by the experimental work in a forging company. As a result of this study, remarkable reduction in the flash, i.e. waste of material, has been achieved with a reasonable number of forging operations. In addition to forging of the steering joint, forging of a chain bracket, which has bent sections with planar parting surface, has also been observed and analyzed during the study. An intermediate bending stage has been proposed to replace the manual hammering stage and satisfactory results have been observed in simulations.

Hot stamping of aluminium alloys allows for increased formability, decreased springback and the possibility of integrating age-hardening heat treatments into the process. However, it can be challenging due to the occurrence of material transfer of aluminium onto the tool, as aluminium is prone to adhesion even at low temperatures. Hence, lubrication is always necessary when forming aluminium, but lubricants can still fail, leading to direct interaction between tool and workpiece and thus material transfer. This phenomenon reduces the efficiency of the process, as interruptions are necessary for the refurbishment of the tools. Understanding of how material transfer takes place is important in order to find new or improved solutions, in terms of lubrication and surface engineering, to prevent adhesion. Nevertheless, current research in high temperature tribology of aluminium, mainly in terms of material transfer mechanisms, is very limited, as many of the works focus on lubricated conditions and do not look into the fundamental interactions between aluminium alloys and tool steels. In this context, the aim of this work is to investigate the mechanisms behind the occurrence of aluminium alloy transfer onto tool steel during sliding at high temperature and in dry conditions. A hot-strip drawing tribometer was used to perform tests at room temperature, 300°C, 400°C, and 500°C, directly after solubilizing the aluminium alloy at 520°C. Two different topographies for the tool steel were used: ground and polished. Material transfer characterization was performed mainly through scanning electron microscopy. It was found that grinding marks (ground tool steel) and carbides (polished tool steel) act as initiation sites for the transfer to occur. Temperature plays a role on the growth mechanisms of the transfer films during sliding, as thermal softening of the aluminium alloy is the dominant factor in determining the growth direction of the transfer layers. A growth towards the trailing edge (shearing and smearing of the transferred aluminium) or a growth towards the leading edge (build-up of transferred aluminium, leading to a thicker and more localized transfer material).

The purpose of the project work was to build knowledge of the tribological behaviour of self-lubricating laser cladding, with and without external solid lubricant during conditions relevant for hot metal forming of aluminium. The materials used during the project were plates coated with a Ni-based self-lubricating clad and a reference sample of work tool steel. The self-lubricating laser clad was applied using a high power direct diode laser. The external solid lubricant used was a graphite dispersion. The external solid lubricant was applied on the samples using a spraying technique, leaving a dry layer of solid graphite on the plates. To test the tribological behaviour of the plates, linear reciprocating friction and wear tests were performed both under lubricated and dry conditions. During the dry tests, different surface roughness of the plates where investigated. The pins for the tribological test were made of AA7075. Parameters chosen for the tribological tests were based on conditions during hot forming of aluminium. After having taken images of the plates using scanning electron microscopy, and using a 3D optical profiler, the wear volume and material transfer was evaluated, and wear mechanism analysis was performed.   The tribological behaviour of polished Ni-based laser clad under dry conditions is comparable to that of the reference sample at its best performance. Using a mirror polished Ni-based laser clad under dry condition can be an option to not using external solid lubricant during hot forming of aluminium. Also, the surface roughness of the self-lubricating clad under dry conditions does not affect the coefficient of friction.

This thesis submits a proposal production technology of single parts from sheet steel No. 11 305, thickness of 2 mm, made from a semi-finished product with a diameter of 246 mm, production run of 50 000 pieces per year. To manufacture the component a technology of hydromechanical drawing is proposed. On the basis of a literary study and calculations a drawing tool was designed, fixed in hydraulic press LPS 4000. To prepare a semi-finished product, flange trimming, and hole punching, a sheet metal cutting is used.

L'anisotropie des proprietes des toles d'aluminium est fonction des textures et microstructures developpees pendant les differentes etapes de la mise en forme. Pour simuler le laminage a chaud, nous avons deforme en compression plane biencastree a chaud (200ct00c, 0. 1. 5, 10#-#3s#-#10#-#1s#-#1) des cristaux d'al et d'al-1%mg (monocristaux, bicristaux et polycristaux). La texture de deformation a 400c de polycristaux d'al est de type l et celle obtenue pour al-1%mg est a dominante c/s. La deformation non imposee #d#l, #d#t des monocristaux d'orientation l d'al a 400c est inferieure a celle obtenue a 20c (facteur 1/3). Ceci est explique par l'activation d'un glissement non octaedrique 112 aux faibles valeurs du parametre de zener-hollomon z. Pour l'orientation c, c'est le glissement 100 qui est observe a t+00c. Le modele viscoplastique avec introduction de glissements non octaedriques simule correctement ces evolutions. A 400c, pour l'orientation l d'al-mg, le glissement est de type octaedrique et #d#l, #d#t est superieure au cas d'al pur. L'orientation cube al-mg se stabilise a chaud de meme que pour al mais pour de plus faibles valeurs de z. Les cellules de dislocations dans les grains d'al s'organisent en deux familles de blocs de cellules formant un damier sous-structural regulier. Les desorientations locales de bloc a bloc sont alternees le plus souvent autour de dt (pour l, quel que soit alors que pour c et s, peut atteindre 20 pour 1). Dans les grains d'al-mg, les tailles de blocs sont deux fois plus petites et les desorientations locales n'excedent pas 3. En recristallisation, le joint de grain presentant la vitesse de migration la plus grande apres deformation a 400c est le joint cube/s (avantage en mobilite et en terme de difference d'energies stockees dans les deux grains). Le joint s+/s- de type migre aussi par siem mais avec une cinetique plus lente. Une nouvelle orientation proche de cube apparait par recristallisation au joint s/l

碩士
國立臺灣大學
機械工程學研究所
107
With the rise of environmental consciousness, automotive manufacturers are committed to developing fuel-efficient, lightweight automotive vehicles. The research on lightweight technology can be broadly divided into two major directions. One is using high-strength materials to reduce the overall vehicle weight and size. The other one is utilizing light-weight metal, such as magnesium-alloy or aluminum-alloy, as the main materials for automotive bodies to achieve weight reduction. However, the forming of high-strength steel and light metals such as aluminum alloy in room temperature is more likely to produce defects such as cracks, wrinkles, or distortion during the forming process. To overcome these challenges, hot stamping technology has become widely used in the auto industry in recent years. In the beginning, hot stamping technology was mainly applied to steel. Due to the characteristics of low flow stress in high temperature, products of hot stamping have fewer forming defects than conventionally formed products. In addition, by the quenching-in-die process in manufacturing, the tensile strength of the final product can reach nearly 1500 MPa. Although hot stamping parts have high strength to resist deformation, their elongation is low, resulting in poor overall toughness. To solve this issue, researchers proposed the concept of tailor properties. The tailor properties suggests that the products have different strength in different areas. Generally, the areas can be divided into the strong zone and the weak zone. The material in the strong zone has high strength and is able to avoid serious deformation during impact, while the material in the weak zone can absorb the crash impact through plastic deformation. Therefore, products with tailor properties can achieve either high strength and high toughness. The technology of hot stamping on other alloys is also growing in recent years. The HFQ(hot-forming-quenching) process on heat treatable aluminum alloys, in particular, has received considerable attention. Combining the heat treatment into the forming process, the process enables its products to achieve the strength as high as the traditional T6 aging treatment can reach and good formability. The technology is still under development; thus, there is still lack of data for the material properties and interface properties of aluminum alloys at high temperatures. Besides, a complete numerical analysis model is still yet to be established. In this thesis, with the analysis software Pamstamp and Abaqus, two models for analyzing the hot stamping process are built, one is for tailor-die quenching of steel, and the other is for aluminum alloy. The characteristics of the commonly used materials in the forming process are analyzed by the model. For hot stamping on steel, a tailor-die quenching process of an A-pillar is analyzed. The influence of process parameters on the final product, and the relationship between the design of the heating system and the distribution of the surface temperature of the die is further discussed. For hot stamping on aluminum alloy, an analytical process for evaluating the quenching effect of the material by TTP curve and numerical simulation is established, and the effect of process parameters on different series of aluminum alloy is discussed. Finally, a simple U-shape and a V-shape forming experiments are chosen to verify the accuracy of the analytical model. The results show that the estimation error between the model and the experiments is less than 10%; as a result, the accuracy of the models are verified.

Aluminum alloys are of great interest to the automobile industry for vehicle mass reduction, which improves vehicle performance and reduces emissions. Hot forming processes, such as superplastic forming (SPF) and quick-plastic forming (QPF) have been developed to take advantage of the improved formability of certain aluminum materials at elevated temperature. Commercial fine-grained aluminum alloy AA5083 sheet is the most commonly used material in the SPF and QPF forming processes. Hot formability of AA5083 is often limited by material cavitation during forming, which makes understanding and controlling cavitation an issue of primary importance for improving hot sheet forming processes. The thermomechanical processing history of AA5083 can strongly affect superplastic performance, causing variations in formability between material lots. These variations are closely related to microstructure, and intermetallic particles are prime suspects for controlling cavitation behavior. However, there has been little more than anecdotal evidence available that these particles nucleate or influence cavitation. Interactions between intermetallic particles and cavities were, thus, analyzed using both two-dimensional (2-D) and three-dimensional (3-D) microstructure characterization techniques. Analysis of 3-D microstructures from AA5083 specimens deformed under conditions similar to the SPF and QPF processes provide conclusive proof that cavities form at specific types of intermetallic particles. Differences in cavitation between materials deformed under the SPF and QPF processes result from differences in deformation mechanisms. These differences are illustrated by the formation of filaments on fracture surfaces of superplastically deformed AA5083 specimens, which have been characterized.
text

Selective corrosion of type 316L austenitic stainless steel welds during the production of organic acids resulted in losses in production due to unscheduled downtimes to perform repairs. Estimated corrosion rates of type 316L filler material welds were an order of magnitude higher than that of the base material. Alternative higher alloyed commercial filler materials were evaluated under actual production conditions. The evaluated filler materials were types 316L, 317L, 309L, 309MoL, 2205, 2507, 625, 825 and 904L. The effect of nitrogen on the corrosion properties of type 309L filler material was evaluated by manipulating the nitrogen concentration of the shielding gas during MIG welding. These changes in nitrogen concentration did not influence the corrosion resistance of the type 309L filler material. No correlation could be established between the corrosion rates, analysed chemical composition of the product and operating temperature during production. In almost all the cases where the chemical composition of the filler material was comparable with that of the base material the corrosion rates of the filler materials were higher than that base material. It might be expected that the ferrite phase with higher molybdenum and chromium should be more corrosion resistant while the austenite should be less resistant. This was, however, not the case with the corrosion of type 309L filler material. It would thus appear that in this case nickel enrichment of the austenite phase had a larger influence on the corrosion resistance of the austenite phase than the chromium and molybdenum had on the corrosion resistance of the ferrite phase. It appears that nickel and molybdenum had the largest contribution to the corrosion resistance of stainless steels welds under these operating conditions. It is, however, believed that a certain minimum concentration of chromium is also required to provide corrosion resistance to these alloys in hot organic acid environments. In contrast with the fact that a substantial alloying content is required to improve corrosion resistance of the filler material, the small difference in composition between ferrite and austenite phases, due to micro segregation, appeared to affect the corrosion resistance on micro scale. This is illustrated by the micrographs, which show corrosion to etch out the dendrite structure. Since the morphology of the austenite and ferrite phases is so similar, it could not always be conclusively established which one of the two phases corroded selectively. Analyses performed on the austenite and ferrite phases did not indicate a concentration difference within the phases itself. However, there were significant differences in the concentration of elements between the phases, with the austenite stabilising elements reporting to the austenite phase and the ferrite stabilizing elements reporting to the ferrite phase, in line with thermodynamic predictions. In the case of the filler materials following the austenite mode of solidification, no significant concentration differences were detected within the matrix. Although all highly alloyed high nickel alloyed filler materials (types 904L, 825 and 625) corroded at a lower rate than the type 316L base material, type 625 filler material was the filler material of choice due to the lack of any pitting of the weld. Pitting was detected in both the 825 and 904L filler materials. Galvanic corrosion was not noted at any of the weld/HAZ interfaces and in no case did the type 316L parent metal adjacent to the weld corrode preferentially to the material further away from the weld. Copyright
Dissertation (MEng)--University of Pretoria, 2010.
Materials Science and Metallurgical Engineering
unrestricted