Academic literature on the topic 'Quality casting'

Mold behavior, mold-related quality and the columnar-to-equiaxed transition influenced by in-mold EMS were examined by performing trials at two steel companies, metallurgical examination of the billet samples and mathematical modelling. The thermal fields in the walls of billet molds (102 X 102mm and 127 X 178mm) and in the cooling water were monitored by a system of thermocouples as the EMS was switched on and off during the continuous casting of several heats. The effect of electro-magnetic stirring on mold heat extraction was found to be negligible. The mold temperatures and cooling water temperatures are strongly dependent on the mold/billet gap which is affected by dynamic distortion of the mold tube. In the case of the square mold, the time-dependent mold distortion resulted from boiling adjacent to the cold face due to low water velocity and poor water quality. In the rectangular mold, differential expansion of the wide and narrow faces of the mold led to periodic wall movement at the midface causing cycling in the mold and water temperature. Both effects completely dominated any potential influence of EMS on mold heat extraction. Cooling water velocities measured in separate experiments and the mold temperature profiles were input to a two-dimensional heat-flow model to establish mold heat-flux profiles. A steep taper of 2.6 %/m in the upper regions of the mold increased heat extraction compared to previously published heat-flux data in 0.8 %/m tapered-molds. However, due to the periodic wall movement in the rectangular mold, the heat flux declines to lower values periodically. The calculated heat flux profiles were employed in a one-dimensional transient heat flow model to predict superheat removal from the liquid pool under a variety of assumed fluid flow conditions. The major heat flow effect of EMS was inferred to be one of increasing the convective heat flow at the solidification front leading to earlier superheat extraction from the liquid steel. Solidification structures in billet samples collected during the trials were examined. The columnar-to-equiaxed transition in continuous casting takes place provided all the superheat is removed from the melt and there is a sufficient density of nuclei present in the pool. At superheats of <20° C in the tundish, high heat extraction in the mold and remelting of the mold generated nuclei facilitate the removal of the superheat well within the mold and the columnar-equiaxed transition is triggered after 10-15 mm of shell growth on both the inside and outside radius faces. At higher superheat in the tundish, the liquid pool leaves the mold with residual superheat which takes longer to remove because of the declining fluid flow. Even though all the superheat is removed lower in the machine, the columnar-equiaxed transition occurs only if dendrite debris generated in the vicinity of the mold has survived in their descent through the superheated liquid. The effect of carbon on the columnar-to-equiaxed transition appears to stem from its influence on facilitating dendrite arm remelting and the survival of the dendrite fragments till the pool reaches sub-liquidus temperature. EMS extracts more superheat by maintaining a steep temperature gradient in the thermal boundary ahead of the solidification front and achieves an earlier columnar-equiaxed transition. Electro-magnetic stirring appears not to affect either the average depth or the variation of depth of oscillation marks across a given face. However, the electro-magnetically driven flow dominates the turbulance at the meniscus due to the input stream and stabilizes a meniscus shape with the result that the oscillation marks are also of a well-defined shape unlike the unstirred billets. No influence of EMS was found on the formation of "hooks" or the fine equiaxed crystal zone near the surface. The influence of EMS on inclining the growing dendrites appears to not come into effect until about 1 mm of shell has formed. It appears that the existence of the momentum boundary layer where the velocity of the rotating steel falls to zero at the surface is the reason for the absence of the influence of EMS on the subsurface solidification. Rhomboidity and off-corner crack formation were found to depend, as reported by previous researchers, on mold distortion and its dynamic nature. The absence of any effect of EMS on these defects is due to its lack of effect on mold heat transfer and thus mold distortion.<br>Applied Science, Faculty of<br>Materials Engineering, Department of<br>Graduate

Mathematical modelling using advanced approach based on the neural networks has been applied to the control and the quality optimisation in the main processes of steelwork such as the ladle metallurgical treatment and continuous casting. Particular importance has been given to the improvement of breakout prediction system and the reduction in the rate of false alarm generated by the conventional breakout detection system. Prediction of the chemical composition and temperature of liquid steel in the ladle has been achieved by neural networks and linear model. This prediction can be considered as a soft sensor. Slab surface temperature stabilisation on the basis of the casting events has been controlled by a neural networks algorithm, that gives an improvement in the surface temperature fluctuation in comparison to the conventional control system which is based on the PID controller. Quality monitoring and classification is also achieved by a neural network which is related to the breakout detection system. This technique achieves a classification of different defects based on the different alarm signal given by the breakout prediction system. Fault detection and process monitoring is developed using neural networks modelling. All models are developed on basis of practical operating database obtained from the iron and steel industry.

Two are the key players of this doctoral thesis: secondary AlSi alloys and the High Pressure Die Casting (HPDC) process. As the levels of CO2 and other greenhouse gasses increase, the industry sector is called to reduce the emissions produced and to contain the energy consumptions. Therefore, secondary Al ingot production shows a 6% annual rate growth, mainly drawn by the transport sector. In particular, automotive industry and, in general, transportation one increasingly needs light components in order to reduce total weights and therefore limit harmful emissions and fuel consumption. To this end, HPDC process is getting more and more crucial. It is, on one hand, versatile and highly productive but, on the other, the elevated amount of defects found in the castings sometimes compromises the characteristics of the final product. The research activities performed during the Ph.D. years focused on the High Pressure Die Casting (HPDC) process and the assessment of the correlations between process parameters, casting quality and mechanical response improvement. The research work comprises three interconnected topics: (i) the effect of the process on the casting quality in terms of defect generation and eutectic Si morphology, (ii) how defects and microstructure influence the mechanical response of the components and (iii) how the mechanical properties can be improved through specific heat treatments. The thesis starts dealing with the work developed within the EU MUSIC project aimed at transforming a production-rate-dominated manufacturing process into a quality/efficiency-driven and integration-oriented one. In the frame of the EU MUSIC project, an investigation procedure was developed which led to the definition of preliminary correlations between process parameters, defect content and static mechanical properties obtained analyzing both reference castings and commercialized industrial demonstrators. A specific focus was always devoted to the possible industrial application in order to narrow down the gap between academic research and industry. The second section deals with the definition of a so-called Penalty Index based on FE simulation model and fractographic investigations. It gave satisfactory predictions of both static strength and fatigue life of castings based on defect locations on the fracture surfaces and their projected areas. The elaborated criterion gives good results even though it does not take into account the real morphology of defects (which can be a very complex 3D morphology in the case of irregular flaws); its main drawback is that it is a "post-mortem" application. However, this limit can be overcome thanks to the wider diffusion of industrial high-resolution Computed Tomography (CT). The third section is mainly dedicated to the study of HPDC microstructure. Two secondary Al-Si alloys, an AlSi12(b) and an AlSi12Cu1(Fe), were used to produce HPDCs. In both cases, there was an evident non-homogeneity of the microstructure, in particular of the eutectic Si morphology which could be correlated, to some extent, to the different as-cast mechanical response of the castings. Heat treatments at 350°C and soaking times ranging from1 to 8 hours were performed Mechanical testing, hardness and metallographic investigations were carried out in order to understand the effect of the heat treatments. Microstructural observations using OM, SEM, TEM and EBSD investigations were also performed. Based on the obtained results, the heat treating procedure was effective since it did not cause surface blistering (a real issue in the heat treatment of die castings) or component distortion. The visible mechanical improvement in terms of both static and fatigue response was determined by Si crystals’ size and morphology. Thanks to heat treatment, the Si particle morphology changed from an interconnected ‘network’ to a more fragmented and smooth one. In terms of mechanical behavior, from a rapid crack propagation which led to a drastic reduction of the load-bearing area during the component use, the heat treatment caused a less instantaneous failure since the crack had to propagate through the ductile Al matrix. The best solution was assessed and proposed to obtain significant mechanical properties and low costs. Preliminary lab tests aimed at verifying the effect on the eutectic Si of common alloying elements such as Cu and Fe were finally conducted. A small mold was designed and manufactured in order to produce castings with solidification velocities comparable with those typical of the HPDC process. The base alloy was an AlSi12(b) ingot which was melt and several elements (i.e. Cu, Fe, Mn) were added to reach the AlSi12Cu1(Fe) alloy ranges. A refining effect of Cu was observed, however further investigations are necessary in order to better understand its joined effect with Fe. This doctoral thesis deals with a complex topic; a high number of sequential actions are involved in the process. This is the reason why the developed work brought interesting and useful results, but it can also be considered a starting point for further investigations. Further analyses can be conducted to understand the causes of the non-uniform eutectic Si morphology and the nature of the Si precipitates within the α-Al phase as well as an enhancement of the elaborated Penalty Index can be obtained.<br>In questa tesi di dottorato vengono presi in considerazione due principali aspetti: le leghe AlSi secondarie e il processo di pressocolata (HPDC). Poiché i livelli di CO2 e gas serra sono in continuo aumento, il settore industriale è chiamato a ridurre le emissioni prodotte e contenere i consumi energetici. Perciò la produzione di Alluminio secondario mostra un aumento annuale del 6%, trainato principalmente dal settore dei trasporti. In particolare, l'industria automotive e, in generale, quella dei trasporti necessitano sempre più di componenti leggeri al fine di ridurre i pesi totali e di conseguenza limitare le emissioni dannose ed il consumo di carburante. A questo fine, il processo di pressocolata sta diventando sempre più cruciale. Esso, da un lato, è versatile e altamente produttivo, ma dall'altro l'elevata quantità di difetti che si possono trovare nei getti talvolta compromette le caratteristiche finali dei getti. L'attività di ricerca condotta durante gli anni di dottorato si è focalizzata sul processo di pressocolata e la valutazione delle correlazioni tra parametri di processo, qualità dei getti e miglioramento della risposta meccanica. Il lavoro di ricerca comprende tre argomenti interconnessi: (i) l'effetto del processo sulla qualità dei getti in termini di generazione di difetti e morfologia del Silicio, (ii) come difetti e microstruttura influenzano la risposta meccanica dei componenti e (iii) come le proprietà meccaniche possono essere migliorate attraverso specifici trattamenti termici. La tesi inizia riportando il lavoro svolto nell'ambito del progetto ricerca Europeo MUSIC finalizzato a trasformare un processo dominato dal livello produttivo in uno guidato da qualità, efficienza ed integrazione. Nel contesto del progetto MUSIC, è stata sviluppata una procedura di indagine in grado di portare alla definizione di alcune correlazioni preliminari tra parametri di processo, contenuto difettologico e proprietà meccaniche statiche ottenute analizzando sia getti di riferimento sia dimostratori industriali già in produzione. E' stato dedicato un focus particolare alle possibili applicazioni industriali al fine di restringere il divario tra ricerca accademica e industria. Nella seconda sezioni si procede alla definizione di un così detto Indice di Penalità basato sulla simulazione FEM e l'analisi frattografica. Esso ha dato correlazioni significative sia con le proprietà meccaniche statiche che a fatica dei getti basandosi sulla posizione dei difetti sulla superficie di frattura e le relative aree proiettate. Il criterio elaborato fornisce buoni risultati anche se non consente di prendere in considerazione la reale morfologia dei difetti (che può essere molto complessa nel caso di difetti irregolari); il principale svantaggio del criterio è il fatto che si tratti di un'applicazione "post-mortem". Tuttavia, il limite potrà essere superato grazie alla vasta diffusione della Tomografia Computerizzata industriale. La terza sezione della tesi è dedicata principalmente allo studio della microstruttura di getti pressocolati. Sono state utilizzate due leghe secondarie, AlSi12(b) e AlSi12Cu1(Fe), per la produzione dei getti. In entrambi i casi si è riscontrata una notevole disomogeneità della microstruttura, in particolare della morfologia del Si correlabile, in parte, alle differenti risposte meccaniche dei getti allo stato as-cast. Sono stati quindi condotti trattamenti termici a 350°C per tempi variabili dall'1 alle 8 ore. Sono state quindi condotte prove meccaniche, di durezza e analisi metallografiche al fine di capire l'effetto dei trattamenti termici. Si sono svolte analisi microstrutturali tramite microscopio ottico, SEM, TEM e EBSD. Sulla base dei risultati ottenuti, la procedura di trattamento termico è stata valutata efficace dal momento che, in primo luogo, non si sono causati blister sulla superficie dei getti o distorsione dei getti. L'evidente miglioramento meccanico sia statico che a fatica è stato determinato dalla dimensione e morfologia dei cristalli di Silicio. A seguito del trattamento termico, si è verificata una modifica del delle particelle di Si e il passaggio da una morfologia interconnessa ad una più frammentata e arrotondata. In termini di risposta meccanica, da una propagazione rapida che porta ad un cedimento drastico dell'area resistente del componente, si è passati ad una rottura meno istantanea per effetto della propagazione della cricca attraverso la matrice duttile di Alluminio. E' stata valutata e proposta la soluzione migliore in grado di fornire un buon compromesso tra risposta meccanica e costi contenuti. Sono stati quindi condotti esperimenti preliminari in laboratorio al fine di verificare l'effetto sul Si eutettico di alliganti comuni quali Cu e Fe. Tramite la costruzione di un piccolo stampo in grado di fornire velocità di raffreddamento comparabili a quelle della pressocolata si sono condotti studi sulle due leghe sopracitate. Si è osservato un effetto affinante dovuto al Cu, tuttavia ulteriori analisi sono necessarie per approfondire l'effetto del Fe. La tesi riguarda perciò una tematica complessa; nel processo di pressocolata sono richieste un numero elevato di azioni consecutive. Questo è il motivo per cui la ricerca condotta ha portato risultati interessati e utili, ma deve allo stesso tempo essere considerata un punto di partenza per ulteriori approfondimenti. Saranno necessarie nel futuro ulteriori analisi ed indagini.

High pressure die casting (HPDC) is a widely used process for manufacturing components with high production rate and complex geometries. However, high pressure die casting is still considered a “defect generating process”, since an amount of 5-10% of scrap is usually detected in castings. For this reason, the availability of new standards and tools for optimizing the process is one of the prominent needs of foundry field. Both these issues are tackled in this Thesis. First, a new classification of defects and reference dies for estimating the static mechanical properties of aluminium alloys have been developed and then published as CEN Technical Reports, satisfying the need for new standards. Then, a novel and comprehensive methodology has been developed for optimizing HPDC process through the definition of meaningful behavioural models. Some newly defined process parameters have been hence introduced, by representing the most important physical phenomena affecting casting quality and the integral nature of HPDC, and an original approach is outlined. In particular, in order to provide an effective representation of the different process stages, the following parameters have been found to be more influential and effective in representing transmitted forces, mechanical energy exchange and heat removal: the root mean square acceleration in the second stage, the energy associated to the flow forces in the whole cycle, the work of the pressure forces in the third stage and the normalized thermal gradient. The first two parameters embody the plunger motion time-history, the third parameter represents the hydraulic pressure time-history, while the last one concentrates on some thermal aspects. These parameters take advantage from signal processing techniques of the measured position and pressure profiles. Four experimental campaigns (in the whole 210 castings) using different injection machines, different alloys and different geometries of the die have been carried out to validate the novel process parameters and to prove the general validity of the approach. The statistical correlation with the measured static mechanical properties, density and percentage of porosity, as well as the metallographic analysis (percentage of oxides on fracture surfaces, analysis of different kinds of internal defect) prove the soundness of the developed method. Given the significance of the plunger motion profile in explaining the casting quality, analytical models for computing the root mean square acceleration and the energy associated to the flow forces have been developed. These analytical models permit selecting in advance the best plunger motion profile, which guarantees reliability and soundness of castings, thus satisfying the need expressed by foundries for effective optimization tools.<br>La pressocolata è un processo ampiamente utilizzato per produrre componenti con elevata produttività e geometria complessa. Comunque, la pressocolata è ancora considerata un “processo generatore di difetti”, dal momento che una percentuale del 5-10% di scarto è rilevata di solito nei getti. Per questa ragione, la disponibilità di nuovi standard e strumenti per l’ottimizzazione di processo è uno dei bisogni più importanti del settore della fonderia. Entrambe queste criticità sono affrontate in questa Tesi. Per prima cosa, è stata sviluppata e successivamente pubblicata come Report Tecnici riconosciuti dal CEN una nuova classificazione dei difetti e degli stampi di riferimento per stimare le proprietà meccaniche statiche delle leghe di alluminio, rispondendo al bisogno di nuovi standard. Poi, è stata sviluppata una metodologia nuova e completa per ottimizzare il processo di pressocolata attraverso la definizione di metamodelli significativi. Quindi, sono stati introdotti dei nuovi parametri di processo, che rappresentano i più importanti fenomeni fisici che influenzano la qualità dei getti e la natura integrale del processo di pressocolata, ed è stato delineato un approccio originale. In particolare, allo scopo di fornire una rappresentazione efficace delle diverse fasi del processo, si è dimostrato che i seguenti parametri sono i più influenti ed efficaci nel rappresentare le forze trasmesse, il flusso di energia meccanica e l’asportazione del calore: il valore quadratico medio dell’accelerazione in seconda fase, l’energia associata alle forze di flusso nell’intero ciclo, il lavoro delle forze di pressione nella terza fase e il gradiente termico normalizzato. I primi due parametri descrivono l’effetto del profilo di moto del pistone; il secondo è invece legato anche alla pressione esercitata dal pistone stesso, mentre l’ultimo rappresenta esclusivamente fenomeni di natura termica. Lo sviluppo ed il calcolo di questi parametri sfrutta le tecniche di elaborazione numerica dei segnali, al fine di estrarre le informazioni dai profili misurati di posizione e di pressione del pistone. La validazione del metodo proposto è stata condotta attraverso quattro diverse campagne sperimentali, con un totale di 210 getti analizzati, nelle quali sono state utilizzate diverse macchine di iniezione, diversi stampi e diverse leghe, al fine di ottenere risultati generali ed estendibili. La correttezza del metodo proposto è confermata dalla correlazione statistica dei parametri proposti con la resistenza meccanica statica, con la densità, la porosità, e da alcune significative analisi metallografiche (percentuale di ossidi sulle superfici di frattura, analisi dei difetti interni). Data l’efficacia dimostrata dei parametri proposti, ed in particolare di quelli legati al profilo di moto del pistone, l’approccio è stato esteso tramite lo sviluppo di alcune relazioni analitiche per calcolare i due parametri cinematici. Ciò rappresenta uno strumento efficace per sintetizzare a priori il profilo di moto ottimale del pistone che, nel rispetto dei vincoli di macchina, consenta di ottimizzare la qualità dei getti.

This thesis work concerns about the importance of the cast surfaces, surface phenomenon such as the formation of the oscillation marks and exudation and related defects including cracks and segregation that happened during the continuous casting. All of the investigated materials were collected during the plant trials while an in-depth analysis on these materials was performed at the laboratory scale with certain explanations supported by the schematic and theoretical models. The work consists on different material classes such as steels and phosphor bronzes with a focus on the surface defects and their improvements. In order to facilitate the theoretical analysis which could be capable of explaining the suggested phenomenon in the thesis, a reduced model is developed which required lesser computational resources with lesser convergence problems.<br><p>QC 20160527</p><br>Oscilation mark formation during continous casting of steel

In this thesis, we study improvement of product quality in manufacturing industry by identifying and optimizing influential process variables that cause defects on the items produced. Real data provided by a manufacturing company from the metal casting industry were studied. Two well-known approaches, logistic regression and decision trees, were used to model the relationship between process variables and defect types. The approaches used were compared.