Le laminage est un procédé de mise en forme à chaud permettant d’obtenir des barres de différents diamètres en partant de blooms issus de la coulée continue. Dans les bruts de coulée se répartissent des inclusions qui sont de natures, de formes et de tailles différentes. Le laminage va permettre de réduire le diamètre de la barre et d’agir sur la microstructure du matériau, notamment, en fragmentant et dispersant les inclusions. L’objectif des travaux de thèse est de contribuer à la compréhension des effets du chargement thermomécanique sur l’évolution des défauts de coulée en laminage. Vu la dimension des installations, les investigations expérimentales directes sur les moyens industriels ne sont pas envisageables. L’idée développée dans les travaux de thèse concerne la mise en place et la validation d’un essai de caractérisation à échelle réduite reproduisant le chemin thermomécanique subi par la matière au cours du laminage. Pour ce faire la ligne de laminage industrielle est modélisée afin d’obtenir le chargement thermomécanique de la barre au cours du laminage. Seules les sollicitations ayant un rôle majeur sur le comportement des défauts sont reproduites. Un essai dit de « forgeage libre » a ainsi été conçu, réalisé, mis en place sur les moyens de mise en forme de la plateforme VULCAIN de l’ENSAM. L’essai a été utilisé avec des défauts artificiels présentant des similarités comme la malléabilité avec les défauts réels. Une campagne expérimentale paramétrique a été menée sur les différents types de défauts. Les échantillons forgés ont été analysés par des méthodes non destructives comme les ultrasons, la radiographie et la tomographie X. Ces analyses ont permis de suivre le comportement du défaut et d’observer certains phénomènes mis en évidence dans la littérature comme l’apparition de cavité à l’interface défaut-matrice / Rolling is a hot forming process dedicated to manufacture bars with different diameters. The initial product is a bloom from the continuous casting. The bloom contains inclusions which have different forms, sizes and distributions. The rolling reduces the diameter of the bar and acts on the material microstructure by fragmenting and dispersing the inclusions. The aim of the thesis work is to understand the behavior of the inclusions with the rolling thermo mechanical loading effects. The direct investigations on the rolling mill are not possible because of the size of the installations. To address this problem, the solution is to develop and validate a small scale characterization test reproducing the thermo mechanical loading of the rolled bar. To achieve this, the rolling mill is modeled. Only the solicitations which have a major role on the behavior of the defects are reproduced. An open-die forging test is designed, manufactured and implemented on the VULCAIN installation of the ENSAM. The artificial defects which have a similar malleability to the real defects are integrated into the sample. A parametric experimental campaign has been conducted on the different defects. The forged samples have been analyzed with non-destructive methods such as ultrasound, radiography and tomography. These analyses allowed to follow the behavior of the defects and to observe certain phenomena illustrated in the literature such as the emergence of a cavity on the defect-matrix interface.

A content of this thesis is an analysis of a steel castings production mismatch in Tatra Metallurgy foundry. Most common defects found on castings are gas holes. Thesis shows a theoretical research on casting defects caused by gases in steel castings. It also describes melting and degassing problematics of molten steel and procedures to reduce gas contents. The last part of theoretical research describes oxidation of steel in contact with silica mold. Practical part of thesis analysis production of steel castings to determine the cause of origin of gas holes. Main attention is paid to mold mixture, chemical composition, casting temperatures and casting time. The last chapter metallographically analysis two defected castings to determine the origin of defects. Main attention is paid to shape and chemical composition of gas holes and to morphology of steel inclusions.

Als Ergebnis der vorliegenden Arbeit wurde ein nützliches Hilfsmittel auf der Basis der kognitiven Herangehensweise zur Verbesserung der Effizienz der Managemententscheidungen für die Gussausschussverringerung und Qualitätsverbesserung in den Gießereien entwickelt. Das Werkzeug hilft dem Technologen, den Mechanismus des Gussfehlerentstehungsprozesses aufzudecken, die Logik der Gussfehlerentstehung zu verstehen und die präventiven Maßnahmen zu testen. Man kann das Werkzeug täglich im Betrieb benutzen, um die strategischen und operativen Entscheidungen rasch und ohne Durchführung der kostspieligen und komplizierten Versuche zu treffen. Auf diese Weise können Kosten und Zeit eingespart werden.

Tensile deformation characteristics of cast aluminum alloys have been investigated extensively. Cast Mg alloys have remained mostly neglected by researchers, despite their potential for weight savings. This present study is motivated by this gap in the literature and consists of two stages; in Stage 1, analysis of tensile data gathered from literature were reanalyzed, and in Stage 2, data generated from tensile testing of 60 specimens of AZ91 Mg alloy castings in both T4 and T6 conditions were analyzed to characterize work hardening behavior. In Stage 1, more than 1600 data were collected from the literature for various Mg alloy families. After plotting these data in yield strength-elongation charts, highest points were identified and interpreted as the maximum ductility, i.e., ductility potential (eFmax). The trend in maximum points indicated a linear relationship with yield strength (σY), expressed as; eF(max) = 41.8 - 0.106σY (1) This ductility potential equation can be used as a metric to compare elongation obtained from tensile specimens to measure the structural quality of Mg alloy castings. Moreover, results indicated that ductility potential was not affected by heat treatment, grain size (within 30-120 μm), casting geometry, size, the type of casting process nor chemical composition. In Phase 2, AZ91 cast Mg alloy specimens in T4 and T6 conditions were tested in tension to obtain stress-strain data for each specimen. Fits of four constitutive equations, namely, the Hollomon, Voce, Ludwik and Swift, to true stress-true plastic strain data in the elastoplastic region were characterized for the specimens with highest elongation values for T4 and T6 specimens. The coefficient of determination, R2, values for all equations were in excess of 0.99, suggesting that all four equations provide excellent fits to tensile data in both conditions. The change in work hardening rate with true stress was investigated for all specimens by using Kocks-Mecking (KM) plots. It was determined that work hardening behavior of Mg alloy castings in T4 and T6 is distinctly different. In T4 specimens, there is a plateau in work hardening rate at approximately E/25 which was observed in all specimens. The presence of this plateau is consistent with results given in the literature for pure Mg. However, this plateau was not observed in any of the T6 specimens. The reasons for the absence of the plateau in T6 specimens are unknown at this time. In both T4 and T6 specimens, the KM work hardening model in which work hardening rate changes linearly with true stress was found to be applicable. This is the first time that KM model was found to be valid for Mg alloys. Moreover in all specimens, there was a sudden drop in work hardening rate just prior to final fracture. This drop was first hypothesized to be due to structural defects in specimens, which was subsequently validated via fractography. Structural defects were found in all specimens whose fracture surfaces were investigated, indicating low to medium levels of quality. The quality index method, originally developed for cast aluminum alloys as the ratio of elongation to ductility potential, was found not to be applicable to Mg alloys, at least in its original form. This is due to the fact that work hardening behavior of cast aluminum alloys follows the KM model and there is no plateau where work hardening rate is constant. Hence the work hardening behavior of cast aluminum alloys and AZ91 specimens in T6 condition was similar. However the plateau of constant work hardening rate had a strong effect on elongation in T4 specimens. Therefore quality index analysis, which is supposed to be independent of alloy condition, did show that T4 and T6 specimens had different quality index levels. This finding contradicted the result from Stage 1 that aging has no effect on ductility potential. However because of the presence of structural defects in all specimens, quality index levels were low (0.30-0.45). Therefore it is unclear at this point whether the work hardening behavior of T4 and T6 specimens would still be different if elongation values were in the proximity of the ductility potential line. More research is needed to characterize work hardening behavior of cast Mg alloys in the absence of major structural defects and also address other questions raised in this study.

Diploma thesis deals with optimization of technological process casting a die cast, which is part of vacuum pump. The die cast is made from alloy AlSi9Cu3(Fe) in foundry KOVOLIT, a. s. There is a problem with inner defects at this cast. Solution of this thesis is to suggest suitable actions to minimize of incidence the defects in the cast. According to analysis of the defects in the cast these defects were classified as combination of shrinkages and gas holes. According to analysis causes of the defects are high temperature of die mould and closed air in die mould. As a result suitable actions were suggested – a change of the die mould tempering and a design adjustment of the cast. In case of implementing a new die mould in the manufacture a new gating system was designed. Suitability of the new gating system was assessed according to simulation of filling die cavity.