Materials Science Forum Vols. 519-521

Abstract: A Compressive Torsion Processing (CTP) is a unique severe plastic deformation process which can easily apply very large strain without shape change to a work piece. Hypereutectic Al-Si alloys have good properties such as low thermal expansion and high wear resistance. It is important for the alloys to control the size of second phase particles (primary and eutectic silicon, intermetallic compounds) as well as grain size of aluminum matrix. In the present work, the CTP was applied to hypereutectic Al-Si alloy (AA390) to investigate the possibility of microstructure refinement of the alloy and the mechanical property of processed alloy was also investigated by tensile test.

Abstract: The important role of dissolved elements, such as manganese or iron, in the production process of rolled material was in the focus of this investigation. In the AlMn-alloys the content of manganese in solid solution is first controlled by other alloying elements such as silicon and iron. The addition of silicon or / and iron reduces the level of manganese in solid solution in the as-cast condition as well as in the following preheating process step. Another major parameter is the final annealing treatment of the rolled products. The preheating treatment and the final annealing are the key parameters to control the level of manganese in solid solution and the size and distribution of the fine dispersoids. In the AlFe-series alloys the iron content in solid solution is first controlled by the casting process DC or CC and the amount of iron in the alloy composition. In the as-cast condition the dissolved iron level is normally higher in the CC-material than in the DC-material. The intermediate annealing treatment also has a great influence on the content of iron in solid solution. They recovery and recrystallisation behaviour is controlled by the content of manganese or iron in solid solution. The precipitation of manganese or iron during thermal treatment leads to optimum dislocation pinning and results in a high thermal stability. On the other hand, if the precipitation of manganese or iron interacts with the occurence of recrystallisation, the resulting grain structure can show coarse grain.

Abstract: The modelling of microstructure, and hence the properties, of cast alloys has so far been attempted assuming the liquid metal is free from defects. The growing appreciation that bifilms, of size measured in millimeters, are usually abundant in liquid metals, and control many features of the casting, has led to this paper in which some of the known and some of the expected effects are reviewed. It is suggested that bifilms cause major effects on microstructure and properties and their presence can vastly overpower the effect of traditionally simulated structural features.

Abstract: Horizontal continuous casting process has been successfully implemented in Alcan for the production of T-ingots of primary aluminium and foundry alloys. Ability to achieve increased productivity targets and reduce production costs relies on a fundamental understanding of key process characteristics and operating parameters. Thanks to the long-standing experience in vertical DC Casting, numerical modelling appeared as a powerful approach to understand phenomena related to metal flow, solidification and ultimately defect formation. As part of a collaborative R&D program, a global model of horizontal casting process, integrating specialized sub-models on critical aspects of the process such as meniscus dynamics, is being developed. Experimental characterization of primary and secondary cooling is performed in parallel with modelling work to provide the information necessary to properly characterize mould heat transfer. This paper will present the development of a 3D process model of T-ingot casting along with its application to solve specific process challenges that have emerged during the first years of production in the plant.

Abstract: Age hardening is one of the most important processes to strengthen aluminium alloys. It usually consists of the steps solution annealing, quenching and aging. For heat treatment simulations as well as for the appropriate choice of quenching processes in heat treatment shops, knowledge of the temperature- and time-dependent precipitation behaviour during continuous cooling is required. Quenching should happen as fast as necessary to reach high strengths, but also as slow as possible, to reduce residual stresses and distortion. This optimal quenching rate of an aluminium component depends on its chemical composition, initial microstructure and solution annealing parameters as well as on its dimensions. Unfortunately continuous cooling transformation (CCT) diagrams of aluminium alloys do almost not exist. Instead isothermal transformation (IT) diagrams or given average quenching rates are used to estimate quenching processes, but they are not satisfying neither for heat treatment simulations nor for heat treatment shops. Thermal analysis, especially Differential Scanning Calorimetry (DSC) provides an approach for CCT-diagrams of aluminium alloys, if the relevant quenching rates can be realized in the DSCequipment. The aluminium alloy Al-4.5Zn-1Mg (7020) is known for its relatively low quenching sensitivity as well as for its technical importance. The complete CCT-diagram of 7020 with cooling rates from a few K/min to some 100 K/min has been recorded. Samples have been solution annealed and quenched with different cooling rates in a high speed DSC. The resulting precipitation heat peaks during cooling have been evaluated for temperature and time of precipitation start, as well as their areas as a measure for the precipitate amount. Quenched samples have been further investigated regarding their microstructure by light and electron microscopy, hardness after aging and precipitation behaviour during re-heating in DSC. The CCT-diagram correlated very well with the microstructure, hardness and re-heating results. A critical cooling rate with no detectable precipitation during continuous cooling 155 K/min could be determined for 7020. A model to integrate the CCT-diagram in heat treatment simulation of aluminium alloys is under development.

Abstract: A mathematical model has been developed and validated to predict deformation, temperature and microstructure evolution during multi-pass hot rolling of an AA5083 aluminum alloy. The validated model was employed to examine the effect of changing the number of rolling passes and the strain partitioning during multi-pass rolling on the material stored energy and the resulting microstructure. Results indicate that the number of rolling passes has a significant effect on the material stored energy. In addition, the way the strain is partitioned in two-pass rolling cases affects the material stored energy with decreasing strain/pass providing the highest stored energy in the material after rolling and vice versa. The reason behind these results was further investigated indicating that the thermal evolution during rolling may significantly influence the material stored energy and subsequent recrystallization kinetics.

Abstract: The effect of extrusion rate and ratio on the Al3Zr induced dynamic recrystallization (DRX) that occurs during hot extrusion of RS-P/M Al-Zn-Mg-Cu-Zr alloys was investigated. An increase in the logarithm of extrusion rate promoted DRX and lead to a monotonic increase in the number of fine grains. Although DRX was also promoted and the grain size reduced by an increase in extrusion ratio from 10 to 20, the DRX behavior hardly changed, even when the extrusion ratio exceeded 20. However, with increasing extrusion ratio, the width of fibrous grain, i.e., the unrecrystallized region, decreased and the tensile strength increased to 879 MPa. When the extrusion rate and ratio exceeded 54 mm/min and 20, respectively, a marked grain coarsening occurred upon solution treatment, and the tensile strength tended to decrease, because of the high dislocation density induced by hot extrusion. By annealing at 563 K before solution treatment, it was possible to prevent grain coarsening, and thus prevent the strength decrease.

Abstract: The process of severe plastic deformation (SPD) makes it possible to reduce the grain size to the submicrometer or nanometer range in many metallic materials. When the SPD process is applied to age hardenable alloys, it may also be possible to control aging behavior. In this study, a technique of equal-channel angular pressing (ECAP) is used as an SPD process and aging behavior is examined on the three selected Al alloy systems such as Al-Ag, Al-Mg-Si and Al-Si-Ge. The microstructures are observed using transmission electron microscopy and the mechanical properties including hardness are measured. It is shown that the SPD process introduces unusual phenomena in the precipitation process and there should be a potential for enhancement of strength over the conventional age-hardening process or for improvement of ductility while keeping the high strength.

Abstract: Thermo-mechanical processing (TMP), coined 50 years ago for steels to describe combined thermal and mechanical treatments that define both product shape and microstructure/properties, has been practiced since the early industrial revolution. The improved function and control in mechanical shaping equipment were easily adapted to newly discovered aluminum, integrating hot forming, cold deformation and annealing. The TMP goals for Al alloys were grain refinement, substructure preservation, texture control and enhanced precipitation. Hot extrusion became widely employed with exploitation of elongated grains with substructure and strong texture and of press heat treatment (solution during deformation, quenching upon exit). Rolling schedules were tuned to generate desired grain size/shape, substructure and texture. This historical account aims to enhance application of metallography to process optimization and innovation that makes metals more competitive with other materials.