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Metallurgical Aspects of Thixoforming of AlMgSi - Wrought Alloys

The objective of this work is to illustrate experimental results concerning microstructural evolution and rheological properties in the semi-solid state of three different wrought AlMgSi alloys. The microstructural evolution has been characterized by conventional parameters (mean grain size and grain shape factor) as a function of holding time and solid/liquid ratio within the semi-solid state. In addition, special attention has been paid to the variation of the solid phase contiguity as a result of grain coalescence and neck sintering connection. The rheological behavior of the alloys in the semi-solid state has been investigated by backward extrusion experiments and correlated to the microstructural changes, particularly the solid phase contiguity. It is shown that the ability for Thixoforming of the investigated AlMgSi alloys strongly depends on the changes of the solid phase contiguity. For homogeneous semi-solid forming a critical contiguity gap (upper and lower limit) seems to exist. Thus, for successful Thixoforming the actual contiguity - which is strongly dependent on holding time and liquid volume fraction - has to be kept within these limits. Finally, a new modified AlMgSi alloy is presented, characterized by a reduced tendency to grain growth and grain coalescence. The new alloy is less sensitive to variations in holding time and solid/liquid volume fraction.