Thixoforming characteristics of thermo-mechanically treated AA 6061 alloy for suspension parts of electric vehicles

Abstract

Thixoforming trials of AA 6061 alloy, which is a wrought aluminum alloy, were undertaken to manufacture the steering knuckles designed for suspension parts of low speed electric vehicles. The billets used as a starting material were thermo-mechanically treated to induce thixoformable fine globular microstructures during subsequent reheating process. After a slug was reheated up to the working temperature (∼645 °C) using the horizontal induction heating device, the part was formed by vertical injection of a heated slug into the die cavity. The well-developed globular microstructures could be observed in most sections of the thixoformed parts except few surface regions close to gate where occasional unrecrystallized large grains are present. However, liquid segregation between near and far gate region could not be avoided, which was probably resulted from high injection speed. The reason for that will be explained using semi-solid characteristic flow behaviors and the method to reduce liquid segregation will be suggested. In order to evaluate the mechanical properties of AA 6061 thixoformed parts, a series of test specimens carefully classified by their cutting location in a part were used for tensile test. From tensile test results, it could be revealed that tensile properties were strongly affected by liquid segregation. Especially, variation of tensile properties was more dominant when the part was heat treated by T6 condition. These results may be attributed to the segregation of alloying elements such as Mg and Si caused by the segregation of liquid phase involving much alloying element. Therefore, it is necessary to reduce liquid phase segregation in order to obtain more uniform tensile properties. By comparing tensile properties of AA 6061 thixoformed parts to those of previously thixoformed parts made from cast grade A357 alloy, it was found that AA 6061 parts showed far better ductility, even though strength level is slightly lower, than A357 parts at both as-formed and T6 condition. In this study, it was confirmed that wrought aluminum alloy, AA 6061 has the reasonable thixoformability and mechanical properties for thixoforming of real parts such as steering knuckles.