Thixo diecasting process for fabrication of thin-type component with wrought aluminum alloys and its formability limitation

Abstract

The reheating conditions to the thixo diecasting process of wrought alloys were given as the control of reheating time and temperature. The reheating process to obtain of a globurized microstructure is performed by various heating step of repeatedly heating process with various temperature steps. The solid fraction and the mean diameter of the semi-solid material after the reheating process are measured by the image analysis. The various problems in fabricating thin wall component by the thixo diecasting process have been investigated with a wrought aluminum alloy. In order to investigate filling phenomena during the thixo diecasting process, the die for the fabrication of the thin-type component was used for this study. In the thixo diecasting process for wrought alloys, the cause of the internal defect phenomenon and microstructure are shown with experimental data.

Introduction

In the automobile-parts industry, components with complicated shapes, are manufactured by casting and forging processes. It is difficult to obtain sound mechanical properties in the components by the casting process because the components have many microscopic defects. Near-net shaping processes in the solid state such as thermal forging and extrusion have an advantage related to the improvement of mechanical properties by eliminating these microscopic defects; however, the problems of these processes are, that high forming pressure is required, productivity is decreased, and additional cost is increased due to secondary forming processes such as cutting. Forging is used for the external appearance of thin parts because pores cannot be allowed on their surfaces, which require surface treatment. However, the metal loss by secondary forming processes is high because net-shape forming of thin parts with wrought aluminum alloys is difficult by the forging process. Therefore, many studies on the forming of near-net shaped components with an almost final shape have been actively conducted [1], [2].

The semi-solid forming process has advantages such as an energy reduction of 35% by forming under a relatively low temperature, and quality improvement through the minimization of casting defects such as porosity and shrinkage [3], [4], [5]. Also, finer and more homogenous globular microstructures related to the internal microstructure of the porous component can be obtained, and the mechanical properties of components fabricated by semi-solid forming are better than those achieved by the conventional forming process such as thermal forging. Also, the forming pressure is reduced, and this forming process has the advantages of minimizing energy expenses and miniaturizing the forming equipment. A near-net shape component with a complicated shape or of a material not being forged in solid state can be obtained due to the low deformation resistance. Therefore, this process has been applied to the production of automobile suspension components up to now, because the thixoforming process has many more advantages than the conventional forming process mentioned above.

At present, mainly studies on the thixoforming process related to a casting alloy have been conducted. However, Kapranos et al. [6] reported after an experiment that the mechanical properties of components fabricated by the thixoforming process with a wrought aluminum alloy were better than those fabricated with a casting alloy, and so proposed the possibility of the thixoforming process with a wrought aluminum alloy. Liu et al. [7] investigated a method for the improvement of mechanical properties through the studies on heat treatment with the A2014 wrought aluminum alloy after the thixoforming process.

As the research trends mentioned above indicated studies of the influences on the filling behavior and mechanical properties regarding the variation of the heating conditions and process conditions related to the semi-solid forming process have not been actively performed. Therefore, the reheating conditions of the wrought alloys for the thixocasting process with a horizontal high-frequency induction heating device were drawn for this study. Various defects by thixocasting of thin parts of thicknesses below 5 mm with a high-strength wrought alloy and the causes of the defects were examined. The effects on the microstructure of a variation in the reheating conditions and extrusive conditions were also investigated in this study.