Effect of mechanical conditions on the microstructures and mechanical properties of thixoformed Al–Cu–Si–Mg alloy

Commercial thixoforming is generally based on conventional aluminum-based casting alloys such as A356 and A357, which provide high fluidity and good castability. However, these alloys do not give such high mechanical properties as the alloys that are conventionally wrought such as Al–Cu–Si–Mg alloys, which generally has wide freezing ranges and tend to be prone to inhomogeneous microstructure and hot tearing. In this paper, one kind of aluminum alloy based on Al–Cu–Si–Mg system was thixoformed under different mechanical conditions. The strain induced melt activated (SIMA) route was used to obtain the semisolid slug for thixoforming. The effect of mechanical conditions on the microstructures and tensile mechanical properties in the T6 condition of the thixoformed aluminum alloy were studied. On proper mechanical condition (the base of component is 15 mm), the thixoformed sample reveals a relatively homogeneous structure among different regions and presents no evidence of macro and microporosities, moreover, the tensile mechanical properties are uniform between the wall and the base. The ultimate tensile strength (376 MPa) is close to the wrought target, and the elongation to fracture (18.7%) is even better, nevertheless, the yield strength (220 MPa) is considerably short of the wrought value. On improper mechanical condition (the base of component is 5 mm), the microstructures are inhomogeneous and inferior with respect to those of the former, and present some microporosities and microcracks. Although, a few cracks were filled by the supplied liquid metal, the mechanical properties are not satisfactory (tensile strength of 346 MPa, yield strength of 216 MPa and elongation to fracture of 11.5%).

► Microstructure and mechanical properties are improved on proper mechanical condition. ► It is essential to make the slugs in three-dimensional compression stress state. ► The mechanical properties of thixoformed aluminum alloy are close to wrought target.