Mechanical properties and microstructures of a modified Al–Si–Cu alloy prepared by thixoforming process for automotive connecting rods

The thixoforming process with feedstock preparation yields a fine microstructure and enhanced mechanical properties relative to other traditional casting processes. However, the thixoforming process via cooling slope is limited to commercial cast alloys and requires specific thixoformability criteria for each type of alloy. Therefore, a modified Al–Si–Cu alloy with 2.0 wt.% Cu, 0.30 wt.% Mg, 0.5 wt.% Mn, and 0.1 wt.% Fe was developed from the thermodynamic analysis result in this work for the fabrication of automotive connecting rods. In this study, the thixoforming process was conducted at different liquid fractions of 0.40, 0.45, and 0.50. A good filling capability of the semisolid slurry was found at a liquid fraction of 0.50 without any defects. The microstructures of the α-Al dendrite and acicular silicon particles were transformed into globular and fibrous shapes in the thixoformed alloy, respectively. The intermetallic compounds of Al2Cu, Al5Cu2Mg8Si5, and β-Al5FeSi had a homogenous and fragmented shape. Due to the microstructural enhancement and the reduction in casting porosity owing to the high pressure during the thixoforming process, the mechanical properties of thixoformed were remarkability improved and reached to the values of 103 HV, 250 MPa and 340 MPa for hardness, yield stress, and ultimate tensile strength, respectively. Also, the T6 heat treatment process was applied to the thixoformed sample. Results indicated 95% and 22% improvements in the ultimate tensile strength and hardness of the thixoformed sample, with the resulting values being 340 MPa and 122 HV, respectively, relative to those of heat-treated gravity-cast alloys.