Development and Characterization of Cast Modified SI-CU-MG Alloys for Heat Resistant Power Train Applications

Abstract

High temperature tensile, compression and low cycle fatigue tests were performed to assess the influence of micro-additions of Ti, V and Zr on the Al-7Si-1Cu-0.5Mg (wt.%) alloy in the as-cast and heat treated conditions. A combination of electron microscopy and high temperature X-ray diffraction was used to identify phases and temperature ranges of their thermal stability. The microstructure of the as-cast Al-7Si-1Cu-0.5Mg (wt.%) base and modified alloys consisted of α-Al, eutectic Si, and Cu-, Mg- and Fe-based phases Al2.1Cu, Al8.5Si2.4Cu, Al7.2Si8.3Cu2Mg6.9 and Al14Si7.1FeMg3.3. In addition, the micro-sized Ti/V/Zr-rich phases Al6.8Si1.4Ti, Al21.4Si4.1Ti3.5VZr3.9, Al6.7Si1.2TiZr1.8, Al2.8Si3.8V1.6Zr and Al5.1Si35.4Ti1.6Zr5.7Fe were identified in the modified alloys. During solution treatment, Cu- and Mg- based phases were completely dissolved, while the eutectic silicon, Fe- and Ti-V-Zr-rich intermetallics partially dissolved. The TEM analysis confirmed the presence of nano-sized precipitates of (AlSi)3(TiVZr) with D023 tetragonal crystal structure. The tensile tests of the alloy in the as-cast state showed that with increasing testing temperature from 25°C to 400°C the yield strength and tensile strength of the studied alloy decreased from 161 MPa to 84 MPa and from 261 MPa to 102 MPa, respectively. Accordingly, the T6 heat treated alloy modified with additions of a higher content of Ti-V-Zr achieved the highest tensile strength of 343 MPa over the base alloy and alloys modified with additions of Ti, Ti-Zr and lower contents of Ti-V-Zr. The fatigue life of the studied alloy in the T6 condition was substantially longer than those of the reference alloys A356 and the same Al-7Si-1Cu-0.5Mg base alloy with lower additions of V, Zr and Ti in the T6 condition. The fractography revealed tensile crack propagation through the eutectic Si and primary phases, exhibiting intergranular fracture along with some cleavage-like features of the plate-shaped Ti-V-Zr-rich intermetallics with a presence of fatigue striations on the latter, indicating their ductile nature. It is believed that the intermetallic precipitates containing Zr, Ti and V improve the fatigue life of the studied alloy in the T6 condition.