ISSUE 3/2011

/// 04
Anyalebechi, P. N. Effect of process route on the structure, tensile, and fatigue properties of aluminum alloy automotive steering knuckles

A quantitative study has been conducted on the effects of process route on the microstructure, tensile, and fatigue properties of automotive steering rear knuckles made of aluminum alloy A356 in the T6 temper condition. The study involved the characterization of the microstructure, strength, and fatigue properties of knuckles produced by the vacuum riserless/pressure riserless casting (VRC/PRC), pressure counter pressure casting (PCPC), and the hybrid CobapressTM (a casting-forging) processes in accordance with the appropriate ASTM standards. Surprisingly, the knuckles produced by the hybrid Cobapress process exhibited the lowest strength. For example, the average 0.2 % yield strength of the Cobapress knuckles was 31 MPa and 40 MPa less than that of the VRC/PRC and PCPC knuckles, respectively. However, between stresses of 150-200 MPa, the fatigue lives of the VRC/PRC, PCPC, and Cobapress knuckles were comparable. But above 200 MPa, the Cobapress knuckles exhibited the lowest fatigue life. The observed differences in the mechanical properties of the different knuckles are attributed to the differences in their microstructures and internal quality. For example, the relatively poor strength and large variability in the strength, ductility, and fatigue properties of the hybrid Cobapress knuckles can be attributed to the comparatively larger dendrite cell size, presence of coarse b-[AlFeSi] constituent phase particles, oxide films, and porosity in the knuckles.

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