Comparative Analysis of Microstructure and Mechanical Properties in Aluminum Alloy Automotive Control Arms Fabricated from Extruded Bars versus Horizontal Direct Chill Cast Bars

Abstract

Addressing the performance-cost trade-off dilemma in the selection of extruded billets and horizontal direct chill cast (HDC) billets for the manufacture of aluminum alloy control arms for automobiles, this study systematically reveals the microstructural inheritance effects and performance evolution mechanisms of the two base materials of 6082 aluminum alloy throughout the entire forging and heat treatment process. The results demonstrate that the gradient structure (surface fine-grain zone/mixed-grain zone/fibrous-grained core) in extruded billets triggers abnormal grain growth after heat treatment due to a drastic reduction in surface second-phase particle area fraction to 0.3% (caused by dissolution of Mg₂Si and coarsening of α-Al₁₂(Fe,Mn)₃Si). This results in inferior control arm properties with tensile strength of merely 335.2 MPa and elongation of 14.7%. In contrast, horizontal direct chill cast billets exhibit a homogeneous equiaxed grain structure. After forging-induced fragmentation of elongated α-Al₁₂(Fe,Mn)₃Si phases, the heat-treated material maintains a higher second-phase area fraction (0.7%). This effectively pins grain boundaries to inhibit coarsening, yielding refined and uniform microstructures. Consequently, the control arm achieves significantly enhanced tensile strength of 367.7 MPa and elongation of 15.8%.