As a near-net-shape forming process combining the advantages of casting and forging, the change in mechanical properties after heat treatment is a core indicator for evaluating the engineering application value of steering knuckles. While squeeze casting achieves a dense microstructure through high-pressure filling and solidification, heat treatment is still necessary to further optimize the microstructure and achieve a balance between strength, plasticity, and toughness. The impact of heat treatment on steering knuckles is mainly reflected in three aspects: grain refinement, phase structure control, and defect repair. These changes collectively contribute to improved mechanical properties.
The microstructure of steering knuckles before heat treatment is already superior to that of traditional castings. Due to the solidification of the alloy liquid under high pressure, its grain size is usually finer, but localized component segregation or residual stress may exist. Heat treatment, through a two-stage process of solution treatment and aging, further eliminates internal defects. During the solution treatment stage, alloying elements fully dissolve in the matrix, forming a supersaturated solid phase. Simultaneously, the high-temperature environment promotes grain recrystallization, eliminating casting stress. During the aging treatment stage, the supersaturated solid undergoes desolvation, precipitating fine, dispersed strengthening phases such as Al₂Cu and Mg₂Si. These second-phase particles significantly improve material strength by hindering dislocation movement.
The strengthening effect of heat treatment on the mechanical properties of steering knuckle is primarily reflected in the increase in tensile strength and yield strength. Solution treatment homogenizes the matrix composition and eliminates dendritic segregation, laying the foundation for age-hardening. The nanoscale strengthening phases precipitated during aging effectively hinder dislocation slip through the Orowan mechanism, resulting in higher macroscopic resistance to deformation. This strengthening effect is particularly significant in Al-Si-Cu-Mg alloys, where the yield strength can approach the level of forgings, meeting the load-bearing requirements of steering knuckles under complex operating conditions.
Improved plasticity is another important contribution of heat treatment to steering knuckle. Although the precipitation of strengthening phases may reduce material ductility, the fine-grained structure formed by the squeeze casting process itself offsets some of the embrittlement effect through grain boundary strengthening. Furthermore, the casting defects (such as shrinkage porosity and cracks) eliminated by heat treatment reduce stress concentration sources, allowing for a more uniform distribution of strain under stress, thus maintaining a certain elongation while increasing strength. This balance between strength and plasticity is crucial for steering knuckles subjected to impact loads.
Heat treatment process parameters are highly sensitive to the performance control of the steering knuckle. The combination of solution temperature and time determines the degree of dissolution of alloying elements: too low a temperature or insufficient time leads to inadequate re-dissolution of strengthening phases, affecting the aging effect; too high a temperature or too long a time may induce grain coarsening, reducing material toughness. Aging temperature and time control the precipitation kinetics of strengthening phases: low-temperature, long-time aging favors the formation of fine, dispersed precipitates, while high-temperature, short-time aging may produce coarse phases, weakening the strengthening effect. Therefore, for specific alloy compositions and casting structures, process optimization is necessary to maximize performance. In practical applications, the steering knuckle, after heat treatment, needs to undergo bench testing to verify its performance reliability. Typical operating conditions include static strength testing and fatigue durability testing. The former assesses the material's resistance to failure under ultimate loads, while the latter simulates crack initiation and propagation under long-term alternating stress. Test results show that the heat-treated steering knuckle is superior to the untreated part in terms of stress distribution uniformity and crack propagation resistance, and its fatigue life can be increased several times, meeting the stringent safety requirements of automotive chassis components.
The heat treatment process of the steering knuckle achieves a significant improvement in mechanical properties through microstructure control. From grain refinement to phase structure optimization, from defect repair to stress relief, each process improvement serves to synergistically enhance the material's strength, plasticity, and toughness. This combination of processes not only enables the steering knuckle to achieve its lightweight goal but also endows it with comprehensive performance comparable to traditional forgings, providing a reliable solution for high-performance manufacturing of automotive chassis components.
