Study On Microstructure And Mechaniacal Properties Of AZ91D Magnesium Alloy Steering Control Arm

The casting process of Magnesium alloy forming is low cost and has outstandingperformance. However, the mechanical properties are low as a result of manymicrostructure defects. Though traditional forging process has high performance, themass production has not been obtained due to its complex process lines and high cost.To solve the problem，the author will analyze the AZ91D auto steering control arm,which is manufactured by squeeze casting forming process. Combining the low cost ofsqueeze casting and the good quality of forging and forming new production technologyof high performance Mg components, and manufacture automotive steering control armof high quality with AZ91D magnesium alloy, on the basis of squeeze casting,die-forging and Casting-Forging Combined Process, the influence of the two formingprocesses on microstructure and the mechanical properties of control arm can beanalyzed comparably.The main conclusions are as follows:①The microstructure of the two processes both consist of α-Mg matrix and blackβ-Mg₁₇Al₁₂phase composition.②In the squeeze casting control arm, low speed made less volume gas, andHigh-pressure solidification made it hard to form the defect shrinkage, so the densitywas good. But, due to chilling action and lack of melt metal in the final solidification, afew defects still exited. The mechanical properties were closely related to themicrostructure: coarse reticular β-Mg₁₇Al₁₂had dissever effect to matrix. Meanwhile,stress concentration in the α-Mg matrix leaded to cracks and caused poor plasticdeformation ability, and the tensile fracture surface appeared to have brittle fracturetoughness mixed mechanism.③The microstructure formed by Casting-Forging combined process presentedstreamline characteristics. Dynamic recrystallization and grain refinement executedfully, the casting imperfections are closed and repaired partly, and the microstructurehad a better density. In the meantime, because the dissever effect was lowered, the grainboundaries’ number increased; and the pinning impact had the dispersion strengtheneffect. Therefore, the comprehensive mechanical properties of the sample enhancedremarkably; the sample had high plastic deformation ability, and the fracture ofcasting-forging combined plastic formation is exhibited typical brittle fracture toughness mixed mechanism.④After aging ttreatment implemented on die-forging samples, β-Mg₁₇Al₁₂wereprecipitated, which enhanced the pinning function of α-Mg and blocked the dislocationmovement, the tensile strength and yield strength improved greatly. However, fracturetended to happen because of the weak binding force between α-Mg and β-Mg₁₇Al₁₂, sothe plastic deformation ability was poor, and the tensile fracture exhibited typical brittlefracture toughness mixed mechanism.To conclude, The microstructure, mechanical properties and fracture way ofAZ91D magnesium alloy car steering control arm under different forming processeswere studied, and the results indicate that, compared to the control arm formed bySqueeze casting, that of die-forging has better microstructure, mechanical properties.The whole research provides a good background of studying Casting-forging combinedforming process.