Residual stress, microstructure, and mechanical properties analysis of HPDC aluminum engine block with cast-in iron liners

Abstract

During the manufacturing of cast aluminum (Al) engine blocks with iron (Fe) cylinder liners, the mismatch of thermal expansion/contraction coefficients between the Al cylinder wall and the Fe liners results in the development of high residual strains/stresses. This evolution of residual stress can surpass the strength of the currently used alloys and can lead to the premature failure of the engine block. Thus, improving the mechanical properties of the engine block alloys reduces the number of failures and allows automotive manufacturers to increase the operating pressures and, therefore, efficiency. High-pressure die casting (HPDC) has been shown to provide increased cooling rates in the casting as compared to sand casting, and therefore manufacturers commonly use this method for improving not just the efficiency of the high-volume production, but also mechanical properties of the cast components. Hence, investigating the impacts of the HPDC process on the evolution of residual stress in engine blocks with Fe liners and, consequently, their durability is of great importance. The present study includes a microstructural, mechanical property, and residual strain analysis on an inline-4 HPDC Al engine block with cast-in Fe liners. The results indicate that the yield and tensile strength at the top of the cylinder were ∼15 % and ∼8 % greater than the bottom, respectively. This trend was primarily attributed to the higher cooling rate experienced by the top region. Moreover, the top and bottom of the Fe liner experienced 450 MPa of compressive residual stress in the axial direction, which is counterbalanced by tensile stresses in the Al cylinder bridge (CB), which reaches approximately 70 % of the alloy’s yield strength. The findings of this study provide automotive industries with important insights on the combined impacts of the HPDC process and cast-in Fe liner on the as-manufactured properties of engine blocks and will assist them in developing the next generation of engine blocks.