Effects of metallurgical parameters on the development of residual stresses in Al-Si alloys used in engine block manufacturing

Using light-weight materials have become an important factor in the automotive industry due to stringent government regulations on fuel consumption. Aluminum alloys are 65% lighter than cast iron enabling significant weight reduction. However, there are several significant challenges associated to the use of hypoeutectic Al-Si alloys in engine block applications. This dissertation investigated the factors influencing the development of residual stresses in I-4 and V-6 engine blocks due to different heat treatments and introducing sub-zero treatment as a mean for reducing residual stresses. The initial section of this thesis explores the development of residual stresses with different casting parameters in A356.1 and B319.1 castings with simple geometries. This analysis involved measurement of tensile properties, microstructural analysis, and residual stress measurement using sectioning technique. The casting parameters include cooling rate, quenching media, aging time and aging temperature. The second phase of this study will investigate the development of residual stress in castings, with more complex shape such as in I-4 and V-6 engine blocks with cast-in gray iron liners, with different heat treatment parameters and sub-zero treatment. An analysis of result shows that higher strength materials, as in B319.1 alloy, produce higher residual stresses compared to material with lower strength, as in the case of A356.1. The results also show that there is direct proportionality between ultimate tensile stresses (UTS) and residual stresses (RS) with quenching rate. The residual stresses were found to gradually decrease with decreasing cooling/quenching rates of the quenching medium. The quenching process develops the highest residual stresses, where quenching in cold water develops the highest, and air cooling the lowest, residual stresses. The relaxation of residual stresses is significantly dependent on aging temperature and proceeds smoothly with the increase in aging