Stress Characterization of Bore-Chilled Sand Cast Aluminum Engine Blocks in As-Cast and T7 Condition with Application of Neutron Diffraction

Abstract

In an effort to improve vehicle fuel efficiency, aluminum (Al) alloys have been gaining upward momentum for use in automotive powertrain components such as engine blocks. Al alloys are lightweight and have good mechanical strength at engine operating temperatures; making them a suitable choice for engine block production. However, during the manufacturing process factors such as inhomogeneous cooling rates and/or coefficients thermal expansion mismatches in multi-material castings can lead to the development of residual stress. This is of particular concern for the relatively thin cylinder bridges, which are exposed to large thermo-mechanical loading during engine operation. The casting process used at Nemak for I6 engine block production does not utilise cast-in liners and therefore may be also be suitable for future mass-produced linerless blocks. This paper utilizes neutron diffraction and SEM/EDS to determine how the elimination of cast-in liners as well as a T7 heat treatment effects the magnitude of residual stress in cast Al (A319 type alloy) engine blocks. It was observed that the T7 treatment resulted in a significant reduction of the strain/stress in the Al cylinder bridge (up to ~50% of the radial stress at the top of the bridge). In addition, the absence of the cast-in Fe liners allowed for unrestricted natural contraction of the Al bridge; leading to a combination of low tension and moderate compression as compared to the typically high tensile stress.