Effect of solution heat treatment on residual stress in Al alloy engine blocks using neutron diffraction

Tensile residual stress in the cylinder bore region is a significant problem associated with Al engine blocks containing cast-in gray iron cylinder liners as it can result in cylinder distortion or engine block fracture. These issues can lead to significant environmental (emissions) and economic (recalls) problems for automotive OEMs. In this study, in-situ and ex-situ neutron diffraction was used to measure the change in residual strain in the cylinder bridge as a function of time during solution heat treatment and map to the stress profiles following heat treatment. A heating system, consisting of coiled tubular heaters in the cylinder bores and strip heaters along the front and side faces of the engine block, was designed and built to heat the blocks to the solution temperatures The results indicated that solutionizing at 470 °C caused a gradual relief of tensile residual strain up to approximately 5 h, where strain was completely relieved. At 500 °C, the strain relieved more rapidly, which resulted in complete relief in under 0.5 h. Cooling to ambient temperature caused the partial re-development of tensile residual stress throughout the cylinder depth due to thermo-mechanical mismatch, however the magnitude was lower than for the as-cast condition.