Analysis of Residual Stress Profiles in the Cylinder Web Region of an As-Cast V6 Al Engine Block with Cast-In Fe Liners Using Neutron Diffraction 2011-01-0036

Continuous efforts to develop a lightweight alloy suitable for the most demanding applications in automotive industry resulted in a number of advanced aluminum (Al) and magnesium alloys and manufacturing routes. One example of this is the application of 319 Al alloy for production of 3.6L V6 gasoline engine blocks. Aluminum is sand cast around Fe-liner cylinder inserts, prior to undergoing the T7 heat treatment process. One of the critical factors determining the quality of the final product is the type, level, and profile of residual stresses along the Fe liners (or extent of liner distortion) that are always present in a cast component.

In this study, neutron diffraction was used to characterize residual stresses along the Al and the Fe liners in the web region of the cast engine block. The strains were measured both in Al and Fe in hoop, radial, and axial orientations. The stresses were subsequently determined using generalized Hooke's law. Further, detailed microscopy and hardness measurements were performed from top to bottom along the interbore region of each cylinder. The microstructure was characterized using an optical and scanning electron microscope (SEM). Further, composition analyses were performed using energy dispersive X-ray spectroscopy (EDX). The results suggest that a variation in cooling rate along the cylinder caused a refinement of Al₂Cu, Al₁₅(Mn,Fe)₃Si₂ and eutectic silicon at the bottom of the cylinder. Increased cooling rate at the bottom of the cylinder also led to a more globular and uniform distribution of second phase particles, thereby resulting in increased hardness.

This study gives invaluable insight on anticipated service properties of the engine block and demonstrates that neutron strain mapping is an efficient tool for optimization of manufacturing technologies.