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.
