Internal Residual Stress Measurement of Aluminum Alloy Castings Using Neutron Diffraction

When designing engine parts of motor vehicles, it is important to evaluate internal residual stresses that cause crack growth and influence the strength of parts. Internal stresses can be measured nondestructively by the neutron diffraction method. However, it is difficult to apply this method to aluminum alloy castings because they consist of coarse crystal grains. As for cylinder heads, the grain size ranges up to approximately 400 μm and there are few grains contributing to intensity of diffraction in each gauge volume. In the case of X-ray diffraction, "the oscillation method" has been employed for materials with coarse grains.

In this study, the applicability of the oscillation method to aluminum alloy castings was investigated with the aim of establishing a method of measuring internal stresses and strains. A related objective was to determine the accuracy of stresses. For the purpose of the study, specimens made of an aluminum alloy casting and measuring 20x20x140 mm in size were prepared and subjected to a bending load. A neutron diffraction experiment with oscillations was conducted, and the strain values were compared with the results measured with strain gauges. The experimental results show that an oscillation range of ±15 degrees provides enough intensity of diffraction to read peak positions. Under this measurement condition, the differences between the strain values obtained by neutron diffraction and with strain gauges were less than 250x10^-6 and the accuracy of the stresses was within 20-25 MPa. We also applied this method to an actual cylinder head. As a result, sufficient intensity of diffraction was observed with oscillations of ±15 degrees. It was found that there were internal tensile residual stresses. We verified the efficacy of the neutron diffraction method with oscillations for aluminum alloy castings.