Residual stress measurement in dissimilar metal joints using neutron diffraction

Joining various metallic parts becomes crucial to lighten the vehicles for automobile industry and thus reduce the greenhouse gas emission while maintaining crashworthiness. The combination of Al and steels is of an interest due to the lightweight of Al alloys and the high strength of steels. Similarly, to classic welds, dissimilar welds accommodate residual stresses. Their quantification is a challenge using conventional approaches. Dissimilar welds produced by Friction Melt Bonding (FMB) technique [1] are used here to verify two different approaches of measuring residual stresses using neutron diffraction at SALSA (Strain Analyser for Large and Small scale engineering Applications) instrument. Firstly, near interface residual stresses are characterized for an Al/steel joint by considering a pseudo strain correction method. Residual stresses near the interface originate from an inhomogeneous distribution of thermal cycles and from the difference in coefficient of thermal expansion for both Al and steel during the cooling stage. These stresses are characterized using this pseudo strain correction method [2]. On the Al/steel interface, longitudinal residual stresses in the steel side reveal an “M” shape distribution while, on the aluminum side, longitudinal residual stresses show a “W” shape. Secondly, four different welds composed of Al/steel produced with various welding conditions (2 different advancing speed and two different backing plates) are characterized by measuring an average residual stress using an oscillating gauge across the thickness [3]. This second approach is particularly beneficial for thin plates with smaller thickness than the gauge’s diagonal width. It is found that the longitudinal residual stress increases for increasing heat input into the welds. The transverse residual stress on the steel side also becomes slightly tensile with increasing heat input into the welds. Thus, it is preferable to produce dissimilar welds with lower heat input in order to minimize the residual stresses in the welds. These two proposed methods enable to quantify the residual stresses in dissimilar welded joints. [1] C. van der Rest, P.J. Jacques, A. Simar, Scr. Mater., 77 (2014) 25-28. [2] N. Jimenez-Mena, T. Sapanathan, J.-M. Drezet, T. Pirling, P.J. Jacques, A. Simar, J. Mater. Process. Technol., 266 (2019) 651-661. [3] T. Sapanathan, J.-M. Drezet, P. J. Jacques, T. Pirling, A. Simar, doi:10.5291/ILL-DATA.1-02-24