Residual stresses and distortion in additively manufactured compositionally graded and dissimilar joints

Additively manufactured compositionally graded joints are potentially attractive to minimize abrupt changes in residual stresses and distortion of dissimilar alloy joints. Performance of these graded joints depends on the residual stresses and distortion governed by the transient temperature field during additive manufacturing and local mechanical properties of the joint. Here we develop, validate and utilize a thermo-mechanical model to provide a definitive way to additively manufacture sound graded joints for minimizing abrupt changes in residual stresses and distortion of the dissimilar joints. This model calculates residual stresses and distortion from accurate temperature fields calculated using a well-tested heat transfer and fluid flow model and temperature dependent alloy properties estimated by thermodynamic calculations. Both graded and dissimilar joints of 2.25Cr-1Mo steel to alloy 800H and Ti-6Al-4V to 800H, fabricated using laser-assisted powder based direct energy deposition process are examined. It is found that the sharp changes in residual stresses in dissimilar joints between Ti-6Al-4V and 800H can be effectively minimized by fabricating a graded joint between them. Although the magnitudes of residual stresses in Ti-6Al-4V to 800H joint are higher than that in 2.25Cr-1Mo steel to 800H joint, the former is less susceptible to warping, buckling and delamination due to the high room temperature yield strength of the Ti-6Al-4V substrate.