Fatigue crack propagation in 15-5PH/316L bi-material steels fabricated by laser powder bed fusion

Multi-material laser powder bed fusion (MM-LPBF) offers the possibility of components with material and compositional complexity, as well as the geometric complexity for which additive manufacturing is known. LPBF materials are susceptible to fatigue failures due to stress concentrating roughness and porosity defects. Understanding fatigue failure processes is therefore important to enable adoption of multi-material parts, and suitable combinations of materials may offer a strategy to enhance fatigue performance by resisting crack propagation. This study focused on fatigue crack propagation in 316L/15-5 precipitation hardened (PH) bi-material stainless steels (SS), and the effect of residual stress distribution and yield stress gradient on fatigue crack propagation through the interface. The expected yield stress gradient effect in bi-materials (soft to hard interface) was simulated using FE models, showing a slight shielding effect with a drop in J-integral value. Contour cutting measurements detected a residual stress distribution near the bi-material interface that was tensile in 316L layer and compressive in 15-5PH layer. Fatigue crack propagation rates in bi-materials deviated from those in the corresponding single-material specimens. A relatively small shielding effect due to the yield stress gradient was detected within a short distance of the crack tip from the interface. However, the effects of residual stress were more pronounced and inhibited the crack growth rate by up to 77.8 % in regions of 15-5PH SS under residual compression, which suggesting that MM-LPBF parts can be designed such that the compressive residual stress is positioned to intercept and suppress propagating cracks to improve damage tolerance.