Grains growing by simultaneous recrystallization and grain growth mechanisms near the surface of shot peened Al

Surface treatments like shot peening (SP) are frequently applied to laser powder bed fusion (LPBF) alloys to introduce beneficial compressive residual stresses, but this process simultaneously creates a gradient microstructure near the surface. The thermal stability of this SP-induced microstructure is critical, as any microstructural evolution during in-service thermal exposure could compromise the beneficial properties. This study analyzes the microstructural evolution of SP-treated LPBF AlSi10Mg during annealing at 500 °C using scanning electron microscopy and electron backscatter diffraction. It is found that the as-built Si network is fragmented by SP and a microstructural gradient is induced: The grains are ultrafine (0.8 μm) at the surface and transition towards coarser sizes reaching 3.0 μm at a depth of 40 μm. Consequently, the annealing response is heterogeneous: The surface layer undergoes rapid coarsening (the high-angle boundary spacing increases from 0.8 μm to 3.4 μm in 5 min), while at the subsurface a few nuclei grow rapidly to an abnormally large size. Subsequently, these grains grow to dominate both the surface and the layer below, facilitated by a reduction in Zener pinning as the Si particles coarsen. This study provides critical insight into a series of competing microstructural mechanisms, which are essential for predicting performance evolution when an SP treated LPBF AlSi10Mg component is in use.