Experimental and analytical investigation of microstructure homogenization, internal defect repair, and material softening in LPBF-printed AlSi10Mg aluminum alloy via surface friction stir treatment

This paper investigates the effects of surface friction stir treatment on the microstructure and defects of LPBF-produced AlSi10Mg alloy. Experimental analyses, supported by numerical simulations, investigate the effects of process conditions on material flow, strain distribution, resulting stir zone (SZ) characteristics, and overall microstructural transformations. The analytical investigation, coupled with statistical analysis, aims to define the links between imposed process conditions, effects on heat generation, heat diffusion, and material flow, and resulting material response in terms of microstructure and microhardness. Nine samples were treated using a pinless rotating tool under identical fabrication conditions. The treatment induced complete dynamic recrystallisation and significant softening within the SZ, with depths ranging from 1213 to 1617 μm, showing good agreement with numerical predictions. Numerical results delineated the origin of the characteristic vortexes as portions of material with the highest level of softening and demonstrated the effects of the processing parameters on their final position within the SZ. A reduction of approximately 30% in porosity and lack-of-fusion defects was achieved in the SZ. Microhardness decreased to 59.58 HV in the SZ and 65.27 HV in the TMAZ compared to 127.12 HV in the as-built material. The SZ depth and softening were strongly dependent on process parameters and were, within the analytical investigations, successfully correlated with dimensionless numbers, including the Péclet number, mechanical Reynolds number, and non-dimensional energy input. Optimal SZ properties were obtained at relatively low processing conditions, where a balance between heat input and plastic deformation promoted continuous dynamic recrystallisation and microstructural refinement.