Microstructure, hook formation and mechanical performance in refill friction stir spot welding of additive manufactured to wrought aluminium joints

• High ultimate lap shear force (> 8.2 kN) for 1000 and 2200 rpm rotational speed. • Medium rotational speeds promote weak bonding in the top sheet (pre-peak cracking). • Clamping ring indent depth correlates with peak temperature in heat-affected zone. • High hook defect reduces quasi-static joint strength. • AlSi10Mg softens due to dissolution of the fine Si-network towards the stir zone. Laser powder bed fusion (L-PBF) enables lightweighting for complex aviation components through design flexibility and near-net-shape manufacturing. Integrating additive-manufactured parts with wrought aluminium structures requires reliable joining methods. Therefore, refill friction stir spot welding (RFSSW or refill FSSW) of L-PBF AlSi10Mg to wrought AA7075-T6 was systematically investigated to maximise the Ultimate Lap Shear Force (ULSF) and analyse geometrical features in the joints. Thermal cycles were recorded at multiple weld locations, and the microstructure was evaluated using high-resolution electron microscopy. The joints exhibited robust ULSF (> 8200 N) at minimum and maximum rotational speed, with the maximum strength governed by the mechanical properties of the L-PBF AlSi10Mg. Pre-peak cracking at intermediate rotational speeds signalled weak bonding at the weld circumference. Reduced absolute hook heights correlated with increased ULSF, underscoring the critical role of hook geometry in load transfer. Longer welding times increased peripheral temperatures but not the peak temperature in the weld centre. In the L-PBF AlSi10Mg, strength degradation centred on the disruption of the Si eutectic network, whereas in AA7075-T6, the complex precipitation sequences, dynamic recrystallisation, and grain refinement affected the resulting microstructure. These findings provide guidelines for tailoring the RFSSW process to achieve high-performance L-PBF to wrought aluminium joints.