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Defect-free welded joints were obtained by placing 316 L stainless steel at the retreating side (case 1) and the advancing side (case 2) of friction stir welds when joining China low activation martensitic (CLAM) steel to 316 L steel. Quenched martensite formed in the weld on the CLAM side, and the precipitates coarsened and aggregated in the outer HAZ, however, uniform austenite microstructures formed on the 316 L side of the weld. The microhardness profiles were asymmetric, and the maximum hardness zones located at the nugget on the CLAM side ranged from 350 to 450 HV. Softening in the outer HAZ on the CLAM side was observed, which was more pronounced in case 2. The welded joints had excellent tensile properties and always reached the tensile strength of the 316 L base metal. Better tensile properties of the welded joint were obtained in case 1, and the bonding strength of interfaces between CLAM and 316 L steels were always excellent in both cases.

A coupled thermo-mechanical model is developed for friction plug welding (FPW) process of AA2219 aluminum alloy, which is validated by the experimental results of the torque and temperature history. Both the deformation of plug and base metal are considered in this model. It is concluded that weak-bonding and unbonding is caused by the poor interface normal force and material flow based on the simulational and experimental results. Significant inhomogeneity in distribution of temperature, effective strain and effective stress is observed along the thickness direction of FPW joint, resulting in the inhomogeneity of microstructure and mechanical property. The peak temperature of bonding interface can reach 508 °C, which is 79% of the melting point of base metal. Higher welding force and rotational speed could promote material flow and improve interface normal force, which is beneficial to obtaining better connection.

Stationary shoulder friction stir welding (SSFSW) was successfully performed on 7075-T651 aluminum alloy. Defect-free joints with smooth weld surface were obtained. The redistribution of constituent particles and the dissolution of η′ precipitates resulted in the low crack initiation energy of the nugget zone. The low crack initiation energy in the heat affected zone (HAZ) can be attributed to both the presence of η precipitates and the redistribution of constituent particles. The presence of η precipitates in the HAZ was the major reason for the decreased tensile strength of SSFSW joint.