Evaluation on Microstructures and Mechanical Properties in Friction Stir Welded Lap Joints of Invar 42/SS 400

This study evaluated the microstructures and mechanical properties of friction stir welded lap joints. Invar 42 and SS 400 as experimental materials were selected, and friction stir welding was carried out at tool rotation speed of 200 rpm and welding speed of 100 mm/min. Applying the friction stir welding was notably effective to reduce the grain size of the stir zone, consequently, the average grain size of Invar 42 was reduced from 11.5 μm in the base material to 6.4 μm in the stir zone. Joint interface between Invar 42 and SS 400 showed a sound weld without voids and cracks, and the intermetallic compounds with Ll₂ type in lap joint interface were partially formed with size of 100 nm. Also, the hook, along the Invar 42 from SS 400, was formed at advancing side, which directly affected an increase in peel strength.