Friction stir lap welded advanced high strength steels: Microstructure and mechanical properties

Friction stir welding was carried out under different heat input and cooling rates to produce lap joints
between high strength martensitic steel sheets. The microstructure of the welds was characterized, and
microhardness was evaluated. Joint efficiency was determined by lap shear test. Variation in processing
conditions governed total heat input, peak temperature and cooling rate during friction stir welding.
Weld nugget microstructure depended principally on cooling rate. The slowest cooling rate promoted
ferrite–pearlite and the fastest cooling rate resulted in martensite formation in the weld nugget. The
weakest region of all the joints was the heat affected zone, which consists of ferrite with small quantities
of pearlite. Fracture during shear testing occurred along the heat affected zone of welded joints. The
width and grain size of ferrite in heat affected zone controlled the joint efficiency.