Material flow and intermixing during friction stir spot welding of steel

• Material flow during FSSW of DP590 is investigated by tracer material technique.
• With increased penetration, material below the pin is displaced upwards.
• A series of material flows are established after shoulder contact.
• The material intermixing between the overlapping sheets creates the bonding ligament.

Tracer materials as inserts have been used to study the material flow during Friction stir spot welding (FSSW) of DP590 steel of 1.6 mm thickness. A PCBN tool with convex scrolled shoulder was used for welding. The progressive development of the material flow with tool penetration has been investigated by tracking the tracer distribution. When flow conditions are established, the shoulder scrolls provide the driving force for material flow by dragging the plasticized materials towards the pin and releasing them there. The released material flow outwards and upwards towards the shoulder periphery along different sub-surface flow routes. Successive flows in this fashion builds up the material circulation within the flow zone. The bonding ligament is created by intermixing of material between the overlapping sheets. The flow zone size and ligament width increases with increasing tool penetration. The flow zone encompasses the pin and a large area below the tool shoulder. Material flow during FSSW can be subdivided in two components, namely rotational flow and through thickness flow. A combination of the two components results in the spiraling motion of plasticized material within the flow zone. A material transport model is developed on the basis of experimental observations, illustrating the material flow during FSSW and the formation of bonding ligament.