Development of a fine-grained microstructure and the properties of a nugget zone in friction stir welded pure copper

Defect-free pure copper welds were achieved under low heat input conditions of 400–800 rpm for a traverse speed of 50 mm min−1. The grain size of the nugget zones decreased from 9 to 3.5 μm with decreasing rotation rate from 800 to 400 rpm. “Onion rings”, consisting of different-sized grain bands, were observed at 400 rpm, but disappeared at 600–800 rpm. Variations of both microhardness and yield strength of the nugget zones with grain size followed the Hall–Petch relationship.