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 mu m 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. (C) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.