Microstructure evolution and microhardness of friction stir welded cast aluminum bronze

• The effect of rotation rate (ω) and traverse speeds (v) on the microstructural characteristics and mechanical properties of friction stir welded cast aluminum bronze have been investigated.
• The obtained results revealed that the ω2/v ratio has a larger effect on the volume of the weld defects than ω/v ratio.
• FSW refined the as-cast coarse microstructure in the base metal.
• FSW led to the formation of four distinct areas containing various coaxial grains within the SZ.
• The effect of grain size on microhardness in the stir zones was satisfied with Hall–Petch relationship.

Microstructural characteristics and mechanical properties of a friction stir welded cast aluminum bronze (Cu–9Al–1Fe), produced by a sand casting method, have been investigated at tool rotation of 850–1500 rpm and traverse speed of 50–100 mm/min. Refinement of the primary coarse cast microstructure in the base metal was seen after friction stir welding. Microstructure of the stir zone was characterized in four distinct areas of non-isometric fine grains while a significant grain growth was noticed in some of the areas. Conditions of grain growth are defined with high heat input intensity and low heat transfer capability. The grain size was observed to decrease after FSW, resulting in a greater microhardness across the welded region from about 100 HV in the base metal to about 150 HV at the center of the stir zone. The increased hardness in the stir zone may have stemmed from the locally refined grain size according to Hall–Petch relation.