Friction stir alloying of AZ61 and mild steel with Cu-CNT additive

Abstract

Dissimilar joining between lightweight magnesium (Mg) alloys and steel is essential to produce lighter vehicles, improve vehicles’ fuel efficiency, and reduce carbon emissions. However, the joining of Mg to steel is impractical due to the immiscible properties between these metals. In this experiment, friction stir alloying (FSA) is proposed to solve this problem. The additive, consisting of different wt% of carbon nanotubes (CNT) in Cu powder was first added into the gap between the workpieces and then friction stir welding (FSW) was performed at varied traverse speed and constant rotational speed. After the joining, microstructure characteristics and mechanical properties of Cu-CNT reinforced Mg/steel joints were investigated. Transmission electron microscopy (TEM) analysis of the Mg/steel joint revealed the formation of IMC at the interface of the joint. Further analysis by X-ray diffraction (XRD) showed a dominant presence of Mg2Cu IMC which indicated the interdiffusion of Cu into Mg element to establish intermetallic bonding. The presence of CNT inside the Mg matrix was also confirmed by TEM which contributed to the strengthening effect of the joint. Tensile and microhardness results revealed a notable enhancement of joint mechanical properties when Cu-CNT additive was added as compared to specimens with only Cu additive, and specimens without additive. The enhanced tensile strength and microhardness of the Cu-CNT reinforced Mg/steel joint was attributed to the dispersion of CNT inside the Mg matrix, which induced multiple dislocations in the surface region, therefore improving the mechanical properties of the joint.