Mg/Al REACTION AND MECHANICAL PROPERTIES OF Al ALLOY/Mg ALLOY FRICTION STIR WELDING JOINTS

Abstract

Mg alloys are increasingly used in aerospace,aircraft and automotives structures due to the low density and good damping properties,especially,the welding of Mg alloy to Al alloys is of practical importance for widening the application of Mg alloys.Although friction stir welding (FSW) has been used to join Mg and Al alloys,defect free joints are achieved only in the thin plate (less than 4 mm).In this paper,three ways of FSW of 6 mm thick 6061-T651 Al alloy and AZ31 Mg alloy plates,offsetting the pin to the seam between the two plates(M4A4),to the Mg alloy side 2 mm(M6A2) and to the Al alloy side 2 mm(A6M2),were studied.The aim is to examine the effects of the relative position between tool and plate on the microstructure and mechanical properties of FSW Mg/Al alloys joint.SEM and XRD analyses revealed the formations of intermetallics Mg_(17)Al_(12) and voids in the interface between Mg and Al alloy plates in the three FSW samples. The voids are resulted from the melting and subsequent solidification of eutectic.In the M4A4 and M6A2 samples,some Al are stirred into the Mg alloy side during FSW,forming the Mg_(17)Al_(12), whereas in the A6M2 sample,some Mg stirred into the Al alloy side are also transformed to the Mg_(17)Al_(12).Some fine pores are observed around the MgnAl_(12).The hardness of the nugget zone exhibits a slight increase due to the formation of a small amount of Mg_(17)Al_(12).The intermetallics and pores at the jointed interface decrease the strength of the joints significantly.