Microstructure and Mechanical Properties of 6061 Al/AZ31 Mg Joints Friction Stir Lap Welded by a Tool with Variable-Pitch Thread Pin

Changshu He,Ni Tian,Zhiqiang Zhang,Gaowu Qin,Jingxun Wei,Chengpeng Qiu,Hao Zhang

doi:10.3390/met11010034

Abstract

Sheets of 6061-T6 aluminum alloy (thickness = 3 mm) and AZ31 magnesium alloy were friction stir lap welded by a tool with a variable-pitch thread pin (coarse-threaded in the upper part and fine-threaded in the lower part). For the same rotation speed and welding speed, the heat input was higher in joints with an upper Al alloy (Configuration Al-Mg) than in those with an upper Mg alloy (Configuration Mg-Al). In Configuration Al-Mg, these two dissimilar metals were poorly mixed and Al dominated the stirred zone (SZ). Many intermetallic compounds (IMCs) of Al3Mg2 formed inside the SZ. In Configuration Mg-Al, Mg alloy bands, flocculent Al12Mg17 bands, and minor Al alloy bands intersected in the SZ, forming a complex onion-ring structure. Moreover, a complex mechanical interlocking structure developed at the bottom interface of the SZ. The maximum tensile shear strengths of the Al-Mg and Mg-Al lap configurations were 160.3 and 217 N/mm, respectively, at 700 rpm. The higher tensile shear strength of the Mg-Al configuration primarily represented less IMCs and complex mechanical interlocking structures in the SZ.

Highlights

Aluminum and magnesium alloys are lightweight metallic materials with several desirable properties: low density, high specific strength, and stiffness
In Configuration Al-Mg at 700 rpm, the stirred zone (SZ) boundary was straight and smooth and the thick intermetallic compounds (IMCs) bands in the SZ were distributed in the Al matrix (Figure 3a)
The complex onion-ring structure indicates that an effective mechanical interlock formed in the SZ [27]

Summary

Introduction

Aluminum and magnesium alloys are lightweight metallic materials with several desirable properties: low density, high specific strength, and stiffness. They are widely used in the automobile, aviation, aerospace, and other industries requiring lightweight structures [1,2]. A hybrid structure formed by welding aluminum and magnesium alloys is expected to inherit the performance advantages of both alloys, and is considered as an alternative approach for structural weight-saving [3]. Friction stir welding (FSW) is a solidstate joining process requiring less heat input than traditional fusion welding. FSW has emerged as a promising method for joining dissimilar aluminum alloy and magnesium alloy [6,7,8]

Results

Discussion

Conclusion