Interface Characteristics and Mechanical Properties of Ultrasonic-Assisted Friction Stir Lap Welded 7075-T6 Aluminium Alloy.

Changshu He,Ying Li,Menggang Zhai,Zhiqiang Zhang,Jingxun Wei,Xiang Zhao,Su Zhao

doi:10.3390/ma13235335

Abstract

In this work, friction stir lap welding (FSLW) and ultrasonic-assisted friction stir lap welding (UAFSLW) was applied to 6-mm-thick 7075-T6 alloy sheets using three welding tools with the same process parameters. The joint formation, microstructural characteristics, and mechanical properties of the resulting lap joints were then investigated. The results showed that ultrasonic vibration significantly promoted the flow of metal at the interface, enlarged the size of the stirred zone (SZ), and reduced the angle between the hook defect and the interface. During lap shear testing, the FSLW and UAFSLW joints fractured in a similar manner. The fracture modes included tensile fracture, shear fracture, and a mixture of both. Cold lap and hook defects may have served as crack-initiation zones within the joint. Under configuration A (i.e., upper sheet on the retreating side (RS)), all joints failed in the shear-fracture mode. The effective lap width (ELW) of the joint welded using tool T2 was the greatest. This resulted in a higher shear fracture strength. The maximum shear fracture strength of the UAFSLW joint was 663.1 N/mm. Under configuration B (i.e., upper sheet on the advancing side (AS)), the shear fracture strength was greatly affected by the fracture mode. The highest shear fracture strength of the UAFSLW joint, 543.7 N/mm, was welded by tool T3. Thus, under otherwise identical conditions, UAFSLW joints can withstand a greater fracture shear strength than FSLW joints, as ultrasonic vibration helps to mix the material at the interface, thus, enlarging the SZ and diminishing the cold lap defects.

Highlights

In the 1990s, The Welding Institute (TWI, United Kingdom) invented friction stir welding, which is characterised by less distortion, fewer defects, and lower production costs and is suitable for joining light alloys [1,2,3]
Because the pin length is similar to the thickness of the upper sheet, the material around the the advancing side (AS) appeared only in the thermo-mechanically affected zone (TMAZ), and cold lap defects on the retreating side (RS) appeared in the stirred zone (SZ)
Hook defects on the AS appeared only in the thermo-mechanically affected zone (TMAZ), and cold lap defects on the RS appeared in the SZ

Summary

Introduction

The different positions of the faying surface and the bending deformation at the lap interface of FSLW joints may cause interface defects, including hook defects and cold lap defects. The presence of these defects generally reduces the effective sheet thickness and effective lap width (ELW), thereby degrading the tensile shear properties of FSLW joints [6,7,8]. Chen et al [13] investigated how, as a function of rotational speed, pin length affects the mechanical properties of FSLW joints of 1.4-mm-thick 2A97 aluminium alloy and found that the joint failure load decreased with increasing pin length and rotational speed. Wang et al [14] found that, as the pin length increases, the hook defect on the advancing side (AS) evolves following an “M”

Methods

Results

Discussion

Conclusion