Study of top sheet thinning during friction stir lap welding of AZ31 magnesium alloy

Abstract

Friction stir lap welding was applied to AZ31 magnesium alloy sheets to investigate the effect of tool designs and welding variables on top sheet thinning. Three tools with different shoulder designs were used. Sheet thinning aroused by the hooking or cold lap feature was quantified, and the lap shear fracture load of the joints was evaluated and correlated with the effective top sheet thickness. The tool geometry has a significant effect on the morphology and extent of both hooking and cold lap features, as well as welding variables such as welding pitch and plunge depth. The morphology and extent of these features influenced the effective top sheet thickness, which exhibited a linear relationship with the unit width fracture load via the welded material strength. High fracture loads can be achieved by suppressing the hooking and cold lap features as well as by strengthening the material in the nugget.

Effect of Welding Speed on Defect Features and Mechanical Performance of Friction Stir Lap Welded 7B04 Aluminum Alloy

“…The hook defect only reduces the effective thickness of the top sheet; however, the cold lap defect can not only lead to the effective thickness reduction in the top sheet, but it can also result in the thinning of the bottom sheet when the welding speed is above 150 mm/min (see Figure 2c,d). This is different from that commonly observed in previous investigations, where only the top sheet thinning occurs due to the cold lap defect [6,[13][14][15]. In order to quantify the levels of sheet thinning caused by the both defects, Figure 5 plots the height of the hook and cold lap defects, which was measured in the optical microscope.…”

“…The author argued that this was because the hook defects on AS did not negatively affect the transmitting load capability of the welded joints, but a deeper explanation was lacking. On the other hand, the phenomenon of higher fracture strength in RS loading compared with AS loading has been reported by other researchers [12,13]. The smaller extent of sheet thinning and lower stress concentration on the RS were considered to be the main reasons for the higher fracture strength of RS loading.…”

“…In the research of Cederqvist [4] and Buffa [11] et al, the AS-loaded lap joints showed higher fracture strength than the RS-loaded lap joints. In contrast, Yang [12] and Yuan [13] claimed that the joint properties obtained in RS loading were superior to those obtained in AS loading. In the present study, both of the cases are observed, and the results indicate that the affecting characteristic of loading configuration on joint performance is actually variable with the welding speed.…”

“…The sheet thinning level caused by the hook and cold lap defects has been demonstrated to play a prominent role in determining the performance of lap joints [6][7][8][9][10][11][12][13]. Yuan et al [13] reported that the crystallographic texture was another factor that influenced the fracture load and fracture path of FSLW joints of AZ31 magnesium alloy. Meanwhile, as far as the heat-treatable aluminum alloy is concerned, the thermal effect during FSW can cause local softening to occur in the joint, leading to the reduction in mechanical properties of the weld relative to the BM [17,18].…”

“…Because of the different characteristic profiles of the hook and cold lap defects, the FSLW joints can be produced in two different loading configurations, which are AS loading and RS loading, respectively [10][11][12][13]. In AS loading, the AS of a lap joint is loaded on the upper plate, while in RS loading, the RS of a lap joint is loaded on the upper plate (see Figure 1).…”