Fatigue and fracture of friction stir linear welded dissimilar aluminum-to-magnesium alloys

Abstract

A comprehensive study of fatigue life and damage mechanisms of friction stir linear welded dissimilar aluminum-to-magnesium alloys in lap-shear configuration is presented. The overlap linear welds were created by welding AA6022 aluminum alloy to AM60B magnesium alloy. The test results showed significant scatter in the fatigue life and the corresponding fracture modes. Two distinct modes of failure were observed for the lap-shear specimens. In one of the failure modes observed, fracture occurred when the dominant fatigue crack propagated into either the magnesium or aluminum sheet in a kink-crack formation. In addition, fretting debris was found at locations of crack initiation for this fracture mode. In the other observed mode of failure, fracture occurred by interfacial weld separation, where fractography analysis suggests that the fatigue cracks initiated at weld defects and then propagated through the brittle intermetallic phase formed during welding.2

The Effectiveness of Al-Si Coatings for Preventing Interfacial Reaction in Al-Mg Dissimilar Metal Welding

“…A similar crack propagation behavior has been reported by other researchers and leads to the poor joint performance generally reported for Al-Mg dissimilar welds. [3][4][5][6][7][8][9] In contrast, in the Al(Si) clad samples, cracks were found to be deflected by the Si particles embedded in the thinner IMC reaction layers (Figure 13(b)). Therefore, in addition to the reduced IMC layer thickness seen in the clad dissimilar welds, a more complex mix-mode failure also contributed to their higher fracture energy and improved joint efficiency.…”

“…However, the metallurgical joining of Al and Mg alloys is very challenging, even when employing solid-state welding techniques such as friction stir welding (FSW). [3][4][5][6][7][8] This is primarily because the rapid formation of Al-Mg intermetallic compounds (IMCs) at the weld interface leads to severe embrittlement. [3,[6][7][8][9] In dissimilar welding and diffusion bonding, an IMC layer comprised the FCC b-Al 3 Mg 2 and BCC c-Al 12 Mg 17 phases is normally seen to form at the joint interface.…”

“…[3][4][5][6][7][8] This is primarily because the rapid formation of Al-Mg intermetallic compounds (IMCs) at the weld interface leads to severe embrittlement. [3,[6][7][8][9] In dissimilar welding and diffusion bonding, an IMC layer comprised the FCC b-Al 3 Mg 2 and BCC c-Al 12 Mg 17 phases is normally seen to form at the joint interface. [4][5][6][7]9,10] The rapid growth of this reaction layer is intrinsically linked to the high diffusion rates of Al and Mg solute, both in each other as a parent solid solution phase and through the Al-Mg IMC phases themselves, when present as a continuous barrier layer at the weld interface.…”

The significance of intermetallic compounds formed during interdiffusion in aluminum and magnesium dissimilar welds

“…Aluminum (Al) and Magnesium (Mg) multi-material structures have potential to reduce weight in the automotive industry [1]. However, currently there is no satisfactory method for welding Al to Mg because the joints are adversely affected by the rapid formation of brittle intermetallic compounds (IMCs) [2][3][4][5]. Although most research to date has focused on solid-state welding (e.g.…”

Effect of overlap orientation on fatigue behavior in friction stir linear welds of magnesium alloy sheets

“…It is important to note that due to the observed hooking and cold-lap features, we assumed that fatigue crack incubation stage was negligible. This assumption is a widely accepted for welded joints and in particular is supported by similar fatigue work on friction stir spot welds of the AZ31 Mg alloy [4,12] and dissimilar Mg-Al alloy lapjoints [17]. For the finite element calculations, constant distributed load was applied as a traction force in order to obtain a fatigue curve.…”