The Effectiveness of Surface Coatings on Preventing Interfacial Reaction During Ultrasonic Welding of Aluminum to Magnesium

Abstract

High power ultrasonic spot welding (USW) is a solid-state joining process that is advantageous for welding difficult dissimilar material couples, like magnesium to aluminum. USW is also a useful technique for testing methods of controlling interfacial reaction in welding as the interface is not greatly displaced by the process. However, the high strain rate deformation in USW has been found to accelerate intermetallic compound (IMC) formation and a thick Al12Mg17 and Al3Mg2 reaction layer forms after relatively short welding times. In this work, we have investigated the potential of two approaches for reducing the IMC reaction rate in dissimilar Al-Mg ultrasonic welds, both involving coatings on the Mg sheet surface to (i) separate the join line from the weld interface, using a 100-μm-thick Al cold spray coating, and (ii) provide a diffusion barrier layer, using a thin manganese physical vapor deposition (PVD) coating. Both methods were found to reduce the level of reaction and increase the failure energy of the welds, but their effectiveness was limited due to issues with coating attachment and survivability during the welding cycle. The effect of the coatings on the joint’s interface microstructure, and the fracture behavior have been investigated in detail. Kinetic modeling has been used to show that the benefit of the cold spray coating can be attributed to the reaction rate reverting to that expected under static conditions. This reduces the IMC growth rate by over 50 pct because at the weld line, the high strain rate dynamic deformation in USW normally enhances diffusion through the IMC layer. In comparison, the thin PVD barrier coating was found to rapidly break up early in USW and become dispersed throughout the deformation layer reducing its effectiveness.

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

“…For example, previous research that employed a manganese barrier layer produced by physical vapor deposition (PVD) found that thin brittle coatings tended to break up during friction welding, compromising their effectiveness. [15] In this investigation, a potential solution has been examined that combines both the above approaches for controlling the interfacial reaction normally seen in Al to Mg dissimilar welding. Based on thermodynamic considerations (e.g., Reference 20), a ductile Al-Si coating material has been selected to (i) function as a partial diffusion barrier layer, and (ii) preferentially form a new IMC phase that can suppress the formation of the highly brittle Al-Mg intermetallic compounds normally seen at the joint interface.…”

Modeling of Intermetallic Compounds Growth Between Dissimilar Metals

“…To produce a bond with good mechanical properties, it is usually essential to maintain the IMC layer thickness below a critical value, since IMC phases are typically brittle and can lead to a marked loss in joint toughness if allowed to become too thick. [5][6][7][8] Therefore, many researchers have studied reactive diffusion in a large number of binary alloy diffusion couples and there have also been several attempts to model the evolution of the IMC layer. [1][2][3][4][5] It is widely accepted that reactive diffusion is mainly governed by volume diffusion, [1][2][3]9,10] and the thickness of an IMC phase generally follows a parabolic relationship with annealing time.…”

Research of Parameters Influence on Shaping and Strength of ABS-steel Lap Joints by RSW

“…The function of ultrasonic assistance used in the process of welding was preliminarily exploded. The first set results, displayed as figure 5, give the fact that ultrasonic assistance indeed have positive effects on joint welding [7,8]. The size of ultrasonic assistance joint reliable area is obviously larger than before with non-ultrasonic assistance in the same parameter 5s, 1.0kA, 0.1Mpa.…”