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

Alexandria Panteli,Ying-Chun Chen,Philip B Prangnell,Joseph D Robson

doi:10.1007/s11661-013-1928-z

Alexandria Panteli, Ying-Chun Chen + Show 2 more

Open Access

https://doi.org/10.1007/s11661-013-1928-z

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Abstract

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.

Highlights

IN the near future, automotive design will be based on a multi-material approach, allowing more efficient use to be made of the best attributes of different classes of materials.[1,2,3] This has resulted in heightened interest in welding dissimilar combinations of light metals, such as aluminum and magnesium
In the work reported here, we have investigated the potential of two approaches for reducing the intermetallic compounds (IMC) reaction rate in the Al-Mg welds, both involving coatings on the magnesium sheet surface, with the aim of (i) separating the join line from the weld interface and (ii) providing a diffusion barrier layer
The cold spray coating had an average thickness of ~100 lm and generated a relatively rough surface compared to that of the original magnesium sheet and contained some porosity, but no evidence could be found of an IMC layer between the coating and the magnesium substrate prior to welding (Figure 1(a))

Summary

Introduction

IN the near future, automotive design will be based on a multi-material approach, allowing more efficient use to be made of the best attributes of different classes of materials.[1,2,3] This has resulted in heightened interest in welding dissimilar combinations of light metals, such as aluminum and magnesium. Fusion welding processes are very difficult to apply to dissimilar aluminum and magnesium alloy joints because of the rapid formation of intermetallic compounds (IMC) which occurs in the welds.[4] As a result, solid-state methods, like friction stir welding (FSW), are attracting increasing interest[5,6,7,8,9] as are other solutions such as selfpiercing rivets (SPR), clinching, and adhesive bonding.[10,11] SPR or clinching and bonding have additional consumable and surface treatment costs, and require sufficient formability of the material.

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