A Metallurgical Approach for Controlling Interfacial Reaction in Aluminium to Magnesium Dissimilar Metal Welding

UoM administered thesis: Phd

  • Authors:
  • Yin Wang


Dissimilar welding of Al to Mg alloys could potentially find significant application in the automobile industry, if the massive production of brittle intermetallic compounds (IMCs) at the joint interface can be prevented. In order to better understand Al-Mg IMC reactions, a comprehensive investigation of the interfacial region in AA6111 - AZ31 diffusion couples was carried out in this research. Three Al-Mg binary IMCs, namely the -Al12Mg17, -AlMg and -Al3Mg2 phases, were observed to form in the Al - Mg diffusion couple. In both the Al3Mg2 and Al12Mg17 layers, residual stresses were detected. The stress components normal to the joint interface were found to be positive, which had the effect of promoting the extension of lateral cracks; while the horizontal components were compressive, which could hinder cracking in the vertical direction. As a result, the fracture resistance of the two IMCs were asymmetric with lower values along the interface than in the vertical direction. The higher stress level in the Al3Mg2 layer made it more susceptible to lateral cracking and hence becoming the weak link in the Al - Mg dissimilar joints. A potential metallurgical solution has been explored involving the introduction of Zn into the material system, so that a new intermetallic compound with better properties can be formed to replace the unfavored Al3Mg2 phase. In this research, an Al-Zn coating alloy was proposed for this purpose. To determine the optimum composition for the alloy, a numerical method that combined CALPHAD thermodynamic calculation and diffusion simulations was developed. The modelling results indicated that Al-20 at. % Zn was the optimum composition for completely suppressing the formation of Al3Mg2, and this has been verified by static diffusion and friction stir spot welding (FSSW) experiments. In both cases, the designed coating alloy was effective in changing the Al-Mg reaction path by forming the mechanically superior (Al,Zn)49Mg32 phase as a substitute for Al3Mg2. The FSS welds prepared with the Zn containing coating alloy exhibited a 6 % increase in lap shear strength, compared to the conventional Al - Mg welds. This lower than expected improvement resulted from the Zn addition reducing the liquation temperature of the material system, resulting in the production of a detrimental eutectic mixture which facilitated debonding of the welds.As a potential alternative solution, Al-Si coating material has been proposed to inhibit the growth of Al-Mg IMC layers, in which the Si phase was expected to form a partial interdiffusion barrier between the substrate materials and change the reaction path by preferentially reacting with Mg. Comparison of long-term static diffusion experiments between the Al-Si coated and Al - Mg dissimilar joints showed that the nucleation and growth of Mg2Si could change the reaction path and greatly reduce the thickness of the Al-Mg IMC layer at the joint interface. Although in actual friction stir spot welding (FSSW), Mg2Si was not formed in a detectable amounts, due to the very short reaction time, the Al-Si coating still led to a significant reduction in the IMC thickness by partially blocking the Al-Mg interdiffusion process. With the coating applied, the Al - Mg dissimilar welds exhibited enhanced mechanical performance with both their strength and fracture energy being markedly increased, through a reduction in the IMC layer thickness and the presence of Si particle toughening the reaction layer by causing crack deflection.


Original languageEnglish
Awarding Institution
Award date31 Dec 2016