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

Abstract

Diffusion couples between Al-AA6111 and Mg-AZ31 have been used to perform a comprehensive review of the IMC phases that develop in this system, including; their growth kinetics, texture, residual stresses, and mechanical properties. The new observations made have been compared to data in the literature with the aim of providing an in-depth understanding of the detrimental effects of interfacial reaction in aluminum–magnesium dissimilar welding. Three Al–Mg binary IMC phases were identified, γ-Al12Mg17 and β-Al3Mg2, as well as ε-Al30Mg23, which has not been previously reported in inter-diffusion studies. The β phase had the highest growth rate and became the dominant constituent of the IMC layer at longer reaction times. The ε-phase only appeared after extended heat treatment, via transformation at the Al12Mg17/Al3Mg2 interface. Intrinsic diffusion coefficients of Al and Mg, calculated in the Al3Mg2 phase, confirmed that Al is the faster diffusing species (by an order of magnitude) during IMC growth. Indentation techniques showed that all the IMC phases had extremely low toughness (<0.1MPam1/2). FIB-assisted ring core milling revealed that both the β-Al3Mg2 and γ-Al12Mg17 phases contained large residual stresses. Their in-situ fracture toughness was consequently found to be asymmetric with lower values parallel to the joint interface. Greater tensile residual stress was measured in the β-Al3Mg2 phase, which, combined with a lower toughness and more rapid growth kinetics, explains why it is generally the ‘weak link’ in Al–Mg dissimilar joints.