Dissimilar MIG-CMT weld-brazing of aluminium to steel: A review

Abstract

Multi-material design is a viable approach to reduce fuel consumption as well as environmentally unfriendly gas emissions. Aluminum is being increasingly used to replace steels in automobile structures due to its high specific strength and good corrosion resistance. Joining aluminum to steel is challenging due to their incompatible thermo-physical properties which leads to several problems viz. formation of hard and brittle layer at the interface composed of Fe-Al intermetallic phases, pores in fusion zone, formation of zinc rich zone, and hot cracking etc. Suppressing the formation and growth of intermetallic compounds is a critical issue because excessive formation of intermetallic compounds deteriorates mechanical properties of the joints. Generally, Al rich intermetallic compounds like needle shaped FeAl3 (towards fusion zone) and tooth shaped Fe2Al5 (towards steel side) form during steel-Al joining. Formation of Fe2Al5 is more problematic due to its uncontrolled growth governed by the structural defect i.e. ∼30% vacancies along the long axis which facilitates the diffusion of Al atoms. MIG-CMT brazing technique utilizes low heat input which is beneficial in retarding the formation and growth of intermetallic compounds at the interface. Desired control over wire feeding is the reason behind the extensive use of CMT brazing to join thin sections in automobile industries. In CMT, during the short-circuiting phase, the retraction of filler wire enables spatter-free metal transfer into the molten pool without addition of EMF. This assessment highlights the key challenges during joining Al to steel and discusses methods to improve the joint properties primarily by suppressing the formation of intermetallic compounds. Effect of process parameters, the chemical composition of filler wire etc. and their influence on the metallurgical and mechanical properties of the brazed joint is also reviewed. In addition, miraculous role of Silicon and Zinc is reviewed systematically.