Gas metal arc brazing behaviour of a galvanised advanced high strength steel in short circuiting and short circuiting with pulsing modes

Brazing of galvanised advanced high strength steels (AHSS) by conventional gas metal arc welding (GMAW)-based processes results in the deterioration of mechanical and corrosion resistance properties in the brazed region, leading to premature failure of the components. Brazing of galvanised AHSS sheets using low melting point fillers with minimal heat input is considered as a best alternative to avoid damage to the base metal microstructure and zinc evaporation. Cold metal transfer is a low heat input variant of GMAW in which droplet transfer generally occurs by the combination of pulsed free flight and controlled dip short circuiting transfers. The productivity of the cold metal transfer brazing process can significantly get affected if the droplet transfer behaviour is not optimised against the mechanical properties of the joints. The objective of this study is to achieve maximum droplet transfer rate with minimum heat input during brazing of a AHSS using copper-based fillers (Cu-3Si-1Mn and Cu-8Al, wt. %). In this study, increased droplet transfer rate is observed when the metal transfer is only by short circuiting (CMT) than short circuiting with pulsed free flight (CMT-Pulse) for the same mean current for both the fillers. Analysis of bead geometry indicated that for a given mean current, increased bead width is observed for Cu-Si filler than for Cu-Al filler due to the narrow solidification temperature range and high liquid metal viscosity of the latter. CMT-Pulse brazed joints exhibit good mechanical properties due to good wetting characteristics than those deposited by CMT process.