Stability analysis of the Cold Metal Transfer (CMT) brazing process for galvanized steel plates with ZnAl₄ filler metal

In order to achieve lower CO₂ emissions and lower fuel consumption, modern motor vehicle industries have been reducing automotive weight with an increasing degree in the past years by replacing steel structures with brazed and welded components of steel and lightweight metals. In this context, the combination of galvanized steel and aluminum admits considerable application potential. Joints of the two cited different types of materials can be produced by partial brazing, through the Cold Metal Transfer (CMT) process with addition of zinc-based wire alloys. However, stability analysis for this process is still not well understood. Most papers focus on metallurgical characteristics such as intermetallic phase formation and joint strength. In order to assess stability, bead on plate samples made of ZnAl₄ deposits on an automotive DX56D + Z140 galvanized steel was statistically correlated to current and voltage oscillograms as well as high-speed images, after variation of the IBoost parameter from 30 to 150 A. Short-circuit and arc burning times were acquired and used to compute the Vilarinho Regularity Index for Short-Circuit Transfer (IVsc), which is based on variation coefficients. The lower the IVsc, the more stable is the metal transfer. The index was used to assess bead homogeneity. The results pointed that bead homogeneity was found only for IBoost values below 97 A. However, stability was reached for IBoost levels of 97 A (with homogeneous bead) and 150 A (without homogeneous bead). This means that to determine homogeneity, the metal transfer stability, determined through IVsc, is an indicator, but other factors such as current level and its effects over the melt pool must also be considered.