Gas Metal Arc Welding of Coated Advanced High Strength Steel (AHSS) - Developments for Improved Weld Quality 2007-01-1360

Gas Metal Arc Welding (GMAW) is commonly used in the automotive industry for joining heavier gauge mild and High Strength Low Alloy (HSLA) uncoated steels, where it is recognized for its versatility and speed. The only constraints typically encountered relate to fatigue performance of the joint as a result of poor design or manufacturing fit-up. Advanced High Strength Steels (AHSS), now being considered for more and more applications, however, do not offer the same ease of welding and process control is significantly more critical. They differ from mild steels in chemical composition and thermal processing, resulting in a different microstructure; designed with a richer metallurgy to have higher strength at equivalent thickness. As a result, the sensitivity to heat input is greater and the process window in which acceptable welds can be achieved is narrower. Coated steels, whether hot dipped galvanized, galvannealed or electro-galvanized, add a level of complexity to the process of achieving quality welds, due to the tendency for the coating materials to volatilize and result in porosity in the weld. Coated steels are the industry norm for body structural applications prone to corrosion in service. This study, the latest in a series involving Dual Phase steels welded to themselves and to different gauges and grades of mild steel, provided a comprehensive understanding of the influence of process variables on weld dimensions and levels of spatter and porosity and, in turn, on the mechanical properties. Optimization of the welding process to control the level of porosity, as a result of quantifying its significant influence on properties such as fatigue strength and life, has been achieved.