Cold Metal Transfer Joining of Aluminum AA6061-T6-to-Galvanized Boron Steel

Along with the development of automobile industry for lightweight vehicles, more and more advanced and ultrahigh strength steels (e.g., hot stamping steel) have been used for automotive applications. Making use of the high strength steels not only reduces the vehicle weight and air emissions but also improves crash safety. Meanwhile, aluminum alloys are one of the lightest structural materials, and they have been widely used in automotive industry due to their many attractive properties such as low density, high specific strength along with good damping capacity. Since both hot stamping steel and aluminum alloys are being widely used for automotive applications, joining of hot stamping steel to aluminum alloys is inevitable. In this study, the feasibility of joining aluminum alloy AA6061-T6 to galvanized boron steel by cold metal transfer (CMT) method using AA4043 filler metal was investigated. The microstructures and chemical compositions of the welded lap joints were examined using scanning electron microscope (SEM) and energy dispersive X-ray spectrometer (EDS), while the static strengths of the joints were measured. Test results showed that a sound weld-brazed joint which consisted of rich zinc zone, reaction interface zone, weld metal zone and fusion zone was formed. The phases and thickness of the reaction layers were analyzed and identified. In addition, the strength of CMT weld-brazed aluminum AA6061-T6 to galvanized boron steel depends on the torch deviation (i.e., distance between the welding torch and the edge of the weld seam). The joints fabricated with a deviation distance of 2 mm had greater strength than that of the joints made a deviation distance of 0 mm. Finally, the effect of temperature exposure of hot stamping on the weldability of CMT joining of joining aluminum AA6061-T6 to galvanized boron steel was investigated. It was found that the surface of galvanized boron steel was severely oxidized after heat treatment process and consequently reduced the weldability in CMT joining AA6061-T6 and galvanized boron steel.