Suppression of cracks by means of added ultrasonic vibration and/or controlled pulse-train in laser spot welding of aluminum alloys

Abstract

When aluminum alloys are joined by laser spot welding, many cracks are formed easily in the melt part because of their high thermal diffusivity. The cracks were formed along boundaries of equiaxed crystal grains and segregation was estimated at the boundaries. The crack was evaluated by a crack ratio which was defined as a rate of crack area against melt one.In order to reduce the cracks at the weld parts, ultrasonic vibration was added to melt part during laser irradiation, and newly developed Nd:YAG laser apparatus with pulse-train of laser beams was used and many kinds of pulse-trains were tried. Used aluminum alloys were A5052(Mg: 2.2-2.8wt%) and A5182(Mg: 4.5-5.0wt%). First of all, lapped spot welding of aluminum alloys and stainless steel(AISI304, 40um in thickness) was carried out, and stirring rate of the melt pool was evaluated by EPMA. As a result, both metals were mixed more in the case of ULW(Ultrasonic vibration Laser Welding) than in the case of BLW(Basic Laser Welding). Then, microstructure of fusion zones was observed with an optical microscope. There were many equiaxed crystal grains in BLW. But, such a grain was not observed in ULW. Change of Vickers’ hardness distribution in a fusion zone was smaller in ULW than in BLW, and also the crack ratio was smaller in ULW than in BLW. New laser apparatus was applied to form a sound weld part in both cases of independent use of controlled pulse-train:PTLW(Pulse-Train Laser Welding) and combined use of pulse-train with ultrasonic vibration:CLW(Combined Laser Welding). Crack ratios were 2.8% in BLW, 2.5% in ULW, 2.0% in PTLW and 1.6% in CLW.When aluminum alloys are joined by laser spot welding, many cracks are formed easily in the melt part because of their high thermal diffusivity. The cracks were formed along boundaries of equiaxed crystal grains and segregation was estimated at the boundaries. The crack was evaluated by a crack ratio which was defined as a rate of crack area against melt one.In order to reduce the cracks at the weld parts, ultrasonic vibration was added to melt part during laser irradiation, and newly developed Nd:YAG laser apparatus with pulse-train of laser beams was used and many kinds of pulse-trains were tried. Used aluminum alloys were A5052(Mg: 2.2-2.8wt%) and A5182(Mg: 4.5-5.0wt%). First of all, lapped spot welding of aluminum alloys and stainless steel(AISI304, 40um in thickness) was carried out, and stirring rate of the melt pool was evaluated by EPMA. As a result, both metals were mixed more in the case of ULW(Ultrasonic vibration Laser Welding) than in the case of BLW(Basic Laser Welding). Then, microstructure...