Results and Discussions The macro morphology of the welded joint (Fig. 3) indicates that a full penetration weld is obtained when the laser power is higher than 3.6 kW. Statistical analysis results of the welded joint indicates that as the laser power increases, the upper and root melting widths of the weld increase from 3.6 mm to 4.0 mm and 2.0 mm to 2.5 mm, respectively, while the reinforcement height decreases slightly (Fig. 4). The central area of the weld possesses the typical as-cast structure, containing several equiaxed grains and little porosity (Fig. 5). When the heat input increases, the cooling rate decreases, which leads to a decrease in subcooling and promotes the growth of columnar crystals. The average width of the columnar crystal band near the bond line is 134 μm, 152 μm, and 232 μm when the laser power is 3.6 kW, 4.2 kW, and 4.8 kW, respectively (Fig. 6). The microhardness distribution patterns of the three welded joints are similar, with the weld and heat affected zone (HAZ) hardnesses being significantly lower than those for the base material (~130 HV). The average hardnesses of the weld and HAZ show a decreasing trend with increasing laser power (Fig. 7). The lower weld hardness and porosity weaken the joint, which eventually leads to the fracture of the welded joint at the seam. The joint tensile strength is the highest (271 MPa) when the laser power is 3.6 kW, while the joint tensile strengths are 244 MPa and 220 MPa for laser powers of 4.2 kW and 4.8 kW, respectively. The highest weld tensile strength occurs at low laser powers, with the joint possessing the tensile strength of approximately 64% of that of the base material at the power of 3.6 kW (Fig. 8).
As a heat-treatable high-strength aluminum alloy with low density, good mechanical properties, easy forming, and corrosion resistance, Al-Mg-Si alloys have been widely used in high-speed railroads, automobiles, aerospace, and other fields. The addition of Cu to Al-Mg-Si alloys is more economical and applicable. It improves the mechanical properties and age-hardening efficiency at peak aging, resulting in ultimate tensile strength of 400 MPa at peak aging for high-strength Al-Mg-Si-Cu alloys, which is significantly higher than that of existing commercial Al-Mg-Si alloys. Softening the heat-affected zone during welding is a major problem that limits the use of heat-treatable aluminum alloys. Laser-CMT composite welding (CMT, cold metal transfer) is a promising high-performance, low-welding distortion technology that utilizes the advantages of laser and CMT welding to overcome the shortcomings of both single-laser and traditional arc welding, while increasing productivity and improving welding quality. Therefore, this study investigates the effects of laser power on the macroscopic morphology and tissue properties of the welded joints of high-strength Al-Mg-Si-Cu alloys and evaluates the possibility of applying laser-CMT composite welding technology to high-strength Al-Mg-Si-Cu alloys.
In this study, a high strength Al-Mg-Si-Cu alloy with a thickness of 2 mm and filler wire with a diameter of 1.2 mm were used. A 6-kW laser and arc welding machine were used for butt-welding laser-CMT hybrid welding experiments. The welding parameters were as follows: laser powers of 3.0, 3.6, 4.2, and 4.8 kW; wire-feeding speed of 4 m/min; welding speed of 4 m/min; laser beam diameter of 0.3 mm, and the distance of 3 mm between the laser beam and arc . The specimens were then etched using the Keller reagent (1 mL HF, 1.5 mL HCl, 2.5 mL HNO3, and 95 mL H2O). The microstructures of the fusion zone and bead dimensions for every welding condition were examined using an optical microscope (OM). The weld microstructure was analyzed using scanning electron microscopy (SEM) after polishing and without a chemical etchant. The Vickers microhardness was measured along the joint cross-section. Dogbone-shaped samples for tensile testing were cut from the weldment perpendicular to the weld fusion line. A tensile strength test of the hybrid joint was performed at room temperature on a tensile machine.
Laser-CMT composite welding is developed to combine high strength Al-Mg-Si-Cu alloys. The characteristics of the welded joint are altered by changing the laser power. Full penetration joints are obtained when the laser power is higher than 3.6 kW. The weld width increases and the reinforcement height decreases slightly with increasing laser power. The weld consists of columnar and equiaxed crystals, and the columnar zone of the weld widens with increasing laser power (from 134 μm at 3.6 kW to 232 μm at 4.8 kW). Increasing the laser power also increases the weld dilution and Mg burnout; this decreases the Si and Mg concentrations in the weld, reducing their solid solution strengthening effects. Consequently, the weld hardness and strength decrease with increasing laser power. Overall, the best mechanical properties of the welded joint are obtained at a laser power of 3.6 kW, with the average hardness and tensile strength of the weld reaching 65% (85 HV) and 64% (271 MPa), respectively, of the base material.
