| Weld bonding is an advanced hybrid joining technology which combines welding with adhesive bonding. Weld bonding can offer many benefits of both welding and adhesive bonding:increased in-plane tensile shear and/or compressive buckling load-carrying capability, enhanced out-of-plane load-carrying capability compared to adhesive bonding only, improved load and stress distribution (uniformity) compared to welding alone, and so on. Hence, this technology has been used in many industries such as spaceflight and automobiles. Many weld bonding technologies has been investigated by researchers, including laser weld bonding, plasma weld bonding, resistance spot weld bonding and YAG laser-tungsten inert gas hybrid weld bonding. The research on these technologies most focues on the mechanical property tests and numerical simulation, but only a few work of joint structure desion and the influence of the adhesive on the welding have been reported.Thus, laser weld bonding and MIG spot weld bonding processes are carried out in this paper, and they are used to join Mg alloy to Mg alloy and Mg alloy to Al alloy. In this paper, through anylizing the stress distributing, the joint structure is optimized, so that the mechanical properties improved. The influence of the adhesive on the molten pool structures and laser induced plasma have been anylized to ensure the interaction of the adhesive and the welding process.Laser weld bonding is applied in joining Mg alloy to Al alloy, and the influence of the adhesive on the microstructures and the mechanical properties are discussed. MIG spot weld bonding is novelly carried out to joining Mg to Al, and the influence of the intermetallic phases on the mechanical properties are researched. The main research contents are as follows:1. In overlap laser welding, the joint fractures at the interface between the sheets, and maximum shear strength can reach 85% of that of the base metal. Off-center moment during tensile shear test can lead to the strength loss, and the weld edge can also influence the strength as a cracking source. Adhesive bonded joint can offer high tensile shear failure force but low peel strength. Laser weld bonded joint offers higher tensile shear failure force than either laser welded joint or adhesive bonded joint, and the improved failure load is due to combined contribution of the weld seam and the adhesive.2. The weld seam can block the adhesive crack propagation, and the adhesive improves the stress distribution, so they can offer synergistic effect. LWB-PA joint offers better performance than LWB-PE joint. This is because the weld seam can increase the peel resistance at the free end of the adhesive bonded part, where a stress concentration exists. Maximum T-peel strengths of LWB joints depend on the laser seam weld. The LWB-PA joint has both the advantages of maximum failure force caused by laser weld and the peel displacement caused by adhesive, so a synergistic effect is obtained in energy absorption.3. Compared with the partial penetration of 2.1mm in laser welding of magnesium direct, a total penetration of 3.0mm can be obtained in laser weld bonding. The adhesive-induced gas caused by the former pulses can make the upper sheet thinner where the next laser pulses interact with the material, so the molten pool structure is changed. This new molten pool structure can result in less energy required for laser keyhole through the upper sheet, so that the surface power density of the lower sheet can be high enough to form keyholes, leading to deeper penetration. The simulation comparison experiments can indirectly verify that this molten pool structure variance can help penetration increase. In LWB, the adhesive between the sheets can generates a mass of gas in front of the keyhole, and when the gas expands and escapes rapidly from the fusion zone, high recoil pressure can be exerted to influence welding process. Compared with the recoil pressure only caused by the evaporation of the target material in laser welding direct, the additional recoil pressure caused by the adhesive-induced gas can enhance the recoil pressure to the lower sheet molten pool to help the depth of melting increase. the adhesive-induced gas in front of the keyhole can influence the plasma behaviors, and the change the plasma traveling direction, and as a result the plasma absorption effect to laser reduces.4. Adhesive can influence the elements distribution. When laser does not interact with adhesive in LWB through gap, intermetallic phase layers form at the weld edge. But the intermetallic phase layers only form in the Al side fusion zone after adhesive is added where laser radiation is incident in LWB through adhesive. Adhesive bonding can change the fracture mode. In laser welding the joint fractures at the Mg-Al interface in the Al side, but in LWB, adhesive bonding changes the mechanics characteristic so that the joint fractures in Mg side, and the fracture mode also changes from brittle fracture to mixture fracture.Adhesive can influence microstructures at the fracture source. LWB through adhesive can show higher strength than LWB through gap. In LWB through adhesive the adhesive-induced gas can prevent mixing of Mg and Al at the weld edge, so that the microstructures at the fracture source are Mg-rich structures, which can give much better performance than intermetallic phase layers in LWB through gap.5. Intermetallic phases are formed in two forms by MIG spot welding:network Al3Mg2 surrounding the a-Al dendritic in the fusion zone and interlayer of Al12g2 and Al12Mg17 at the fusion line. In tensile shear test, the MIG spot welded joint fractures in the region of Al-rich dendritics in the FZ, and the shear strength is determined by the content of Al3Mg2 and maximum shear strength can reach 130 MPa,.Increased heat input can result in large amount of Al3Mg2 in the FZ and deteriorate the shear strength. Adhesive can help increase the failure force of the joint. IMPs can significantly influence the peel strength of the joint. When small amount of network IMPs in the FZ are formed, the joint fractures in the interlay er of IMPs, leading to a relatively high failure force. Severe formation of network IMPs in the FZ can result in the interfacial fracture with a low failure force. The MIG spot welded joint offers low torsion strength, because high shear stress is loaded on the interlayer of IMPs. Adhesive can be added to increase in-plane load-carrying capability in MIG spot weld bonding. |
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