Research On Laser Weld Bonding Mg To Al Technology And Microstructures

Mg and Al are two kinds of light metals widely used in the industry. The joining technology has attracted much attention of abroad and home reaserchers. The well joining of Mg and Al alloys could improve the using area of these kinds of light metals. In order to solve the problem in melting welding Mg to Al process, laser weld bonding as a new kind of hybrid welding technology is used to join Mg to Al sucessfully. Laser weld bonding technology is a synergy of metallurgy joining and mechnical joining, and develops in which the tensile and peel performance of the weld-bonded structure exceeds that of either technology alone. In this thesis, a study would be done on the process laser weld bonding technoloy, microstructure in the fusion zone, microcracks in the joint and the crystal structure in the fusion zone.1. In this thesies, AZ31B Mg alloy and 6061 Al alloy are the subject investigated. Laser weld bonding (LWB) technology is used to join Mg to Al sucessfully firstly. By analyzing laser welding and LWB Mg to Al process, it is found that when the laser welding mode in Al alloy is in conductive mode, there is less Mg-Al intermetallic forming in the fusion zone. However, when the welding mode in Al alloy is in keyhole mode, there is more Mg-Al intermetallic forming in the fusion zone, which makes obviously harmful effect on the property of the joint. Therefore, conductive mode should be used in laser welding and LWB process. The LWB Mg to Al joint without cured has a better tensile shear and peel properties than that of laser welding joint. Thus the addition of the adhesive would make effect on the microstructure of Mg and Al joint.2. By studying the microstuctures of LWB Mg to Al joint in conductive mode, it is found that when the welding depth in Al alloy of LWB joint is less than 0.3mm, there is some resudial adhesive in the Al fusion zone because of the low laser power, which makes harmful effect on the property of the joint; when the welding depth in Al alloy of LWB joint is about 0.5mm, there is relatively less Mg-Al intermetallic, and the best shear load is obtained in this kind of joint; when the welding depth in Al alloy in LWB joint is larger than 0.8mm, there is a little more Mg-Al intermetallics forming in the fusion zone, which makes the property of the joint decreased.3. Through the comparative experiments of laser welding and LWB joint, it is found that the addition of adhesive could change the surface state of Al alloy, and increase the laser absorptive of Al alloy. In same welding parameters, the welding depth of LWB Mg to Al joint has a deeper penetration than that of laser welding joint. By analyzing the species and distributions of Mg-Al intermetallics in laser welding fusion zone, there is a continous Mg-Al intermetallics layer at the bottom of the fusion zone in laser welding Mg to Al joint, which should has a lowest the temperature and freeze the firstly in the fusion zone. As the temperature at the bottom of the Al fusion zone in laser welding joint was the lowest, the intermetallics would be freezing firstly, and then the melted Mg alloy would freeze. The shrink stress of Mg freezing and the laser welding recoil pressure would act on the interface between the Mg17Al12 and Mg2Al3. Therefore some microcracks could form at this interface, which make harmful effect on the property of the joint. In LWB Mg to Al process, the addition of adhesive makes effect on the distribution of Mg-Al intermetallics, which locates in the middle of the fusion zone. The thickness of Mg-Al intermetallics is decreased in extent, and the intermetallics in the fusion zone are composed by a mainly Mg17Al12 and few Mg2Al3, which makes less harmful effect on the joint.4. The microcracks in laser welding and LWB joint are analyzed in comparison. In LWB Mg to Al joint, the intermetallics distribution is deconcentration, the content of intermetallics is singleness relatively, and the thickness of intermetallics is decreased. Therefore the tendency of microcracks formation in LWB Mg to Al joint is less than that in laser welding joint. Through the observation of the temperature in Mg and Al fusion zone, the temperature contrast between Mg and Al fusion zone in LWB joint is less than that in laser welding joint. Therefore the thermal stress acting on the Mg and Al interface in LWB joint should be less than that in laser welding joint. Therefore the microcracks forming tendency in LWB joint should be less than that in laser welding joint.5. Transmission electron microscope (TEM) is used to analysis crystal strucuture in different parts of the LWB Mg to Al joint. By analysis the crystal of the intermetallics, the Mg17Al12 intermetallics has a similar structure with that of Mg alloy and could forming a steady mixing eutectic phase with Mg alloy. However the Mg2Al3 intermetallic has a similar structure with that of Al alloy, which would make bad effect on the LWB joint. A number of dislocations are found in Al grains, which is formed by the impact stress of the laser welding recoil pressure and the.shrink stress of the upper alloy freezing. Therefore, there is obviously pressure stress at the bottom of the fusion zone in laser welding Mg to Al joint, which makes the tendency of microcracks in intermetallics enlarged.