| ZK series magnesium alloys possess low density, superior properties for absorbing vibration or shock energy, and insulating electromagnetic interference, etc, and they have been considered as one of the most excellent commercial magnesium alloy systems, owning to their higher strength, better stress corrosion resistance and heat-treatability. Their weight saving potential when used as structural materials in aircraft and automobile industries is beneficial to fuel economy and reducing CO2emissions. However, these alloys with high Zn content are more difficult to be welded by conventional arc welding methods, wherein hot cracking occurs as a consequence of a rather wide melting temperature range and excessive heat input, and thus they are not recommended for welded structures. As a result, after a deeper comprehension of the effect of Zn content on the microstructural evolution and mechanism of hot cracking during welding, it is of important values both in theory and engineering to develop a more reliable joining process for widening the application of ZK series magnesium alloys. In the present dissertation, the CO2laser beam welding (LBW) with low heat input was applied to join ZK series magnesium alloys, and the appearances, microstructures, mechanical properties and cracking mechanism of the autogenously welded joints were systematically investigated. Based on these results, LBW with filler strip process has been firstly applied in the joining of ZK60alloy which has the highest Zn content among the ZK series alloys, and the surface defects and solidification cracks in the welds are avoided successfully.The results of the study on technology optimization suggest that the welding parameters with too high welding speed or cooling rate in the joint are not recommended for welding ZK series alloys, and a good anti-solidification cracking ability of the weld could be obtained as the welding speed ranged from2to3m/min. The highest depth-width ratio and deepest penetration could be achieved when the focal point was adjusted on the surface of workpiece, i.e. the defocus distance Zf=0mm. For welding ZK series alloy plate of2mm in thickness, the heat input must be higher than19J/mm to avoid incomplete penetration in the joints. The optimum joints of ZK21and ZK40alloys could be obtained with the laser power of1200W and the welding speed of3m/min, and the ultimate tensile strength (UTS) of the joints were289MPa and315MPa, which are up to94%and91%of those of their base metals, respectively.The welding parameters and Zn content of the base metals significantly affect the microstructures of welds of ZK series alloys welded by autogenous LBW. As the Zn content increased, the morphology of grains in the fusion zone (FZ) adjacent to fusion boundary changes from cellular, to equiaxed and finally to equiaxed dendritic, and the grain size of weld metals decreases in the order of ZK60, ZK21and ZK40. So the finest grains with average size of4.8μm can be obtained in the weld of ZK40alloy, by which the mechanical properties of ZK40welded joint is higher than those of ZK60and ZK21. Besides, the substructure of columnar grains in the weld center of ZK21is characteristic of both equiaxed dendritic and cellular, which caused by the increase of solidification parameter (G/R) and low nucleation ratio during solidification, therefore equiaxed grains can be obtained by high welding speed. Whereas, the morphology of grains in ZK40weld changed from equiaxed to equiaxed dendritic with increasing welding speed.Due to low heat input, the ZK21and ZK40alloys possess good weldability during autogenous LBW, but the ZK60alloy is still susceptible to hot cracking including solidification cracking in the FZ and liquation cracking in the partially melted zone (PMZ). The former is mainly caused by the net-like distribution of more eutectic phase of Mg7Zn3with low melting point along grain boundaries (GBs), and the latter originates from the constitutional liquation of residual MgZn phase particles in the base metal.The continuous porosities-like liquation cracks and cavities were firstly observed in the PMZ of as-rolled ZK60welded joints, and the formation mechanism is discussed as follows:as the heat source approaching, the continuously distributed residual MgZn particles melt and constitutional liquation occurs in the PMZ, wherein big particles changed into larger global fusion pools and the liquation of small particles broadens liquation layers along GBs which act as a channel between the FZ and the small fusion pools in the PMZ. After the leaving of the heat source, although the beginning of solidification in PMZ is prior to FZ, there still residues some liquid with high Zn content in the PMZ when the molten metal of FZ is cooling, as a results, residual liquid will be sucked into the FZ under the large force produced by cooling shrinkage of the molten metal in FZ. At the terminal stage of solidification, the gaps-like liquation cracks and cavities are formed due to the absence of liquid which caused by the block of the channel for the liquid filling back from the FZ.The as-rolled ZK60magnesium alloy with high Zn content was successfully welded by the process of LBW with filler strip, in which surface cracks and pits often occurring in the autogenously welded joints are moved up into the filler strip, and a high quality welded joint freedom from defects can be obtained after removal of the residual filler strip. Optimal joint is obtained by LBW with filler strip of ZK40alloy due to the mitigation of hot cracking and refinement of grains in the weld metal, and it possesses the highest UTS of322MPa and good elongation of6.5%, which are up to90.7%and63.8%of those of the base metal, respectively. |
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