| Compared with other structural materials, magnesium alloys have a large of advantages, and they have extremely important value in the automotive, electronics, transportation, aviation, aerospace and defense, military industry and many other field. Magnesium alloys have been paid more attention to because of its rich resources with the traditional metal mineral resources depleted. However, the connection technology is still technology bottleneck which restricts the further application of magnesium alloy due to its magnesium alloy poor welding metallurgy performance. Therefore, study on the connection technology of magnesium alloy has important practical significance using laser which has many advantages.In this paper, laser welding properties of AZ31B magnesium alloy thin sheet are researched using 500W pulsed Nd:YAG solid state laser, hoping to discover the general principles which can guide pulsed YAG laser welding of magnesium alloy; the phase, microstructure , mechanical properties, fracture morphology and the electrochemical corrosion of the YAG laser butt joints are studied; the mechanism and the major defects of the laser welding process are analysed , and relevant prevention measures and solutions are put forward.Firstly, laser self-fusion welding processing properties of 2mm thick AZ31B magnesium alloy are studied, finding different pulse width and pulse frequency have a large influence on weld width and deep penetration under the same average power , the peak power and pulse width are the main parameters which affected weld width and deep penetration. To improve the coupling efficiency between pulsed YAG laser and magnesium alloys, pulsed YAG laser welding of magnesium alloys should be followed by the principle of a larger width(>3ms), low frequency(<60Hz), proper peak power.Based on the general principles of pulsed YAG laser welding of magnesium alloys AZ31B, 1mm AZ31B magnesium alloy is welded in the butt-welded form, the optimum process parameters: pulse width(tp)=4.5ms, pulse frequency(f)=30Hz, average power(P)=253W, peak power(pp)=1.87kW, welding speed(v)=5mm/s, duty cycle (CT)=16%, protective gas is Ar, side protection gas flow 15mL/min, defocusing (△)=+3mm.The microstructures of YAG laser butt-welded joints mainly consist of fineα-Mg grain andβ-Mg17Al12 phases which dispersed in theα-Mg solid solution. grains in weld area is significantly refined by large undercooling rate during laser welding, grain size is refined from the original 35μm to 10μm. In addition to few columnar crystals near the fusion line, other regions consist of fine equiaxed grains. The grain size has significant change from fusion line to weld center, changing from columnar crystals area to the fine equiaxed grains area to more fine equiaxed grains area, and then to fine equiaxed grains area in weld center, and the transition interface is clearly visible. the grain size along weld depth direction has little change. Tensile strength of butt-welded joint is 180.63 MPa, which is 76.78% of that of the base material. The form of base metal fracture is mainly cleavage fracture, fracture of welded joints form is the mixed fracture of quasi-cleavage fracture and ductile fracture, brittle fracture tendency of the base metal of welded joints than big. compared with the base metal, micro-hardness of butt-welded joint increased by 12.7%~30.9% in the interaction of grain refinement and precipitation strengthening. the maximum value of macrohardness is 72HV0.05, but the location in which microhardness is maximum is not weld center. the average corrosion potential of YAG laser butt-welded joint is -1384mV, compared with substrate materail, increasing by 87mV. The average corrosion current of base metal is 1184mA, and corrosion current of the welded joint is 1128mA, compared with substrate materail, reducing 56mA. improving corrosion resistance.Finally, massive experimental results show that major defects in YAG laser butt welding and self-fusible welding of AZ31B magnesium alloy and butt welding are welding cracks and burning of alloying elements. Welding cracks are mainly solidification cracking, and magnesium alloy have greater tendency to solidification cracking. few crack appears the phenomenon of many-times bifurcation, there is no cracks is observed in the sample for 48 hours under certain restraint stress After surface examination and anatomical, finding none form of delayed crack. |
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