| Friction stir spot welding(FSSW) is a new kind of connection technology, which has been successfully used in aerospace, rail transportation and other industry fields. Compared with conventional melting spot welding and riveting, this technology has many advantages, such as high joint strength, good quality, energy saving and small investments, and it is also a "green spot connection technology". Scholars have studied the effect of overlapping manners and processing parameters on the microstructure, hardness and other mechanical properties of the FSSW joints. At present, the research and application of FSSW mainly focus on the field of aluminum alloy, while FSSW of magnesium alloy is not fully studied. Due to the complexity of joints’ formation mechanism, it is better for us to study the processing of heat input and material flow during FSSW, in order to better understanding the nature of FSSW for processing design. In this paper, influence of the FSSW processing parameters on the AZ31 magnesium alloy joints’ material flow, the temperature history, the joints’ organization and performance was studied, as well as the new technology of underwater FSSW for AZ31 magnesium alloy.The effect of processing parameters on the connection quality of the FSSW joints of AZ31 magnesium alloy sheet was studied, and the processing parameters were optimized using a Taguchi orthogonal array. The results showed that the influence sequence of friction stir spot-welded parameters on joints’ ultimate tensile load from large to small was tool rotational speed, dwell time and plunge depth, which contributed 72.15%, 19.68% and 2.36% to the joints’ ultimate tensile load, respectively. The optimized parameters were as follows: tool rotational speed of 1000 rpm,dwell time of 5s, and plunge depth of 4.6mm.According to the microstructure examination and microhardness test, joints of specimens with ultimate tensile load had the largest size of metallurgical bonded section, the largest tensile deformation value, however, the lowest hardness of the stir zone. SEM fracture morphologies indicated that the joints failed through a mixture mode of ductile and brittle fracture. When choosing improper FSSW processing parameters or assembling unreasonably, defects of the joints would appear, such as flying edge, burr, discontinuous forming of the joint, oxidation wearing of plastic ring or so on.The effect of processing parameters on the temperature curve tested by thermocouple and the material flow processing identificated by the copper foil of the friction stir spot-welded joints of AZ31 magnesium alloy was studied. The results showed that the main factor of temperature curve during FSSW was the rotational speed, and the grain size of stir zone of the joints was greatly affected by temperature. With the other parameters remaining the same, the peak temperature of joints increased with the speed increasing, and the material flowing enhanced, but the stir zone grains roughened, and the hardness reduced. The peak temperature of joints fluctuated upward with the plunge depth increased. However, changing the plunge depth did not significantly changed the temperature of the joints, and the material flow was stabilized after the first increasing. When the plunge depth was small(eg 4.3mm), obvious growing of the dynamic recrystallized grains in stir zone could not be observed, which led to high hardness. When the plunge depth was large(≥4.5mm), obvious grain coarsening in stir zone could be observed, which reduced the hardness value of the stir zone. In this study, when the dwell time was 5s, the highest peak temperature of the joints could be achieved, which led to the maximum grain size and minimum hardness of stir zone. In addition, the sequence of joints’ hardness value of friction stir spot-welded AZ31 magnesium alloy from large to small was stir zone, thermo-mechanically affected zone and heat affected zone.AZ31 magnesium alloy was friction stir spot welded in air and cooling water respectively, and the effect of the enhanced cooling on the microstructure and mechanical properties of the joints was analyzed. Due to the enhanced cooling effect of the flowing water, the average grain size in stir zone was far smaller than that of the joints prepared in air cooling condition. Under the condition of enhanced cooling, the microhardness in stir zone significantly increased, the ultimate tensile load(~ 3.99kN) increased by 15.7%, and the tensile deformation value(~ 3.65 mm) increased by 62.2%. Dimples in SEM fracture morphologies indicated the better plastic deformation capacity of joints prepared in cooling water, which failed through a mixture mode of ductile and brittle fracture. |
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