Study on Microstructure and Mechanical Properties of 5A06/AZ31 Dissimilar Alloy RFSSW Joint
Refilling friction stir spot welding(RFSSW)experiments of 5A06 aluminum alloy top plate/AZ31 magnesium alloy bottom plate were carried out by using different rotational speed and plunge depth.The macroscopic morphologies of the solder joints were observed,the cross-sectional hardness distribution and tensile shear load capacity of the RFSSW joints were tested by microhardness test and tensile shear test.The grains and IMC layer of the interface beneath the stirring sleeve were characterized and the composition distribution was quantitatively analyzed.The results show that surface forming of the solder joints is good,while the rotational speed and plunge depth are too big,there are lots of overlap on the surface of the solder joints.When the rotational speed is 1900 r/min,the plunge depth is 1.6 mm,the tensile shear strength is the maximum,which is only 57.65 MPa.Under the tensile shear load,it breaks along the horizontal direction of the interface,and the fracture exhibits brittle fracture characteristics.With the increase of welding heat input,the IMC reaction layer at the interface becomes thicker,and the reaction product consists of β-Al3Mg2 and γ-Al12Mg17.Due to thermal stress between the brittle phase and alloy matrix,microcracks appear at the interface.The plastic deformation is produced at the metals on both sides of the joint interface below the stirring sleeve produce and gradually accumulates,which promotes dynamic recrystallization cores and the recrystallization grains form.
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