Effect of welding speed on microstructure and properties of Q&P980 steel friction stir welding joints
The Q&P980 steel was welded by friction stir welding with the fixed rotation speed of 400 r·min-1 and welding speed of 50 min-1(low welding speed)and 600 mm·min-1(high welding speed),respectively.The microstructure and mechanical properties of the welded joints were characterized by optical microscopy,scanning electron microscopy,electron backscatter diffraction,microhardness and tensile property tests.The effects of welding speed on the microstructure and mechanical properties of the joints were studied.The re-sults show that compared with the base metal,the overall retained austenite content of the joints is reduced,the grains in the stirring zone are obviously refined and homogenized,and a large amount of martensite are formed.The grains in the heat-affected zone are slightly coarse,and the microhardness decreases.Compared with the low welding speed joint,the peak temperature of the high welding speed joint decreases,the material flow range is narrow,and the thermal coupling effect is weakened.The macro-size of the heat-affected zone decreases,the content of retained austenite increases,the microhardness is higher,the deformation coordination of the joint is better.The tensile strength and elongation of the joint increase from 1074 MPa,6.0％to 1268 MPa,15.3％,respectively.
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