| In order to improve the mechanical properties of light alloys,researchers at home and abroad have begun to make metal matrix composites by using light metals(such as aluminum,magnesium,titanium)as the matrix.It is beneficial to improve the mechanical properties of magnesium alloy,meet the current industrial demand,and realize the industrial application of magnesium alloy.Nano SiC particles have high hardness.FSW can realize the effective connection of magnesium alloy and improve the mechanical properties of welded joints.Therefore,in this paper,the 6 mm thick AZ31 magnesium alloy is used as the base metal matrix.By adding 30 nm or 300 nm nano SiC reinforcing particles into the powder filling tank of the base metal,the friction stir welding technology is used to prepare the welded joints of magnesium matrix composite materials,and different welding process parameters are used to weld,and the weld quality,microstructure,microhardness,tensile strength and yield strength of the welded joints are analyzed The influence law of degree.Finally,the effect of post weld heat treatment on the grain size and microhardness of the weld nugget was discussed.The main contents of this paper are as follows:(1)Effect of heat input on grain size and microhardness of welded jointThe decrease of spindle speed and the increase of welding movement speed(the decrease of welding heat input)are beneficial to the refinement of grain size and the improvement of microhardness of nugget zone.With other welding parameters unchanged,the grain size in the nugget area decreases with the decrease of the spindle speed;with the increase of the welding speed,the grain size in the nugget area decreases with the change of the welding speed..The average microhardness value of the base metal area is 63 hv.When the spindle speed is 1200 rpm and the welding speed is 70 mm / min,the average microhardness value of the nugget area is the highest,and the highest point is 98.3hv.(2)Effect of machining passes on grain size and microhardness of welded jointsThe increase of welding passes is beneficial to the uniform distribution of nano SiC particles in the matrix of magnesium alloy and to the improvement of microhardness of the nugget zone.During the sic-1pass welding,the nano SiC particles are not evenly dispersed,and the SiC particles are agglomerated and distributed in a strip shape.The microhardness value increases,but the fluctuation between the test points is large.When the sic-4 pass welding is carried out,the uniform distribution of SiC particles in the magnesium matrix and the microhardness value of the nugget area can be effectively strengthened by increasing the number of welding passes highest.(3)Effect of SiC particles on grain size and microhardness of welded jointsFor the welded joint that has experienced four passes of welding,increasing the amount of nano SiC particles or using nano SiC particles with smaller average diameter is conducive to the uniform distribution of nano SiC particles in the AZ31 magnesium matrix,the grain size of the welding nugget area is smaller,and the microhardness of the welding nugget area is also increased.(4)Effect of PWHT on grain size and microhardness of welded jointWhen the friction stir welded joint is heat-treated at 300 ?,there is a phenomenon that the microhardness of the nugget area without adding nano SiC particles decreases due to the grain growth,while the nugget area with adding nano SiC particles,because the nano SiC particles have a good "pinning effect",effectively restrain the grain growth,thus avoiding the decrease of microhardness value.(5)Tensile test of welded joint with the highest microhardnessThe tensile test was carried out on the welded joint(the highest microhardness value in the nugget area)with the spindle speed of1200 rpm and the welding speed of 70 mm / min.the tensile property and yield strength of the joint were 254 mpa and 208.6mpa respectively.The thermo mechanical influence zone of the tensile fracture on the forward side of the weld is brittle fracture. |
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