Microstructure And Mechanical Properties Of Onion Ring Of Aluminum Alloy7050 Friction Stir Welded Joints

The butt welding process of friction stir welding was used on 5 mm thick7050 aluminum alloy.Effect of welding parameters on the macrostructure of onion ring were analyzed by the changing of the rotation rate and the welding speed.Then,we observed the microstructure of onion ring.The influences of precipitate microstructure morphology and distribution on the microstructure of onion ring were investigated.The relationships between the microstructure of onion ring and fracture characteristics of the joint combined with the Vickers hardness and the tensile strength tests were researched.The experimental results showed that,if we used stirring pin with threads welding,a approximate elliptic area appear at welding nugget zone.The area which is similar to the cross section morphology of onion named onion ring. With the change of welding parameter,the macrostructure of onion ring also changed.When the welding speed is 80mm/min and rotation speed is 600r/min,the onion ring is obvious.It is symmetrical about welding nuclear center which like half arc shape.With the increase of rotational speed,per unit length of welding heat input increased, the number and fluidity of plastic metal increased,too.It made the metal flow irregular,so the onion ring gradually blurred.While the welding speed is40mm/min and the rotation speed is 800r/min,onion ring occupied the entire welding nugget zone.With the increase of welding speed,onion rings gradually become smaller.It is mainly due to the increase of welding speed made the heat input reduced and the liquidity of plastic metal dropped.Moreover,the increase of welding speed made the stagnation time of rotational tool per unit length in the welding nugget zone reduced.Thereby it reduced the number of plastic metal which is along the thread from top to down made the onion ring be smaller.The microstructure of onion ring is studied.The result showed that the onion ring,in fact,is made up of recrystallized grains whose sizes are uneven.The fine grains which gathered along the deformation direction formed cambered fine grains belt constitute the “ring”.While,between two adjacent “ring” is coarse grain belt which form with coarse grains.It is mainly due to the blocking effects of precipitate which dissolve out during the cooling process after welding on the growth of recrystallization grains.If the size of precipitate particle is bigger and the distance between the precipitated phase particles is farther,the growth of recrystallization grains is promoted.The reason is precipitated phase particles block grain boundarymovement’s force is smaller,if the diameter of particles is bigger and the number is smaller.With the movement of grain boundary,fine grain gradually being merged into the neighboring grain formed a coarse grain at last.While,part of the recrystallized grains which are hampered by precipitation phase that hinder the grain boundary migration be fine grains in the process of growing up.Therefore, the onion ring is made up of coarse grain belt and fine grain belt under the micro condition.In addition,with the increase of rotational speed,the grains of onion rings increased gradually.It is mainly because of the rise of rotation speed made the peak temperature of WNZ increased.With the increase of the peak temperature of WNZ,the activity of the grain boundary is increased significantly.With grain boundary migration increase made the equiaxed recrystallization grain grew up.Thus,with the increase of rotation speed, grain size gradually increases. Finally,for the onion rings take different aging treatment,the result showed that the onion ring grains gradually homogenize and nubbly precipitation phase particles which is on the grain boundary formed a long stick after T6 aging treatment. Moreover,with the increase of aging treatment temperature, precipitation phase particles partly dissolved made its density decline. The small precipitate particles distribute on the grain boundary.We found that the micro hardness of FSW joint like “W” on the whole.But, the micro hardness of welding nugget zone like ”zigzag”.It is mainly related to the onion ring.The above studies proved that the onion ring is made up of coarse grain belt and fine grain belt.The grain of coarse belt is thick, the density of second phase is small and the size of second phase is big.So, the micro hardness is low in this place.While the grain and second phase of fine belt is small,the density of second phase is large.Thus,the micro hardness is high here.After manufacturing a gap in the weld joint center we begin to do tensile test.We got that the cracks are along the gap and through the coarse grain boundary extension.It is demonstrate that the emergence of coarse grain belt accelerate the expansion of crack in the welding nugget zone.