Effects Of The Welding Parameters On Mechanical Properties Of Friction Stir Welded 5A06-6061 Aluminum Alloys

5A06 aluminum alloy has high strength and good corrosion resistance.6061 aluminum alloy has good comprehensive performance,wide application and high yield strength.The high-quality dissimilar weldments of these two aluminum alloys can exert their respective advantages and meet the needs of different materials for various working conditions.However,the conventional welding method is difficult to achieve welding of dissimilar metals.Friction stir welding(FSW)as a solid phase connection technology effectively solves the above problem.However,great heat generated during the welding process would weaken the mechanical properties of the joint.How to optimize the welding process and further improve the mechanical properties of the welded joint has become an urgent problem to be solved.In this paper,5A06-H112 and 6061-T651 aluminum alloys with a thickness of 5 mm were friction stir butt welded in natural cooling and forced air cooling conditions.Systematic study of rotational speed(300,600,900 and 1200 rpm),welding speed(100 and 200 mm/min)and cooling mode(natural cooling and forced air cooling)on the performance of welded joints.The microstructure of the joint was characterized by optical microscope and scanning electron microscope.The mechanical properties of the joint were evaluated by means of tensile test and hardness test.The mechanism of joint strengthening or weakening was explored to obtain the optimal welding parameters.In natural cooling condition,the strength of the welded joint increases first and then decreases with the increase of the rotational speed/transverse speed ratio.When the rotational speed/transverse speed ratio is too low(300/200 r/mm),the joint is brittle fracture in the weld nugget zone due to insufficient material mixing.The tensile strength of the joint is significantly reduced.When the rotational speed/transverse speed ratio is increased to 600/200 r/mm,the joint has a maximum tensile strength of 198.23 MPa,which is 71% of the tensile strength of the base material.As the rotational speed/transverse speed ratio increases to 1200/100 r/mm,the tensile strength gradually decreased to 184.48 MPa.The hardness of each area of the joints also decreased to some extent.The mechanical properties of the heat affected zone of 6061 aluminum alloy decreasd significatly,becoming a weakened area.The weakening mechanism is: 6061 belongs to the heat treatable aluminum alloy.With the increase of the rotational speed/transverse speed ratio,the heat input during the welding process increases(the temperature rises),and the the high temperature thermal cycle in the process of FSW destroyed the rolling hardening state of the 6061 base material.The increase of heat input promoted the dissolution and coarsening of strengthen phases and the growth of grain size of the heat affected zone on 6061 side,thus weakening the mechanical properties of the joint.Forced air cooling treatment can effectly improve the mechanical properties of the joint.With the aid of forced air cooling,the tensile strength of the joints in various welding parameters have an average increasement about 10%,and the yield strength and elongation at break are slightly improved.The range of weakened area of the joint(the heat affected zone of 6061)is significantly reduced,and the hardness significantly increased.The main reason is that the cooling effect of forced air accelerates the cooling speed of the joint,thus reduces the affecting time of high temperature and suppress the coarsening of the grains and the dissolution of the precipitate phases in the heat affected zone of 6061,contributing to the improvement of the joint mechanical properties.The joint fracture angle decreases with the increase of heat input.The fracture angle of the joint is approximately equal to the inclination of the weakened area.When the heat input is lower(600/200 r/mm),the width of the joint weakened area is narrow and the inclination angle is large(about 45?).During the stretching process,the joint experiences a shear fracture at nearly 45? along the region with the weakest mechanical properties.With the increase of heat input,the width of the weakened area of the joint increases,the inclination angle decreases,and the joint tends to have a smaller fracture angle.The fracture mode gradually evolves from shear fracture to normal fracture.Correspondingly,with the increase of heat input,the dimples on the the fracture surface are transformed from shear dimples to equiaxed dimples.In summary,the mechanical properties and fracture behavior of FSWed joint of heat treatable aluminum alloys are easily affected by welding heat inout.Therefore,the optimization principle of dissimilar 5A06-6061 aluminum alloys for firction stir welding is: in the premise of ensuring sufficient material mixing in the weld nugget zone,the rotational speed/transverse speed ratio should be as low as possible.In addition,forced cooling methods such asforced air are adopted to reduce the heat input in the welding process and shorten the affecting time of high temperature,so as to inhibit the dissolution and coarsening of the strengthening phases and the growth of grain size,thus effectively improving the overall mechanical properties of the joint.