Study On BT-FSW Process Of Aluminum Alloy

Bobbin tool friction stir welding(BT-FSW)is an improved to FSW,BT-FSW not only retains the advantages of friction stir welding which Joint is forming by Semi-solid metal,and Improve the shortcoming of the FSW likes low efficiency,root defects of the welding.It has caused very high concern and has been thought has high research and application value no matter in in and out.Using Butt welding test on 6mm LF21 aluminum alloy by the optimization design of the welding tool with double shoulderThe experiment analyzed the Influence of the weld joint of the morphology and forming effect,microstructure and mechanical property changes,the second phase composition and distribution,change of the resistivity by the different of the rotate speed,the welding speed,the pressure volume of he shoulders.Comparing the microstructure,mechanical properties,resistivity and corrosion resistance of the welding of BT-FSW with The Traditional FSW and Traditional FSW with double side.To fing the influence of different kinds of stirring head on the weld seam.The welding test shows that the weld by BT-FSW can form the weld with high quality,whose surface is Smooth and tidy and with no defects in inside and outside the weld,within a certain welding process.The shape of the cross section of the weld is like a saddle shaped.The microhardness test showed the microhardness distribution of the upper,middle and lower parts of the welded joint is like a “W”.The heat affected zone and the heat affected zone of the weld are softened with the increase of rotating speed.the fracture surface shows a mixed fracture morphology when the rotational speed is in a low level;With the increase of the rotational speed,the tensile strength of the weld increases,and the fracture surface shows ductile fracture morphology.With the increase of the rotational speed,the tensile strength of the weld is improved,and the fracture became a ductile fracture.When the rotational speed is further increased,the tensile strength of the weld is decreased,and the fracture is still a ductile fracture;The welding of BT-FSW is very sensitive to the shoulder pressure,The quality of weld is change very obviously with the change of the a shoulder pressure.With the increase of welding speed,the heat input per unit length is reduced,the tensile strength and elongation are improved..The welding resistance increases first and then decreases,With the increase of welding speed.Comparing the characteristic among the welding of BT-FSW with The Traditional FSW and Traditional FSW with double side.The experimental results show that the weld of T-FSW is like a “funnel”,the weld of BT-FSW is like a “saddle”,there is a "spoon" shape of the intersection area in the middle of the weld,the weld of D-FSW is similar as BT-FSW,but there isn't a "spoon" shape of the intersection area in the middle of the weld.The resistance of the weld is the BM<BT-FSW<D-FSW<T-FSW,which shows that BT-FSW weld has a more complete crystal structure than the others.It is similar in the tensile strength among the different FSW weld.From the strength-displacement curves,but it is found that the cold hardening phenomenon is T-FSW>BT-FSW>D-FSW,lead to the weld of BT-FSW had the greatest breaking energy before the fracture,by integrating the strength and displacement curves.The Electrochemical corrosion test says,consistent with the surface corrosion test,The upper and lower surface corrosion resistance makes the strongest of corrosion resistance,and the corrosion resistance of single-shoulder friction stir welding surface is less than BT-FSW.The electrochemical corrosion test Surface double-shoulder friction stir welding The upper and lower surface corrosion resistance with the strongest,single-shoulder friction stir welding surface corrosion resistance of the worst,consistent with the surface corrosion test.The corrosion resistance is the worst in the lower surface of the friction stir welding seam.Analysis of the corroded surface by backscattering and energy spectra revealed that the contact surface of the weld and the shoulder had a larger and more diffused Mg and Si second phase,which could lead to the formation of dense oxides to prevent further corrosion.