Microstructure And Properties Improvment Of Friction Stir Welded Dissimilar 2198-t3 And 7A04-T6 Aluminum Alloy

In the aircraft manufacturing process,2198 aluminum lithium alloy is often used as the fuselage and wing skin material.Due to the addition of Li,Ag and other elements,the manufacturing cost is high.The traditional forming process of the aluminum alloy fuselage skin is to use the entire sheet to directly stretch the shape.After the forming,the remaining material on both sides of the jaw is cut off to become a waste product,which results in low material utilization and high production costs.If the jaw surplus material is replaced by a cheap and high-strength 7A04-T6 aluminum alloy plate for FSW tailored welding with 2198-T3 plate,the utilization rate of the material can be improved,the requirements for the width of the formed plate can be reduced,and finally the purpose of reducing production cost can be realized.As is known to all,during the friction stir welding of heat treatable strengthened aluminum alloy,the precipitated particles are dissolved and coarsened by the welding thermal cycle,the joints are softened,and it is easy to preferentially deform and fracture in the softened zone during the stretch forming process.In this study,the method of applying forced cooling during the process of FSW was used to control the dissolution and coarsening behavior of the precipitated particles in the softened zone of the joint.The weak zone of the joint is strengthened by low-temperature artificial aging and natural aging treatment,and the tailor welded joint meets the requirements of skin stretch forming,which provides the experimental basis for the application of FSW tailor welded forming technology in the production and manufacturing of fuselage skin.In this thesis,the 2198-T3/7A04-T6 dissimilar aluminum alloy were welded under different cooling conditions.The microstructure characteristics and mechanical properties of FSW joints after welding,natural and artificial aging were studied systematically,and the key factors affecting the microstructure and property evolution of joints were revealed.Based on the experimental research conditions of this paper,the 2198-T3 aluminum alloy and the 7A04-T6 aluminum alloy are placed on the advancing side and the retreating side,respectively,the distance between the steel clamp and the center of the weld is 10 mm,and the forming state of the stir zone(SZ)in the 2198-T3/7A04-T6 dissimilar FSW joint is the best,with the rotation speed of 1200 rpm and the welding speed of 100 mm/min.Under water-cooled copper clamp and underwater welding conditions,the peak temperature of the heat affected zone(HAZ)of the joint is reduced,and the residence time of high-temperature is shortened.Therefore,the strengthening precipitate of T1 and ?' dissolution and coarsening at the lowest hardness zone(LHZ)on the 2198-T3 side of the joint were limited,resulting in a lower reduction of microhardness value at the LHZ and a significant increase in the tensile strength of the joint.By comparing three different cooling conditions,the joints obtained by underwater welding have the best comprehensive mechanical properties.The joints welded under different cooling conditions were treated with low temperature artificial aging and natural aging,the tensile properties were significantly improved.This is due to that a large number of fine-grained T1 and ?' strengthening phases precipitated at the LHZ on the 2198-T3 side of the joint.However,the aging strengthening ability of the joint obtained under cooling condition is strong.This contributes to the reduction of the coarsening of the T1 and ?' strengthening phase at the LHZ on the 2198-T3 side of the joint under the cooling condition,which indicates that the precipitation behavior of the ?-Al supersaturated solid solution is suppressed under the welding thermal cycle in this position,and a relatively high aging strengthening ability is maintained.After 60 days of natural aging,the joints obtained by underwater welding showed the best tensile properties.The yield strength and tensile strength were 319 MPa and 445 MPa,respectively,and the elongation of the base material area of the joint was 10.2%.