| Al-Li alloys are known as ideal lightweight materials in aerospace industry due to its low density,high specific strength and high specific stiffness,the alloys also have low fatigue crack growth rate and good formability.The diffusion bonding/superplastic forming(DB/SPF)technology has unique advantages in reducing structural weight.improving structural integrity and bearing efficiency.Therefore,the development of DB/SPF technology for Al-Li alloys can give full play to the material characteristics and technological advantages,and obtain lightweight integral components with complex structure,which has promising application prospect in the aerospace industry.However,the stable oxide film which exists on the surface of Al-Li alloys seriously affects the diffusion bonding quality and restricts the development of DB/SPF technology.In the present study,the 1420 Al-Li alloy was taken as the main research object.The experiment were conducted through diffusion bonding of 1420 Al-Li alloys,pure A1 interlayer assisted diffusion bonding of the 1420 Al-Li alloys and diffusion bonding between the 1420 Al-Li alloy and the 7B04 Al alloy.The effect of bonding parameters,interlayer,alloy elements diffusion and interfacial reaction on the interface microstructure and mechanical properties were investigated in detail.The mechanism of interfacial reaction and its effect on bonding quality were analyzed.Based on the high bonding quality between the 1420 Al-Li alloy and the 7B04 Al alloy,the DB/SPF experiment was carried out.The main conclusions are shown as follows:Increasing the bonding temperature,the bonding pressure,prolonging the bonding time can significantly improve the bond ratios and mechanical properties of 1420 Al-Li alloy diffusion bonding joints.A sound bond joint can be obtained when diffusion bonding was conducted at 520?-6 MPa-90 min.The microstructure of the base materials is stable,the shear tests show the bond strength of the joints can be as high as 185.2 MPa,which is 90%of the shear strength of 1420 Al-Li alloys.There are deformation dimples and shear tearing edges exist on the fracture surface,which indicates that the current fracture mechanism is a mixed fracture dominated by ductile fracture.On this basis,to further increase the bonding temperature,pressure or prolong the bonding time do not have a significant influence on joint strength and fracture mechanism,but resulted in large plastic deformation or grain growth of the 1420 base plates.Pure A1 interlayer assisted diffusion bonding of 1420 Al-Li alloy can improve the bonding quality effectively.In the initial stage of diffusion bonding,the plasticity of pure aluminum interlayer can increase the physical contact areas between the surface to be bonded,meanwhile the alloying elements diffusion between the interlayer and the Al-Li alloy can further improve the interface bonding quality.The results show that a defect-free joint can be obtained when diffusion bonding was conducted at 520?-6 MPa-60 min.The shear strength of joint is 148.2 MPa.about 72%of the shear strength of 1420 Al-Li alloy.The decrease of shear strength is due to the diffusion bonding process can not make the joints between the pure aluminium interlayer and Al-Li alloy disappear completely.and the lower strength of the pure Al interlayer leads to the failure.The interface microstructure and mechanical properties of the joints obtained by diffusion bonding between the 1420 Al-Li alloy and 7B04 A1 alloy were improved obviously due to the alloying elements diffusion and interfacial reaction.With the rise of bonding temperature.the inter-diffusion distance of Mg and Zn across the bonding interface increases obviously,and the crystalline oxide(Al2MgO4)appears inside of the grain close to the interface.This can be ascribed to the diffusion coefficient of alloying elements increases with the bonding temperature,the alloying elements diffusion resulted in the increase of diffusion fluxes across the interface:on the one hand,it promotes the interface reaction between the magnesium and oxide film,resulted in the transformation of oxide film into crystalline oxide and dissolves into the grain;on the other hand,the increased diffusion fluxes of alloying elements can accelerate the closure of interfacial voids and result in the improvement of the bonding quality.When diffusion bonding between the 1420 Al-Li alloy and the 7B04 A1 alloy were conducted under 460-490?(6 MPa-60 min),the alloy elements segregate at the interface and form AlZnMgCu quaternary phases with size of 2-10 um.The formation of the interfacial phases caused brittle fracture of the joint.When diffusion bonding were conducted under the parameters of 520?-6 MPa-60 min,a sound diffusion bonding joint is obtained without interfacial phases and voids.The microstructure of base metal around the interface is stable,the shear strength of joint can be as high as 190 MPa,which is 92%of the 1420 Al-Li alloy.The shear fracture surface includes dimples and shear tearing edges,the fracture mechanism is a mixed mode dominated by ductile fracture.The DB/SPF experiments were conducted between the 1420 Al-Li alloy as substrate and the 7B04 Al alloy as forming plate.The results show that the optimum superplasticity conditions of the 7B04 Al alloy is 520? and 3×10-4 s-1.The elongation can be 1105%,and the strain rate sensitivity index m is 0.57 at 520 C.The superplastic forming process of 7B04 aluminium alloy was simulated by ABAQUS software creep forming module,and the forming parameters were optimized.The DB/SPF experiment of the 1420 Al-Li alloy and the 7B04 aluminium alloy were completed in a single thermal cycle.The component fit well with the mould,the wall thickness distribution in the forming area change evenly.A sound bonding interface was obtained at the diffusion bonding areas of the component.The quality of the diffusion bonding zones and the superplastic forming zones of the component conform well with the experimental expectation. |
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