Research On Ion Bombardment Assisted Solid-State Diffusion Bonding Process And Mechanism Of 1060 Pure Aluminum

Aluminum（Al）microchannel heat exchanger is of great significance to the development of high efficiency,miniaturization and lightweight for heat exchanger due to the advantages of low density,compactness,small size and high heat transfer coefficient.However,the deformation of the microchannel will block the cooling medium flow in the channel and reduce the heat exchanger efficiency.On the basis of ensuring the bonding strength of joints in the heat exchanger,it is necessary to control the bonded joint deformation to achieve the coordination between the performance and shape of heat exchanger.In addition to the bonding parameters,the oxide film and surface morphology are the inevitable factors for solid-state diffusion bonding of Al under low temperatures.In this paper,the effect of Ar ion bombardment on oxide film,morphology and structure of Al surface were firstly investigated and the evolution mechanism of surface structure during the bombardment was clarified.The bombardment parameters（energy&dose）were optimized subsequently to remove the surface oxide film and reduce the surface roughness.Based on the Ar ion bombardment,the solid-state diffusion bonding process was carried out to study the effect of bonding parameters on the strength and deformation of joints,as well as reveal the influence mechanism of Ar ion bombardment on the bonding interface.According to the results,the mathematical models for shear strength,deformation of joint with bonding parameter were established to optimizing the bonding parameters.And in this way to obtaining the Al bonded joints that meet the requirements of deformation and strength.With the enhancement of bombardment energy and dose,the surface oxide film was gradually removed and the surface roughness（Rms）first decreased and then increased.When the bombardment parameters were 1ke V–2×10¹⁸ions/cm²,a flat Al surface without oxide film was obtained and the Rms reached the lowest value of0.61nm.Accompanied by numerous small dot-like defect clusters and dislocation loops,the surface showed residual compressive stress of-85.2MPa and a hardened state.By further increasing the energy&dose(>1ke V–2×10¹⁸ions/cm²),the irradiation swelling and even porous morphology were generated under the growth of Ar bubbles,which roughened the Al surface.The stress state on the surface was changed from compressive stress to tensile stress.Moreover,the porous morphology induced by high energy bombardment reduced the hardness and elastic modulus of the surface.The changes of surface oxide film and morphology caused by Ar ion bombardment with different parameters also affected the quality of pure Al solid-state diffusion bonding.Under the bombardment conditions of low energy&dose(<1ke V–2×10¹⁸ions/cm²),the presence of residual oxide film led to large unbonded regions with a minimum joint strength of 13.2MPa.The removal of the oxide film and low roughness both facilitated the formation of bonding interface,and the high bonding ratio of 97.7%with a strength of 45.9MPa was achieved in the case of1ke V–2×10¹⁸ions/cm².The corresponding failure mode was ductile fracture with dimples.However,the surface swelling and brittle porous morphology produced by high energy&dose bombardment(>1ke V–2×10¹⁸ions/cm²)deteriorated the joint quality with the lowest bonding ratio of 10.2%and strength of 8.5MPa.The fractographs of joint showed a flat failure morphology without plastic deformation.The influence of the bonding parameters on the strength and deformation of joints was in the following order:Temperature（T）>Pressure（P）>Holding time（t）.Both of strength and deformation of joints were increased with the enhancement of bonding parameters.Under the bombardment condition of 1ke V–2×10¹⁸ions/cm²,the mathematical formula between the bonding parameters and the shear strength of joints based on the experimental data was established as Y=f（T,P,t）=–217.58+0.543T–4.215P+0.145t+0.252P²–0.0012t²+0.005TP+0.033Pt.The formula between the bonding parameters and the deformation of joints was established as D=f（T,P,t）=264.4–1.1935T–6.557P–0.0996t+0.001335T²+0.000276Tt+0.016803TP.According to the formula,the calculated shear strength and deformation of joint bonded at 450℃–5MPa–90min were 41.4MPa and 4.89%,respectively.The experimental results showed that the well bonded joint with bonding ratio of more than 95%was obtained.The actual shear strength were45.9MPa with deformation of 4.82%,which matched well with calculated value.The interfacial atoms preferentially diffused into the Al matrix along the defects created by bombardment,and the corresponding mechanism was mainly the vacancy diffusion mechanism with the minimized diffusion energy barriers of0.58e V.The increase of vacancies significantly reduced the diffusion activation energy and enhanced the atomic transition frequency and diffusion coefficient.The simulation results showed that the diffusion activation energy of the interfacial Al atoms after the introduction of vacancies was Q_Al-v=0.59e V,which was 0.12e V lower than that of interfacial Al atoms in perfect crystal with Q_Al=0.71e V.The removal of oxide film and the reduction of surface roughness caused by Ar ion bombardment provided good contact conditions for the initial contact stage in diffusion bonding.The bombardment-induced defects allow more interfacial atoms diffused into the opposite aluminum substrate,which resulted in grain boundary migration.Moreover,the increase of temperature promoted the recrystallization and more grain boundaries cross the bonding interface,which led to the voids shrinkage and finally formed the effective bonding.