AC-assisted Al/Cu Plasma Arc Brazing By Adding Nano-powder And Interfacial Diffusion Dynamics Analysis

In the manufacturing of Al/Cu composite structures,the effective and reliable bonding of Al/Cu dissimilar metals due to the differences in melting properties and thermophysical properties is a key problem that needs to be solved urgently.At the same time,the i nterface behavior of the joint has an important influence on the performance,so it is of great significance to analyze the interface behavior from many angles and aspects.Based on the low cost and high operability of traditional arc welding,AC assisted Al/Cu plasma arc brazing with adding SiO₂ nano-powder was proposed to realize the Al/Cu dissimilar metal joints with good forming and mechanical properties from the aspects of process improvement and interface regulation.At the same time,the internal mechanism of the formation of microstructure at the interface was analyzed through experimental analysis and molecular dynamics simulation at micro and nano scales to explore the internal relationship between atomic diffusion behavior at the micro scale and external factors.Further improve the welding process,achieve good joints and interface analysis provide new ideas,technology,and theoretical support.The AC assisted Al/Cu plasma arc brazing with adding SiO ₂ nano-powder was established to conduct the test process.The introduction of the auxiliary AC arc significantly affect the behavior of the arc,broken material surface oxidation film.With the increase of the AC current value,the width of the weld seam increases and the height gradually decreases.The wetting angle of the surfacing process also gradually decreases from the initial 70°to 27°.The wetting and spreading performance shows a positive correlation effect with the current value.With the increase of AC auxiliary current value,the effective connection length of Al/Cu lap joint increases from 12.25mm to 15.2mm.On the basis of the above experiments,the internal relationship of the difference in the structure and performance of the AC-assisted Al/Cu plasma arc welding joints with different parameters added with SiO ₂ nano-powder is deeply analyzed.The joint structure area is mainly Al ₂Cu and Al₄Cu₉ intermetallic compound layer area,Al-Cu eutectic area and mesh weld area.The thickness of the intermet allic compound layer shows a positive correlation that increases with the increase of the AC auxiliary current value.The thickness of intermetallic compound layer at the interface with SiO ₂ nano-powders decreases when the AC auxiliary current value is 45A,and trace Si element is distributed in the interface area.The AC auxiliary current value has no effect on the phase composition of the joint.Due to the relatively high surface tension and specific surface area,SiO ₂ nanoparticles adsorbed on the interf acial IMC and Cu surface acted as a diffusion barrier,impeding the reaction and mutual diffusion of Al and Cu atoms,thus effectively reducing the thickness of interfacial IMC in the welding process.With the addition of SiO ₂ powder and different AC auxil iary current values,the hardness of the structure in the weld area increases slightly with the increase of heat input.It presents the distribution law of increasing hardness in the aluminum and copper base metal area,weld area and interface intermetalli c compound area.Affected by the effective connection length and the thickness of the intermetallic compound layer,the joint with SiO₂ added and the AC auxiliary current value of 45A can withstand a maximum load of 0.85k N.The fracture position is located in the heat-affected zone on the aluminum side,with good mechanical properties and fracture morphology It is a form of ductile fractu re with dimples.Finally,the Al/Cu interface model was established through molecular dynamics simulation,and the interface atomic diffusion dynamics analysis was carried out to explore the internal relationship between atomic diffusion behavior and exter nal factors at the microscopic scale.At lower temperatures,the diffusion coefficients of Al and Cu atoms in the solid-solid Al/Cu interface increase with increasing temperature,and the diffusion coefficients of Cu atoms are more sensitive to temperature than Al atoms.Cu and Al atoms are at the interface.The activation energies during the diffusion process were 52.24 k J/mol and 80.23 k J/mol,respectively,which verified the accuracy of the model.After the interface is completely cooled to room temperat ure,the main component isθ-Al₂Cu.The number of Cu atoms diffused across the initial interface is significantly greater than the number of Al atoms.The higher the temperature,the more obvious the phenomenon.The contribution to the growth of the diffusion layer is mainly provided by the diffusion of Cu atoms in the liquid Al,which is basically consistent with the interface element distribution and types of the test results.The atomic interdiffusion behavior of Al and Cu at the solid-liquid interface a t800～950K is similar to that at 600～800K.Due to the differences in the structure and dilution thermal properties between Cu atoms and Al atoms,the K irkendall effect exists in the interface diffusion model,and the interface gradually moves towards the copper side migrates.At the same time,analysis shows that the activation energy of diffusion is not constant at high temperature,but is phased.The value of activation energy is related to the state and diffusion direction of the substance under stud y;there is a relationship between the thickness of the diffusion layer and the temperature:D=k·T,k=0.548?/K,which is consistent with the phenomenon that the thickness of the compound layer increases with the increase of heat input in the experiment.Du e to the stress change induced by the diffusion process,the stress value on the Al side is significantly greater than that on the Cu side,and the temperature has a significant effect on the atomic shear stress at the interface.As the diffusion duration increases,there is a parabolic relationship between diffusion depth and time D=k t^1/2.The introduction of pressure reduces the distance between atoms and hinders the process of atomic diffusion,so that the thickness of the diffusion layer and the diffus ion coefficients of Al and Cu atoms show a negative correlation trend that decreases with the increase of pressure.