Interfacial Characteristics Of In-situ Particle Composite Reinforced Sn9Zn Solder Wetting Aluminum Alloy At Low Temperature

Aluminum alloy is widely used in various fields because of its low density,high strength and good post-weld processing.As a precise joining technology,low temperature soldering is a reliable method for welding aluminum alloys.Sn9Zn solder is applied to low-temperature soldering of aluminum alloy due to its excellent performance,but there is still the problem that the solder is easily oxidized and the oxide layer on the surface of the aluminum alloy is difficult to be wetted.In order to solve these problems,there are usually two methods of alloy modification and second phase particle enhancement.However,both methods have certain limitations.Therefore,it is of great significance to develop new soldering filler metals that are modificated using the second phase particles and elements together and to explore the wetting phenomena and mechanism of the filler metals for aluminium alloy wetting.A certain amount of Al₂O₃ nanoparticles and different amounts of Cu element are added to the Sn9Zn filler metal.The effects of the Cu element content on the properties of the filler metal and the micro-morphology and mechanical properties of 6061 aluminium alloy wetted by low temperature soldering with the filler metal are systematically studied.The mechanism of the in-situ particle Cu₅Zn₈ on the mechanical properties of the solder is also studied.The effect of copper content on heat dissipation of 6061 aluminum alloy wetted by the filler metal is observed.Ultrasound assisted and the prepared solder are applied to the connection of the aluminum back electrode of the polycrystalline Si solar panel and the 6061 aluminum alloy to explore the optimal process parameters.After adding Cu element to the Sn9Zn-1Al₂O₃ solder,the microstructure and overall properties of the composite solder are improved.The melting point of Sn9Zn-1Al₂O₃-4.5Cu is 0.6°C lower than that of Sn9Zn-1Al₂O₃ composite solder.The SnZn eutectic structure in Sn9Zn-1Al₂O₃-4.5Cu is the smallest,and the black Cu₅Zn₈ phase appears in the soldering filler metal.The microhardness and tensile strength of Sn9Zn-1Al₂O₃-4.5Cu composite soldering fillers reach45.8Hv_0.3.3 and 67.6MPa,respectively.The tensile strength is 56.49%higher than that of Sn9Zn-1Al₂O₃,it's mainly due to the pinning effect of Al₂O₃nanoparticles,the dispersion strengthening effect of Cu₅Zn₈,and fine grain strengthening.Sn9Zn-1Al₂O₃-xCu composite solder has good wettability for 6061aluminum alloy.Sn9Zn-1Al₂O₃-4.5Cu exhibits the best wetting effect,a precursor film with a composition of Al-Cu-Zn solid solution is formed,and the width is 1095?m.Its spread area is 0.75cm²,which is 27.12%higher than that of Sn9Zn-1Al₂O₃.Its wetting angle is 28.6°,which is 32.39%smaller than that of Sn9Zn-1Al₂O₃.The length of the wetting interface is 15.32?m and the micro interface thickness is 0.67?m.Strong diffusion of Zn occurs at the wetting interface.The joints of 6061 aluminum alloy soldered with Sn9Zn-1Al₂O₃-xCu composite soldering filler metal are analyzed.Al₄Cu₃Zn solid solution layer appears at the interface junction of the joint.The thickness of the solid solution layer first decreases and then increases with the increase of Cu element.When the content of Cu is 4.5wt.%,the thickness of the solid solution layer is 0.65?m.The hardness and shear strength of interface solid solution layer also show the law of first increasing and then decreasing with the increasing of Cu element.When the content of Cu is 4.5wt.%,the hardness and the shear strength of interface solid solution layer are 47.17HV_0.3.3 and 91.7MPa respectively,which are 28.14%and 75.6%higher than that of Sn9Zn-1Al₂O₃.There are dimples in the joint fracture.In Sn9Zn-1Al₂O₃-4.5Cu soldered joint,dimples are the most abundant and more dispersed.Sn9Zn-1Al₂O₃-4.5Cu solder is used to connect the polycrystalline Si sheet aluminum back electrode and 6061 aluminum alloy.When the ultrasonic parameter is at 350W ultrasound power,on 220?soldering temperature and at10s ultrasonic time,a large number of cellular Al appear in the diffusion layer of soldering filler metal and aluminium back pole.The diffusion thickness reaches22.3?m,the hardness of the diffusion layer is 99.97Hv_0.3,and the shear strength of the joint is 12.4MPa.The fracture of the joint occurs entirely on the silicon chip,and the size of the small pits on the micro-fracture surface of the joint is the smallest.The heat dissipation process of 6061 aluminum alloy wetted by Sn9Zn-1Al₂O₃-xCu is analyzed by thermal imaging.The solder portion is heated for about 2-3 minutes before the temperature is significantly lowered.The initial temperature of the solder portion:intermediate area>edge area>central area.The average cooling rate at different positions of the same solder:intermediate region is the fastest.And the higher the content of Cu element in the soldering filler metal is,the higher the average cooling rate in the intermediate portion of the soldering filler metal will be.