Jointing Of ZA Alloy And Microstructure And Properties Of Joint

ZA alloy is based on Zn and composed of Al, Cu, Mg added as alloying elements. It is a kindof environmental friendly material that possesses characteristics of high strength, better plasticity,excellent wear resisting property, low friction coefficient and high specific strength, low meltingpoint and no magnetic. ZA alloy has been widely applicated in the fabrication of mechanical partsand models, such as bearing, axle sleeve, pressing plate and turbine. However, the joiningproblem of ZA alloy that hasn't been solved has limited its application in some fields. That hasbecome one of the strongly desired to be solved problem. The welding and soldering mechanismsof ZA alloy, and the microstructure and properties of their joints were systematically studied inthis paper. Studies were specially stressed on ZA alloy joining material, joining technology,practical application and joining theory.Results of ZA alloy welding study show that the main factors to affect the weldability of ZAalloy were Zn evaporation, oxidation, excessive penetration, porosity and inclusion. The Znevaporation is the most important factor. By using the newly developed TIG welding fluxes andtaking measurements, for example left-hand welding and strict control of welding parameters(heat input), the evaporation of Zn happened during welding could effectively be reduced andprevented. The technological problem in ZA alloy welding has been solved successfully.The TIG welding fluxes and their acting mechanism in the process of ZA alloy being weldedwas studied for the first time. It was observed that fluxes increased the cross—section of the arc,and made the anode spot get more obvious. Surface of weld with flux was finer. Flux also reducedthe loss of Zn content in weld. The mechanism of the fluxes to reduce the loss of Zn includes: 1)Zn transited into weld through metallurgical reaction. 2) The liquid film formed by flux covered the welding pool acted as a pan cap to protect welding pool and prevent Zn from evaporation andoxidization. 3) The liquid film prevented the are heat from transferring into the welding pool,which resulted into the decrease of the welding pool temperature. Then, the evaporation of Zn wasreduced.The weld mainly composed of elongated directional crystal, coarse dendrite and lamellastructure has been observed through investigating the characteristic,microstructure and propertiesof ZA alloy TIG welding joint. The lamella structure, whose spacing is less than 1μm, arranged asfine pearlite-like shape. The microstructure of fusion zone was mainly composed of columnardendrite crystal, eutectic and compounds. HAZ was made up of recrystallization zone and partialrecrystallization zone. No overheated microstructure was found in HAZ. The phases of the jointwere hexagonal latticeη—Zn, face-centered cubic latticeα—Al, body centered cubicβ′-CuZnand intermetallic compounds such as Al₄Cu₉, Al₇Cu₃Mg₆, CuTi₂. There were many fine polygonalsub-grains inη—Zn, whose size varied from 0.1μm to0.3μm. A great deal of dislocation and smalldislocation loop existed inα—Al in weld as well as fusion zone. While small amount ofdislocation and dislocation loop existed inη—Zn. Lattice constant of cubic lattice Al₄Cu₉ whichexisted inη—Zn or grain boundary as the banding compound is a=0.8704nm. There was fixedlattice orientation relationship between Al₄Cu₉ andη—Zn, that is [211]_Al4Cu9//[212]_η-Zn, (011)_Al4Cu9//(101)_η-Zn, (111)_Al4Cu9//(120)_η-Zn. The black compound particles distributed in matrix werebody centered cubicβ′—CuZn, whose lattice constant is a=0.295nm.β′—CuZn which was hard,brittle, without plasticity and made weld brittle was sequentially transformed phase. The bodycentered cubic Al₇Cu₃Mg₆ (lattice constant is a=1.212nm) and small amount of tetragonal CuTi₂(lattice constant is a=0.297nm,c=1.090nm) also exited in weld.The investigation of ZA alloy soldering ability indicates that the soldering ability of ZA alloyis poor. The soldering joint with good quality, high strength and excellent mechanical propertiescould be yielded by developed flux and selected employing solders of low melted temperature, high strength, and excellent wettability, and with reasonable soldering parameters. Thus, theservice performance was satisfied. By studying on the microstructure and properties of solderedjoint, the soldering seams composed of solid solutions (Cd,Sn,Zn), oxidations(SnO,SnO₂,CdO)and intermetallic compounds(MgZn, Mg₂Sn,CdSn_1.9) have been found. The phases at interfacewere made up of solid solution (Cd,Sn,Zn,Al), oxidation(SnO,SnO₂,CdO) and intermetalliccompounds (MgZn,Mg₂Sn,Al₄Cu₉,Mg₂Cu₆Al₅). The lattice orientation relationship, [102]_Cd/[111]_Mg2Cu6Al5, exited between Cd and Mg₂Cu₆Al₅. During soldering, the inter-diffusion between solderand base metal prompt the wider diffusion layer to form at interface, and improved the combinedstrength between ZA alloy soldering seam and base metal. Therefore, the mechanical properties ofthe soldering joint are increased.