Microstructures And Mechanical Properties Of Welded Dissimilar Casting And Wrought AZ Series Magnesium Alloys

As the lightest structural metals,magnesium alloys have wide potential applications inthe fields of aeronautics,automotive,and electronics.Magnesium alloys are divided into two categories according to the way they are processed: casting magnesium alloys and wrought magnesium alloys.Casting magnesium alloys have the advantages of low cost and high production efficiency.Wrought magnesium alloys have higher tensile strength and plasticity.Therefore,for the reduction of the production cost and retention of desired performance of the whole structures,an advanced welding technology and composite structures of casting and wrought magnesium alloys matched must be developed.Cast and wrought dissimilar magnesium alloys have significant differences in composition,processing status,microstructure,and properties.These differences render forming control,process stability,and composition homogenization difficult to achieve.Thus,a welding method with excellent adaptability is required.To date,studies on dissimilar magnesium alloys are mostly focused on the connection of two casting or two wrought magnesium alloys.By contrast,research about connections for casting and wrought dissimilar magnesium alloys are limited.Thus,casting magnesium alloy AZ80 and wrought magnesium alloy AZ31 B and AZ61 subjected to TIG filler wire welding and laser–TIG hybrid filler wire welding were investigated in this paper.The changes and reasons in the formation,microstructure,and properties of dissimilar joints under two heat sources were systematically analyzed.We aimed to explore the feasibility of welding AZ31 B and AZ80 plates through the use of two welding methods and provided support of process and mechanical properties for practical industrial applications.Firstly,welding process on the butt joint of cast and wrought dissimilar magnesium alloys AZ80-AZ31 B was studied.The welding appearance of joints were sensitive to heat input.After the process parameters optimization,the heat input range of TIG welding was only 138.60 J/mm~142.15 J/mm.While the hybrid welding had a wider heat input range,which was between 95.05 J/mm~104.55 J/mm.According to the thermodynamic analysis of weld pool,the smaller the penetrated width W2,the wider the forming parameter range.The hybrid welding with concentrated heat source distribution created a smaller W2 and consequently broadened the welding parameters compared with TIG welding.Besides,the bead shapes of the two joints were characterized by an irregular anchor-shaped weld with an‘angle' region on the AZ31 B side.A formation model of the angle region was constructed to reveal that the angle region was caused by the difference between AZ80 and AZ31 B with respect to liquefaction temperature and melting latent heat.The size of the "angle" can be reduced by shifting the welding position in hybrid heat source.The microstructure and properties based on the optimized welded joints were studied.The results showed that the AZ80-AZ31 B joints were mainly divided into five parts: AZ31 base metal(BM),AZ31 heat affected zone(HAZ),fusion zone(FZ),AZ80 partially melted zone(PMZ)and AZ80 base metal.The microstructure of joints mainly consisted of ?-Mg phases and ?-Mg17Al12 phases and divorced eutectic ? phases were found in the FZ,AZ80-BM and AZ80-PMZ.The structure of FZ was observed as fine equiaxed dendrites.The microstructure of the heat affected zone(HAZ)on the AZ31 B side of joints is coarse.A lot of precipitation phase were observed in the AZ80-PMZ.The Al content in the joints evidently showed three gradient changes: AZ80 > fusion zone > AZ31 B and was more uniform in the hybrid welding.The average hardness of the weld in the joints was between that of BM–AZ31B and the BM–AZ80.The HAZ of AZ31 B exhibited the lowest hardness and the PMZ of AZ80 exhibited the highest hardness.Good tensile strength can be achieved both in joints of two heat sources.The ultimate tensile strength(UTS)of both joints were over155.43 MPa,and fractured at the AZ80 base metal.As showed in the microstructure and mechanical properties studies,the redistribution of the microstructure in PMZ had a good effect on the microhardness and tensile strength of the joints.In addition,the microstructure of the hybrid joints in PMZ was more finely dispersed,which was different from the TIG joints.Therefore,the morphology of microstructure in PMZ was intensively studied.The results showed that increasing with heat input,the remelted eutectic structure of PMZ on the AZ80 side had four typical shapes: network-like,cluster-like,fine-dendritic-like,and coarse-dendritic-like.The fine-dendritic-like remelted eutectic structure had the finest grain size,the most uniform morphology and element distribution,and the highest hardness performance.The hybrid heat source could quickly increase the degree of dispersed distribution in PMZ when the heat input is low,and slowed down the coarsening of the remelting eutectic at the time of large heat input.Therefore,it was more conducive to refinement and dispersion of the remelted microstructure than the TIG heat source.Based on the above researches on the microstructure and properties of the cast magnesium alloy AZ80 and wrought magnesium alloy AZ31 B butt joints,TIG and laser-TIG hybrid heat sources were used to study the process and performance of the AZ80-AZ61 step structures required for actual industrial production.The results showed that the TIG welding heat source was difficult to achieve the high quality welding of the step structure due to the shallow weld penetration and the large range of heat sources.While in the laser-TIG hybridwelding,after the process optimization,by using two parameters(300 W laser power,150 A current and 700 mm/min welding speed;900 W laser power,150 A current and 1000 mm/min welding speed),the welded step structures with beautiful butt-appearance and reliable lap-joined can be achieved.A positive correlation was found between the tensile strength of the step structure and the lap joint width of the step surface.The better the lap combination is,the closer the fracture position is to the edge of the step,and the higher the maximum tensile load value is.The joint with the parameter of 900 W laser power,150 A current and 1000mm/min welding speed had the highest tensile maximum load,which is 3858.6 N and is85.01% of the AZ61 base metal.