Investigation On Weld Formation Mechanism And Properties Of AZ31 Magnesium Alloy Electron Beam Welding

As an ideal light alloy,magnesium alloy is widely used in aerospace,national defense and military industry,vehicle transportation and other industrial manufacturing fields.However,magnesium alloy has low melting point,high thermal conductivity and thermal expansion coefficient,active chemical properties and easy-oxidation.These characteristics lead to many difficulties in the welding process of magnesium alloy.As an advanced vacuum welding manufacturing technology,electron beam welding(EBW)has the characteristics of high energy density,high vacuum in chamber and high welding quality,which can well realize the deep penetration welding of magnesium alloy.The mechanical properties and corrosion resistance of electron beam welded joints are better than those of other welding processes.However,when electron beam welding magnesium alloy,the welding parameters are strictly required.If the welding parameters are not selected properly,it is difficult to obtain the welded joint with good shape and excellent performance.Therefore,it is of great significance to investigate the weld formation mechanism and optimize the welding parameters for improving the quality of electron beam welding of magnesium alloy.Firstly,in order to solve the problem that the single heat source model is difficult to reflect the actual welding process in EBW simulation,a composite heat source model composed of conical Gauss heat source and double ellipsoid heat source was established based on the characteristics of electron beam deep penetration welding.Fluent software was used to simulate the molten pool formation behavior of magnesium alloy during electron beam welding,including transient molten pool heat transfer,metal liquid flow,keyhole formation and metal backfill after welding.The temperature field and stress field of AZ31 magnesium alloy during electron beam welding were simulated by Sysweld software.The forming regularity of welds,thermal cycle curves and residual stress distributions were obtained during EBW of AZ31 magnesium alloy with different welding powers and welding speeds.It has laid a theoretical foundation for the establishment of electron beam welding process.Secondly,the effects of different electron beam welding parameters on the forming regularity of AZ31 magnesium alloy welded joints were studied by electron beam surfacing test,and the welding process parameters were optimized.The results showed that the increase of accelerating voltage or electron beam current increased the weld penetration and weld width,and the increase of penetration depth was more obvious than the weld width during electron beam welding of AZ31 magnesium alloy.After the base metal was fully welded,the weld shape changed from nail to straight with the increase of heat inputs.When the focus position was on the surface,with the increase of focusing current,the farther the focus deviates from the surface focusing position,the larger the weld width and the smaller the penetration depth.With the increase of welding speed,the weld penetration depth and weld width gradually decreased.Thirdly,the butt joint tests of AZ31 magnesium alloy sheets were carried out based on the optimized penetration welding process.The effects of different heat inputs on mechanical properties and microstructure of electron beam welded joints of AZ31 magnesium alloy were studied.The results showed that with the increase of heat input,the strength and plasticity of the joints increased at first and then decreased.The increase of welding heat input led to the increase of grain size and the number of Mg₁₇Al₁₂precipitates in the fusion zone.During the tensile plastic deformation of the welded joints,the precipitates can effectively pin the dislocation line,restrict the movement of the dislocations,and improve the strength of the welded joints.When the plastic deformation was 3%,the slip of base plane was dominant.However,when the plastic deformation reached 8%,a lot of twins were found in the joints,and the widths of twins increased.The fracture locations of welded joints all occurred near the fusion lines.The fracture morphology of fusion zone was obviously different from that of base metal.In the joint,the fracture on the side of the base metal was characterized by intergranular brittle fracture.The fracture surface of the weld zone of the joint showed a certain ductile fracture morphology.Then,the corrosion behavior and mechanism in 3.5wt.%Na Cl solution of electron beam welded joint of AZ31 magnesium alloy under different welding parameters were studied by means of microstructure observation,electrochemical corrosion test and corrosion morphology observation.The results showed that the main corrosion pattern of base metal was filiform corrosion.Corrosion was initiated in the form of pitting on the base metal.Subsequently,the pitting corrosion propagated along the grain boundary of the base metal,resulting in continuous filiform corrosion pits.When the filamentous corrosion spread to the fusion line,the precipitates near the fusion line which distributed along the grain boundaries hindered the corrosion propagation effectively.In the fusion zone,the first corrosion occurred near the precipitates,and the corrosion area can not extend to all around due to the influence of precipitates.Corrosion will continue to nucleate in other areas.The corrosion area increased with the corrosion time.Macroscopically,the fusion zone was characterized by uniform corrosion.The results of immersion tests and electrochemical corrosion tests showed that with the increase of heat inputs,the corrosion resistance of electron beam joints of AZ31 magnesium alloy presented a trend of first improving and then decreasing.The heat input of 180 J/mm magnesium alloy welding joints had the best corrosion resistance.The charge transfer resistance R_ctof the joint was 9.5 times higher than that of the base metal,and the value of R_ctincreased by an order of magnitude.And its value of Ecorr was the largest,which was 0.061 V higher than that of base metal,its icorr was only 27%of the base metal.Finally,the performances of electron beam welded joint of AZ31magnesium alloy with heat input of 180 J/mm,which was well welded,were improved by post weld heat treatment and cryogenic treatment.The results showed that the precipitated phase in the welded joints of AZ31 magnesium alloy were completely dissolved after solution treatment at 450?/0.5h.The plasticity of AZ31 magnesium alloy was greatly improved,and the elongation was about 15.7%,was 3.0%higher than that of untreated.After cryogenic treatment,the grain sizes in the weld zone of AZ31 magnesium alloy joints were decreased,the uniformity of microstructure was further improved,and the precipitates were refined and tend to disperse distribution.The preferred orientation of crystal deviated from(10^—10)to(10^—11).The corrosion resistance of AZ31 magnesium alloy joints were improved.When the cryogenic time were fixed,the corrosion resistance order of AZ31 magnesium alloy joints under different cryogenic processes was as follows:-140?/4h>-180?/4h>-100?/4h.The charge transfer resistance R_ctof-140?/4h cryogenic treatment was 1.82 times higher than that of the non-cryogenic treatment,and its Ecorr value was the largest,which was-1.421 V,0.051 V higher than that of untreated.When the cryogenic temperature was constant,the order of corrosion resistance of joints under different cryogenic processes was as follows:-140?/6h>-140?/4h>-140?/8h>-140?/2h.The charge transfer resistance R_ctof-140?/6h cryogenic treatment was 2.41 times higher than that of the non-cryogenic treatment,and its Ecorr value was the largest,which was-1.402V,0.07 V larger than that of untreated.While value of icorr was only 31.5%of untreated.