Research On Melting Characteristics And Behavior Of Cast Magnesium Alloy He+Ar Mixed Gas Shielded Welding

Compared with traditional metal materials,magnesium alloys have many advantages such as low density and high specific strength,and magnesium resources are quite abundant,which is enough to meet people's needs.However,due to its active chemical properties and large coefficient of thermal expansion,magnesium alloys have poor weldability,especially for cast magnesium alloys.This reason also limits the application of cast magnesium alloys,so it is necessary to study the welding of cast magnesium alloys.At present,the welding methods of casting magnesium alloy mainly include laser welding,friction stir welding and tungsten inert gas shielded welding(TIG).However,laser welding is prone to air holes.Friction stir welding has extremely high requirements on the shape and size of the workpiece.Tungsten arc welding has a small penetration depth,and it is also easy to generate air holes.The tungsten electrode He+Ar mixed gas shielded welding has a large penetration depth,good forming quality,and strong applicability,which can meet the requirements of casting magnesium alloy welding processing.However,the influence rules and reasons of the proportion of helium gas on the penetration of magnesium alloy welds,as well as how to take into account good weld forming and welding processability in the case of obtaining large penetration,still need further experiments.Therefore,this article will systematically study the melting characteristics and behavior of the cast magnesium alloy tungsten electrode He+Ar mixed gas shielded welding.In order to formulate a well-formed welding program,first,a self-fusion welding experiment under different parameters was performed.The influence of shielding gas flow rate and tungsten electrode extension length on the weld morphology of AZ91 D magnesium alloy was analyzed,and the effects of welding current,welding speed and tungsten electrode height on welding quality and forming dimensions were summarized.The weld metallographic structure obtained by welding is observed.The weld structure is mainly composed of ?-Mg and ?-Mg17Al12,of which ?-Mg17Al12 is distributed in a network at the grain boundaries.The grain size gradually decreases from the depth of the weld to the surface,and the content of ?-Mg17Al12 increases.The effects of different helium contents in shielding gas on the weld forming quality and microstructure and properties are analyzed.With the increase of the helium content in the shielding gas,the penetration depth and the aspect ratio of the weld seam are greatly improved.The quality of the weld seam is improved first and then decreased.The grain size gradually becomes larger,but the increase is smaller.The hardness difference of the samples obtained by welding under different helium content shielding gas is small.The experimental results show that the addition of helium in the shielding gas can effectively improve the penetration of magnesium alloy welds without reducing the weld mechanical properties.In order to study the reason for the increase of penetration depth when helium gas is added to the shielding gas,the welding electrical signal is measured and the arc shape is observed by high-speed camera.It was found that as the content of helium in the shielding gas increased,the absolute values of positive voltage and negative voltage both increased.At the same time,the arc shape became more restrictive and the anode radius of the arc decreased.Combining the two changes,the energy density at the weld increases,which increases the weld penetration.At present,there are few studies on the influence of helium in shielding gas on the temperature field and stress field of cast magnesium alloy welds.Therefore,it is very necessary to simulate the welding process of cast magnesium alloy tungsten electrode He+Ar mixed gas shielded welding.The variation law of temperature field and residual stress field under different welding parameters was explored.As the content of helium in the protective gas increases,the peak value of the thermal cycle curve of the temperature field increases,and the holding time is longer.The lateral stress in the residual stress increases first and then decreases,while the longitudinal stress and the Mises equivalent stress continue to decrease.The stress curves of lateral stress,longitudinal stress and Mises equivalent stress become more gentle.Experiments show that adding helium to the shielding gas will reduce the overall stress level.As the welding speed increases,the penetration depth and width of the welding seam decrease,the peak of transverse stress first increases and then decreases,and the longitudinal stress continues to increase.As the welding current increases,the heat input increases,and the penetration depth and width increase.The peak lateral stress also increases first and then decreases,and the longitudinal stress continues to decrease.