Microstructure And Mechanical Properties Of TIG Welded Dissimilar Light Alloy Joints And The Numerical Simulation

With the expanding and deepening of the lightweight structural materials application field,magnesium alloys have attracted tremendous interest due to its low density and high specific strength.Magnesium alloys can significantly reduce the weight of parts and structures without any reduction of the strength.Recently,the increasing variety of newly developed magnesium alloys and its engineering requirements drive the development of the magnesium alloys welding technology.However,the welding of dissimilar novel magnesium alloys and the welding of magnesium and light metal aluminum alloys face technical difficulties and limit its practical application.Therefore,obtaining welded joints in dissimilar light metal materials with good formability and mechanical properties by improving welding technology or optimizing welding forming parameters is of technical importance.Tungsten inert gas shielded welding(TIG)and active welding(A-TIG)have become one of the most effectively used welding methods for bonding magnesium alloys,due to the advantages of centralized heating,small deformation and good arc stability.However,few reports have been reported on their application in the welding of dissimilar magnesium alloys and magnesium/aluminum alloys.Moreover,the optimizing and improving TIG welding process of dissimilar magnesium alloys by numerical simulation method is very rare.In this paper,the microstructure and mechanicals properties of dissimilar magnesium alloys and Mg/Al alloy welded by TIG and A-TIG welding methods were studied theoretically and experimentally.Taking three different groups of dissimilar materials(AZ61 magnesium alloy/ZK60 magnesium alloy,AZ61 magnesium alloy/AM60 magnesium alloy and AZ31 magnesium alloy/6061 aluminum alloy)as research objects,the effects of welding parameters on the forming quality,microstructure and mechanical properties of welded joints were investigated and the mathematical model of dissimilar light alloy welding pool basing on the action of three-dimensional transient moving heat source was established.The main contents and achievements of this paper are as follows.(1)Aiming at achieving the high-quality joint of magnesium alloy AZ61/ZK60,TIG welding and A-TIG welding processes were used and the welding process parameters dissimilar magnesium alloy were explored systematically.Prior to ensuring good weld forming quality and no macro-defects,the effect of welding current,welding speed and coating thickness of activator on the microstructure,alloy composition and tensile test and hardness test of welded joints were carried out.The observed microstructure shows that the dissimilar magnesium alloy AZ61/ZK60 presents well weldability.Base on the relationship between welding parameters and the mechanical properties,the optimum welding parameters are obtained.The welding current is 80A and welding speed is 180mm/min for TIG welding.The welding current is 80A and the active flux coating is 15mg/cm2 for A-TIG welding.(2)Aiming at obtaining the heat distribution of the welded AZ61/ZK60 dissimilar magnesium alloys,the mathematical model of welding pool of dissimilar magnesium alloys under the action of three-dimensional transient moving heat source is established.Besides,the characteristics of temperature field distribution and thermal cycle law of dissimilar magnesium alloys are simulated.The results show that the thermal conductivity of AZ61 and ZK60 is less than ZK60 at the fixed temperature and thus the temperature exhibit asymmetrical distribution.(3)The AZ61/AM60 alloy combination was welded by TIG(A-TIG)method.The effect of the welding current on the microstructure quality and macro defects of the weld was studied.The microstructure-related mechanical propertis,such as tensile strength and hardness tests,were carried out in welded joints and the weldability of dissimilar magnesium alloys by A-TIG method was revealed.The optimum welding process parameters were as follows.The welding current was 115A and the welding speed is 180 mm/min for TIG welding method.The welding current was 110A and the coating amount of active flux was 15mg/cm2 for A-TIG welding approach.Theoretically,the characteristics of temperature field distribution,thermal cycle and pool shape of AZ61/AM60 dissimilar magnesium alloys were simulated and analyzed,compared with the experimental values.The results show that the temperature field of TIG welding of AZ61-AM60 is the same as that of AZ61-ZK60,but the asymmetry of temperature distribution in AZ61-ZK60 is more pronounced.This distinct is attributed to the gradient thermal conductivity of AZ61,ZK60 and AM60,which shows the magnitude relation AZ61<AM60<ZK60.Additionally,the remarked difference of thermal conductivity between AZ61 and ZK60 causes the more obvious asymmetry temperature distribution.(4)Aiming at realizing the combination of light metal magnesium alloy AZ31 and aluminum alloy 6061,TIG(A-TIG)method was successfully used to weld AZ31/6061 magnesium alloy.Also,the residual stresses of dissimilar magnesium/aluminum alloy AZ31/6061 welded joints were simulated numerically and analyzed.The theoretical model accorded well with the experimental results.The transverse stress is compressive at both ends of the weld and tensile at the middle part.