Research On Microstructure Performance Of Welded Joints And Arc Characteristics Of Aluminum Alloy Welding With Shielding Of Multiple Gases

As with the development of the lightweight and high strength material research and development,most countries of the world have stopped the manufacturing of trains with carbon steel and turn to use the lightweight aluminum alloy to manufacture in order to achieve the aim of a lightweight and high speed railway transit system.In order to accelerate the development of domestic high speed rail transit process, China has made a lot of investment in the high speed train research.Nowadays the MIG welding with pure Ar gas is the mainly adopted welding method in the welding production of high-speed trains in our country.But this method exists the faults of insufficient static strength and fatigue performance of welded joint faults.So it is of great importance to improve the service performance of joints. If we can through changing the type and composition of the welding protective gas to achieve the goal of improve the organization and comprehensive mechanical properties of joints, it will play a a role to the rapid development of industry of our country’s high-speed train.In this article,we will use the pulse MIG welding method to weld the A7N01 S aluminum alloy of 12 mm thickness with the welding protective gas of pure Ar、Ar +He and Ar + He + N2.Then we will study the similarities and differences of the electrical signals 、the shape of the arcs and the organization and performance of the welded joints. In the bead welding,the arc current with pure Ar as protective gas is higher than that with multiple gases under the same parameters,while the rule of voltage of weld arcs is just opposite.And the arcs morphology with different welding protective gas are all bell shape.Due to the more concentrated heat source and its high concentration factor,the welding arcs contract significantly comparing to the arc with pure Ar as the welding protective gas.So does the the width of the welding arc.Using the parameters of the welding joint in butt welding, we can set up the finite element model and obtain the result of the temperature field simulation of welding.The results says the area aside the weld metal which is above a certain temperature are not all the same. The area of the joint welded with multiple mixed gases are smaller than that of the joint welded with pure argon.This also shows the difference of the temperature fields changing with the different protective gases on the other hand.While it also lead to the difference of the organization and mechanical properties of thewelded joints. Through the electron back scattering and EDS analysis,we find that the distribution, shape and number of the white phase in base area and the heat affected zone are roughly the same,but is quite different with that in weld metal.The distribution of the white phase in weld metal has obvious trends of along the grain boundaries,namely the grain boundary segregation phenomenon.The EBSD analysis results show that the grains of base metal are elongated plate strip, but the HAZ grains size are significantly bigger than base metal grains.The grains in the center area of the weld metal of three joints welded with different protective gases are all equiaxial crystal morphology,but the grains size of joint welded with pure argon are significantly bigger than that of joints welded with multiple gases,especially the joint welded with Ar + He+ N2 mixed gases has the smallest grains size.There is no N element in the weld metal of joint welded with ternary mixed gases in the STEM analysis,which states that N dose not take part in the metallurgical reaction in molten pool.Mechanical performance analysis are carried out on the three kinds welded joints,it reveals that those three kinds welded joints have the same changing rule of hardness,but the joint which is welded with pure argon has the lowest hardness.Meanwhile other mechanical performance of the joints welded with pure argon such as the tensile strength,impact toughness and fatigue strength are also lower than that of the joints welded with other two kinds mixed gases.In conclusion,the joints welded with multiple mixed gases have better comprehensive performance than that with pure argon as the protective gas.