Effect Of Repeatedly Repair Welding And MIG Welding Process Parameters On Microstructure And Mechanical Properties Of A7N01P-T4 Aluminum Welding Joints

Aluminum alloy more and more widely used in railway transportation especially application of rail vehicles,a new aluminum alloy material—A7N01P aluminum alloy was introduced into the vehicles,but its welding procedure will bring some inconvenience,the repair welding on the reasonable welding process and the number of welding is imminent in particular.At the same time,the right welding parameter of A7N01P aluminum alloy selection is crucial.This article introduced on A7N01P-T4 aluminum alloy MIG welding of 4mm and16mm thickness butt joints in different times repair welding and different welding process under nondestructive test,tensile test,bending test,micro hardness test and metallographic test,the results of the study show that:All butt joints forming in different number of repair welding and different MIG welding process is good,the grades of X—ray flaw detection test are B,and bending properties of all joints are also good.Tensile strength of thin and thick plate joints after repair welding are not up to standard and decreases with the increase of the number of repair welding,but the coefficient of thin plate of repair welding one time and two times and thick plate of repair welding one time are very close to the standard.The Tensile strength of joints under different MIG welding process almost all achieves the standard requirements,tensile strength of thin plate under current line energy and no preheating are higher and that of thick plate under current line energy and preheating and interlayer temperature both 70-90?are higher.The hardness on weld zone of thin plate and thick plate joints is lowest,thin plate joints have no softening phenomenon,thick plate joints have obvious softening phenomenon just under repair welding,140-160?preheating temperature and directly welding of welding interlayer temperature.Parent metal microstructure of thin and thick plate joints are all fibrous grain extended along the rolling direction and the MgZn₂ strengthening phase distributes on??Al?substrate during the aging process;Heat affected zone microstructure is similar to the parent metal,but grain is slightly bulky and the amount of precipitated phase is more than the parent metal,expressly thin plate joints in big line energy and 140-160?preheating temperature and thick plate joints in big line energy;weld zone microstructure is mainly for the??Al?substrate with??MgZn₂?precipitation phase partly distributes on??Al?,its grain shape is Isometric grain,weld zone grain of thick plate joints in big line energy is bulky;fusion zone grain is relatively bulky and columnar crystal grain is also very obvious when thick plate joints during the increase of number of repair welding and under the big line energy,preheat temperature increasing and interlayer temperature for cooling to room temperature.Repair welding process is eliminating all parts of the butt joint in addition to 1 mm allowance from the original weld fusion line before each time of repair welding,so thin plate joints can be carried out repair welding one time and two times and thick plate joints can be carried out repair welding only one time.Reasonable MIG welding processes of thin plate joints are no preheating before welding and selecting current line energy during welding and thick plate joints are 70-90?preheating temperature before welding and selecting current line energy and 70-90?interlayer temperature during welding.