GMA Welding Technologies And Porosity Formation Mechanism In Weld Joint Of Galvanized Steel DP780

Dual phase steel, as an important steel, is widely used in the automotive industry due to its high strength, no yield extension and no room-temperature aging, low yield ratio, high initial work-hardening rate and good weldability. GMA welding is a widely used technique for its high efficiency and low cost. It provides a lot of convenience as reducing the labor intensity and improving the working environment for the automated production due to its combination with industrial robot technique. Coated zinc layer on the surface is a useful method to prevent the corrosion of steel, but the zinc layer affects the stability of welding process and may cause defects, one of the difficulties is to control the porosity.In this study, the research object is GMA welding technologies and porosity formation mechanism of the 1.4mm thick galvanized steel DP780 with lap fillet joint. Effects of welding speed, wire feed speed, position and angle of the welding torch, wire genuine length and shield gas flow on the stability of welding process were tested under the pulse mode of GMA welding, and the welding parameter ranges under different speed and weld mode were also tested. Result show that the best wire feed speed is 3.2m/min when the welding speed is 10mm/s. The bottom plate is easy to burn through when the heat input is too high, on the contrary, the defects like bite edges and incompletely filled occur. As the best posture, the wire should aim to the root of lap position, the torch work angle and marching angle are 70° and 80 °, respectively. A good weld seam can be obtained when the wire genuine length is 12 mm and the gas flow is 25L/min. Under different welding speed, DC and CMT mode has the best adaption, the adaption of CMT+P mode is the worst, the adaption of P and DP mode becomes worse with the increasing of welding speed.The X-ray non-destructive test was used to analyze the porosity defects in weld seams. The molten pool keeps longer at higher temperature when the heat input is much higher, and the less the porosities. The porosity quantity is the lowest of DP mode due to the molten pool agitation by arc under different welding speed. The arc temperature of mixture gas increases with the increasing of the proportion of CO2, therefore, the molten pool keep in high temperature for longer time and the zinc air bubbles escape more easily, the porosity quantity of welds using 82%Ar+18%CO2 shield gas is less than that of using 90%Ar+10%CO2 shield gas. The gap between the plates is conducive to the spread of zinc vapor, the zinc vapor in molten pool and the quantity of porosities reduce. The quantity of porosities in welds increases with the increasing of cooling speed.The microstructure and mechanical properties of the joint were analyzed, a large number of low strength microstructure were not found. The hardness in the junction of base metal and heat affected zone is the lowest. Result of tensile-shear test show that the strength of joints using 82%Ar+18%CO2 shield gas are higher than that of using 90%Ar+10%CO2 shield gas. The higher heat input lead to less quantity of porosities. When using the same gas, the strength of joint is higher while the heat input is higher. The joints of DC、P and DP mode fracture in the base metal, and the joints of CMT mode fracture in the weld zone.