Study On Porosity Formation Mechanisms And Processes In Welding Of Galvanized Steel Sheets For A Zero-gap Lap Joint Configuration

With the improvement of the industrial level,people put forward higher requirements on the quality and service life of products.For the automotive industry,the life of a product is closely linked to its corrosion resistance.Galvanized steel sheet has been widely used in the field of automotive due to its good corrosion resistance.However,since the boiling point of zinc is906?,and the temperature of the molten pool is often above 1500?during the fusion welding process.Therefore,the zinc coating would be heated and vaporized to form a large amount of zinc vapor.If the zinc vapor enters the molten pool and doesn't escape,porosity will form in the weldment.A large number of porosities would deteriorate the quality of weldment,and affect the quality of welding joint.In order to reduce the porosities in the welding of galvanized steel sheets for a zero-gap configuration,the cross sections of weldments under different welding conditions were studied in this thesis.The results show that with the increase of heat input,the number and size of the pores in the weldment also increase,and the shape of the pores change from point and ellipse to strip and tadpole.At the same time,the oxide layer exists on the surface at the rear of molten pool.The thickness of oxide layer increases with increasing heat input,and the oxide layer impedes the escape of zinc bubble.According to the observation,the model of porosity formation mechanisms in welding of galvanized steel was established.On the basis of the model,there are two methods to suppress porosities in the welding of galvanized steel sheets:promote the escape of zinc vapor from the high temperature zone;reduce the amount of zinc vapor entering the molten pool.Based on the theory of accelerating the release of zinc vapor in high temperature region,CMT welding mode was adopted,and the influences of different process parameters were investigated.The results show that the porosities could be reduced by using the welding wire with low content of Si and the appropriate protective gas in the condition of little torch angle and suitable drag angle.Based on the theory of reducing the amount of zinc vapor entering the melt pool,Pulse-MAG welding was used in this thesis,and the influences of different process parameters on the opening of molten pool were studied.With higher peak current,moderate voltage,suitable welding speed and lower CO₂ content,stable opening of molten pool could be created,and thus the weldment has lower porosities and good weld quality.Both methods can achieve a satisfied lap joint strength.