Study On Microstructure And Properties Of TWIP Steel Joints Welded By AC Resistance Spot Welding

Reducing weight, inducing the oil consumption, lowering the emission and adding security are the important development direction of today’s car manufacturing technology. However, TWIP (Twinning Induced Plasticity) has higher tensile strength (about600Mpa), higher elongation (>60%), impact absorbing energy and lower temperature brittle transition temperature. Therefore, using TWIP steel for automobile steel sheet can significantly reduce weight, enhance body impact resistant ability and increase the safety of the car. Thus,TWIP steel becomes one of the great potential lightweight materials as automobile manufacturing industry nowadays. With the widely application of the TWIP steel, the welding problem is prominent increasingly in the production, so the TWIP steel welding problems need to be solved. For TWIP steel, study on microstructure and properties of ac resistance spot welding joint is few, so the inpact of the welding heat、energizing time and the pressure time on mechanical properties of1mm TWIP steel joints welded by ac resistance spot welding was investigated. In addition, microstructure, mechanical properties and corrosion of the different delivery conditions (different annealing state) of the welding joints were studied. Main conclusions are as follows:1. The inpact of the welding heat、energizing time and the pressure time on mechanical properties of1mm TWIP steel joints welded by ac resistance spot welding was investigated. Under the same energizing time and pressure time, with the increase of the welding heat, indentation and patter increase gradually and the shear force and nugget diameter also increase. Under the same welding heat and pressure time, with the increase of energizing time, indentation and patter increase gradually and the shear force and nugget diameter also increase, whose principle is similar with the welding heat. Under the same welding heat and energizing time, with the increase of the pressure time, indentation、patter and nugget diameter increase, while the shear force reduces when the value of it reaches the max. In the best welding process parameters (welding heat:2.1KJ;energizing time:1.9s;pressure time:5s),the joints forming is good and the nugget diameter achieves5.54mm. the value of tensile shear force reaches4.939KJ.2. The microstructure of TWIP steel welded by ac resistance spot welding is mainly composed of nuclear zone and heat affected zone(HAZ). Solidification process of welding nugget begins with the boundary, which belongs from spontaneous nucleation. And the nugget edge characteristics epitaxial solidification obviously. The molten metal within the nugget forms nucleation from unmelted metal grain. Under the lower degree of supercooling, the columnar crystal perpendicular to the fusion lion grows to nugget center. The grains of TWIP steel HAZ grow up obviously and there is no phase transformation occurs. In fact, it is still pure austenitic organization. The microstructure of joint melt nuclear zone is mostly columnar crystals which are perpendicular to the fusion line. The average microhardness of the melt nuclear area is259.9HV which is significantly higher than the base metal and HAZ appears to soften. The crack of the welded joints are the one along the crystal boundaries.The ambient fracture of the welded joints is ductile fracture with dimples and the central is brittle fracture which appears river pattern.3. The great changes of TWIP microstructure have taken place after welded by ac resistance with different annealing time. With the increase of annealing time, annealing twin in parent metal increases and size of them grows rapidly. At the same time, austenite grains in HAZ grow up and isometric crystals in molten core zone increase gradually. With the increase of annealing time, microhardness of parent metal, heat affected and core zone are increased and the corrosion of welding joint improves.