Weldability Of The Advanced Hot Forming Steel With Large Product Of Strength And Elongation And Its Expulsion Mechanism

In recent years,with the continuous improvement of lightweight and safety requirements for car bodies,hot stamping steel is increasingly used as key parts of car bodies.Recently,a new type of advanced hot forming steel(AHF)with the product of strength and elongation more than 20GPa%has been developed successfully.The parts are thinner,and can be produced by existing hot forming process and equipment.At present,as one of the main connection processes of automobile body,resistance spot welding technology accounts for about 90%of the workload of automobile body in white assembly.However,the weldability of this new type of hot forming steel,especially its match with resistance spot welding process,has not been verified.Therefore,the weldability and expulsion mechanism of resistance spot welding of AHF steel were studied in this paper,and the resistance spot welding process matching its physical metallurgical characteristics under actual working conditions was developed.The main research contents of this paper are as follows:(1)Orthogonal design was adopted to establish a regression mathematical model in which welding current,welding time and electrode force were process parameters,and the welding nugget diameter and the Maximum shear force of spot welding joints were target quantity.Then the relationship between the weld nugget diameter and the process parameters and the relationship between the maximum shear force and the process parameters were established.The four-dimensional diagrams were drawn,and they reveals the change rules of the quality of the spot welding joint with the process parameters.(2)Genetic algorithm was used to optimize the resistance spot welding process,and the optimal resistance spot welding process parameters for AHF steel were obtained as follows:welding current 8kA,electrode force 2kN,and welding time 20cyc.The calculated maximum shear force of the spot welded joints under the optimal process condition is 19.4kN,and the deviation from the test value is 5.9%.(3)The austenite equilibrium transition temperature and grain size of AHF steel were tested.Combined with the parent material composition,JMatPro software was used to simulate the phase transition point and phase transition process of AHF steel under the continuous cooling condition.The isothermal transition curve(TTT diagram)and continuous cooling transition curve(CCT diagram)of the supercooled austenite were obtained.The temperature-expansion curve and final micro structure information of AHF steel under continuous cooling condition were measured by phase change meter,and the CCT curve obtained by simulation calculation was calibrated.Meanwhile,according to the TTT curve,some necessary physical properties of AHF steel,such as density,resistivity,thermal conductivity,rheological stress and elastic modulus,were calculated to provide data to the simulation of resistance spot welding process.(4)A numerical model for resistance spot welding of AHF steel was developed based on the simufact welding software platform to reveal the current density distribution,temperature field distribution and the thermal cycling curve of spot welding joints.The evolution process of the microstructure of spot welding joints was obtained.The results show that the microstructure.of the spot welding joint is mainly martensite and residual austenite.The higher the mass fraction of carbon in the austenite,the lower the Ms and Mf points,which leads to the difficulty in the transformation from austenite to martensite and is conducive to the formation of carbon-rich austenite residue.The numerical simulation of multi-spot welding indicates that the welding spacing of AHF steel with a thickness of 1mm should be no less than 20mm.(5)Logistic method was used to reveal the expulsion mechanism of spot welding under complex conditions(with influences of external factors such as workpiece assembly,equipment stability and operation stability),establish the expulsion curve of resistance spot welding nugget,and predict the expulsion probability of this material under complex working conditions.On this basis,the weldability lobe of AHF steel with expulsion probability information was established with the minimum nugget diameter and expulsion of spot welding as boundary conditions.Power balance model about expulsion was built.Expulsion was determined by expansion energy of liquid nugget and the fracture work.The relationship between expansion forces and the temperature of liquid nugget was established.Constitutive equation of high temperature rheological peak stress of plastic rings was also established.At the same time,the expansion force and rheological peak stress of plastic ring in molten nugget of AHF steel at high temperature were calculated.The Expulsion boundary of resistance spot welding was established.The results in this paper have accumulated reference data for the research on the weldability of high strength steel plates and provided experimental basis to establish the welding process and operation specifications of automobile body.