| Rail transit is a key national infrastructure and an important basic industry,which plays an irreplaceable overall supporting role in China’s economic and social development,improvement of people’s livelihood and national security.The next generation of high-speed trains will be faster,safer,more environmentally friendly,more economical and more comfortable.On the premise of ensuring the strength and stiffness,reducing the weight of the vehicle as much as possible can not only reduce the energy consumption,but also have a significant impact on the stability and safety of the train movement,which is conducive to increasing the comfort of the ride and extending the service life of the vehicle structure and line.The important way to realize the lightweight of the next generation high-speed train is to use lightweight and high-performance structural materials.The bogie structure of rolling stock is generally made of stainless steel or low carbon alloy steel by MAG Welding.In order to further improve train reliability and complete train upgrade manufacturing,it is necessary to explore the welding applicability of higher strength steel grades and carry out lightweight design of structure.In this paper,the low carbon bainitic high strength steel with good Mechanical properties obtained through TMCP（Thermo Mechanical Control Process）cold-controlled rolling Process is selected as the research object,and laser-MAG composite welding Process is adopted to conduct in-depth research on its weldability and joint performance.In this paper,the characteristics of laser-MAG composite welding and the stability of welding process are analyzed by graphic method,and the process evaluation and optimization are carried out.The effects of laser power,welding speed and wire feeding speed on weld forming quality are studied.By analyzing the graphic characteristics of arc shape,a method to evaluate weld forming quality was proposed by using its probability density curve.Finally,the relationship among graphic characteristics of arc shape-composite welding process characteristics-weld forming quality and defect formation was established,according to which the optimized process parameters were obtained:Power4.8k W,welding current 308 A,welding voltage 29.9 V,welding speed 26 mm/s,wire feeding speed 13 m/min,group pair gap 1.2mm.Then,physical field using COMSOL simulation software of quasi steady state method of laser-MAG compound for numerical simulation of welding process,taking into account the arc,drop,such as heat source,and the molten pool stress（shear force,electromagnetic force,plasma Ma Lange,etc.）influence on the molten pool,the analysis of laser-MAG compound welding the temperature field and flow field in the vicinity of keyhole evolution process,Based on the numerical simulation application in this paper,a flow-thermal coupling simulation strategy suitable for large size test plate is proposed to improve the simulation accuracy of the temperature field of laser-arc composite welding.The thermal cycle characteristics of laser-MAG composite welding was obtained.Furthermore,based on the characteristics of thermal cycle parameters in each microzone of laser-MAG composite welding joint,the Gleeble thermal simulation equipment was used to fit the thermal cycle process of each microzone in the heat affected zone during welding process,and simulate the thermal physics process in the heat affected zone of the welding joint under the characteristics of laser-MAG composite heat source.The influence law of thermal cycling parameters on microstructure of micro-zone in welding heat affected zone was obtained,and the relationship between welding process,micro-zone microstructure characteristics and basic mechanical properties of low carbon bainite high strength steel was established.Based on this,the microstructural control strategy of laser-MAG composite welding joint of bainitic high strength steel was established.When the control line energy is less than 5.55 k J/mm and the coarse-grain zone t8/5 is greater than 10 s,the laser-MAG composite welding joint of bainitic high strength steel with excellent mechanical properties can be obtained under the premise of good joint forming.Finally,the laser-MAG high strength steel weld of low carbon bainite zone,coarse grain zone,incomplete phase transition area and the base area to study the low-temperature impact properties,using OM and EBSD to analysize the characteristics of impact process of crack propagation and fracture morphology.Based on the microstructue of joint group differences,characteristics of grain orientation and analysis of different HAZ impact fracture mechanism of the microstructure were revealed.In order to evaluate the service performance of the welding joint of the low carbon bainite high strength steel,the fatigue of the laser-arc hybrid welding joint was tested.By fatigue strength test and the microscopic fracture characteristics of fatigue crack observation,to analyze its fatigue failure mechanism,and to establish conforms to the low carbon bainite steel compound laser and electric arc welding head of fatigue life prediction model,the model of predicting the fatigue life and fatigue life in double real error band,has a high prediction precision. |
