Research On Micro-morphology And Prediction Of Microstructure Of RSW Nugget

Resistance spot welding (RSW) has the advantage of high efficiency, easy automation, reliability, and stability, which made it an essential material processing method. It is an important analysis method in material processing to optimize welding parameters by numerical simulation and therefore enhance the joint quality. Now there is little numerical simulation on the microstructure of spot welding, so this paper studies the microstructure, and the morphology of the spot welding nugget based on the numerical simulation.Firstly, established a model based on heat, electrical coupling to calculate the temperature field, which considered Joule heat supply, convection boundary conditions, and the thermal physical properties of the material varying with temperature. The spot welding temperature field is calculated based on the ANSYS heat and electrical element; we received the heating characteristic and the thermal cycle curve of resistance spot welding. Then according to the nugget cooling rate and CCT chart, we got the microstructure of spot welding nugget, and established a method to prediction the microstructure of the spot welding nugget. In this way, we researched the nugget microstructure of Q235, with the experiment parameters as follows: 8.6KA current, 240ms electrified time, 1600N pressure. The nugget microstructure is mainly made up of bainite, with little martensite and ferrite. Then we verified the correction of the conclusion by experimental actions.Also established a model to simulate the morphology of the spot welding micro- structure based on cellular automata method, which calculated by refining the grid based on the macro temperature field calculation. Crystals are heterogeneous nucleated by continuous forming formula, and grown up by capturing the liquid state cells all around, and the effect of temperature gradient and ingredient overcooling was considered. The complex calculation results were displayed in real-time images with different colors means different crystals, which makes the result visual and straightforward.The two typical kinds of microstructure of spot welding nugget were obtained through the MATLAB programming and calculating: the columnar microstructure and the columnar-equiaxed microstructure. The experiment indicated that the low carbon steel spot welding nugget have thick columnar crystals, and aluminum alloy spot welding nugget are made up of columnar- equiaxed crystals, which coincides with the numerical simulation result, and meanwhile verified the microstructure morphology simulation method based on the cellular automata method is feasible.