| As the country of more automobile production,fuel consumption and traffic safely are becoming more and more prominent in our country.The factors such as economy,environmental protection and security are paid great attention by people gradually.Automotive light-weight is one of the most direct and effective ways to solve these problems.Hot-dip galvanized DP steel with good corrosion,stamping and good stamping performance has become light quality and high safety of furture developmening main materials.Usually,the ordinary passenger has about 3000?6000 resistance spot welding spot.The strength of solder joint directly influences the lifetime of the car and safety performance.The research on the resistance spot welding process and the dynamic performance of spot welding joint of hot-dip galvanized DP steel has become the technical basis for advancing the application of advanced high strength steel material in the domestic automobile manufacturing industry.Based on the hot-dip galvanized DP steel(DP600,DP800 and DP 1000)as the research object,the resistance spot welding process and the microstructure and properties of spot welding joint were systematically studied.The main conclusions of this paper are as follows:(1)The resistance spot welding process of hot-dip galvanized DP steel was studied.The welding process was optimized and the welding window was established.The research indicates:The optimized welding current of DP600 was 7.5kA?9.0kA.The optimized welding time of DP600 was 12cyc?28cyc.The optimized electrode pressure of DP600 was 3.OkA?4.5kN.The optimized welding current of DP800 was 7.0kA?9.0kA.The optimized welding time of DP800 was 16cyc?28cyc.The optimized electrode pressure of DP800 was 3.0kA?4.5kN.The optimized welding current of DP1000 was 7.0kA?8.5kA.The optimized welding time of DP1000 was 20cyc?24cyc.The optimized electrode pressure of DP1000 was 3.0kA?4.5kN.(2)The microstructures and hardness distributions of spot welded joints of hot-dip galvanized DP steel under different welding processes were studied by using electron microscope and hardness tester.Experimental results show:The spot welding joints of hot-dip galvanized DP steel had three parts that are fusion zone,heat affected zone and base metal.The structure of the fusion zone was mainly composed of coarse lath martensite,showing columnar crystal state.The structure of the heat affected zone was mainly composed if ferrite and acicular martensite-The structure of the base metal was mainly ferrite and island martensite.The hardness of the fusion zone was highest.And hardness of heat affected zone decreases rapidly.The spot weld joint of DP1000 were affected by softening.The hardness of the heat affected zone near the base material was lower than that of the base material.(3)The dynamic performance of hot-dip galvanized DP steel was studied by dynamic impact test.Research indicates:in the optimized welding process window,the dynamic impact property of spot welding joint of hot-dip galvanized DP steel was highter than that of static tensile property.If the diameter of the welding core was the larger,the dynamic impact performance was better.The failure of spot welding joint was divided into interface separation and welding nucleus extraction.The mechanical properties of spot welding joint with failure mode of welding nucleus extraction were better.(4)The resistance spot welding process,microstructure and properties of hot-dip galvanized DP steel with diffrerent strength were analyzed.As the strength of hot-dip galvanized DP steel increasing,welding process window became narrow,weldable became worse and the more prone to splash welding.In the optimized welding zone,strength of hot dip-galvanized DP steel was increased,its welding core diameter,static tensile property and impact resistance were improved;martensite structure in the spot welding joint increases and organization was more coarse;hardness of spot welding joint increases. |
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