Research On Parameters And Mechanical Properties Of Resistance Spot Welding Joints Of 22MnB5 High Strength Steel

In order to comply with the fuel consumption in 2020 required by “Energy saving and new energy automobile industry development planning(the year from 2012 to 2020)”,the FAW Group proposed technical solutions in terms of reducing energy needed for an automotive to run,improving the conversion efficiency from heat energy to kinetic energy,reducing the loss of energy transfer,reducing auxiliary energy consumption and optimizing the management of automotive energy.Among all these solutions,the most substantial and effective one is to reduce the weight of automotive body to achieve lightweight auto-body.The application of hot stamping high-strength steel can reduce the weight of auto-body effectively in the premise of improving automotive collision performance,which achieves energysaving and emission-reduction.The FAW Group plans to substantially increase the proportion of hot stamping steel in the developing models.Therefore,it is important to study the spot welding parameters and mechanical property of the welding joint for realizing lightweight auto-body and complying with the requirement of fuel consumption.In this thesis,the spot welding parameters and mechanical properties of the welded joint were studied.The influence of welding current,electrify time and electrode force on mechanical property of the welded joint is studied through orthogonal experiment.The results showed welding current contributed great influence,electrify time was less critical and electrode force was the least critical.The optimal welding parameters also was found in this thesis.This thesis focused on the influence of welding current and electrify time on nugget diameter,indentation depth,microstructure and mechanical property.The results showed that with the increase of welding current the nugget diameter first ly increased and then decreased,the indentation depth increased,the martensite grains grew dramatically in the fusion zone,the area of HAZ expanded and the quantity of Martensite-Austenite constituents and carbonide increased,the bearing capacity of the welded joint firstly increased and then decreased.With the increase of electrify time,the nugget diameter firstly increased and then became stable with no growth,the indentation depth increased,the martensite grains grew dramatically in the fusion zone,the area of HAZ expanded and the quantity of Martensite-Austenite constituents and carbonide increased,he bearing capacity of the welded joint firstly increased and then decreased.Moreover,the defects in spot welding of hot stamping steel were researched and the process measures were proposed to prevent the defects in this thesis,it was found that the welding current contribute a noticeable impact on welding spatter,with the increase of welding current,the welding spatter was more serious.With proper welding parameters,the welding spatter may be prevented.A low heat input was not recommended as it may result in shrinkage.Finally,the research results were applied on the FAW new developing Model D090,a qualified rate of 99.5% was achieved in the welded joints.