Study On Low Cycle Fatigue Property And Behavior Of High-strength Steel Weld Joints For Automobile Application

High strength steel can achieve in reducing vehicle weight and emissions without reducing vehicle safety.Tensile strength and elongation of current high-strength steel can cover most of the automobile parts.Static mechanical properties and processing properties of high-strength steel has been understood deeply.However,vehicles are affected by alternating load because of the impact blow from the ground on dynamic condition.Using tensile property only cannot describe the properties of the material.The fatigue property of the material directly determines the service life of the automobile.In addition,welding process is a necessary process in automobile manufacturing.Owing to the welding thermal cycle,significant changes in the microstructure and properties happen to the joint,and there will be residual stress distribution in joint.These mean that during manufacturing vehicles not only the fatigue property of the base metal should be considered,but also the fatigue property and fatigue mechanism of the joint should be considered.In this paper,the fatigue property and fatigue mechanism of the HR550LA+Z and its butt joint are concentrated on.By material testing and microstructure analysis,types of fatigue crack sources are identified,and the reasons why fatigue failure of HR550 LA is caused are discussed,in order to provide experimental and theoretical basis for the life estimation of HR550 LA in automobile structure.First,the GMAW welding process of 2.4mm HR550LA+Z is tested.Short-circuiting,CMT and pulse current mode are used.The processing windows of different transfer modes are got by taking shaping,zinc layer burning,welding efficiency and microstructure.Compared with CMT and short-circuiting mode,GMAW-P weld can achieve in high speed welding and low heat input,making welding efficiency and zinc layer protecting leveraged with current from 100 A to 160 A and welding speed from 55cm/min to 125cm/min.For 2.4mm HR550LA+Z,GMAW-P weld is the recommended mode.At the same time,HR550LA+Z laser welding butt joint is got by processing test.In addition,by using the recommend GMAW-P weld butt joint and laser-weld joint,fatigue properties are tested,getting the fatigue parameters by regression.The statistic result shows that both of the laser-weld joint and GMAW joint have worse fatigue performance no matter what kind of load.The larger amplitude is,the larger fatigue life decrease happens to the joints.Under different kind of loads,laser-weld joint and GMAW joint performs differently on fatigue property.Under the same strain amplitude,GMAW-P weld joint has a better fatigue performance but under the same stress amplitude,laser-weld joint performs better.The author also does the statistics on HR550 LA crack initiations and crack position by SEM observation.It has been found by EDS analysis and hardness test that under large stain amplitude,PSB crack initiation happens on fusion zone outside because of the soft zone,and that under small strain amplitude,Si and Al oxide serve as the crack initiation in the welding zone.