Micorstructures,Mechanical Properties And Failure Analysis Of High Strength 590 MPa Welded Steel Joints Of Wheel Rims

The use of lightweight materials in the auto industry,such as aluminium alloys and high-strength steels,is an effective way to achieve automobile light weighting.The wheel rim is an important component of the car,and considering the bearing capacity and safety performance of the wheel rims,high-strength low-alloy(HSLA)has been widely used in the production of automobile rims.As a typical resistance welding,flash butt welding is the mainstream method in the production of wheel rims because of the advantage of high welding speed,efficiency and high strength in welded joints.With the increase of HSLA steel strength,the cracking frequency of the flash butt welded joints in the forming process increases.Therefore,the 590 CL high strength steel plate was employed as the base metal of wheel rims.First of all,the microstructures and mechanical properties of flash butt welded joints were investigated.Secondly,the effect of welding parameters on microstructures and mechanical properties of flash butt welded joints by adjusting welding parameters separately was evaluated.Then the fracture mechanisms of welded joints in the flaring and expanding procedure of were analyzed.Finally,the microstructures and mechanical properties of welded joints in the condition of air cooling were studied,and the strain of wheel rim in the flaring process was obtained by ARGUS strain measuring system.The results indicated that the composition of 590 CL HSLA steel met the standard of rims steel,the weld was composed of Widmanstatten ferrites and granular bainite,and the base metal showed the ferrite grain and pearlite with a banded structure.The ultimate tensile strength and impact absorbing energy of welded joints were 642 MPa and 96.6J,respectively.The maximum micro-hardness value occurred at the weld,which was mainly related to the acicular ferrite and granular bainite.The welding parameters such as flash allowance,upset allowance and upset pressure obviously affected the microstructures and mechanical properties of flash butt welded joints.With the increase from 4 to 8mm of flash allowance,the Widmanstatten ferrite in the weld become serious and the amount of inclusions increased,mechanical propertiesof welded joints reduced,which was due to the welding heat input.As the upset allowance changed from 2.5 to 5.5mm,the amount of inclusions in the weld tended to decrease.The Widmantatten ferrites in the weld got serious and the amount of inclusions decreased with the increase from 6 to 9MPa of upset pressure.Combined with the practical joints cracking data of wheel rims,when the flash allowance,upset allowance and upset pressure were 4mm,3.5mm and 8MPa,the mechanical properties of welded joints was the optimal,the cracking frequency of flash butt welded joints was lowest at 2%.The fracture mechanisms of flash butt welded joints in the flaring and expanding procedure were investigated from the macroscopic and microcosmic angle,the failure mode showed the brittle fracture and ductile fracture.The coarse Widmanstatten ferrite in the weld was the major reason for joints cracking in the flaring procedure,the crack propagated from the weld to HAZ due to the brittle-ductile transition caused by the coarse grain.Comprehensive factors such as thinning phenomenon in the HAZ,weld inclusions,the amount of micro cracks and welding residual stress concentration were the cause of joints cracking in the expanding process.The mechanical properties of welded joints would be improved with the method of air cooling after welding.The microstructure in the weld showed the granular bainite and without Widmantatten ferrites.The tensile strength and impact absorbing energy of welded joints after air cooling were 652.7MPa and 101.1J,respectively.In addition,the principal strain,secondary strain,and thickness reduction ratio of rim in the flaring process were obtained by ARGUS strain measuring system.The deformation of wheel rim in the flaring process was not big as a whole,the deformation mainly concentrated on the edge of the rims.The strain in the marginal region contained welded joints was lower than other region,which indicated that the welded joints in the edge of rim tended to fracture under the huge flaring force.