Research On The Viscous Pressure Bulging(VPB) Tests Of The Ahss Sheet

Recently,as the increasing demand for light weight of automobile,The advance high steel sheet(AHSS)have been used in automobile.Unfortunately,the room temperature formability of AHSS is poor.The common defects,fracture,wrinkling and springback,usually occur during the pressure forming process,which limit their further applications.Viscous pressure forming(VPF)technology is a newly proposed flexible die forming technology.Previous researches show that VPF technology can improve the room temperature formability of metal sheet.It can be used to the forming of complex shaped parts with difficult deformed materials..Therefore,the mechanical properties and formability of three AHSS with strength of about 1000 MPa are investigated though the viscous pressure bulging(VPB)tests in this work.Firstly,the mechanical properties of AHSS with strength of about 1000 MPa are investigated through the uniaxial tensile tests.The tensile tests with respect to the rolling direction of 0oand 90 oare conducted to analysis the anisotropy of the three AHSS.The plasticity of the three AHSS is studied,comparatively.Secondly,the effect of different degree of deformation on the microstructure of the three AHSS is researched through the uniaxial tensile tests.The microstructures of the three AHSS at different degree of deformation are observed.The relationship between the deformation degree and the microstructure are captured.Finally,the VPB tests of the three AHSS with strength of about 1000 MPa were carried out to investigate the formability of complex AHSS parts under biaxial stress state.The curves of bulging pressure and bulging height against time were obtained.Meanwhile,the ultimate forming stress of three kinds of AHSS was determined by bulging to fracture.The ultimate strain values and strain distributions of different AHSS were studied by testing the strain of typical bulging samples on ASAME(Automated Strain Analysis and Measurement Environment).Furthermore,the flow stress curves of steel sheet under biaxial stress were determined by analyzing the curvature radius and wall thickness at the highest point of bulging parts.Based on the biaxial tensile test through the experiment of VPB,the data of bulging specimens with the application of DP980,QP980 and TWIP980 are 28 mm,32.3mm and 44.8mm in terms of maximum height respectively,1490 MPa,1456MPa and1417 MPa in terms of maximum effective stress respectively,0.3,0.4 and 0.77 in terms of maximum effective strain respectively.By comparing among all the materials employed,the formability of TWIP980 could be sorted as the best one.For the remaining two kinds of AHSS,QP980 is better than DP980 in respect offormability.With further study,the curve illustrating the relationship between the bulging height and pressure is more approximated by power function.In comparison between the uniaxial tensile test and the biaxial tensile test conducted by VPB using the same material,the true stress measured by the latter one is larger than the other under the circumstance of identical strain.