Research On Effect Of Pre-stretch Deformation On Formability Of Aluminum Alloy 5052-O Sheet Metal

Aluminum alloy sheet is widely used in aerospace,rail transportation,automobile industry,electric power equipment,construction engineering and other fields because of its high specific strength,high specific stiffness and excellent comprehensive mechanical properties.As the criterion of instability fracture of sheet metal plastic forming,forming limit diagram(FLD)can assist in the formulation and evaluation of aluminum alloy sheet metal forming process effectively.However,the conventional FLD can only reflect the forming limit strain of sheet metal under a single strain path.Once multiple composite strain paths appear in the process,the FLD will lose its effect.Therefore,taking the 5052-O aluminum alloy sheet with 1 mm thickness as the research object,the purpose of this work is to study the influence of bilinear composite strain paths on mechanical properties,and determine the FLD of pre-stretching sheet metal by combining experiment and finite element simulation,which provides theoretical support for formulating the forming process of aluminum alloy sheet rationally.The main work and conclusions are as follows:(1)A set of fixtures and samples for pre-stretching of the large sheet metal is designed by ABAQUS finite element simulation software.Three parameters are considered in the design of the fixture:the length-width ratio of the sample,the number and distribution structure of the pin holes,and the blank holder clearance.The pre-stretching of sheet metal with stretching width of 190mm could be obtained,which can minimize stress concentration,avoid skidding,and ensure that the pre-strain is distributed along the center of the sheet evenly.The pre-strain of 1.5%,3%,5.5%and 7.5%are obtained for the self-designed type Ⅰ pre-stretching samples,and 3%,5.5%,7.5%and 10%are obtained for the self-designed type Ⅱ pre-stretching samples,using the pre-stretching fixture.(2)To study the effects of pre-strain levels and direction on mechanical properties such as flow behaviors and anisotropy,the uniaxial tensile tests of pre-strain aluminum alloy sheet are carried out.The results show that the pre-strain mainly affects the yield deformation stage of the material.When the direction of pre-strain and sample are different,the sample shows transient effect that tends to be obvious with the increase of the level of pre-strain.It has little effect on the mechanical properties of materials in the uniform plastic deformation stage.(3)The forming limit curve(FLC)is obtained by the hemispherical punch bulging experiment of pre-stretching aluminum alloy sheet,and the conversion between FLC and forming limit stress curve(FLSC)is realized based on MATLAB.The influence of pre-strain on FLC and FLSC of sheet metal is studied,and it is found that the pre-strain causes the FLC of sheet metal to shift along the pre-strain path,while it has little effect on the FLSC.(4)The finite element model of hemispheric punch bulging experiment is established by ABAQUS,and the influence of FLSC fracture criterion and conventional instability criterion on FLC under bilinear composite strain paths is compared in the finite element simulation.It is found that the FLSC fracture criterion is more accurate,which is consistent with the experimental results.