Formability Of Sheets During Multi-directional Dieless Incremental Hole-flanging

Multi-directional incremental sheet forming is the extension of traditional incremental forming technology,which has very different force and deformation behavior from the ordinary bi-axial stretching type incremental forming.Dieless incremental hole-flanging as a type of MISF,studies have shown that it can improve the forming limit of sheet metal,but the related mechanism is still lack of in-depth and comprehensive understanding.This paper selected the SUS304 and AL6061 blank as experiment material and first studied the effects of the lubrication conditions,precut hole diameter,horizontal step size and shaping path on the forming limit of multi-directional dieless incremental hole-flanging,and compared the forming limit of traditional punch working hole-flanging with the forming limit of MISF hole-flanging;Secondly,according to the deformation characteristics of MISF,discontinuous tensile experiment was designed to investigate discontinuous deformation on the dieless incremental hole-flanging deformation;Based on the material characteristics of SUS304,microstructure analysis and ferromagnetic test were used to study the deformation induced martensite phase transformation phenomenon influence on SUS304 incremental hole-flanging forming limit.Main conclusions are as follows:(1)Experiment found that compared with the punch working hole-flanging,whether AL6061 or SUS304 plate,multi-directional dieless incremental hole-flanging can improve the forming limit,to some extent.and the forming limit of AL6061 plate improves more than SUS304 plate.Overall,dieless incremental hole-flanging forming limit of AL6061 sheet is greater than the SUS304 sheet.(2)For AL6061,good lubrication conditions can greatly improve the hole-flanging forming limit.However,the surface lubrication condition has little effect on the forming limit of SUS304 plate.The lubrication effect of molybdenum disulfide is better than oil,but it's more convenient to use oil;The effect of precut hole size on hole-flanging forming limit by MISF is relatively obvious.With the increase of precut hole size,the forming limit of hole-flanging is smaller;The horizontal step size ?X has a significant effect on the forming limit of the MISF hole-flanging.For AL6061,When the horizontal step size ?X is small,the hole-flanging forming limit gradually decrease as the ?X increases,but along with further increase of horizontal step size ?X,its effect on the hole-flanging forming limit is gradually reduced.On the contrary,in the case of SUS304 plate,with the increase of the horizontal step size ?X,the incremental forming limit is gradually increase;The forming limit of contour forming path and spiral forming path are generally consistent,both of the shaping paths can get good results.(3)Discontinuous tensile deformation(multiple scattered deformation)is helpful to improve elongation of AL6061 plate,as the load interval ?S decrease(repeated loading times increase),the elongation increase gradually.But for SUS304 plate,discontinuous tensile deformation will causes the elongation to decreases,and the elongation reduces further as ?S decreases.Overall,the discontinuous deformation has limited impact on the forming limit of dieless incremental hole-flanging.(4)Along with the rising of strain,the deformation induced martensite content also increases;The discontinuous tensile deformation is helpful to transform austenite to martensite.With repeated loading times increase,the amount of austenite to martensite transformation is greater.But in discontinuous tensile deformation and dieless incremental hole-flanging of SUS304 plate,multiple scattered deformation induced martensite do not have enough plasticizing effects.