Experimental Study On Friction Size Effect In Micro Sheet Forming Of Stainless Steel Thin Sheet

With the development of high and new technology and industry,micro system technology and micro electro mechanical system are favored,which makes the demand of micro forming parts and other products increase rapidly.The advantages of sheet metal micro plastic forming technology are prominent,however,due to miniaturization of parts,material mechanical properties and forming mechanism show obvious size effect.Therefore,traditional sheet metal forming technology can not be directly scaled down to micro forming,and friction is a popular research direction in sheet metal micro plastic forming,all aspects need to be further improved.SUS304 stainless steel ultra thin sheet as test object in this paper,effects of material mechanical property size effect and process parameters on the friction of thin sheet were studied,mechanism of friction size effect ws revealed,and friction size effect phenomenon of actual micro deep drawing process ws discussed.Main research contents as follows:1)Different specifications of thin sheets for uniaxial tensile test,the results show that flow stress increases with the decrease of thin sheet thickness or grain size,showing obvious mechanical property size effect of “the smaller,the stronger”;Based on surface passivation film reinforcement,strain gradient strengthening effect,and fine grain strengthening theory of polycrystalline materials,the mechanism of mechanical property size effect was discussed,and the tensile fracture morphology was analyzed.2)Linear reciprocating ball-disc friction test was carried out respectively for thin sheets with different thickness and grain size.The results show that mechanical property size effect does not change friction coefficient to a great extent.The effect of process parameters on friction coefficient of thin sheet with different sizes was studied.Through wear scar morphology observation and section size measurement,the results show that under dry friction condition,friction coefficient of thin sheet increases significantly with the increase of steel ball surface roughness,decreases slowly with the increase of linear reciprocating friction speed,increases slowly with the increase of specimen surface roughness and friction load;Process parameters remain unchanged,thickness thinning has no influence to mechanical mesh or molecular attraction between friction ball and thin sheet surface,which showed no friction size effect.3)Using cylindrical pin-strip specimen contact way,different specifications of strip thin sheets for linear reciprocating pin-disc friction test,the results show that under fluid lubrication condition,friction coefficient increases significantly with the decrease of strip thin sheet width,showing obvious friction size effect;Based on friction morphology observation and model analysis,the friction size effect mechanism was revealed.Using multi-scale precision micro rectangular drawing mold,strip thin sheet of micro deep drawing forming was carried out.The results show that under fluid lubrication condition,as specimen size or mold scale decreases,the ratio of friction force to drawing forming force increases,friction coefficient increases,friction size effect has significant effects on the forming force.