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1 CharacteristicsCharacteristicsCharacteristicsCharacteristics ofofofof StampingStampingStampingStamping andandandand PropertiesPropertiesPropertiesProperties ofofofof SheetSheetSheetSheet MetalMetalMetalMetal FormingFormingFormingForming Stamping is a kind of plastic forming process in which a part is produced by means of the plastic forming of the material under the action of a die Stamping is usually carried out under cold state so it is also called cold stamping Heat stamping is used only when the blank thickness is greater than 8 100mm The blank material for stamping is usually in the form of sheet or strip and therefore it is also called sheet metal forming Some non metal sheets such as plywood mica sheet asbestos leather can also be formed by stamping Stamping is widely used in various fields of the metalworking industry and it plays a crucial role in the industries for manufacturing automobiles instruments instruments military parts and household electrical appliances etc The process equipment and die are the three foundational problems that needed to be studied in stamping The characteristics of the sheet metal forming are as follows 1 High material utilization 2 Capacity to produce thin walled parts of complex shape 3 Good interchangeability between stamping parts due to precision in shape and dimension 4 Parts with lightweight high strength and fine rigidity can be obtained 5 Highproductivity easytooperateandtorealizemechanizationand autoimmunization The manufacture of the stamping die is costly and therefore it only fits to mass production For the manufacture of products in small batch and rich variety the simple stamping die and the new equipment such as a stamping machining center are usually adopted to meet the market demands The materials for sheet metal stamping include mild steel copper aluminum magnesium alloy and high plasticity alloy steel etc Stamping equipment includes plate shear and punching press The former shears plate 2 into strips with a definite width which would be pressed later The later can be used both in shearing and forming 1 11 11 11 1CharacteristicsCharacteristicsCharacteristicsCharacteristics ofofofof stampingstampingstampingstamping formingformingformingforming There are various processes of stamping forming with different working patterns and names But these processes are similar to each other in plastic deformation There are following conspicuous characteristics in stamping 1 The force per unit area perpendicular to the blank surface is not large but is enough to cause the material plastic deformation It is much less than the inner stresses on the plate plane directions In most cases stamping forming can be treated approximately as that of the plane stress state to simply vastly the theoretical analysis and the calculation of the process parameters 2 Due to the small relative thickness the anti instability capability of the blank is weak under compressive stress As a result the stamping process is difficult to proceed successfully without using the anti instability device such as blank holder Therefore the varieties of the stamping processes dominated by tensile stress are more than those dominated by compressive stress 3 During stamping forming the inner stress of the blank is equal to or sometimes less than the yield stress of the material In this point the stamping is different from the bulk forming During stamping forming the influence of the hydrostatic pressure of the stress state in the deformation zone to the forming limit and the deformation resistance is not so important as to the bulk forming In some circumstances such influence may be neglected Even in the case when this influence should be considerer the treating method is also different from that of bulk forming 4 In stamping forming the restrain action of the die to the blank is not severe as in the case of the bulk forming such as die forging In bulk forming the constraint forming is proceeded by the die with exactly the same shape of the part Whereas in stamping in most case the blank has a certain degree of freedom only one surface of the blank contacts with the die In some extra case such as the forming of the suspended region of sphere of cone and curling at the end of tube neither sides of the blank on the deforming zone contact with 3 die The deformation in these regions are caused and controlled by the die applying an external force to its adjacent area Due to the characteristics of stamping deformation and mechanics mentioned above the stamping technique is different from the bulk metal forming 1 The importance of the strength and rigidity of the die in stamping forming is less than that in bulk forming because the blank can be formed without applying large pressure per unit area on its surface Instead the techniques of the simple die and the pneumatic and hydraulic forming are developed 2 Due to the plane stress or simple strain state in comparison with bulk forming more research on deformation or force and power parameters has been done Stamping forming can be performed by more reasonable scientific methods Based on the real time measurement and analysis on the sheet metal properties and stamping parameters by means of computer and some modern testing apparatus research on the intellectualized control of stamping process is also in proceeding 3 It is shown that there is a close relationship between stamping forming and raw material The research on the properties of the stamping that is forming ability and shape stability has become a key point in stamping technology The research on the development but also enhances the manufacturing technique of iron and steel industry and provides a reliable foundation for increasing sheet metal quality 1 21 21 21 2CategoriesCategoriesCategoriesCategories ofofofof stampingstampingstampingstamping formingformingformingforming Many deformation processes can be done by stamping the basic processes of the stamping can be divided into two kinds cutting and forming Cutting is a shearing process that one part of the blank is cut from the other It mainly includes blanking punching trimming parting and shaving where punching and banking are the most widely used Forming is a process that one part of the blank has some displacement from the other It mainly includes deep drawing bending local forming bulging flanging necking sizing and spinning In substance stamping forming is such that the plastic deformation occurs in the deformation zone of the stamping blank caused by the external force The stress state and 4 deformation characteristic of the deformation zone are the basic factors to decide the properties of the stamping Based on the stress state and deformation characteristics of the deformation zone the forming methods can be divided into several categories with the same forming properties and to be studied systematically The deformation zone in almost all types of stamping forming is in the plane stress state Usually there is no force or only small force applied on the blank surface When it is assumed that the stress perpendicular to the blank surface equals to zero two principal stresses perpendicular to each other and act on the blank surface produce the plastic deformation of the material Due to the small thickness of the blank it is assumed approximately that the two principal stresses distribute uniformly along the thickness direction Based on this analysis the stress state and the deformation characteristics of the deformation zone in all kinds of stamping forming can be denoted by the points in the coordinates of the plane principal stresses diagram of the stamping stress and the coordinates of the corresponding plane principal strains diagram of the stamping strain The different points in the figures of the stamping stress and strain possess different stress state and deformation characteristics 1 When the deformation zone of the stamping blank is subjected to the plane tensile stresses itcanbedividedintotwocases that0 0 tr and 0 0 tr In both case the stress with the maximum absolute value is always a tensile stress These two cases are analyzed respectively as follows 1 In the case that0 0 tr and according to the integral theory the relationships between stresses and strains are k mt t mmr r 1 1 where r and t are the principal strains of the radial tangential and thickness directions of the exial symmetrical stamping forming r and t are the principal stresses of the radial tangential and thickness directions of the axial symmetrical stamping forming m is the average stress 3 tr m kis a constant 5 In plane stress state Equation 1 1 is rewritten as following form k r t tr r 3 2 3 2 3 1 2 Since0 r so02 rand0 r It indicates that in plane stress state with two axial tensile stresses if the tensile stress with the maximum absolute value is r the principal strain in this direction must be positive that is the deformation belongs to tensile forming In addition because0 r therefore0 rand0 r 0 And when 2 The range of is0 r In the equibiaxial tensile stress r according to Equation 1 2 0 rand0 tr and according to Equation 1 2 02 r and 0 This result shows that for the plane stress state with two tensile stresses when the absolute value of is the maximum the strain in this direction must be positive that is it must be in the state of tensile forming Also because0 r therefore0 rand0 0 r that is in equibiaxial tensile stress state the tensile deformation with the same values occurs in the two tensile stress directions when0 r 02 r that is in uniaxial tensile stress state the deformation characteristic in this case is the same as that of the ordinary uniaxial tensile This kind of deformation is in the region AON of the diagram of the stamping strain see Fig 1 1 and in the region GOH of the diagram of the stamping stress see Fig 1 2 Between above two cases of stamping deforming the properties of and r and the deformation caused by them are the same only the direction of the maximum stress is different These two deformations are exactly the same for isotropic homogeneous material 2 When the deformation zone of the stamping blank is subjected to two compressive stresses r and 0 t it can also be divided into two cases which are0 r 0 t and0 r 0 t 1 When0 r and0 t according to Equation1 2 02 rand 0 r This result shows that in the plane stress state with two compressive stresses if the stress with the maximum absolute value is0 r the strain in this direction must be negative 7 that is in the state of compressive forming Also because0 rand0 t The strain in the thickness direction of the blank t is positive and the thickness increases The deformation condition in the tangential direction depends on the values of r and When 2 r 0 when 2 r 0 and when 2 The range of is0 r When r it is in equibiaxial tensile stress state hence0 r when0 it is in uniaxial tensile stress state hence2 r This kind of deformation condition is in the region EOG of the diagram of the stamping strain see Fig 1 1 and region COD of the diagram of stamping stress see Fig 1 2 2 When0 r and0 t according to Equation 1 2 02 r and0 This result shows that in the plane stress state with two compressive stresses if the stress with the maximum absolute value is the strain in this direction must be negative that is in the state of compressive forming Also because0 rand0 t The strain in the thickness direction of the blank t is positive and the thickness increases The deformation condition in the radial direction depends on the values of r and When r 2 0 r When r 2 0 r and when r 2 r The range of r is0 r When r it is equibiaxial tensilestress state hence0 r This kind of deformation is in the region GOL of the diagram of the stamping strain see Fig 1 1 and in the region DOE of diagram of the stamping stress see Fig 1 2 3 The deformation zone of the stamping blank is subjected to two stresses with opposite signs and the absolute value of tensile stress is larger than that of the compressive stress 8 There exist two cases to be analyzed as follows 1 When0 r 0 according to Equation 1 2 02 r and0 r This result shows that in the plane stress state with opposite signs if the stress with the maximum absolute value is tensile the strain in the maximum stress direction is positive that is in the state of tensile forming Also because0 r 0 therefore0 r 0 r 0 r it is the uniaxial tensile stress state This kind of deformation condition is in the region MON of the diagram of the stamping strains see Fig 1 1 and in the region FOG of the diagram of the stamping stresses see Fig 1 2 2 When0 0 r according to Equation 1 2 by means of the same analysis mentioned above 0 that is the deformation zone is in the plane stress state with opposite signs If the stress with the maximum absolute value is tensile stress the strain in this direction is positive that is in the state of compressive forming The strain in the radial direction is negative 0 r that is in the state of compressive forming The range of r is r 0 when r then0 0 0 r 0 r according to Equation 1 2 02 r and0 r and0 rand0 r The strain in the tensile stress direction is positive or in the state of tensile forming The range of r is r 0 When r then0 r 0 r 0 0 according to Equation 1 2 and by means of the same analysis mentioned above 0 0 0 r r AONGOH Tensile r AOCAOH Tensile Biaxial compress with state 0 0 r r EOGCOD Compressive r MONFOG Tensile r LOMEOF Compressive Stateof stresseswith opposite signs 0 0 r r CODAOB Tensile r DOEBOC Compressive TableTableTableTable 1 21 21 21 2 ComparisonComparisonComparisonComparison betweenbetweenbetweenbetween tensiletensiletensiletensile andandandand compressivecompressivecompressivecompressive formingformingformingforming ItemTensile formingCompressive forming Representationofthe qualityprobleminthe deformation zone Fracture in the deformation zoneduetoexcessive deformation Instability wrinklecaused by compressive stress Forming limit 1 Mainly depends on the plasticityofthe material andis irrelevanttothe thickness 2 Can be estimated by the extensibilityorthe forming limit DLF 1 Mainly depends on the loading capability in the force transferring zone 2 Dependsonthe anti instability capability 3 Has certain relationship to the blank thickness 12 Variation of the the blank thicknessinthe deformation zone ThinningThickening Methodstoimprove forming limit 1 Improve the plasticity of the material 2 Decreaselocal deformation and increasedeformation uniformity 3 Adopt an intermediate heattreattreatment process 1 Adopt multi pass forming process 2 Change the mechanics relationship between the forcetransferringand deformation zones 3 Adoptanti wrinkle measures Stamping forming limit Forming limit in the force transferringForming limit in thedeformation zone Compression forming Tensile formingCapabilityofanti wrinkle underthetensileand compressive stresses Strength Deformatio uniformity and extension capability Capability of anti necking Plasticity Deformation force and its varation plasticityCapability of anti wrinkle Anisotrotropy Value of Hardening characteristics Deformation resistance Deformation conditions Structure Chemistry component Relative thickness The value of the n and r Mechanical property Die shape Hardening characteristics Gradient of strain State of stress Hardening characteristics Deformation conditions Structure Chemisty component Fig 1 3Examples for systematic research methods Table 1 2 clearly shows that in the deformation zone of the blank the characteristics of the force and deformation and the patterns relevant to the deformation for each stamping method are the same Therefore in addition to the research on the detail stamping method it s feasible to study stamping systematically and comprehensively The characteristic research is 13 to study the common principle of all different types of stamping methods The results of the systematic research are applicable to all stamping methods The research on the properties and limit of the sheet metal stamping has been carried out in certain extent The contents of the research on the stamping forming limit by using systematic method are shown in Fig 1 3 1 31 31 31 3MethodsMethodsMethodsMethods totototo studystudystudystudy stampingstampingstampingstamping formingformingformingforming The objective of the stamping forming research is to realize and apply the principles to solve various engineering problems through study on the stamping forming procedure Recently the research on stamping forming has been carried out in wide range with various methods The research can be summed up substantially into following types Fig 1 4 Diagram of stamping research methods The double line arrows in Fig 1 4 denote the procedure of formal theoretical research In such research the actions of the equipment and die are decomposed into micro ingredients of the force and applied on the microstructure of the metal according to the multi crystal microstructure of the deformation metal the deformation of the metal microstructure is then investigated and the deformation of microstructure to macro deformation of the sheet metal is summed up This scientific method is perfect but current development in mechanics and metallurgy cannot meet such demand Therefore this kind of formal theoretical research is still Equipment and die Mechanics Ideal Homogeneous Solid Metal Stamping 14 in burgeoning and is not applicable in practice Since the formal theoretical research method encounters formidable difficulty a kind of simplified theoretical method appears in the research field of the stamping forming The characteristic of this kind of research method is denoted by continuous line in Fig 1 4 which is the main trend of the recent research This research assumes metal as the ideal homogeneous solid and simplifies the property parameters the boundary conditions and the blank geometrical parameters of the metal the stamping process and principle are analyzed and described by mathematical