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Characteristics and Sheet Metal Forming 'b FEhQ ? 1．The article overview 1pWz~3e)o ? Stamping is a kind of plastic forming process in which a part is produced by means of the plastic forming the material under the action of a die. Stamping is usually carried out under cold state, so it is also called 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. )O 4~lE7/ ? ? Stamping is widely used in various fields of the metalworking industry, and it plays a crucial role in the industries for manufacturing automobiles, instruments, military parts and household electrical appliances, etc. 2
E[?( ? ? The process, equipment and die are the three foundational problems that needed to be studied in stamping. o r]j&LW ? ? The characteristics of the sheet metal forming are as follows: #\A%}B, j` ? (1)? ? High material utilization Zm0Je1u.% ? (2)? ? Capacity to produce thin-walled parts of complex shape. hn R-Umy ? (3)? ? Good interchangeability between stamping parts due to precision in shape 8:Ho*jo ? and dimension. v$M_cO =c ? (4)? ? Parts with lightweight, high-strength and fine rigidity can be obtained. /b*( y@{ ? (5)? ? High productivity, easy to operate and to realize mechanization and? ? automatization. R;2r&r~ ? ? ? 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. O2QzX]t;W ? ? The materials for sheet metal stamping include mild steel, copper, aluminum, magnesium alloy and high-plasticity alloy-steel, etc. T3


NM ? ? Stamping equipment includes plate shear punching press. The former shears plate into strips with a definite width, which would be pressed later. The later can be used both in shearing and forming. pb"8]LFUuH ? 2．Characteristics of stamping forming m{IkZ~%D ? 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: dl4jgw)% ? （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 simplify vastly the theoretical analysis and the calculation of the process parameters. '%%Q[o- ? （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 dominated by compressive stress. Km!HlKdy| ? （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 considered, the treating method is also different from that of bulk forming. HvL
H
L? ? （4）．In stamping forming, the restrain action of the die to the blank is not severs 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 cases, the blank has a certain degree of freedom, only one surface of the blank contacts with the die. In some extra cases, such as the forming of the blank on the deforming zone contact with the die. The deformation in these regions are caused and controlled by the die applying an external force to its adjacent area. E_J)wzM@ ? Due to the characteristics of stamping deformation and mechanics mentioned above, the stamping technique is different form the bulk metal forming: Is `A{Uw} ? （1）．The importance or 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. ".k!Z$,X ? （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. ZfpI>#? ? （3）．It is shown that there is a close relationship between stamping forming and raw material. The research on the properties of the stamping forming, that is, forming ability and shape stability, has become a key point in stamping technology development, but also enhances the manufacturing technique of iron and steel industry, and provides a reliable foundation for increasing sheet metal quality. QeXUoR'VF2 ? 3．Categories of stamping forming (]U,<Z"
. ? ? ? Many deformation processes can be done by stamping, the basic processes of the stamping can be divided into two kinds: cutting and forming. N0y9=& ? ? ? 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 blanking 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. VmO=.1Wa ? 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 deformation characteristic of the deformation zone are the basic factors to decide the properties of the stamping forming. 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 be studied systematically. GHM7rWF~ ? ? 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 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 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 and the coordinates of the corresponding plane principal strains. ONCSGa$` ? Wrinkling that occurs in the stamping of tapered square cups and stepped rectangular cups is investigated. A common characteristic of these two types of wrinkling is that the wrinkles are found at the draw wall that is relatively unsupported. In the stamping of a tapered square cup, the effect of process parameters, such as the die gap and blank-holder force, on the occurrence of wrinkling is examined using finiteelement simulations. The simulation results show that the larger the die gap, the more severe is the wrinkling, and such wrinkling cannot be suppressed by increasing the blank-holder force. In the analysis of wrinkling that occurred in the stamping of a stepped rectangular cup, an actual production part that has a similar type of geometry was examined. The wrinkles found at the draw wall are attributed to the unbalanced stretching of the sheet metal between the punch head and the step edge. An optimum die design for the purpose of eliminating the wrinkles is determined using finite-element analysis. The good agreement between the simulation results and those observed in the wrinkle-free production part validates the accuracy of the finite-element analysis, and demonstrates the advantage of using finite-element analysis for stamping die design. OpFyd^mlr ?Wrinkling is one of the major defects that occur in the sheet metal forming process. For both functional and visual reasons, wrinkles are usually not acceptable in a finished part. There are three types of wrinkle which frequently occur in the sheet metal forming process: flange wrinkling, wall wrinkling, and elastic buckling of the undeformed area owing to residual elastic compressive stresses. In the forming operation of stamping a complex shape, draw-wall wrinkling means the occurrence of wrinkles in the die cavity. Since the sheet metal in the wall area is relatively unsupported by the tool, the elimination of wall wrinkles is more difficult than the suppression of flange wrinkles. It is well known that additional stretching of the material in the unsupported wall area may prevent wrinkling, and this can be achieved in practice by increasing the blank-holder force; but the application of excessive tensile stresses leads to failure by tearing. Hence, the blank-holder force must lie within a narrow range, above that necessary to suppress wrinkles on the one hand, and below that which produces fracture on the other. This narrow range of blank-holder force is difficult to determine. For wrinkles occurring in the central area of a stamped part with a complex shape, a workable range of blank-holder force does not even exist.uXqw&+P(8d 沖壓成形與板材沖壓 x?Hv ? ? ? 分離工序是使胚料的一部分與另一部分相互分離的工藝方法，主要有落料、沖孔、切邊、剖切、修整等。其中又以沖孔、落料應(yīng)用最廣。變形工序是使胚料的一部分相對于另一部分產(chǎn)生位移而不破裂的工藝方法，主要有拉深、彎曲、局部成形、脹形、翻邊、縮徑、校形、旋壓等。 0D}zj yoLX ? ? ? 從本質(zhì)上看，沖壓成形就是毛胚的變形區(qū)在外力的作用下產(chǎn)生相應(yīng)的塑性變形，所以變形區(qū)內(nèi)的應(yīng)力狀態(tài)和變形特點景象的沖壓成形分類，可以把成形性質(zhì)相同的成形方法概括成同一個類型并進行體系化的研究。 WC]2(s~-w= ? ? ? 絕大多數(shù)沖壓成形時毛胚變形區(qū)均處于平面應(yīng)力狀態(tài)。通常認為在板材表面上不受外力的作用，即使有外力作用，其數(shù)值也是較小的，所以可以認為垂直于板面方向上的應(yīng)力為零，使板材毛胚產(chǎn)生塑性變形的是作用于板面方向上相互的兩個主應(yīng)力。由于板厚較小，通常都近似地認為這兩個主應(yīng)力在厚度方向上是均勻分布的?；谶@樣的分析，可以把各種形式?jīng)_壓成型中的毛陪變形區(qū)的受力狀態(tài)與變形特點，在平面應(yīng)力的應(yīng)力坐標系中與相應(yīng)的兩向應(yīng)變坐標系中以應(yīng)力與應(yīng)變坐標決定的位置來表示。 )qWKd9 ? 5．板材沖壓性能及其鑒定方法 n(|A1( ? ? ? 板材是指對沖壓加工的適應(yīng)能力。對板材沖壓性能的研究具有飛行重要的意義。為了能夠運用最科學與最經(jīng)濟合理的沖壓工藝過程與工藝參數(shù)制造出沖壓零件，必須對作為加工對象的板材的性能具有十分清楚的了解，這樣才有可能充分地利用板材在加工方面的潛在能力。另一方面，為了能夠依據(jù)沖壓件的形狀與尺寸特點及其所需的成形工藝等基本因素，正確、合理地選用板材，也必須對板材的沖壓性能有一個科學的認識與正確的判斷。 Km{H kJk ? ? ? 評定板材沖壓性能的方法有直接試驗法與間接試驗法。 q`{ MdF@ ? ? ? 實物沖壓試驗是最直接的板材沖壓性能的評定方法。利用實際生產(chǎn)設(shè)備與模具，在與生產(chǎn)完全相同的條件下進行實際沖壓零件的性能評定，當然能夠的最可靠的結(jié)果。但是，這種評定方法不具有普遍意義，不能作為行業(yè)之間的通用標準進行信息的交流。 ^27%[6 ? ? ? 模擬試驗是把生產(chǎn)中實際存在的沖壓成形方法進行歸納與簡單化處理，消除許多過于復(fù)雜的因素，利用軸對稱的簡化了的成形方法，在保證試驗中板材的變形性質(zhì)與應(yīng)力狀態(tài)都與實際沖壓成形相同的條件下進行的沖壓性能的評定工作。為了保證模擬試驗結(jié)果的可靠性與通用性，規(guī)定了私分具體的關(guān)于試驗用工具的幾何形狀與尺寸、毛胚的尺寸、試驗條件（沖壓速度、潤滑方法、壓邊力等）。 B']7Xb ? ? ? 間接試驗法也叫做基礎(chǔ)試驗法。間接試驗法的特點是：在對板材在塑性變形過程中所表現(xiàn)出的基本性質(zhì)與規(guī)律進行分析與研究的基礎(chǔ)上，進一步把它和具體的沖壓成形中板材的塑性變形參數(shù)聯(lián)系起來，建立間接試驗結(jié)果（間接試驗值）與具體的沖壓成形性能（工藝參數(shù)）之間的相關(guān)性。由于間接試驗時所用試件的形狀與尺寸以及加載的方式等都不同于具體的沖壓成形過程，所以它的變形性質(zhì)和應(yīng)力狀態(tài)也不同于沖壓變形。因此間接試驗所得的結(jié)果（試驗值）并不是沖壓成形的工藝參數(shù)，而是可以用來表示板材沖壓性能的基礎(chǔ)性參數(shù)。 mJKl4\Q ? 在沖壓臺階方盒形件、錐形方盒形件中發(fā)生的褶皺現(xiàn)象（的原因）已經(jīng)被調(diào)查出。兩種褶皺的共同特點是褶皺都出現(xiàn)在相對無支撐的拉延壁上。沖壓錐形方盒形件時，工藝參數(shù)如凸凹模間隙和壓邊力等對產(chǎn)生褶皺的影響可以用有限元分析檢查出來。（有限元分析的）模擬結(jié)果顯示凸凹模間隙越大,褶皺就越嚴重,并且這種褶皺不能通過增加壓邊力來消減.在對沖壓臺階方盒形件中產(chǎn)生的褶皺的分析中,對一個有相似結(jié)構(gòu)(臺階結(jié)構(gòu))的實際生產(chǎn)中的零件進行檢查.在拉延壁上發(fā)現(xiàn)的褶皺歸因于在沖頭和臺階之間的金屬板材受到的拉伸力的不平衡.通過有限元分析能夠確定最合理的可以減小褶皺的模具的設(shè)計.模擬結(jié)果和對不發(fā)生褶皺產(chǎn)品的觀察結(jié)果的吻合驗證了有限元分析的精確性,并且證明了對沖壓模具設(shè)計進行有限元分析的優(yōu)勢所在. 起皺是金屬板料成型中常見的失效形式之一。 由于功能的和視覺效果的原因, 起皺通常不能為零件制品所接受。在金屬板料成型加工中通常存在三種類型的起皺現(xiàn)象：法蘭起皺，側(cè)壁起皺，和由于殘余壓應(yīng)力在未變形區(qū)產(chǎn)生的彈性變形。在沖壓復(fù)雜形狀之間時，拉伸壁起皺就是在模具型腔中形成的起皺。由于金屬板