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沖壓成形與板材沖壓
1． 概述
通過模具使板材產生塑性變形而獲得成品零件的一次成形工藝方法叫做沖壓。由于沖壓通常在冷態(tài)下進行，因此也稱為冷沖壓。只有當板材厚度超過8~100mm時，才采用熱沖壓。沖壓加工的原材料一般為板材或帶材，故也稱板材沖壓。某些非金屬板材（如膠木板、云母片、石棉、皮革等）亦可采用沖壓成形工藝進行加工。
沖壓廣泛應用于金屬制品各行業(yè)中，尤其在汽車、儀表、軍工、家用電器等工業(yè)中占有極其重要的地位。沖壓成形需研究工藝設備和模具三類基本問題。
? 板材沖壓具有下列特點：
（1）．高的材料利用率。
（2）．可加工薄壁、形狀復雜的零件。
（3）．沖壓件在形狀和尺寸方面的互換性好。
（4）．能獲得質量輕而強度高、剛性好的零件。
（5）．生產率高，操作簡單，容易實現機械化和自動化。
沖壓模具制作成本高，因此適合大批量生產。對于小批量、多品種生產，常采用簡易沖模，同時引進沖壓加工中心等新型設備，以滿足市場求新求變的需求。板材沖壓常用的金屬材料有低碳鋼、銅、鋁、鎂合金及高塑性的合金剛等。如前所述，材料形狀有板材和帶材。
沖壓生產設備有剪床和沖床。剪床是用來將板材剪切成具有一定寬度的條料，以供后續(xù)沖壓工序使用，沖床可用于剪切及成形。
2． 沖壓成形的特點
生產時間中所采用的沖壓成形工藝方法有很多，具有多種形式餓名稱，但塑性變形本質是相同的。沖壓成形具有如下幾個非常突出的特點。
（1）．垂直于板面方向的單位面積上的壓力，其數值不大便足以在板面方向上使??板材產生塑性變形。由于垂直于板面方向上的單位面積上壓力的素質遠小于板面方向上的內應力，所以大多數的沖壓變形都可以近似地當作平面應力狀態(tài)來處理，使其變形力學的分析和工藝參數的計算大呢感工作都得到很大的簡化。
（2）．由于沖壓成形用的板材毛胚的相對厚度很小，在壓應力作用下的抗失穩(wěn)能力也很差，所以在沒有抗失穩(wěn)裝置（如壓邊圈等）的條件下，很難在自由狀態(tài)下順利地完成沖壓成形過程。因此，以拉應力作用為主的伸長類沖壓成形過程多于以壓應力作用為主的壓縮類成形過程。
（3）．沖壓成形時，板材毛胚內應力的數值等于或小于材料的屈服應力。在這一點上，沖壓成形與體積成形的差別很大。因此，在沖壓成形時變形區(qū)應力狀態(tài)中的靜水壓力成分對成形極限與變形抗力的影響，已失去其在體積成形時的重要程度，有些情況下，甚至可以完全不予考慮，即使有必要考慮時，其處理方法也不相同。
（4）．在沖壓成形時，模具對板材毛胚作用力所形成的約束作用較輕，不像體積成形（如模鍛）是靠與制件形狀完全相同的型腔對毛胚進行全面接觸而實現的強制成形。在沖壓成形中，大多數情況下，板材毛胚都有某種程度的自由度，常常是只有一個表面與模具接觸，甚至有時存在板材兩側表面都有于模具接觸的變形部分。在這種情況下，這部分毛胚的變形是靠模具對其相鄰部分施加的外力實現其控制作用的。例如，球面和錐面零件成形時的懸空部分和管胚端部的卷邊成形都屬這種情況。
? ?由于沖壓成形具有上述一些在變形與力學方面的特點，致使沖壓技術也形成了一些與體積成形不同的特點。由于不需要在板材毛的表面施加很大的單位壓力即可使其成形，所以在沖壓技術中關于模具強度與剛度的研究并不十分重要，相反卻發(fā)展了學多簡易模具技術。
由于相同原因，也促使靠氣體或液體壓力成形的工藝方法得以發(fā)展。因沖壓成形時的平面應力狀態(tài)或更為單純的應變狀態(tài)（與體積成形相比），當前對沖壓成形匯中毛胚的變形與 力能參數方面的研究較為深入，有條件運用合理的科學方法進行沖壓加工。借助于電子計算機與先進的測試手段，在對板材性能與沖壓變形參數進行實時測量與分析基礎上，實現沖壓過程智能化控制的研究工作也在開展。人們在對沖壓成形過程有離開較為深入的了解后，已經認識到沖壓成型與原材料有十分密切的關系。所以，對板材沖壓性能即成形性與形狀穩(wěn)定性的研究，目前已成為沖壓技術的一個重要內容。對板材沖壓性能的研究工作不僅是沖壓技術發(fā)展的需要，而且也促進了鋼鐵工業(yè)生產技術的發(fā)展，為其提高板材的質量提供了一個可靠的基礎與依據。
3．沖壓變形的分類
? ?沖壓變形工藝可完成多種工序，其基本工序可分為分離工序和變形工序兩大類。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、落料應用最廣。變形工序是使胚料的一部分相對于另一部分產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。
從本質上看，沖壓成形就是毛胚的變形區(qū)在外力的作用下產生相應的塑性變形，所以變形區(qū)內的應力狀態(tài)和變形特點景象的沖壓成形分類，可以把成形性質相同的成形方法概括成同一個類型并進行體系化的研究。
絕大多數沖壓成形時毛胚變形區(qū)均處于平面應力狀態(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，其數值也是較小的，所以可以認為垂直于板面方向上的應力為零，使板材毛胚產生塑性變形的是作用于板面方向上相互的兩個主應力。由于板厚較小，通常都近似地認為這兩個主應力在厚度方向上是均勻分布的?；谶@樣的分析，可以把各種形式沖壓成型中的毛陪變形區(qū)的受力狀態(tài)與變形特點，在平面應力的應力坐標系中與相應的兩向應變坐標系中以應力與應變坐標決定的位置來表示。
4.沖壓用原材料
? ?沖壓加工用原材料有很多種，它們的性能也有很大的差別，所以必須根據原材料的性能與特點，采用不同的沖壓成形方法、工藝參數和模具結構，才能達到沖壓加工的目的。由于人們對沖壓成形過程板材毛胚的變形行為有了較為深入的認識，已經相當清楚的建立了由原材料的化學成分、組織等因素所決定的材料性能與沖壓成形之間的關系，這就使原材料生產部門不但按照沖壓件的工作條件與使用要求進行原材料的設計工作，而且也根據沖壓件加工過程對板材性能的要求進行新型材料的開發(fā)工作，這是沖壓技術在原材料研究方面的一個重要方向。對沖壓用原材料沖壓性能方面的研究工作有
（1）原材料沖壓性能的含義。
（2）判斷原材料沖壓性能的科學方法，確定可以確切反映材料沖壓性能的參數，建立沖壓性能的參數與實際沖壓成形間的關系，以及沖壓性能參數的測試方法等。
（3）建立原材料的化學成分、組織和制造過程與沖壓性能之間的關系。沖壓用原材料主要是各種金屬與非金屬板材。金屬板材包括各種黑色技術和有色金屬板材。雖然在沖壓生產中所用金屬板材的種類很多，但最多的原材料蛀牙是鋼板、不銹鋼板、鋁合金板及各種復合金屬板。
5．板材沖壓性能及其鑒定方法
? ? 板材是指對沖壓加工的適應能力。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濟合理的沖壓工藝過程與工藝參數制造出沖壓零件，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。評定板材沖壓性能的方法有直接試驗法與間接試驗法。
? ?實物沖壓試驗是最直接的板材沖壓性能的評定方法。利用實際生產設備與模具，在與生產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。
? ?模擬試驗是把生產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與應力狀態(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果的可靠性與通用性，規(guī)定了私分具體的關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）。
? ?間接試驗法也叫做基礎試驗法。間接試驗法的特點是：在對板材在塑性變形過程中所表現出的基本性質與規(guī)律進行分析與研究的基礎上，進一步把它和具體的沖壓成形中板材的塑性變形參數聯系起來，建立間接試驗結果（間接試驗值）與具體的沖壓成形性能（工藝參數）之間的相關性。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，所以它的變形性質和應力狀態(tài)也不同于沖壓變形。因此間接試驗所得的結果（試驗值）并不是沖壓成形的工藝參數，而是可以用來表示板材沖壓性能的基礎性參數。
Characteristics and Sheet Metal Forming
1． The article overview
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.
?? 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.
? ?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)? ? High productivity, easy to operate and to realize mechanization and? ? automatization.
? ? 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 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.
2．Characteristics of stamping forming
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 simplify 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 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 considered, 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 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.
Due to the characteristics of stamping deformation and mechanics mentioned above, the stamping technique is different form the bulk metal forming: 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.
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. 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.
3．Categories of stamping forming
? ? 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 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.
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.
??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.
4．Raw materials for stamping forming
There are a lot of raw materials used in stamping forming, and the properties of these materials may have large difference. The stamping forming can be succeeded only by determining the stamping method, the forming parameters and the die structures according to the properties and characteristics of the raw materials. The deformation of the blank during stamping forming has been investigated quite thoroughly. The relationships between the material properties decided by the chemistry component and structure of the material and the stamping forming has been established clearly. Not only the proper material can be selected based on the working condition and usage demand, but also the new material can be developed according to the demands of the blank properties during processing the stamping part. This is an important domain in stamping forming research. The research on the material properties for stamping forming is as follows:
（1）．Definition of the stamping property of the material.
（2）．Method to judge the stamping property of the material, find parameters to express the definitely material property of the stamping forming, establish the relationship between the property parameters and the practical stamping forming, and investigate the testing methods of the property parameters.
（3）．Establish the relationship among the chemical component, structure, manufacturing process and stamping property.
?? The raw materials for stamping forming mainly include various metals and nonmetal plate. Sheet metal includes both ferrous and nonferrous metals. Although a lot of sheet metals are used in stamping forming, the most widely used materials are steel, stainless steel, aluminum alloy and various composite metal plates.
5．Stamping forming property of sheet metal and its assessing method
The stamping forming property of the sheet metal is the adaptation capability of the sheet metal to stamping forming. It has crucial meaning to the investigation of the stamping forming property of the sheet metal. In order to produce stamping forming parts with most scientific, economic and rational stamping forming process and forming parameters, it is necessary to understand clearly the properties of the sheet metal, so as to utilize the potential of the sheet metal fully in the production. On the other hand, to select plate material accurately and rationally in accordance with the characteristics of the shape and dimension of the stamping forming part and its forming technique is also necessary so that a scientific understanding and accurate judgment to the stamping forming properties of the sheet metal may be achieved.
There are direct and indirect testing methods to assess the stamping property of the sheet metal?.Practicality stamping test is the most direct method to assess stamping forming property of the sheet metal. This test is done exactly in the same condition as actual production by using the practical equipment and dies. Surely, this test result is most reliable. But this kind of assessing method is not comprehensively applicable, and cannot be shared as a commonly used standard between factories.
? ? The simulation test is a kind of assessing method that after simplifying and summing up actual stamping forming methods, as well as eliminating many trivial factors, the stamping properties of the sheet metal are assessed, based on simplified axial-symmetric forming method under the same deformation and stress states between the testing plate and the actual forming states. In order to guarantee the reliability and generality of simulation results, a lot of factors are regulated in detail, such as the shape and dimension of tools for test, blank dimension and testing conditions(stamping velocity, lubrication method and blank holding force, etc).???Indirect testing method is also called basic testing method its characteristic is to connect analysis and research on fundamental property and principle of the sheet metal during plastic deformation, and with the plastic deformation parameters of the sheet metal in actual stamping forming, and then to establish the relationship between the indirect testing results(indirect testing value) and the actual stamping forming property (forming parameters). Because the shape and dimension of the specimen and the loading pattern of the indirect testing are different from the actual stamping forming, the deformation characteristics and stress states of the indirect test are different from those of the actual one. So, the results obtained form the indirect test are not the stamping forming parameters, but are the fundamental parameters that can be used to represent the stamping forming property of the sheet metal.