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1 沖壓變形 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 分離工序是使坯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 其中有以沖孔 落料應用最廣 變形工序是使坯 料的一部分相對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 沖壓成形就是毛坯的變形區(qū)在外力的作用下產生相應的塑性 變形 所以變形區(qū)的應力狀態(tài)和變形性質是決定沖壓成形性質的基本因素 因 此 根據變形區(qū)應力狀態(tài)和變形特點進行的沖壓成形分類 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系統(tǒng)化的研究 絕大多數沖壓成形時毛坯變形區(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 板面方向的應力為零 使板材毛坯產生塑性變形的是作用于板面方向上相互垂 直的兩個主應力 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 基于這樣的分析 可以把各種形式沖壓成形中的毛坯變形區(qū) 的受力狀態(tài)與變形特點 在平面應力的應力坐標系中 沖壓應力圖 與相應的兩 向應變坐標系中 沖壓應變圖 以應力與 應變坐標決定的位置來表示 也就是說 沖壓 應力圖與沖壓應變圖中的不同位置都代表著不同的受力情況與變形特點 1 沖壓毛坯變形區(qū)受兩向拉應力作用時 可以分為兩種情況 即 0 t 0 和 0 t 0 再這兩種情況下 絕對值最大的應力都是拉應力 以下 對這兩種情況進行分析 1 當 0且 t 0時 安全量理論可以寫出如下應力與應變的關系式 1 1 m m t t m k 式中 t 分 別 是 軸對稱沖壓 成 形時 的 徑向 主 應變 切向主 應 變 和厚度方向上的主 應變 t 分 別 是 軸對稱沖壓 成 形時 的 徑向 主 應 力 切向主 應 力和厚度 方向上的主 應 力 m 平均 應 力 m t 3 k 常數 在平面 應 力 狀態(tài) 式 1 1 具有如下形式 3 2 3 2 t 3 t t k 1 2 因為 0 所以必定有 2 0 與 0 這個結 果表明 在 兩向 2 拉應 力的平面 應 力 狀態(tài)時 如果 絕對 值 最大 拉應 力是 則在這個方向上的主 應變一定是正應變 即是伸長變形 又因為 0 所以必定有 t 0 與 t2 時 0 當 0 的變化范圍是 0 在雙向等拉力狀態(tài)時 有 式 1 2 得 0 及 t 0 且 t 0 時 有式 1 2 可知 因為 0 所以 1 定有 2 0 與 0 這個結果表明 對于兩向拉應力的平面應力狀 態(tài) 當 的絕對值最大時 則在這個方向上的應變一定時正的 即一定是 伸長變形 又因為 0 所以必定有 t 0 與 t 0 當 0 的變化范圍是 0 當 時 0 也就是 在 雙向等拉 力 狀態(tài)下 在 兩個拉應 力方向 上產 生 數 值相同的伸 長變形 在受 單 向拉應 力 狀態(tài)時 當 0 時 2 也就是說 在受 單向拉應 力 狀態(tài) 下 其 變形 性 質 與一般的 簡單 拉伸是完全一 樣 的 這種變形與受力情況 處于沖壓應變圖中的 AOC 范圍內 見圖 1 1 而 在沖壓應力圖中則處于 AOH 范圍內 見圖 1 2 上述兩種沖壓情況 僅在最大應力的方向上不同 而兩個應力的性質以及 它們引起的變形都是一樣的 因此 對于各向同性的均質材料 這兩種變形是 完全相同的 1 沖壓毛坯變形區(qū)受兩向壓應力的作用 這種變形也分兩種情況分析 即 t 0 和 0 t 0 1 當 0 且 t 0 時 有式 1 2 可知 因 為 0 一定有 2 0 與 0 這個結 果表明 在 兩向壓應 力的平面 應 力 狀態(tài)時 如果 3 絕對 值最大 拉應 力是 0 則在這個方向上的主應變一定是負應變 即是壓 縮變形 又因為 0 與 t 0 即在板料厚度方 向上的 應變 是正的 板料增厚 在 方向上的變形取決于 與 的數值 當 2 時 0 當 2 時 0 當 0 這時 的變化范圍是 與 0 之間 當 時 是雙向等 壓 力狀態(tài) 時 故有 0 當 0 時 是受 單 向 壓應 力 狀態(tài) 所以 2 這種變形情況處于沖壓應變圖中的 EOG 范圍內 見圖 1 1 而在沖壓應力圖 中則處于 COD 范圍內 見圖 1 2 2 當 0 且 t 0 時 有式 1 2 可知 因為 0 所以 一定有 2 0 與 0 這個結果表明 對于兩向 壓 應力的平面應力狀 態(tài) 如果絕對值最大是 則在這個方向上的應變一定時負的 即一定是壓 縮變形 又因為 0 與 t 0 即在板料厚度方 向上的 應變 是正的 即 為壓縮變形 板厚增大 在 方向上的變形取決于 與 的數值 當 2 時 0 當 2 0 當 0 這時 的數值只能在 0 之間變化 當 時 是 雙向 等壓力狀態(tài) 所以 0 這種變形與受力情況 處于沖壓應變圖中的 GOL 范圍內 見圖 1 1 而在沖壓應力圖中則處于 DOE 范圍內 見圖 1 2 1 沖壓毛坯變形區(qū)受兩個異號應力的作用 而且拉應力的絕對值大于壓應 力的絕對 值 這種變形共有兩種情況 分別作如下分析 1 當 0 時 由式 1 2 可知 因 為 0 所以一定 有 2 0 及 0 這個結 果表明 在異 號 的 平面 應 力 狀態(tài)時 如果 絕對 值最大 應 力是 拉應 力 則在這個絕對值最大的拉應 力方向上應變一定是正應變 即是伸長變形 又因為 0 所以必定有 0 0 0 時 由式 1 2 可知 用與前 項相同的方法分析可得 0 即在異 號應 力作用的平面 應 力 狀態(tài)下 如果 絕 對 值最大 應 力是 拉應 力 則在這個方向上的應變是正的 是伸長變形 而在 壓應力 方向上的應變是負的 0 0 0 時 由式 1 2 可知 因 為 0 所以一定有 2 0 及 0 0 必定有 2 0 即在 拉應 力方向上 的 應變 是正的 是伸長變形 這時 的變化范圍只能在 與 0 的范圍內 當 時 0 0 0 時 由式 1 2 可知 用與前 項相同的方法分析可得 0 0 0 0 AON GOH 伸長類 AOC AOH 伸長類 雙向受壓 0 0 EOG COD 壓縮類 0 MON FOG 伸長 類 LOM EOF 壓縮類 異號應力 0 COD AOB 伸長類 DOE BOC 壓縮類 7 變形區(qū)質量問題的表 現(xiàn)形式 變形程度過大引起變形區(qū) 產生破裂現(xiàn)象 壓力作用下失穩(wěn)起皺 成形極限 1 主要取決于板材的塑 性 與厚度無關 2 可用伸長率及成形極 限 DLF 判斷 1 主要取決于傳力區(qū)的 承載能力 2 取決于抗失穩(wěn)能力 3 與板厚有關 變形區(qū)板厚的變化 減薄 增厚 提高成形極限的方法 1 改善板材塑性 2 使變形均勻化 降低局 部變形程度 3 工序間熱處理 1 采用多道工序成形 2 改變傳力區(qū)與變形區(qū) 的力學關系 3 采用防起皺措施 伸 長 類 成 形 脹 形 拉 深 翻 邊 壓 縮 類 成 形 壓 縮 類 成 形 擴 口 拉 深 脹 形 伸 長 類 成 形 縮 口 縮 口 擴口 4 4 翻 邊 圖 1 3 沖壓應變圖 8 沖壓成形 極限 變形區(qū)的 成形極限 傳動區(qū)的 成形極限 伸長類 變 形 壓縮類 變 形 強 度 抗拉與抗壓 縮失衡能力 塑 性 抗縮頸 能 力 變形均 化與擴 展能力 塑 性 抗起皺 能 力 變形力及 其 變 化 各向異性 值 硬化性能 變形抗力 化學成分 組 織 變形條件 硬化性能 應力狀態(tài) 應變梯度 硬化性能 模具狀態(tài) 力學性能 值與 值 相對厚度 化學成分 組 織 變形條件 圖 1 3 體系化研究方法舉例 9 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 form 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 form 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 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 equal 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 10 the deformation characteristics of the deformation zone in all kind of stamping forming can be denoted by the point in the coordinates of the plane princ ipal stress diagram of the stamping stress and the coordinates of the corresponding plane principal stains 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 toplanetensile stresses it can be divided into two cases that is 0 t 0and 0 t 0 In both cases the stress with the maximum absolute value is always a tensile stress These two cases are analyzed respectively as follows 2 In the case that 0and t 0 according to the integral theory the relationships between stresses and strains are m m t t m k 1 1 where t are the principal strains of the radial tangential and thickness directions of the axial symmetrical stamping forming and tare the principal stresses of the radial tangential and thickness directions of the axial symmetrical stamping forming m is the average stress m t 3 k is a constant In plane stress state Equation 1 1 3 2 3 2 t 3 t t k 1 2 Since 0 so 2 0 and 0 It indicates that in plane stress state with two axial tensile stresses if the tensile stress with the maximum absolute value is the principal strain in this direction must be positive that is the deformation belongs 11 to tensile forming In addition because 0 therefore t 0 and t2 0 and when 0 The range of is 0 In the equibiaxial tensile stress state according to Equation 1 2 0 and t 0 and t 0 according to Equation 1 2 2 0 and 0 This result shows that for the plane stress state with two tensile stresses when the absoluste value of is the strain in this direction must be positive that is it must be in the state of tensile forming Also because 0 therefore t 0 and t 0 and when 0 12 The range of is 0 When 0 that is in equibiaxial tensile stress state the tensile deformation with the same values occurs in the two tensile stress directions when 0 2 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 deformation the properties of and and the deformation caused by them are the same only the direction of the maximum stress is different These two deformations are same for isotropic homogeneous material 1 When the deformation zone of stamping blank is subjected to two compressive stresses and t 0 it can also be divided into two cases which are 0 t 0 and 0 t 0 1 When 0 and t 0 according to Equation 1 2 2 0 與 0 This result shows that in the plane stress state with two compressive stresses if the stress with the maximum absolute value is 0 the strain in this direction must be negative that is in the state of compressive forming Also because 0 and t 0 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 13 of and When 2 0 when 2 0 and when 0 The range of is 0 When it is in equibiaxial tensile stress state hence 0 when 0 it is in uniaxial tensile stress state hence 2 This kind of deformation condition is in the region EOG of the diagram of the stamping strain see Fig 1 1 and in the region COD of the diagram of the stamping stress see Fig 1 2 2 When 0and t 0 according to Equation 1 2 2 0 and 0 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 because 0 and t 0 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 and When 2 0 when 2 0 and when 0 The range of is 0 When it is in equibiaxial tensile stress state hence 0 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 the 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 the tensile stress is larger than that of the compressive stress There exist two cases to be analyzed as follow 14 1 When 0 according to Equation 1 2 2 0 and 0 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 because 0 therefore When then 0 0 0 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 tensile forming The strain in the radial direction is negative When then 0 0 0 according to Equation 1 2 2 0 and 0 and 0 therefore 2 0 The strain in the tensile stress direction is positive or in the state of tensile forming The range of is 0 When then 0 0 0 according to Equation 1 2 and by means of the same analysis mentioned above When then 0 0 0 0 AON GOH Tensile AOC AOH Tensile Biaxial compressive stress state 0 0 EOG COD Compress ive 0 MON FOG Tensile LOM EOF Compress ive State of stress with opposite signs 0 COD AOB Tensile DOE BOC Compress ive 20 Table 1 2 Comparison between tensile and compressive forming Item Tensile forming Compressive forming Representation of the quality problem in the deformation zone Fracture in the deformation zone due to excessive deformation Instability wrinkle caused by compressive stress Forming limit 3 Mainly depends on the plasticity of the material and is irrelevant to the thickness 4 Can be estimated by extensibility or the forming limit DLF 4 Mainly depends on the loading capability in the force transferring zone 5 Depends on the anti instability capability 6 Has certain relationship to the blank thickness Variation of the blank thickness in the deformation zone Thinning Thickening Methods to improve forming limit 4 Improve the plasticity of the material 5 Decrease local 4 Adopt multi pass forming process 5 Change the mechanics 21 deformation and increase deformation uniformity 6 Adopt an intermediate heat treatment process relationship between the force transferring and deformation zones 6 Adopt anti wrinkle measures Fig 1 1 Diagram of stamping strain tensile forming bulging deep drawing flanging compressive forming compressive forming expanding deep drawing bulging tensile forming necking necking expanding 4 4 flanging Fig 1 2 Diagram of stamping stress 22 Ten sile for ming Com pres sion for ming St re ngth Cap abil ity of an ti w rinkle und er t he t ensi le and com pres sive st re sses Plasticity Cap abil ity of an ti n ecking Def orma tion uniformit y an d ex te nsion ca pa bility Pl as ticity Cap abil ity of an ti w rinkle Def orma tion for ce a nd i ts Ani sotr opy valu e of r Har deni ng c hara cter isti cs Deformation r es is ta nc e Che mist ry c ompo nent Str uctu re Deformation c on di ti on s Har deni ng c hara cter isti cs Sta te o f st ress Gradient of s tr ai n Har deni ng c hara cter isti cs Die sha pe Mechanical pr oe rt y The value of t he n a nd r Relative th ic kn es s Che mist ry c ompo nent Str uctu re Deformation c on di ti on s Fig 1 3 Examples for systematic research methods