冷挤压组合凹模失效分析及挤压过程数值模拟
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摘要
冷挤压作为一种高精、高效、优质低耗的先进生产工艺技术,已被广泛应用于工业生产。冷挤压金属在三向压应力作用下变形,使得挤压模具承受的挤压力非常大,模具寿命短,因此研究挤压过程中凹模的应力变化对分析模具失效、提高模具寿命起到了重要作用。
本文通过螺钉挤压成形实验,观察冷挤压组合凹模模具的失效现象;同时基于弹塑性有限元模拟技术,采用有限元软件DEFORM-2D对螺钉挤压过程进行模拟,改变凹模的挤压半锥角、长径比及挤压比,分析冷挤压组合凹模应力的分布及变化情况,对冷挤压组合凹模的疲劳破坏机理进行探讨,并建立了模具疲劳寿命估算模型,得出以下主要结论:
(1)冷挤压组合凹模的失效主要是疲劳失效,疲劳裂纹主要出现在挤压锥角附近。在挤压锥角入口处和挤压锥角出口附近,裂纹主要沿与凹模内表面成45°方向产生并向模具外表面扩展,而在挤压锥角入口后方数毫米处,内表面裂纹主要沿90°方向产生并向模具外表面扩展。
(2)由DEFORM-2D软件对组合凹模的挤压过程模拟的结果可知,在组合凹模的挤压过程中,凹模的切向受力由压应力逐渐变化成拉应力,切向拉应力和最大等效应力主要发生在挤压锥角入口到入口后方数毫米之间和挤压锥角出口前方数毫米处,与实验中裂纹出现的位置相一致。
(3)凹模挤压半锥角、长径比及挤压比对模具应力产生影响,当挤压半锥角和模具长径比减小时,凹模挤压锥角处的最大等效应力增大;当挤压比减小时,挤压锥角处的最大等效应力减小。
(4)将有限元模拟结果与局部应力-应变法相结合,建立模具疲劳寿命估算模型,为挤压凹模的优化设计提供新的方法和途径。
As a high-precision, high-efficiency, high quality and low cost advanced production technology, cold extrusion has been widely used in the industrial. The extrusion die’s life short when the unit extruding force is very large because of the cold extrusion metal deformed under the three-dimensional compressive stress. Analyzing the stress’s distribution and change of the cold extrusion combination concave die is very important to improve the die’s life.
The cold extrusion combination concave die’s failure was observed through the experiment of the screw cold extrusion forming. At the same time we based on the elastic-plastic finite element model (FEM) simulated the cold extrusion process of screw by using DEFORM-2D. Analyzed the stress’s distribution and change and discussed the mechanism of the die’s fatigue failure when we changed the die’s semi-cone angle, aspect ratio and the extrusion ratio, then established the die fatigue life prediction model. We can get some conclusions as follows:
(1)The fatigue crack was the main failure form of the cold extrusion combination concave die and the crack generated mainly in the extrusion cone angle. In the entrance of the extrusion cone angle and near exit of the extrusion cone angle the crack along the 45 degrees expanded to outward and a few millimeters after the entrance of the extrusion cone angle the crack along the 90 degrees expanded.
(2)The simulation results by using of the DEFORM-2D software was consonant with the experiment. In the process of the extrusion the die’s tangential force gradually changes from compressive stress into tensile stress, the tangential tensile stress and maximum equivalent stress occured mainly between the entrance and a few millimeters after the entrance and before the exit of the extrusion cone angle.
(3)The semi-cone angle, aspect ratio and the extrusion ratio influenced the stress of the die. When the semi-cone angle and aspect ratio decreased the maximum equivalent stress increased and the extrusion ratio decreased the maximum equivalent stress decreased.
(4)The finite element simulation results and local stress-strain method were combined to establish the die life prediction model. This provided new ways to optimizing the design of extrusion die.
本文通过螺钉挤压成形实验,观察冷挤压组合凹模模具的失效现象;同时基于弹塑性有限元模拟技术,采用有限元软件DEFORM-2D对螺钉挤压过程进行模拟,改变凹模的挤压半锥角、长径比及挤压比,分析冷挤压组合凹模应力的分布及变化情况,对冷挤压组合凹模的疲劳破坏机理进行探讨,并建立了模具疲劳寿命估算模型,得出以下主要结论:
(1)冷挤压组合凹模的失效主要是疲劳失效,疲劳裂纹主要出现在挤压锥角附近。在挤压锥角入口处和挤压锥角出口附近,裂纹主要沿与凹模内表面成45°方向产生并向模具外表面扩展,而在挤压锥角入口后方数毫米处,内表面裂纹主要沿90°方向产生并向模具外表面扩展。
(2)由DEFORM-2D软件对组合凹模的挤压过程模拟的结果可知,在组合凹模的挤压过程中,凹模的切向受力由压应力逐渐变化成拉应力,切向拉应力和最大等效应力主要发生在挤压锥角入口到入口后方数毫米之间和挤压锥角出口前方数毫米处,与实验中裂纹出现的位置相一致。
(3)凹模挤压半锥角、长径比及挤压比对模具应力产生影响,当挤压半锥角和模具长径比减小时,凹模挤压锥角处的最大等效应力增大;当挤压比减小时,挤压锥角处的最大等效应力减小。
(4)将有限元模拟结果与局部应力-应变法相结合,建立模具疲劳寿命估算模型,为挤压凹模的优化设计提供新的方法和途径。
As a high-precision, high-efficiency, high quality and low cost advanced production technology, cold extrusion has been widely used in the industrial. The extrusion die’s life short when the unit extruding force is very large because of the cold extrusion metal deformed under the three-dimensional compressive stress. Analyzing the stress’s distribution and change of the cold extrusion combination concave die is very important to improve the die’s life.
The cold extrusion combination concave die’s failure was observed through the experiment of the screw cold extrusion forming. At the same time we based on the elastic-plastic finite element model (FEM) simulated the cold extrusion process of screw by using DEFORM-2D. Analyzed the stress’s distribution and change and discussed the mechanism of the die’s fatigue failure when we changed the die’s semi-cone angle, aspect ratio and the extrusion ratio, then established the die fatigue life prediction model. We can get some conclusions as follows:
(1)The fatigue crack was the main failure form of the cold extrusion combination concave die and the crack generated mainly in the extrusion cone angle. In the entrance of the extrusion cone angle and near exit of the extrusion cone angle the crack along the 45 degrees expanded to outward and a few millimeters after the entrance of the extrusion cone angle the crack along the 90 degrees expanded.
(2)The simulation results by using of the DEFORM-2D software was consonant with the experiment. In the process of the extrusion the die’s tangential force gradually changes from compressive stress into tensile stress, the tangential tensile stress and maximum equivalent stress occured mainly between the entrance and a few millimeters after the entrance and before the exit of the extrusion cone angle.
(3)The semi-cone angle, aspect ratio and the extrusion ratio influenced the stress of the die. When the semi-cone angle and aspect ratio decreased the maximum equivalent stress increased and the extrusion ratio decreased the maximum equivalent stress decreased.
(4)The finite element simulation results and local stress-strain method were combined to establish the die life prediction model. This provided new ways to optimizing the design of extrusion die.
引文
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[12] Jongung Choi, Hae-Yong Cho, Chang-Yong Jo. Forging of spur with internal serrations and design of the dies[J]. Journal of Materials Proeessing Teehnology. 2000, (104): l-7.
[13] Jens Groenbaek, Torben Birker. Innovation in cold forging die design[J]. Materials Processing Technology, 2000, 98 (2): 155-161.
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