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激光冲击金属箔材塑性成形是一个瞬时、高速、非线性的动态过程,模拟时需要考虑到高应变率下金属材料塑性成形的特点。ABAQUS/Explicit是基于动态显式算法的求解器,非常适合爆炸和冲击等瞬态事件[17]。
由于作用在材料上的激光光斑以及夹具孔是圆的,因此模拟过程中采用轴对称模型,这样可大大缩短计算时间。模拟中对厚度为40μm的T2紫铜箔材进行计算,激光的光斑直径为3mm,材料厚度方向的网格长度设为0.4μm,远小于光斑半径,能够获得足够的计算精度。由于夹具的受力情况不是关注的重点,因此可以看作是刚体。材料在强激光诱导冲击波作用下的动态响应是高应变率冷塑性变形过程,应变率高达108s-1[18]。Johnson-Cook本构模型非常适合金属在高应变率的塑性变形[19],因此使用Johnson-Cook本构模型来表征材料参量。表 1中为铜的Johnson-Cook模型参量,其中, ρ为材料密度,E为弹性模量,ν为泊松比,M为材料熔点,A, B, C, n, m分别为材料常数。
Table 1. Johnson-Cook parameters of copper
material ρ /(g·cm-3) E/MPa ν M/K A/MPa B/MPa C/MPa n m Cu 8.93 119000 0.326 1356 90 292 0.025 0.31 1.09 由Fabbro公式[20]将激光能量转换为冲击波压力。激光冲击脉宽约为20ns,冲击波是持续时间为激光脉宽2倍~3倍的三角波[21],则压力脉冲脉宽为60ns。仿真过程中通过设置夹具与箔材之间的约束来控制成形方式,仿真模型如图 2所示。
铜箔激光冲击微成形微观组织与残余应力研究
Research of microstructure and residual stress of copper foils processed by laser shock forming
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摘要: 为了研究激光成形方式对成形轮廓和微观组织的影响,采用厚度为40μm和80μm的T2铜箔进行激光冲击微胀形和微拉深实验。同时使用ABAQUS有限元仿真对实验进行模拟,研究不同变形方式下箔材位移和残余应力场。结果表明,激光冲击微胀形后铜箔变形区域出现颈缩,激光作用区域内变形机制主要为位错滑移、变形扭曲晶粒和机械孪晶;箔材上表面(激光冲击表面)为残余拉应力,最大值约为372.3MPa,箔材下表面(背向激光冲击面)为残余压应力,最大值约为-218.7MPa;而对于微拉深,箔材成形轮廓过渡圆滑,厚度分布均匀,光斑作用区域内出现大量位错露头和一些机械孪晶,箔材上表面为残余压应力,最大值约为-365.6MPa,箔材下表面为残余拉应力,最大值约为203MPa。这一结果对激光冲击箔材成形控制是有帮助的。Abstract: In order to study influence of laser forming methods on forming profile and microstructure, T2 copper foils with thickness of 40μm and 80μm were used to do experiments of laser shock micro bulging and micro deep drawing. At the same time, ABAQUS finite element simulation was used to simulate the experiment, and the displacement and residual stress field of the foil under different deformation modes were studied. The results show that, after bulging, necking occurs in the deformed region of copper foils. The deformation mechanism mainly includes dislocation sliding, deformation distortion grain and mechanical twinning in the laser processed region. The upper surface of the foil (laser shock surface) is residual tensile stress and the maximum value is about 372.3MPa. The lower surface of the foil (the opposite of laser shock surface) is residual compressive stress and the maximum value is about -218.7MPa. For drawing, foil forming profile is smooth and has uniform thickness distribution. A large number of dislocations and mechanical twinning appear in laser processed region. The upper surface of the foil is residual compressive stress and the maximum value is about -365.6MPa. The lower surface of the foil is residual tensile stress and the maximum value is about 203MPa. This result is helpful for the control of laser shock forming of foil.
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Key words:
- laser technique /
- laser shock forming /
- microstructure /
- finite element simulation /
- residual stress
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Table 1. Johnson-Cook parameters of copper
material ρ /(g·cm-3) E/MPa ν M/K A/MPa B/MPa C/MPa n m Cu 8.93 119000 0.326 1356 90 292 0.025 0.31 1.09 -
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