【作者】 刘杰；

【导师】 孙胜；

【作者基本信息】 山东大学 ， 材料加工工程， 2008， 博士

【摘要】 激光弯曲成形作为一种新的无外力,无模具板材柔性成形方法,具有常规机械加工方法无法比拟的优点。尤其是激光束可以聚焦到很小的尺寸,非常适合微尺度精密加工。深入研究激光微尺度弯曲成形机理以及工艺控制策略,将对微细加工理论和实践有所贡献,因而具有重要科学意义和工程应用价值。本文围绕厚度为0.1mm及以下尺寸的微尺度弯曲件的弯曲方向控制及变形量控制这两个问题,针对微型不锈钢工件的激光弯曲过程,采用有限元仿真以及实验方法进行了研究。在对激光微尺度弯曲成形特点分析基础上,考虑了随温度变化的材料热物理性能参数（热传导率和比热）和材料的力学性能参数（热膨胀系数、屈服强度以及弹性模量等）,以及应变强化和应变速率对屈服强度的影响等本构特点,建立了激光微尺度弯曲成形热力耦合有限元模拟模型。模拟结果表明,微尺度工件仍可实现基于温度梯度机理的激光弯曲。温度梯度机理下工件厚度方向存在强烈的温度梯度,随着加热-冷却变化,该梯度导致的应力变化,使工件上表面压应力和压缩变形量均大于下表面,因此发生朝向激光束的正向弯曲变形。加热过程中温度场沿扫描路径分布不均引起扫描路径上不同位置的位移不同是造成“边缘效应”现象的根源之一。采用小功率CO₂激光器、数控机械平台、专用夹具与检测装置,建立了激光微尺度弯曲成形实验装置。经大量实验,证明有限元分析结果与实验值吻合良好,同时研究表明,工件表面峰值温度在材料熔点以下时,弯曲角度随激光功率增大而增大,随扫描速度的增大而减小,当峰值温度高于材料熔点,弯曲角度随激光功率增大而减小,随扫描速度增大而增大;通过对多道次激光扫描弯曲成形进行实验研究,分析了扫描步距及工件长度对微尺度工件弯曲成形的影响,根据研究结果,提出了根据单条扫描路径获得的弯曲角度来确定单曲率圆柱面成形的扫描路径与扫描步距的工艺设计新方法。对屈曲机理的激光微尺度弯曲成形进行研究,揭示了屈曲机理的变形机制,发现由于温度梯度小,工件的初始应力易对其弯曲方向产生影响,屈曲将在存在较大压应力的表面优先发生。针对这种情况,通过一定方式给工件施加可控的预应力,并对其进行数值模拟与实验研究,详细分析了预应力对弯曲过程的影响,结果表明预约束应力可显著改变激光弯曲成形过程中的应力分布,通过控制预约束应力大小与方向可以稳定实现工件反向弯曲,也可灵活控制工件的弯曲变形量。研究了激光偏振特性在激光弯曲成形中的作用。由于激光偏振特性引起激光吸收率的变化,以及累积弯曲角度引起光斑面积的变化,实际作用于板材的激光能量是变化的,这就是造成沿同一路径多次激光扫描,每次扫描弯曲角度变化的主要原因。改变入射角度可以适应激光的偏振特性,在实验研究变入射角度下微型工件弯曲变形规律的基础上,提出了通过改变激光入射角度进行工件弯曲变形量控制的方法,并通过实验进行了验证。实现了有限元软件MARC与优化设计软件iSIGHT的有机集成,探究了激光弯曲成形工艺优化的改进方案。在理论模拟与实验分析的基础上,将优化过程中热力耦合模拟转换为温度场模拟,大幅提高了优化效率。利用改进型工艺优化策略和变入射角工件弯曲变形量控制方法,提出了预定角度弯曲成形工艺设计新路线,并通过实验进行了验证。更多还原

【Abstract】 Laser bending or laser forming (LF) is a newly developed, flexible technique for forming sheet metal by means of thermal stress instead of external force or hard tools, which has many advantages compared with cold and mechanical processing. Especially, the laser can be focused into very small beam and is very suitable for precision micro-scale processing. The study works of laser micro-bending mechanisms analyzing and process controlling have an important significance to micro manufacture both in theory and practice.This thesis focuses on bending direction and deformation controlling of micro-scale stainless steel specimens. The study contents includes: investigation of temperature gradient mechanism(TGM) and buckling mechanism(BM) of laser micro-bending, laser micro-bending process with pre-stress, laser polarization affection and laser micro-bending process optimization .Based on the analysis of the characteristics of laser micro-bending, the numerical model was developed by using the FEM software MSC.Marc. The simulation results show that the bending in TGM also could be obtained on micro-scale specimens. The heat affection zone was almost constrained in the area of the laser beam during heating process. Large temperature gradient and stress gradient were generated along thickness of the specimen. Both compress stress and plastic strain induced on the top surfaces were larger than that on the bottom surface, thus positive bending towards to laser beam was produced. The main reason of the edge effect during laser bending process is that the peak temperature distribution along the scanning path is non-uniform.Laser micro-bending experimental system was built for studying the forming law of the micro-parts in TGM. The FEM model was verified under different powers and scanning speeds. The experimental results indicated: the peak temperature on the top surface is a critical point, when it is lower than the material’s melting point, the bending angle increased with increasing power and decreasing scanning speed; when it is higher than the melting point, the bending angle decreased with increasing laser and decreasing scanning speed. Based on the affection of the distance between multi-passes and length of the specimen on the bending angle, a process design method for forming single-curvature surface was proposed and verified through experiments.The FEM model was developed for laser micro-bending process simulation in BM, and experiments also were carried out. The results showed that the bending direction in BM was strongly affected by the initial stress of the specimen. For controlling the bending direction, pre determined pre-stress was introduced in the specimen’s heated zone and the effect of the pre-stress was studied by numerical simulation and lots of experiments. The results showed that the pre-stress could enlarge the stress gradient and the bending direction could be controlled freely by controlling the value and direction of the pre-stress.The influence of the beam polarization in laser bending process was experimental studied. The results showed that the laser absorption of the metallic specimen could be enhanced by increasing the incident angle when using linearly polarized laser, while at the same time, the laser beam area increased with increasing incident angle, thus the laser energy and bending angle are influenced by those two factors during multi-scanning process. When the deformation part is parallel to the laser incident direction, the worked laser energy decreased drastically and the deformation couldn’t be produced any more, thus a strategy of bending angle controlling by changing the incident angle was proposed. The experimental results showed that the precision of bending angle can reach±0.1°.Considering the computation efficiency, the improved strategy for parameters optimization of laser bending process was developed. The FEM software and optimization design software were integrated through secondary development. Based on the analysis of the deformation law and temperature distribution during laser forming process, the temperature field computation was used instead of the previous coupled thermal-mechanical simulation in the optimization process. The optimization efficiency was considerably improved by the premise of guaranteeing the precision of the results. A process design route was developed based on optimization results and bending angle controlling strategy, experiments were also carried out to verify this process design method.更多还原