【作者】 周益军；

【导师】 张永康；

【作者基本信息】 江苏大学 ， 机械制造及其自动化， 2007， 博士

【摘要】 在分析国内外金属板料无模成形技术研究的基础上,重点分析了基于激光热应力的板料柔性、无模成形技术存在的问题,并提出了目前板材热应力成形技术应用于生产所需要研究的内容。为了对激光热应力成形机理及其关键技术进行系统的研究,对激光诱导的热应力作用下板料的变形现象进行了实验和相关分析。实验中以AISl304、TA2、QT450、YG10板料等为研究对象,在2.5kw RS2000SM快轴流CO₂激光器上进行热应力弯曲成形试验。实验过程中采用空气冷却和水冷却两种方式。采用水冷却时,须保持试样表面露出水面。采用单因素试验法时,改变激光束能量、光斑直径、扫描速度、扫描次数等参数,使其他参数保持一定。为了保证扫描路径和相连两次扫描的时间间隔一致,激光束相对于工作台的运动由数控程序保证。为了确保实验数据的有效性,在设计实验方案前,先根据已知理论进行分析,或者先进行摸索试验,然后估计出能产生热应力弯曲的大致工艺参数组合,最后再进行实验方案的设计。通过多组工艺参数的试验,研究了厚度一定的不锈钢钢板弯曲成形时的工艺参数对弯曲角度的影响,并探讨了基于空气冷却和水冷却的AISI304不锈钢板材热应力成形规律。相关实验结果表明:当其它各参数不变时,增加光束功率后,能量密度（入射到单位表面积上的能量）随之加大,导致板料的弯曲角度显著增大;上述规律不是线性的,有一个最佳的工艺参数。随着扫描次数的增加,诱发热应力成形角度呈近似线性增大;增加扫描速度,则引起能量密度降低,使板料弯曲角减小,该规律是近似线性的。弯曲角度随光斑直径的增大反而下降;该规律是近似线性的。光斑直径满足加工要求是先决条件,线能量密度（P/V值）是关键参数;对0.6mm厚AISI304不锈钢冷轧钢板而言,采用空气冷却时,它的最佳的线能量密度应在30～40J/mm之间变化,否则成形的效率会很低,或者工件表面会产生烧伤现象;采用水冷却时,线能量小于80J/mm时,工件几乎不产生变形;在150～180J/mm范围内,热应力成形的效果最佳。在水冷却条件下,工件的表面烧蚀现象较空气冷却时大为改观,而且成形的稳定性较高。扫描路径为圆弧时,板材弯曲的幅度相对于直线扫描时弯曲的幅度要小得多。为了优化激光热应力成形的工艺参数,以正交试验理论为指导,通过改变激光束能量、光斑直径、扫描次数、扫描速度、板料厚度对0.6mm厚的AISI304不锈钢冷轧板进行激光热应力成形的正交试验,旨在评价上述参数的耦合作用,根据正交试验的原理,可判别相互影响的参数产生作用的主次关系。试验结果表明:在AISI304不锈钢冷轧板激光热应力成形时,正交试验中研究的五个工艺参数产生的作用是不同的,按其变化对弯曲变形量影响的大小排序,它们依次是扫描次数、激光束能量、板材厚度、光斑直径、扫描速度（在水冷却的条件下）。其中扫描次数、激光束能量是两个重要的影响因素,它们对增加弯曲变形都有较大潜力。为了对工艺参数进一步优化,本文对遗传算法的应用的方法进行了研究,阐述了遗传算法在激光热应力成形工艺参数优化上的应用思路。认为原始种群的规模必须较大,而且必须经过摸索试验获得种群。在选择优化模型时,要针对具体的成形条件,先确定其成形机理,再套用对应的计算模型。在研究成形区板料的力学性能、材料组织的变化时,实验中以HVS-1000显微硬度测试仪、X-350A型X射线应力测定仪为工具,使用2.5kW RS2000SM快轴流CO₂激光器对0.6mm厚的TA2板料进行扫描,按照正交试验理论安排成形时的工艺参数,研究TA2板料弯曲成形时主要工艺参数对弯曲角度的影响,以及试样表面残余应力的分布和试样断面上的显微硬度变化情况。结果表明:正交试验中研究的4个工艺参数产生的作用是不同的,按其变化对弯曲变形量影响的大小排序,它们依次是扫描次数、光斑直径、激光束功率、扫描速度。成形参数对试样表面的残余应力分布也存在一定的影响。在试样变形区断面上的显微硬度变化呈现出一定的规律。变形区组织产生了变化。为了有效地控制成形的质量,提出了激光热应力成形质量评估这一问题,研究了激光热应力成形工件表面质量评价指标;分别介绍了表面粗糙度及其相关检验方法、表面缺陷的检验方法和表面综合机械性能及其检验方法。探讨了激光热应力成形工件成形质量评价体系构建方法,为对激光热应力成形工件表面质量进行全面评价提供了参考。为了拓宽了热应力成形的工艺范畴。实验中以球墨铸铁QT-450、硬质合金YG10为研究对象,用CO₂激光器的激光束对球墨铸铁板条及YG10板条进行扫描,通过改变成形工艺参数,研究脆性材料成形的规律,研究试样热应力成形后表面形貌、性能和变形区组织的变化。为了研究试样表面的氧化情况,实验中采用有氮气保护和无氮气保护进行对比,并分析成形区组织变化情况和各元素含量的变化情况。研究结果表明:脆性材料在激光束的热效应作用下也可以弯曲成形。成形后的板料在变形区的组织、硬度和力学性能产生了变化。在有保护气、有水冷却的条件下进行试验效果明显优于在无保护气、无水冷却的条件下进行试验效果。变形区和其他区域的各元素含量相同。更多还原

【Abstract】 In the light of analyzing existent problems of flexible and die-less sheet metal forming technology by laser induced thermal stress as a high spot, the research on die-less sheet metal forming technology at home and abroad was studied. And what is in need of research in applying sheet metal forming technology at present has been put forward.In order to make a systematic study the mechanism of laser forming with thermal stress and its key technology, the phenomenon of forming for sheet steel by laser thermal stress was studied and analyzed. In the experiment, sheets of AISI304 stainless steel, TA2 and QT450 were selected as test samples, the laser beam was produced by a 2.5kW CO₂ laser. Test samples were cooled by water or air. With water-cooling, the surface of the samples must be exposed out of the water. When the pattern of cooling was selected, the experiment adopts the single-factor method, which means that only one of the parameters such as laser power, laser spot, scan speed, number of passes is changed to investigate the mechanism of laser induced thermal stress forming. The scanning type is reciprocating and continuous, and the scan path and the time interval are consistent. It is carried out by numeral control. In order to ensure validity of experimental data, it is necessary to analyze according to known theory before designing experiment plan, and estimate combination process paramerters of the forming to bring bending, then design the experiment, The thickness of the test samples was a constant in the research, and the process parameters were multipacket, air-cooling and water-cooling were used in investigating the relationship of AISI304 stainless steel sheet laser bending. The results show that there is no bending for 0.6 mm thickness of AISI 304 sheet steel under the condition of water cooling when linear energy density is less than 80J/mm; the best value of the linear energy density is between 150 and 180J/mm for it; the technological parameters such as the thickness of sheet metal, laser spot and scanning times have great effect on the bending angle. And the ablation and stability of bending for AISI304-sheet metal in the experiment is evaluated as well. The new idea of sheet forming by laser thermal stress based on water-cooling is put forward, which provides the experimental foundation to control accurately the sheet forming by laser induced thermal stress. The above experimental result shows that the power density increases with the increase of laser power when other parameters keep constant, which eventually results in the distinct increase of the bending angle. The above-mentioned mechanisms are non-linear and there is an optimal process parameter. Bending angle induced by laser with thermal stress increases approximately in the linear proportion to the feed number. Increasing scan speed may decrease laser energy density, and then lessens bending angle. The effect of scan speed on the bending angle is approximately linear. Bending angle decreases when laser spot increases while other parameters keeps constant. The relationship mentioned above is approximately linear. It is obvious that the reasonable size of laser spot is one of the prerequisites in laser forming with thermal stress; the value （P/v） of the linear power density is the key process parameter. The best value of the linear power density is 30-40J/mm for 0.6 mm thickness AISI 304 sheet steel under the condition of air-cooling. Otherwise, bending angle is very small and ablation occurs on the surface. When the linear power density is less than 80 J/mm, there is no bending angle under the condition of water-cooling. And the best linear power density is 150-180J/mm; there is slight ablation on the surface of the samples, and the stability of forming is superior, the acquired angle of curve bending drops sharply, and decreases with the increase of the curvature.In order to optimize process parameters of bending with thermal stress induced by laser, an orthotropic experimental method was adopted here. In this experiment, laser bending of the 0.6mm thick AISI 304 sheet was studied by changing the laser power density, laser spot, scan speed, number of scan passes and thickness of sheet, and it can make a comprehensive evaluation on coupling of process parameters. According to the principle of orthotropic experimental method, the relationships of the interacted parameters can be distinguished. The results show that the five parameters develop different effects under the condition of water-cooling. Number of passes ranks first, followed by power density, thickness of sheet, laser spot and scan speed in terms of their effects on the bending angle, and the two most potential important parameters are the number of passes and the power density, They have strong potential to increase bending angle in laser bending.In the interest of further optimization of process parameters, Genetic algorithms was studied here, and the applying plan to optimize process parameters of bending with thermal stress induced by laser using Genetic algorithms was set forth. It is thought that the scope of population should be large, and the population should come from grope test. The condition of forming should be considered firstly when choosing optimization model, choose the mechanism of laser forming, and then apply mechanically the computational model.For the sake of studying mechanical performance and metallographic phase of forming area, a 2.5 kW CO₂ laser, a HVS-1000 Micro-hardness measuring instrument and an X-350A X-ray stress-measuring-instrument were adopted in the experiment where 0.6mm thickness sheet steel of TA2 was studied to evaluate the process parameters of bending with thermal stress induced by laser. Bending angle was measured as the function of main processing parameters. These processing parameters were arranged according to the theory of orthotropic experiment. Distribution of residual stress on the surface and variation of micro-hardness on the cross section of samples were studied after forming. The results show that these parameters bring different effects. The number of passes ranks first, followed by laser spot in terms of their effect on the bending angle, with power of laser beam and scan speed being the last two factors. Parameters of laser forming also have some influence on the distribution of residual stress on the surface of samples. Certain regularity appears in the variation of micro-hardness on the cross section of samples. The structure in the deforming area changed.To control forming quality effectively, the problem of how to evaluate forming quality was raised; the evaluating index of the surface quality on the samples after laser bending were studied here; surface roughness and its measurement, surface flaw and its measurement, surface association mechanical property and its measurement were introduced. The method of how to establish quality-evaluating system was discussed, which provides reference to a comprehensive evaluation on the surface quality for test sample after laser bending.By way of widening the forming technology, a RS2000SM CO2 laser of 2.5kW was used here to irradiate the nodular cast iron plate and hard alloy plate, the forming rule of brittle material QT-450 and YG10 was studied by changing the parameters of the laser beam and the machine tool; and the change of the surface properties of the QT-450 plate after laser forming was investigated. In order to study the oxidation in laser forming, Nitrogen-protecting and no protecting are adopted in the test; the structure and the content of elements of forming area was investigated. The analyses showed that the brittle material, such as nodular cast iron, can also be bent by thermal effect of laser beam; the structure, appearance, hardness and the mechanical performance of the plate in the deforming area also changed; and the case with Nitrogen-protecting and water-cooling is more excellent than that without air-protecting and water-cooling. The content of elements of forming area and other area is sameness.更多还原