预处理法铜-钢层合板激光切割试验研究 
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Objective Copper-steel laminated plates exhibit the advantages of both copper and steel. Such plates are extensively used in the aerospace, instrumental, and military fields. However, some problems, such as high reflectivity, serious oxidation of the oxygen-assisted cutting surface, and large seam widths after cutting, can be observed when cutting 1-mm-thick copper-steel laminated plates with a fiber laser. Although some solution methods have been proposed in the existing researches, the protection of the original properties of the material surface, the processing precision, and the product processing technologies are still limited. The effects of lasers and materials can be improved using the pretreatment method proposed in this study, in which the element compositions, morphologies, and physicochemical properties of processing areas are adjusted to ensure processing quality, expand the processing range of a 1064 nm fiber laser, and design the experiment for processing slit gratings. The feasibility of this method is verified.
Methods Based on the analysis of the material properties and the laser processing characteristics, a shallow melting surface could be obtained by controlling the energy input. The plate surface was scanned using a low-power fiber laser by considering oxygen as the auxiliary gas to obtain morphology changes and material properties conducive to laser processing. The processing quality of this method is compared with those of other methods. A three-dimensional (3D) microscope was used to observe and measure the surface morphology, heat-affected zone (HAZ), and slit width. In addition, the Plus software was used to estimate the oxide distribution in the shallow melting area formed by pretreatment. Thus, the proportion of oxide area in the shallow melting area could be calculated. The surface roughness was measured using a Keynes laser confocal microscope. Further, the surface morphological changes in the shallow melting area were analyzed to investigate the influence of pretreatment on laser absorption and subsequent processing. The advantages of this method with respect to quality and technology were deduced via experiments in which different pretreatment methods were used. After reasonable processing parameters were obtained, the slit grating was processed by planning the processing path, and the deformation and accuracy were controlled within the required range.
Results and Discussions In this study, the plate surface is guided to obtain shallow melting and black oxide via pretreatment. The width of shallow melting on the material surface is smaller than the spot diameter, and the oxide and molten substance overlap with each other. Therefore, the main energy absorption area with respect to the spot diameter is segmented and reduced, and black oxide can effectively improve the laser absorption rate. Thus, the problem of low laser absorptivity on the surface of the copper-steel laminated plates can be solved and the appropriate energy utilization can be achieved. Hence, most of them are used to remove the materials within the irradiation range and reduce heat accumulation at the edge of the slit. The slit width is smaller than the spot diameter, and the HAZ of the slit is reduced. Thus, the cut accuracy and quality of the laser-cut copper-steel laminated plates are improved when the proposed pretreatment method is applied. Through pretreatment, a pretreated surface containing 36.54%--77.84% of oxidation area can be obtained. The larger the proportion of the oxidation area, the lesser is the amount of oxide copper spots in the shallow melting zone, the greater is the density of the molten pool, and the more uniform is the edge line. Compared with that of the oxygen-assisted cutting method, the seam width of the pretreatment method decreases by approximately 45% and that of the carbon black coating method decreases by 36%.
Conclusions The material surface melts in the form of a shallow layer when the energy density is controlled and the oxidation reaction is guided. The width of shallow melting on the material surface is smaller than the spot diameter. Compared with other methods, the material around the slit is not affected by oxidation or pollution before and after processing. Compared with other methods, the laser pretreatment method achieves a better cutting quality and requires shorter time. By conducting the complete experiment with respect to two factors and three levels, a pretreated surface in agreement with the shallow melting state can be obtained. In this treatment range, the processing parameters are selected based on the proportion of oxidation area to perform the cutting experiment, and the grating sample is produced. The results show that a slit grating satisfying all the quality requirements can be obtained using the proposed method. Further, the HAZ is considerably smaller after cutting, and there is no slag on the back. The properties of copper cladding before and after cutting are protected to ensure that the quality of the copper-steel laminated plates cut by the fiber laser meets the application requirements.
1 引言
铜-钢层合板是由覆层紫铜及碳钢基板经冷轧制备而成,兼具铜材优良的导热性、耐腐蚀性和耐磨性以及碳钢的强度和刚度。层合材料具有优于两种单一金属的特性并节约了铜材,在航空航天、**以及精密仪器仪表领域有着极高的应用价值。在光纤激光切割领域,激光吸收率低是铜材切割面临的难题之一,波长为1064 nm的光纤激光吸收率不足10%[1]。为了解决这一难题,国内外学者提出了很多方法。Dubey等[2]等研究了激光束在精密切割金属薄板特别是铝等反光金属薄板方面的应用,并研究了Nd∶YAG激光切割铝合金板的最佳切削参数。Robinson等[3]研究了180 fs、775 nm脉冲激光在不同辅助气压下与铝表面的相互作用,分析了材料再定位、残余表面粗糙度和烧蚀速率等参数。Riveiro等[4]研究了不同辅助气体对典型Al-Cu合金激光切割刃口质量及其表面化学性质的影响。倪超等[5]利用飞秒激光超短脉冲和超高峰值功率的特性,在厚度约为18 μm的覆铜板上实现了完全刻蚀。张菲等[6]研究了波长为355 nm的紫外激光器和波长为1064 nm的红外激光器刻蚀铜的效果,研究结果表明,铜对紫外激光的吸收率较高且缝隙表面热影响区较小。侯红玲等[7]利用4种涂层材料对反光材料表面进行了表面处理,并检测了吸收率的大小。焦俊科等[8-9]利用不同激光器先对紫铜表面进行处理,再进行焊接加工。孙文峰等[10]利用脉冲激光辐照,有效调谐了Ag、Cu、Al三种金属薄膜的等离子特性。张晓等[11]利用飞秒激光对铜板进行刻蚀试验以实现线路成形制造等。以上研究在材料表面原有性质保护、加工精密度以及产品加工工艺方面鲜有涉及。
铜-钢层合板兼具两种材料的性能优点,满足零件的使用需求,但也存在加工难度。以典型零件狭缝光栅为例,覆层铜保证光栅件有抗腐蚀、物化性质稳定的光洁表面,基体碳钢保证了光栅件的强度。就零件的结构及精度而言,激光切割是最理想的加工方式,但该加工方式面临着材料反光、断栅、材料污染和精度要求高等难题。本文提出了预处理法,并加工了铜-钢层合板,保证了加工质量,拓展了1064 nm波长光纤激光器的加工范围。设计了狭缝光栅的加工试验,验证了该方法的可行性。
2 试验材料及设备
2.1 试验材料
试验采用的材料为1 mm厚的铜-钢层合板。该层合板采用冷轧复合的方式,将异性金属轧制在一起,上下覆层为0.05 mm厚的T2紫铜,中间基板为0.9 mm厚的SPCC碳钢。
2.2 试验条件
试验使用正天激光生产的光纤激光切割机,激光输出波长为1064 nm,最大输出功率为750 W,试验采用的切割速度范围为2~10 m/min。试验分别采用体积分数为99.95%的氧气和体积分数为99.99%的氮气作为辅助气体,预处理气体及切割气体的压力大小相同,皆取1 MPa,经查阅相关设备参数,算得气体流量为14.34 m3/h。
预处理试验参数:采用104 W/cm2和105 W/cm2两种数量级的能量密度及6~10 m/min的扫描速度进行预处理表面改性,改性采用单次扫描,辅助气体为氧气,光斑聚焦在材料的表面,切割路径即改性路径。设置扫描结束后激光头返回初始点,为正式切割做准备。
切割试验参数:功率为600~750 W,速度为6~10 m/min,离焦量为0,氮气作为辅助气体。切割路径按照预处理路径进行,操作过程仅需更改工艺参数的功率、速度、气体即可,其他加工工艺参数在整个过程中不变,设置如
表 1. 激光切割工艺参数
Table 1. Process parameters for laser cutting
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2.3 检测设备及方法
使用日本KEYENCE公司的VHX-600E型超景深数码显微镜检测预处理后材料表面的浅层熔化区域及切缝宽度,并拍摄放大500倍后的显微图像, 最高分辨率可以达到5400×104 pixel。利用FEI公司生产的Q45扫描电子显微镜自带的能谱仪(EDS),对预处理后的材料表面进行元素检测,分析切割后的材料成分构成。利用基恩士激光扫描共聚焦显微镜(LSCM)VK-X260K检测浅层熔化区的表面粗糙度。将拍摄的预处理表面图片导入IPP(Image-Pro Plus)软件,利用物质颜色的差异来测定氧化物在切割区域的面积占比,并以此为基础研究预处理法与切割质量之间的关系。
3 铜-钢层合板的预处理
3.1 预处理后板材表面元素的检测
为了进一步验证预处理后材料表面浅层熔化区内的材料状态变化,用扫描电子显微镜自带的能谱仪对浅层熔化区进行元素检测,以此确定预处理后材料表面的元素成分,论证材料状态变化与激光吸收率的关系。
在试验中,氧气作为辅助气体,所以铜表面的氧化反应可表述为
结合(1)式及元素含量的检测结果可知,表面的铜与氧气发生氧化反应,生成了黑色的氧化铜,且氧元素的含量在浅层熔化区明显较高。通过研究分析可知,由于氧化铜的导电性差,因此激光的吸收率得到提高。
3.2 预处理后板材表面粗糙度的检测
为了进一步研究预处理对铜-钢层合板激光吸收率的影响,利用激光共聚焦显微镜对浅层熔化区内的粗糙度及截面曲线进行了检测,其结果如
由
图 2. 不同粗糙度的预处理区域。 (a)小表面粗糙度;(b)大表面粗糙度
Fig. 2. Pretreated areas with different roughness. (a) Low surface roughness; (b) high surface roughness
预处理后的表面粗糙度范围为0.59~1.56,而板材原表面的粗糙度为0.35。浅层熔化后,当表面粗糙度较小时,熔斑之间的衔接处比较平滑,难以实现激光的多次反射吸收,如
3.3 预处理试验与图像处理
3.3.1 材料表面的预处理试验
根据浅层熔化的能量密度范围要求,通过控制预处理功率、扫描速度两个参数,可以得到各类型的浅层熔化表面。按照“正交试验法”设计2因素3水平完全试验,选取具有不同氧化面积占比的典型表面以进行切割试验。后续根据加工效率以及切割后的缝宽、挂渣情况、氧化面积大小等工艺需求确定预处理参数范围。其中,功率分别取用7.5,15,37.5 W,并且每一个功率分别对应6,8,10 m/min的扫描速度进行预处理实验。
3.3.2 利用图像处理法分析预处理表面
预处理后材料表面生成了黑色氧化铜,分析可知氧化铜沿扫描线分布,且由于功率小、扫描速度快,浅层熔化区域的氧化铜分布不连续。利用IPP图像处理软件研究激光预处理后的材料表面,分析不同功率密度及扫描速度影响下的氧化面积占比,并以此研究切割质量。
氧化面积占比是指因氧化反应,预处理材料表面产生的氧化物在浅层熔化区域内所占面积的比例。如
利用该方法对前述预处理表面进行氧化物面积占比测定,结果表明,当功率为7.5~37.5 W,扫描速度为6~10 m/min时,可获得氧化物面积占比为36.54%~77.84%的预处理表面。基于此结果进行切割试验。
4 铜-钢层合板切割试验与分析
4.1 试验要求及板材处理
为了提高狭缝光栅件的切割质量,选取现有的辅助激光加工的工艺措施,进行切割质量对比试验。通过对切缝宽度、热影响区大小等进行测量比较,论证在精密加工需求下预处理法的优点。
板材处理:在部分铜-钢层合板表面涂覆碳黑,其余用乙醇擦拭以去除表面污垢,晾干备用。
预处理法所用材料:根据预处理试验结果,分别选取氧化面积占比为40%~50%、50%~60%、60%~70%、70%~80%的板材进行切割试验以研究切割质量的相关问题。
4.2 铜-钢层合板的切割质量分析
研究表明,预处理法能够获得小于光斑直径的缝宽。在同样的切割参数下,预处理法所得的切缝宽度小且均匀、无热影响区、无挂渣;氧气辅助切割的切缝宽且热影响区大,在切缝背面留有飞溅的熔渣;预加涂层则污染材料表面的缝宽居于前两者之间且切缝边缘呈锯齿状。
经超景深显微镜测量,氧气辅助切割的缝宽为176.07 μm,如
图 5. 不同方法下的板材切割质量。 (a)(d)预处理法;(b)(e) 氮气辅助切割;(c)(f)氧气辅助切割
Fig. 5. Cutting quality under each method. (a) Pretreatment; (b)(e) N2-auxiliary cutting; (c)(f) O2-auxiliary cutting
分析可知,预处理法有效解决了覆层铜吸收率低的问题,改变了光斑内能量吸收区域的分布。浅层熔化区域内的氧化物吸收了大部分激光的入射能量,且由于预处理的特点,浅层熔化区域的大小也限制了能量的吸收范围。所以吸收的能量主要用于材料的去除,减少了热扩散导致的材料物化性质的改变。
4.3 狭缝光栅件的激光切割试验
为了进一步验证该方法的实用性,对狭缝光栅件进行加工。相对于普通光栅件,狭缝光栅对切缝的精度要求更严格。普通光栅件的切缝在毫米甚至厘米级,而狭缝光栅的切缝要求在微米级,超精密光栅的切缝要求甚至在纳米级。对于大部分光栅件,微米级切割精度的使用需求最大。
预处理实现了材料表面光斑区域内的浅层熔化,由
图 6. 狭缝光栅的激光切割试验。 (a)狭缝光栅的预处理实物照片; (b)狭缝光栅的激光切割实物照片
Fig. 6. Laser cutting test of slit grating. (a) Pretreatment picture of slit grating; (b) laser cutting picture of slit grating
5 结论
预处理法相对于其他方法,能控制能量密度并引导氧化反应,使材料表面发生浅层熔化,材料表面的浅层熔化宽度在光斑直径以内。同比其他方法,切缝周边材料在加工前后不因氧化或污染而影响材料物化性质。利用预处理法,获得了氧化面积占比为36.54%~77.84%的预处理表面,氧化面积占比越大,浅层熔化区内的未氧化铜斑越少,熔池密度越大,边线分布越均匀。其中,当氧化面积占比为46.20%、50.53%时,获得了宽度小于光斑直径的均匀切缝,最小缝宽为90.42 μm,同比氧气辅助切割,缝宽减小了44.94%。通过设计2因素3水平的完全试验,获得了符合浅层熔化条件的预处理表面,按照氧化面积占比大小,选取对应预处理后的板材进行切割试验,并加工了光栅样件。通过切割质量对比发现,同比其他方法,激光预处理法的切割质量更优,且用时较短。
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Article Outline
王坤, 王续跃. 预处理法铜-钢层合板激光切割试验研究[J]. 中国激光, 2021, 48(6): 0602105. Kun Wang, Xuyue Wang. Laser Cutting Test of Copper-Steel Laminated Plates by Pretreatment Method[J]. Chinese Journal of Lasers, 2021, 48(6): 0602105.
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