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本实验中选用的SiCp/Al复合材料是以6061铝合金和SiC颗粒为原料,采用粉末冶金方法制成,其中将原料按照不同的比例混合以达到不同颗粒体积分数的要求。材料的规格是100mm×150mm×4mm的板材,SiC颗粒平均尺寸是60μm,体积分数为0.70。图 1为其材料显微图。图中白色区域为SiCp增强体,黑色区域为Al基体,材料微观结构显示SiCp均匀分布在Al基体中。
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为了研究皮秒激光参数对孔重铸层厚度的影响规律,结合皮秒激光加工系统的性能,设置各加工参数的单因素实验。实验中采用的激光器为DL-600P,输出激光波长为355nm、脉冲宽度为10ps的紫外皮秒激光器,该设备最大功率为12W,透镜聚焦激光束焦距为100mm,在实验过程中使用压缩空气作为辅助气体帮助去除熔融材料。已有的研究表明,激光功率百分比和扫描速率对重铸层厚度有显著性的影响[16-17],因此, 本文中的重点研究这两个参数对厚板铝基复合材料皮秒激光制孔重铸层的影响规律。皮秒超短脉冲激光实验平台如图 2所示,激光器参数如表 1所示, 实验参数如表 2所示。在各组实验中,为保证对目标参数单一变量的研究,其余参数均保持不变。为保证实验数据的可靠性,避免实验中的偶然误差,选取多个位置多次测量后取平均值作为实验结果。
Table 1. Laser system parameters
parameter value laser power 12W wavelength 355nm spot diameter 10μm pulse width 10ps laser frequency 500kHz Table 2. Experimental parameters (hole diameter is 1mm, gas pressure is 1MPa)
No. percentage of power/% scanning speed/ (mm·s-1) 1 70 300 2 80 300 3 90 300 4 100 300 5 100 400 6 100 200 7 100 100 -
皮秒激光加工实验台的光路系统如图 3a所示,实验目标为在SiCp/Al复合材料基板表面加工直径为1mm的通孔。激光路径图如图 3b所示,激光经光路系统最终到达样品表面的实际加工聚集光斑直径为10μm。激光头按照由外向内的同心圆路径进行扫描加工,画一个直径为R的圆(本实验中R=500μm),扫描路径致密填充直至填满该圆(本实验中设置的填充间距H=10μm),激光加工时逐圈扫描直至孔中央。激光扫描速率为v,完成一次由外向内的同心圆加工等同于完成一次扫描,可以设定多次扫描。
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加工前,先要对试件进行处理,将试件放在乙醇溶液中经超声波清洗20min, 以去除SiCp/Al复合材料表面的杂质。加工后,采用线切割、砂纸打磨、超声振动清洗、机械抛光、吹风机吹干、溶液腐蚀等技术手段对样品进行处理,此后需要在工件表面喷金进而使其获得导电层。喷金设备如图 4a所示,喷金后的样品使用台式发射扫描电子显微镜(型号:TM3030)观察孔内壁重铸层的形貌;使用图 4b所示的Leica显微镜(型号:DVM6)在不同倍率下观察孔的出入口形貌、表面轮廓和孔的形态。
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图 5所示为不同功率百分比加工孔时产生的重铸层及其裂纹放大图。裂纹的产生是在激光加工提供的高温环境下,该复合材料与氧气发生反应,生成SiO2和Al2O3,如下式所示:
$ 4 \mathrm{Al}+3 \mathrm{O}_{2}=2 \mathrm{Al}_{2} \mathrm{O}_{3} $
(1) $ \mathrm{SiC}+2 \mathrm{O}_{2}=\mathrm{SiO}_{2}+\mathrm{CO}_{2} $
(2) 当温度从高温下降到环境温度时,会产生残余应力[17]。生成物SiO2(5×10-5/K)和Al2O3(7.2×10-6/K)的热膨胀系数不同是残余应力产生的最主要原因,因此在冷凝的重铸层中产生许多裂纹。图 5a所示在重铸层厚度较小且表面没有明显的裂纹。图 5b所示为在重铸层表面产生的裂纹。图 5c所示为裂纹加深,从表面深入到重铸层内部。图 5d所示为内部的横向和纵向的裂纹交错,使得重铸层发生大面积的断裂和脱落[18]。由图 5可知,在固定的扫描速率(300mm/s)下,增加激光功率百分比,重铸层的表面裂纹逐渐加深,从单一的横向或纵向发展为裂纹纵横交叉,从而导致重铸层的脱落,而且重铸层中的SiC颗粒还阻碍裂纹的延伸。
图 6所示为重铸层厚度随激光功率百分比变化的趋势图。由图 6可知,随着激光功率百分比的增加,重铸层的厚度并未呈现单调性的变化趋势而是先增加后减小。这是因为增加了激光功率,孔内部产生的等离子体就增多,随着电子密度增加,等离子体密度增加并超过临界值,等离子体屏蔽阵面形成,使得后续入射的激光脉冲发生反射,从而阻碍激光继续向下照射,抑制了烧蚀的进行,喷射压力减小,熔融物无法迅速排出孔外,导致重铸层冷凝变厚[19];但是在100%的激光功率下,重铸层厚度最小,这是由于增加功率引起的增强烧蚀作用大于等离子体的阻碍作用,使得产生金属蒸气压力增大,带走的液相物质增多,减少了熔融物的冷却重凝。
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图 7所示为激光功率百分比是100%时不同扫描速率下加工孔产生的的重铸层及其裂纹放大图。图 7a所示孔壁边缘有重铸层脱落,并且有少许SiC颗粒夹杂其中;图 7b所示为重铸层脱落之后的形貌,重铸层与基体贴合较为紧密;图 7c所示横向和纵向的裂纹交叉,导致重铸层脱落[20];图 7d所示熔融物和基体的结合性能较弱,有很大的缝隙存在,这是由于SiC与氧气的氧化反应中会产生CO2,CO2从重铸层逸出,使得重铸层与基体出现缝隙。在本实验中,在200mm/s的扫描速率下,加工750次,所需时间约为24min,与在400mm/s的扫描速率下加工1500次所需时间接近。
图 8所示为重铸层厚度随激光扫描速率变化的趋势图。平均厚度和最大厚度随着扫描速率的增加而增加,参考文献[12]中指出,这是由于扫描速率增加,比能减少,更高的比能以及高压辅助气体喷射有助于有效地去除重铸层和孔出口处的残渣。由图 8可知,在一定激光功率百分比下,较低的扫描速率下使得激光在材料表面停留的时间较长,脉冲重叠率较高,导致材料局部温度升高。由于材料对激光的吸收程度与温度有关,温度越高,材料对激光的吸收效果越好,熔融效果越明显,“库伦爆炸”越剧烈,而且加工孔所需的时间更短。
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图 9a所示为通孔形貌,孔的锥度为5.342°。在测量并记录重铸层厚度时发现,孔壁重铸层的厚度分布不均匀,呈现两端薄,中间厚的弧状形貌,原因是该实验板材相对较厚,孔内的熔融物向外喷射距离变长,在靠近孔入口处,激光烧蚀产生的等离子体云容易喷射出孔外,靠近底部的熔融物又能被高压辅助气体从出口排出,而中部由于熔池对流而未能迅速排出的熔融物重新凝结并形成较厚的重铸层。在激光作用的初始阶段,熔池逐渐形成,此时熔融物厚度分布不均,底部最薄顶部最厚,且有熔融物外溢和喷溅排出现象;另外,熔融物流速各部分也有差别,参考文献[10]中指出, 毫秒激光打孔喷溅而出的熔融物速率约为100mm/s,而熔池内熔融物最大流速达到了274mm/s。皮秒激光属于“冷加工”,材料直接变成等离子体并在熔池和辅助气体的的反冲压力作用下发生逸散[21],成孔的孔壁热影响区较小,烧蚀过程中的等离子体也不会轻易重新凝结成重铸层。但是长时间的激光加工依然会出现较多热量,还是会形成重铸层,当无激光能量的输入时,熔融物很快冷却结晶形成重铸层,厚度不再变化。
SiCp/Al复合材料厚板皮秒激光制孔重铸层影响研究
Study on the influence of picosecond laser hole recasting layer of SiCp/Al composite thick plate
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摘要: 为了探究激光加工高体积分数碳化硅颗粒增强铝基(SiCp/Al)复合材料厚板的成孔特征,采用激光旋切法对厚度为4mm的SiCp/Al复合材料进行直径为1mm的制孔实验,分析紫外皮秒激光制孔中重铸层及其表面裂纹的形成机理,获得了关键激光加工参数(激光功率百分比和扫描速率)对孔重铸层的影响规律。结果表明,SiCp/Al复合材料厚板皮秒激光加工中存在百微米级别的重铸层,重铸层厚度不随激光功率的增加而单调变化,在100%的激光功率下厚度最小, 其值为99.5μm;而随激光扫描速率的增加而增加,在100mm/s的扫描速率下厚度最小,其值为71.2μm; 厚板紫外皮秒激光加工SiCp/Al复合材料的重铸层呈现“月牙型”的弧状形貌特征,同时重铸层表面显著存在横向和纵向等多种裂纹。该研究为实现SiCp/Al复合材料皮秒激光低损伤微小孔加工提供了一定的理论指导。
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关键词:
- 激光技术 /
- 重铸层厚度 /
- 紫外皮秒激光 /
- SiCp/Al复合材料
Abstract: In order to research the hole-making characteristics of high volume fraction silicon carbide particle reinforced aluminum matrix (SiCp/Al) composite material thick plates processed by laser, the hole making test of 4mm thick plate was carried out using different laser processing parameters. And the formation mechanism of the recast layer and its surface cracks in the hole made by the ultraviolet picosecond laser was analyzed. The influence of key laser processing parameters (laser power percentage and scanning speed) on the hole recast layer was obtained. The results of the experiment show that there is a recast layer of hundred microns in the picosecond laser processing of SiCp/Al composite thick plates. The thickness of the recast layer does not change monotonously with the increase of laser power, and the thickness is the smallest at 100% laser power with value of 99.5μm; the thickness increases with the increase of the laser scanning speed, and the smallest value is 71.2μm at a scanning speed of 100mm/s with. In addition, the recast layer of the thick plate ultraviolet picosecond laser processed SiCp/Al composite has a "crescent" arc-shaped morphology. At the same time, there are significant horizontal and vertical cracks on the surface of the recast layer. This research provides theoretical guidance for low-damage micro-hole machining of SiCp/Al composites with picosecond laser. -
Table 1. Laser system parameters
parameter value laser power 12W wavelength 355nm spot diameter 10μm pulse width 10ps laser frequency 500kHz Table 2. Experimental parameters (hole diameter is 1mm, gas pressure is 1MPa)
No. percentage of power/% scanning speed/ (mm·s-1) 1 70 300 2 80 300 3 90 300 4 100 300 5 100 400 6 100 200 7 100 100 -
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