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激光清洗对象为Q235钢板。Q235的质量分数分别如下:w(C)=0.0120~0.0130,w(Mn)=0.0040~0.0060,w(Si)=0.0010~0.0070,w(P)≤0.0003,w(S)≤0.0003,w(Cr)=0.0475~0.0550,w(V)= 0.0080~0.0140,w(Mo)=0.0090~0.0140,其余为Fe。
试验中采用的Q235钢板材规格为60mm×40mm× 1mm,用75%酒精清洗样品,保证样品表面无其它污染物,待表面干燥清洁后备用。试验样品如图 4所示。图 4b显示样品表面整体锈蚀,并出现大量斑点腐蚀,点蚀的深度往往较其它区域的腐蚀更深。
本试验中所使用的激光清洗系统如图 5所示。该激光清洗系统采用IS-0604QCW型Nd∶ YAG准连续光纤输出激光器,额定功率600W、波长1064nm,脉冲宽度0ns~100ns,扫描宽度10mm~100mm可调,扫描速率约5000mm/min可调。该系统可实现能量、脉冲宽度、扫描速率、扫描宽度等工艺参量调控,通过控制卡和软件可实现自动清洗。本试验时采用100mm场镜,聚焦光斑直径0.5mm。
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针对Q235钢板的特性,影响激光清洗效果的工艺参量和因素较多,在现有条件下,只能针对主要参量和因素。激光能量密度影响激光清洗阈值,对清洗过程去除机理起决定的作用[23]。清洗速度和扫描宽度影响清洗效率,并且对搭接率也产生影响;离焦量影响聚焦表面激光能量的分布;脉冲重复频率又影响到激光能量密度,该参量是影响激光清洗阈值的主要参量[24]。而激光脉宽影响材料表面的热扩散,对短脉冲造成的细微影响[25]。因此, 本试验中分别对激光能量密度、清洗速度、扫描宽度、离焦量等进行单因素试验。在单因素试验基础上进行多因素正交试验。
在试验时,针对Q235钢板的物理特性和激光清洗设备参量,在合适的参量区间内对激光能量密度、清洗速率、离焦量、扫描宽度和脉冲重复频率进行单因素试验。(1)激光能量密度:在试验时,根据锈蚀层和基材情况选择激光能量密度为5.0J/cm2,7.5J/cm2,10.0J/cm2,结果表明,在5.0J/cm2时除锈效果不理想,在7.5J/cm2左右除锈效果良好,在10.0J/cm2左右除锈效果又较差,并出现二次氧化现象,说明能量密度过大,对基材产生了破坏,如图 6所示;(2)清洗速率:清洗速率在900mm/min~1500mm/min区间内选择900mm/min,1200mm/min和1500mm/min,结果表明,随着清洗速率的增加,清洗效果越来越差,最优清洗速率为900mm/min;(3)离焦量:离焦量在0mm~2mm区间内选择0mm,1mm和2mm,结果表明,在1mm离焦附近时,除锈效果最好;(4)扫描宽度:扫描宽度在30mm~50mm区间内选择30mm,40mm和50mm,试验结果观察显示,最佳的扫描宽度为30mm~ 40mm之间;(5)激光脉冲频率:脉冲重复频率在15kHz~25kHz区间内选择15kHz,20kHz和25kHz进行试验,结果表明,在该脉冲重复频率范围内清洗效果相差不大。因此,本试验中分别选取激光能量密度、清洗速率、离焦量和扫描宽度4个因素进行正交试验,正交试验因素水平如表 1所示,选用L9(34)正交试验。
Table 1. Factor level of orthogonal experiment
level factor laser energy density/(J·cm-2) cleaning speed/(mm·min-1) defocusing distance/mm sweep width/mm 1 6.4 900 0 30 2 7.6 1000 1 35 3 8.9 1200 2 40 根据表 1中的激光工艺参量进行试验。对正交试验结果数据进行综合评分。综合评分以清洗后的表面粗糙度值、锈蚀去除程度、表面形貌评为依据。试验后的9组试验清洗效果在光学显微镜下观察。根据正交试验结果,确定因子的主次顺序,通过对比综合评分的大小,确定各因子主次顺序依次为:清洗速率、离焦量、激光能量密度、扫描宽度。综合评分以清洗后的表面粗糙度值、锈蚀去除程度、表面形貌评为依据,获得Q235钢最佳激光工艺参量,分别为清洗速率900mm/min、离焦量1mm、能量密度7.6J/cm2、扫描宽度30mm。
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目前,激光清洗尚未制定通用质量标准,对金属材料表面污染物清洗质量的表征主要为油脂、污物、氧化皮、铁锈、油漆涂层和杂质去除后,表面具有均匀的金属色泽及残留污染物。激光在去除材料表面污染层的同时,也会对材料表面性能产生影响。输入激光能量过大,会造成基体材料的烧伤,输入能量过小会影响污染层的去除效果,并且输入的能量不同会对基体表面产生不同的热影响。因此,需要对在最佳工艺参量下激光除锈后、未除锈、无锈的样品进行检测,研究激光清洗对Q235钢清洗质量和表面性能的影响。清洗质量主要从材料表面锈蚀层清洗程度和表面的粗糙度来考察。
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激光除锈能有效地去除生锈样品表面的锈层,同样会引起除锈后样品表面粗糙度值的变化。利用NanoMap-D光学双模式3维形貌仪,分别检测了未生锈样品、生锈样品和最佳激光除锈工艺参量下的样品粗糙度值。
图 7为试样的3维形貌图。生锈的样品3维形貌图存在大量锈蚀物,且锈蚀分布不均匀、锈蚀深度不均匀。未生锈的清洁样品表面均匀度好,极少量的绿色和红色是由于样品表面存在微小的划痕。而在最佳除锈工艺参量下除锈后样品表面主要由黄色构成,还存在少量的绿色,说明激光基本上已经把锈蚀清除干净,而且获得了平整的表面。
从检测结果可知,激光光斑重叠均匀,能量在基材表面均匀分布,使辐射后的基材表面平整;由于锈蚀在基材表面的锈蚀程度不均匀,锈蚀厚的位置被激光清除后会留下微小的凹坑;除锈后基材表面留下凹坑取决于锈层的分布。综上所述,激光除锈能改善基材表面的总体粗糙度,但粗糙度的峰值取决于锈层的分布和锈蚀深度的一致性。
图 8为试样的表面轮廓变化状态。从图 8纵坐标值可以看出,锈蚀的样品表面的峰值高度较大,整体曲线存在一个较大的波峰和波谷,说明锈蚀层存在较大的突起和凹陷。而比较未生锈样品和最佳工艺参量下除锈的样品,趋势的起伏相对锈蚀样品小,起伏的程度较为均匀,没有出现较长的波峰,波谷段。对比未生锈的样品和最佳工艺参量下除锈的样品,发现在x方向上形貌的趋势总体上一致,而且纵坐标可以看出,表面整体起伏也一致。
表 2为生锈试件、未生锈试件和激光清洗试件三者表面粗糙度的检测结果。表 2中的数据可以说明,激光除锈之后,材料表面粗糙度Ra比生锈试件有明显减小,表面整体粗糙度接近未生锈试件。由于生锈试件的锈蚀层厚度不均匀,影响到激光清洗试件表面的波峰R1和R2波谷。
Table 2. Roughness test results
type roughness value
Ra/μmcrest
R1/μmvalley
R2/μmunrusted sample 1.430 15.193 -7.231 rusty sample 2.339 23.119 -8.410 laser cleaning sample 1.646 19.532 -8.197 综上所述,在最佳工艺参量下激光除锈可以改善基材的表面形貌,获得接近于未生锈样品的表面形貌。
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对除锈前后的样品做扫描电镜检测,如图 9所示。由对比结果可直观分析表面微结构。其中,图 9a是锈蚀层的扫描电镜图,图 9b是激光除锈后的扫描电镜图。
由图 9可知,激光除锈能彻底清除金属表面的锈蚀层。在扫描电镜图中,可以看到显示材料表面凹坑的黑色区块,这种凹坑的形成有两种原因。其一,材料表面原始锈蚀层形成的凹坑。从锈蚀层的扫描电镜图中,可以看到锈层疏松的表面结构,也可以明显看到锈蚀层表面存在疏松的微孔,黑色锈蚀区域厚度明显小于疏松微孔区。这也是导致激光除锈后表面出现微小凹坑的原因,锈蚀层厚度的不一,锈蚀程度不均匀,当锈蚀层吸收均匀的激光能量情况下,就会导致激光去除锈蚀的厚度不一致,从而在锈蚀深度更深的微小区域留下凹坑。其二,激光扫描在烧蚀锈蚀薄层时形成的。由于激光光束为高斯光束,基材吸收的激光能量同样也会呈高斯分布,光斑中心吸收的能量密度比周围的能量密度高,导致光斑中心区域基材的烧蚀,从而形成凹坑。由图可知,激光除锈能彻底清除金属表面的锈蚀层,清洗质量受激光光束和锈蚀层的锈蚀程度影响。
最后,对试样进行了能谱分析。采用X射线能量色散谱仪对样品表面和断面做能谱分析,分析结果如图 10所示。
由图 10能谱图可以看出,除锈前后试件表面O,K,Fe,Si和Ca各元素的含量变化,其中激光除锈后材料表面的Fe元素的含量增多,O元素含量明显减少,K元素的含量也有降低,Si和Ca元素的含量变化不明显,说明Si元素和Ca元素并未发生氧化分解反应。Fe元素和O元素含量在清洗前后的相对变化,说明锈蚀层在激光的辐照下发生了脱氧的反应。所以可以得出结论:Q235钢表面的氧化物锈蚀已经基本清除干净;另外,从X射线能量色散谱仪所检测的Q235钢表面元素整体分布情况看,由于锈蚀试件表面锈蚀层度不均匀,尽管除锈后O元素明显减少,但由于锈蚀层厚度分布不均匀,锈蚀层厚的地方会残留微量的O元素,锈蚀层的厚度不均匀会影响激光清洗的效果。
激光参量对碳钢表面清洗质量的影响
Influences of laser parameters on the cleaning quality of carbon steel surface
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摘要: 为了研究激光工艺参量对碳钢表面锈蚀污染层清洗质量的影响规律,采用Nd∶ YAG准连续激光器对Q235钢板试样进行了激光除锈清洗试验,并观测了试样表面清洗质量。分析了激光清洗污染物烧蚀的工作机理,并试验研究了激光能量密度、扫描速率、扫描宽度、离焦量和重复频率等工艺参量对清洗质量的影响,在此基础上进行了正交试验;通过对激光清洗前后试件进行表面粗糙度、形貌和能谱分析,获得了Q235钢最佳激光工艺参量分别为扫描速率900mm/min、离焦量1mm、能量密度7.6J/cm2、扫描宽度30mm。结果表明,激光除锈后表面粗糙度和3维形貌得到改善,激光清洗样品的粗糙度值接近无锈样品,激光清洗样品的形貌与无锈样品基本相同;激光清洗后材料表面的锈蚀层基本去除,表面存在重焰的微结构。激光清洗能满足对碳钢表面锈蚀污染层的清洗质量要求。Abstract: In order to study the influence of laser process parameters on the cleaning quality of carbon steel surface contaminant layer, the Nd∶ YAG quasi continuous laser was used to clean the surface rust contaminant of Q235 steel sample. The effect of laser cleaning on the sample surface was observed, and the working mechanism of laser cleaning contaminant ablation was analyzed. Then the cleaning effects and the cleaning quality with different process parameters such as laser energy density, cleaning rate, scanning width, defocusing amount, and repetition frequency were studied experimentally, and based on this, the orthogonal test was carried out to obtain the optimum laser process parameters to the Q235 steel; three-dimensional morphology, scanning electron microscopy, and energy spectrum were conducted to analyse the cleaning effect. And the optimal process parameters were obtained: Laser cleaning rate is 900mm/min, defocusing amount is 1mm, energy density is 7.6J/cm2, scanning width is 30mm respectively. Research shows that the surface roughness and three-dimensional morphology of the laser cleaning samples are improved, the roughness value of the laser cleaning samples is close to that of the rust-free samples, and the morphology of the laser cleaning samples is basically the same as that of the rust-free samples; after laser cleaning, the rust layer on the surface of the material is basically removed, and there are remelted microstructures on the surface. It can be concluded that laser cleaning can meet the cleaning quality requirements for carbon steel.
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Key words:
- laser technique /
- laser cleaning /
- derusting /
- Q235 steel /
- laser parameters /
- contaminant
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Table 1. Factor level of orthogonal experiment
level factor laser energy density/(J·cm-2) cleaning speed/(mm·min-1) defocusing distance/mm sweep width/mm 1 6.4 900 0 30 2 7.6 1000 1 35 3 8.9 1200 2 40 Table 2. Roughness test results
type roughness value
Ra/μmcrest
R1/μmvalley
R2/μmunrusted sample 1.430 15.193 -7.231 rusty sample 2.339 23.119 -8.410 laser cleaning sample 1.646 19.532 -8.197 -
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