-
试验材料为6005A-T6铝合金车体部件型材,激光清洗试验试样尺寸为10 mm×10 mm×4 mm方形小片试样,激光清洗-电弧焊接试样尺寸为长1 m的车体部件型材,焊接接头形式为搭接,搭接区域上下板厚为4 mm。试验采用杰普特纳秒脉冲光纤激光器,激光经准直、均匀化、偏转后聚焦在工件表面形成高斯光斑,激光扫描路径如图1所示,其中X方向为清洗速度V方向,Y方向为振镜往复扫描方向,扫描线宽L即为清洗宽度,焊接方向与X方向一致。激光器参数如表1所示。
表 1 激光器参数
Table 1. Parameters of laser
Parameter Symbol Value Wavelength/nm λ 1064 Average power/W P 0-200 Repetition frequency/kHz F 1-4000 Pulse width/ns l 8-500 Scanning frequency/Hz f 0-300 Focal length/mm f1 254 Spot diameter/μm D 92.9 Laser mode parameter M2 1.8 -
在铝合金激光清洗过程中,影响清洗效果的典型工艺参数包含激光平均功率P、激光重复频率F、振镜扫描频率f、清洗速度V等。针对上述参数,将清洗速度与焊接速度匹配,调整其余工艺参数,得到合适的焊前激光清洗工艺参数组合。基于现场实际加工条件及大量前期基础研究,同时匹配焊接速度,保证清洗范围和效率,清洗过程中扫描线宽L需要保持在45 mm以上。
因此,为实现清洗效率的最大化,保持激光功率200 W,在不同清洗速度下,为保证激光搭接率,需要对激光重复频率及振镜扫描频率进行调整。与不同焊接速度匹配后,进行焊前激光清洗工艺试验,工艺参数组合如表2所示。
表 2 焊前清洗工艺参数组合
Table 2. Combination of cleaning process parameters before welding
No. Average
power,
P/WRepetition
frequency,
F/kHzLine
width,
L/mmCleaning
speed, V/
m·min–1Scanning
frequency,
f/Hz(a) Untreated (b) 200 160 45 0.5 100 (c) 200 170 45 0.6 105 (d) 200 180 45 0.7 110 (e) 200 200 45 0.8 120 (f) 200 220 45 0.9 130 (g) 200 250 45 1.0 145 (h) 200 260 45 1.1 150 利用光学显微镜观测激光清洗前后铝合金表面形貌,基于理论计算,通过改变振镜扫描频率及激光重复频率,建立可匹配不同焊接速度的焊前激光清洗工艺参数预测模型。
为研究激光清洗前后铝合金表面氧化膜的变化情况,采用场发射透射电子显微镜(TEM)观测激光清洗前后截面Al、O元素分布;利用X射线光电子能谱仪(XPS)分析激光清洗前后样品深度方向氧含量变化情况;使用激光共聚焦显微镜测量激光清洗去除深度。
通过分析实际生产过程中激光清洗-电弧焊接同步加工问题,设计清焊一体化装置,该装置可实现清洗在前,焊接在后,清洗和焊接工序同步进行。为进一步验证去除铝合金表面氧化膜对焊接性能的改善,以焊接接头剪切性能等典型焊接性能为目标,对焊缝性能展开研究,剪切试验试样以焊缝区域为中线取样。基于获得的焊前激光清洗工艺参数,对同步激光清洗后的焊接接头进行力学性能测试,以验证焊前激光清洗对铝合金产品焊接性能的影响;采用渗透探伤方式检测激光清洗前后焊缝气孔产生情况;取激光清洗后的焊缝试样,使用自动磨抛机对试样进行机械打磨抛光,抛光后的铝合金试样通过Kroll试剂进行金相腐蚀。将制好的样品置于光学显微镜中,对激光清洗后焊接接头的截面形貌及微观结构进行观察与分析。
-
为匹配焊接速度,保证光斑搭接,结合实际生产中的工况,通过理论计算建立匹配不同焊接速度的清洗工艺参数预测模型。为保证有效去除铝合金表面氧化膜,激光能量密度需达到铝合金表面氧化膜清洗阈值[12-13],应维持在8.75~26.25 J/cm2[12],并满足激光光斑的密排分布。
由激光能量密度计算公式可知:
$$ E=\frac{4P}{\pi F{D}^{2}} $$ (1) 式中:E为激光能量密度,则重复频率F需满足112.4~337.2 kHz。此外,针对激光光斑分布,需满足下述计算公式。
$$ \left\{\begin{array}{c}{N}_{X}=\dfrac{2Df}{V}\\ {N}_{Y}=\dfrac{DF}{2fL}\\ N=\sqrt{\dfrac{{N}_{X}^{2}{N}_{Y}^{2}}{{N}_{X}^{2}+{N}_{Y}^{2}}}\end{array}\right. $$ (2) 式中:NX、NY和N分别为清洗方向X、扫描方向Y和斜向光斑搭接个数。为满足光斑斜向搭接(即N≥1),需保证NX,NY≥
$ \sqrt{2} $ 。在实际生产过程中,光斑直径D和扫描线宽L均为定值,清洗速度V与焊接速度相匹配,则有:$$ \left\{\begin{array}{l}\dfrac{\sqrt{2}V}{2D} \leqslant f \leqslant {f}_{{\rm{max}}}\\ \dfrac{2LV}{{D}^{2}} \leqslant F \leqslant \dfrac{4P}{\pi {D}^{2}{E}_{{\rm{min}}}}\end{array}\right. $$ (3) 式中:fmax为激光器最大扫描频率;Emin为激光能量密度最低阈值。在实际生产过程中,考虑振镜偏差及行进过程中焊接机器人的抖动,可在公式(3)计算结果的基础上适当增大扫描频率f和重复频率F的值。
利用上述焊前同步激光清洗工艺参数预测模型,可实现200 W脉冲激光器,保持线宽45 mm情况下,快速匹配焊接速度0.5~1.1 m/min的激光清洗工艺参数。
通过不同工艺参数组合的激光清洗试验对上述模型进行验证。不同工艺下激光清洗后铝合金表面微观形貌如图2所示,图2(a)~(h)对应表2(a)~(h)的工艺参数。
从图2(a)~(h)中可以看出,表2所述工艺参数能确保激光光斑有效搭接,清洗效果良好,工艺参数均满足公式(3),试验结果与模型预测结果匹配良好。结合图2和表2可以看出,随着清洗速度从0.5~1.1 m/min变化时,图2(b)~(h)的光斑搭接逐渐稀疏,但仍能满足光斑整体覆盖的要求,且清洗效率逐步上升。
-
针对实际生产过程,采用平均功率200 W,重复频率250 kHz,扫描线宽45 mm,清洗速度1.0 m/min,扫描频率145 Hz,开展激光清洗对铝合金表面氧化膜的影响研究。通过分析表面氧化膜厚度及形貌变化,直观深入地评价铝合金表面氧化膜的去除状况。
为研究激光清洗前后铝合金表面氧化膜厚度变化,进一步观察铝合金表面氧化膜变化,TEM结果如图3所示。未处理的样品原生氧化膜厚度约为200 nm;激光清洗后,由于铝合金表面在自然环境下短时间内发生氧化,稳定后生成纳米级氧化膜[15-16],表面观察到50 nm相对疏松的氧化膜。铝合金氧化膜厚度明显减薄,表面状态也较为疏松。
图 3 激光清洗前后TEM结果。(a)未处理;(b)清洗后
Figure 3. Results of TEM before and after laser cleaning. (a) Untreated; (b) After cleaning
利用XPS测量激光清洗前后铝合金表面氧含量随深度方向的变化,如图4所示。
未处理的样品氧元素含量在200 nm左右处发生骤降,激光清洗后的样品在深度方向上50~100 nm氧元素含量逐渐下降。由于表面刻蚀时存在一定角度且样片表面不平整,故测试时刻蚀到一定深度依旧可以采集到氧化膜。该结果与TEM结果保持一致,证明原生氧化膜厚度仅为200 nm左右。
使用激光共聚焦显微镜测量激光清洗的去除深度,结果如图5所示。
从图5中可以看出,激光清洗的去除深度约为3 μm,远远大于铝合金表面原生氧化膜厚度,证明原生氧化膜经激光清洗后被去除,说明图3和图4中观察到的约50 nm厚度的氧化膜为激光清洗后铝基底与空气接触氧化再次生成的疏松氧化膜。这在即清即焊、清焊一体的加工过程中,可有效避免再生氧化膜的影响。
可见,在所选工艺参数下,激光作用在铝合金产品表面时,铝合金表面氧化膜和污物在热、力作用下被有效去除,激光清洗可以有效降低铝合金表面氧含量,且未对基材造成较大损伤。
Study on synchronous laser cleaning technology of aluminum alloy carbody welding (invited)
-
摘要: 针对轨道交通车辆车体铝合金型材焊接同步激光清洗,开展了相关工艺、测试与应用技术研究。通过工艺研究得出铝合金氧化膜激光清洗工艺窗口,建立了匹配不同焊接速度的激光清洗参数预测模型,并分析了激光清洗对铝合金表面氧化膜的影响;针对实际工况,设计了激光清洗-电弧焊接同步清焊一体化装置,并开展焊接同步激光清洗工艺验证,实现了焊前高质高效去除氧化膜;通过分析焊接接头力学性能,检测焊缝缺陷,观测焊接接头截面微观组织,评判焊接同步激光清洗焊缝质量。研究表明,采用200 W脉冲激光,清洗速度0.5~1.1 m/min可调的情况下,可以实现线宽45 mm的焊接同步氧化膜的有效清洗,经激光清洗后,原生氧化膜被完全去除,且可避免再生氧化膜的影响。经激光清洗后的焊接接头剪切强度及应变相较未处理的接头分别提升了26.4%和9.98%,较人工打磨后的接头强度分别提升了3.53%和1.43%,焊缝中心组织主要由α基体和β (Mg2Al3)相组成,焊接性能满足轨道交通车辆车体制造要求,可有效替代人工打磨。Abstract: The processes, testing and application technology research were carried out for the welding synchronous laser cleaning of aluminum alloy for rail transit vehicle carbodies. The process window of laser cleaning for oxide film of aluminum alloy was obtained through the study of process. The prediction model of laser cleaning parameters before matching different welding speeds was established. And the effect of laser cleaning on the superficial oxide film of aluminum alloy was analyzed. According to the actual working conditions, an integrated laser cleaning and arc welding synchronous device was designed. And the pre-welding synchronous laser cleaning process verification was carried out. High-quality and high-efficiency removal of oxide film before welding was achieved. By analyzing the mechanical properties of welded joints, detecting weld defects and observing the microstructure of the cross-section of the welded joint, the quality of welding synchronous laser cleaning welds was evaluated. The research shows that with 200 W pulsed laser, the cleaning speed is adjustable from 0.5 to 1.1 m/min. Welding synchronous laser cleaning of oxide film with a line width of 45 mm can be effectively achieved. After laser cleaning, the primary oxide film is completely removed and the effect of regenerative oxide film can be avoided. The shear strength and strain of the welded joint after laser cleaning before welding increased by 26.4% and 9.98% compared with the untreated joint. The shear strength and strain of welded joints after laser cleaning increased by 3.53% and 1.43% respectively compared to those after manual grinding. And the central microstructure of the weld was mainly composed of α matrix and β (Mg2Al3) phase. The performance of the welding met the requirements of rail transit vehicle carbody manufacturing. Laser cleaning with welding can effectively replace manual grinding technology.
-
Key words:
- aluminum alloy /
- laser cleaning /
- oxide film /
- integrated laser cleaning and welding
-
表 1 激光器参数
Table 1. Parameters of laser
Parameter Symbol Value Wavelength/nm λ 1064 Average power/W P 0-200 Repetition frequency/kHz F 1-4000 Pulse width/ns l 8-500 Scanning frequency/Hz f 0-300 Focal length/mm f1 254 Spot diameter/μm D 92.9 Laser mode parameter M2 1.8 表 2 焊前清洗工艺参数组合
Table 2. Combination of cleaning process parameters before welding
No. Average
power,
P/WRepetition
frequency,
F/kHzLine
width,
L/mmCleaning
speed, V/
m·min–1Scanning
frequency,
f/Hz(a) Untreated (b) 200 160 45 0.5 100 (c) 200 170 45 0.6 105 (d) 200 180 45 0.7 110 (e) 200 200 45 0.8 120 (f) 200 220 45 0.9 130 (g) 200 250 45 1.0 145 (h) 200 260 45 1.1 150 -
[1] Han Xiaohui, Qi Xiansheng. Engineering application and prospect of high-efficiency and high-quality laser cleaning technology for rail passenger cars [J]. Machinist Metal Forming, 2020(3): 11-14. (in Chinese) [2] Jin Wentao, Lu Anjin, Dai Zhongchen. Application of laser cleaning technology in automatic welding of aluminum alloy body in rail vehicles [J]. Machinist Metal Forming, 2019(9): 17-20. (in Chinese) [3] Wang W, Shen J, Liu W J, et al. Effect of laser energy density on surface physical characteristics and corrosion resistance of 7075 aluminum alloy in laser cleaning [J]. Optics & Laser Technology, 2022, 148: 107742. [4] Zhang G X, Hua X M, Huang Y, et al. Investigation on mechanism of oxide removal and plasma behavior during laser cleaning on aluminum alloy [J]. Applied Surface Science, 2020, 506: 144666. doi: 10.1016/j.apsusc.2019.144666 [5] Li R Y, Yue J, Shao X Y, et al. A study of thick plate ultra-narrow-gap multi-pass multi-layer laser welding technology combined with laser cleaning [J]. International Journal of Advanced Manufacturing Technology, 2015, 81(1-4): 113-127. doi: 10.1007/s00170-015-7193-0 [6] Guo L Y, Li Y Q, Geng S N, et al. Numerical and experimental analysis for morphology evolution of 6061 aluminum alloy during nanosecond pulsed laser cleaning [J]. Surface and Coatings Technology, 2022, 432: 128056. doi: 10.1016/j.surfcoat.2021.128056 [7] Alshaer A W, Li L, Mistry A. The effects of short pulse laser surface cleaning on porosity formation and reduction in laser welding of aluminum alloy for automotive component manufacture [J]. Optics & Laser Technology, 2014, 64(4): 162-171. [8] Chen Yiming, Zhou Longzao, Yan Fei, et al. Mechanism and quality evaluation of laser cleaning of aluminum alloy [J]. Chinese Journal of Lasers, 2017, 44(12): 1202005. (in Chinese) [9] Guo Zhicheng, Wang Qiuying, Qin Wenzhao. Influence of laser cleaning on the machanical properties of A5083-H111 aluminum processed with MIG welding [J]. Rail Transportation Equipment and Technology, 2020(3): 14-16. (in Chinese) [10] Shi T Y, Wang C M, Mi G Y, et al. A study of microstructure and mechanical properties of aluminum alloy using laser cleaning [J]. Journal of Manufacturing Processes, 2019, 42: 60-66. doi: 10.1016/j.jmapro.2019.04.015 [11] Zhou C, Li H G, Chen G Y, et al. Effect of single pulsed picosecond and 100 nanosecond laser cleaning on surface morphology and welding quality of aluminum alloy [J]. Optics and Laser Technology, 2020, 127: 106197. doi: 10.1016/j.optlastec.2020.106197 [12] Liu B W, Wang C M, Mi G Y, et al. Oxygen content and morphology of laser cleaned 5083 aluminum alloy and its influences on weld porosity [J]. Optics and Laser Technology, 2021, 140: 107031. doi: 10.1016/j.optlastec.2021.107031 [13] Ren Y, Wang L M, Li J F, et al. The surface properties of an aviation aluminum alloy after laser cleaning [J]. Coatings, 2022, 12(2): 273. doi: 10.3390/coatings12020273 [14] Deng J, Zhao G R, Lei J H, et al. Research progress and challenges in laser-controlled cleaning of aluminum alloy surfaces [J]. Materials, 2022, 15(16): 5469. doi: 10.3390/ma15165469 [15] Fehlner F P, Mott N F. Low-temperature oxidation [J]. Oxidation of Metals, 1970, 2: 59-99. doi: 10.1007/BF00603582 [16] Kuzik L A, Yakovlev V A. Effect of a noble metal coating on a natural aluminum oxide film [J]. Thin Solid Films, 1999, 340(1-2): 288-291. doi: 10.1016/S0040-6090(98)00854-2 [17] Dong Shiyun, Song Chaoqun, Yan Shixing, et al. Effect of cleaning pretreatment on laser welding formation of 7A52 aluminum alloy [J]. Journal of Armored Forces, 2017, 31(4): 100-105. (in Chinese) doi: 10.3969/j.issn.1672-1497.2017.04.019 [18] Xia Peiyun, Yin Yuhuan, Cai Aijun, et al. Laser cleaning process of 2219 aluminum alloy anodic oxide film before welding [J]. Chinese Journal of Lasers, 2019, 46(1): 0102005. (in Chinese)