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喻杰, 韦东波, 王岩, 吕鹏翔, 狄士春. 激光重熔改性铝合金微弧氧化膜层的组织与性能. 无机材料学报, 2013, 28(8): 859-863
YU Jie, WEI Dong-Bo, WANG Yan, LÜ Peng-Xiang, DI Shi-Chun. Structure and Property of Micro-arc Oxidation Coating Modified by Laser Melting and Solidifying on Aluminum Alloy. Journal of Inorganic Materials, 2013, 28(8): 859-863  
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激光重熔改性铝合金微弧氧化膜层的组织与性能
喻杰, 韦东波, 王岩, 吕鹏翔, 狄士春
哈尔滨工业大学 机电工程学院, 哈尔滨 150001
狄士春, 教授. E-mail:dishichun@126.com

喻 杰(1986-), 男, 博士研究生. E-mail:yujie163.ok@163.com

摘要

为了改善微弧氧化(MAO)膜层多孔疏松的组织和性能, 对其进行了激光重熔处理, 并制备了两种实验膜层: (1)选择双向电流脉冲和Na2SiO3-KOH体系的工作液, 在6082铝合金基体上制备平均厚度为18 μm的MAO膜层; (2)采用Nd:YAG激光器对上述MAO膜层进行激光重熔(LSM)处理, 获得MAO+LSM膜层。利用扫描电子显微镜(SEM)、X射线衍射仪、超显微硬度计和电化学分析仪分别检测上述两种膜层的微观形貌、相组成、表面硬度和耐蚀性能。结果表明: 激光重熔后的膜层由内往外分为致密层、中间层和重熔层, 组织致密、气孔率低的重熔层取代了MAO疏松层, MAO+LSM膜层中α-Al2O3相的比例得到提高, 硬度和耐蚀性能也进一步得到改善, 且保持了MAO膜层与基体的结合方式。

关键词: 铝合金; 微弧氧化; 激光重熔; 耐蚀性
中图分类号:TG174   文献标志码:A    文章编号:1000-324X(2013)08-0859-05
Structure and Property of Micro-arc Oxidation Coating Modified by Laser Melting and Solidifying on Aluminum Alloy
YU Jie, WEI Dong-Bo, WANG Yan, L#cod#x000dc; Peng-Xiang, DI Shi-Chun
School of Mechatronics Engineering, Harbin Institute of Technology, Harbin 150001, China
Abstract

In order to improve performance and microstructure of micro-arc oxidation (MAO) coating, especially loose and porous characteristic, a laser melting and solidifying process (LSM) was introduced. Two kinds of samples were prepared: (1) MAO coatings, 18 μm average thickness, were produced on 6082 aluminum alloy by bipolar current pulse in Na2SiO3-KOH solution. (2) a melting process using a Nd:YAG laser was employed to modify above-mentioned MAO coatings to obtain MAO+LSM coating. Microstructure of two kinds of coatings (MAO coating and MAO+LSM coating) were examined by scanning electron microscopy. X-ray diffraction was used to determine the phase composition of the coatings. Coating hardness was tested by ultra-micro hardness tester, and corrosion performance was investigated by polarization test instrument. The results show that the MAO+LSM coating is composed of dense layer, intermediate layer and melting layer from inside to surface. The loose layer of MAO film is replaced by a dense and low porosity melting layer after LSM treatment. The occupancy of α-Al2O3 phase in MAO+LSM is improved compared with MAO coating. Hardness and anticorrosion performance of MAO+LSM coating are also further strengthened while the remelted coating keeps the same binding manner as MAO coating.

Keyword: 6082 aluminum alloy; micro-arc oxidation; laser melting and solidifying; anticorrosion behavior

铝合金具有重量轻、比强度高、成本低等特点, 被广泛应用于现代工业生产中, 但其硬度、耐磨性等机械性能和耐蚀性能有待提高[ 1, 2, 3, 4, 5, 6]。微弧氧化是在电化学反应和等离子放电作用下, 在Al、Mg、Ti等阀金属及其合金表面原位生长陶瓷膜层的表面改性技术[ 6, 7, 8]。在众多表面处理技术中, MAO膜层与基体具有较好的结合力[ 9, 10, 11], 但膜层外部一般为疏松层, 内部为致密层, 疏松层比致密层厚, 组织松散, 且主要由硬度较低的γ-Al2O3组成[ 12, 13, 14]。MAO的等离子体放电通道内熔融物喷射淬冷后形成了膜层的多孔结构, 随着放电的持续孔径逐渐变大, 并且一些较大的孔隙贯通了膜层表面和基体, MAO膜层的这些特点降低了膜层的耐磨和耐蚀性能[ 15]。激光重熔是在激光束热作用下改变材料表面微观结构和相组成, 提高材料的耐磨、防腐和抗氧化性能的表面改性技术[ 16, 17, 18]。许多学者研究表明激光重熔有利于消除材料的缺陷, 改善陶瓷体力学性能, 重熔和再凝固可实现多孔陶瓷的封孔, 膜层表面变得光滑[ 19, 20, 21, 22]。Mateos研究等离子喷涂膜层的激光重熔时, 发现重熔后大量的孔隙消失, 微观组织的一致性也变 好[ 23]。本工作结合MAO和LSM两种工艺有望获得结合力好、整体组织致密和硬度高的膜层, 提高膜层的防腐和耐磨性能。

1 实验方法

实验样件材料为6082铝合金, MAO样件尺寸: 100mm×50mm×4mm。

实验步骤分为两步:(a)在铝合金基体上制备MAO膜层。铝合金样件经1000#砂纸打磨后, 先后在自来水和去离子水中清洗, 然后烘干。在Na2SiO3-KOH体系的工作液中, 对预处理后的铝合金用MAO工艺处理40 min, 工作液中各溶质浓度分别为 CNa2SiO3=20 g/L, CKOH=1 g/L和 C(NaPO3)6=8 g/L, 溶剂为去离子水, 装有工作液的不锈钢槽体为负极, 样件为正极, 采用双向恒流加工方式, 正向电流10 A, 负向电流4 A, 频率500 Hz, 保持电解液温度30~35℃, 电源为哈尔滨迪斯数控设备有限公司研制的15 kW微弧氧化实验专用脉冲电源;(b) 激光重熔多孔微弧氧化膜层。利用线切割机床将MAO样件分割成尺寸为15 mm×15 mm的小样件, 然后经肥皂水、自来水多次清洗后, 最后在酒精中进行超声波清洗5~10 min, 直至表面洁净后烘干。利用Nd:YAG激光器对小样件的MAO膜层进行重熔处理, 优化工艺后, 激光重熔参数如下: 焦点至膜层表面距离为12~15 mm, 激光功率23~31 W, 扫描速度2~6 mm/s, 激光扫描重叠率为50%。

激光器是英国GUS公司生产的型号为LUMONICS JK700。采用美国FEI公司生产的SIRION扫描电子显微镜(SEM)分析MAO和MAO+LSM膜层的表面和截面的微观形貌和组织结构。采用日本理学株式会社生产的D/max-rB旋转阳极X射线衍射仪对MAO涂层和MAO+LSM膜层的物相成分进行分析。利用日本株式会所生产的岛津DUH-W201S型超显微硬度计测量膜层表面硬度。采用上海晨华仪器仪器公司生产的电化学分析仪测定了膜层在3.5%NaCl水溶液中的动电位极化曲线。

2 实验结果与讨论
2.1 激光重熔对MAO膜层的微观形貌的影响

图1是MAO膜层和微弧氧化膜层经激光重熔(MAO+LSM)后的表面形貌。MAO膜层生长主要是在等离子体放电作用下完成的, 单次等离子体放电通常经历介质击穿形成放电通道, 放电通道附近的金属及其氧化物熔化, 在高温和高压的放电通道内的熔融物往外部喷射, 在工作液中淬冷后, 膜层表面形成大量的“火山口”, 并且“火山口”附近堆积了许多冷却物, 形成小突起, 且分布大量的微孔, 膜层也因此而变得粗糙, 如图1(a)和图1(b)所示。MAO膜层中微孔边沿的小突起为附近区域的“制高点”, 如图1(b)所示, 激光束照射到MAO膜层后, 小突起距离激光焦点距离近而获得激光辐射的能量密度较高, 激光束在突起中心的微孔内反复折射后, 突起微区辐射的能量密度进一步提高, 因此, MAO膜层在激光重熔过程中先在突起处形成较小熔池, 然后熔池逐渐往外生长, 相邻熔池的熔融物流动、汇集后形成熔融区, 熔融区之间相互连接, 在大气环境下冷却后形成表面光滑, 微孔稀疏, 组织致密的膜层结构, 如图1(c)所示。若照射到膜层激光光斑能量密度较低, 扫描速度较快, 形成的熔池较小且熔融物保持高温时间短熔池不易长大, 难以形成连续的熔融区, 重熔后膜层表面间隔地分布着MAO和MAO+LSM的形貌, 如图1(d)和图1(e)所示, 黑色曲线区域为LSM+MAO膜层区域, 其余为MAO膜层。图1(f)反映了MAO和MAO+LSM膜层边界区域的形貌, 激光重熔后膜层表面的微孔减少, 封孔效果明显, 但表面出现凹陷区域, 可能为熔池中的熔融物流入较大的MAO膜层孔隙, 导致其附近出现凹陷, 另有边沿规则的圆孔可能为重熔过程中膜层内部的排气通道。

图1 MAO和MAO+LSM膜层的表面SEM照片Fig. 1 SEM images of MAO and MAO+LSM surface topography(a) MAO coating; (b) Tower around crate; (c) Continuous MAO+LSM coating; (d,e) Interrupted MAO+LSM coating; (f) Boundary surface micro topography between MAO and MAO+LSM

图2为MAO和MAO+LSM膜层的截面形貌, MAO膜层内部一般为组织较好的致密层, 但致密层所占整个膜层厚度的比例不高, 外部为疏松多孔, 防腐和耐磨性能较差的疏松层, MAO膜层内部存在因微弧放电留下的盲孔, 以及贯穿基体和膜层外部的通孔。另外, 放电通道内的高温熔融物喷射后在工作液中淬冷, 产生大量裂纹等组织缺陷, 使得外部疏松层内聚力弱, 容易脱落, 如图2(a)所示。图2(b)所示为重熔后的膜层截面形貌, 原MAO膜层中致密层组织变化较小, 保留了MAO膜层与基体结合力好的优点, 而外部疏松层经激光处理后形成了致密的重熔层, 重熔层内部微孔、裂纹明显减少, 且极大地降低了连通基体和膜层外部通孔出现的几率, 膜层表面的小突起和疏松颗粒减少, 表面变得整齐、平直。在图2(b)所示重熔后的膜层中, 激光重熔层和微弧氧化致密层之间有一较窄的中间层, 内含少量的较小孔隙, 此部分在MAO膜层中为靠内的疏松层, 在激光热能往膜层内部传递过程中, 热作用逐渐减弱, 激光热源改变膜层组织的能力也随之降低, 因此, 中间层还“继承”了MAO疏松层的组织特征, 但该层的存在保护了MAO致密层和基体的组织以及二者的结合方式。因此, 根据截面形貌特征可将MAO+LSM膜层为三层, 由内往外分别为致密层、中间层和重熔层。

图2 MAO和MAO+LSM膜层的截面SEM照片Fig. 2 SEM images of MAO and MAO+LSM cross-sectional topography(a) MAO coating; (b) MAO+LSM coating

2.2 激光重熔对膜层相组成的影响

在铝合金的MAO膜层中, 通常以γ-Al2O3和α-Al2O3相为主, γ-Al2O3和α-Al2O3的相变温度分别为560℃和1370℃。在此实验条件下制备的MAO 样件中, 并未检测到含有α-Al2O3晶相, 如图3所示。实验观察和相关研究表明微弧氧化膜层生长前期火花密度大, 火花移动速度快, 放电通道内能量较小[ 24], 不易形成较长时间的“高温烧结”环境, 故MAO前期难以生成α-Al2O3。在激光束扫描MAO膜层过程中, 具有高能量密度的激光光斑照射到膜层后, 其局部区域温度迅速上升至数千摄氏度以致部分膜层融化形成熔池, 熔池往基体方向为热传率低的氧化铝陶瓷, 外部与大气接触, 不同于MAO工艺条件的工作液, 周围热绝缘性相对较好的介质有利于保护熔池的高温环境, 促进高温区域内熔融物由γ-Al2O3向α-Al2O3转变, 图3表明激光重熔后出现了高温陶瓷相的α-Al2O3。相对于MAO过程大部分能量迅速传递给工作液, LSM过程中能量利用率较高。

图3 MAO和MAO+LSM膜层XRD图谱Fig. 3 XRD patterns of the MAO and MAO+LSM samples

2.3 激光重熔对膜层硬度的影响

铝合金基体、MAO和MAO+LSM膜层的表面硬度值见表1。基体的显微硬度为105.324 HV, MAO和MAO+LSM膜层的硬度分别为432.022 HV和962.768 HV, 相比基体的硬度均有较大提高。而MAO膜层经激光重熔和重结晶后具有更好的硬度: 一方面, 由于激光重熔提高了MAO膜层中高硬度α-Al2O3的比例; 另一方面, 激光重熔后外部膜层组织由疏松多孔变得致密、紧凑、孔隙率低, 大幅增加了具有致密组织的膜层厚度, 提高了膜层的内聚力。

表1 基体、MAO和MAO+LSM膜层的硬度和极化参数 Table 1 Vickers hardness and polarisation data of substrate, MAO and MAO+LSM coating
2.4 激光重熔对膜层耐蚀性的影响

铝合金基体、MAO和MAO+LSM膜层的耐蚀实验测试参数见表1。基体的腐蚀电流密度672.1 nA/cm2, 腐蚀电位为0.914 V, MAO和MAO+LSM处理后耐蚀性相对基体材均有大幅提高。MAO膜层的腐蚀电流密度为4.740 nA/cm2, 腐蚀电位为0.212 V, 激光重熔后腐蚀电流密度和腐蚀电位分别为3.465×10-9A/cm2和0.142 V, 耐蚀性能相对MAO膜层好。膜层的厚度、组织形貌和相组成是影响耐蚀性的重要因素, 激光重熔后, 膜层的厚度降低了, 但致密性好的膜层比例发生显著变化, 由疏松层为16~17 μm, 致密层1~2 μm的MAO膜层变为重熔层7~8 μm, 中间层1~2 μm, 致密层1~2 μm的LSM+MAO膜层结构, 致密膜层比例由10%左右提高至80%左右, 有效防腐层显著变厚; 重熔对多孔微弧氧化膜层的封孔效果明显, 尤其降低了贯穿膜层表面和基体的通孔的几率, 另外也增加了耐蚀性能好的α-Al2O3比例, 故激光重熔进一步提高了膜层的耐蚀性能, 如图4所示。

图4 基体、MAO和MAO+LSM膜层的极化曲线图Fig. 4 Potentiodynamic polarization curves of substrate and MAO, MAO+LSM coatings

3 结论

1) 激光重熔能将MAO膜层表面“火山口”边沿的小突起铺平, 微弧放电通道形成的大量微孔被填充, 膜层表面变得光滑, 微孔稀疏。

2) 激光重熔后, 形成了由内往外分别为致密层、中间层和重熔层结构的MAO+LSM膜层。致密层仍为与基体结合力好的MAO膜层, 重熔层改变了原MAO外层疏松、多孔的组织, 形成了致密、紧凑的膜层, 中间层继承了MAO膜层的多孔性, 但有所改善, 且厚度可控制在整体膜层的20%以下。

3) 激光束辐射的高密度能量促使了膜层中的γ-Al2O3向α-Al2O3转变, 提高了膜层的硬度。

4) MAO+LSM膜层具有致密层和重熔层双层有效防腐膜层, 占到膜层总厚度的80%以上, 且激光重熔对MAO膜层的封孔效果显著, 尤其降低了通孔率, 故MAO+LSM膜层较MAO表现出更好的耐蚀性能。

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