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图 2中给出了不同物质的量比粉体熔覆后涂层的界面形貌及涂层中部典型微观组织的SEM图像。图 2a、图 2c、图 2e分别为A1(n(TiAl)∶n(TiN)=1∶1)、A2(n(TiAl)∶n(TiN)=1.1∶1)、A3(n(TiAl)∶n(TiN)∶n(Al)=1∶1∶0.1)试样涂层的界面微观形貌。由左下角的涂层界面的宏观形貌可以看出,涂层A1, A3结合界面较为平直,A2结合界面较为曲折,各涂层界面处均未出现宏观的裂纹和孔洞,未观察到未熔合和其它缺陷,整体涂层的厚度在1mm~1.6mm之间,涂层和基体之间呈现良好的冶金结合。从涂层与基体结合界面的微观形貌发现,涂层A1界面附近存在较多的黑色不规则块状相和少量树枝状相,其中大块黑色颗粒可能为未熔化反应的TiN颗粒;A2试样界面处仅有少量黑色的粗大颗粒,而A3试样界面处基本无黑色块状颗粒,主要以基体和熔凝析出的树枝状晶体为主,且A3界面附近的组织相比A1和A2更加均匀。可见,增加Al元素物质的量比的A2和A3试样能够获得组织更为均匀的界面结构,采用添加少量Al粉的方式更能够进一步改善界面的组织均匀性。
图 2b、图 2d、图 2f中给出了涂层A1, A2, A3中部显微组织的SEM图像。可以看出,不同物质的量比粉体熔覆后涂层中部的组织特征没有太大的区别,均为基体上分布着边缘为亮白色的树枝状晶和少量的白色等轴状颗粒。图 2d与图 2b相比,随着TiAl含量的增加,不存在未熔的TiN颗粒,快速熔凝形成的树枝晶的含量也明显增多,枝晶发展不够充分,其尺寸也更为细小。从图 2f可以看出,加入少量的Al元素粉后,树枝状相变得更细小弥散,等轴状颗粒明显增加,基体组织与A1和A2试样明显不同,表现为细小致密的短杆状组织。这表明加入的0.1mol Al不仅能够补充熔覆过程中Al的烧损,Al元素的熔化还能够促进熔池中TiAl和TiN的反应,改善熔凝组织的均匀性。
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图 3中给出了不同物质的量比粉体熔覆后涂层的XRD图谱。可以看出,涂层A1, A2和A3中的物相并无区别,主要由TiAl基体(TiAl和Ti3Al双相组织)、MAX相Ti2AlN和TiN陶瓷组成。从各物相的峰强比可以看出,A1试样中TiN含量最高,这与显微组织分析残留未熔的TiN颗粒相一致;A2试样多添加了0.1mol TiAl,熔覆组织中TiAl基体和TiN的衍射峰均较强,Ti2AlN相含量相对较少;A3在加入了0.1mol Al元素粉后,TiAl含量最高,Ti2AlN峰的强度有较明显的增强。对比分析可见,A3试样中Ti2AlN相含量有明显增加。
图 4中进一步给出了各试样涂层中部典型部位的能谱分析(energy dispersive spectroscopy,EDS)。表 1中为对应测试点的EDS定量分析结果。结合图 4和表 1中的数据分析可知,黑色基体为TiAl合金典型的γ-TiAl+α2-Ti3Al双相组织,灰黑色树枝状晶体为熔池凝固过程中重新析出的TiN相,而弥散分布的亮白色颗粒和深灰色杆状相(见图 4c中点能谱8)均为Ti2AlN相。可见,A1和A2试样组织为典型的在TiAl基体上分布着大量树枝状的TiN相和少量颗粒状的Ti2AlN相,而A3试样是以TiAl+Ti2AlN为基体,分布着少量的树枝状的TiN相和少量颗粒状的Ti2AlN相。此外可以看出,TiN树枝状晶体边缘呈现与Ti2AlN相类似的亮白色衬度,而细小的枝晶末端甚至转变为亮白色的Ti2AlN颗粒。说明颗粒状的Ti2AlN相一部分为从熔池中直接反应析出,而另一部分则表现为TiN相与TiAl基体局部反应形成。A3试样中除及少量的亮白色Ti2AlN颗粒外,大部分Ti2AlN相呈深灰色与TiAl相交错分布形成涂层基体,说明Al粉的加入改变了熔池中Ti2AlN相的形成过程。
Table 1. EDS analysis of different area in Fig. 4
position composition (atomic fraction) possible phase Ti Al N 1 0.5648 0.0207 0.4145 TiN 2 0.5568 0.1482 0.2950 Ti2AlN 3 0.6276 0.2752 0.0973 Ti3Al 4 0.4341 0.0152 0.5507 TiN 5 0.4787 0.2257 0.2271 Ti2AlN 6 0.5513 0.4203 — TiAl 7 0.4854 0.0094 0.5044 TiN 8 0.5957 0.2301 0.1698 Ti2AlN 9 0.6845 0.2987 — Ti3Al 根据上述分析可以总结出激光熔覆条件下TiN-TiAl复合粉末体系下涂层的原位合成过程:当激光能量输入时,TiAl粉末和TC4基体首先形成Ti-Al熔池,而后TiN颗粒逐渐熔解到熔池中,部分TiN与Ti-Al熔池反应生成等轴状的Ti2AlN颗粒;随着温度降低,TiN首先以树枝状形态析出,并伴以少量等轴状Ti2AlN颗粒的析出;同时,先析出的TiN树枝晶边缘与Ti-Al熔池反应扩散生成白亮的Ti2AlN相;最后,Ti-Al熔池凝固得到最终的复合涂层。应该注意到,由于激光熔覆条件下熔池存在时间很短,TiN-TiAl反应生成Ti2AlN相的进程受到抑制。A1和A2成分的涂层在熔覆界面处尚存在少量未完全熔化进入熔池的TiN颗粒,而A3试样因添加了Al粉能够更早地形成Ti-Al熔池,更多的Al原子不仅能够直接与TiN反应也能促进TiN与Ti-Al熔池的反应,最终形成大量的Ti2AlN相,而凝固析出的TiN树枝晶显著减少。这说明通过添加少量Al粉能够提供足够的游离Al原子,从而显著促进TiN-TiAl向Ti2AlN相转变的反应程度,增加最终复合涂层中Ti2AlN MAX相的含量。
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图 5中给出了不同涂层横截面上的显微硬度分布。结果表明,A1, A2和A3试样涂层内部的平均硬度分别约为717.3HV, 761.4HV和673.8HV,分别约为基体(325.7HV)的2.20倍、2.34倍和2.07倍。这与RICHARDSON等人[31]采用激光熔覆工艺制备出的Ti2AlC复合涂层的硬度(811HV)接近。涂层与基体之间存在热影响区(heat affected zone,HAZ),涂层部分的显微硬度分布相对稳定,而在涂层与基体界面处具有较高的硬度值。这主要是由于近基体处熔池凝固较快,TiN形核率较高,形成了弥散细小的分布状态,提高了涂层表面的硬度;同时,在近界面处,TC4基体中Ti元素扩散导致界面处基体的强化;此外,少量未熔的TiN颗粒也增加了涂层界面处组织硬度分布的离散性。综合显微组织及硬度分布可以看出,采用激光熔覆工艺能够获得组织均匀致密、界面冶金结合良好的高硬度熔覆涂层,有望满足高温氧化、磨损等使用工况的应用需求。
TC4表面激光熔覆Ti-Al-N复合涂层的组织与性能
Microstructure and properties of laser cladding Ti-Al-N composite coating on TC4 surface
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摘要: 为了在TC4钛合金表面开发新型MAX相复合涂层,利用激光熔覆在TC4钛合金表面原位合成了含Ti2AlN MAX相的Ti-Al-N复合涂层, 分析了涂层的组织结构特征及硬度分布,研究了复合涂层的原位合成机理。结果表明,不同物质的量比粉体熔覆后的涂层与基体呈现良好的冶金结合,涂层由TiAl基体、Ti2AlN MAX相和TiN树枝晶组成,涂层平均硬度约为基体的2倍以上,涂层厚度在1mm~1.6mm之间;添加少量Al粉能够促进熔池中TiN和TiAl的反应,从而显著提高了涂层中Ti2AlN MAX相的含量。此研究结果在明晰MAX相原位合成机理的基础上,对采用激光熔覆技术制备MAX相复合涂层具有重要意义。
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关键词:
- 激光技术 /
- TC4钛合金 /
- 激光熔覆 /
- 合成机理 /
- Ti2AlN MAX相
Abstract: In order to develop a new MAX phase composite coating on the surface of TC4 titanium alloy, a Ti-Al-N composite coating containing Ti2AlN MAX phase was synthesized on the surface of TC4 titanium alloy by laser cladding. The structure characteristics and hardness distribution of the coating were analyzed, and the in-situ synthesis mechanism of the composite coating was studied. The results indicate that the coating and the substrate after powder cladding with different ratios of amount of substance show a good metallurgical bond. The coating is mainly composed of TiAl matrix, Ti2AlN MAX phase, and TiN dendrites. The average hardness of the coating is about twice that of the substrate. The coating thickness is between 1mm~1.6mm. Adding a small amount of Al powder can promote the reaction of TiN and TiAl in the molten pool, thereby significantly increasing the content of Ti2AlN MAX phase in the coating. The research results are of great significance to the preparation of MAX phase composite coatings by laser cladding technology based on the clarification of the MAX phase in-situ synthesis mechanism.-
Key words:
- laser technique /
- TC4 titanium alloy /
- laser cladding /
- synthesis mechanism /
- Ti2AlN MAX phase
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Table 1. EDS analysis of different area in Fig. 4
position composition (atomic fraction) possible phase Ti Al N 1 0.5648 0.0207 0.4145 TiN 2 0.5568 0.1482 0.2950 Ti2AlN 3 0.6276 0.2752 0.0973 Ti3Al 4 0.4341 0.0152 0.5507 TiN 5 0.4787 0.2257 0.2271 Ti2AlN 6 0.5513 0.4203 — TiAl 7 0.4854 0.0094 0.5044 TiN 8 0.5957 0.2301 0.1698 Ti2AlN 9 0.6845 0.2987 — Ti3Al -
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