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Microstructure and antioxidant properties of TC11 alloy irradiated by intense pulsed laser

  • School of Mechanical Engineering, Chongqing Vocational Institute of Engineering, Chongqing 402260, China

  • Received Date: 2019-10-11
    Accepted Date: 2019-11-28
  • In order to improve the performance of TC11 titanium alloy surface, the strong laser irradiation treatment was used, and then the influence of laser pulse energy density on TC11 titanium alloy surface morphology and performance was studied. The results show that after the laser irradiation by strong laser pulse of 1J/cm2, the surface of the alloy begins to melt, and with the increase of pulse energy density, the outer diameter of the molten hole in the alloy surface becomes larger. After laser irradiation, a micropore with outside diameter of about 1.5μm formed at the surface of the alloy, whose constituent elements were mainly Mg, O, S and C. The microhardness of TC11 alloy was significantly increased after pulse irradiation. When the pulse energy density increases, the microhardness of the alloy increases, and the anti-oxidation performance of the alloy sample is obviously improved. The oxides formed on the surface of the alloy consist of two types: Granule and lamellar. The oxide is mainly composed of Al2O3 and Cr2O3. The study is helpful to improve the surface properties of TC11 titanium alloy.
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    LIU Ch Y, SUN L D, FU X, et al. Microstructure and fluorescence detection of silicon surface irradiated by 355nm nanosecond pulse lasers [J]. Chinese Journal of Lasers, 2010, 37(8): 2139-2142(in Chinese). doi: 10.3788/CJL20103708.2139
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Microstructure and antioxidant properties of TC11 alloy irradiated by intense pulsed laser

  • School of Mechanical Engineering, Chongqing Vocational Institute of Engineering, Chongqing 402260, China
  • Received Date: 2019-10-11
  • Accepted Date: 2019-11-28
  • Available Online: 2020-09-25

Abstract: In order to improve the performance of TC11 titanium alloy surface, the strong laser irradiation treatment was used, and then the influence of laser pulse energy density on TC11 titanium alloy surface morphology and performance was studied. The results show that after the laser irradiation by strong laser pulse of 1J/cm2, the surface of the alloy begins to melt, and with the increase of pulse energy density, the outer diameter of the molten hole in the alloy surface becomes larger. After laser irradiation, a micropore with outside diameter of about 1.5μm formed at the surface of the alloy, whose constituent elements were mainly Mg, O, S and C. The microhardness of TC11 alloy was significantly increased after pulse irradiation. When the pulse energy density increases, the microhardness of the alloy increases, and the anti-oxidation performance of the alloy sample is obviously improved. The oxides formed on the surface of the alloy consist of two types: Granule and lamellar. The oxide is mainly composed of Al2O3 and Cr2O3. The study is helpful to improve the surface properties of TC11 titanium alloy.

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引言

  • 利用强脉冲激光照射对物体进行照射可以将大量能量在极短时间内辐射到物体表面，使被辐射物体的表面出现熔化、烧蚀等[1-7]。当材料受到剧烈辐射后, 其表面温度将会发生迅速改变从而形成明显的热应力集中现象，同时材料内部则会因为受到烧蚀等离子体喷发作用形成反冲应力波[8-11]。在上述各项作用的综合影响下，辐射靶材的表层组织将会发生结构改变，并进一步引起靶材表面耐蚀性、摩擦性能以及显微硬度等的变化，达到材料表面改性的目的[12-13]。由于钛合金可以在较高温度范围内(600℃~900℃)保持良好的机械强度以及优异的抗高温氧化与耐腐蚀性能，目前该合金已被广泛应用在汽车发动机、航空航天等重要工业领域[14-17]。其中，TC11合金属于一类通过沉淀强化方式得到的变性高温合金，在飞机发动机叶片、机匣等部件制造领域得到了大量应用[18]。

    本文中主要研究了利用强脉冲激光照射技术对钛合金进行性能改性的过程，分析了TC11钛合金受到强脉冲激光照射之后发生的组织结构与力学性能方面的变化情况，探讨了对TC11钛合金采用不同脉冲参量强脉冲激光照射之后，其表面微观结构、抗氧化性、显微组织结构与硬度等的变化规律。

1.   材料及方法

  • 本实验中采用TEMP-4M强脉冲激光加速器对合金材料进行强脉冲激光照射测试。参量如下：波长为800nm，脉宽为250fs，频率为1kHz, 最小输出功率为1W，移动范围为300μm×300μm×200μm，精度为1μm，负载为12kg, 速率不小于300mm/s。该加速器的峰值电压为250kV，生成的强脉冲激光照射脉冲离子成分由30%氢离子和70%碳离子构成，并控制在0J/cm2~5J/cm2的脉冲能量密度。用于测试的样品尺寸为20mm×8mm×6mm，在测试前需要对试样采用SiC砂纸打磨处理，再对其实施超声清洗并用酒精擦拭，之后再对材料表面照射测试。在实验过程中，将强脉冲激光照射的脉冲能量密度分别控制在1J/cm2, 3J/cm2与5J/cm2。

    对试样表面微观形貌进行了JSM6010扫描电镜与光学显微镜表征，同时采用Thermal能谱仪对合金材料的表面元素组成情况进行了测试。采用型号为Rigaku Ultimal V的X射线衍射仪对试样表面的物相结构进行了测试。以HV-1000(Z)显微硬度计测试试样的显微硬度，对各试样进行5次随机测量并取平均值作为测试结果。参照HB-5258标准测试了试样的抗高温氧化性能，测试过程在坩埚内进行，在空气中将试样升温至9200℃并保温60h，通过电子天平对试样的质量进行了称量。

2.   结果与讨论

    2.1.   表面形貌及成分

  • 对照射处理前以及经过不同脉冲能量密度强脉冲激光照射处理后的TC11合金材料的表面微观情况进行观察, 得到图 1所示的形貌。根据图 1可知，对TC11合金照射处理前，对试样不同部位的表面形貌进行观察结果基本相同，大多是由机械抛光产生的许多小划痕以及部分金属碎屑。经过1J/cm2的强脉冲激光对材料表面进行脉冲照射后，可以发现合金材料的表面出现了熔融的情况，明显看到有部分小划痕受熔体流动的影响后呈现为圆滑形态。此外，还可以观察到试样表面形成了许多熔坑，这些熔坑的外径基本处于5μm~10μm之间。随着强脉冲激光照射脉冲能量密度增大到3J/cm2之后，合金表层的熔坑外径也随之变大到10μm~20μm。之后，强脉冲激光照射脉冲能量密度继续增大达到5J/cm2时，熔坑外径达到了50μm~70μm。并且当能量密度增大后，TC11合金的熔融时间及其熔化层深度也相应增加，此时在合金表层中已不存在最初的机械划痕结构。

    Figure 1.  Surface morphologies of original and irradiated TC11 alloy

    对图 1所示的合金表面区域进行透射电子显微镜(energy dispersive spectroscopy, EDS)测试后，得到了表 1所示的各成分组成情况。经过强脉冲激光照射处理后，合金表层形成了约1.5μm外径的微孔，对这些微孔部位进行EDS测试发现其构成元素主要为Mg, O, S, C。对TC11合金的初始原料粉末化学成分进行分析可知，微孔区域的元素基本都是在熔炼阶段形成的杂质或痕量元素。这些元素的熔点比较低，无法在γ相中固溶，因此通常是以化合物或单质颗粒的状态分布于相界与晶界区域。经过强脉冲激光照射后，试样中的夹杂物先熔融或汽化，同时在受热或朝边缘膨胀的阶段还会引起亚表层喷发的现象。在强脉冲激光照射处理结束后，合金表层依然保持了原先的形貌状态并凝固，最终生成熔坑结构。

    point Ti Cr Al Ni Mo Mg W C N O S
    A 0.3826 0.1624 0.1224 0.0778 0.0068 0.1062 0.0086 0.0268 0.0068 0.0128 0.0868
    B 0.0362 0.0124 0.0236 0.2286 0.0128 0.1264 0.0224 0.3826 0.0142 0.1320 0.0088
    C 0.1226 0.0328 0.0320 0.1156 0.0074 0.0828 0.0162 0.2324 0.0106 0.2652 0.0824

    • 2.2.   截面组织形貌

  • 图 2中显示了经过5J/cm2的脉冲照射处理得到的TC11合金截面形貌。其中，合金基体为面心立方结构，γ相中固溶了Fe, Cr, Mo等多种元素。对合金进行微观形貌表征可知，呈弥散态分布的γ′相尺寸在100mm~300nm范围内。

    Figure 2.  Cross section morphology of TC11 alloy irradiated by 5J/cm2

    • 2.3.   硬度分析

  • 从表 2中可以看到对TC11合金表面强脉冲激光照射处理后再对其进行显微硬度测试所得的结果。对比照射处理前后的合金表面显微硬度数据可以发现，经过强脉冲激光照射处理后，材料显微硬度明显增大。并且当脉冲能量密度增加后，合金显微硬度也随之上升。出现硬度上升的原因是合金表面受到照射之后发生了晶格发形变以及晶粒滑移的结果，最终引起冷加工硬化的结果。

    pulse energy intensity/(J·cm-2) 0 1 3 5
    microhardness/HV 302 326 346 375

    • 2.4.   抗高温氧化性能

  • 表 3显示了在900℃下对TC11合金进行高温氧化处理后得到的试样增重变化。根据变化规律可以发现，在氧化初始阶段试样的重量快速增加，之后氧化速率降低并达到稳定状态。其中，在初始氧化阶段试样表面只形成了部分氧化层，此时氧化速率取决于合金材料与气体的反应界面，因此具有较快的氧化速率。当氧化时间增加后，试样表层的氧化膜形成了完整的覆盖情况，此时元素在氧化层内的扩散过程对整个氧化反应的速率起到决定性影响。经过强脉冲激光照射处理后，合金试样的抗高温氧化性能得到了明显的提升。

    exposure time/h 10 20 40 60 80 100
    weight change/
    (mg·cm-2)    		 before 0.42 0.74 0.86 1.02 1.08 1.16
    after 0.36 0.58 0.68 0.81 0.86 0.94

    Table 3.  Weight change of TC11 alloy before and after irradiation of 5J/cm2

    对氧化处理100h得到的TC11合金试样进行表面形貌观察与EDS表征后，得到如图 3和表 4所示的结果。可以发现，合金表层形成的氧化物包含两种形貌类型：第1种是颗粒状的氧化物，第2种是具有致密结构的层状氧化物。对颗粒氧化物进行EDS表征可以看到，该氧化物含有O与Ni两种元素，层状氧化物则含有Cr, O, Al多种元素。可以推断，颗粒状氧化物主要由NiO构成，层状氧化物则主要由Al2O3与Cr2O3构成。同时还可看到，对试样照射之后，其表面的层状氧化物数量开始增加，说明此时的试样表面存在更多Al2O3与Cr2O3。与原始试样相比，经过照射处理后，Al, Ti更易扩散到合金表层区域，形成连续性Al2O3膜。由于Al2O3与Cr2O3的致密度比NiO更高，可以对O元素的扩散过程起到更明显的阻碍作用，使试样氧化速率显著减小，从而有效改善TC11合金的抗高温氧化能力。

    Figure 3.  Surface morphologies of samples after oxidating for 100h

    Ti Cr Al Ni O
    A 0.5863 0.0916 0.0064 0.0762 0.2068
    B 0.1624 0.1684 0.0962 0.0668 0.4364

    Table 4.  EDS (mass fraction) of samples in Fig. 3

3.   结论

  • 经过1J/cm2的强脉冲激光照射后，发现合金表面出现了熔融，随着强脉冲激光照射脉冲能量密度增大，合金表层的熔坑外径也随之变大。经过强脉冲激光照射处理后，合金表层形成了约1.5μm外径的微孔，其构成元素主要为Mg, O, S, C。

    经过5J/cm2的脉冲照射处理得到的TC11合金截面形貌呈弥散态分布, γ′相尺寸在100mm~300nm范围内。经过强脉冲激光照射处理后，材料显微硬度明显增大。并且当脉冲能量密度增加后，合金显微硬度也随之上升。

    当氧化时间增加后，试样表层的氧化膜形成了完整的覆盖情况。经过强脉冲激光照射处理后，合金试样的抗高温氧化性能得到明显的提升。合金表层形成的氧化物包含颗粒状和层状两种形貌类型，氧化物主要由Al2O3与Cr2O3构成。

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