激光金属沉积技术制备高熵合金组织及性能研究

RESEARCH ON MICROSTRUCTURE AND PROPERTIES OF HIGH ENTROPY ALLOYS BY LASER METAL DEPOSITION

李瑶

11930442

硕士

材料工程

刘朝阳

2021-05-20

2021-06-07

南方科技大学

深圳

高熵合金拥有高强度、高硬度、良好的耐腐蚀和抗氧化性等优秀性能，在航空航天、海洋工程、轨道交通和新能源等领域有广泛的应用前景。目前制备块体高熵合金的方法主要为电弧熔炼和铸造的方式，这些传统的制备方法会限制合金尺寸大小，容易有孔洞，疏松等缺陷，难以用作高性能的材料的制备手段。激光金属沉积技术作为一种高效的激光增材制造技术，具有不需要模具、自由度高、制造材料广、制造周期短等诸多优点，能够定制净成形复杂构件。通过激光金属沉积技术制备高熵合金的研究仍处于初步阶段。本课题使用激光金属沉积技术成功制备出FeCoCrNiMn高熵合金薄壁试样，并以其为研究对象开展了一系列的研究，主要完成工作如下： （1）研究了工艺参数对高熵合金薄壁试样的成形质量、表面形貌、组织结构、物相变化和力学性能等的影响。研究表明，相较于1400 W和1800 W，激光功率为1000 W时制备的试样成形质量更优，无明显宏观缺陷，致密度最高。激光功率为1800 W时，由于线能量密度过高，成形失败。成形件的底部为等轴树枝晶和柱状晶混合枝晶，中、顶部为趋向于热流方向生长的柱状树枝晶组织。 （2）研究了激光金属沉积技术制备的高熵合金薄壁试样的硬度、拉伸性能、高温氧化和耐腐蚀性能等性能。硬度测试结果表明，沉积态试样的硬度范围在155~175 HV之间，成形件整体硬度分布均匀，室温拉伸结果显示，激光功率为1400 W时，试样抗拉强度451 MPa，延伸率20%；1000 W时的抗拉强度为505.31 MPa，延伸率约为30%。与铸造制备的FeCoCrNiMn高熵合金试样（抗拉强度447 MPa）相比，抗拉强度有一定提高。比起船板用钢的开路电位（-0.34 V），1400 W时制备的FeCoCrNiMn高熵合金的开路电位为-0.097 V，FeCoCrNiMn高熵合金的耐腐蚀性能优秀，可做短时间腐蚀使用。 （3）研究了热处理温度不同对激光金属沉积高熵合金组织性能的影响。本研究选择三种退火热处理温度，分别为400℃、600℃、800℃。退火热处理工艺保温时长为2小时。研究表明，经过400℃、600℃、800℃热处理后的试样仍存在元素偏析，枝晶间富集Fe和Ni，枝晶臂富集Mn、Cr和O元素。1000 W下制备的试样经过400℃/2 h热处理后延伸率与沉积态相比提高了30%。而1400 W下制备的试样经过400℃/2 h热处理后的延伸率为30.71%，与沉积态相比提高了50%，延伸率得到了显著提高，且其抗拉强度也提高了10%。 （4）研究了退火热处理时长对激光金属沉积高熵合金薄壁试样组织和性能的影响。退火热处理温度1000℃，退火时长分别为4 h、8 h、16 h和32 h。研究结果表明，经过1000℃退火不同时长处理后的FeCoCrNiMn高熵合金均为面心立方结构，FeCoCrNiMn高熵合金高温相的稳定性高。经过1000℃退火处理后，不论退火时长如何变化，组织均发生了变化，打印态组织消失，各元素分布均匀，无明显偏析。但是由于该高熵合金耐高温性能较差，开裂处往往有着退火孪晶生成。因为其高温易开裂，故不同时长热处理后的试样拉伸性能较差，经过退火处理后的硬度下降，硬度不随着退火时间的增长而下降，不同热处理后试样的硬度稳定在130 HV左右。

High entropy alloys have excellent properties such as high strength, high hardness, good corrosion resistance and oxidation resistance. It is a new type of alloy with great application prospect in aerospace, marine exploration, refractories and other fields in the future. At present, most of the methods for producing bulk high entropy alloys are arc melting and casting. However, due to the limitation of the alloys’ size, the traditional preparation methods are difficult to be used for the preparation of high performance materials. Laser metal deposition(LMD) is a kind of laser additive manufacturing technology, which has many advantages, such as no need of mold, high degree of freedom, customized net forming of complex components, wide range of manufacturing materials, short manufacturing cycle and so on. The preparation of high entropy alloys by laser metal deposition is still in the initial stage. In this thesis, FeCoCrNiMn high entropy alloy has been successfully prepared by laser metal deposition technology. The main findings and conclusions are as follows: (1) The effects of processing parameters on the forming quality, surface morphology, microstructure, phase change and mechanical properties of thin-walled specimens were studied. The results show that compared with 1400 W and 1800 W, the sample with 1000 W laser power has better forming quality, no obvious macro defects and the highest density. When the laser power is 1800 W, the forming fails due to the high linear energy density. The results show that the mixed equiaxed and columnar dendrites are formed at the bottom of the part, with the columnar dendrites tend to grow in the direction of heat flow at the middle and top of the part. (2) The properties such as hardness, tensile properties, high temperature oxidation and corrosion resistance of thin-walled specimens of high entropy alloy prepared by laser metal deposition were studied. The hardness test results show that the hardness range of the deposited sample is 155~175 HV, and the overall hardness distribution of the formed part is uniform. The room temperature tensile test results show that when the laser power is 1400 W, the tensile strength of the sample is 451 MPa, and the elongation is 20%; when the laser power is 1000 W, the tensile strength is 505.31 MPa, and the elongation is about 30%. Compared with the FeCoCrNiMn high entropy alloy (tensile strength 447 MPa) prepared by casting, the tensile strength is improved to a certain extent. Compared with the open circuit potential (-0.34 V) of ship plate steel, the open circuit potential of FeCoCrNiMn high entropy alloy prepared at 1400W is -0.097 V. The corrosion resistance of FeCoCrNiMn high entropy alloy is excellent and can be used for short time corrosion. (3) The effect of heat treatment temperature on microstructure and properties of laser metal deposition high entropy alloy was studied. Three annealing temperatures, i.e., 400℃, 600℃ and 800℃, were selected in this study. The duration of annealing heat treatment is 2 hours. The results show that the samples after heat treatment at 400℃, 600℃ and 800℃ still have element segregation, the dendrites are enriched in Fe and Ni, and the dendrite arms are enriched in Mn, Cr and O. After heat treatment at 400℃ for 2 h, the elongation of the sample prepared at 1000 W is 30% higher than that of the as deposited sample. The elongation of the sample prepared at 1400 W after 400℃/ 2 h heat treatment is 30.71%, which is 50% higher than that of the as deposited sample, the elongation is significantly improved, and the tensile strength is also increased by 10%. (4) The effect of annealing time on microstructure and properties of thin-walled samples of laser metal deposition high entropy alloy was studied. The annealing time is 4h, 8h, 16h and 32h respectively. The results show that the FeCoCrNiMn high entropy alloy annealed at 1000℃ for different time is face-centered-cubic(FCC) structure, and the high temperature phase stability of FeCoCrNiMn high entropy alloy is high. After annealing at 1000℃,the printed structure disappears, the distribution of elements is uniform, and there is no obvious segregation. However, due to the poor high temperature resistance of the high entropy alloy, annealing twins are often formed at the crack. Because it is easy to crack at high temperature, the tensile properties of the samples after different heat treatments are poor. The hardness of the samples after annealing treatment decreases. The hardness does not decrease with the increase of annealing time. The hardness of the samples after different heat treatments is stable at about 130 HV.

激光金属沉积 高熵合金 组织性能 热处理

laser metal deposition high entropy alloys microstructure and properties heat treatment

中文

独立培养

南方科技大学

李瑶. 激光金属沉积技术制备高熵合金组织及性能研究[D]. 深圳. 南方科技大学,2021.