【作者】 李杰；

【导师】 何立子； 刘晓涛；

【作者基本信息】 东北大学 ， 材料加工工程， 2013， 硕士

【摘要】 在实际生产中,机械零部件的失效与其表面密切相关。从材料的内在属性出发,改善金属表面性能方面不足的最佳途径是对其进行表面改性处理。而激光技术作为一种新的表面处理技术,与传统表面处理技术相比具有很多优点,激光功率密度大,用激光束强化金属加热速度快,基体自冷速度高；输入热量少,工件处理后的热变形很小；高熵合金是近年来兴起的一种新型合金材料,它的最大特点是突破了传统合金只以一种或两种金属元素为主的设计框架,多主元高熵合金是以五种或者五种以上主要元素按等摩尔比或近等摩尔比组合而成具有金属特性的材料,晶体结构倾向于形成简单体心或简单面心结构的固溶体而非金属间化合物,使高熵合金的性能比传统合金具有较大优势。本文通过优化激光参数,利用Nd:YAG脉冲固体激光器,通过激光表面改性技术在低碳钢和H13钢表面分别激光合金化和激光熔覆高熵合金涂层。主要利用金相显微镜、扫描电镜、X射线衍射仪、显微硬度仪和高温高速磨损试验机等设备对激光合金化层和熔覆层的显微组织、截面成分、物相、显微硬度以及耐磨性能等进行了分析研究。实验得到了优化后的多道搭接工艺参数,低碳钢激光合金化：电流275A、扫描速度5.5mm/s、离焦量-14mm；低碳钢和H13钢激光熔覆：电流275A、扫描速度3mm/s、离焦量-14mm。在低碳钢和H13钢表面得到了组织性能较好的高熵合金涂层,涂层组织致密,有少量裂纹等缺陷,与基体形成了较好的结合。合金化层和熔覆层具有典型的激光快速凝固组织特征。低碳钢激光合金化层组织为平面晶、胞状晶和树枝晶的混合结构；合金化层与基体界面结合处为平面晶,合金化层底部及中部多为垂直于结合面生长的胞状晶和树枝晶,多表现为“带”状组织,合金化层表层为方向紊乱的树枝晶；低碳钢激光熔覆层组织主要为细小的树枝晶组织；H13钢激光熔覆层组织同样为平面晶、胞状晶和树枝晶的混合结构；熔覆层与基体界面结合处为平面晶,熔覆层底部及中部多为垂直于结合面生长的胞状晶和树枝晶,熔覆层表层为方向紊乱的树枝晶。低碳钢激光合金化层组织主要Fe基体和Cr7C3组成,Cr7C3的存在是使合金化层硬度得到明显提高的主要原因,合金化层硬度最高达650HV,约是低碳钢基体硬度(165HV)的4倍；低碳钢和H13钢激光熔覆层组织主要由具有BCC和FCC结构的固溶体组成,细晶强化和固溶强化是熔覆层硬度提高的主要原因,低碳钢和H13钢熔覆层硬度分别最高达639HV和476HV,分别是低碳钢基体硬度(165HV)和H13钢基体硬度(240HV)的4倍和2倍。高温磨损试验结果表明合金化层和熔覆层的耐磨性相比于基体都得到了很大提高,转数为4000转(16min)时,低碳钢基体的失重分别是合金化层和熔覆层的2.0倍和2.7倍,H13钢基体的失重是熔覆层的3.0倍；转数为8000转(32min)时,低碳钢基体的失重分别是合金化层和熔覆层的1.60倍和1.65倍,H13钢基体的失重是熔覆层的1.90倍。低碳钢和H13钢基体的磨损机制主要是粘着磨损,合金化层和熔覆层的磨损机制主要是磨粒磨损,Cr7C3硬质碳化物颗粒和固溶体的形成分别是合金化层和熔覆层耐磨性提高的主要原因。更多还原

【Abstract】 In the practical production, the failure of mechanical components is closely related to the surface properties of the components. Surface modification treatment is a promising solution to improve the surface property of the materials. Laser treatment, as a new surface treatment method, owns many merits compared to the traditional surface treatment methods due to its high power density, fast healing rate and cooling rate, minor heat influence on the substrate. As a novel materials system beyond traditional metallurgical design, the high entropy alloys (HEAs) consist of multiple principal elements in equiatomic or close-to-equiatomic ratios. In HEAs, simple solid solutions with FCC or BCC structure tend to form with the absence of intermetallic compounds and elaborate phases during solidification.In this paper, the low-carbon steel and HI3 steel were modified using high entropy alloys by laser surface alloying and laser cladding techniques, respectively. A Nd:YAG pulsed solid laser device was used in the study and the laser parameters were optimized. The cross-sectional microstructure, phase assemblages, micro-hardness and abrasive resistance of the laser alloying layer and cladding layer were characterized with optical microscopy, scanning electronic microscopy with EDS, X-ray diffractometer, micro-hardness tester and high temperature and high speed abrasion tester.The multipass lap fabrication parameters for the laser alloying of low-carbon steel were obtained:current 275A, scanning rate 5.5mm/s, defocusing amount-14mm, whereas for the laser cladding of low-carbon steel and H13steel:current 275A, scanning rate 3mm/s, defocusing amount-14mm. The high entropy alloy coating with good microstructure and property was prepared on the low-carbon steel and H13 steel. The coating is compact with a few cracks and well bonded with the substrate.The laser alloying layer and cladding layer own the typical microstructure of laser rapid solicitation. The laser alloying layer is a hybrid structure composed with planar, columnar and dendritic crystals; planar crystal exists at the interface of the alloying layer and substrate; the bottom and middle side of the alloying layer are mainly composed of columnar and dendritic crystals which grow perpendicularly to the bonding face, in the form of "band" structure. The surface of alloying layer is dendritic crystals which are in disordered direction. The laser cladding layer of the low-carbon steel is mainly composed with fine dendritic crystals. The laser cladding layer of the H13 steel is also a hybrid structure composed with planar, columnar and dendritic crystals. Planar crystals exist at the interface of the cladding layer and substrate. The bottom and middle side of the cladding layer are mainly composed of columnar and dendritic crystals which grow perpendicularly to the bonding face. The surface of cladding layer is dendritic crystals which are in disordered direction. The laser alloying layer of the low-carbon steel is composed of Fe and Cr7C3. The existence of O7C3 is the main contributor to the enhancement of the hardness of the alloying layer. The highest hardness of the alloying layer is about 650HV, about four times of that of the low-carbon steel substrate. The laser cladding layer of the H13 steel is mainly composed of solid solution with BCC and FCC structure. The hardness of the cladding layer is enhanced mainly through the refining strengthening and solidification strengthening mechanism. The highest hardness of low-carbon steel and H13 steel are 639HV and 476HV, which are four times of that of the of low-carbon steel and two times of that of the HI3 steel substrate. The results of high-temperature abrasion test demonstrated that the abrasive resistance of alloying layer and cladding layer was greatly enhanced compared with the substrate. The weight loss of the low-carbon steel substrate is 2.0 times of that of the alloying layer and 2.7times of that of the cladding layer after 4000r (16min) of test. The weight loss of the low-carbon steel substrate is 1.60 times of that of the alloying layer and 1.65times of that of the cladding layer after 8000r (32min) of test. Meanwhile, the weight loss of the H13 substrate is 1.90 times of that of the cladding layer. The abrasion mechanisms of the low-carbon steel and H13 steel substrate is adhesive wear, whereas the main abrasion mechanism of alloying layer and cladding layer is abrasive wear. The formation of Cr7C3 hard particles and solid solution is the main reason for the enhancement of the abrasive resistance of the alloying layer and cladding layer, respectively.更多还原