【作者】 贺海燕；

【导师】 曹晓卿；

【作者基本信息】 太原理工大学 ， 材料加工工程， 2010， 硕士

【摘要】 H13（4Cr5MoSiV1）是国际上广泛使用的热作模具钢,它具有较高的热强度和硬度、高的耐磨性和韧性,以及较好的耐热疲劳性能,广泛应用于制造各种锻模、热挤压模,以及镁、铝及其合金的压铸模。热作模具在使用过程中承受着磨损、热疲劳、冲蚀、应力腐蚀、表面热焊合等物理、化学作用,因而其使用寿命较低。众所周知,模具的失效首先是从表面开始的,因此改善表面组织及性能可有效提高模具的使用寿命。H13钢表面激光熔覆是利用激光的高辐射、高亮度、高方向性、高单色性特点,使材料的表面性能得到提高,特别是材料的表面硬度、耐磨性、耐蚀性和耐高温性的改进,从而提高热作模具的使用寿命。实际生产中需要对产品进行大面积熔覆,但通常入射激光光斑直径较小,最简单易行的办法是通过激光扫描带间的相互搭接来满足要求。本文采用5kW横流CO₂激光器,通过多道搭接大面积熔覆在H13热作模具钢表面制备了Ni基、Co基合金熔覆层,利用光学显微镜（OM）、X-射线衍射仪（XRD）等手段分析了熔覆层的显微组织结构、相组成,观察了高温氧化后的组织形貌;借助HVS-1000A型数显显微硬度计对激光熔覆层的硬度进行了测量;利用磨损试验机对基体及熔覆层的磨损性能进行了研究。组织分析表明,激光熔覆Ni基、Co基合金熔覆层在组织结构上分为熔覆区、结合区和热影响区三部分。Co基熔覆层的显微组织为大体上垂直于界面生长的平面晶和胞状晶,向熔池中部过渡为多方向生长的树枝晶区,近表面为平行于激光扫描速度方向生长的细小枝晶区,熔覆层与基体之间实现了良好的冶金结合,具有较好的综合力学性能。Ni60A熔覆层的组织主要由Cr₂₃C₆、Ni2Si等强化相和过饱和固溶体γ（Ni-Fe）组成;Co50熔覆层的组织主要由（Cr-Fe）₇C₃等强化相和过饱和固溶体γ-Co组成;Co42熔覆层的组织主要由M₂₃C₆、CoC_X等碳化物强化相和少量过饱和固溶体γ-Co组成。激光熔覆层细小的组织使其具有较高的表面硬度。多道搭接熔覆后进行的显微硬度测量表明,Ni60A显微硬度在800HV0.2～900HV0.2之间;Co50显微硬度约为500HV0.2;Co42显微硬度约为475HV0.2;而H13钢基体的硬度约为450HV0.2,即硬度由高到低依次是Ni60A→Co50→Co42→H13。通过长时间高温氧化试验,发现Co基合金高温氧化后硬度并没有下降,即优于Ni基合金。在600℃的温度下长时间保温后Co基合金仍具有良好的热稳定性;从H13钢基体及Ni60A、Co50、Co42熔覆层在不同载荷下的磨损量可知,ΔM Co50>ΔM Co42>ΔM H13>ΔM Ni60A,即Ni60A熔覆层的耐磨性最好。压铸模具在使用过程中主要的失效形式是热疲劳、冲蚀、轻金属腐蚀、高温腐蚀、表面热焊合等,不追求过高的硬度。因此在H13压铸模具表面激光熔覆时应优先选用高温性能较好的Co基合金粉末。更多还原

【Abstract】 H13（4Cr5MoSiV1）, the mostly widely used hot work die steel, which has high thermal strength, wear resistance, impact-toughness and excellent thermal fatigue resistance, is widely used to manufacture various forging dies, hot extrusion dies and casting dies of magnesium and aluminium alloys. Because hot work dies suffer from various physical and chemical actions, such as wear, thermal fatigue, erosion, stress corrosion and surface thermal wielding in service, their service life are not long. It is well known that failure of mould starts from the surface. To improve the surface microstructure and mechanical properties is an effective way to extend its service life. Method of surface laser cladding is used to improve surface properties of H13 steel, eapecially its surface hardness, wear resistance, corrosion resistance and heat resistance by making use of high radiation, high brightness, sound directivity and monochromaticity of laser beam, and so its service life is extend. In practical, large area cladding should be done, but the diameter of the incident laser spot is usually too small to meet the requirement. Over lapping between contiguous laser scanning zones is the simplest solution.In this paper, a 5kW power carbon dioxide flow transverse laser system was adopted for preparation of Ni-based, co-based alloy cladding layer on the surface of H13 hot die steel by means of overlapping large area cladding. The microstructure and phase composition of cladding layer as well as its microstructure and morphology after high temperature oxidation are analyzed using optical microscope （OM） and X-ray diffraction （XRD）. The hardness of cladding layer was tested using HVS-1000A microhardness tester and its wear resistance was analyzed by wear testing machine.Microstructure analysis showed that the laser cladding Ni-based and co-based alloy coatings were divided into three areas from the aspect of organizational structure: the cladding, the combining area and the heat affected zone. At the cladding layer bottom of Co-based alloy were mostly planar and cellular grains growing normal to the interface. In the middle were dendritic crystals growing multi-directionally, and at the top zone were small dendritic crystals growing parallelly to the laser scanning direction. Good metallurgical bonding between the coating and the substrate and good comprehensive performances were obtained. The phases in Ni60A cladding layer mainly consisted of Cr₂₃C₆ and Ni2Si reinforced phase and theγ（Fe-Ni） supersaturated solid solution; the phases in Co50 cladding layer mainly consisted of （Cr-Fe）₇C₃ reinforced phase andγ-Co supersaturated solid solution; and the phases in Co42 mainly consisted of reinforced phases of carbides such as M₂₃C₆ and a littleγ-Co supersaturated solid solution.The laser cladding coating had a high surface hardness because of its fine structure. In this paper, the multi-track laser cladding coating was adopted, and the wear test results showed that Ni60A microhardness ranged from 800HV0.2 to 900HV0.2, Co50 microhardness was around 500HV0.2, Co42 microhardness was approximately 475HV0.2, and that hardness of the H13 steel matrix was around 450HV0.2. In other words, descending order of the hardness was as follows: Ni60A→Co50→Co42→H13.It’s discovered that the hardness of Co-based alloy keep stable and it is higher than that of Ni-based alloy after long-time high-temperature oxidation test, which showed that the Co-based alloy enjoyed a good thermal stability even at 600℃after long time holding. The wear amount curve of H13 steel matrix and cladding coating of Ni60A, Co50 and Co42 under different loads showedΔM Co50 >ΔM Co42 >ΔM H13 >ΔM Ni60A, which meant that wear resistance of the Ni60A cladding coating was the best.The failure mold of die cast mold in service is thermal fatigue, erosion, light metal corrosion, high temperature corrosion and surface thermal welding. So it is not necessary for die cast mold to possess extortionate hardness. Therefore, heat-resistant Co-based alloy is the best choice in the surface cladding process of H13 die cast mold.更多还原