【作者】 Pham Thi Hong Nga（范氏红娥）；

【导师】 蒋业华；

【作者基本信息】 昆明理工大学 ， 材料加工工程， 2013， 博士

【摘要】 H13钢是国际上广泛使用的热作模具钢,但其经淬火和回火后的硬度不足,加上恶劣的工作环境,在使用过程中表面常会因热磨损和热疲劳而失效,从而使模具报废,严重影响了生产效率。激光熔覆作为一种新兴的表面改性技术,与传统表面改性技术相比,具有冷却速度快、涂层稀释率低、可选区熔覆以及易于实现自动化等突出特点,在大型贵重设备失效部位的表面修复中具有潜在优势。Co基自熔性合金及TiC具有良好的高温强度和耐磨性、但激光熔覆纯TiC粉末存在易开裂、成型性差等缺点,而Co基合金在熔化时具有很好的润湿性,有利于获得致密性好和光滑平整的熔覆层。故选择TiC/Co基混合粉末可获得质量很好的熔覆层,且涂层具有良好抗高温氧化和耐磨等性能。为改善H13热作模具钢表面的热磨损性能,采用预置粉末激光熔覆技术在其表面制备了Co50合金涂层和不同TiC含量的TiC/Co50复合涂层(10%TiC+Co50.20%TiC+Ci50和30%TiC+Co50).借助XRD、OM和SEM对比分析了涂层与基材的结合特征、涂层的物相组成和截面显微组织形貌；通过显微硬度计和高温摩擦磨损试验机测试了涂层截面的显微硬度分布和不同温度下的摩擦磨损性能。重点研究了复合涂层中TiC的形态特征和生长机制对其显微硬度和高温摩擦磨损性能的影响规律。结果表明：(1)Co50合金涂层和粉末预置层TiC含量小于等于20%(wt.%)时的TiC/Co基复合涂层均与H13钢基材呈良好冶金结合,而TiC含量大于20%时复合涂层与基材未能实现较好的结合,说明激光熔覆技术在模具修复领域具有可行性。Co50合金涂层主要由初生γ-Co枝晶及其间的共晶组织组成,而TiC/Co基复合涂层主要由TiC颗粒、枝晶及细小的共晶组织组成。(2)Co50涂层主要由γ-Co固溶体和Cr1.12Ni2.88相组成,但随着预置层粉末TiC含量的增加,TiC/Co基复合涂层中基体相种类减少。10%TiC+Co50复合涂层的基体相由TiCo3、Cr2Ni3和Cr-Ni-Fe-C组成,20%TiC+Co50复合涂层的基体相由Cr2Ni3和γ-Co组成,而30%TiC+Co50复合涂层的基体相为)γ-Co固溶体。(3)TiC/Co基复合涂层中的TiC主要源于两部分,一是混合粉末中原有的TiC,二是TiC分解后在后续凝固过程中原位自生的TiC。其中,原位自生TiC是由于TiC含量较少时,预置层粉末中部分TiC颗粒分解出Ti和C,同时重新析出所造成,且原位自生的TiC表现出多种形态。另外,预置粉末层TiC含量对复合涂层中的TiC形态有较大影响,且涂层不同区域的TiC形貌也有所区别。10%TiC+Co50复合涂层中原位自生的TiC以细小颗粒、沉淀析出,呈多边形和花瓣状,而从涂层底部向上,TiC花瓣状数量不断增多、粒径愈来愈大。20%TiC+Co50复合涂层中原位自生的TiC主要为细小颗粒和发达树枝晶,而从涂层底部到表层,未全熔TiC粒径呈现明显的梯度变化。30%TiC+Co50复合涂层中未出现原位自生的TiC,且未全熔TiC粒径没有明显梯度变化。(4)经激光熔覆处理后,H13模具钢的截面显微硬度得到显著改善。Co50合金涂层和TiC/Co基复合涂层截面平均显微硬度明显高于H13钢,分别为499HV0.2、552HV0.2、590HV0.2和824HV0.2,各是H13钢基材显微硬度(208HV0.2)的2.4倍、2.7倍、2.8倍和4.0倍。其中,添加TiC陶瓷颗粒与未添加TiC的激光熔覆层相比,复合涂层平均显微硬度提高53～325HV0.2,且随着预置层中TiC含量的增加,TiC/Co50复合涂层的截面平均显微硬度呈上升趋势,最高可达824HV0.2,约为基材的4倍。(5)随着载荷的增大,Co50涂层和H13钢的摩擦系数呈递减趋势。随着滑动速度的增大,Co50涂层和H13钢的摩擦系数均表现出先减小后增大的趋势。H13钢髓着滑动速度的增加,磨损率呈先增高后减小的趋势,在滑动速度为200r·min-1时,磨损率达到最大值；而Co50涂层磨损率呈先增大后减小的趋势。温度对Co50涂层的摩擦系数有较大的影响,在室温、200℃和700℃,摩擦副具有较大的摩擦系数；而温度对20%TiC+Co50涂层的摩擦系数影响较小,摩擦系数比较平稳。(6)20%TiC+Co50复合涂层的高温耐磨性比H13钢、C050合金涂层和10%TiC+Co50复合涂层显著提高,摩擦系数平稳。涂层在室温下的磨损机理主要为脆性剥落,粘着磨损和犁削；而在700℃时复合涂层表面存在大量氧化物,磨损机制主要为氧化磨损和疲劳磨损。因此,与H13钢基材相比,20%TiC+Co50复合涂层具有良好的综合性能,显微硬度和高温耐磨性均得到明显改善。更多还原

【Abstract】 AISI H13steel is widely used in hot work die steel, but after quenching and tempering, its hardness is lack. Additionally, with poor working conditions, its surface often fails due to thermal wear and thermal fatigue that make the mould scrap, and seriously affect production efficiency. Laser cladding (LC) is a new kind of surface modification technologies, compared with conventional surface technology, it has many outstanding characteristics, such as fast cooling speed, low dilution rate, optional area cladding as well as easy automation, so LC has good prospects for development and potential advantage of surface repair, especially in parts of large and expensive equipment partial failures. Co-based self-fluxing alloy and TiC has good high temperature strength and wear resistance, but laser cladding pure TiC powder has the disadvantages of easy cracking, poor formability and so on, while Co-based alloy has good wettability on melting, this is helpful to obtain good dense and smooth cladding layer. Therefore, using TiC/Co-based mixed powder is available to obtained not only good quality, but also good high temperature oxidation resistance and wear resistance of cladding layer.In order to improve the wear resistance of AISI H13hot work tool steel, Co50alloy coating and TiC/Co-based composite coatings with different compositions of Co50-TiC precursor powders (10%TiC+Co50,20%TiC+Co50and30%TiC+Co50), were prepared on the H13steel surface by6kW transverse-flow CO2laser. The bonding characteristics, phase composition, microstructure morphology of the coatings were investigated by XRD, SEM and OM; using micro-hardness tester and high-temperature wear tester studied the micro-hardness distribution and the wear behaviors at different temperatures of the coatings. The research was focused on the morphological characteristics and growth mechanism of TiC whose influence on the microhardness and high-temperature wear properties of TiC composite coatings were also investigated. The results indicated that:(1) Co50alloy coating as well as TiC/Co composite coatings with the contents of TiC (wt.%) less than20%showed good metallurgical bonding characteristics with the H13steel substrate surface, but when the TiC content was more than20%, the composite coating failed either at the coating/substrate interface. Co50alloy coating was mainly composed of y-Co dendrite and eutectic between y-Co dendrites, while TiC/Co composite coating which contained of TiC particles, dendrite and fine eutectic.(2) Co50alloy coating mainly contained of y-Co solid solution and Cr1.12Ni2.88phase, but with the increasing of TiC content, the substrate phase composition of TiC/Co based coatings tended to be simple. Phase composition of Co+10%TiC coating was consisted of TiCo3, Cr2Ni3as well as Cr-Ni-Fe-C phases, and so on, while phase composition of Co+20%TiC coating was Cr2Ni3and y-Co, and phase composition of Co+30%TiC coating mainly composed of y-Co solid solution.(3) TiC/Co based coatings were presented in terms of TiC origins which can be divided into two kinds, i.e. undissolved TiC and in-situ TiC particles. In-situ TiC particles is highly dependent on the TiC content. When TiC contents were low, TiC particles in preset powder layers decomposed Ti and C, caused re-precipitation in-situ TiC at the same time. In addition, the contents of TiC in preset powder layers were found to have a significant effect on the morphology characteristics of TiC particles, as well as TiC morphologies in various regions of TiC/Co-based composite coatings were different. In10%TiC+Co50composite coating, in-situ TiC contained of fine TiC, eutectic mixture TiC, polygon TiC and flower-buster; from the coating bottom upwards, the number of flower-buster TiC increased, the diameter of TiC particles were growing. In-situ TiC of20%TiC+Co50coating mainly consisted of fine TiC and developed dendrite TiC; from the bottom to the coating surface, the diameter of undissolved TiC showed obvious gradient.30%TiC+Co50coating had no in-situ TiC and undissolved TiC did not appear significant gradient.(4) After laser cladding processing, cross-section microhardness of H13steel surfaces have been substantially improved, the average cross-section microhardness of Co-based coating and TiC/Co based coatings were significantly higher than that of H13steel substrate (208HV0.2), are499HV0.2,552HV0.2,590HV0.2and824HV0.2, respectively, about2.4-4.0times higher than the H13steel. In comparision, microhardness of TiC/Co based coatings were higher than that of Co-based coating, with the increasing of TiC content, microhardness of TiC/Co based coatings was increased, the highest hardness could be increased up to824HVo.2, about4times greater than the substrate. This result plays a beneficial role to improve the wear resistance of H13steel substrate surface. (5) With the increasing of the wear load, in all cases, friction coefficient of Co50coating and H13steel showed a decreasing tendency. As the sliding velocity increased, friction coefficient of Co50coating and H13steel showed a trend of first decreasing then increasing. With the increasing of the sliding velocity, the wear rate of H13steel was first increased and then decreased, when the sliding speed of200r·min-1, the wear rate reached the maximum value; while Co50coating was first decreased and then increased to the contrary. Friction coefficient of the Co50coating was found to be strongly influenced by temperatures, at room temperature,200℃and700℃the coating has a large friction coefficient. In contrast, not much difference in the friction coefficient of20%TiC+Co50coating was as observed as various temperature, further, the average friction coefficient was relatively stable.(6) Co+20%TiC coating showed better wear behavior than H13steel, Co50coating and10%TiC+Co50coating, as well as had a more stable fiction coefficient; The wear mechanism at room temperature was caused by brittle spalling wear, adhesive wear and plough wear; while high-temperature wear mechanism was mainly caused by oxidation wear and fatigue wear.Overall, comparing with the H13steel substrate, Co+20%TiC composite coating has good comprehensive properties, its hardness and high-temperature wear resistance can meet the use requirements.更多还原