【作者】 牟军伟；

【导师】 赵玉民；

【作者基本信息】 武汉理工大学 ， 材料学， 2008， 硕士

【摘要】 目前,模具技术已经成为衡量一个国家产品制造水平的重要标志,而模具寿命是直接影响产品质量、加工效率和成本的重要因素之一。热疲劳是热作模具主要失效形式,据统计,热作模具中由于热疲劳导致失效的占失效总量的60～70%。因此通过模具表面技术提高热作模具的寿命具有重大的国民经济意义。本文通过激光熔覆和真空熔结两种技术在H13钢表面制备了钴基合金覆层,利用光学显微镜、扫描电镜（附带EDS）、XRD和显微硬度计等试验设备分析了覆层的组织结构、硬度等性能。研究了两种技术制备的覆层的界面结合情况及热疲劳等性能。试验结果表明,激光熔覆钴基合金覆层在组织结构上分为熔覆区、结合区和热影响区。激光熔覆层组织随扫描速度增大、激光功率减小而变得细小,根据试验分析得出了较佳的工艺组合。多道搭接熔覆层由于基体材料温度升高,凝固时界面结合区温度梯度减小,不形成激冷层,界面处没有平面晶区。真空熔结钴基合金覆层组织致密,与激光熔覆层相比组织粗大,在界面处发生较高元素扩散,形成扩散结合界面。激光熔覆层组成相主要为γ-Co、Cr₂₃C₆及Ni_2.9Cr_0.7Fe_0.36等相;真空熔结层主要由Co、Cr₇C₃和Ni_2.9Cr_0.7Fe_0.36等相组成。激光熔覆层细小的组织使其具有较高的表面硬度,硬度随着激光扫描速度的增大及激光功率的减小而增大,表层由于合金元素的烧损,硬度值较次表层稍有下降,真空熔结层硬度较低。多道搭接熔覆时硬度比单道熔覆硬度低。热疲劳失效是热作模具的主要失效形式,试验结果表明,激光熔覆钴基合金覆层出现宏观热疲劳裂纹时承受的热循环次数最多,裂纹宽度最小为14um,热疲劳性能最好,而未经表面处理H13钢裂纹最宽,热疲劳性能最差。由于钴基合金具有高的热稳定性能,热循环前后表面硬度下降幅度较小。真空熔结层在热循环后,围绕块状共晶化合物产生较多的微裂纹,并最终剥落形成凹坑。更多还原

【Abstract】 At present,mould technology has been become an important symbol to measure the level of a country’s product manufacturing.Mould life is not only the important factor that affecting product quality,machining efficiency and cost,but also an important guide to measure the level of manufacturing.Thermal fatigue is a leading failure of hot working die steels.According to statistics,60～70%die failure are caused by thermal fatigue of hot working die.So there have great national economic significance to raising the service life of hot work die by surface strengthening technology.Co-based alloy coatings are prepared on H13 steel by laser cladding and vacuum fusion sintering technology.Microstructure,Microhardness of the coatings have been tested using optical microscope,scanning electron microscope（SEM,including EDS microanalysis）,X-ray diffraction instrument（XRD）and microhardness tester.The interracial bonding and thermal fatigue properties of coatings were studied.The experiment results showed that the microstructure of single track laser cladding Co-based alloy coating include the cladding,combined and heat-affected area.When increasing the laser scanning speed or decreasing the laser power the microstructure of laser cladding coating become finer.With the experiment,the optimal laser cladding parameters have been obtained.In multi-track laser cladding coating there have no planar crystal in interface,because the temperature of substrate raised leading to the temperature gradient in interface decrease,so it can’t form chilled layer.The vacuum fusion sintering coating has compact structure and coarse microstructure.In interface,there happen the element diffusion;by diffusion reaction a good diffuse combination interface was formed.The results showed that there are primary phasesγ-Co,Cr₂₃C₆ and Ni_2.9Cr_0.7Fe_0.36in the laser cladding coating.Co, Cr₇C₃ and Ni_2.9Cr_0.7Fe_0.36phases are shown in vacuum fusion sintering coating.The laser lading coating has higher surface microhardness than vacuum fusion sintering coating.When increasing the laser scanning speed or decreasing the laser power the microhardness of laser cladding coating increased.Because of the burning of alloy elements in surface,the microhardness in surface is a little lower than subsurface.The microhardness of multi-track laser cladding coating is lower than single track laser cladding coating.Thermal fatigue is a leading failure of hot working die steels.The experiment results showed that laser cladding coating has the most excellent thermal fatigue property.The laser cladding coating suffers the most thermal cycling times before appear macroscopic thermal fatigue crack,and the crack width is the least,the crack width is 14um.H13 steal without surface treatment has the poorest thermal fatigue property.The Co-based alloy coating has high thermal stability,so the descend range of microhardness is small after thermal cycling.But the descend range of H13 steel is large.After thermal cycling,microcracks will generate encircle the massive eutectic compound of vacuum fusion sintering coating,spalling and become pit lastly.更多还原