【作者】 汪潇；

【导师】 王华昌；

【作者基本信息】 武汉理工大学 ， 材料加工工程， 2010， 硕士

【摘要】 热锻模的寿命问题一直受到业界人士的广泛关注。要提高模具寿命,最重要的是缓解热锻模温度波动区的热应力幅值。本文试图就热锻模模膛表面层区材料进行实验研制和优选,来探索缓解热锻模表面热应力的方法。涉及主要制备方法有等离子喷涂、等离子重熔以及等离子喷焊。本文首先总结了热锻模的各种失效形式,以及热锻模寿命和应力的关系,影响热锻模寿命的热物理性能参数；然后介绍了本文涉及到的等离子喷涂、等离子重熔和喷焊技术的研究现状。第二部分介绍了实验材料和实验方法。选W6Mo5Cr4V2高速钢为基体；粘结金属和粘结底层材料选用Ni60粉；陶瓷粉未选择了SiC粉,Cr₃C₂粉和WC粉。将Ni60粉和三种碳化物陶瓷粉末分别以70%／30%、80%／20%、90%／10%的比例混合,制成实验所需九种粉。然后设计了等离子喷涂-重熔和等离子喷焊实验,基体上喷涂Ni60-SiC涂层,Ni60-Cr3C2涂层,Ni60-WC涂层,再利用等离子弧对喷涂表面进行重熔；同时对另一部分试样采用喷焊法获得喷焊层。最后对等离子喷涂层和“一步法”“两步法”得到的覆层进行比较制定了对涂层进行检测的方案,主要包括采用金相显微分析、显微硬度测定和对主要热物理性能,包括材料的热膨胀系数口热传导系数兄和比热容C进行测定。第三部分对检测结果进行纵向对比分析。按陶瓷粉末种类分为三组,对每组中三种配比粉末的喷涂层,重熔层和喷焊层进行比较。首先分析了Ni60-SiC试样。从显微结构上,等离子喷涂层呈典型的层状结构,等离子重熔层基体和覆层结合良好,基体对覆层有一定程度的稀释,等离子喷焊层显微组织均匀致密；从显微硬度上进行分析,三种方法中喷焊的硬度最高,喷涂其次,重熔最差。随着SiC含量的增多,涂层的显微硬度增大；三种处理方法中喷焊试样的热物性参数是最好的。等离子喷焊的热膨胀系数和基体最接近,30%SiC试样的热膨胀系数是最小的。结论：30%SiC粉末采用喷焊方法得到的覆层最有利于缓解锻模的热应力。接着分析了Ni60-Cr3C2试样。从显微结构上和SiC类似,喷焊层组织均匀致密,是结合最好的；硬度上,喷焊的硬度最高,喷涂其次,重熔最差,随着Cr3C2含量的减少,覆层的显微硬度反而增大；三种处理方法中喷焊试样的热物性参数是最好的。结论：10%Cr3C2粉末采用喷焊方法得到的覆层最有利于缓解锻模的热应力。最后对Ni60-WC试样分析。显微结构是喷焊层最好；显微硬度分析表明,三种方法中喷焊的硬度最高,喷涂其次,重熔最差,随着WC含量的增加,覆层的显微硬度增大；三种处理方法中喷焊试样的热物性参数是最好的。结论：30%WC粉末采用喷焊方法得到的覆层最有利于缓解锻模的热应力。第四部分在第三部分检测结果的基础上,对30%SiC,10%Cr3C2,30%WC粉末试样进行横向的对比分析。从宏观形貌上来说,喷涂层表面粗糙,容易产生裂纹,喷涂厚度因为内应力大而受到制约；重熔层中间有一个烧蚀的凹坑,是重熔时喷枪反复经过中间,加热过度造成的；喷焊层表面光滑致密,厚度达到可2mm,覆层和基体结合良好。从微观形貌上分析,喷涂层是层状结构；重熔层组织细密,基体对覆层有不同程度的稀释,但涂层中没有硬质相；喷焊层覆层和基体形成良好的冶金结合,组织均匀细密,在过渡区形成增碳层或者平面晶带,增大了结合强度。在显微硬度方面,喷涂层的硬度和基体相当,但是在过渡区都有一个硬度的下降点；重熔层硬度比基体低,没有达到增大基体硬度的效果；30%WC喷焊层硬度和基体相近,并没有很大的提高,而30%SiC喷焊层和10%Cr3C2喷焊层硬度都远高于基体,达到强化基体硬度的作用。在热物性参数的分析中,热膨胀系数是对综合应力最敏感的。30%SiC喷焊层在比热容和热传导上是最满足耐热性要求的。在和理论最优粉末质量配比方案匹配方面,10%Cr3C2和理论值相差很大,加上缺少热膨胀系数实验数据,所以不能作为最优配比材料。30%WC喷焊层显微硬度没有达到预期效果,并且配比和理论值相差大,所以不能作为最优配比材料；30%SiC喷焊层实验效果比较好,并且配比和理论值很接近。综上所述：本实验中一步法比二步法的效果要好。采用喷焊的方法,选用30%SiC粉末得到的覆层,从宏观形貌,微观形貌,显微硬度和热物性参数上都满足热锻模表面的使用要求。更多还原

【Abstract】 Life expectancy of hot forging die has been under much concern in the industry. The key solution to increase the life is releasing thermal stress amplitude. In this study, the method to mitigate hot forging die surface thermal stress is discussed, in terms of material testing and optimization of hot forging die surface. Principal preparation method includes plasma spraying, plasma remelting and plasma welding.In the first part, failure modes of hot forging die, correlation between life and thermal stress, and parameters that affect the useful life of hot forging die have been brought out. Cutting edge developments of plasma spraying, plasma remelting and plasma welding are also presented.In the second part, testing material and method are described. High Speed Steel W6Mo5Cr4V2 is adopted as substrate, bonding metal and bonding base material is Ni60 powder. Ceramic powder is SiC, Cr3C2 and WC. Ni60 and the above mentioned three type of ceramic powder are mixed by the composition of 70%/30%,80%/20%,90%/10%, thus nine composition of powder mixtures are used in the tests.Design of plasma spraying-remelting and plasma welding experiment is described thereafter. Firstly, spray Ni60-SiC, Ni60-Cr3C2, Ni60-WC on substrate; Secondly, remelting the plasma spraying layer on plasma arc. At the same time, use plasma overlay welding to obtain plasma spray welding layer. finally, comparing the plasma spraying layer, plasma re-melting layer(called two step method) and plasma spray welding layer (called one step method). The test methods include microscopy, micro-hardness and determine thermal constant which including coefficient of thermal expansion, thermo-conductivity and specific conductance.The third part of this article is comparison among the testing results. Each of the three types of Ceramic is mixed by three ratios, and the results of plasma spraying layer, plasma re-melting layer and plasma spray welding layer in each ceramic powder are compared.Firstiy, analyzing on Ni60-SiC sample. At metallomicroscopy, the plasma spraying layer is tytical layer structure; in plasma re-melting layer, and overlying strata bond well, we can see overlying strata is dilutioned by; plasma spray welding layer has homogeneous and dense microstructure. At microhardness, comparing three kind of preparation method, plasma spray welding layer has the highest hardness, plasma spraying layer in the middle, plasma re-melting layer has the lowest hardness. Microhardness of layer increases with SiC content increasing. Plasma spray welding sample has the best thermal constant. Coefficient of thermal expansion of plasma spray welding layer is the closest to, and coefficient of thermal expansion of 30%SiC is the lowest. In conclusion, plasma spray welding layer of 30%SiC is the best one to release heat stress amplitude.Then, analyzing on Ni60-Cr3C2 sample. It’s the same as Ni60-SiC sample at metallomicroscopy. Plasma spray welding layer is the best one because it has homogeneous and dense microstructure. At microhardness, plasma spray welding layer has the highest hardness, plasma spraying layer is in the middle, plasma re-melting layer has the lowest hardness. Microhardness of layer decreases with Cr3C2 content increasing. Plasma spray welding sample has the best thermal constant. In conclusion, plasma spray welding layer of 10%Cr3C2 is the best one to release heat stress amplitude.In the end, analyzing on Ni60-WC sample. At metallomicroscopy, plasma spray welding layer is the best one. At microhardness, plasma spray welding layer has the highest hardness, plasma spraying layer is in the middle, plasma re-melting layer has the lowest hardness. Microhardness of layer increases with WC content increasing. Plasma spray welding sample has the best thermal constant. In conclusion,plasma spray welding layer of 30%WC is the best one to release heat stress amplitude.The fourth part of this article compared the sample between 30%SiC,10% Cr3C2 and 30%WC, at the base of part three.At macroscopic feature, plasma spraying layer has roughening surface and crack easily, and thickness of spray has been restricted by internal stress; plasma re-melting layer has a ablation dent, caused by overheat of spray gun; plasma spray welding layer is smooth and dense, the thickness can achieve 2mm, and overlying strata bond well. At microcosmic feature, plasma spraying layer has layer structure; plasma re-melting layer has homogeneous microstructure, overlying strata is diluted by, lacking hard phase in layer; plasma spray welding layer and overlying strata bond well with plasma spray welding layer, has carbon-enriched layer or plane zone in transition region which can enhance bonding strength.At micro-hardness, plasma spraying layer is the same as, but in transition region has a suddenly decent point; plasma re-melting layer is lower than, far away from the target; Plasma spray welding layer of 30%WC is the same as, don’t enhance much hardness. Plasma spray welding layers of 30%SiC and 10%Cr3C2 are higher, achieving the effects to improve hardness.In thermal constant analyse, coefficient of thermal expansion is sensitive to comprehensive stress. Plasma spray welding layer of 30%SiC can satisfied heat resistance well.According to best proportion of powder in theory,10%Cr3C2 and 30%WC can’t meet the demand, only 30%SiC can match it.In conclusion, one step method is better than two step method; plasma spray welding layer of 30%SiC are more reliable, showing longer life expectancy under operational condition.更多还原