【作者】 彭元东；

【导师】 易健宏；

【作者基本信息】 中南大学 ， 材料学， 2011， 博士

【摘要】 微波烧结技术是利用微波与物质相互作用的介质损耗而产生热量,使整个材料加热至被烧结温度而达到致密化的一种方法,其具有体积加热、选择性加热、非热效应等特点,是快速制备高质量的新材料和具有新的性能的传统材料的一种重要技术手段。论文首先探索了微波烧结纯金属粉末的微波加热特性及发热机理,并以还原铁粉为对象,研究了其烧结动力学及机制,为深入探讨微波烧结粉末冶金材料进行了基础性研究。在此基础上选取了在粉末冶金烧结中具有代表性的Fe-Cu-C铁基粉末冶金合金(少液相烧结或固相烧结)、W-Ni-Fe高密度合金(金属液相烧结)以及WC/Co硬质合金(金属与陶瓷液相烧结)为对象,研究了微波烧结此系列粉末冶金材料的工艺、性能、组织结构的特点与特性,并与常规烧结进行了对比。通过以上研究工作,得到以下主要研究结果：(1)金属粉末微波加热的机理存在电导损耗、磁损耗(涡流损耗、磁滞损耗、剩余损耗)和电弧放电等损耗机制,不同元素的金属粉末主导损耗机制不同。在磁性材质中,涡流损耗、磁滞损耗、剩余损耗比非磁性材料加热作用大；电导率大的材料电导损耗效果明显。同种金属粉末压坯的微波加热升温曲线依赖于粉末颗粒尺寸、孔隙度和微波输出功率。颗粒尺寸越小,加热速率越快,最终所能加热的温度也越高；孔隙度越高,加热速率越快,而最终所能加热的温度与起始孔隙度无关；微波输出功率越大,加热速度越快。在低温下,SiC可大量缩短金属压坯的加热时间,而高温下不影响Fe粉压坯的加热曲线形状,是一种较为理想的低温辅热材料。(2)微波烧结还原铁粉,在fcc晶体区域,试样密度和烧结温度满足黄培云综合作用烧结理论。还原铁粉微波烧结的主要机制是晶界扩散,也存在体积扩散。微波烧结的表观活化能为76.21 kJ/mol,和常规烧结相接近。(3)相比于常规烧结Fe-2Cu-0.6C粉末冶金材料,微波加热过程升温速度快,烧结时间显著缩短。微波烧结样品在氮氢混合气氛中,保温10min时,在1150℃得到最佳的烧结性能,密度为7.20g/cm3、洛氏硬度为HRB75、抗拉强度为413.9MPa、伸长率为6%。在1150℃微波烧结时,保温15min试样的各项性能达到最佳,密度为7.22g/cm3、硬度为HRB78、抗拉强度为416.8MPa、延伸率为5.5%。与常规烧结相比,微波烧结试样洛氏硬度稍低,拉伸性能则显著提高。微波烧结对材料力学性能改善在于其良好的微观结构。微波烧结有着更少的孔隙,即小的、近圆形的、分布较为均匀的孔隙,而不同于常规烧结的较大的、有尖角的、分布不均匀的孔隙。且微波烧结有着更为均匀的孔隙结构,即中心致密边缘多孔,而不同于常规烧结的非均匀结构,即边缘致密中心相对多孔的结构。常规烧结样品的组织主要是由大量铁素体、极少量珠光体以及大小不一的各种孔洞组成；而微波烧结样品的组织主要是由铁素体、片状和粒状珠光体以及极少量的孔隙组成。常规烧结样品属于脆性穿晶断裂,而微波烧结为脆性穿晶断裂和韧窝型的穿晶韧性断裂的混合型断裂,提高了材料的韧性和塑性。(4)90W-7Ni-3Fe合金在相同烧结温度下,原料粉末的粒度越小,性能越优异；但是在高温下,细粉颗粒性能反而降低。同种样品不同温度烧结时,有一最佳烧结温度,可获得最优的综合性能。在微波烧结中：1440℃下A试样的抗拉强度为919MPa、硬度为HRC35.8,1460℃下B试样的抗拉强度为899MPa、硬度为HRC34.7,1480℃下C试样的抗拉强度为884MPa、硬度为HRC34.5。相对于常规烧结,微波烧结试样W晶粒更细小、均匀,从而力学性能较好。TEM表明微波烧结合金中发现局部区域具有定向的条纹,这是微波“非热效应”作用的结果。(5)在保温15min,微波烧结YG8试样在1500℃时达到最佳的硬度HRA90.3。与常规烧结试样相比,微波烧结试样的硬度较高,但抗弯强度明显降低；合金的显微组织细小、均匀,但明显存在脱碳相η。微波烧结气氛中含有氧是脱碳相η存在原因之一。钴含量不同的试样,在1460℃时WC-6Co硬度最高,而WC-9Co抗弯强度最高。用同等含量的Ni粉、Fe粉代替Co粉,所得试样与WC-6Co试样相比性能低。更多还原

【Abstract】 Microwave sintering is a densification method through using the heating produced by the couple of microwave with the microstructure of materials and which have these features on bulk heating, selective heating and non-therm effect. It is a significant process capable of fabricating advanced materials with high performance and traditional materials with new properties. In this paper, the behaviour and mechnical of microwave heating pure metal powders was investigated, and the kinetics and mechanism of microwave sintering deoxidized ferrous powder was researched. The process, properties, microstructure and feature of microwave sintered Fe-Cu-C alloy, W-Ni-Fe alloy and WC/Co alloy are studied, and compared with conventional sintered alloy. Through these studies, we could know the microwave sintering characteristic on powder metallurgy materials. On the base of study, the major results of this paper are shown as following.(1)The mechnical of microwave heating metal powder existed electricity conducting loss, magnetism loss (eddy current loss, magnetism hysteresis loss, remain loss) and electron arc discharge loss. The sort of metal powders decided the effect degree of the microwave loss mechnical. The effect of eddy current loss, magnetism hysteresis loss and remain loss are obvious in magnetic materials, and electricity conducting loss is more in the materials with higher conductivity. The microwave heating therm profiles of same kind metal powders are dependent on partical size, porosity and output power. The little of partical size is, the faster of heating rate and higher of final temperature are. The larger of porosity is, the faster of heating rate is, but the final temperature don’t depend on the porosity. And the higher of output power is, the faster of heating rate is. SiC is fast heated at low temperature and don’t effect the Fe green heated at high temperature, so it is a ideal susceptor materials for metal green reducing the microwave heating time. (2) The relationship between microwave sintered densities and temperature of deoxidized ferrous powder with fcc structure was accorded with Huang Peiyun’s synthesized sintering theory. The microwave sintering main mechanism of deoxidized ferrous powder is grain boundary diffusion, and there also exists volume diffusion. The apparent activated energy is 76.21 kJ/mol and it is close to conventional sintered.(3) Compared with the conventional sintering, the microwave heating rate is faster and reduce processing time for Fe-2Cu-0.6C steel alloy. Soaking time for 10min, Fe-2Cu-0.6C steel alloy sintered at 1150℃via microwave irradiation has a superior properties. The as-sintered sample has a density, HRB, tensile strength and elongation up to 7.20 g/cm3,75,413.9 MPa and 6.0%, respectively. At temperature 1150℃, Fe-2Cu-0.6C steel alloy sintered for 15min via microwave irradiation has a superior properties. The as-sintered sample has a density, HRB, tensile strength and elongation up to 7.22 g/cm3,78,416.8 MPa and 5.5%, respectively. In comparison with the conventional sintered, the microwave sintered sample has slightly low HRB and obviously high tensile properties. Sintering mode influences the pore morphology and porosity in the sintered alloy. Lower porosity, small and isolated pores are formed in the microwave sintered sample, in contrast with the large, connected and angular ones in conventional sintered counterpart. Flaky and granular pearlite plus ferrite are presented in the alloy after microwave sintering, different from the predominant ferrite and minor pearlite in conventional sintered sample. Microstructure discrepancy elucidates the different properties possessed by the sintered alloys. Fracture analysis indicates that microwave sintering generates a mixed fracture mode consisting of ductile and brittle manners, other than the single brittle one in conventional sintered sample.(4) The little of W particle size is, the higher of the properties of 90W-7Ni-3Fe alloy sintered at the same temperature are. On the contrary little particle has low performance at high temperature. There are optimal properties at appropriate temperature for every kind of sample. The A sample sintered at 1440℃via microwave irradiation has tensile strength 919 MPa and hardness HRC35.8, and B sample at 1460℃has tensile strength 899 MPa and hardness HRC34.5, then C sample at 1480℃has tensile strength 884 MPa and hardness HRC34.5. Compared with conventional sintered, microwave sintered sample has more little distortion and more small and uniform tungsten grain. TEM shown that some local orient stripes were found in W-Ni-Fe alloy sintered via microwave irradition, it was the result of microwave non-therm effect.(5) Soaking time for 15min, YG8 cemented carbide sintered at 1500℃via microwave irradition has a optimal properties. The as-sintered sample has a HRA up to 90.3. Compared with conventional sintered, microwave sintered sample had higher hardness and badly lower bending strength, it existedηphase and lost carbon in microwave sintering sample, and the same time WC grain dissolution-deposition is slow. Microstructural investigations showed that microwave sintered sample were small and uniform in grain sizes. The formation ofηphase attributes to oxygen gas existed in the microwave sintering furnace. The samples sintered at 1460℃WC-6Co has the maximum hardness, and WC-9Co has the best bending strength. Compared with WC-6Co sample, WC-3Ni-3Fe sintered via microwave irradition has low properties.更多还原