【作者】 谭天亚；

【导师】 傅正义；

【作者基本信息】 武汉理工大学 ， 材料学， 2003， 硕士

【摘要】 脉冲电流热加工(Pulse Electric Current Heat Treatment，比如烧结，焊接等)是九十年代发展起来的一种材料快速制备新技术，它包括放电等离子烧结与焊接、等离子活化烧结与焊接、脉冲大电流扩散焊接等。它具有升温、降温速度快、能在较低的温度下烧结或焊接以及时间短的特点。该技术已应用于梯度功能材料、金属基复合材料、纤维增强复合材料、纳米材料、多孔材料等多种材料的烧结和不相容材料之间的连接。 脉冲大电流烧结、焊接为什么如此之快是各国研究人员探索的热点，多数学者从证明形成等离子的角度来解释这一现象。实际上，脉冲大电流热加工过程是由原子的扩散过程来实现的。因此，原子的扩散是决定脉冲大电流热加工的重要因素。在脉冲大电流加工过程中原子是如何扩散的，与一般的加热烧结、焊接是否一样，脉冲大电流形成的特殊电场及磁场是否对原子的扩散有大的推动作用，弄清这些问题成为进一步揭示脉冲大电流热加工机理的关键。本文设计出三种样品预构件，研究脉冲大电流加热条件下片状材料、线状材料和球形颗粒之间的原子扩散过程，弄清脉冲大电流加热条件下原子扩散与一般烧结和焊接过程中原子扩散的区别，以证实特殊的电场和磁场是否对原子的扩散有推动作用，揭示脉冲大电流加热条件下原子的扩散过程，探索脉冲大电流热加工技术快速高效的原因。 其它条件相同时，分别在750℃、800℃、850℃、900℃的温度下采用脉冲大电流加热连接Cu和Ni片状材料，结果表明接触面处Cu和Ni的扩散系数随温度升高而增大。与辐射加热相比，接触面处原子扩散速率加快。其它条件相同时，将试样分别以200℃／min、260℃／min、370℃／min的升温速率升温，结果表明，升温速率在260℃／min左右时接触面处Cu和Ni的扩散系数最大。其它条件相同时，分别以6／1、12／2、24／4、48／8的脉冲比(ON／OFF)扩散连接Cu和Ni片状材料，结果表明脉冲大电流加热条件下，电流的脉冲比(ON／OFF)对接触面处原子的扩散也有影响。其它条件相同时，改变装模方式以获得不同的加热条件和电场条件，结果表明，无模套时接触面处Cu和Ni的扩散系数比有模套时大。 脉冲大电流加热条件下，其它条件相同时，分别在800℃、900℃的温度 武汉理土大学硕士学位论文下扩散连接Cu丝和Ni丝，结果表明接触线处Cu和Ni的扩散系数随温度的升高而增大。与辐射加热条件下的比较研究表明:脉冲大电流加热促进了接触线处原子的扩散。 脉冲大电流烧结Fe颗粒材料，烧结体孔径平均值随温度呈波浪关系;辐射加热烧结体的孔径平均值随温度的升高先缓慢减小，继续升高温度，孔径平均值急剧减小。通过两种加热条件下烧结体孔径分布的比较，表明脉冲大电流烧结在相对较低的温度就能获得更高的致密度。其它条件相同，分别以6/1、12/2、24/4、48/8的脉冲比(ON/O FF)烧结Fe，结果表明脉冲比为12/2时获得的相对密度最高。对同温度下脉冲大电流和辐射加热烧结体的扫描电镜分析表明，前者颗粒间接触区面积比后者大，证明脉冲大电流加热条件下物质迁移速率大于辐射加热。比较脉冲大电流和辐射加热烧结过程中样品的收缩，结果表明前者在相对较低的温度即发生明显的收缩，并且收缩速率大于后者。更多还原

【Abstract】 Pulse electric current heat treatment （PECHT） developed by Sodick, Ltd., of Japan, is a recently developed material processing method which consists of spark plasma sintering and welding, plasma activated sintering and welding, big pulse electric current （BPEC） diffusion welding etc. The following are basic merits of PECHT: rapid heating and cooling; short sintering or welding time; lowering sintering or welding temperature. This process has been successfully used to sinter FGM, MMC, FRC and so on, and joining difficult-to-weld materials.Why PECHT is so quick is a hot topic and many scholars are trying to explain it by proving plasma generating. As a matter of fact, PECHT is achieved by atom diffusing. Thus atom diffusion is a’key factor to PECHT. How do atoms diffuse under BPEC heating? And is it the same as under radiation heating? Do the special electric and magnetic fields produced by BPEC accelerate atom diffusion? Understanding the above questions fully is a key to expose the mechanism of PECHT. In this paper, three unique models were designed to research atom diffusion at interface, contacting line and joining point under BPEC heating and compare with it under radiation heating in order to find out if BPEC speed atom diffusion.The first model was aimed to research atom diffusion at interface under BPEC heating. Experiment condition of BPEC diffusion welding sheet Cu and Ni : direct diffusion welding, at a pressure of l0MPa, welded at 750⁹00℃, with heat rate of 200⁴00℃/min for 10mm, ON/OFF of 6/l⁴8/8, with die or not, vacuum of 6Pa. The samples were welded at 750℃, 800℃, 850℃ and 900 ℃.The diffusion coefficients （D） of Cu and Ni at interface increased with the increase of welding temperature. And comparing with radiation heating, atom diffusion velocity at interface quickened. The samples were joined with heat rate of 200℃/min, 260℃/min, 370℃/min. DCu and DNJ reached a maxim at rate ofaround 260℃/min. The study on ON/OFF indicated that ON/OFF had an effecton D_cu and D_Ni- Two different methods of samples placed in the die were also researched: with die or not. D_cu and D_Ni were higher without die than with die.In the second model, wirelike Cu/Ni was welded directly by BPEC heating. The following is the Experiment condition: at a pressure of l0MPa, welded at 800 and 900℃, with heat rate of 200℃/min forlOmin, ON/OFF of 12/2, vacuum of 6Pa. The samples were welded at 800℃, 900℃. D_cu and Dni at contacting line increased with increase of welding temperature. And atom diffusion at contacting line welded by BPEC heating was much quicker than by radiation heating.The third model was designed to investigate atom diffusion at contacting point. Fe was sintered at 500℃, 550℃ > 600℃ , 650℃ > 700℃ under BPEC and radiation heating. The test results of image processing indicated that the average pore size took on wavelike with temperature under BPEC heating. While sintered by radiation heating, the average pore size decreased slowly with increase of temperature at first, and then fell sharply, showing that the PECHT technique could achieve consolidated structure at relative low temperature. Fe was sintered at ON/OFF of 6/1,12/2,24/4,48/8. Measured by the Archimedes method, the high relative density could achieve at ON/OFF of 12/2. The test results of SEM indicated that contacting area sintered by BPEC was bigger than by radiaton heating, demonstrating that material transferring under BPEC heating was quicker than under radiation heating. Comparing shrinkage under BPEC and conventional heating, we can conclude that obvious shrinkage occurred at relative lower temperature and velocity of shrinkage was greater under BPEC heating.更多还原