超细硬质合金激光焊接接头“异常晶粒长大”机理及其抑制

Abnormal grain growth (AGG) is an abrupt growth phenomenon at elevated temperature and tungsten carbide (WC) grains have a strong tendency for AGG in ultra-fine cemented carbide. This is devastating for the mechanical properties since these abnormal grains act as initiation points for cracking and breakage. The proposed research focuses on solving critical scientific problems related to AGG mechanism and its inhibition in ultra-fine cemented carbide during multi-layer laser welding and annealing process both experimentally and theoretically (in combine with numerical simulation). The effects of chemical composition and procedure on AGG, microstructure and macro properties are addressed; clarifying the fundamental rules of diffusion, interfacial migration and chemical reaction between abnormal grains and binding phase; revealing the thermodynamics conditions and kinematic behavior of AGG, and dislocation modeling and its influence on nucleation and growth of abnormal grain will be employed to provide the further understanding of driven for AGG. On the basis of characterization of dislocation, slip deformation and WC/Co(Fe, Ni) interface, in combination with experimental results and numerical simulation on temperature field and stress field distribution, this project will model the WC AGG for as-welded and as-annealed cemented carbide welded joint. Research goals include revealing the mechanism of abnormal grain growth in laser-welded ultra-fine cemented carbide and AGG inhibition. The resulting mechanistic understanding of AGG will be used to improve the weldability of cemented carbide during dissimilar welding of cemented carbide and steels, and provide scientific bases for cemented carbide-steel application in "next generation" oil and gas transportation equipment under conditions of technological importance。

异常晶粒长大是发生在加热过程中的晶粒突然长大现象，特别是在超细硬质合金中碳化钨具有强烈的长大倾向，严重降低机械性能。本项目拟采用理论计算、结合物理实验和数值模拟方法，研究超细硬质合金激光多层焊及焊后退火条件下"异常晶粒长大"机理及抑制的关键科学问题。重点研究成分和工艺对晶粒异常长大、微观组织、宏观性能的影响规律；明确异常晶粒与粘结相的润湿性机理及扩散、界面迁移及化学反应的基本规律；揭示位错形成及其对焊接接头异常晶粒形核与长大的驱动规律，确定异常晶粒长大的热力学条件及动力学行为。在焊接接头位错、滑移变形与WC/Co(Fe, Ni)界面结构表征基础上，结合基于多场耦合的温度场、应力场数值模拟及实测结果，建立焊态和退火态接头碳化钨异常晶粒长大模型。本项目旨在揭示激光焊焊接接头碳化钨异常晶粒长大机理，提出抑制理论方法，为提高硬质合金与钢的连接能力、推进其在下一代油气运输装备中的应用提供科学基础。

碳化钨晶粒的异常长大行为破坏硬质合金的致密度和碳化钨晶粒的均匀性，严重降低机械性能，是硬质合金零部件制造的瓶颈之一。本项目利用激光焊接方法，分别采用因瓦合金焊丝/纯镍丝、预置因瓦合金中间层以及无填充的方式实现了硬质合金与钢的焊接。在焊接接头的硬质合金侧形成一个约30微米宽的熔化边界区域，其高温边界由碳化钨（碳化钛或碳化钽）的熔点决定，低温边界由钴的熔点决定。实验发现：异常长大的碳化钨晶粒常常形成于硬质合金侧熔化边界区域和硬质合金侧焊缝，富钨低碳是异常长大的晶粒的形核特征，贫钨富钴是异常长大的晶粒的环境特征。主导硬质合金侧熔化边界区域的碳化钨晶粒异常长大行为的机制是界面扩散（界面能），主导近硬质合金焊缝的碳化钨晶粒长大速率的机制是固-液界面反应与激光/碳化钨的相互作用。研究表明：硬质合金化学成分、碳化钨晶粒尺寸和形状显著地影响碳化钨晶粒的异常长大、微观组织和宏观性能。采用较低的碳含量、高粘结相硬质合金、提高C/W比值、加入TiC/TaC晶粒长大抑制剂降低碳化钨晶粒的异常长大的驱动力。微观组织几何尺寸的不均匀性、化学成分的不均匀性以及激光与碳化钨的相互作用是碳化钨晶粒异常长大的驱动力的主要来源。在激光的能量/物质向熔池传递的过程中，碳化钨之间表现为四种WC-WC的“碰撞”模式，颈部的晶界及液相膜的迁移是驱动力的主要组成部分。热力学计算表明：平直晶界无法摆脱气相的钉扎而独自运动，当弯曲晶界-气相的两面角大于75°、晶界曲率角35°时，系统的自由能变化为负，碳化钨可以摆脱约束而运动，晶面(10-10)是碳化钨晶粒异常长大的优先生长面，在碳化钨颗粒变成异常长大的晶粒的过程中，液相膜和扩散都能破除颗粒中细小晶粒的晶界。通过因瓦合金中间层，选择合适的焊接工艺参数及激光作用位置与方式，控制焊前、焊接过程凝固后段温度以及焊后适当的退火处理的方式，能够控制碳化钨晶粒的异常长大行为。本研究为提高硬质合金与钢的连接能力，推进其在下一代油气运输装备中的应用提供了重要的科学基础。

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