引入梯度复合中间层的大厚度硬质合金/钢电子束焊接机理研究

Cracks easily occurred in the fusion weld between large thick (30-100mm) cemented carbide and steel due to the brittle intermetallics and welding stress, thus caused the poor weldability between the two materials. In this project, the gradient multi-interlayer was introduced to improve weldability. The welding mechanism will be studied by analyzing the unsteady diffusion behavior of elements at the liquid/solid interface, blocking effect and stress relaxation effect of the diffusion layer. Then achieve the qualified weld between large thick cemented carbide and steel by electron beam welding. The content of the research mainly includes: First, establish composition design rule of the gradient multi-interlayer and investigate the boundary effect between the gradient multi-interlayer and the heat source; Second, calculation of the heat and mass transfer at the liquid/solid interface and characteristics of the temperature and flow field distribution. Besides, the metallurgical barrier effect of the primary diffusion layer will be analyzed with the calculation of interfacial diffusion by Fick's second law. Third, the gradient relaxing effect of the multi-interlayer on the welding residual stress of the joint will be studied, and then an evaluation standard for the assessment of the joint will be built based on the stress gradient. The new method in this project of electron beam welding of large thick cemented carbide to steel will providing the theory basis for the research on dissimilar materials joining by electron beam with gradient multi-interlayer and the research on unsteady interfacial behavior at the liquid/solid interface.

大厚度（30-100mm）硬质合金与钢熔化焊接性较差，因焊缝生成脆性化合物及焊接应力而造成接头开裂。本项目引入梯度复合中间层对大尺度硬质合金与钢电子束焊接机理进行研究，并对熔池与硬质合金非稳态液/固界面扩散行为进行理论分析，阐明扩散层阻隔作用机理和应力梯度-松弛效应，实现硬质合金与钢电子束焊优质连接。研究内容主要包括：综合多种因素建立梯度复合中间层化学成分设计准则，研究复合中间层几何特征和热源作用的边界效应；计算熔池与硬质合金液/固界面处的传热传质、温度场及流场分布特征，利用Fick第二定律求解液/固界面的扩散结果，分析先生成扩散层的冶金阻隔作用；研究添加梯度复合中间层对接头焊接应力的梯度-松弛效应，建立以应力梯度为考核指标的评价依据。在研制出适于大厚度硬质合金与钢电子束焊接新方法的基础上，为异种材料电子束焊接引入梯度复合中间层连接提供理论依据，也为非稳态液/固界面行为的研究提供理论基础。

本项目针对大厚度WC-Co硬质合金和钢异种材料连接的要求，提出了利用梯度复合中间层的方式实现硬质合金和钢的有效连接。本项目以界面元素扩散和应力缓释机理为切入点，重点研究了脆性相的形成机理及界面元素的扩散机制。分析了梯度复合中间层对界面脆性相的抑制作用及接头应力缓释机理。通过计算Fe原子的扩散行为对比分析中间层对脆性相的抑制机理，最终利用电子束焊接技术实现了大厚度WC-Co硬质合金和钢异种材料的有效连接。在此基础上，研究了截齿结构WC-Co硬质合金和40Cr钢环焊缝的电子束焊接。利用电子束单一热源通过一次加热过程同时获得两种焊接接头形式，将硬质合金齿头与40Cr钢基座环壁通过电子束熔焊接头实现高强度有效连接，同时两者底面形成钎焊接头增强了齿头工作过程中的散热。本项目丰富了异种材料连接的基础理论，促进了硬质合金与钢连接技术的发展。