Microstructure and crystal orientation of pure tungsten formed by selective electron beam melting
As an important refractory metal material, tungsten has the characteristics of high melting point, high thermal shock resistance, high thermal conductivity, low expansion coefficient, low vapor pressure and good radiation resistance. It has extremely important applications in aerospace, nuclear industry, chemical industry and other extreme environment fields. However, due to its high melting point and ductile brittle transition temperature, it is difficult to form tungsten. Usually, tungsten parts are prepared by powder metallurgy combined with hot working. However, conventional sintered tungsten has many disadvantages, such as low density, low strength, poor plasticity, and difficult to control the impurity content, so its application range is greatly limited. However, after hot processing such as extrusion and rolling, the tungsten is prone to recrystallization embrittlement and other problems. At the same time, in practical application, the structure of parts is often more complex, which usually has the characteristics of curved surface, curved pipe, hole, groove and so on. In order to overcome the shortcomings of traditional forming methods, it is necessary to adopt some new forming technologies. In recent years, the rapid development of metal additive manufacturing technology (or 3D printing technology) is to use high-energy heat source to melt metal powder and realize the direct manufacturing of three-dimensional solid parts by the method of point by point, line by line and layer by layer. It can be an effective way to prepare high melting point tungsten materials. At present, the technology has been applied to the manufacturing of complex parts such as superalloys and titanium alloys [5-7], However, the research on additive manufacturing of tungsten and tungsten based alloys was carried out relatively late, and the laser selective melting forming method in additive manufacturing technology was mostly adopted. However, due to the relatively low energy utilization rate of laser, it is difficult to realize the full densification of high melting point pure tungsten parts. Therefore, most of the researches are focused on the tungsten based alloys with high specific gravity containing Ni, Fe, Cu and so on There are few reports on the production of pure tungsten materials by liquid phase sintering of melting point elements. The reported results show that 15, spheroidization and cracking caused by thermal stress are obvious. Selective melting of electron beam is a kind of additive manufacturing process with high energy electron beam as energy source. The energy of electron beam can reach 3KW. Because there is no reflection problem, the energy utilization rate is high. At the same time, the electron beam deflection is magnetically controlled deflection, and the maximum scanning speed of electron beam can reach 8000 M / s, the powder bed can be quickly preheated, and the preheating temperature can be more than 1000 ° C, which can effectively reduce the thermal stress caused by temperature gradient and avoid cracks. In this study, pure tungsten samples were prepared by electron beam selective melting (EBM) technology, and the microstructure characteristics of the materials formed under the special thermal field conditions of multi-layer repeated thermal cycles were preliminarily explored, so as to play a guiding role in the performance control of pure tungsten formed by selective electron beam melting.
