Electron beam melting efficiency at multiple hafnium e-beam processing

The method of electron beam melting and vacuum refining has clear advantages over other metallurgical methods since it enables manufacturing of refractory and chemically active metals. This study focuses on the efficiency of removing impurities from technogenic hafnium under multiple electron beam melting. Assessments are performed on the efficiency of double and triple e-beam melting processing of refractory metal hafnium. The influence of different e-beam melting technological modes on the refining effectiveness is investigated. A highest hafnium purity of 99.2% was achieved after double and triple e-beam refinements of the investigated materials, with the highest process efficiency reaching 61.58% and 51.07%, respectively.


Introduction
Hafnium (Hf) is a strategic metal due to its exceptional thermophysical and mechanical properties, including a high melting temperature, impressive strength, excellent ductility, and remarkable resistance to corrosion in aqueous and gaseous environments.These attributes make hafnium wellsuited for various applications in the high-temperature zones of nuclear reactors, due to its unique ability to absorb neutrons.Hafnium and its isotopes have a high value of the cross section of absorption of thermal neutrons which ensures the preservation of the elements' efficiency during their operation in the reactors.It serves as a structural material in the production of components responsible for critical functions within certainty and monitoring systems.Hf-based oxide material, specifically undoped and Ti-doped HfO 2 , demonstrates luminescent properties with distinct characteristics and the fast decay observed in undoped HfO 2 can be attributed to intrinsic matrix defects, such as oxygen vacancies, and conversely, the slow decay in Ti-doped HfO 2 is attributed to luminescence associated with Ti impurities [1].
The presence of impurities profoundly influences the characteristics and structural integrity of hafnium.Obtaining pure hafnium poses an additional challenge due to its inseparable association with zirconium, a companion element found in the natural raw material.Various techniques have been • τ = 30 min employed to address these challenges, including solvent extraction, carbochlorination, Kroll reduction, vacuum distillation, and electron beam melting [2][3][4][5].
Electron beam melting and refining (EBMR) method is an environmentally clean and waste-free method, providing a high degree of purification of metal materials by removing metallic and nonmetallic impurities [6].Studies on the processing of wastes of refractory metals such as molybdenum, tantalum, CoMoCr alloys, and other metal materials have shown that EBMR is a suitable recycling method [6][7][8].By combining high operating temperature and a vacuum environment, the EBMR provides good conditions for refinement, mobility in the selection and regulation of technological parameters, and the ability to obtain ingots of various sizes with uniform chemical composition, homogeneous microstructure, and improved mechanical properties.Lowering the pressure allows reactions that are not impossible at atmospheric pressure such as reduction, degassing, distillation of volatile components, etc.
This study is a continuation of our investigations on technogenic hafnium purification [5] and the work focuses on the effectiveness of hafnium processing using multiple (double and triple refining) ebeam melting processes (subsequent refining processing) for the purification of refractory metal hafnium.Various e-beam melting modes were examined and discussed in terms of their impact on the refining efficiency at EBMR of two types of starting Hf materials.

Experimental
The experiments related to the e-beam melting of technogenic hafnium were carried out utilizing the EBM furnace model ELIT-60 (Leybold GmbH, Germany) with a power capacity of 60 kW in IE-BAS.The ELIT-60 furnace is equipped with a melting chamber and a single electron gun with an accelerating voltage of 24 kV.The electron beam current range is 550 -750 mA and 20 mm is the ebeam diameter.The operation takes place under a controlled vacuum environment, maintaining a pressure of 1 × 10 -3 Pa.
Experiments were conducted using two types of starting (initial) hafnium materials (Hf st1 and Hf st2 ), and their composition is provided in table 1.The initial hafnium materials significantly differ in their content of zirconium, nickel, and iron.The content of the main metallic impurities in the starting hafnium materials and in the metal after EBMR processing was determined by emission spectral analysis (UBI 1, Carl Zeiss, Jena, Germany).Experimental tests were performed under different technological modes (TM1 and TM2) for double (figure 1 (а)) and triple refining (figure 1 (b)).

Results and discussion
Table 2 provides the melting temperatures and densities of hafnium and its accompanying impurities.It is evident that within the operational temperature range of 2600 -3000 K, all monitored metallic impurities (Zr, Ni, Cr, Fe, and Zn) will be in a liquid state.In EBMR, the refining processes take place mainly on the reaction surface of the liquid metal, with impurities moving from the volume of the liquid bath to its surface, depending on their relative weights.From the values presented in table 2, one can see that the densities of the metal impurities such as Zn, Ni, Fe, Zr, and Cr are much lower than that of Hf and will therefore float to the liquid metal/vacuum interface where they can be removed.In EBMR, an important role in the refining processes is played by the vapor pressure of the impurities, and depending on their type and thermodynamic parameters (operating temperature and vacuum pressure), the refining processes can proceed through degassing or distillation [6].To achieve high efficiency of the refining processes, the partial pressure of the metal impurities must be higher than that of the base metal (Hf).   2 presents the vapor pressure values of the metal impurities for the studied working parameters.The results show that in the operating temperature range (2600 -3000 K), metal impurities such as Zn, Ni, Fe, and Cr can be removed by evaporation -their vapor pressure is significantly higher than that of hafnium.The vapor pressure of Zr is slightly higher than that of hafnium, indicating that the process is thermodynamically possible, but will also be accompanied by losses of the base metal.
The impurity contents in the investigated materials after double and triple e-beam remelting are shown in table 3. The efficiency of the double and triple e-beam remelting regimes (TM1 and TM2) for hafnium processing was calculated, and the results are presented in figure 3.  It can be seen that increasing the number of refining processes enhances the overall process efficiency.In the case of a single remelting at T = 2600 K and τ = 10 min, the removal efficiency is approximately 33% for zirconium, ~85% for nickel, and ~92% for iron, while for the other metallic impurities such as chromium and zinc, it reaches 100%.Under all other conditions, nickel, iron, chromium, and zinc are completely removed.The maximum removal efficiency for zirconium (60.78%) was achieved through double remelting of Hf st1 at a temperature of T = 2800 K for 30 min.The overall process efficiency is 61.58%, resulting in a hafnium purity of 99.2%.The same hafnium purity was obtained after triple remelting of Hf st2 at a higher temperature (3000 K) and shorter time (10 min).In this case, the overall process efficiency is 51.07%(figure 3).The lower overall process efficiency during the triple remelting of Hf st2 (with lower Zr content) can be explained by the more challenging removal of zirconium (figure 2) and higher weight losses of the base metal.
Figure 4 shows the microstructures of initial technogenic hafnium Hf st1 and of a specimen after double melting processing at the highest process effectiveness (61.58%).A metallographic inverted microscope IM-3MET (Optika) with a digital microscope camera Axiocam ERC5S-5MP was used for microstructures' characterization.On the micrograph of the initial hafnium (figure 4 (a), large polyhedral grains are observed, while under TM1-II mode with the highest efficiency of impurities' removal, the obtained hafnium structure is visibly changed (figure 4 (b)).This is due to both the high removal rate of metallic impurities such as nickel, chromium, iron, and zinc and the partial removal of zirconium and the resulting structure is fine-grained and homogeneous.Along the boundaries of the grains (close to hexagonal shape) traces of solid solution remain between zirconium and hafnium.

Conclusions
In this work, the influence of EBMR on the efficiency of technogenic hafnium processing under different e-beam melting treatment (number of remelting cycles, melting temperature, and refining time) has been studied.It has been found that multiple refining (increasing the number of refining processes) enhances the overall process efficiency.Metal impurities such as nickel, iron, chromium, and zinc were completely removed after double remelting.The highest achieved zirconium removal rate (60.78%) was obtained following double remelting at temperatures of 2600 K (first melting) and 2800 K (second refining) for 30 min.The highest achieved purity is Hf 99.2%, which results from double and triple e-beam remelting processes with the highest process efficiency being 61.58% and 51.07%, respectively.The results indicate that after multiple e-beam refining, the hafnium samples are with improved structures.The obtained results provide insight into the possibility of using EBMR and multiple refining for efficient hafnium purification and show that this melting method yields high metal purity and better structure.

Figure 1 .
Figure 1.Flow chart of the EBMR regimes at multiple refining of the studied hafnium materials: (a) double e-beam processing of Hf st1 and (b) triple e-beam processing of Hf st2 .

Figure 2 .
Figure 2. Vapor pressure of hafnium and metal impurities.

Figure
Figure 2 presents the vapor pressure values of the metal impurities for the studied working parameters.The results show that in the operating temperature range (2600 -3000 K), metal impurities

Figure 3 .
Figure 3.Effect of the operating parameters and number of remelting cycles (single, double, and triple EBM processing) on the refining efficiency at hafnium e-beam treatment.

Table 1 .
Concentration of metallic impurities in the investigated starting hafnium materials (in mass %).

Table 2 .
Density and melting temperature of Hf and metallic impurities.

Table 3 .
Impurities content in hafnium materials after multiple e-beam refining.