Influence of process parameters on quality of copper in electron-beam melting

Results are presented and discussed obtained by experimental and theoretical investigation concerning the influence of process parameters (melting power and refining time) on the quality of copper metal ingots in electron-beam melting. The thermodynamic and kinetic conditions are analyzed. Possible refining process mechanisms under different technological conditions are investigated. Appropriate processing regimes are proposed for efficient refining of copper and for obtaining high-quality metal after e-beam melting.


Introduction
Analyses of the world's raw material resource for copper production show that copper ore reserves are decreasing, while the global copper consumption continues to grow every year [1]. Copper is the third most important metal after iron and aluminum used in a variety of applications (energy and electrical systems and networks, architecture, electronics and telecommunications, transportation, industrial machinery and equipment, chemical industry, agriculture, etc.) that are necessary for a reasonable standard of living. The requirements for the chemical composition, structure and quality of the final product are also increasing. The development of effective methods for the production of metals and alloys with a low content of metal, non-metal and gas impurities and the preservation of the chemical composition obtained during further processing are important problems that modern metallurgy is successfully solving.
As part of the vacuum metallurgy, electron-beam melting (EBM) for refining is a key method for producing new micro-and nanoelectronics materials that require high purity and quality. This method combines well the advantages of the electron beam (EB) as an unconventional source of heat without the limitations of the achieved temperature and the high vacuum as an environment for refining processes [2][3][4].
In the EBM process, favorable conditions are created for carrying out fully reactions and processes that involve a gas phase, since it is possible to shift their thermodynamic equilibrium in the desired direction. In EBM, degassing, deoxidation, reduction, and evaporation of volatile impurities occur that are unrealizable at atmospheric pressure. Refining takes place at the boundary surface between superheated liquid metal and vacuum (reaction surface), where various processes run simultaneously, such as mass transfer (from volume to boundary surface and vice versa), chemical interactions 1 To whom any correspondence should be addressed. between components (elements and chemical compounds) present in the surface region, and evaporation from the boundary surface of impurities in atomic form or bonded in compounds [3,5].
To provide an efficient realization of a concrete refining process in EBM processing, it is important to know the concentration of the investigated components at thermodynamic equilibrium under concrete conditions (temperature and pressure) giving in principle a possibility of refining. Also, to achieve a thermodynamic equilibrium, the rates and limits of the processes and the parameters influencing them should be known.
In this paper, we analyze the thermodynamic conditions of and the kinetic limitations on the flow of refining processes and the possible refining mechanisms in electron beam refining of copper. The influence of the beam power (temperature) and the duration of the melting process on the quality of the refined metal is studied as well.

Results and discussion
The experiments on EBM of copper disks with a height of 50 mm and diameter of 60 mm were conducted using the 60-kW ELIT 60 plant at the Physical Problems of the EB Technologies Laboratory of the Institute of Electronics, Bulgarian Academy of Sciences. The EBM furnace consists of a melting chamber, one electron gun with an accelerating voltage of 25 kV, a vacuum system, and an extraction system [6]. The operation vacuum pressure is 3  6×10 -3 Pa.
The investigated material contained 99.83% Cu and impurities such as As, Sb, Pb, Sn, Ni, Bi, Zn, Ag and O 2 . The initial material was processed at melting powers of 7.5 kW (T = 1400 K), 10 kW (T = 1500 K) and 15 kW (T = 1700 K) for different refining times -3 min, 10 min, 15 min, 20 min and 30 min. Using emission spectral analysis, the chemical composition of the samples before and after EBM was determined.
During EBM of Cu, the liquid metal is a complex system of Cu, Cu 2 O, metal impurities, and their oxides, in a liquid and solid state depending on the operating temperature. The refining processes take place simultaneously and should be considered in their interdependence. By analyzing the thermodynamic and kinetic refining conditions under each of the technological regimes studied, the qualitative composition of the metals and oxides reaching the reaction surface can be predicted.
Equations (1-3) present possible chemical interactions between the base metal (Cu) and metal components (Me) with the oxygen: where ΔF T , ΔF T Cu/Cu2O and ΔF T Me/MeO are the free energies of the respective process. According to the thermodynamics laws [7], these reactions can take when the value of the respective free energy is negative. In EBM, dissociation of oxides stable at atmospheric pressure is possible and proceeds completely due to the continuous separation of the gas phase from the reaction surface (the respective free energy should be negative).
Depending on the thermodynamic melting conditions and the type of individual impurities, the refining of the metal at the boundary surface can proceed by (a) degassing  evaporation of impurities having a vapor pressure (p i ) higher than the vapor pressure of copper (the base metal), i.e. p i > p Cu ; (b) distillation  evaporation of volatile compounds of metallic impurities when p MeO > p Me , which also applies to the re-melted metal oxide. Effective refining requires the implementation of the following inequalities: (p MeО ) > (p Me ) > (p Cu2O ) >(p Cu ). Table 1 shows data on the concentration of metallic impurities during e-beam refining of Cu, with the oxygen content varying from 1500 ppm (in the initial material) to 10 ppm (after EBM). It can be seen that the removal of the impurities Sb, Pb, Sn depends on the temperature of the liquid metal. Increasing the temperature (T) enhances the removal of these impurities (table 1). At T = 1700 K and melting time τ = 30 min, the refining efficiency (η i , %) for each of them reaches 90%. Under the  The results show that Zn release is possible during the first 10 min at T = 1700 K. With continuous refining at T = 1700 K, the content of Ag in the base metal increases. For all impurities tested, the removal efficiency was the highest during the first 3-10 min, after which the refining time τ did not significantly affect their removal. The highest purity of Cu (99.991%) and an oxygen content of 20 ppm was obtained at T = 1700 K for τ = 10 min; the degree of refining was 94.7%. The evaluated values of the overall removal efficiency (removal efficiency of all the impurities in the sample) are presented in table 2. It is seen that the increase of the temperature (beam power) and also the increase of τ up to 10 min lead to an increase of the overall removal efficiency of the controlled impurities. Table 3 presents data for the material losses which are mainly due to evaporation and also to splashes. The results show that prolonged melting does not increase the refining efficiency due to the higher weight losses of the base metal (tables 2, 3). increases with increasing the temperature, which explains the greater weight loss at T = 1700 K. Under certain thermodynamic conditions, an interaction is possible between a metal impurity and the molecule of Cu 2 O (equation (1)), with the impurity reducing that molecule to Cu and the free energy ΔF T = ΔF T Me/MeO  ΔF T Cu/Cu2O . The values of ΔF T were calculated; the results show that ΔF T > 0 for the impurities Pb, Bi, and Sb, i.e. withdrawal of oxygen from the Cu 2 O molecule is not possible. For Zn, Ni, As, and Sn, ΔF T < 0, i.e. these impurities exhibit reducing properties and extract O 2 from the liquid metal. Subsequent distillation of the metal oxides reduces the content of these impurities and of O 2 in the base metal. At 1700 K, the distillation from the reaction surface mainly involves SnO 2 and Sb 2 O 3 and to a lesser extent NiO (table 4). In EBM of Cu with a high content of Sn, Sb, Ni, the removal of oxygen and of these metal impurities is more efficient due to their good reducing properties.

Conclusions
The thermodynamic and kinetic conditions for EBM of copper were investigated in view of process optimization. Possible mechanisms and reactions for refining processes under different technological conditions were analyzed. The results show that the refining processes are most intense during the first 3  10 minutes. Separation of the impurities Sb, Pb, Sn depends on the processing temperature, while it affects only slightly the removal of Ni, Bi, As, Ag. It was also found that by distilling the oxides of the possible reducers Ni, Sn and Sb, mainly Sb and Sn are released from the reaction surface, resulting in a reduction in the oxygen content of the refined metal. The contribution of the Ni reducer to the removal of oxygen from Cu is smaller. Separation of As is possible by degassing. Prolonged melting processing does not reduce significantly the content of the impurities investigated due to the higher weight losses of the refined metal; even a relative increase in the concentration of some of the impurities is possible. The highest purity of Cu (99.991%) and an oxygen content of 20 ppm was obtained at 1700 K for a melting time of 10 min with a removal efficiency of 94.7%.