J. Cent. South Univ. (2020) 27: 1-9

DOI: https://doi.org/10.1007/s11771-020-4272-7

Electrode heating effects on preparation of Al-65V alloy

WAN He-li(万贺利)^{1, 2, 3, 4}, XU Bao-qiang(徐宝强)^{1, 2, 3}, LI Lan-jie(李兰杰)⁴,

YANG Bin(杨斌)^{1, 2, 3}, WANG Li(王丽)^{1, 3, 4}, DAI Yong-nian(戴永年)^{1, 2, 3}

1. National Engineering Laboratory for Vacuum Metallurgy, Kunming University of Science and Technology, Kunming 650093, China;

2. Research Center of Engineering on Aluminum Industry of Yunnan Province, Kunming 650093, China;

3. Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China;

4. Chengde Iron and Steel Company, Chengde 067002, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: In this study, Al-65V alloy has been produced by electrode heating for 3-15 min. The impurities content, microstructure and the vanadium content of the alloy in the slag are investigated. The results indicate that different heating time shows obviously impacts not only on the impurities and microstructure of AlV alloy, but also on the vanadium recovery rate of slag. The impurities contents of C, O and N are 0.085%, 0.022% and 0.036%, respectively, which meet the commercial standard when the electric heating time is controlled to 10 min, and the content of vanadium in slag is 0.18%.

Key words: Al-65V alloy; electrode heating; impurities content; microstructure

Cite this article as: WAN He-li, XU Bao-qiang, LI Lan-jie, YANG Bin, WANG Li, DAI Yong-nian. Electrode heating effects on preparation of Al-65V alloy [J]. Journal of Central South University, 2020, 27(1): 1-9. DOI: https://doi.org/10.1007/s11771-020-4272-7.

1 Introduction

Since the titanium-based alloys with lower content of aluminum are brittle and provide moderate resistance to oxidation, alloying with vanadium enhances their ductility, strength, oxidation, and corrosion resistance [1-4]. In addition, the comprehensive mechanical properties of the titanium-based alloys are significantly improved when vanadium (V) and aluminum (Al) are added to titanium alloy; therefore, V and Al are the key elements in the titanium alloy. For example, Ti-6Al-4V and Ti-8Al-Mo-V are the two most important alloys of titanium alloy [5, 6]. The quality of intermediate alloys (AlV) is significant to titanium alloys.

Currently, V₂O₅ powder and Al powder are used as raw materials in production Al-V alloys [7, 8], and the conventional Al-V alloy production methods include the two steps process [9, 10], aluminothermic reduction [11, 12], and self propagating [13-16]. By two-step process, it has very high purity and low impurity content. However, it is typically complicated and high production cost [17].

For aluminothermic reduction and self propagating methods, the V content is low (50 wt%-60 wt %). The content of V in Al-V alloy is difficult to be increased in aluminothermic reduction techniques. Because the segregation in product is formed, which is not conducive to the uniformity of product components [18].

It is worth mentioning that, although these thermal reduction methods have been applied for commercial production, many ignition agents, such as magnesium or potassium chlorate, are consumed during the reduction process. Meanwhile, some impurities are diffused into the AlV alloy, and its fragility is increased [19]. As is widely known, the application field and the price of the alloy depend on the content of V and the impurities in the alloy. Therefore, making a high quality and low cost alloy has become a glaring technological challenge in the titanium alloy manufacturing industry [20]. There exists no single process that can produce Al-65V alloy with all the required characteristics: uniform components, the desired V content (60 wt%-70 wt% in alloy), low impurities (O≤0.1 wt%, Fe≤0.18 wt%, Si≤0.18 wt%), and low cost. The objective of this research is to develop such a process to achieve all the above characteristics.

In this paper, a new process is designed and demonstrated. It combines metal thermal reduction and electrode heating to form the Al-65V alloy. The raw material was composed of vanadium oxide (V₂O₅) granules and Al granules, and the calcium oxide (CaO) was used as additive. The effects of different electrode heating time on the alloy composition and microstructures were investigated. Furthermore, the effects of electrode heating on the content of impurities and the recovery of vanadium from slag are discussed. This research can provide a theoretical basis and technical support for determining reasonable electrode heating conditions for the production of the alloy, thereby improving the quality of the alloy and increasing the recovery of vanadium.

2 Experimental

The raw materials used in this study are 98.5% pure V₂O₅ from Cheng Steel Group Co., Ltd. and 99.5% commercial grade Al particles of 5-10 mm, respectively. Additionally, 99.9% pure CaO powder was used as the additive in the experiment. The mass ratio of Al/V₂O₅ = 0.82:1 was adopted, and each set of materials was mixed in a certain proportion in a sealed container. The parameters of the graphite electrode were as follows: length L=1600 mm, diameter D=250 mm, resistivity was 6.5-7.0 mΩ·m, the density of the electrode was 1.6-1.7 g/cm³, and carbon mass fraction was greater than 99.5%. The materials were mechanically mixed for 20-30 min and transferred into the furnace. Before adding materials, the preheated furnace was heated to 473-623 K to eliminate water and to avoid heat loss in the later reaction. The early electrode voltage control was controlled in 140-160 V, which had a sufficient energy to ignite the raw materials and to heat the products in the later stage. After the reaction, the slag and alloy were separated when the alloy was cooled at room temperature. In order to meet the commercial applications, the AlV alloy was further classified by crushing and screening, and the entire process is illustrated in Figure 1.

Figure 1 Schematic diagram of preparation of Al-65V alloy by electrode heating

The phase of products was identified by X-ray diffraction (XRD) using a Rigaku(Tokyo, Japan) D/max-3B X-ray diffractometer equipped with graphite-monochromatized Cu K_α radiation. Scanning electron microscope (SEM) of S3400 N (Hitachi, Tokyo, Japan) was used to observe the microstructure of the alloy products and its energy dispersive spectrometer (EDS) (Hitachi, Tokyo, Japan) was used to detect the element distribution of Al and V. The elements distribution in alloy samples was analyzed by EPMA microanalyzer (EPMA: JXA8230).

3 Results and discussion

Figure 2(a) shows the sample when the electrode heating time in the preparation process is controlled in 8 min. The alloy and slag are depicted in Figures 2(b) and (c), respectively. The alloy and the slag are completely separated, as shown in Figure 2(c). The microstructure cross-section of the alloy is displayed in Figure 2(d), from which it can be seen that the section composition of alloy sample is uniform and without obvious segregation. Furthermore, Figure 2(d) shows a magnified image of the composition of the Al-65V alloy sample obtained by the electrode-assisted heating process. Notably, two zones are discernible, as shown by the dotted lines, and exists in the gray and black regions. Figures 2(e)-(f) present the EDS results of the alloy generated by the electrode-assisted heating process. According to the Spot 1, the content of Al is 44.82 wt%, but the content of V is 55.18 wt%. According to the Spot 2, the content of Al is 32.99 wt%, while the content of V is relatively lager, and is close to 67.01 wt%. Furthermore, WDS mapping (Figure 3) shows the compositional distribution of the alloy sample, which confirms that the gray is V-rich phase and the black is Al-rich phase. According to the mass of V and Al atoms and the EDS analysis results, the composition of the gray area is approximated to n(Al):n(V)=1:3 (AlV₃), and the composition of the black area is approximated to n(Al):n(V)=8:5(Al₈V₅). The two compositions are evenly distributed, and the gray region of AlV₃ is obviously greater than the black region of Al₈V₅. It indicates that the Al-65V alloy is a non- stoichiometric compound with a V content ranging from 65 wt% to 68 wt%.

Figure 2 SEM micrograph of product:

Figure 3 WDS mapping of Al-65V alloy

3.1 Impurities content in Al-65V alloy

The main impurities in the alloy product are Si, Fe, C, N and O. For this reason, ICP-AES is used to determine the Si, Fe contents, and the C, N, O contents are determined using an LECO TCH 600 and LECO C/S 230. The analysis of impurities for the experimental products under different heating time are provided in Figures 4(a) and (b). As was determined by the analysis of the sample composition, and as can be seen in Figure 4(a), the content of C in the product, notably increased by more than 0.103% when the electrode heating time was controlled in 14 min, which does not meet the commercial standard (TS/T 579-2014). An increase of C in the alloy was observed because C entered the alloy through the slag layer. Therefore, the electrode heating time should be controlled in 8-10 min in the experiments.

The contents of O and N in the Al-65V alloy directly depend on the heating time of the electrode heating process, as shown in Figure 4(a). In this research, the electrode heating time is controlled from 0 to 18 min. As the heating time is prolonged, the contents of O and N in the alloy exhibit a gradually trend. Because lots of the heat are released in the early stage of the reaction, the residual heat can not maintain a uniform alloy composition. It causes segregation and cracks, and therefore the impurities of O and N are seeped into the alloy. Table 1 lists the chemical compositions of the alloy samples generated by the electrode heating process, which show that the alloy product meets the requirements. Impressively, the O and N contents of the alloy are even lower than the requirements of the commercial standard. In fact,Figure 4(a) shows that the O and N contents in the final alloy product can be tailored in the ranges of 0.036 wt%-0.156 wt% and 0.014 wt%-0.054 wt%, respectively, if so desired. Understandably, the O and N contents in the alloy changed drastically throughout the electric heating process, as the process was designed for that purpose. The issue of controlling the content of impurities in the product is further discussed below.

Figure 4 Chemical compositions of Al-65V alloy by electrode heating process

Table 1 Chemical compositions of Al-65V alloy after electrode heating process

However, in the electrode heating process, Al and V are fully diffused into the alloy when the heating is effectively increased. It should be noted that, when the electrode heating time is increased, the contents of the impurities of Si and Fe in the alloy are not significantly reduced, as demonstrated in Figure 4(b). Thus, these impurities are reduced by controlling the material composition selection. Additionally, the content of V in the Al-65V alloy is shown in Figure 4(c), and meets the titanium alloy requirements of TS/T 579-2014 for chemical composition [21, 22].

3.2 Recovery rate of V in slag

The content of V in the slag layer is reduced with the increasing in electrode heating time, as presented in Figure 5(a). It can be found that the V content in the slag changed drastically when the electrode heating time is controlled in 2-14 min. Therefore, the content of V in the slag is less than 0.08 wt% when the electrode heating time exceeds 14 min. Figures 5(b) and (c) show the slag phase XRD patterns of different slag samples obtained by the electrode heating process and the self- propagating reaction process, respectively. The analyses also reveal that the content of vanadium oxide (VO) in the slag phase is significantly decreased during the electrode heating process. As previously mentioned, VO is reduced during the heating process [23-25]. This fact indicates that the slag layer and the alloy cooling time are extended during the electrode heating process, and the VO is reduced in the slag phase, as exhibited in Figure 6(a).

Figure 5 XRD patterns of slag samples by different processes:

Additionally, Figure 6(b) shows that V is concentrated in the bottom of the alloy through the liquid alumina slag layer. Therefore, the content of V in the slag can be reduced and the recovery of V is increased.

3.3 Microstructures

Figure 7 shows the XRD pattern of the Al-65V alloy produced by the electrode heating process.

Figure 6 Schematic diagram of slag phase composition change:

Figure 7 XRD pattern of Al-65V alloy product with electrode heating time for 8 min

The alloy sample only exhibits the characteristic peaks of AlV₃ and Al₈V₅, which proves their high purity. Additionally, the spectra of AlV₃ have a higher peaks than those of Al₈V₅, which implies that AlV₃ is more crystalline or has a larger particle size. It is possible to form the other compounds of Al and V. However, this compound distribution is substantially uniform, the concentration is less than 5%(wt), and it cannot be detected by X-ray diffraction analyses [26].

The SEM images of the alloy produced by different electrode heating time are shown in Figure 8. The SEM images verify the Al₈V₅ and AlV₃ component distribution and crystalline structure of the prepared alloy. In addition, the Al₈V₅ particles are relatively larger than those of AlV₃, which is in agreement with the XRD pattern. Figures 8(a)-(d) show that the dendritic Al₈V₅ particles are gradually decreased and become snowflake-like Al₈V₅ particles when the heating time is extended from 0 to 8 min. However, the composition of the product changes significantly when the heating time is further extended to 12-14 min; the particles size of Al₈V₅ is gradually reduced and uniformly distributed in AlV₃ as shown in Figures 8(e)-(f). Furthermore, the V content in the alloy is increased, which is in agreement with the results previously shown Figure 3, although their assisted heating to fabricate the alloy has a different heating route [27, 28]. These results are mainly due to two reasons. Firstly, with the extension of heating time, the Al is burned and lost in the alloy. Second, because the heating is conducted via electrode assisted heating, the V and Al in some liquid phases are directly converted into AlV₃ compounds in the high temperature zone.

Figure 8 Typical cross-section microstructures with various electrode heating time in experiments:

4 Conclusions

In the present study, the electrode heating is adopted. It assists in reduction, and reduces the segregation and impurities of Al-65V alloy. The main findings include:

The purpose of this study is to discover a better way to produce of the alloy. The product has fine microstructures and low impurity content due to the electrode assisted heating process. The content of V in the slag is diminished, and it is used as the furnace lining material in this method.

The alloy is a non-stoichiometric compound, the main components are Al₈V₅ and AlV₃. With the extension of heating time, the particle size of Al₈V₅ is gradually decreased and evenly distributed in the phase AlV₃ phase.

Al and V are uniformly composed, and a low impurity content is obtained, for the Al-65V alloy. The micro structure and the content of impurities in the alloy illustrate its dependence on electrode heating time. The composition of the alloy product meets the commercial standard, and the content of V in the slag is 0.18% when the heating time is controlled at 10 min.

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(Edited by HE Yun-bin)

中文导读

电极加热对制备Al-65V合金的影响

摘要：本文研究了电极加热制备Al-65V合金过程中得到合金产物中的杂质含量、微观结构和渣中钒含量的变化情况，电极加热时间控制在3~15 min。研究结果表明，不同的电极加热时间不仅对合金产物中的杂质含量和合金的微观结构有明显影响，而且对炉渣中钒的回收率也有显著影响。实验过程中，将电极加热时间控制在10 min时，得到的钒铝合金中的杂质碳、氧、氮含量分别为0.085%、0.022%和0.036%，合金成分满足商业化应用标准，同时，得到的炉渣中钒的含量为0.18%。

关键词：Al-65V合金；电极加热；杂质含量；微观结构

Foundation item: Project(51734006) supported by the National Natural Science Foundation of China; Project(2014HA003) supported by the Cultivating Plan Program for the Leader in Science and Technology of Yunnan Province of China; Project(2017HB009) supported by the Science and Technological Talent Cultivation Plan of Yunnan Province, China; Project(2017HA006) supported by the Program for the Academician Free Exploration Fund of Yunnan Province, China

Received date: 2018-11-16; Accepted date: 2019-07-04

Corresponding author: XU Bao-qiang, PhD, Professor; Tel: +86-13608864121; E-mail: kmxbq@126.com; ORCID: 0000-0001-6886- 9560