PRIMARY MELTING OF TITANIUM WITH PLASMA ARC Tatsuo Fujiwara, Koshi Kato, Kiyoo Ono and Hiroyuki Yamada Daido Steel Co., Ltd. Japan Introduction Plasma arc has a good characteristic as a heat source for melting metals and alloys. That is, its temperature is higher than that of ordinary electric arc and its power can be controll­ ed very easily. When an inert gas like argon is used as. a plasma forming gas, very clean heat, or non-reactive heat, is obtained. Furthermore, the contamination by plasma torch may be substan­ tially none. Accordingly, plasma torch is thought to be a very useful nonconsumable electrode. Recently, the use of plasma arc for melting metals and alloys has been well studied. In USSR and GDR two types of plasma arc furnaces have been developed and working on an in­ dustrial scale. One is a plasma arc furnace in which the carbon electrodes and its construction were, as a whole, similar to those of the conventional arc furnace (1,2). And the other is the plasma arc remelting (PAR) furnace (3,4). In the meantime, in Japan, our company has been doing the research on plasma arc melting of steels and superalloys for more than 10 years, and has already developed an industrial Plasma Induction Furnace (5,6), which is the combination of plasma arc and induction heating. We also have studied the use of plasma arc for melting titanium. This may be thought to be one of the most promising fields in which the characteristics of a plasma arc -high temperature, easily controllable and clean heat source would be made the best use of. At present, conventional double vacuum arc melting (VAR) process for titanium has such problems that, as generally known, it is necessary to make a primary consumable electrode from titanium sponge, .but it is a very troublesome process and recy­ cling of various shaped titanium scrap is difficult. Concerning titanium melting with plasma arc we made a presentation of the results on our own PPC-flasma frogressive Casting-furnace at the 6th International Vacuum Metallurgy Con­ ference held in San Diego, April 23-27, 1979. In this we de­ scribed the test results obtained from an examination of avail­ ability of the PPC process for melting of titanium, and it was concluded that it was more economical to utilize the PPC as the primary melting process, that is, the double melting consisting of the primary melting with PPC and the secondary melting with VAR was particularly effective for recycling of scrap materials. 2136 T. Fujiwara et al. After this, we fully made an examination of the i'PC primary melt­ ing conditions from the qualitative and economical point of view. Description of Plasma Progressive Casting Furnace Plasma arc furnace designed and constructed for this experi­ ment is shown in Fig.l. We call this furnace PPC-Elasma Erogres­ sive Casting-furnace. A transfer type plasma torch was set ver­ tically just above the mold. The section of the plasma torch is shown in Fig.2. The W-Th cathode is attached at the end of water cooled copper pipe and the cathode is encircled by the water cooled copper nozzle. Argon gas, which we used as a plasma form­ ing gas, is flowed between these, and a part of argon gas is ionized by the arc and pinched at the nozzle, so that the high energy density plasma arc is gained. The anode was the molten metal in the mold. The water cooled copper mold llOmm in dia. shown in Fig.l installed a stirring coil and was combined with a movable bottom plate, which was withdrawn progressively and proportionally to the melt rate of the melting materials from the hopper so that the pool surface might be kept at the settled position. Before starting the melting, the furnace chamber was evac­ uated to the reduced pressure of the order of 10-3 Torr and then argon gas of five-nine purity was supplied to latm. Then the plasma arc was ignited. The argon gas from the torch was flowed out through the leak valve of the furnace. So melting was done under argon atmosphere of latm. The power source of this furnace had a maximum capacity of o.c. 150KW and the ingot obtained was maximum 25Kg in weight. Photo.l shows the upper and lower view of the PPC furnace. It can be seen the plasma torch and its moving mechanism, the hopper, the viewing port and the melting chamber in the upper view, the mold, of which jacket for water cooling is only seen, and the ingot cooling chamber in the lower view. This PPC furnace was used for studying the plasma arc remelting of steels and superalloys, too (7). PPC Primary Melting Primary meltings were done under the various melting con­ ditions -plasma power, feeding rate and type of melting materials­ using titanium sponge or scraps and Ti-6Al-4V alloy scrap. The chemical compositions of these melting materials are shown in Table 1. One of the requirements to the PPC primary melted ingots is that they can be safely used as the electrodes for the VAR. This requirement is satisfied by proper selection of the melting conditions. We adopted the apparent densities of the primary melted ingots as the parameter of the soundness. PRIMARY MELTING BY PLASMA ARC 2137 Table 1 Chemical compositions of melting materials used for this experiments (ppm) Melting Material 0 N c Fe H Mg Cl Ti sponge(JIS Grade1) 500 50 100 200 20 800 400 Ti sponge(JIS Grade 2) 700 70 100 300 30 800 400 Ti cut wire scrap 900 80 120 500 13 - - Ti cut plate scrap 1070 80 120 300 15 Al(°lo) V(°lo) Ti-6Al-4V cut wire scrap 1500 150 200 1000 50 6.25 4.10 Fig.3 indicates the results of the examination, in which the dependency of the apparent densities of the PPC primary melted ingots on the plasma power and the feeding rate of the melting materials is shown. At the same power level, the inverse rela­ tion between the apparent densities and the feeding rates was seen. And also at the high plasma power the feeding rate was made higher to obtain the same density. Photo.2 shows the surface appearances of the PPC primary ingots melted from the various melting materials. The reasons why the primary melted ingots had the lower apparent densities than the true density of 4.5gr/cm3 were that the fed materials had not been completely melted, as seen in Photo.2, and that there had been blow holes in the interior of the ingots (Photo.3) But from Photo.3 their interior quality was proved to be fairly better than that expected from the ingot surfaces. Therefore, they were thought to be satisfactory as the electrodes for the subsequent VAR. However, for establishing the proper conditions for PPC primary melting it is necessary to determine the lower limit of the apparent density usable as the electrode .for VAR. So, we examined the density distributions on the horizontal sections of the primary ingots with various apparent densities. The results of the examination are shown in Fig.4. In this figure, A and B were melted at the high and low melt rate at 90KW, respectively, and c were at the high melt rate at llOKW. And the base planes of the figures correspond to the density of 4.0gr/cml. comparing these, it is clearly seen that the lower the melt rate and the higher the plasma power, the higher the density of the ingot became. Also there could be seen the tendency that when the melt rate was high, the density of the outer side of the ingot became lower, even though the plasma power had been high. From these examination, it was confirmed that the PPC pri­ mary melted ingots which had the apparent densities over 3.5 gr/crri1 had satisfactory qualities and could be safely used as 2138 T. Fujiwara et al. the electrodes for VAR. All of those shown in Fig.3 had the apparent densities over 3.Sgr/cm3 and so they can be used as the electrodes for VAR. In the meantime, the melting efficiency is also a matter of concern. Fig.S shows the melting efficiencies of PPC primary melting under the various melting conditions. It is seen that the melting efficiencies were dependent on the feeding rate at all plasma power level tested, and that the high efficiencies could be obtained under the higher power conditions, because the feeding rate could be increased more than that under the lower power conditions without any decrease in the apparent densities. Here the typical melting conditions and the apparent dens­ ities of the ingots are shown in Table 2, in which titanium sponge only, mix of titanium sponge and scrap and Ti-6Al-4V alloy scrap only were respectively used as the melting materials. And Table 3 shows the results of the chemical analysis of the PPC primary ingots obtained from titanium sponge only, as well as those of sponge. Table 2 Melting conditions at PPC primary melting and the apparent densities of the ingots obtained Heat PPC Condition Melting Ingot Appar. Mel ting Material Efficiency Density No. Power(KW) F~~i~pte (Kg/KWH) (gr/cm3) 125 Ti, sponge (JIS Grade 1) 110 1.16 0.54 3.8 713 Ti, sponge (JIS Grade 2) 110 0.96 0.43 3.9 128 Ti, cut wire scrap(5)+sponge(1) 110 1. 17 0.53 3.8 118 Ti, cut plate scrap(S)+sponge(l) 110 1.