Ti-2007 Science and Technology, edited by M. Ninomi, S. Akiyama, M. Ikeda, M. Hagiwara, K. Maruyama The Japan Institute of Metals (2007) Skull Melting, an Alternative Melting Process for the Production of Ti 6Al-4V Aerospace Plates

Dietmar Fischer1, Fabio Guglielmi2

1ThyssenKrupp Titanium GmbH, Westendstrasse 15; 45143 Essen, German 2ThyssenKrupp Titanium S.p.A., Viale B. Brin, 218; 05100 Terni, Italy

The manuscript describes skull melting as an alternative melting process for the production of Ti 6Al-4V aerospace plates.

Keywords: Vacuum-Arc-Skull melting (VASM), Titanium (Ti) 6 Aluminium (Al) — 4 Vanadium (V), Refining, Single melting, Aerospace application

In addition to the traditional vacuum melting processes the skull melting process offers a broader range of possibilities. Shortages of raw materials make use of recycling to a key issue. In skull melting different raw materials are melted in a crucible and poured into a mould. The charge can be bulk weldables, sponge, master alloys, turnings, briquettes and others, suitable to the configuration of the crucible for melting. A high degree of metal homogeneity is achieved as the material is mixed well in the liquid state. High interstitial defects and/or high/low-density inclusions are dissolved or remain in the skull. The described procedure uses the residual skull from the previous heat as the consumable electrode for the next one. The chemical composition of numerous single skull molten ingots in Ti 6Al-4V grade was analysed. The statistical evaluation showed a high degree of homogeneity throughout the cast. The structure, the mechanical properties and the internal integrity of produced plates of Ti 6Al-4V were evaluated. At the end it was shown that hot rolled plates from the skull melting furnace fulfil the Aerospace AMS 6945 requirements regarding single molten slabs to a high extent.

1.Introduction These processes take place in a vacuum-tight chamber and the electric arc is ignited between the electrode and the A variety of methods are used for melting titanium ingots water-cooled crucible. and slabs. The selection of an appropriate technique is The liquid metal solidifies forming a skull on the copper dependent on factors such as the metal grade being melted, surface which remains in the crucible after the pouring of the the amount of molten material required for production and metal in the mould. the area and environment available to accommodate the In case of the described process, the skull is taken as the melting equipment and auxiliaries. new consumable electrode for the next melt. For titanium and titanium alloys the conventional methods As well as refining the melt from refractories and impurities, used on a large scale are classic vacuum arc remelting of homogenisation of the chemical composition is obtained by only round ingots with consumable electrodes (VAR), a long soaking time of the liquid metal and the heavier plasma remelting (PM), electron beam melting (EB) and particles remaining in the skull. vacuum arc skull melting (VASM). Mainly PM, EB and Renewed melting of the skull during the campaign will VASM are used to remelt scrap up to 100 % in shapes of dissolve any impurities to a wide extent with the result of round ingots, blooms or slabs. By using cold hearth receiving impurity-free melts. The scrap input needs less technology during PM or EB remelting, refining of the melts processing compared with other melting technologies. by removing refractory metals, oxides, nitrides and/or The VASM process using the skull as consumable electrode carbonitrides is achieved. has been developed by experts of All-Russia Institute of Electron beam melting and plasma melting and subsequent Light Metals, Moscow, Russia and is described in a number casting are carried out stepwise producing an ingot or slab. of papers1-3) also with the experimental and theoretical To achieve sufficient homogeneity to reproducibly meet in background producing ingots on an industrial scale. particular aerospace requirements an additional VAR In 1998 a demonstration plant using innovative technology remelting process is needed by many specifications. concerning equipment and process planning and process Nevertheless uniformity of chemical composition in the control was developed and installed with the support of a standard scatter and keeping the aluminium content under European R&D project. With around 1000 mt per year control in case of electron beam melting has to be a special capacity, the aim was to manufacture products of focus of attention. commercially pure titanium (CP), low alloyed titanium and The alternative scrap melting process avoiding these titanium alloys in the form of ingots and near net shape slabs problems is known as the Vacuum Arc Skull Melting and blooms, ready for direct hot forming by forging or process originally developed for the production of castings rolling. using a regular consumable electrode melting scrap and other raw material in a water-cooled copper crucible.

1267 Another aim was to improve cost efficiency by reducing energy and labour costs by up to 30% compared with conventional processes as well as positive environmental effects were strived for.

Figure 2. Schematic section of the skull melting furnace

1. Vacuum chamber (melting + pouring) 2. Crucible 3. Consumable electrode 4. Mould 5. Ram 6. Upper lid 7. Furnace dome 8. Lid opening cylinders

2. Features of the melting process

Before the furnace loading the bottom charge is prepared Figure 1. Skull melting furnace and the copper crucible is lined with titanium sheets to protect against electric arc damage. In Figure 1 the skull melting furnace, a direct current (DC) After loading and inserting the skull electrode, a vacuum is high power vacuum electrical discharge arc furnace with 2.0 created and melting, soaking and casting are carried out. — 2.5 MW, is shown as realised at the site while Figure 2 Ingot and skull extraction has to be done after sufficient illustrates the schematic section of the furnace. cooling of the cast in the steel mould. Clearly visible are the water cooled copper crucible, the The production cycles are short because no special electrode consumable electrode formed by the solidified previous skull from sponge or other material has to be produced externally; and the steel mould for ingots or slabs. vice versa the skull electrode is produced together with the Before melting starts, the crucible is charged with feeding cast. material such as scrap in any form available, bulk weldables, The single melting process generates homogeneous products feed stock, sheets, cobbles and chips. The preparation of the both in composition and in structure. The latter comprises scrap is low on labour and in total cost effective. Material fine small grains due to the thermal conditions during without fragmentation can be used as well as contaminated pouring. This homogeneous and fine structure is preferable material from e.g. flame cutting. Also low grade titanium considering the subsequent hot forming. sponge can be used up to a certain extent. The furnace offers high flexibility with regard to product shapes and sizes. Melting of titanium alloys with relatively volatile elements such as aluminium can be done without any losses. In Figure 3 the aluminium content of 39 heats determined on top and bottom of heats are plotted in the frame of the referenced ASTM limits as well as vanadium in Figure 4.

1268 Besides being in the limits of specification the top and bottom values are close together. Considering the oxygen and nitrogen content the statistics of 111 heats of ASTM grade 2 are shown likewise for top and bottom of ingots/slabs. Also in this case the specified contents are evenly distributed in a close range of the required specification.

3. Production of slabs for aerospace application

As already mentioned the main importance of the process is to use the residual skull remaining from the previous melt as Figure 3. Aluminium content Grade 5 / Ti 6Al-4V new consumable electrode. The melting process itself is carried out in a specially designed water-cooled copper crucible. Consumable skull electrode, copper crucible and mould are combined in a vacuum tight chamber. After having the first skull melting electrode no special preparation is required, but for every titanium grade a consumable skull electrode with a chemical composition meeting the alloying requirements is needed. Compared with the needs of other melting processes the scrap preparation was reduced significantly because shape, size and surfaces are not decisive eventually. Descaling by sand blasting is an appropriate and sufficient method. According to the calculated chemical composition the total charge for the aerospace slab consisted of 58% skull from the Figure 4. Vanadium content Grade 5 / Ti 6Al-4V previous heat, 22% commercially pure sponge, chips and plate crops, 2.5% alloying elements and 17.5% of solids and plate crops. The commercially pure material had to be added mainly to set the right oxygen content. After charging, the melt chamber was closed, a high vacuum was pumped and the ignition of the arc by direct current was started similar to the conventional vacuum-arc-remelting process. In around 1.5 hours the charge and the electrode were liquefied and soaked. The metal was poured quickly followed by argon cooling for around 4.5 hours. The metal weight of around 1580 kg has crystallised in mild steel mould producing a slab 180 x 1100 mm in cross section together with a sample appendix while the unpoured skull material combined with a preset stub part, was removed later to be used again as consumable electrode. After sufficient cooling under argon the mould was taken out Figure 5. Oxygen content Grade 2/ Commercially Pure of the crucible and was cooled down until disassembling the mould for slab extraction. The surfaces of the slabs were clean without a high degree of skin or condensates. The bottom samples were taken for the determination of the chemical analysis. Due to the fact that the complete cast was liquefied and soaked sufficiently the chemical composition of the whole cast part is uniform throughout the length and cross section as shown by the statistics in Figure 3 - 6. The head of the slab with the shrinkage part was removed and used again for melting in the furnace. The remaining slab was ready for hot rolling to aerospace plates of 16 mm thickness. Typical yield from raw material to slab after removing the head is around 86%. per kg of poured product approx 1.60 KWh can be

Figure 6. Nitrogen content Grade 2 / Commercially Pure

1269 expected compared to VAR melting, which consumes around 0.8 - 1.0 KWh per kg single molten material or 1.6 — 2.0 KWh per kg double VAR molten ingot.

4. Production of aerospace plates from skull melted slabs and test results

The slab of 180 mm thickness was transformed by a Trio Plate Mill to 16 mm thickness applying cross rolling technology both in the high temperature and in the alpha plus beta temperature range. The plates were pre-cut and vacuum creep flattened, sand blasted, pickled and ultrasonically tested. Besides mechanical properties, the microstructure and hydrogen content was tested. Figure 7. Comparison of microstructures of aerospace plates 16 mm thick from Ti 6Al-4V, VASM versus VAR Table 1. Chemical composition of the investigated VASM heat of Ti 6Al- 4V used for the production of aerospace plates The microstructure after annealing is shown in Figure 7 exhibiting a quite uniform alpha plus beta structure. At the end the ultrasonic tests were made on the basis of AMS 2631 Class A1 without findings and deviations.

5. Summary and conclusion

This study has shown that skull melting of titanium and titanium alloys can be considered as an effective technology for producing slabs with homogeneous composition and a high grade of reliability concerning absence of refractory metals and/or nitrides, carbonitrides and oxygen inclusions. The chemical composition as calculated and as received is The charge of up to 100% of low processed scrap gives shown in Table 1 compared with the requirements of ASTM economical advantages in availability of scrap at lower B 265, Grade 5 as well as AMS 6945 for single melt plate labour costs. 6Al-4V and versus a similar plate out of the standard double The possibility to cast direct near net shape slabs for VAR-process. The mechanical properties in longitudinal and producing low-cost aerospace material generates high transverse direction are given in Table 2 also meeting the integrity plates of Ti 6Al-4V. These are equivalent in all aerospace standards of AS 6945, Rev. August 2005. tested properties to material described in Aerospace Material Specification AMS 6945 for single melt plates, produced by Table 2. Mechanical properties of the investigated aerospace plates 16 mm thick from VASM heat, grade Ti 6Al-4V cold hearth melting using plasma arc or electron beam. A comparison with similar plates produced from double molten VAR-ingot shows equivalent properties. The finally obtained 16 mm plates have already found application in an aerospace project.

Acknowledgement The authors wish to thank Mr. Filippo Oreglia of ThyssenKrupp Titanium S.p.A. for making the slab and Mr. Sergio Cervo of ThyssenKrupp Titanium S.p.A. for the testing and the statistical analysis.

REFERENCES 1) M.I. Musatov, A. Sh. Freedman and B. N. Sukhorosov: Five-Tonne Titanium Ingots produced by skull melting (Titanium '92 Science and Technology, Vol. III, 1993) pp. 2,415-2,422 2) A.L. Andreev and A.I. Mescheryako: Exploitation and development trends of large vacuum-ark skull melting furnaces (Titanium and Titanium Alloys, Scientific and Technological Aspects, Vol. I, 1976) pp. 169-178 3) M.I. Musatov, A. Sh. Friedman, Tschubarov: Quality of ingots melted in a skull furnace (Titanium and Titanium Alloys, Scientific and Technological Aspects, Vol. I, 1976) pp. 179-187 4) Ti-Tech brochure (Italy, April 2003)

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