Deoxidation Limits of Titanium Alloys during Pressure Electro Slag Remelting

This paper focuses on deoxidation of titanium alloys produced by aluminothermic reduction (ATR) and subsequent homogenizing and alloying by vacuum induction melting (VIM). The main goal of the performed research work is to outline the deoxidation limit during pressure electro slag remelting (PESR) of the described material. To obtain electrodes for deoxidation, a Ti-24Al-16V masteralloy was produced by ATR and afterwards melted in a 0.5 litre calcium- zirconate (lab scale) or 14 litres high purity calcia (pilot scale) crucibles with continuous addition of Ti-sponge after reaching liquid state in order to obtain a final Ti-6Al-4V alloy. During melting, in both cases evaporation of calcium was noticed. The cast ingots were analysed for oxygen using inert gas fusion method, matrix and alloying elements were analysed by XRF. Results show oxygen levels between 0.5 and 0.95 wt.-% for the ingots which were melted in calcium-zirconate crucibles and approx. 1 - 1.2 wt.-% for the material produced by utilization of calcia crucibles. The subsequent deoxidation was carried out in lab and pilot scale electroslag remelting furnaces using a commercially pure calcium fluoride slag and metallic calcium as deoxidation agent. It could be shown, that deoxidation of the highly contaminated material is possible applying this method to a certain limit. Pilot scale trials showed a reduction of oxygen contents by 1500 - 3500 ppm. Oxygen levels in lab scale trials showed weaker deoxidation effects. In order to describe the achieved deoxidation effects in a quantitative way, the analyzed oxygen contents of the obtained ingots are compared with calculated data resulting from a mathematical kinetic model. The modelled datasets are in good agreement with experimental oxygen values.


Introduction
Titanium and its alloys combine several attractive properties such as high strength, low density, corrosion resistance and excellent biocompatibility. Despite these characteristics, the wider application of titanium and titanium alloys is limited due to the high manufacturing costs. Therefore, since approximately 10 years the IME, RWTH Aachen University, is working on the application of a new synthesis and recycling process route for titanium alloys with special regard on decreasing the level of oxygen due to its negative influence on the ductility. Detailed investigations on each process step were performed by Lochbichler [1] Stoephasius [2], and Reitz [3]. An important aspect of the process route is to resort to conventional and established metallurgical processes such as Vacuum Induction Melting (VIM), Electroslag Remelting (ESR) or Vacuum Arc Remelting (VAR).
According to the type of material (scrap, raw materials) and the composition, different process steps are combined to achieve the desired product quality. Main principle to control the oxygen level is the introduction of metallic calcium to the liquid slag or melt as a deoxidant. Especially for the material class of  -TiAl it could be shown, that deoxidation of these alloys from investment casting scraps during ESR is feasible resulting in final oxygen contents below 500 ppm in the product [3]. The research presented in this paper focuses on an alternative process route for Ti6Al4V alloys with special regard on the PESR deoxidation.

Fundamentals
One of the most challenging tasks in the refining process of titanium alloys is the removal of oxygen present in titanium as dissolved TiO. Besides the high stability of TiO (G f = -357 kj/mol @ 1750°C) also the oxygen solubility of up to 33 at% in the system Ti-O indicates that the removal is difficult. The deoxidation by vacuum distillation is not feasible due to the necessary oxygen partial pressure in the furnace atmosphere (p o < 10^2 0 bar for 1000ppm in a cp-Ti melt) which is so low, that even present oxygen from the gas phase will be nearly fully dissolved. Thus, the use of deoxidation agents becomes evident. Regarding the Vacuum Induction Melting of titanium and titanium alloys, the choice of the refractory material is crucial. According to standard Gibbs free energy calculations of oxides, only CaO, Y 2 O 3 and ZrO 2 seem to be sufficiently stable against titanium, whereas CaO should be most stable. Tsukihashi et al. [4] investigated the calcium and oxygen uptake in c.p. titanium and TiAl melts in equilibrium with solid CaO. The experiments were conducted in a closed system by using a lid to avoid Ca evaporation. His research shows, that the reaction according to (1) is causal between for the equilibrium between calcium and oxygen in titanium melts, which depends strongly on temperature. [ Figure 1: Necessary Ca content in the slag in order to achieve specification conform oxygen contents in titanium and titanium aluminide [5] 3. Experimental

Masteralloy synthesis by Aluminothermic Reduction (ATR)
The feasibility of aluminothermic reduction (ATR) of TiO 2 to produce Ti alloys was shown in previous works [7] [8]. The main challenge for the ATR process is to produce Ti alloys with high titanium and low aluminium content. However, the high oxygen solubility of titanium does not allow producing Ti alloys with low aluminium and simultaneous low oxygen contents. Because Al is used as alloying element in many Ti materials, residual Al content in the ATR product can be tolerated.  [8]. Therefore, a subsequent deoxidation step has to be performed for ATR Ti in order to meet the specifications of 2000 ppm. It is of particular interest to provide aluminothermic reduced titanium for further titanium based alloy production. By co-reduction of TiO 2 and other oxides of alloying elements such as vanadium the synthesis of a master alloy is possible. In this respective, Ti-6Al-4V, representing the most utilized titanium alloy, gets relevant.. For the present research the composition of the master alloy is chosen as Ti with 60 wt.-%, Al with 24 wt.-% and V 16 wt.-%. For the subsequent processing by VIM titanium sponge has to be charged with a ratio of 3:1.
The reduction of TiO 2 with aluminium requires further heat input to obtain the reaction selfpropagated. KClO 4 is added to the reaction mixture with the required amount of aluminium. For synthesis of the master alloy, the combined reduction of oxides of titanium and vanadium is performed. There are different oxides available, such as V 2 O 3 , V 2 O 4 , V 2 O 5 and mixtures of theses oxides whereat V 2 O 5 provides the highest enthalpy input. In order to decrease the KClO 4 amount and hereby decreasing Cl-bearing off-gas V 2 O 5 is used in every experiment. In order to decrease the liquidus temperature of the alumina slag, lime is added to the ATR mixture as flux. All input materials are added in powder form and blended before charging. Experiments on two different scales (20 L ≙ 6kg of product and 90 L ≙ 30 kg) are performed in the present work. The initial ignition is carried out by a filament on top of the input mixture in all trials. After reaction, the produced metal ingot is analyzed with respect to total oxygen as well as base elements. The ingot is cleaned from residual slag attached to the surface and crushed for the subsequent processing steps.   remelting and cooling for several hours, the ingot is stripped, the cap-slag and slag skin are removed, the ingot is sectioned and analyzed by IGF for oxygen.  Table 1 shows the oxygen contents of the obtained master alloys. With respect to the lower titanium content and higher aluminium content, the lower oxygen amount in the small scale trial is evident and corresponds with literature data. For further processing, the 6kg batch was used for lab scale VIM trials (VIM1a-3a), the 30kg batch for pilot scale (VIM1b-3b) melts.  Table 2 show the concentration of the major elements in the obtained electrodes from lab and pilot scale tests. Since one lab scale ingot consists of two castings, an oxygen concentration gradient from top to bottom can be measured. In general, the oxygen pickup when using CaZrO 3 crucibles is lower, which indicate their stability. On the contrary, a Zr enrichment in the metal can be measured. A detailed description of the CaZrO 3 -Ti-interaction is given by [10]. Melting in pure lime crucibles leads to an oxygen increase to approx. 1-1.2 wt.-%. All cast electrodes are further processed in the electroslag remelting process.