EVALUATION OF Y 2 O 3 AS FRONT LAYER OF CERAMIC CRUCIBLES FOR VACCUM INDUCTION MELTING OF TiAl BASED ALLOYS

During the last decades titanium alloys were found to be valuable engineering materials for many different applications. Formerly used in criti al applications like aerospace, aeronautic and military equipment, where the factor cost is no t relevant, titanium alloys are finding now new and different markets. However, the development of such new markets will d epend on an effective cost reduction of titanium parts, in order to achieve a selling cost sui able with its application in consumer goods. A possible solution to decrease production costs mi ght be the use of traditional casting techniques to produce near net shape functional par ts. During the last years, the authors have developed extensive research work on this field, an d new technique both for melting and moulding, using ceramic multi-layered crucibles and i vestment casting shells was developed. This paper presents some of the results obtained du ring that research work: Ti-48Al alloy were melted and cooled inside CaO, MgO and Y 2O3 stabilized ZrO2 crucibles with inside layer of Y2O3. The chemical composition, hardness and microstruc tu e at the metal-crucible interface, studied by secondary ion mass spectrometry, SEM/EDS and XRS are presented. On a second step, the same alloy was melted on the same crucibl es, and poured into graphite moulds, and the crucibles wall was characterized by SEM/EDS and XRS . Introduction Foundry is one of the technologies available to pro cess titanium alloys. However, the high reactivity of those alloys with almost every elemen t makes very difficult the use of traditional casting techniques and equipment to obtain sound ca stings. For such reason, specially designed furnaces (cooled copper crucible induction or arc f urnaces) are used for vacuum melting preprepared (by multiple remelting) melting charges wi th the desired chemical composition, and the molten metal is poured in ceramic moulds obtain ed by the investment casting process. In many cases, the molten alloy is poured in the form f billets that are postly processed by forming in order to obtain approximate final shapes . The present processing technology often EPD Congress 2005 Edited by M.E. Schlesinger TMS (The Minerals, Metals & Materials Society), 2005 574 leads to castings with a surface hard-case, usually c ed “alpha-case”, due to the presence of contaminants on that region, mainly oxides, that mu st be removed by chemical milling. For such reasons, titanium alloys parts are extremely e xpensive, and its use is limited to special and critical applications where the factor cost is almo st irrelevant, like in the aerospace, aeronautical or military industries. During the last years, an effort to develop a casti ng echnique suitable to produce Ti based castings at competitive cost, based on the traditio nal ceramic crucible induction melting technique, has been done by several researchers, wh ere t e authors are included. Results obtained so far show that no material accomplishes t melting crucibles two main demands: inertness facing the titanium alloy and high therma l-shock resistance. Due to the high reactivity of such alloys, no material has been found absolute ly in rt facing titanium, and the yttrium and calcium oxides were found to be those chemically mo re stable when used as crucible materials [1-8]. However, in both cases, the crucibles therma l-shock resistance was found to be poor, and there are references to crucibles that cracked duri ng melting. Besides, calcium oxide reveals manipulation problems, due to its high higroscopici ty. The crucibles thermal-shock resistance can be impro ved if partially or fully stabilized zirconium dioxide is used on its production. Howeve r, its reactivity with molten titanium alloys is higher, and its use usually leads to the contami n tion of the alloys with Zr and O [1,4,5,8]. Finally, the extremely high cost of those refractor ies, mainly in what concerns Y 2O3, makes its use on the production of ceramic crucibles almost f rbidden. Facing such results, the authors developed a two-la yer ceramic crucible to be used to melt TiAl. In order to grant the desired thermal-shock resista nce and an affordable cost, crucibles are based on stabilized ZrO 2, and present an Y 2O3 inside layer in order to achieve the desired chemical stability facing TiAl, and so decreasing t he probability of a metal-crucible interaction to occur. Results obtained using the developed tech nique are quite satisfactory, and very low contaminated castings were produced, revealing no “ alpha-case” and excellent surface finishing. Experimental Technique Melting operation Ti-48Al alloys have been produced, prepared from pu re aluminium and titanium, melted in multi-layer crucibles. ZrO2 crucibles partially stabilized with Y 2O3, MgO and CaO were used as base crucibles, in order to study the influence of the stabilizer compound on the crucible thermal-shock resistance. Every crucible was inside coated with an Y2O3 film 200 μm thick, in order to achieve the desired chemical stability. Melting was performed on an induction furnace, insi de a sealed chamber, where a controlled atmosphere of commercial pure argon was maintained. The melting procedure was the same as used in previous work and published elsewhere [1,5] . Melting stock weights 100 g, made of titanium rod grade II and commercial pure 99,8% alu minium. In order to evaluate the effect of a slower cooling rate upon the residual element con ent, the melts have been allowed to solidify and cool inside the melting crucible until room tem perature. Some melts have been EPD Congress 2005 Edited by M.E. Schlesinger TMS (The Minerals, Metals & Materials Society), 2005 575 centrifugally poured into graphite moulds, to allow the evaluation of the crucible wall after melting. Melting temperature was 1600oC Samples characterization Cast specimens obtained by letting the alloy to sol idify inside the melting crucible were cylinders 40 mm in diameter and 22 mm long. Samples for characterization were collected from the middle section of them, by cutting the cylinder s at half their height, and prepared using traditional metallographic techniques. Samples characterization includes microstructure id ent fication, chemical composition and microhardness values of each microconstituent, and “ lpha-case” characterization. Chemical composition measurement was performed by quantitati ve EDS analysis with standards of pure Ti, Al, Zr and Y, using a JEOL JSM 35C scanning ele ctron microscope. Overall oxygen content was measured using the Inert Gas Fusion (IGF) techn ique. Oxygen content variation from the surface to the inside of samples was evaluated by S econdary Ion Mass Spectroscopy (SIMS). SIMS has been performed in a Multitechnique Surface Analysis System detailed described elsewhere [9]. A 4 keV Ar , 200 nA beam focused in a 50μm diameter was used. S condary ions produced by sputtering where mass analyzed by a quadrupole type mass spectrometer. Both positive and negative spectra were taken. Base pressure during analysis was in the order of 10 mbar. X-Ray diffraction was used to identify compo unds present on the microstructure. Microhardness was evaluated on a Shimadzu hardness tester, using a 50g load, for 15 seconds. Experimental Results Samples microstructure As shown in Figure 1, the microstructure is very si milar for every sample, no matter the crucible used to produce them: a lamellar dendritic constituent with two phases ( α2 + γ) and an interdendritic γ phase. The lamellar constituent is present on a vo lume fraction of around 80% that increases from the samples surface to the insi de, following the decrease of the cooling rate. Every sample reveals the presence of small inclusio ns that the EDS spectrum (Figure 2 c)) suggests to be yttrium oxide. Microconstituents were identified through its chemi cal analysis (Table I) and EDS spectrum (Figure 2), and later confirmed by X-Ray Diffractio n (Figure 3). The same X-Ray Diffraction evaluation proves that the small inclusions are in fact Y2O3 particles which origin is believed to be the erosion of the Y 2O3 film present on the crucible interior wall. When c ompared with samples produce before in single Y 2O3 stabilized ZrO2 crucibles (Figure 1 d)) [1], the presence of aluminum oxide is not detected, what suggests th e high chemical stability of the Y 2O3 inside layer. EPD Congress 2005 Edited by M.E. Schlesinger TMS (The Minerals, Metals & Materials Society), 2005