Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D21/00—Casting non-ferrous metals or metallic compounds so far as their metallurgical properties are of importance for the casting procedure; Selection of compositions therefor
  • B22D21/002—Castings of light metals
  • B22D21/005—Castings of light metals with high melting point, e.g. Be 1280 degrees C, Ti 1725 degrees C

Definitions

• This invention respects a foundry process for reactive alloys, namely titanium alloys, using conventional melting and moulding techniques, to which small alterations were made, either in terms of equipment, and in respect to the materials being used.
• titanium casting alloys includes multiple remelting before the final melting charge is reached, with the purpose of obtaining the desired chemical composition and its homogenisation.
• the final melt is performed in arc or induction skull melting furnaces (using cold wall copper crucibles), under controlled atmosphere.
• Ceramic crucible induction melting might be a valuable alternative to the actual melting routes, not only for titanium alloys but for reactive alloys in general. Although its use is actually limited to research purposes at lab scale, there are some references to a few industrial applications, where the castings quality is not critical [1,2].
• the US Patent 4710481 refers the melting of commercially pure titanium in crucibles of calcium oxide (CaO) that, in spite of the high thermodynamic stability that they present, drove to oxygen incorporation in the cast metal.
• CaO calcium oxide
• Sato [1,2] that refers an incorporation of 0.4% oxygen in weight for commercially pure Ti, besides porosity formation inside the castings, after solidification.
• the use of CaO is, however, delicate, given the high hygroscopicity it presents.
• the US 5102450 and EP 0526159B1 Patents also refer the use of CaO crucibles in the melting of intermetallic alloys based on the ⁇ -TiAl, to which 2 to 8% of Nb and 2% of Cr was added, with the purpose of reducing the tendency for oxygen absorption by the metallic bath.
• the metallic charges were introduced in the CaO crucibles in a predefined sequence (in first place Al, Cr and Nb, were melted and only after the melting of the alloy so obtained the titanium was introduced, with the purpose of shortening the time of contact of the titanium with the crucible).
• the alloys so produced revealed oxygen absorption between 0.1 and 0.25%, depending on the chemical composition of the alloy.
• the use of this process for the obtaining ⁇ -TiAl is not viable, since the decrease of the contamination of the alloy with oxygen is reached by significantly altering the chemical composition of the alloy itself.
• the US Patent 4040845 respects the melting of the alloys of the Ti-Be system in crucibles constituted by a refractory material composed by about 60% of Y 2 O 3 , the remaining being a mixt ⁇ re of rare earth elements, namely Dy 2 O 3 , Yb 2 O 3 , Er 2 O 3 , Gd 2 O 3 , CeO 2 , La 2 0 3 , Sm 2 0 3 , Nd 2 O 3 e Pr 2 O 3 , and still CaO, Al 2 O 3 and TiO 2 .
• Y 2 O 3 , MgO and CaO stabilized ZrO 2 were also evaluated as crucible materials for titanium alloys. It's use lead to high incorporation of oxygen (1,55 to 6%) [5,6] and zirconium (1,3%) [6] in the molten metal.
• the time of contact between metal and crucible is one of the maia factors responsible for the contamination of the metallic alloys.
• the US5299619 Patent refers the development of a method to decrease that time of contact and, so, to reduce metal contamination.
• This invention proposes the use of two melting charges, molten in different equipments that react exothermically after entering in contact, so decreasing the total time of melting, and consequently the time of metal-crucible contact.
• the process When applied to the production of TiAl, the process consists on the melting of an aluminium charge in a conventional furnace, the transport of the liquid metal to the furnace where the mixture Ti- Al will be done, and the pouring, under air conditions, of the liquid aluminium in the resident crucible of this furnace. This process doesn't avoid the surface oxidation of the molten aluminium during the melting, transport and pouring operations.
• a graphite crucible is united to a ceramic mould obtained by successive coatings of an expanded polystyrene pattern, to obtain a sealed setup, with a direct connection between the crucible's mouth and the mould' s ingate.
• the mould-crucible set-up is then heated at high temperature, so producing the vaporization of the polystyrene and the complete drying of the mould.
• the melting charge (a billet with the geometry of the interior of the crucible and with the desired final chemical composition) is introduced in the crucible through its bottom, and the set-up is placed with the crucible inverted in the upper position in the melting furnace.
• the melting of the charge is very fast, since the crucible was already pre-warmed, so reducing the incorporation of carbon in the metal. This process demands, however, the use of pre-melting, to obtain the billet with the desired final chemical composition.
• the US4787439 Patent describes a process for obtaining moulds by the lost wax moulding process, using contact arid filling layers of different nature.
• the contact layers are produced starting from slurries consisting in Y 2 0 3 , ZrO 2 , mixtures of Y 2 O 3 and ZrO 2 and mixtures of Zr0 2 and Si0 2 .
• the contact layers are made with a slurry of colloidal silica and alumina silicate, using a coating of higher granulometry alumina silicate.
• the contact layers are produced starting from slurries consisting in mixtures of Y 2 O 3 and ZrO 2 , in proportions varying between 100 and 75% of Y 2 0 3 , colloidal Si0 2 being used as binder.
• the grapes After the immersion of the pattern in the slurry, the grapes are covered with granular alumina.
• the outer layers are obtained starting from a slurry consisting of alumina using ethyl silicate as binder.
• the coatings after immersion in the slurry are made with alumina of larger granulometry. With this process it is possible to obtain Ti6A14V castings with "alpha-case" extensions between 25 and 200 ⁇ m, according to the thickness of the part.
• the main subject of this invention is the development of a casting technique for ⁇ TiAl alloys, suitable to obtain contamination free castings, with characteristics that allow its industrial use.
• the invention suggests the melting of ⁇ TiAl in low cost ceramic crucibles, suitable to achieve the main task of the invention.
• the developed ceramic crucibles reveal high thermal-shock resistance and are thermodynamically stable facing ⁇ TiAl alloys.
• the developed crucibles are not hygroscopic, which makes easier their manipulation.
• this invention suggests the use of melting stocks consisting in commercially pure titanium and aluminium, in order to avoid pre-melting operations needed to obtain sub-products (billets) with the desired final chemical composition.
• Another aspect of the invention concerns the moulding process and materials used to obtain the desired moulds.
• the invention anticipates the production of multi-layer moulds using the investment casting process, using refractories chemically stable facing ⁇ TiAl, without significant changes on the methodology, procedures and equipment of that moulding process.
• the moulding technique allows the production of very low surface contaminated castings with good surface finishing, similar to that of ferrous castings obtained by the traditional investment casting process.
• ⁇ -TiAl parts by foundry is not usual. Such parts are usually produced by mechanical forming processes, from billets with a composition previously adjusted through a complex sequence of remeltings. Titanium and traditional titanium alloys castings are produced by the traditional investment casting process, but they usually present many casting problems, like porosities, chemical composition heterogeneity, and hardness variation. Moreover, the actual production costs are very high, as consequence of high investment costs in equipment and the high production costs themselves (melting operation, finishing costs, raw materials).
• the developed technique allows the elimination, or significant decrease, of those problems, namely the absence of casting porosities and very low contamination of the base metal in elements known to be harmful to the performance of any part (oxygen for example).
• the "alpha-case” extension is lower than 25 ⁇ m, nevertheless the hardness value is still in agreement with the existent standards for ⁇ -TiAl, and therefore " being not needed any later operation of removal of the surface metal.
• the developed technique relies upon low cost equipments, so decreasing the global production costs, and allows its use by any foundry that can use the traditional lost wax: moulding process, without need of great modifications of its production processes.
• FIG. 1 Schematic representation of the centrifugation structure of the melting furnace used on this work
• Figure 5 Graphic representation of the oxygen concentration variation, versus the distance to the surface of the samples
• This invention concerns the development of a casting process to obtain ⁇ -TiAl parts.
• melting is performed starting from a charge of pure aluminium and titanium, technique until now not used to produce ⁇ -TiAl.
• a casting is obtained by only one single melting operation, in the same way of the traditional casting of metallic alloys, so avoiding the multiple previous remeltings needed to correct the chemical composition.
• the furnace is constituted by a hermetic chamber (1), used both for melting and pouring, having a glass window (2) for introduction of a thermocouple for temperature reading and visualization of the course of the melting operation.
• a quartz crucible (7) that, during melting, is involved by an induction coil (10), that guarantees the heating of the melting charge.
• an induction coil (10) that guarantees the heating of the melting charge.
• a set consisting of an inner crucible (fusion) (6) and an outer crucible with a spout, for protection of the quartz and pouring crucibles (5), is placed inside the chamber, and inside the quartz crucible.
• the mould (3) is placed in horizontal position, inside the same chamber, hold by an appropriate support (4), linked to the spout of the pouring crucible by a ceramic positioner (8).
• the chamber is tied up to a metallic shaft structure, which is fixed to the top of a vertical axis by a screwing device (12), around which the whole apparatus rotates.
• a counterbalance exists (13), at the opposite side of the chamber, relatively to the rotation axis, to guarantee the balance of the apparatus. Pouring is done centrifugally, by the rotation of the whole set described, around the referred axis.
• Crucibles used in the melting operations are commercially available crucibles, made of ZrO 2 stabilized with 9% of Y 2 O 3 , produced by the company Marketech International, U.S.A., with porosity from 10 to 30%, to which interior a coating of Y 2 O 3 is applied.
• the Y 2 O 3 coating is applied as an emulsion with the characteristics presented in the table 1, produced by the company ZYP Coatings, U.S.A.
• Table 1 Characteristics of the Yttrium oxide (Y 2 0 3 ) suspension used to inside coating the multi-layered crucibles.
• Solid component Y 2 O 3 (99.9%) Liquid component Ethyl alcohol wt % solid component 54 wt % liquid component 46 Viscosity, ⁇ 120 cps pH 7 Composition after drying Y 2 0 3 — 99.7 wt% C-0.3 wt%
• the coating operation of the crucibles is done by filling them with the Y 2 O 3 suspension, followed by 30 seconds of permanence inside it, followed by dripping out for 5 minutes. With this procedure a uniform thickness coating is obtained, about 100 ⁇ m thick. After this operation, crucibles are dried at 200°C for 24 hours, and the mentioned operations repeated once again, except for the time and drying temperature that will be 24 hours at 500°C. With this procedure an interior coating of the crucible is obtained with a film of Y 2 0 3 , with a uniform thickness of about 200 ⁇ m.
• the mould is obtained by the lost wax moulding process, using successive coatings, of variable composition.
• the purpose of the use of different coating compositions is to decrease the production costs, since the material used in the outer layers is of significantly lower cost than the one used in the inside layer.
• the moulding process begins with the constr ⁇ ction of one, or several, wax patterns and their filling runners and ingates (grape), using commercially available wax for lost wax moulding.
• the different elements of the grape are obtained by gravity pouring the wax in metallic counter-moulds built for that purpose.
• the pouring temperature of the wax is 11 5°C, and the temperature of the counter-moulds is room temperature.
• the stabilization of the metallic counter-moulds temperature is attained by their immersion in cold water, immediately after each pouring.
• silicon oil is used, applied by aspersion.
• the wax grape is later immersed in a solution of ethyl alcohol (50%) and acetone (50%), with the purpose of degreasing the surface and improving the adhesion characteristics of the first ceramic coating.
• the slurry is prepared using traditional mixdng techniques, using a stem mixer of variable rotation speed.
• preparation begins with the addition of urea to the binder (with the wetability promoter and the anti-foam already included), and later addition of the refractory. This addition is done along 120 minutes, at a rate of 25% every 30 minutes. Finally the acetic acid is added, in bulk, but very slowly (in about 5 minutes).
• urea is to decrease the tendency for the slurry to jellify, so guaranteeing that that phenomenon is not verified for values of pH lower than 8.5.
• acetic acid is needed to significantly lower the initial pH of the slurry to values that guarantee its acid nature, since the basic nature of the Y 2 0 3 refractory would cause its jellification.
• the pH of the slurry is going to be adjusted along its time life, with small additions of acetic acid.
• the grape After coated with the slurry, the grape is dried for 24 hours under controlled atmosphere (temperature and moisture).
• the grape is successively covered by immersion in another slurry, immediately followed by coating with a granular refractory material, being followed by new drying under controlled atmosphere.
• the grain size of the refractory used is increased from coating to coating, in order to avoid sudden variations of the morphology of the coatings used in each shell, to avoid possible detachments between successive coating layers, during the drying or during the thermal processing of the mould. This series of operations is repeated as many times as considered necessary to give the shell the mechanical strength needed to its handling.
• the nature of the outer coatings is different from that of the inner coating (Y 2 0 3 ), in order to reduce costs.
• table 5 the composition of the slurry used is shown, and in the tables 6 and 7 are shown the characteristics of the binder and the refractory used in its preparation.
• Table 8 presents the characteristics of the granular refractory material used in the outer coating operations after coating with the slurry.
• the grape After this operation and the cooling of the shell, the grape is introduced in a metallic box, the remaining space inside the box being filled with sand agglomerated by the sodium silicate/CO 2 process. This way, the mould for the pouring operation is built.
• the melting operation is done using an electric induction furnace, and the pouring is accomplished by centrifugation.
• the melting crucible is introduced inside an alumina crucible with a spout, for protection of the induction coil in case of fracture of the melting crucible.
• the spout of the crucible has a direct connection with an existent hole in the mould for its filling.
• the whole process occurs inside a chamber under controlled atmosphere (pressure and nature) (figure 1).
• the set crucible + mould + melting charge is subjected to a heating stage at 300°C for 2 hours with the purpose of eliminating any moisture that may be present.
• the described process allows the obtaining of ⁇ -TiAl castings of thickness up to 20 mm, in which the level of contaminant elements doesn't surpass 0.11 at% of Y and 0.66 at% of O (»0.29 wt%), free from inside porosities, without need of removal of the "alpha-case", without formation of surface oxides films and with surface finishing identical to the usually presented by the ferrous alloys castings obtained by the lost wax moulding process.
• Example 1 In sequence, an example of sample production using the described process is presented.
• Example 2 In sequence, an example of sample production using the described process is presented.
• Cylindrical samples with 20 mm diameter and 85 mm length were produced in ⁇ -TiAl.
• Melting charges were made of commercially pure Ti and Al, with the composition indicated in table 9. 100 g charges were used, with 52% Ti and 48% Al.
• the set crucibles-mould-melting charge was placed inside the melting furnace chamber, in the position schematically shown in figure 2.
• the chamber was evacuated up to the pressure of 5 x 10 " bar, and after, a washing operation ⁇ vas done with high purity argon (N60 Argon- O 2 ⁇ 10 ppm, N 2 ⁇ 5 ppm, H 2 O ⁇ 1 ppm), with a 10 "1 m /minute caudal under the pressure of 2.5 bar.
• This "washing" operation was performed three times, and the argon pressure was then stabilized at the level of 10 "3 bar, and then the effective melting process was initiated.
• the beginning of the melting operation started very slowly, to avoid the occurrence of thermal shock on the crucibles.
• the heating began using 1 kW power, kept for 10 minutes, time enough to allow the aluminium to change to the mushy state.
• heating was performed under a 2 kW power, for 5 minutes, followed by a heating stage under 3 kW power, for 4 minutes, the beginning of the melting of the alloy being verified by the end of it.
• the molten alloy was maintained under the same power for 60 seconds, the pouring being executed afterwards, around 1550°C, and the samples were allowed to cool inside the chamber until room temperature, maintaining the argon atmosphere. Pouring was performed by centrifugation, at a rotation speed of 418 rpm, with an acceleration in agreement with the curve shown in figure 2.
• Moulds were produced according to the described techniques, the first coating was made with an Y 2 O 3 slurry, without any refractory coating, and the following coatings used a ZrO 2 slurry and the ZrO 2 granulated refractories of the types Rl, R2 and R3. The procedure used is described bellow in detail:
• the viscosity values were ascertained through the measure of the flow time of the slurries through the hole of a Zahn #4 cup. After the production of the shells a thermal shock operation to eliminate the wax was done. The shells were placed in an adequate support on the floor of a dedicated furnace, which was later properly lifted to the interior of that furnace, previously heated to the temperature of 900°C. After elimination of the wax, the shells were left to cool until room temperature. The final thermal processing of the shells was done according to the thermal cycle represented in figure 3.
• Samples obtained by this process present a microstructure consisting in a lamellar ⁇ 2 + ⁇ microconstituent and a ⁇ phase.
• table 11 chemical composition of the microconstituents versus its distance to the surface of the sample is shown.
• the alloy revealed a small contamination with Yttrium (0,11 at. %), being lightly superior at the metal-mould interface (0,15 at. %).
• Microhardness values are uniform along the whole sample, presenting a slightly higher value (about 3% higher) at the metal-mould interface, in the ⁇ 2 + ⁇ constituent (figure 4).
• the registered increase is a characteristic of all casting parts, as a consequence of different cooling speeds on the periphery and the interior of the castings.
• the oxygen concentration determined by SIMS, is also uniform, a slight increase being registered for distances to the surface of the sample less than 20 ⁇ m ( Figure 5). This increase, is however non significant, since its influence in the microhardness value in the same zone is negligible.
• Table 11 Variation of the chemical composition of the micro constituents versus the distance to the surface of the sample, in a sample of Ti-48A1, obtained by melting it in a ZrO 2 crucible inside coated with Y O3 and pouring in multi- layered ZrO 2 + Y 2 O 3 ceramic moulds.
• the surface roughness value (Ra) obtained for the samples was 1.6 ⁇ m (figure 6) and the surface finishing is in agreement with the 3/OS1 pattern of the 359-01 Technical Recommendation of BNIF.
• the samples haven't developed any superficial film of oxides, being only visible by electronic microscopy small Y 2 O 3 portions adherent to the surface, with a thickness non greater than 2 ⁇ m, easily removable using the cleaning processes conventionally in foundry.
• the characteristics of the castings obtained by this process allow its direct industrial use, except in the cases in which the geometrical complexity demands the use of local machining operations.

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• 2003
  • 2003-09-12 PT PT103018A patent/PT103018A/pt unknown
• 2004
  • 2004-09-10 WO PCT/PT2004/000023 patent/WO2005025778A1/en active Application Filing
  • 2004-09-10 EP EP04775189A patent/EP1670604A1/en not_active Withdrawn
  • 2004-09-10 US US10/571,677 patent/US20070267165A1/en not_active Abandoned

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