The use of a palladium alloy to pin a core in the casting of turbine blades

Abstract: Pinning wires suitable for use in turbine blade manufacture comprise palladium alloyed with one or more noble and/or refractory metals, and are substantially more cost effective than conventional pinning wires.

Description: This invention relates the use of Pd alloy pinning wires in the costing of turbine blades. Advanced gas turbines are required to operate at as high a temperature as possible to maximise fuel efficiency. The turbine blades in these engines must be air cooled to maintain adequate strength. This is achieved by casting blades into patterns which are ceramic moulds containing special ceramic cores which are removed prior to service. Unfortunately, due to the complex nature of these poorly supported patterns, drift or movement can occur during production which causes high scrap rates. Core pinning technology using fine platinum wires has been developed to overcome these problems. In a typical case seven to ten pins, each of 5 to 10mm in length are required for a 2 inch blade. The pins are inserted into a wax preform and butt against the ceramic core. The wax is coated with a zirconium silicate/alumina shell mould and fired at 850°C to 1130°C in air, for between 1 and 50 hours. After firing and burning out of the wax the mould assemblies are heated to approximately 1475°C in a vacuum for 20 minutes, prior to pouring of the molten superalloy at a temperature of approximately 1550°C, into the mould. The pinning wires dissolve in the molten superalloy. Finally the mould is withdrawn out of the bottom of the furnace, at a controlled rate which aids optimum grain structure in the turbine blade. In use, therefore, the pinning wire must be capable of surviving and maintaining adequate strength at temperatures of the order of 850°C to 1130°C in air with minimal oxidation and approximately 1475°C in vacuum with minimal metal loss. In addition, it must dissolve evenly in the molten casting alloy without producing any detrimental effects on the physical or mechanical characteristics of the finished turbine blade, such as spurious grain nucleation. Presently, pure platinum wire or grain stabilised platinum wire is employed. The high cost of platinum makes the pinning wires very expensive. GB 2118 078 (Howmet Turbine Components Corporation) discloses a system for pinning cores during the casting of turbine blades. The use of wires of Palladium, platinum and other platinum group metals is disclosed. Accordingly, the present invention provides the use of a palladium alloy wire as defined in claim 1, with preferred embodiments in claims 2 and 3. Oxide dispersion strengthening and/or grain stabilising may be promoted in the alloys through the minor additions (up to 1% of the total weight of alloy) of metals such as Zr, Ni, Co, Mn, V, and Ti. The pinning wires according to the invention are normally of 0.5-0.6mm in diameter, although for certain applications diameters may range from 0.3-1.5mm. They may be prepared by conventional wire drawing, and may be supplied as reels of wire or pre-cut into pins which are usually 6-8mm in length, although for large blades the pins may be up to 2cm in length. The invention will now be described by example only. EXAMPLE The samples produced were: Group I (0.6mm diameter wires) (i) Pd-20%W (ii) Pd-15%Mo (iii) Pd-20%W (Pt-coated to 5µm) (iv) Pd-15%Mo (Pt-coated to 5µm) Group II (sheets-for test comparisons only) (i) Pd-20%W (ii) Pd-15%Mo (iii) Pd-16%W-4Ir (iv) Pd-11%Mo-4Ir (v) Pd-15%W-5Pt (vi) Pd-10%Mo-5Pt (vii) Pd-10%Mo-5Ta (viii) Pd-15%W-10Au (ix) Pd-20%W-10Au All the above samples have a melting point higher than that of Pd. Two tests were performed on the manufactured wire/sheet: Group I (wires) 1. Oxidation Test - eighteen hours in air at 850°C 2. High temperature vacuum test - one hour at 1450°C in vacuum. Group II (sheets) 1. Oxidation test - 8 hours in air at 1075°C 2. High temperature vacuum test - 30 minutes at 1475°C in vacuum. RESULTS Oxidation Test - Group I After 18 hours in air at 850°C the Pd-Mo and Pd-W samples all showed signs of a thin blue/pink surface oxide. There was no thick oxide or spalling on any of the samples. The diameter of each of the wires was unchanged by the oxidation treatment. The Pt-coated Pd-W wire behaved in a very similar manner to the uncoated specimen recording a very small weight gain and diameter increase. However, the Pt-coated Pd-Mo wire behaved very differently compared to its uncoated counterpart. The coated wire 'swelled' so that its diameter was increased by 17.5% while the wire suffered a 14% mass reduction. Metallography of the samples was carried out to assess any internal damage to the wires; Group I Sample Oxidation Damage Pt no damage Pd-Mo voids in sub-surface layer (to around 1/50th of wire diameter) Pd-W voids near surface and porosity to 1/5th of wire diameter Pd-Mo (coated) suffers 14% weight loss and the wire 'swells' by 17.5% (diameter) Pd-W (coated) very small weight gain High Temperature Vacuum Test - Group I A visual examination of the samples following a one hour treatment at 1475°C showed that all the surfaces were a dull grey. Those which previously were coated with a thin oxide had substantially different appearance after the high temperature treatment. Metallography of the samples was conducted to assess any internal damage. The samples were also weighed and their dimensions recorded prior to, and following the testing. Table 2 summarises the weight losses, section size changes and metallographic information of the samples. Also included for comparison with Group I results are data for Pd and Pt wires which underwent similar oxidation and high temperature vacuum treatments; Samples %Diameter reduction Weight loss % Observations Pt 0 0 no loss of material Pd-Mo 7 20 large surface voids collapsed/volatilised leaving rough surface Pd-Mo (coated) 0 62 massive metal loss leading to a 'spongy' final wire with no strength, cracks appeared in the Pt coat Pd-W 16 32 heavy voiding to 1/5th of wire diameter Pd-W(coated) 4 17 some cracks appeared in the Pt coat Pd 75 95 massive metal loss Oxidation Test and High Temperature Vacuum Test - Group II Stage 1. Oxidation test; cool to room temperature. Stage 2. High temperature vacuum test; cool to room temperature. Metallography of the samples was conducted to assess any internal damage. The samples were also weighed and their dimensions recorded prior to, and following the testing. Table 3 summarises the weight losses and metallographic information of the samples. The Tables show variation in properties as the amount of Pt is reduced. However, it is clear that all the Pd alloy based wires performed to a level where any of them are potential new pinning wires. The suitability of the Pd alloy based wires as pinning wires is particularly surprising when compared with the inadequate performance of pure Pd. The substitution of 15%Mo and 20%W into Pd has a remarkable effect on the metal loss by volatilisation at 1475°C in a vacuum. In addition these wires suffered far less grain growth at high temperatures than did the Pt, Pd and Pd-Pt-refractory metal samples. The oxidation problems anticipated with these materials appear manageable. Neither wire suffered catastrophic oxidation which is surprising since neither the Mo or W form 'protective' oxides. Particularly interesting was the behaviour of the Pd-Mo wire. After oxidation at 850°C, voids formed under the oxidised surface. Subsequently during the high temperature vacuum treatment the surface appeared to be lost possibly due to the volatile nature of the oxide layer, leaving a rough but clean pin. In this case, coating of the wire resulted in a greatly increased mass loss. However, coating may be beneficial in other cases - the effect of coating the Pd-W sample appears to have been beneficial halving the weight loss and reducing the diameter reduction to a quarter of the value recorded for the uncoated wire. It is obviously important that any potential pinning wire material does not have deleterious effects on the host alloy. In the first instance it is important that the pinning wire elements are dispersed uniformly. Casting trials have been performed to produce aerofoil shapes. Analysis of these for the elements in the pinning wires was performed and the results are contained in Table 4 below. Analysis of Investment Cast Aerofoil Shapes Pinning Wire Alloy Nominal Concentration in Aerofoil Analysis Site Analysed Concentration in Aerofoil Pt% Pd% Pt(%)±0.05 Pd(%)±0.05 Pd-15%Mo - 0.21 Root - 0.12 - 0.21 Blade - 0.15 - 0.21 Tip - 0.15 Pd-20%W (Pt Coated) 0.01 0.19 Root - 0.1 0.01 0.19 Blade 0.1 0.14 0.01 0.19 Tip 0.02 0.11 Pt 0.25 - Root 0.36 - 0.25 - Blade 0.1 - 0.25 - Tip 0.27 - These results indicate that palladium disperses through the nickel based casting alloys at least as well as platinum. This is beneficial since concentration of one element may lead to localised variation in blade properties, which must be avoided. There is considerable difficulty in obtaining satisfactory results of this type but the indications are that palladium and non-platinum bearing palladium alloys dispose through the host nickel alloys more easily than platinum or the palladium alloys being platinum. Two nickel superalloy compositions (A and B) containing the individual dissolved pinning wire alloys were tested for stress rupture. Three pinning wires according to the invention were selected (wire X is Pd20W coated with Pt; Y is Pd15Mo;). Special blocks were directionally solidified and samples machined from them. The test conditions and results are presented in Table 5. The results demonstrated that the use of these alloys is not deleterious to longitudinal stress rupture properties in the alloys tested when compared to the current standard material, platinum. Indeed, marginal benefits may be achievable.

Claims: The use of a palladium alloy wire comprising 15 to 30% by weight of the alloy of one or more noble and/or refractory metals selected from the group Ta, Mo, W, Nb, Hf, Re, Pt, Ru, Ir, Os and Rh, optionally 0-10% of one or more of Cu and Al and optionally up to 1 % of one or more of Zr, Ni, Co, Mn, V and Ti, the balance being palladium including unavoidable impurities, to pin a core in the costing of turbine blades. The use according to claim 1, wherein said alloy consists of Pd alloyed with one or more of Ta, Mo, W, Pt and Rh. The use amounting to claim 1 or 2, wherein said wire is coated with Pr, Pd, Ir or Rh.

Inventors: COUPLAND DUNCAN ROY, DOYLE MARK LAURENCE, COUPLAND, DUNCAN ROY, DOYLE, MARK LAURENCE
Applicants: JOHNSON MATTHEY PLC, JOHNSON MATTHEY PUBLIC LIMITED COMPANY
Application year: 1992
Main classification: B22C21
IPC codes: B22C21, F01D5, C22C5, B22C9