A water-soluble alumina core for single-crystal blades

As the gas temperature ahead of the turbine has risen, blade alloys have reached their limit, and cooling has become the main reserve – complex internal passages formed by a ceramic core. The applicant states plainly that core technology is a key secret in every country, compositions are patented, and some plants regard it as the "lifeline" of the business.

Three core types dominate – silica-based, alumina-based and zirconia-based. Silica cores expand little on heating and dissolve easily in caustic and hydrofluoric acid, but they are weak, work at lower temperatures and react with the aluminium, hafnium and carbon in the alloy. Alumina cores are chemically more stable and resist creep better – they suit superalloy blades better. But they have their own problem: they barely dissolve in concentrated acids or alkalis even when heated, and getting them out of a cavity is very hard.

The Jiangsu University patent proposes making an alumina core partly water-soluble.

What came before

The patent's review of prior art is useful in itself:

• in the USA alumina cores existed in the 1970s but did not spread because of complex processing and equipment;
• in the USSR in the 1970s alumina cores were made for columnar-grain blades – 90–98% α-Al₂O₃ and 2–10% SiO₂; they were hard to remove;
• in China from the 1990s the Beijing Institute of Aeronautical Materials developed alumina cores AC-1 and AC-2; AC-1 is 92–99% α-Al₂O₃ of mixed grain sizes (60–75% coarse), 1–8% of an additive TC-1 and 13–18% plasticiser on top;
• the Institute of Metal Research of the Chinese Academy of Sciences made Al₂O₃–SiO₂ nanocomposite cores – stronger, but less soluble;
• abroad, alumina cores are removed with fluoride salts (sometimes with high-pressure washing) or caustic with pressure washing.

The closest prior art is an earlier Chinese patent on an alumina core with the same sodium salts (85–98% α-Al₂O₃, 0.5–9% NaAlO₂, 1.5–6% Na₃PO₄·12H₂O and water). It dissolved well but was too weak in tension and bending for single-crystal blades. The applicant sees the novelty in the quartz and MgO, the polyethylene glycol and, above all, in high-temperature sintering: the earlier composition was only heated, without sintering.

Composition and the role of each component

The claimed composition, wt%: fused alumina α-Al₂O₃ 85–90, quartz 3–8, MgO 3–5, sodium phosphate Na₃PO₄·12H₂O 3–5, sodium aluminate NaAlO₂ 1–5. On top of that, 3–5 of polyethylene glycol (PEG-600) as a plasticiser and 5–15 of water. Powders: alumina 200 mesh, quartz 320 mesh, MgO 100 mesh, analytically pure salts.

The roles are as follows. A small amount of quartz sinters with the alumina and gives strength – ordinary powder, not nano-silica as in composite cores. MgO acts as a mineraliser, mainly lowering the reaction temperature and speeding up sintering, with some taking part in the reaction. Sodium phosphate and sodium aluminate bind the mix and make the core water-soluble.

How it is made

• MgO, quartz, alumina and sodium aluminate are mixed in a ball mill;
• sodium phosphate, PEG and water are added in a drum mixer and mixed until uniform;
• the green core is formed in a core box;
• drying – heating with the furnace to 300 ± 10 °C and holding for 1 h;
• firing – heating with the furnace to 1500 ± 20 °C and holding for 30 min;
• furnace cooling to 110 °C, then to room temperature.

Claimed

23–30 MPabending strength at room temperature

6–10 MPatensile strength

2–4 hoursdissolution in 40% NaOH or water at 90 °C

1500 °Cfiring temperature, 30-minute hold

What the examples showed

The examples sit within the claimed range. The first: 85% alumina, 7% quartz, 3% sodium phosphate, 2% sodium aluminate, 3% MgO, plus 3% PEG and 10% water; drying at 300 °C, firing at 1500 °C. Tensile strength 6.8 MPa, bending strength 24.4 MPa; in 40% NaOH most of the core dissolves in 2 hours, and fully with stirring.

The second: 88% alumina, 3% quartz, 3% phosphate, 2% aluminate, 4% MgO, plus 2% PEG and 12% water; drying at 290 °C, firing at 1480 °C. 8.5 and 27.5 MPa; in hot water at 90 °C most of it dissolves in 3.5 hours. PEG here is 2% although the claim requires 3–5% – a small inconsistency in the document itself.

What the patent lacks

High-temperature properties – sag and creep at pouring temperature – which for single-crystal casting is the main requirement for a core. Interaction with the alloy. Any trial casting of blades. Everything was tested on the cores alone at room temperature. Sodium salts in a core that sits in the mould at superalloy pouring temperature are a separate question: they need checking both for sag and on the surface of the cast cavity.

The patent was granted in 2016 and, according to Google Patents, later lapsed for non-payment of fees. As a reference point for your own trials with a soluble binder in an alumina core body, it is useful simply because it gives the full composition and cycle.