Higher temperatures and pressures increase gas turbine efficiency and reduce emissions, but at a cost. Components downstream of the combustors suffer more fatigue and wear, resulting in shorter lifetimes and more repair work. Gas turbine component manufacturers are responding with advances in how these parts are made.

A Challenging Environment

In a gas turbine engine, continuous combustion generates gases that drive a series of turbines. These gases are around 2,300°F (1,260°C) and include corrosive compounds like NOx and SOx. Steel lacks strength at these temperatures, so nozzle components are made from creep-resistant superalloys.

A second issue is that fast-moving, corrosive gases erode surfaces they pass over. This changes dimensions and reduces performance, requiring frequent engine rebuilds.

Today, gas turbine manufacturers are improving efficiency and reducing emissions by raising combustion temperatures and pressures. This makes the environment even more challenging for the components used.

Investment Casting for Cooling Features

Superalloys have creep and corrosion resistance, but their melting point is below the temperatures in the engine. This is addressed by feeding cooler air into and over the surface of the aerofoil sections in the nozzle assemblies.

To achieve this, each hollow section is pierced by small holes that let air fed into the interior bleed out over the exterior surfaces. Careful design of the aerofoil profile and precise hole positioning result in a cooler boundary layer that protects the nozzle components from the worst of the combustion gases.

High strength makes superalloys difficult to machine, so these cooling features are produced by investment casting, (and sometimes by electro-discharge machining (EDM.))

In investment casting a wax pattern is used to create a mold. This is given a wet slurry coating that dries to form a hard shell. The wax is then melted out, leaving a cavity for metal to fill.

Cores placed in the wax pattern form hollow regions and small passages. These remain in place when the wax is melted and dissolve after the shell is broken apart.

Advances in Manufacturing Technology

Additive manufacturing, sometimes called 3D printing, is revolutionizing low volume production of metal parts. In laser powder bed fusion (LPBF), parts are built by depositing thin layers of metal powder, then selectively sintering by laser.

LPBF is especially effective for parts with complex internal features, like those needed for fluid flow. This gives designers a way to consolidate what were previously separate components into a single piece.

Advantages of LPBF include:

• Reduced piece counts
• Weight reduction
• Lead time reduction

However, the process has some limitations. Slow build speed, combined with the cost of the machines and metal powders, makes it expensive. Furthermore, to assure customers of the integrity of the part, CT scanning is often required.

Applying Additive Methods to Gas Turbine Nozzle Assemblies

While technically feasible to “print’ these parts, economics and the difficulty of obtaining powders that provide the necessary metallurgy, make it less attractive than investment casting. However, an alternative approach is to “print” the wax patterns and cores.

Conventionally, these are produced by injection molding. While a quick process, the need to release the pattern from a mold imposes design limitations. Additive processes can create overhangs and re-entrant features that are otherwise impossible to achieve. For increased part complexity, a pattern can be assembled from multiple wax pieces.

Build time is slower than using injection molding, and striations (lines resulting from building layer-by-layer), must be removed from the surface of the pattern. However, as the part itself is still produced by the proven investment casting method, concerns over alloy availability and structural integrity are addressed.

Minimize Risk: Partner With Impro

The pursuit of higher efficiency and lower emissions is creating challenges for manufacturers of gas turbine components like nozzle assemblies. Superalloys, already at their limits, need even better cooling if acceptable lifetimes are to be achieved.

Investment casting, already the preferred manufacturing method, can help. Where wax patterns are not feasible, owing to molding challenges, additive processes provide an alternative that lets gas turbine users stick with tried-and-tested alloys rather than switching to new and relatively unknown metal powders.

As a proven supplier of gas turbine engine components, Impro understands the challenges and concerns. Contact us to discuss your nozzle assembly needs.