The heart of the axial flow turbine engine is the combustion chamber. This is where fuel and air are mixed and ignited, with air velocity determining how far through the chamber the flame front extends.

Parts used in the combustion system need a complex combination of properties. Creep resistance, corrosion resistance, fatigue resistance and high-temperature strength are the most important, although gas turbine engineers could doubtless list more. Accordingly, these parts are produced in alloys that are difficult to cast and machine, which makes their manufacture a challenge. Here’s how we do it.

Alloys for Combustion Parts

Alloy selection is dictated by the properties needed. Nickel and cobalt-based superalloys like Inconel, Nimonic, Haynes and Mar-M are used when creep and corrosion resistance are paramount, but stainless steel and titanium have roles to play too.

These metals have high melting points, which pose challenges for the foundry casting them. Some also need vacuum melting and pouring to minimize impurities and defects. For most, machinability is poor, due to the combination of high strength, hardness and toughness.

Manufacturing Requirements for Part Function

Weight minimization and effective control of gas flow are two of the highest priorities in part design. These result in intricate geometries with thin walls and complex internal spaces. Smooth gas flow requires excellent surface finish and precisely defined curves.

Some of these features can be machined into the parts after casting. Others can only be produced in the casting process, and for a significant subset there’s a choice to be made between casting and precision machining.

Casting Processes for Combustion Parts

These parts are usually cast by:

• The permanent mold process
• Shell molding
• Investment casting

Permanent mold casting is generally restricted to high volume production of small parts with lower melting points. Shell molding and investment casting can both handle high temperatures but have significant differences.

Using resin sand applied over a steel pattern, shell molding is limited in its ability to reproduce fine detail and internal voids. Shell molding also needs a draft on vertical walls, which is not the case for investment casting.

Investment casting, where a ceramic shell is built up over a wax pattern, has a greater ability to reproduce fine detail and thin walls. Surface finish is better than shell molding and the process can maintain tighter tolerances.

Heat Treatment

Depending on the exact treatment applied, this can increase surface hardness, raise toughness and make parts less brittle. Heat treatment is also sometimes used to reduce stress induced by machining.

Precision Machining Combustion Parts

While turning and milling are sufficient for some simple parts like flanges and brackets, many b from 5-axis machining. This improves part accuracy and surface finish, and depending on machine type, can enable milling of complex contours.

Inspection

The demands placed on combustion parts, coupled with their complexity and low factors of safety, make inspection an important part of the manufacturing process. Parts can undergo magnetic particle or fluorescent dye inspection to find surface cracks and X-ray analysis to detect internal defects. Where part function requires it, pressure testing is carried out to assure integrity.

Welding and Brazing

Fuel system components, swirl vanes and other combustion system parts often need assembly. This usually involves welding or brazing. Both processes require strict control to ensure the joints are free from defects that would otherwise compromise strength and durability.

Casting and Precision Machining

If you’re looking for complex parts cast in challenging alloys, Impro Aerotek has the experience and capabilities you need. As a leader in investment casting, we pour a wide range of alloys, and we have the precision machining and assembly resources to finish your parts to the standard required. We have aerospace quality certifications and a commitment to satisfying the expectations of our customers.

To learn more about what we do, or to discuss your needs for aircraft engine and APU combustion parts, contact us.