Manufacturing Techniques for Gas Turbine Nozzle Assemblies
In a gas turbine engine the nozzle assembly comes just after the combustion chamber. It directs fast moving gasses onto the turbine blades that generate the rotary motion needed for power generation. It’s a complex piece of engineering, produced in exotic alloys and manufactured to tight tolerances. Impro is one of few vertically-integrated manufacturers with the capabilities to cast, machine, assemble and inspect these parts.
Here are the steps involved.
Step 1: Investment Casting
Nickel-based superalloys provide the high-temperature strength and corrosion resistance necessary for survival in the combustion gas stream but their melting point is below the gas temperature. To overcome this potentially fatal limitation, nozzle assemblies incorporate features like cooling channels and a hollow core.
The only practical way of making parts like these is by investment casting. In this process a wax pattern replicates the part to be cast. The pattern is injection-molded with cores being incorporated that will be removed after casting to leave voids.
After attaching more wax moldings that will form the metal delivery passages, the pattern is coated with a slurry that dries to form a hard ceramic shell. The wax is then melted out with the cavity that remains forming the part mold. Metal is melted and poured into the mold under vacuum to minimize the occurrence of casting defects. Once the metal has solidified the shell and cores are broken apart to leave the cast parts.
Step 2: Precision Machining
Investment casting is a near net shape process capable of holding tight tolerances and producing smooth surfaces. Despite this, some machining is needed to create tightly toleranced mounting points and interfaces.
5 axis CNC machining eliminates the need for multiple setups, which improves precision. Some types of 5 axis milling machine can angle the workpiece as it’s being processed in order to produce complex contours.
Step 3: Assembly
Components are joined by either electron beam welding (EBW) or vacuum brazing. EBW focuses energy into a very small area, which reduces the heat-affected zone around the joint. It’s performed in a vacuum to prevent the beam being scattered, which also eliminates oxide formation and weld embrittlement. The EBW process is highly automated, which results in good part-to-part consistency.
In brazing separate metal pieces are joined with a lower melting point filler metal. This filler is selected for its ability to form strong bonds with the metal of the components. It’s applied as a paste to the join with the components clamped precisely in position. The assembly then moves into an oven where the filler melts to form the bond. Brazing under vacuum avoids the need for an oxidation-preventing flux.
Step 4: Heat Treatment
Heat treatment is used to increase strength, hardness and toughness, and also to reduce brittleness and relieve stresses. It’s often used with superalloys to address segregation caused by differing melting points of the elements in the alloy. While listed here as Step 4, heat treatment may also be performed before machining to improve machinability.
Step 5: Inspection
Rigorous process control and a QA system certified to the aerospace AS9100 standard minimize the occurrence of defects, but inspection provides complete assurance of quality. In addition to visual and dimensional checks, this can include crack detection for surface flaws and X-ray imaging for internal defects.
Your Partner For Manufacturing High Engineered Products
Impro is a vertically-integrated manufacturer with the expertise, resources and capabilities necessary to produce highly engineered components like those used in gas turbines. Whether you’re looking for gas turbine components or need other complex parts made in difficult or unusual alloys, we can help. Contact us to discuss your needs.