Aerospace turbine blades

01

Choose the alloy and crystal structure

A TMS compilation of nominal cast superalloy compositions (wt%, nickel balance) lists CMSX-4 as a single crystal alloy with 6.5 Cr, 9 Co, 0.6 Mo, 5.6 Al, 1 Ti, 6.5 Ta, 6 W, 0.1 Hf and 3 Re, hardened by about 70 vol% gamma prime; René N5 with 7 Cr, 8 Co, 2 Mo, 6.2 Al, 6 Ta, 5 W, 0.2 Hf and 3 Re; PWA 1484 with 5.0 Cr, 10.1 Co, 1.9 Mo, 5.7 Al, 8.7 Ta, 5.9 W and 3.0 Re (no Ti); and the equiaxed casting alloy Inconel 738LC with 16 Cr, 8.5 Co, 1.7 Mo, 3.5 Al, 3.5 Ti, 1.7 Ta, 2.5 W and 2 Nb. The same table lists CM186LC as a rhenium-bearing alloy designed for use in the directionally solidified form. [1]

A NASA Glenn technical memorandum states that single crystal nickel-base superalloy turbine blades can have much higher creep and fatigue resistance than polycrystalline superalloys because weak grain boundaries are removed and because the elastic modulus is lower in the [001] orientation, the primary loading direction. Single crystal airfoils are grain etched and inspected before and after heat treatment to confirm that crystal orientation and single crystal defects — low angle boundaries, higher angle boundaries, freckles and new grains — stay within acceptability limits. It describes CMSX-4 as used in a variety of aerospace gas turbine engines as turbine blades and vanes. [2]

The same report warns that plastic deformation from machining, handling or mold stresses during casting can cause new grains to form (recrystallization) during later heat treatment or service: studies found plastic strain exceeding 1 percent sufficient, with recrystallization at temperatures above 1,200 °C. For CMSX-4 the full solution treatment ramps slowly to about 1,310 °C; the double aging treatment is 1,150 °C for 4 h and 871 °C for 20 h. [2]

Editorial note: ask the foundry to state alloy specification, crystal structure (single crystal, directionally solidified or equiaxed), allowed primary orientation deviation from [001], heat treatment route and how recrystallization after machining is checked.

Give the supplier the engine model and stage, part number and drawing revision, alloy and material specification, crystal structure and orientation limits, heat treatment, cooling configuration, coating system, inspection methods and acceptance standards, certification basis (type-certificate holder or PMA) and quantity.

02

Specify cooling, coating and inspection

In NASA Lewis turbojet tests, coating systems of plasma-sprayed zirconia stabilized with yttria, magnesia or calcia over a thin alloy bond coat were applied to air-cooled rotating blades. The coatings were in good condition after up to 500 two-minute cycles between full power at a gas temperature of 1644 K and flameout, or up to 150 hours of steady-state operation at gas temperatures up to 1644 K with 35 start and stop cycles; on durability and processing cost, yttria-stabilized zirconia was judged the best of the three. [3]

For the cyclic-test blades, airfoil surfaces and platforms were grit-blasted with white alumina; a NiCrAlY bond coat was sprayed to 0.010 ± 0.005 cm with a measured roughness of 5 micrometers rms, followed by stabilized zirconia to 0.038 ± 0.008 cm. Coatings were applied first to the leading edge, then the trailing edge, then the pressure and suction surfaces, because seams along small radii can lead to coating cracking. [3]

Editorial calculation: 1644 K − 273 = about 1,371 °C gas temperature, and 0.038 cm = 380 µm of ceramic over 0.010 cm = 100 µm of bond coat.

A Polish Maritime Research review of gas turbine blade diagnostics notes gas working-medium temperatures of 1100 to 1600 K depending on blade material, cooling and coating, and identifies overheating and thermal fatigue as the main causes of blade damage, starting with failure of the protective coating. It describes X-ray computed tomography as showing the actual thickness of inner walls, internal geometry against tolerance, cracks and blocked internal canals, but only with the blade removed and using a costly CT scanner. [4]

Editorial note: the reviewed sources do not give fluorescent penetrant or film radiography acceptance criteria; ask the supplier which penetrant sensitivity level, radiographic method and CT wall-thickness limits apply, and for the cooling-hole airflow check.

03

FAA material, containment and PMA rules a blade supplier must meet

Wuxi Turbine Blade Co., Ltd. has a product page for aviation turbine blades; the FAA rules below set what its approval basis, or any other supplier's, has to cover.

14 CFR 33.15 requires engine materials to have suitability and durability established by experience or tests and to conform to approved specifications. 14 CFR 33.19 requires turbine rotor cases to contain damage from rotor blade failure, and 14 CFR 33.94 requires engine tests showing containment, without fire or mount failure, for at least 15 seconds after failure of the critical turbine blade at maximum permissible r.p.m., chosen by blade weight and case strength. [5]

For a replacement part under Parts Manufacturer Approval, 14 CFR 21.303 requires the applicant to submit drawings and specifications, dimensions, materials and processes, test reports and computations showing airworthiness, and to make inspections and tests showing that materials conform to the design specifications and that the manufacturing processes conform to those specified. [5]

When requesting quotations, ask each supplier for its approval basis (type-certificate holder production approval or PMA), material certificates, casting and coating process specifications, inspection records and first-article reports.

04

Airfoil, platform, root, cooling and coatings

05

Temperature, alloy and inspection data for a blade quote

Published values below come from the NASA, TMS and Polish Maritime Research sources; chord, weight, fatigue, creep and conductivity data have to come from the drawing and the foundry's test reports.

06

Send the engine, stage and approval basis together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Engine and stage: [model; HPT/LPT stage; part number; drawing revision]
Alloy: [grade; specification; crystal structure; orientation limit]
Heat treatment: [solution; aging]
Cooling: [passages; airflow check]
Coating: [bond coat; TBC; thickness]
Inspection: [grain etch; penetrant; X-ray/CT; acceptance standard]
Approval: [TC holder; PMA]
Quantity: [blades]

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