Rejuvenating Coated Superalloy Components via Selective Grit Blasting
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Solution Overview
Problem
Current methods for removing and repairing diffusion aluminide coatings on superalloy components, such as those used in gas turbine engines, often result in substrate alloy depletion and altered airflow characteristics, leading to component scrapping due to the need for complete removal of the coating layers.
Innovation Solution
A method involving controlled grit blasting to selectively remove a portion of the additive layer of a diffusion aluminide coating, followed by air blasting and application of a new aluminide coating using a gel process, with diffusion heat treatment to form a rejuvenated protective coating, which maintains the original chemistry and microstructure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of repair
If complete removal of diffusion aluminide coating is performed using acid strip, then the additive layer is removed, but the diffusion zone is also removed causing substrate alloy depletion and altered airflow characteristics
Solution Approach 1:
The coating removal process is segmented into selective removal of the additive layer while preserving the diffusion zone. This is achieved through controlled grit blasting parameters that target only the outer additive layer, creating a stepped profile that maintains the protective diffusion zone and prevents substrate alloy depletion.
Solution Approach 2:
The treatment is applied locally to only the additive layer with different intensity than the diffusion zone. The grit blasting parameters (pressure, media type, angle) are optimized to selectively remove aluminum-rich additive layer while leaving the intermetallic diffusion zone intact, creating a localized treatment effect that preserves critical substrate material.
2Ease of repair
If complete removal of diffusion aluminide coating is performed using multiple grit blastings, then the coating is removed, but excessive time and labor are required
Solution Approach 1:
Instead of complete removal, only partial removal of the additive layer (25-75% thickness) is performed. This partial action is sufficient to expose the diffusion zone and create proper surface profile for recoating, eliminating the need for multiple sequential blasting operations and significantly reducing repair time.
Solution Approach 2:
The traditional multi-step mechanical removal process (multiple grit blastings) is replaced by a single optimized grit blasting step followed by chemical etching. This substitution reduces process complexity and time while achieving the same coating removal objective.
3Ease of repair
If complete removal of diffusion aluminide coating is performed using acid strip, then the coating is removed, but harsh chemicals and special facilities are required
Solution Approach 1:
A chemical etching solution serves as an intermediary that selectively removes the additive layer after grit blasting. This intermediate chemical step completes the removal process without requiring the extensive acid strip facilities, using milder chemicals that can be applied in standard shop environments.
4Loss of substance
If controlled grit blasting is used to selectively remove portion of additive layer, then substrate material is preserved, but precise control of removal depth is required
Solution Approach 1:
The grit blasting process incorporates feedback through visual inspection and surface profile measurement to control the removal depth. The process is stopped when the characteristic stepped profile is achieved, with the diffusion zone visibly exposed but not eroded, providing real-time feedback that ensures precise control without requiring complex measurement equipment.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method allows for the rejuvenation of superalloy components with reduced time, labor, and cost, while preserving the original coating's chemistry and microstructure, maintaining dimensional and airflow requirements, and minimizing wall thickness loss compared to full-stripping repairs.
Implementation Method 1
selectively removing the oxide layer and a portion of the additive layer by grit blasting
Implementation Method 2
The MAl intermetallic is the result of deposited aluminum and an outward diffusion of iron, nickel and/or cobalt from the substrate
Implementation Method 3
diffusion heat treating at a preselected elevated temperature to form a rejuvenated protective aluminide coating
Implementation Method 4
During high temperature exposure in air, the MAl intermetallic forms a protective aluminum oxide (alumina) scale or oxide layer that inhibits oxidation of the diffusion coating and the underlying substrate
Data Source
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AI summary
A method (400) for controlled removal of a portion (52) of a diffusion coating from a coated superalloy component (10) and a method (500) for rejuvenating a coated superalloy component (10) are provided. The methods (400, 500) include providing (401, 501) the component (10) having an oxide layer (40), an additive layer (50) between the oxide layer (40) and a diffusion zone (60), the diffusion zone (60) being between the additive layer (50) and a superalloy substrate (70) of the superalloy component (10). The methods (400, 500) include selectively removing (403, 503) the oxide layer (40) and a portion (52) of the additive layer (50) by grit blasting, wherein removing creates an exposed portion (56). Rejuvenating (500) includes applying (505) an aluminide coating to the exposed portion (56) and heat treating (507) at a preselected elevated temperature to form a rejuvenated protective aluminide coating (90) on the superalloy component (10).