Superalloy Casting Grain Selectors for Weldability
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Solution Overview
Problem
Superalloys used in gas turbine components exhibit poor weldability, leading to liquation and strain age cracking in the heat affected zone (HAZ) due to grain boundaries, which is a concern for defect size requirements under operational stress and temperature conditions.
Innovation Solution
The process involves selectively growing large single grains in areas where welding will occur, eliminating grain boundaries and reducing cracking by using single crystal casting techniques and grain selectors, such as 'pig tails', during the casting operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If fusion welding is performed on superalloy components, then fabrication and repair operations can be completed, but liquation and strain age cracking occur in the heat affected zone due to grain boundaries
Solution Approach 1:
The patent applies local quality by creating a specific single crystal grain structure only in the regions where welding will be performed, while other regions of the component can maintain their conventional equiaxed or directionally solidified structure. This localized modification of grain structure eliminates grain boundaries at weld locations, preventing cracking without requiring single crystal structure throughout the entire component.
Solution Approach 2:
The patent employs preliminary action by establishing the single crystal grain structure during the initial casting process before welding operations are performed. Grain selectors are placed in the mold during casting to ensure that the regions designated for welding develop as single crystal grains, so that when welding occurs later, the material is already in the optimal crack-resistant state.
2Productivity
If multiple grains are present in the casting, then the casting process is simpler, but grain boundaries are created that lead to cracking in the heat affected zone
Solution Approach 1:
The patent uses grain selectors as intermediary elements during the casting process. These selectors are placed in the mold cavity and act as mediators that interact with the solidifying metal to promote the formation of single crystal grains in specific regions. The grain selectors enable control over grain structure without requiring complex modifications to the overall casting process or equipment.
3Reliability
If grain boundaries are eliminated by growing large single grains, then cracking in the heat affected zone is reduced or eliminated, but the casting process becomes more complex
Solution Approach 1:
The patent applies local quality by creating a specific single crystal grain structure only in the regions where welding will be performed, while other regions of the component can maintain their conventional equiaxed or directionally solidified structure. This localized modification of grain structure eliminates grain boundaries at weld locations, preventing cracking without requiring single crystal structure throughout the entire component.
Solution Approach 2:
The patent employs grain selectors that are disposable elements placed in the mold during casting. These selectors serve their purpose of guiding grain formation and are then removed or discarded, avoiding the need for complex, expensive, or permanent modifications to the casting equipment or process infrastructure.
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 approach effectively minimizes or eliminates cracking in the weld and heat affected zone, enhancing quality, reducing rework, and eliminating scrap, while meeting design requirements.
Implementation Method 1
Castings employed in gas turbines are either equiaxed, directionally solidified or single crystal in nature
Data Source
AI summary
Process for reducing cracking in superalloy metal components by selectively growing large single grains during the casting operation at a region where fusion welding will be required.