Directional Solidification Mould Notch for Grain Boundary Control
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Solution Overview
Problem
In directional solidification casting, thermal strains cause dendrites to bend, leading to misorientation and the formation of undesirable grain boundaries, especially in smaller diameters and sharply angled mould features, which can result in secondary growth dominance and compromised component integrity.
Innovation Solution
A mould with a sacrificial geometry portion featuring a notch that contains axial dendritic growth, positioned to manage dendritic growth direction and reduce secondary grain structure competition, allowing controlled solidification and eliminating the need for a spiral 'pig-tail' grain selector.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If directional solidification is used to create single crystal structure, then component mechanical properties are improved, but thermal strains cause dendrite bending and secondary grain growth that compromises integrity
Solution Approach 1:
The invention converts the harmful effect of thermal strains causing dendrite bending into a beneficial outcome by designing a sacrificial geometry that intentionally accommodates and directs the bent dendrites into a designated zone. The notches in the sacrificial geometry guide the secondary grain growth into controlled regions that will be removed, thereby eliminating the harmful effect while maintaining the primary single crystal structure integrity.
Solution Approach 2:
The sacrificial geometry acts as an intermediary element between the primary grain structure and the mould walls. It provides a buffer zone with notches that intercept and contain the bent dendrites and secondary grain growth, preventing them from compromising the primary single crystal structure while being easily removable after casting.
2Manufacturing precision
If cylindrical tube grain selectors are used to block secondary grain growth, then primary grain growth is protected, but dendrite bending increases and misorientation reaches up to 15°
Solution Approach 1:
The invention extracts the problematic function of cylindrical tube grain selectors entirely from the system. Instead of using rigid tubes that cause dendrite bending, the patent employs a sacrificial geometry with notches that passively accommodates bent dendrites without imposing geometric constraints that cause misorientation.
Solution Approach 2:
The invention changes the geometric parameters of the grain control mechanism from rigid cylindrical tubes with fixed diameters to a flexible sacrificial geometry with notches of varying shapes and positions. This allows the system to adapt to dendrite bending without causing additional misorientation, as the notches can be designed to match the expected dendrite paths.
3Manufacturing precision
If smaller diameter cylindrical tubes are used in grain selectors, then secondary grain growth is better blocked, but dendritic bending is accentuated and misorientation increases
Solution Approach 1:
The invention replaces the complex, precision-engineered cylindrical tube grain selectors with a simple, disposable sacrificial geometry made from inexpensive materials like wax or plastic. This sacrificial geometry is removed after casting, having served its purpose of guiding dendrite growth without causing bending issues.
Solution Approach 2:
The grain control function is segmented into multiple notches within the sacrificial geometry rather than relying on a single continuous cylindrical tube. This segmentation allows each notch to independently manage dendrite growth in specific regions, reducing overall complexity while maintaining effective grain structure control.
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
The notch design significantly reduces secondary grain structure occurrence, enhancing the integrity of cast components by maintaining the primary grain structure orientation and preventing cracking during mould removal, thus improving the mechanical properties of cast components like turbine blades.
Implementation Method 1
The starter blocks 4b sit on a chill plate 5 which is maintained generally at a temperature below the melting point of the material M creating a temperature gradient from the bottom to the top of the moulds 3
Implementation Method 2
The combination of the grain selector 4a and starter 4b with controlled cooling encourages growth of a single crystal structure in the solidifying casting
Implementation Method 3
During the pouring process, the moulds 3 are enclosed by a heat source 6 which encircles the cup 1 and the array of moulds 3
Implementation Method 4
a seed crystal having the required directional dendritic growth is located in a sacrificial portion of the mould and is partially melted
Implementation Method 5
crystal growth in terms of dendritic growth during solidification will converge upon a wall of the mould or diverge from the wall
Implementation Method 6
Thermal strains experienced by the primary dendrites as they grow through the grain selector can result in clumps of dendrites within the primary grain growth bending away from the initial direction of growth
Data Source
AI summary
A mould for casting a component in a directional solidification casting process having a preferred direction of grain growth (non-axial <001>) comprises a shell defining a cavity for receiving molten material. The cavity defines a three dimensional shape made up of a finished component geometry portion (42, 43, 44) and a sacrificial geometry portion (45) wherein the sacrificial geometry portion (45) includes a notch (48) which is shaped and positioned so as to, in use, contain high angle grain boundaries between dendritic growth in the preferred direction (non-axial <001>) and dendritic growth in a competing direction to the preferred direction (axial <001>) within the sacrificial geometry portion of a casting solidifying in the mould.