How do you correct your melt in order to control stability ? What is shrinkage casting defects potential in the melt ? In this article, we’re not referring to gating design, casting geometry or casting procedures. Today, let’s discuss the metallurgical shrinkage tendency of the melt itself. David Sparkman at MeltLab presented this theory at the Ductile Iron Society last October 2015.
Last year, we partnered with MeltLab because it’s the only thermal analysis system we know that works with the higher order derivatives ( 3,4,5th ). Their trade secret ( smoothing technique ) allows magnifying the data by more than 10,000 times to measure solidification properties of the melt. Here are some example :
For iron : Shrinkage, nodularity, nodule count, recalescence, carbides, etc.
For Aluminum : Shrinkage, modification efficiency, inoculation efficiency, copper phase for heat treatment, etc.
In this paper by David Sparkman, it’s explained that shrinkage appears as an endothermic reaction on the cooling curve because stress is relieved to form a void ( micro, macro or suck-in ). MeltLab measures this bump and provides you a value. Then, your furnace or pouring operator’s job is to correct the melt with the right additions, which MeltLab calculates for him, so he can pour a melt within your quality ranges. That’s how we manufacture consistent castings.
Figure 1 : Cooling rate ( green ) and its derivative ( blue ) from a thermal analysis iron sample calculated by MeltLab. By using higher order derivatives, we can calculte the % energy released by the creation of a void due to metallurgical shrinkage through stress relief. That’s how you can correct your melt before pouring it, and stay in your quality range.
Now, I invite you to read this newsletter published by David Sparkman last week ( click for full article ):
Although iron and aluminum are quite different from each other, there are many common physical reactions that can teach us something about the physics that is driving shrinkage. Here is what we now understand about the relationship of stress and shrinkage:
1. Stress can be thought of as a low pressure area of the castings tending toward a partial vacuum.
2. Shrinkage/stress cannot occur until the casting is rigid enough that metal cannot easily flow to the higher stressed areas.
3. Stress at an atomic level is when the atomic bonds between atoms are elastically stretched without actually breaking.
4. Internal shrinkage occurs on an atomic level when the atomic bonds between atoms are broken thus forming an interior surface within the casting.
5. External shrinkage occurs when an external wall of the casting is pulled in (suck-in) by the pushing of atmospheric pressure outside the casting while the partial vacuum inside the casting is strong enough to bend the casting wall inward.
6. Both stress and shrinkage absorb energy and so are endothermic reactions on the cooling curve of thermal analysis.
7. A shrinkage occurrence reduces the residual stresses in the casting. This reduced residual stress can be used to discover the degree of shrinkage – at least in iron. We are still looking at this for aluminum.
8. Consolidating the shrinkage into one major shrinkage hole is clearly undesirable and degrades the physical properties of the metal, i.e. tensile, yield, and elongation.
So what do you think about this stress / shrinkage concept ? Several foundries produce better castings because of this. Their gating design and casting design work better because they have a more consistent metal filling the mold melt after melt.
To conclude, here’s an article on avoiding shrinkage defects by D.White published in the 2012 AFS transactions. MeltLab helps foundries with points 1 to 5 of the list below.
- Maximize the Graphite Precipitation Expansion Effect Without Nodule Flotation
- Time the Carbon Precipitation Expansion Effect Correctly
- Keep Base Sulfur Content Consistent
- Avoid Long Hold Periods
- Use Special Nodulizers to Avoid Shrinkage
- Increase Freezing Rate Rather Than Risering
- Produce Uniformly Strong, Rigid Molds
Thank you for supporting us at SF with our projects. Since last week we’re receiving an awesome demand for our casting defect mobile app. Metallurgists from around the world started to contribute to the app content with pictures and comments. Next article will be on controlling greensand moisture or how SF works with mechanical design engineers who buy from foundries.