Determining Shrink by Thermal AnalysisDavid Sparkman August 2014

The ProblemShrinkage in medium to large castings is a constant (common) issue in foundries around the world. There are multiple causes: gating, risering, pouring temperature, graphite growth/ and shape, nucleation, chemistry, oxide films, and a host of other unknowns. For this informational paper, let’s assume that on some days we make good castings, and on other days we don’t, using on the same pattern. That removes the issue problem of the gating and risering design. The problem then comes down to the iron: it’s its temperature, chemistry, cleanliness, and degree of inoculation/modification. What can Thermal Analysis tell us about the tendency of the metal to shrink on a day to day and hour to hour basis?

Can Thermal Analysis Predict Shrink?Some time ago both MeltLab® and NovaCast’s ATAS® systems found an interesting correlation between with the shape and size of the solidus arrest and shrinkage tendency. NovaCast, which used massive correlation analysis on the problem, found that a higher rate of cooling at the solidus point meant less shrinkage. Of course the math does not tell explain the reason for why the correlation. We at MeltLab were puzzled about this strange relationship— the strong endothermic reaction of solidus being good, while an earlier endothermic reaction preceding the solidus was indicating a sure sign of shrinkage and bad. So we have two endothermic reactions: one good and one bad. One means shrink did happen and one means shrink didn’t happen.

We at MeltLab identified the first endothermic arrest as actual shrink, and the second as only the buildup of stress in the casting. Actual shrink reduces the (good) residual stress in the casting, whereas a lot of stress energy in the solidus shows that shrinkage didn’t happen. So what we did was to measure the stress energy of the solidus by using the Calculus method of integration and compare it to an expected value. If the stress energy falls below the expected value we have shrink; if not we have good metal.

This was all good, and we felt that we had a solid way of measuring shrink tendency. In some cases we could even see the shrink arrest happening, though it was always very small. (carriage return 2x)

Then we got into the magnification effect of the 2nd derivative…

Magnification 100x and 1000xDerivative Thermal Analysis is a way of magnifying small energy producing or energy adsorbing events in a sample of metal. If you Let’s consider that the basic thermal analysis temperature curve of iron has its critical range from say 2300 down to 1900 degrees Fahrenheit. That is a range of 400 degrees.

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The first derivative or cooling rate has its critical data from about 4 degrees per second down to 0 degrees per second. That is a range of 4 degrees per second or a magnification factor of 100 times. It becomes 100 times easier to see events in the cooling rate than in the temperature curve. This is also the magnification commonly used with microscopes to determine overall structure of iron castings.

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Taking this a step further, we can progress to the 2nd derivative with a magnification factor of 10 times the 1st derivative or 1,000 times magnification of the temperature curve. Some shrink is evident as well as the graphite growth arrests of a 95% nodularity.

The progression of Thermal AnalysisFoundry Thermal Analysis has concentrated on just the two major arrests found in the temperature curve: liquidus and eutectic. The eutectic was for a long time misnamed as the solidus by ElectroNite® and Leeds & Northrup®. With MeltLab’s introduction in 1991, we added the end of freezing (solidus)

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and the first derivative. NovaCast followed suit adding the 1st derivative, and ElectroNite, after many years, got around to properly naming it the eutectic, and grudgingly included the solidus point.

We at MeltLab (believe we?)have finally reached a full understanding of most of the arrests of Iron and the major Aluminum alloys that show up in the temperature curve and the 1st derivative. The important ones are: (carriage return)

Iron Chemistry Aluminum Chemistry

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Carbon Equalivant Carbon Silicon Silicon

Iron Liquidus Aluminum LiquidusStart of Liquidus Start of LiquidusLiquidus LiquidusIntegrated energy of Liquidus Integrated energy of Liquidus

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Liquidus undercooling/recalesence Liquidus undercooling/recalesence Liquidus coherency pointDendritic Arm Spacing (hypoeutectic)CuP Silicon grain refinement (hypereutectic)TiB Grain refinement (hypoeutectic)

End of Liquidus End of Liquidus

Iron between Liquidus and Eutectic Aluminum between Liquidus and Eutectic

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Pro-eutectic carbides Beta crystal temperature Beta crystal integrated energy

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Iron Eutectic Aluminum EutecticStart of Eutectic arrest Start of Eutectic arrestEutectic undercooling/recalesence Eutectic undercooling/recalesence Nodularity (ductile) Eutectic modification (suppression)Gray Iron Gas GasEnd of eutectic arrest End of eutectic arrest

Iron Post Eutectic Aluminum Post Eutectic

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Nodule count (ductile) Copper arrest temperature Post eutectic carbides Copper arrest integrated energy

Magnesium arrest temperatureMagnesium arrest integrated energy

Shrinkage indicator Shrinkage indicatorShrinkage integration

Solidus/End of freezing Solidus/End of freezing Steady State Cooling of solid Steady State Cooling of solid

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Cup fullness Cup fullness

Direct Measurement of Shrinkage in AluminumAluminum is very prone to shrinkage and often, the shrinkage event and energy can be seen in the Rate of Cooling curve (1st derivative). Going(Taking the data) to the 4th derivative to pick up where the “going negative” zero passes are, we can bound bind the arrest, calculate the end points and integrate

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the energy. “Zero pass” refers to the derivative passing through a scale value of zero. We can also take the “going positive” zero pass to find the strong point of the arrest. (moved “zero past” definition up)

For the purist(? May be instead “for the more curious, you may notice”), please note that on the black curve to the right, there is a sudden increase in stress above background just before the shrink is triggered (blue arrow). If we knew what that was, we would understand shrink better.

New Territory

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Moving on to the (Looking at the) second derivative opens up provides additional information on the microstructure of the casting. The zero crossing point of the 2nd derivative often indicates the strong point of a crystallization event (exothermic) or a shrinkage event (endothermic). From there the third and fourth derivatives can pinpoint the beginning and end of such events allowing integration of the event’s energy.

Quite possibly But the most attractive characteristic of the second derivative appears to be its ability to characterize the form of shrinkage in both iron and aluminum castings.

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Shrinkage in Ductile IronThe indicators of shrinkage in ductile are counter intuitive so some explanation is required. Shrinkage is the natural state of ductile iron in the temperature range of 1900 degrees. The steel matrix of the casting has lost volume faster than the graphite has grown. Most of the matrix volume change occurs when the casting goes from liquid to solid. The graphite on the other hand continues to grow down to the eutectoid temperature where it finally “catches up” with loss of volume of the steel. At that point the graphite volume is approximately 10% while the matrix shrinkage is about the same. 10%.

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As stresses build up in the casting, there are two things that can happen: either the stress concentrates and a void is formed (which relieves the stress permanently) or the stress moves into the grain boundaries and remains defuse through the casting until the further growth of the graphite compacts the grain boundaries and removes the stress. So residual stresses at the solidus point are beneficial, and but relieving stresses have been relieved by void formation are is not beneficial. The energy to form the void is subtracted from the residual stress leaving less stress.

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In the examples below, the first two curves indicate high residual stresses but no void formation (shrinkage). The remainder all show some degree of stress relief, some drastic, and some minor. Those with drastic quick release (we believe) are most likely larger voids while the ones with just a flattening (#6 and #8) are probably fine micro-shrinkage. Good curves with no shrinkage are #1 and #2. Very bad shrinkage is shown in #3, #5, and #9.

2nd Derivative curves of the Solidus arrest showing various levels of shrinkage

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1 2 3 4 5

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6 7 8 9

What can be done about shrinkage?

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Now that there is a tool to directly look at the shrinkage tendency of a metal, the question is what can be done to reduce shrinkage? What role do tramp elements play by reducing the solidus point and extending the freezing time of the grain boundaries? Do Rare Earth Metals help? We don’t know. But now we do now have a tool to measure the cause and effect. and we feel confident that one of our users will unlock more of these mysteries.