Hot Topics on Thermal Analysis by David Sparkman
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Bonus August 2010 Quantifying an Arrest in Thermal Analysis
Quantifying and arrest means to take the measure of the arrest - putting a number or, in this case, several numbers on an arrest.
From the Paper
So, although there is a great deal of information available in a single arrest, it takes a very well-trained person looking carefully at the curve to
see it. What is needed is a way to extract the information in a reliable way automatically. So we introduce Calculus, a form of mathematics invented
by Newton and Leibniz, to the system. Calculus is the mathematics of change, the study of acceleration and deceleration (derivatives), as well as the
summing of complex accelerations (integration) which can be used to calculate the heat energies and then derive the percent of different microstructures.
Conclusion
These tools then should allow a more thorough investigation of the thermal analysis of metals. They do not in of themselves tell us what each arrest is.
That is the undertaking of metallurgists with other tools such as phase diagrams, microscopes, spectrometers, etc. Further, the rates of the reactions
can be explored and equations calculated to estimate grain/cell counts, dendritic arm spacing, or, in ductile iron, nodule count.
August 2010 Measuring Copper and Magnesium-silicide microscopic phases in Aluminum
Microscopic events down to 0.1% and perhaps lower can be measured using thermal analysis!
From the Paper
Copper is commonly used in the 319 alloys to provide matrix strength through heat treating. Most of the copper precipitates out
during the casting’s solidification, but some copper remains in the matrix as xxx. Typical composition in total copper is 3 to 4%.
During heat treating, the casting is heated up to just below the solidus temperature and copper is absorbed from the grain boundaries
back into the casting. On cooling, the copper stresses the matrix and prevents movement of the aluminum giving it added strength.
Magnesium is added to aluminum principally to provide hard spots in the aluminum for wear resistance. It forms a complex alloy
called magnesium-silicide. (Magnesium also has a limited effect on blunting the tips of Beta crystals.) Typical ranges are 0.1%
to 0.5%. Using some of the same techniques described last month (5th derivative), we have taught the program to locate the start
and stop points of these arrests.
Measuring Microscopic phases using Aluminum Beta Crystals as an example July 2010
Microscopic events down to 0.1% and perhaps lower can be measured using thermal analysis!
From the Paper
...giving a standard deviation during the arrest of 5.1 sigma caused by about 0.4% iron. Clearly this is a real event and not noise.
The obvious omission in this mathematics is the specific heat of the alloy verses the specific heat of the beta phase.
The problem is that these specific heats are unknown at the present time. Our solution is to provide a ratio factor where
the customer can compare the MeltLab percentage of energy to what is found under the microscope and enter in a correction
factor for specific heat. Since these ratios should remain consistent for any alloy class, we can move the science of
thermal analysis forward.
Balancing graphite growth and shrinkage in ductile iron June 2010
Though Ductile Iron is a mirical metal, it has a weak spot - shrinkage. This paper explains why as never before.
From the Paper
Based on these numbers let's look at what is happening. When the iron first changes from liquid to solid it goes
through what is called a phase change (the way the atoms arrange themselves) and, typical of phase changes, it goes
though a volume change as well. It appears from my data to be about a 5% volume change . The graphite that forms
at this time only occupies 4.3% of the casting volume leaving a deficit of about 0.7%. This deficit results in the
stress arrest we see. The graphite growth will not catch up with the volume lost until about 1880 degrees F and so all
internal shrink has occurred before this temperature.
Measuring Shrinkage in Ductile Iron by Thermal Analysis May 2010
This is the first scentific explaination of how shrinkage relates to the stress of the grain boundaries in thermal analysis.
Forget what you have heard before and see how new metrics can help you control shrinkage.
From the Paper
The problem of shrinkage has haunted pattern engineers for ages. Is it the pattern or is it the iron, was the argument
I had many a time when I was a Quality Control/Plant Metallurgist with Dana. Looking in retrospect, it was mostly the iron.
The concept of timing graphite growth with the freezing off of the in-gates was not well understood at the time and we made
a lot of solid non-feeding risers. So let's go over the concepts and see how thermal analysis can help.
Metrics of Thermal Analysis part 2 – Measuring Inoculation April 2010
This is a continuation of how inoculation levels can be measured by thermal analysis derivatives. It comes down to
how fast a reaction starts up or dies out. And that depends firstly on the number in inoculation sites and therefore
on the distance atoms have to diffuse to reach a nucelation site. We call it the diffusion path length and it is indicated
by the strongest point in the 2nd derivative during the starting or stopping of any arrest.
From the Paper
In this scope, the term “inoculate” means to add a material (catalyst) to a molten metal to increase the cell
count and reduce the under-cooling of a phase in the metal. In gray iron, we inoculate to promote more A/B flake
and decrease undercooling which leads to D and E flake. In ductile iron, we inoculate to promote the formation of
late graphite, increase nodule count and prevent carbides. In hypoeutectic aluminum we inoculate to promote smaller
dendrites, and prevent undercooling and hot tearing. In hypereutectic aluminum we inoculate to promote the formation
of smaller silicon particles. There are many other metals that also use various materials to promote cell count and
prevent undercooling. This article discusses how all these various methods may be measured and quantified.
Metrics for Thermal Analysis Feb 2010
I would like to warn everyone that this is a difficult paper. It would be best explained through a
presentation at the Modern Castings show, but that will have to wait 'til next year. But this explanation
sets the ground work for being able to calculate and measure the amount of different crystalline phases in the
solidified metals we pour. Please take the time to understand how we can automatically and in real time, measure
the amount of carbides in iron or steel, or the amount of Mgsilicide in aluminum. This then extends to any
phase in any metal. The lower detection limit with a proper installation seems to be at 0.05% or slightly better
of total energy. That can be equated to total volume with a small correction factor for specific heat.
Next month we will do the same for rates of reactions in the metal which will then give us grain size in
aluminum and nodularity and nodule count in iron.
From the Paper
MeltLab took a bold step in introducing not just the first derivative but a whole series of
derivative curves into the analysis system. A few research papers and an expensive instrument
called a Digital Thermal Analysis (DTA) offer first derivatives, but no one else talks about
the use of higher level derivatives. MeltLab has broken ground with noise filtering and smoothing
techniques that are unique and allow as many derivatives as desired. Why use derivatives,
and what do they give us? Let's talk.
Solidification mode and feeding ductile castings Jan 2010
From the Paper
To fully understand how castings solidify, it is important to understand
how the different components of iron form in the casting. These components
are: austenite free of graphite in the form of dendrites, graphite in the
liquid melt that is free of austenite and eutectic material where graphite
forms in austenite shells. The metal solidifies based on chemistry,
magnesium treatment, which causes a form of graphite modification, and
inoculation levels can be seen though a thermal analysis sample of the metal.
The chemistry alone is insufficient to predict the solidification mode, and the
foundries that depend solely on chemistry are often disappointed with the casting results.
Sample cups for thermal analysis Nov 2009
From the Paper
Sampling cups are one of the consumables and can represent the major
cost of thermal analysis. In this topic we will examine the different
types of cups, the different results of each style and where they are
most appropriately used, and the different kinds of problems each style
has.
Oxygen in Base Iron Oct 2009
From the Paper
Oxygen plays an important
role in the production of iron as an undesirable element. Problems
related to high oxygen content is excessive slag, higher consumption of
inoculants and, in the case of ductile iron, higher magnesium
consumption or lower recovery. Excessive slag can increase the labor
involved in slagging the furnace, and, if not controlled, can appear as
slag stringers in castings or block up filters that have been added at
extra cost to prevent slag from reaching the casting. For those
unfamiliar: slag is a metal oxide, a glass that can be present in a
casting as a defect. It has no strength so it acts as an internal crack
in the casting.
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