Mohawk College - Introduction to Metal Casting
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MATL MTB70 Module 4 - Course Notes...
Solidification
of Metals.
This module is
primarily concerned this what happens after that metal actually is poured into a
mold. The process of solidification, through a series of transitions, and how
the alloy being poured impacts the process as well as the mold material's
affect.
The metal being
poured can be considered in terms of a;
Pure Metal: Single
element metal with a clean phase transition between liquid and solid
Eutectic Alloy: The
alloy which has the lowest melting point possible for a given composition.
Metals in Solid
Solution type alloys: A single
solid homogeneous crystalline phase containing two or more chemical species
The definitions
above are far from useful... Eutectic being a particularly difficult concept for
me to fully understand... for the context of alloying, an eutectic alloy would
be the alloy of 2 or more elements that combines to a lower melting point than
any of the constituent elements For example, eutectic solder paste has a
composition of 63% tin (Sn) and 37% lead (Pb), and has a eutectic temperature of
183șC (straight from google), similarly Eutectic alloys exhibit finer grain
sizes than the pure metal and precipitate 2 or more phases simultaneously (this
will be revisited)...
The idea of a
solid Solution Alloy would be every other combination of Lead (Pb) and Tin (Sn)
that is not the Eutectic with exception of 100% Lead (Pb) & 0% Tin (Sn), and
obviously 0% Lead (Pb) & 100% Tin (Sn), which would be pure metals.
Beyond the lecture
notes, Assigned Text (Chapter 13 Not 7 as noted in the course notes), Google is
proving to be fantastic, and Steve Chastain's Sand casting Manual Vol 1 & 2
is yet another perspective, with enough variation in presentation to make the
idea's less vague.
This following
level of detail is not required to make castings, but I fear that more practical
information will be based on a sound understanding of this... I think a
legitimate concern.
The transition of
phase (from Liquid to Solid) is called the "heat of
fusion" and is represented as unit measure energy as a function of
unit measure weight (of element or solution) released at transition unit measure
temperature (s) of solidification. Ipso Facto Ergo Sum...
Water is a classic
example... 144BTU of energy transfer occurs to change 1 pound of water into 1
pound of ice at 32F. The energy is transferred from the Liquid water to the air,
earth what ever... When a metal is poured a similar energy transfer occurs
between the metal mold interface, presumably on a much larger scale.
Properties
of Thermal Transfer within a Casting
The application of
thermal energy transfer theory to metal casting requires consideration in the
following properties.
The shape and size
of the casting. The shape and complexity of a casting influences the efficiency
with which the thermal transfer occurs, as it is related to the surface area of
the metal-mold interface. The size of the casting (including sprue, runners,
gates and risers) determines the amount of energy that must be transferred to
complete the phase transition from liquid to solid.
Molding Material
is the recipient of the energy being transferred, so the composition, thermal
characteristics and density of the material also influence the solidification
process of a given casting.
Physical
Dimensioning through the Phase Transition.
Metals typically
have a lower density when molten then when solid, unlike water that has a lower
density when Solid (frozen) than in it's liquid state (Ice Floats).
As density is a
function of weight and volumetric displacement, and the weight of the metal
remains constant, the volumetric displacement must be reduced to raise the
density of a specific qty of metal.
The manner in
which the casting solidifies (as determined by the alloy, mold, shape, size,
sprue, gate, runner and riser system combine to either yield a dimensionally
correct casting or one that has defects such as gross
shrinkage cavities (pipe) or
dispersed porosity.
As molten metals
typically have a lower density, there is an expectation that the casting will be
proportionally smaller than the pattern from which it was cast. This is a
predictable result and the percentile increase in pattern size to achieve an
accurate cast size is a prerequisite to useful castings. The
"Pattern-Maker's Ruler" is found in most every pattern shop to
accurately scale the pattern for a given alloy to the required solidified
dimensions.
Directional
Solidification
The solidification
process begins at the mold-metal interface, meaning the entire outer skin of the
casting, the energy transfer continues through the layer of solid metal toward
the mold material. As the energy is traveling in one direction the
solidification process is traveling in the opposite direction.
Logically, thin
sections of castings will solidify before thick sections of the casting, as the
volume of metal is decreasing in relation to the percentage of metal that has
solidified, a source of liquid metal has to be drawn upon to keep the casting
dimensionally accurate. The source of liquid metal is from the Risers that act
as reservoirs to feed the casting during the solidification phase.
Essentially there
will always be a cavity defect, but the idea is to position the defect in the
riser not the casting itself. This is accomplished by gate and runner placement
such that the thinnest sections cool first and the placement of the risers
ensure a source of molten metal.
In closing the
process of "Directional Solidification" is accomplished by orienting
the Sprue, gates, runners, pattern and risers such that the thinnest sections
through thickest are forming a wedge that ends with the riser(s) being the
thickest section
The image to the
right certainly is overly simplistic but illustrated the basic idea.
More complex
castings would require additional risers and supporting runners and gating.
Progressive
Solidification
Although the
lecture notes clearly state that this term must be understood, the Glossary of
terms simply refers to directional solidification. So the inequities of the
college system of Ontario again falls on the shoulders of theworkshop.ca (when
will it end???)
The basic concept
is the formation of crests and troughs within the leading edge of the
solidifying metal as it propagates through the liquid metal.
The ideal in the
illustration to the right is that over time the wave grows from the right toward
the left, with the understanding that the left section continues on toward an
even thicker section...
The right most end
of the cross-section has a greater surface of mold/metal interface and is not
only growing inward from the top and bottom but also from the right.
Also of note is
that the points in time A, B & C are for illustration, and the freeze wave propagates
over a continuum in a semi-linear fashion, and does not have such exaggerated
demarcation points.
Lastly The concept
of a Freeze Wave is allegorical in nature only, subsequent section(s) takes this
conceptual idea and illustrates it in more accurate detail.
Three Stages
of Contraction (Shrinkage)
This is a complex
time in the life of a casting, The liquid Metal has a Volume "A", as
it solidifies it shrinks during the phase transition to solid and reflects a new
volume "B", and lastly the solidified casting further contracts
(shrinks) through the cooling process (Starting at Temperature of
solidification through to ambient temperature) settling on Volume
"C"...
Obviously the
world around us is in constant dimensional change as the ambient temperature is
always in a state of transition, but the minor variations in volumetric
displacement are negligible compared to the variances that occur from
"A" to "B" and lastly to "C".
In
Consideration of Pure Metals
Although the vast
majority of castings produced are alloys of varying elemental composition, the
point to start at is with a pure elemental composition. Pure metals exhibit
ideal solidification characteristics and introduce a point of reference when
consideration of solidification is turned to alloyed compositions.
The illustrations
to the upper right depict a pure metal and the flat thermal property of
solidification. The blue line shows the point of fusion with a predictable
dissipation of energy with no drop in temperature until the solidification
process has completed.
The lower right
illustration is of an alloy, and the point of fusion has been expanded to cover
a range of temperature drop over the energy dissipation.
Two new terms are
introduced "Liquidus & Solidus" that denote the points in the
graph where phase transition begins and ends for the solidification process...
Curiously the area between Liquidus & Solidus has a rather mundane name
"Mushy Zone"...
I personally would
have preferred Zonis Indeterminus, or Soliqus...
Control of
the Solidification Rate
By understanding
the factors associated with the solidification process, we can influence the
speed at which solidification occurs, through the use of molding material, and
orientation of casting components, such as risers etc...
The speed of
solidification has a direct bearing in such issues as grain size within the
solid casting, Shrinkage and associate defects, as well as a new issue
"Segregation".
As an Alloy has 2
or more constituent elements, the individual elements potential can solidify at
varying rates creating concentrations or isolated pockets of that element within
the liquid solution during the phase transition.
The solidification
rate can be graphed for various alloys, mold materials etc... This is mathematically
a function of the casting thickness (assuming a flat surface) against the sq
root of time as well as the introduction of a Constant "K"... I've
opted to pass on the diagrams and the math as it's not an essential building
block (or so I hope)...
Solidification
Modeling
The illustration
on the right has 5 steps shown A thru E.
A) is the initial
nucleus that forms on the smallest scale as the outermost skin of molten metal
enters the Mushy Zone by the energy transfer between the mold-metal interface...
millions of nucleus form near simultaneously.
B) is the
extension of the Primary axis into the molten metal
C) is the
formations of secondary axes at right angles to the primary Axis
D) is the extended
growth of the now complete dendrite with ever thickening branches and
trunks as seen in E...
The crystalline or
lattice structure is growing by the solidification of the molten metal, and this
is the freeze wave discussed earlier. The energy transfer between the liquid and
solid interface is conducted through this dendrite structure toward the metal
mold interface.
Grain
Structure and Growth
The illustrations
are getting looser and dirtier as we go along here, but at least I know what I'm
trying to depict.
At the base of the
structure to the right are fine grains that formed very quickly at the
metal-mold interface, in the center are longer "Columnar" grains
that form directionally in opposition to the energy flow of the point of
fusion, toward the top are smaller random "Equiaxed" grains that form
at a slower rate.
This cross-section
is supposed to illustrate the region of solidification from the mold wall upward
to the liquid/solid interface.
At this point get
a beer, have a smoke and review the disjointed factoids presented thus far...
So... If
directional solidification is encouraged though proper placement of Sprue,
gates, runners and Risers, we know that the nucleation of dentrites will form
and grow in opposition to the energy flow inward making a mushy zone that is
being impregnated by dendrite growth.
Since the majority
of castings are alloys (2 or more elemental constituents) the dendrite growth
will be displacing higher point of fusion elements into the liquid or mushy
center. This segregation of constituents impacts grains size and composition.
The Alloy
composition is impacted by the thermal gradient of the mold-metal interface and
the efficiency with which it can transfer the energy released by the point of
fusion of the elemental constituents. Since any given alloy has a static Mushy
Zone based upon it's composition and the mass of the casting, the only variable
that can influence grain formation and reduce elemental segregation is the time
required to transfer the point of fusion energy.
The generalization
that wide mushy zones (slow phase transitions) promote Equiaxed Grains and micro
segregation, while narrow mushy zones (fast phase transitions) promote columnar
grain structures is still somewhat meaningless, except that columnar grains are
stronger and contain a better elemental distribution through the alloy.
In summary the
faster the better... I think that this is what Metallurgy & Heat treating
will be based upon.
And again, (as
this is being repeated it has to be on the final exam) Mold-Metal interface is
the chill zone with fine grain structure, The next is the columnar zone that are
longer directional grains that grow toward the liquid metal, the third is the
equiaxed zone of small random grains in the center of the casting.
Porosity
Defects
The most subtle of
the porosity defects is caused by alloyed melts that have a wide mushy zone
transition phase to solid. The elemental nucleation and dendritic growth
displaces molten elements and results in micro porosity.
The most obvious
porosity defects are caused by the entrapment of gases within the molten
solution. Typically hydrogen precipitates into melts by contact with the
atmosphere, or poor foundry practices, such as introducing wet charge materials
into the melt.
Since Hydrogen is
highly soluble in molten metal, it is best to avoid, super-heating metals beyond
their melting temperature, and to avoid holding the metal in a molten state any
longer than is required.
Gases can be
scavenged from the molten metal by introducing either an inert gas such as argon
or nitrogen and bubbling it through the melt, or using a solid such as chlorine
with an inverted cup that will plunge the chlorine to the bottom of the melt. As
the chlorine turns to a gas it will form hydrogen chloride, scavenging the
hydrogen in the process.
To reduce the
absorption of gases from the atmosphere leave any slag or dross cover over the
molten metal until just prior to pouring into the mold.