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Figure 1. Grain Structure of a solid casting.

Experiment A: Solidification and Casting

Introduction:

The purpose of this experiment is to introduce students to the concepts of solidification and tostudy the development of solidification microstructures. The lab is divided into three parts:

Part 1: Solidification of a pure elementObservation of the dendritic growth in pure lead. Examination of a cross-section of a leadcasting.

Part 2: Solidification of the ammonium chloride/water systemUsing this system as a transparent model for metal alloy casting. A saturated solution ofammonium chloride will be used to simulate a superheated liquid metal undergoing differentsupercoolings. The solidification behaviour will then be observed.

Part 3: Solidification microstructuresThe microstructures of cast alloy systems will be viewed under the light microscope and relatedto the appropriate phase diagram.

Background:

Casting

The fabrication of most metallic and many nonmetallic materials involves melting the rawmaterials and pouring the resulting liquid into a mould which produces a solid of manageablesize and shape. Solidification usually proceeds inward from the mold wall, as heat is extractedout through the wall. As a result, the grains that form are often columnar or long, narrow andrun perpendicular to the mold wall. The grains usually do not grow homogeneously andinstantaneously. Each grain forms a skeletal structure of planes first, the remaining liquid

between the planes solidifying later. Theskeletal framework of a grain is called adendrite and is similar to the snowflakestructure found in nature.

A typical casting shows three distinct zones(Figure 1), a thin chill-cast zone adjacent tothe mold wall formed by heterogeneoussolid nucleation at the mold wall-liquidinterface, a columnar zone formed bypreferential growth of dendrites and acentral equiaxed zone.

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Figure 2. Crystals oriented like (a) will grow furtherinto the liquid in a given time than crystals oriented like(b): (b)-type crystals will get wedged out and (a) -typecrystals will dominate eventually becoming columnargrains.

Figure 3. Dendritic growth of metallic crystals from a liquid state. A through C the dendritesnucleate and grow. The grains of a solid pure metal are depicted in D. Dendritic growth is notevident since all the atoms are identical. E shows an impure metal where the impurities havebeen carried to the regions between the dendrite arms, thus indicating the initial skeleton of themetal structure.

The progressive development of the dendritic structure is illustrated in Figure 2 below.

The cast structure is far from ideal. The firstproblem is one of segregation, as long columnargrains grow they push impurities ahead of them. If, as is usually the case, the alloy is being cast,this segregation can result in big compositionaldifferences and therefore differences inproperties between the outside and the inside ofthe casting. The second problem is one of grainsize. Fine-grained materials are harder thancoarse-grained ones. Indeed, the strength ofsteel can be doubled by a ten-times decrease ingrain-size. Obviously, the big columnar grainsin a typical casting are a source of weakness. But how do we get rid of them?

One cure is to cast at the equilibrium temperature. If, instead of using an undersaturatedsolution, we pour a saturated solution into the mold, we get what is called “big-bang” nucleation. As the freshly poured solution swirls past the mold walls, heterogeneous nuclei form in large

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Figure 4. The microstructure of a cored,cast bronze or copper-tin alloy. 12X.

numbers. These nuclei are then swept back into the bulk of the solution where they act asgrowth centres for equiaxed grains. The final structure is then almost entirely equiaxed, withonly a small columnar region. For some alloys, this technique (or a modification of it called“rheocasting”) works well.

The more traditional cure is to use inoculants. Small catalystparticles are added to the melt just before pouring (or evenpoured into the mold with the melt) in order to nucleate asmany crystals as possible. This gets rid of the columnarregion altogether and produces a fine-grained equiaxedstructure throughout the casting. This important applicationof heterogeneous nucleation sounds straightforward, but agreat deal of trial and error is needed to find effectivecatalysts.

Coring during solidification

If a molten binary alloy solidifies through a liquid plus a solidregion under equilibrium conditions, the compositions of theliquid and solid phases must readjust continuously as thetemperature is lowered. Such readjustments are affected by

the diffusion of both atomic species in both phases. But since the diffusion rate in the solid statetends to be slow, an extremely long time may be required to even out the composition gradients. In practice, cooling rates are almost always so rapid that the composition gradients remain, sucha microstructure is said to be cored because the first region solidify (the “cores”) havecompositions different from those of the last material to solidify. Since a chemical etch oftenattacks regions of different compositions at different rates, cored regions can be delineated in amicrostructure.

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Figure 5 – One way of considering the development of a cored structure. The alloy is not considered to becompletely solid until its composition line crosses the “nonequilibrium solidus” at T5.

Figure 5 shows the process by which a cored structure forms. Consider a molten alloy of over-allcomposition Co at temperature To; as it is cooled, the first solid to form has composition αl. Weassume that the solid forming at the solid-liquid interface at temperatures T2, T3, and T4 hascompositions α2, α3, and α4, that is, that its composition is given by the equilibrium solidus. Ifthe cooling rate is so rapid that each increment of solid formed maintains its initial compositions,we may picture the average composition of all solid formed proceeding along a “nonequilibrium

solidus” from α1’ to α2’ to α3’ and so on. The last liquid disappears only when the averagecomposition of the alloy, that is, when the nonequilibrium solidus crosses the vertical line at Co.

Eutectics

An eutectic reaction represents an easy way by which two (or more) constituents fit togetherduring solidification. During the reaction in a binary alloy, two types of crystal phases intergrowat a constant temperature to give a variety of characteristic patterns. Alloys to left and right of

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Figure 6 - Hypothetical binary equilibrium diagram for elements A and B whichare completely soluble in each other in all proportions in the liquid state but only to a limited extent in the solid state. TA and TB are the melting points of pure A andpure B; Te is the eutectic temperature.

the actual eutectic composition develop primary crystals of the excess phase before the eutecticreaction sets in. One of the types of equilibrium diagrams which may result when there is onlylimited solubility in the solid state is a binary eutectic diagram, illustrated schematically in Figure 6. Consider alloy CO, which exits as a single-phase liquid at point a: when it is cooled topoint b, the composition of the first solid to form is given by the other boundary of the two-phaseregion, Cα1. On further cooling to point c, a solid phase of composition Cα and a liquid ofcomposition C1 are at equilibrium. If we ignore non-equilibrium effects (such as coring) therelative amounts of the two phases in equilibrium may be calculated by the lever rule. At point c,the fraction which is α phase is (Cl - CO)/( Cl - Cα), and the fraction which is liquid phase is (CO -Cα)( Cl - Cα).

If the material is cooled still further below point c, more solid forms, and the composition of theliquid follows the liquidus down to the point e, which is called the eutectic point. With furtherextraction of heat, the eutectic liquid of composition Ce solidifies isothermally at the eutectictemperature Te. This is an invariant of the system; since the three phases are in equilibrium

during solidification of the eutectic liquid, there are no degrees of freedom. The temperature, thecomposition of the liquid phase, and the compositions of both solid phases are fixed.

The solid state microstructure having composition Ce in Figure 6 will be an intimate mixture oftwo phases. The α and β phases in such a eutectic material may be in the form of thin (of the

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Figure 7. Etched cross-section of a lead ingot

order of a micron) plates and rods or tiny particles. A material with composition between Cαeand Ce is called hypoeutectic and, in general, will have a microstructure containing primary α ina matrix of eutectic.

Safety

It is the responsibility of each TA and each student to be aware of the many hazards in thislaboratory and make use the appropriate safety equipment when performing this lab. The mainpotential hazards in this experiment are heat, cryogenic materials and hazardous chemicals. Thefollowing MSDS are available: Lead, Ammonium Chloride and Methanol.

Liquid nitrogen and dry ice expand rapidly at room temperature taking up large volumes of air. Under no circumstances, place solutions containing liquid nitrogen or dry ice in sealed containersor an explosion may result.

An important note about lead: Lead is a designated substance under the Ontario Health andSafety Act, under no circumstances should the lead metal be touched or removed from the

crucibles. Look but don’t touch!

Part 1: Solidification of a pureelement

The samples of lead have alreadybeen prepared in fireclay crucibles. Lead shot was heated to 420oC in thefireclay crucibles until melted usinga muffle furnace. The crucibles werethen air cooled and the oxideremoved from the molten metalsurface. Once a thick skin formedon the molten metal surface, theremaining molten lead was pouredout of the crucible. The molten leadwas allowed to cool to a point wheredendrites have started to form on thecrucible walls. The retaineddendrites on the crucible walls are

clearly visible in the crucibles.

Part 1: Lab Report

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Figure 9. Setup for Part 2 of the experiment.

Figure 8. Early stages of growth of an idealized metallicdendrite.

Observe the lead structures visible in the fireclay pots, sketch of a few of the dendrites as seenunder the stereo microscope. Include a description of the size and direction of growth of thedendrites with your sketches. How does the rate of heat removal from a casting affect the sizeand direction of growth?

Part 1: Lab Report (cont.)

Figure 7 shows the cross-sectionof a cast lead ingot that has beenpolished and etched with anammonium molybdate solution. Sketch the microstructurelabelling the different zones (i.e.,chill zone, columnar zone, andequiaxed zone). If the lead werecast into a chilled mold, howwould the size of the dendrites beaffected? What would be theeffect on the relative sizes of thedifferent zones of the casting?

Part 2: Solidification of theammonium chloride/watersystem

In this part, a transparent analogue

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of metal alloy solidification is used to illustrate the solidification process that occurs when metalsare cast. A slightly saturated solution of ammonium chloride will be made at 50°C. The solutiontemperature is then raised to 75°C (to stimulate superheat) and poured into a mold chilled withliquid nitrogen, minus 196°C (to simulate chill casting). The procedure is then repeated with a mold cooled to minus 50°C (to simulate sand casting).

Part 2: Procedure

Heat 50 ml of water to 50°C (ie. Hot Plate set on low with gentle stirring). Maintaining thetemperature at 50°C, slowly add enough ammonium chloride to make a slightly saturatedsolution (i.e., a few ammonium chloride crystals should remain undissolved). This isapproximately 30g.

Heat the solution to 75°C (25°C of superheat). Meanwhile, cool the solidification cell by pouringthe liquid nitrogen (to a level even with the top of the cell holder). Do not immerse any of theportion of the plastic windows in the liquid nitrogen.

Pour the solution at 75°C into the funnel positioned above the cell until the cell is just filled asdepicted in Figure 9.

Observe the solidification process. Using a magnifying glass and propping a black card behindthe cell will make the process easier to see. Initially and every few minutes squirt a littlemethanol on the windows to keep them frost-free. If the windows frost up, squirt a small amountof methanol on the windows. Once complete, clean the glassware and cell under running water,and dry.

DO NOT PUT THE COLD CELL UNDER HOT WATER – THIS MAY CRACK THEWINDOW!

Repeat the procedure but instead of liquid nitrogen use propylene glycol/dry ice mixture., andadd dry ice to the propylene glycol until the temperature is approximately minus 50°C.

Part 2: Lab Report

Include the following:Describe the solidification process for both cases (illustrations would be useful).Why are the dendrites smaller for the liquid nitrogen case?

What effect does the degree of supercooling have on cast structures, in terms of the variouszones?

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What are the limitations of this model? Is cooling with liquid nitrogen a good simulation of chillcasting? Is cooling with dry ice in propylene glycol a good simulation of sand casting?

Part 3: Solidification microstructures

Part 3: Procedure

Each specimen has already been mounted, polished and etched and is designated by the numberon the bottom of the mount. If the specimens require repolishing or etching, please contact thetechnical staff. Each specimen should be observed visually and at high and low magnificationswith a bench microscope. Systematically scan the whole section. Select regions that arerepresentative of the majority of the specimen. Sketch the observed microstructures on blankwhite paper, which will be provided. The sketches indicate whether or not a clear understandingof the basic structures observed has been achieved. Each sketch should show the principalcharacteristics of each specimen. The solidification section of the 3T04 atlas should also beexamined as it contains additional images that may be helpful. The phase diagrams for thespecimen alloys are provided at the end of this write-up. Please return the specimens to thedesiccators after observations are completed. The following three specimens are used toillustrate the coring phenomenon:

Specimen D5. 5% Tin Bronze (chill cast)This specimen was made from cathode copper and high-purity tin. The copper was deoxidizedbefore adding the tin with an addition 0.5% zinc. Pictures of the “as polished” angular oxideinclusions are in the 3T04 Atlas. Etching reveals predominately equiaxed grains. Inside thedifferent coloured grains, a dark pattern is apparent surrounding the dendrites. This representscoring and segregation, or an uneven distribution of tin in the copper. Between the dendrites andinterdendritic regions, a blue-grey delta compound (non-equilibrium) can be observed. Picturesin the solidification section of the 3T04 Atlas show the eutectoid patterns in these particles. Some shrinkage voids are apparent.

Specimen X2. 4% Tin Bronze (sand cast)Slightly elongated or columnar grains withvarying degrees of shading can be seen by eye atthe outer edge of the specimen. With themicroscope this difference in shading betweenthe grains can also be observed. The grainboundaries appear as thin black lines which inthis case follow irregular paths. A dark almostskeleton pattern can be observed inside thegrains. This represents coring and unevennessin composition or uneven distribution of tin in

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the copper. A small amount of blue-grey delta compound (non-equilibrium) can bedistinguished at the interdendritic positions. This specimen also contains shrinkage voids. Pictures in the 3T04 Atlas show the eutectoid pattern apparent in some of the particles.

Specimen X3. 4% Tin Bronze (sand cast, annealed 700°C for 2 hrs)As with X2, the grains show varying degrees of shading. However, the thin black grainboundaries are more regular in appearance. No coring or the blue gray delta compound isvisible. The annealing has allowed the grains to become uniform in composition and themicrostructure is now very similar to a pure metal. The grain size is also significantly smallerthan X2. However, porosity is still apparent within the specimen.

D5, X2 and X3 may show strain markings as a result of deformation during preparation.

The following three specimens are from the eutectic in the Cu-Cu3P system.

Specimen X5 Copper/ 8.4% phosphorus, eutectic alloy (sand cast)The surface of the specimen has an iridescent appearance. The columnar structure and severeporosity of the specimen are visible by eye. Microscopic examination reveals that the grains arecomposed of colonies of fine eutectic structure. High magnification will allow most of thestructure to be resolved. Copper-rich crystals (solid solution alpha) and crystals of copperphosphide have intergrown in a lamellar or laminated pattern. Each lamellar colony has grownradially with quite often a coarsening of the structure at the colony boundaries. The copper-richcrystals appear dark or brown as they are attacked preferentially by the etching solution, whereasthe copper phosphide appears white. Occasional, free pieces of copper phosphide may be seen.

Specimen X6 Copper/ 4.5% phosphorus, hypo-eutectic (sand cast)The specimen is dark in colour. No clear grain structure is apparent to the eye. As this alloycontains excess copper with respect to the eutectic composition (i.e., it is of hypo-eutecticcomposition), there are separate copper-rich crystals, alpha phase, together with the eutectic,which is in a distinctly coarser form than that in X5. Further, the eutectic regions have a fringeof copper phosphide. The copper-rich crystals contain a relatively small amount of phosphorusin solid solution. They are cored, and range in shade from dark blue to light brown or orange. These crystals grew first in the melt and they have developed in characteristic dendritic shape. Infact, the dendritic form is not well developed, the crystals are short and rounded. Some of theapparently isolated, round shapes probably represent regions where the cross-section has passedthrough a dendrite arm. There is a small amount of shrinkage porosity.

Specimen X7 Copper/ 10.5% phosphorus, hyper-eutectic alloy (sand cast)In effect, the reverse of X6, in that rounded dendrites of copper phosphide are set in abackground of eutectic. The dendrites (white in appearance) seem to be better developed thanthose in X6. However, the present dendrites are not cored because copper phosphide does notexist in a range of compositions. The degree of fineness of the eutectic is approximately similarto that of X5. Some porosity is also apparent.

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Part 3: Lab Report

Include labelled sketches of the various specimens indicating the different phases, and/orregions. Indicate the magnification used. Relate what is observed in each specimen to theequilibrium phase diagram.

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Copper - Tin Phase Diagramsα - fcc phase with a maximum solubility for tin of 15.8% at 520oC. An equilibrium state occurs slowly allowing100% α alloys with up to 12 percent or more tin to be created.β - bcc phase formed by peritectic reaction between solid α and residual liquid.γ − bcc phase formed by peritectic reaction between solid β and residual liquid. γ changes to an eutectoid mixture α and δ at 520oC. Beta and Gamma phases are not normal found in commercial alloys at ambient temperatures.δ − intermetallic compound with a γ brass-type structureε - is an orthorhombic structure. The eutectoid transformation of δ phase to α + ε occurs very slowly underequilibrium conditions at 350oC. Usually chill cast tin bronzes will be composed of α + δ.

Equilibrium Phase Diagram Industrial Phase Diagram - Non-Equilibrium(Below 520oC very long annealing times) (Normal annealing times)

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