Experiment A: Solidification and Casting

Transcription

1 Experiment A: Solidification and Casting Introduction: The purpose of this experiment is to introduce students to the concepts of solidification and to study the development of solidification microstructures. The lab is divided into three parts: Part 1: Solidification of a pure element Observation of the dendritic growth in pure lead. Examination of a cross-section of a lead casting. Part 2: Solidification of the ammonium chloride/water system Using this system as a transparent model for metal alloy casting. A saturated solution of ammonium chloride will be used to simulate a superheated liquid metal undergoing different supercoolings. The solidification behaviour will then be observed. Part 3: Solidification microstructures The microstructures of cast alloy systems will be viewed under the light microscope and related to the appropriate phase diagram. Background: Casting The fabrication of most metallic and many nonmetallic materials involves melting the raw materials and pouring the resulting liquid into a mould which produces a solid of manageable size and shape. Solidification usually proceeds inward from the mold wall, as heat is extracted out through the wall. As a result, the grains that form are often columnar or long, narrow and run perpendicular to the mold wall. The grains usually do not grow homogeneously and instantaneously. Each grain forms a skeletal structure of planes first, the remaining liquid between the planes solidifying later. The skeletal framework of a grain is called a dendrite and is similar to the snowflake structure found in nature. Figure 1. Grain Structure of a solid casting. A typical casting shows three distinct zones (Figure 1), a thin chill-cast zone adjacent to the mold wall formed by heterogeneous solid nucleation at the mold wall-liquid interface, a columnar zone formed by preferential growth of dendrites and a central equiaxed zone. A1

2 The progressive development of the dendritic structure is illustrated in Figure 2 below. Figure 2. Crystals oriented like (a) will grow further into the liquid in a given time than crystals oriented like (b): (b)-type crystals will get wedged out and (a) -type crystals will dominate eventually becoming columnar grains. The cast structure is far from ideal. The first problem is one of segregation, as long columnar grains grow they push impurities ahead of them. If, as is usually the case, the alloy is being cast, this segregation can result in big compositional differences and therefore differences in properties between the outside and the inside of the casting. The second problem is one of grain size. Fine-grained materials are harder than coarse-grained ones. Indeed, the strength of steel can be doubled by a ten-times decrease in grain-size. Obviously, the big columnar grains in a typical casting are a source of weakness. But how do we get rid of them? Figure 3. Dendritic growth of metallic crystals from a liquid state. A through C the dendrites nucleate and grow. The grains of a solid pure metal are depicted in D. Dendritic growth is not evident since all the atoms are identical. E shows an impure metal where the impurities have been carried to the regions between the dendrite arms, thus indicating the initial skeleton of the metal structure. One cure is to cast at the equilibrium temperature. If, instead of using an undersaturated solution, 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 A2

3 numbers. These nuclei are then swept back into the bulk of the solution where they act as growth centres for equiaxed grains. The final structure is then almost entirely equiaxed, with only 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 catalyst particles are added to the melt just before pouring (or even poured into the mold with the melt) in order to nucleate as many crystals as possible. This gets rid of the columnar region altogether and produces a fine-grained equiaxed structure throughout the casting. This important application of heterogeneous nucleation sounds straightforward, but a great deal of trial and error is needed to find effective catalysts. Figure 4. The microstructure of a cored, cast bronze or copper-tin alloy. 12X. Coring during solidification If a molten binary alloy solidifies through a liquid plus a solid region under equilibrium conditions, the compositions of the liquid and solid phases must readjust continuously as the temperature is lowered. Such readjustments are affected by the diffusion of both atomic species in both phases. But since the diffusion rate in the solid state tends 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, such a microstructure is said to be cored because the first region solidify (the cores ) have compositions different from those of the last material to solidify. Since a chemical etch often attacks regions of different compositions at different rates, cored regions can be delineated in a microstructure. A3

4 Figure 5 shows the process by which a cored structure forms. Consider a molten alloy of over-all composition C o at temperature T o ; as it is cooled, the first solid to form has composition α l. We assume that the solid forming at the solid-liquid interface at temperatures T 2, T 3, and T 4 has compositions α 2, α 3, and α 4, that is, that its composition is given by the equilibrium solidus. If the 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 Figure 5 One way of considering the development of a cored structure. The alloy is not considered to be completely solid until its composition line crosses the nonequilibrium solidus at T 5. solidus from α 1 to α 2 to α 3 and so on. The last liquid disappears only when the average composition of the alloy, that is, when the nonequilibrium solidus crosses the vertical line at C o. Eutectics An eutectic reaction represents an easy way by which two (or more) constituents fit together during solidification. During the reaction in a binary alloy, two types of crystal phases intergrow at a constant temperature to give a variety of characteristic patterns. Alloys to left and right of A4

5 the actual eutectic composition develop primary crystals of the excess phase before the eutectic reaction sets in. One of the types of equilibrium diagrams which may result when there is only limited solubility in the solid state is a binary eutectic diagram, illustrated schematically in Figure 6. Consider alloy C O, which exits as a single-phase liquid at point a: when it is cooled to point b, the composition of the first solid to form is given by the other boundary of the two-phase region, C α1. On further cooling to point c, a solid phase of composition C α and a liquid of composition C 1 are at equilibrium. If we ignore non-equilibrium effects (such as coring) the relative amounts of the two phases in equilibrium may be calculated by the lever rule. At point c, the fraction which is α phase is (C l - C O )/( C l - C α ), and the fraction which is liquid phase is (C O - C α )( C l - C α ). If the material is cooled still further below point c, more solid forms, and the composition of the liquid follows the liquidus down to the point e, which is called the eutectic point. With further extraction of heat, the eutectic liquid of composition C e solidifies isothermally at the eutectic temperature T e. This is an invariant of the system; since the three phases are in equilibrium Figure 6 - Hypothetical binary equilibrium diagram for elements A and B which are completely soluble in each other in all proportions in the liquid state but only to a limited extent in the solid state. T A and T B are the melting points of pure A and pure B; T e is the eutectic temperature. during solidification of the eutectic liquid, there are no degrees of freedom. The temperature, the composition of the liquid phase, and the compositions of both solid phases are fixed. The solid state microstructure having composition C e in Figure 6 will be an intimate mixture of two phases. The α and β phases in such a eutectic material may be in the form of thin (of the A5

6 order of a micron) plates and rods or tiny particles. A material with composition between C αe and C e is called hypoeutectic and, in general, will have a microstructure containing primary α in a matrix of eutectic. Safety It is the responsibility of each TA and each student to be aware of the many hazards in this laboratory and make use the appropriate safety equipment when performing this lab. The main potential hazards in this experiment are heat, cryogenic materials and hazardous chemicals. The following 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 containers or an explosion may result. An important note about lead: Lead is a designated substance under the Ontario Health and Safety 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 pure element Figure 7. Etched cross-section of a lead ingot clearly visible in the crucibles. The samples of lead have already been prepared in fireclay crucibles. Lead shot was heated to 420 o C in the fireclay crucibles until melted using a muffle furnace. The crucibles were then air cooled and the oxide removed from the molten metal surface. Once a thick skin formed on the molten metal surface, the remaining molten lead was poured out of the crucible. The molten lead was allowed to cool to a point where dendrites have started to form on the crucible walls. The retained dendrites on the crucible walls are Part 1: Lab Report A6

7 Observe the lead structures visible in the fireclay pots, sketch of a few of the dendrites as seen under the stereo microscope. Include a description of the size and direction of growth of the dendrites with your sketches. How does the rate of heat removal from a casting affect the size and direction of growth? Part 1: Lab Report (cont.) Figure 8. Early stages of growth of an idealized metallic dendrite. Figure 7 shows the cross-section of a cast lead ingot that has been polished and etched with an ammonium molybdate solution. Sketch the microstructure labelling the different zones (i.e., chill zone, columnar zone, and equiaxed zone). If the lead were cast into a chilled mold, how would the size of the dendrites be affected? What would be the effect on the relative sizes of the different zones of the casting? Figure 9. Setup for Part 2 of the experiment. Part 2: Solidification of the ammonium chloride/water system In this part, a transparent analogue A7

8 of metal alloy solidification is used to illustrate the solidification process that occurs when metals are cast. A slightly saturated solution of ammonium chloride will be made at 50 C. The solution temperature is then raised to 75 C (to stimulate superheat) and poured into a mold chilled with liquid 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 the temperature at 50 C, slowly add enough ammonium chloride to make a slightly saturated solution (i.e., a few ammonium chloride crystals should remain undissolved). This is approximately 30g. Heat the solution to 75 C (25 C of superheat). Meanwhile, cool the solidification cell by pouring the liquid nitrogen (to a level even with the top of the cell holder). Do not immerse any of the portion 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 as depicted in Figure 9. Observe the solidification process. Using a magnifying glass and propping a black card behind the cell will make the process easier to see. Initially and every few minutes squirt a little methanol on the windows to keep them frost-free. If the windows frost up, squirt a small amount of 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 THE WINDOW! Repeat the procedure but instead of liquid nitrogen use propylene glycol/dry ice mixture., and add 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 various zones? A8

9 What are the limitations of this model? Is cooling with liquid nitrogen a good simulation of chill casting? 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 number on the bottom of the mount. If the specimens require repolishing or etching, please contact the technical staff. Each specimen should be observed visually and at high and low magnifications with a bench microscope. Systematically scan the whole section. Select regions that are representative of the majority of the specimen. Sketch the observed microstructures on blank white paper, which will be provided. The sketches indicate whether or not a clear understanding of the basic structures observed has been achieved. Each sketch should show the principal characteristics of each specimen. The solidification section of the 3T04 atlas should also be examined as it contains additional images that may be helpful. The phase diagrams for the specimen alloys are provided at the end of this write-up. Please return the specimens to the desiccators after observations are completed. The following three specimens are used to illustrate the coring phenomenon: Specimen D5. 5% Tin Bronze (chill cast) This specimen was made from cathode copper and high-purity tin. The copper was deoxidized before adding the tin with an addition 0.5% zinc. Pictures of the as polished angular oxide inclusions are in the 3T04 Atlas. Etching reveals predominately equiaxed grains. Inside the different coloured grains, a dark pattern is apparent surrounding the dendrites. This represents coring and segregation, or an uneven distribution of tin in the copper. Between the dendrites and interdendritic regions, a blue-grey delta compound (non-equilibrium) can be observed. Pictures in 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 with varying degrees of shading can be seen by eye at the outer edge of the specimen. With the microscope this difference in shading between the grains can also be observed. The grain boundaries appear as thin black lines which in this case follow irregular paths. A dark almost skeleton pattern can be observed inside the grains. This represents coring and unevenness in composition or uneven distribution of tin in Figure 10: Crystallization Cell A9

10 the copper. A small amount of blue-grey delta compound (non-equilibrium) can be distinguished 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 grain boundaries are more regular in appearance. No coring or the blue gray delta compound is visible. The annealing has allowed the grains to become uniform in composition and the microstructure is now very similar to a pure metal. The grain size is also significantly smaller than 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-Cu 3 P system. Specimen X5 Copper/ 8.4% phosphorus, eutectic alloy (sand cast) The surface of the specimen has an iridescent appearance. The columnar structure and severe porosity of the specimen are visible by eye. Microscopic examination reveals that the grains are composed of colonies of fine eutectic structure. High magnification will allow most of the structure to be resolved. Copper-rich crystals (solid solution alpha) and crystals of copper phosphide have intergrown in a lamellar or laminated pattern. Each lamellar colony has grown radially with quite often a coarsening of the structure at the colony boundaries. The copper-rich crystals appear dark or brown as they are attacked preferentially by the etching solution, whereas the 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 alloy contains excess copper with respect to the eutectic composition (i.e., it is of hypo-eutectic composition), 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 fringe of copper phosphide. The copper-rich crystals contain a relatively small amount of phosphorus in 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. In fact, the dendritic form is not well developed, the crystals are short and rounded. Some of the apparently isolated, round shapes probably represent regions where the cross-section has passed through 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 a background of eutectic. The dendrites (white in appearance) seem to be better developed than those in X6. However, the present dendrites are not cored because copper phosphide does not exist in a range of compositions. The degree of fineness of the eutectic is approximately similar to that of X5. Some porosity is also apparent. A10

11 Part 3: Lab Report Include labelled sketches of the various specimens indicating the different phases, and/or regions. Indicate the magnification used. Relate what is observed in each specimen to the equilibrium phase diagram. A11

12 Copper - Tin Phase Diagrams α - fcc phase with a maximum solubility for tin of 15.8% at 520 o C. An equilibrium state occurs slowly allowing 100% 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 520 o C. Beta and Gamma phases are not normal found in commercial alloys at ambient temperatures. δ intermetallic compound with a γ brass-type structure ε - is an orthorhombic structure. The eutectoid transformation of δ phase to α + ε occurs very slowly under equilibrium conditions at 350 o C. Usually chill cast tin bronzes will be composed of α + δ. Equilibrium Phase Diagram (Below 520 o C very long annealing times) Industrial Phase Diagram - Non-Equilibrium (Normal annealing times) A12

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