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CHAPTER 2 DR . Ahmad Hassan
1-Solidification of metals: After molten metal is poured into a mold, a series of events takes place during solidification of the casting and it’s cooling to ambient temperature. These events greatly influence the, size, shape, uniformity, and chemical composition of the grains formed throughout the casting, which in turn influence its overall properties. The significant factors affecting these events are: 1)the type of metal 2) the thermal properties of both the metal and the mold 3) the geometric relationship between volume and surface area of the casting 4) and the shape of the mold.
1.1-Pure metals solidification: Because a pure metal has a clearly defined melting or freezing point, it solidifies at a constant temperature. Pure aluminum, for example, solidifies at 660 ºC, iron at 1537 ºC, and tungsten at 3410 ºC. When the temperature of the molten metal is reduced to its freezing point, its temperature remains constant while the latent heat of fusion is given off. The solidification front moves through the molten metal, solidifying from the mold wall in towards the center. The solidified metal, which we now call the casting, is then taken out of the mold and begins to cool to ambient temperature. The grain structure of a pure metal cast in a square mold is shown in Fi.1. At the mold walls, the metal cools rapidly since the walls at ambient temperature. Rapid cooling produces a solid skin, or shell, of fine equiaxed grains. The grains grow in the direction opposite to the heat transfer out through the metal. Those grains that have favorable orientation will grow preferentially and are called columnar grains, Fig.2. As the driving force of the heat transfer is reduced away from the mold walls, the grain becomes equiaxed and coarse.
Fig.1 Schematic illustration of three cast structures of metal solidified in a square mold: (a) pure metals; (b) solid-solution alloys; and structure oriented by using nucleating agents.
Fig.2 Development of a preferred texture at a cool mold wall. Note that only favorable oriented grains grow away from the surface of the mold.
1.2- Alloys solidification : Solidification in alloys begins when the temperature drops below the liquidus, TL, and is complete when it reaches the solidus, TS, Fig.3. Within this temperature range, the alloy is in a mushy or pasty state with columnar dendrites. Fig.3 Schematic illustration of alloy solidification and temperature distribution in the solidifying metal. Note the formation of dendrites in the mushy zone.
Note the presence of liquid metal between the dendrite arms. Dendrites have three-dimensional arms and branches (secondary arms) and they eventually interlock, as shown in Fig.4. The width of the mushy zone, where both liquid and solid phases are present, is an important factor during solidification. This zone can be described in terms of a temperature difference, known as the freezing range, as follows: Freezing range = TL – TS. In Fig.3, it can be seen that the pure metals have a freezing range that approaches zero and that the solidification front moves as a plane front, without forming a mushy zone. Eutectics solidify in a similar manner with an approximately plane front.
Fig.4 (a) Solidification pattern for gray cast iron in a square casting. Note that after 11 min of cooling, dendrites reach each other, but the casting is still mushy throughout. It takes about two hours for this casting to solidify completely. (b) Solidification of carbon steels in sand and shell mold. Note the difference in solidification patterns as carbon content increases.
1.3- Effect of cooling rates : 1-Slow cooling rates, on the order of 102 K/s, or long local solidification times result in coarse dendritic structures with large spacing between the dendrite arms. 2-For faster cooling rates, on the order of 104 K/s, or short local solidification times, the structure becomes finer with smaller dendrite arm spacing. The structures developed and the resulting grain size, in turn, influence the properties of the casting. As grain size decreases, (a)the strength and ductility of the cast alloy increases, (b)micro porosity (indendritic shrinkage voids) in the casting decreases, and (c) the tendency for the casting to crack (hot cracking) during solidification decreases. 1.4Fluidity of molten metal: Fluidity is a term commonly used to describe the capability of the molten metal to fill mold cavity. This term consists of two basic factors: (1) characteristics of the molten metal and (2) casting parameters.
1-Effect of molten metal characteristics on fluidity: a) Viscosity: As viscosity and its sensitivity to temperature increases, fluidity decreases. b) Surface tension:A high surface tension of the liquid metal reduces fluidity. Oxides films developed on the surface of the molten metal thus have a significant adverse effect on fluidity. For example, the oxide film on the surface of pure molten aluminum triples the surface tension. c) Inclusions: As insoluble particles, inclusions can have a significant adverse effect on fluidity. This effect can be verified by observing the viscosity of a liquid such as oil with and without sand particles in it; the former has higher viscosity. d) Solidification pattern of the alloy: The manner in which solidification occurs can influence fluidity. Moreover, fluidity is inversely proportional to the freezing range. Thus the shorter the range, the higher the fluidity becomes. Consequently, alloys with long freezing ranges have lower fluidity.
2-Effect of casting parameters on fluidity: a) Mold design:The design and dimensions of components such as the sprue, runners, and risers all influence fluidity. b) Mold material and its surface characteristics: The higher the thermal conductivity of the mold and the rougher its surfaces, the low the fluidity of the molten metal becomes. Heating the mold improves fluidity, even though it slows down solidification of the metal and the casting develops coarse grains; hence it has less strength. c) Degree of superheat: It is defined as the increment of temperature above the melting point of the alloy. Superheat improves fluidity by delaying solidification. d) Rate of pouring: The slower the rate of pouring the molten metal into the mold, the lower the fluidity becomes because of the faster rate of cooling. e) Heat transfer: This factor directly affects the viscosity of the molten metal.
Fluidity tests: Although none is accepted universally, several tests have been developed to quantify fluidity. One such test is shown in Fig.5, where the molten metal is made to flow along a channel at room temperature. Obviously this length is a function of the thermal properties of the metal and the mold, as well as the design of the channel.The fluidity index is the length of the solidified metal in the spiral passage. The greater the length of the solidified metal, the greater is its fluidity. Fig.5 A test for fluidity using a spiral mold.
1.5 Solidification time: During the early stages of solidification, a thin solidified skin begins to form at the cool mold walls and, as time passes, the skin thickens. With flat mold walls, this thickness is proportional to the square root of time. Thus doubling the time will make the skin (2)0.5 =1.14 times, or 41 percent, thicker. The solidification time is a function of the volume of a casting and its surface area. Solidification time = C ( volume / surface area)2, (1) Where C is a constant that reflects mold material, metal properties and temperature. Thus large sphere solidifies and cools to ambient temperature at a much slower rate than dose a smaller sphere. The reason is that the volume of the sphere is proportional to the cube of its diameter, and the surface area is proportional to the square of its diameter. The effects of mold geometry and elapsed time on skin thickness and shape are shown in Fig.6. As illustrated, the un-solidified molten metal has been pored from them the mold at different time intervals, ranging from 5 s to 6 min. note that the skin thickness increases with elapsed time but the skin is thinner at internal angles (location A in Figure) than at external angles (location B). Slower cooling at internal angles than at external angles causes this latter condition. A process called slush casting, which is based on this principle, makes hollow ornamental and decorative objects.
Fig.6 A steel casting solidified skin. The remaining molten metal is poured out at the times indicated in the figure.
Example: Solidification times for different shapes: Three pieces being cast have the same volume but different shapes. One is a sphere; one is a cube, and the other a cylinder with a height equal to its diameter. Whichpiece will solidify the fastest and which one the slowest?
Solution: The volume is unity, so we have from equation (1): Solidification time 1 / surface area The respective surface areas are: Sphere: V = (4/3) лr3, r = (3/4 л)1/3, And A = 4 лr2 =4 л (3/4 л)2/3 = 4.84; Cube: V = a3, a = 1, A = 6a2 = 6; Cylinder: V = лr2b = 2 лr3, r = (1/2 л)1/3, and A = 2 лr2 + 2 лrb = 6 лr2 = 6 л(1/2 л)2/3 = 5.54 Thus the respective solidification time’s t are tsphere = 0.043 C, tcube = 0.028 C, and t cylinder = 0.033 C. Hence the cube-shaped casting will solidify the faster and the sphere-shaped casting will solidify the slowest.
1.6 Shrinkage: Because of their thermal expansion characteristics, metal shrink (contract) during solidification and cooling. Shrinkage, which causes dimensional changes- and, sometimes, cracking-is the result of: Contraction of the molten metal as it cools prior to its solidification Contraction of the molten metal during phase change from liquid to solid Contraction of the solidified metal (the casting) as its temperature drops to ambient temperature. The largest amount of shrinkage occurs during cooling of the casting. The amount of contraction for various metals during solidification is shown in Table 2.1.
Table 2.1 Solidification contraction for various cast metals
1.7Defects: As we well see in this section, various defects can result in manufacturing processes, depending on factors such as materials, part design, and processing techniques. While some defects affect only the appearance of parts, others can have major adverse effects on the structural integrity of parts made. The following defects can develop in castings: Metallic projections, consisting of fins, flash, or massive projections such as swell and rough surfaces. 1- Cavities, consisting of rounded or rough internal or exposed cavities, including blowholes, pinholes, and shrinkage cavities. 2- Discontinuities, such as cracks, cold or hot tearing, and cold shuts, as shown in Figs.7&8. 3- Defective surface, such as surface folds, laps, scars, adhering sand layers, and oxide scale. 4- Incomplete casting, such as misruns, insufficient volume of metal poured, and run out. 5-Incorrect dimensions or shape, owing to factors such as improper shrinkage allowance, pattern mounting error, irregular contraction, deformed pattern, or warped casting. 6- Inclusions, which form during melting, solidification, and molding. Generally nonmetallic, they are regarded as harmful because they act like stress risers and reduce the strength of the casting.
Fig.7 Examples of hot tears in castings. These defects occur because the casting cannot shrink freely during cooling, owing to constrains in various portions of the molds and cores
Fig.8 Examples of common defects in castings. These defects can be minimized or eliminated by proper pouring design and preparation of molds and control of procedures.
