Ch 10

10-15 Explain the effects of mold materials on fluid flow and heat transfer in casting operations.

o 10-15 Mold materials can have dramatic effect on the pouring, cooling and final strength properties of the casting. If the mold allows the metal to cool to quickly, it may freeze before the mold is full, trap gas bubbles that cause porosity defects, and have time to tears. If the casting material allows the metal to cool too fast, it will not have time to develop the proper grain and will thus lower the strength and other properties. o If the mold material has a low coeffienicient of heat transfer, it will give the metal plenty of time to fill the mole and cool to its final grain structure, but may add an extraordinary amount of time to the process before mold can be broken and then part removed. This slows production rates.

10-16 if it known that pouring metal at a high rate into a mold can have certain disadvantages. Are there any disadvantages to pouring it very slowly?

o 10-16 The slower the rate of pouring molten into the mold, the lower the fluidity because if the higher rate of cooling when poured slowly. Poring metal too slowly may allow it to solidify in the mold before it is completely full, leaving behind a partial casting. The problem can be overcome by using mold materials with a low thermal conductivity or by preheating the mold.

10-18 Would you be concerned about the fact that portions of internal chills are left within the casting? Explain.

o 10-18 Leaving an internal chill in place within the casting does not cause major concerns if the poured alloy and chill material are identical or very similar in composition. If the remaining chill does not form with the pour, area of weakness may occur in the finished casting. o If any gas is entrained in the pour metal when it hits the chill, it may freeze the gas bubbles into the part, creating high porosity and thus stress concentrations in those areas, leaving the part weaker.

10-19 what partical demonstration can you suggest to indicated the relationship of the solidification time to the volume and surface area of a casting?

o 10-19 Take a one cup metal measuring cup, fill in to the top with water, and then dump that water onto a metal cookie tray. Refill the metal cup with water. Place both in the freezer. The water in the tray will freeze long before the water in the cup. o Asphalt heated at the plant and shipped to a road construction site in a dump truch stays fluid for the entire trip, once it is poured onto the roadway, it is drivable within an hour.

10-20 Explain why casting may have to be subjected to various heat treatments.

o 10-20 Because the metal in a casting may not always freeze at the same rate, there are variation in internal stresses and in the quality of grain structure frozen into the piece. Heat treating and tempering allow the metal to reheat and relax any area of high strain. As well as create a homogenous grain structure which will make the part stronger.

10-21 Why does porosity have detrimental effects on the mechanical properties of casting? Would physical properties, such as thermal and electrical conductivity also be adversely affected by porosity? Explain.

o 10-21 Porosity leaves voids in the casting. When the past is subjected to stress, these pores provide absolutely no assistance in distributing the stress, and in fact require the metal around them to carry more loads for the same resistance. This makes the part weaker in tension, ductility and toughness. o Because air and most other gasses very poor conductors, heat and electricity transfer will decrease with increasing porosity.

10-22 A spoked handwheel is to be cast in gray iron. IN order to prevent hot tearing of the spokes, would you insulate the spokes or chill them? Explain.

o 10-22 During cooling, the spokes will contract. The tendency for hot tearing increase due to coarse grain and the presence of low melting point segregates along the grain boundaries. Hot tearing decreases with increasing strength. Therefore, to keep the spokes away from hot tearing, they should be chilled to harden them faster.

10.1 Explain why casting is an important manufacturing process.

o 10.1 Casting is the ideal method manufacturing complex shapes. Compared to the alternative manufacturing process, casting these types of shapes can be vary easily done. In addition to its ease of application, casting allows for a higher internal pressure during manufacture. Casting withstand compressive residual stresses.

10.10 Explain the reasons for hot tearing in castings.

o 10.10 Along with cracks and cold shuts, hot tearing can be caused by a multitude of factors. One such factor is a coarse grain size, which can greatly increase the occurrence of hot tearing. o Another factor is the presence of low melting point segregates around the edges of the boundaries.

10.11 why is it important to remove dross or slag during the pouring of molten metal into the mold? What methods are used to remove them?

o 10.11 the Reynolds number value less than or equal to 2000 represents laminar flow the idea flow for a fating system. Between 2000 and 20000 represents a mind, generally inconsequential mixture of laminar and turbulent flow, Beyond 20000 represents a severe turbulent flow, which can cause aeration of the molten metal and the formation of dross. o Dross is a scum that forms on the surface of molten metal resulting from a reaction between the metal and gases. Removing dross is important and should be done during the pouring process. A vacuum casting can Eliminate dross or slag completely.

10.12 Why is Bernoulli's equation important in casting?

o 10.12 Bernoulli's equation represents in inverse relationship between the fluidity of the liquid and pressure. This means that the flow must be monitored and maintained throughout the casting system. o The wall of the vesting system is especially important. Too permeable, and the liquid metal will absorb though the walls, decreasing the overall flow rate the liquid.

10.13 Describe thixicasting and rheocasting.

o 10.13 In thixicasting, a pre cast billet is used which does not contain dendrites. Once molten, the material is die cast. The primary advantage if thixocasting is an extremely high production quality and consistency. The primary disadvantage is an increase expense from using specially cast billets. o In rheocasting, a semi-solid slurry is produced from a typical die casting process. This results in a lower cost than thixcasting, but less overall consistency in the end products.

10.14 Describe the stages involved in the contraction if metal during casting.

o 10.14 1 this is the contractionof the liquid before it begins to solidify. It is not necessarily important in casting design. o 2 it is the shrinkage of the metal as it cools from liquid into crystallized solid metal. The amount of solidification shrinkage various greatly between alloys. o 3 It is the contraction that occurs after the metal has completely solidified and is cooling to ambient temperature. The solid casting shrinks away from the mod. Assumes final dimensions that must be predicted by the patternmaker. This contraction is another important casting design consideration because it varies and is critical to dimensional accuracy.

10.2 what are the differences between the solidification of pure metals and metal alloys?

o 10.2 Pure metals are cast at a consistent temperature. This casting is then allowed to cool at environmental temperatures. In addition, pure metal have a very low freezing point, often approaching zero. These factors allow for the solidification to occur on a plane, without forming a soft area in the casting. o Alloys, on the other hand, begin solidification only when the temperature drops below the liquidus completing at the solidus during this range, the alloy becomes soft and pasty. The size of this zone is an important factor in the solidification process. The temperature range of this zone is called the freezing ranging for the alloy.

10-25 are external chills as effective as internals chills? Explain.

o 10.25 External chills are just as effective as internal chills when they are used in the proper situations. Internal chills are used to help cool the inside of a cast part, whereas external chills are used to chill the surface of the part first. Depending on how the grain structure is to be developed and whether or not directional solidification is essential to park design or strength will determine which type of chill will be most effective. o Internal chills are made of the same material as the casting and are left in the casting and this may give rise to problems like proper fusion of the internal chills with the casting. Thus foundries generally avoid the use of internal chills. Where as external chills can be made or iron, copper or graphite.

10.3 what are dendrites? Why are they called so?

o 10.3 Dendrites comes from the Greek, dendrites, meaning, literally, similar to a tree. They are so named due to their structure. They have arms or branches, that branch out and eventually intertwine. o Dendrite structure is a complex field of study. Its importance's stems from the impact of a dendrites structure on its compositional variations, segregation, and micro-porosity.

10.4 Describe the difference between short and long freezing ranges.

o 10.4 A short freezing rang represents a temperature difference less than or equal to 50 degrees, centigrade. A freezing range above 50 degrees represents a long freezing range. A freezing range represents the range of temperatures for which a composite is a soft, mushy paste. This occurs when the temperature drops below the freezing point of the liquidus but has bot yet reached the freezing point of the solidus. o The freezing range is determined simple by taking the difference between the freezing point of the liquidus, and that of the solidus. That is the freezing range is given by the formula Tl-Ts.

10.5 What is superheat? Is it important?

o 10.5 Superheat is the difference between the pouring temperature of a metal and its solidification temperature. o This difference is important to calculate in order to maintain a consistent flow in the molten during casting. It expresses a heat variance tolerance within the flow system.

10.6 Define shrinkage and porosity. How can you tell whether cavities in a casting are due to porosity or to shrinkage?

o 10.6 shrinkage is the contraction of a material during solidification. In addition to dimensional changes, cracking can occur. o Porosity has a detrimental effect on the ductility of a casting and the overall surface finish. It can make the casting permeable. Negatively impacting pressure tightness. Porosity is often caused by either shrinkage during casting or the presense of various gases. o There are three stages during which contraction can occur. The first is a contraction of the molten metal prior to solidification. Comtraction can also occur while a molten metal is solidifying. That is, when it is changing from a liquid to a solid. Finally, contraction can occur when an already solid cast is cooling down to ambient temperature.

10.7 What is the function of chills? What are they made of?

o 10.7 An external or internal chilling is used during sand casting in order to prevent or reduce the occurance of porosity causing shrinkage. o Additional measures may be used in order to reduce the occurrence of porosity due to shrinkage. For instance, ensuring adequate molten metal will help avoid cavities cussed by the shrinkage.

10.8 Why is the Reynolds number important in casting?

o 10.8 Reynolds's number represents a ratio relating inertial forces to viscosity. Essentially, Reynolds's number relates both forces to that of flow rate and conditions. o Reynolds's number is important in casting in order to calculate the flow condtions of molten metal's throught a given flow system. Because it is a dimensionless number, it can be based on known values rather than on variables such as material volumes.

10.9 how is Fluidity defined? Why is it important?

o 10.9 Fluidity refers to the ability of the molten metal to completely fill a given mold. The fluidity depends on both the properties of the molten metal as well as the parameters set up during casting. o Proper fluidity for a full casting mold and reduces the occurrence of shrinkage and subsequent porosity.

10-31 It has long been observed that a low pouring temperature promote the formation of equiaxed grain over columnar grains and (b) equiaxed grains become finer as the pouring temperature decreases. Explain these two phenomena.

o Equiaxed grains form when metal is cooled quickly. These form at the surface of a casting when molten metal, barely above the pour point, is introduced to a cold mold surface and freezes quickly. o As more and more heat is removed from the casting metal, and it is poured closer to its freeze point, the metal solidifies even faster when it hits the cold surface of the mold. The faster promotes smaller or finer equiaxed grains because more of the available energy in the metal (heat) is transferred to the cold mold.

10-38 assume that you have methods of measuring porosity in casting. Could you have used the information to accurately predict the strength of the casting? Explain

o Even if the total porosity of the cast part was known, it would not provide reliable results in testing. This is because of the strength of the part is also affected by how that porosity is distributed throughout the casting. If it is in one large bubble, the strength of the part will be much different than if the voids were evenly distributed throughout the entire casting. o Knowing the porosity would only be helpful if you could ensure the metal and pores were blended homogenously.

10-33 If you inspect a typical cube of ice, you are likely see air pickets and cracks in the cube. Some ice cubes, however, are tubular in shape and do not have noticeable air pocket or cracks in their structure. Explain this phenomenon.

o Ice formed in a tray or mold cools are freezes from the outside in. Thus, once outer surface had hardened, the inner liquid continues to shrink, shrink, creating mirco cracks in the process. Also, as the liquid inside cools, gasses that were in the water and were soluble are now insoluble in the ice, and come out of solution. Since the outer surface is already frozen, however, the gasses cannot escape and get trapped inside the frozen cube. o Ice in the shape of a tube is formed by running water over the outside of a chilled metal tube. Thus, the ice freezes from the inside out. Gasses cannot be trapped by this method since the surface of the ice is the last portion to freeze, and any internal stressors set up by the expanding frozen ice are dissipated because there is still a liquid layer above that can yield to the expansion.

10-29 Is there any difference in the tendency for shrinkage void formation in metals with short and long freezing ranges, respectively? Explain.

o If the metal has a long freezing range, the part will tend to develop a mushy zone toward the solidification center, creating microporostiy is also developed when the liquid metal solifies and shrinks between dendrites and between dendrite branches. If the metal has a short freezing range, the part is more likely to have large shrinkage cavities in the are near the solidification center. If gates and risers are properly placed, the metal with the shorter freezing range will be more controllable to prevent shrinkage.

10-35 describe the drawbacks to having a riser that is (a) too large and (b) too small.

o Large risers add to the cooling time of the part, slowing production. Also, large risers, when they are removed, leave behind larger areas on the cast part that must be removed or dressed up with some areas to solidify too soon and leave voids behind. o Risers that are too small may not hold enough reserve metal to account for part shrinkage during cooling, or they may freeze too quickly, leaving the mold incompletely filled, small risers can require the casting metal to be heated to a higher temperature. Which increases the chances of forming shrinkage pores, which leads to a lower quality part.

10-34 how can you tell whether cavities in a casting are due to shrinkage or entrained air bubbles?

o The easiest way to tell if bubbles are the result of entrainment or shrinkage is by looking at them under a magnifying glass. Entrained gas will exist as a sphere in the liquid metal because of surface tension, and will harden in the shape of a hollow sphere. o Shrinkage cavities will be very rough and jagged because they were formed after the metal solidified and the surface in that area fractured.

10-32 In casting metal alloys, what would you expect to occur if the mold were agitated (vibrated) aggressively after the molten metal had been in the mold for a sufficient amount of time to form a skin?

o The part would cool more quickly because the vibration would induce convectin within the metal that would increase heat transfer, and would induce convection at the mold exterior/atmosphere interfere. o Dendrite formation would be reduced as the agitation would continuously break them up as them formed in the slushy phase.