REVIEW QUESTIONS.doc

B级要求 10REVIEW QUESTIONS10.1 Why is casting an important manufacturing process?As with all other manufacturing processes, an understanding of the fundamentals is essential- both for the production of good quality and economical castings, and to establish proper techniques for mold design and casting practice.10.2 What is the difference between the solidification of pure metals and metal alloys?The flow of molten metals into the mold cavity is discussed in terms of mold design and fluidflow characteristics.10.3 What are dendrites?Note the formation of dendrites in the mushy zone.10.4 State the difference between short and long freezing ranges. How is range determined?The shorter the range (as in pure metals and eutectics),the higher the fluidity. Conversely, alloys with long freezing ranges (such assolid-solution alloys) have lower fluidity. 1Mold design. 2Mold material and its surface characteristics. 3Degree of superheat. R4ate of pouring, 5Heat transfer.10.5 Describe the parameters on which solidification time depends.This lack of convection affects solidification structure and distribution of impurities. Recent experiments involve the growth of crystals for production of cadmium-zinc telluride, mercury-zinc telluride, and selenium-doped gallium arsenide semiconductor samples.10.6 Define shrinkage and porosity. How can you tell whether cavities in a casting are due to porosity or to shrinkage?Porosity caused by gases evolved during solidification can be a significant problem, particularly because of its adverse effect on the mechanical properties of the castings. Various defects can develop in castings from lack of control of material and process variables.10.7 What is the function of chills?Internal chills are usually made of the same material as the casting and are left in the casting10.8 How are dissolved gases removed from molten metal?Dissolved gases may be removed from the molten metal by flushing or purging with an inert gas, or by melting and pouring the metal in a vacuum. If the dissolved gas is oxygen, the molten metal can be deoxidized.10.9 Describe the features of a gating system.The metal may low through a variety of passages (including pouring basins, sprues, runners, risers, and gating systems) before reaching the final mold cavity. Bernoullis thcorcm, the continuity law, and the Reynolds number are the an alytical tools used in designing, with the goals of an appropriate flow rate and the elimination of defects associated with fluid flow.QUALITATIVE PROBLEMS10.18 Describe the stages involved in the contraction of metals during casting.Although most metals shrink during solidification, gray cast iron and some aluminum alloys actually expand. Dimensional changes and cracking (hot tearing) are difficulties which can arise during solidification and cooling. Seven basic categories of casting defects have been identified.QUANTITATIVE PROBLEMS10.46 For rhe spw &btxi in Problem 10.45, what diameter runner is needed to ensure a Reynolds number of 2000? How long will a 25 in3 casting take to fill with such a runner?Fiber materials can be graphite, aluminum oxide, silicon carbide, boron, molybdenum, and tungsten. The elastic modulus of nonmetallic fibers ranges between 200 GPa and 400 GPa, with tensile strengths being in the range from 2000 MPa to 3000 MPa.From the information given, we estimate the weld nugget volume to be 30 mn1 (0.0018 in3). Assume that the density for steel (Table 3.1) is 8000 kg/m3 (0.008 g/mm3); then the weld nugget has a mass of 0.24 g.11REVIEW QUESTIONS11.1 Describe the differences between expendable and permanent molds.Expendable-mold casting usually involves lower mold and equipment costs, but producesless dimensional accuracy. Permanent molds, which are made of metals that maintain their strength at high temperatures.As the name implies, they are used repeatedly and are designed in such a way that the casting can be easily removed and the mold used for the next casting11.2 Name the important factors in selecting sand for molds.As its name implies, the centrifugal-casting process utilizes the inertial forces caused by rotationto distribute the molten metal into the mold cavities. This method was first suggested in the early 1800s. There are three types of centrifugal casting: true centrifugal casting, semicentrifugal casting, and centrifuging11.3 What are the major types of sand molds? What are their characteristics? 1. The mold itself, which is supported by a flask. Wo-piece molds consist of a cope on top and a drag on the bottom. The seam between them is the parting line. When more than two pieces are used, the additional parts are called cheeks.2. A pouring basin or pouring cup, into which the molten metal is poured.3. A sprue, through which the molten metal flows downward.4. The runner system, which has channels that carry the molten metal from the sprue to the mold cavity. Gates are the inlets into the mold cavity.5. Risers, which supply additional metal to the casting as it shrinks during solidification. Figure 11.4 shows two different types of risers: a blind riser and an open riser.6. Cores, which are inserts made from sand. They are placed in the mold to form hollow regions or otherwise define the interior surface of the casting. Cores are also used on the outside of the casting to form features such as lettering on the surface of a casting or deep external pockets.7. Vents, which are placed in molds to cany off gases produced when the molten metal comes into contact with the sand in the mold and core. They also exhaust air from the mold cavity as the molten metal flows into the mold.11.4 List important considerations when selecting pattern materials.In selecting insert materials, the possibility of galvanic corrosion should be taken into account. Steel, brass, and bronze inserts, for example, are commonly used in die casting alloys. If galvanic corrosion is a potential problem, the insert can be insulated, plated, or surface-treated,11.5 What is the function of a core? What are core prints?Qpical core materials are oil-bonded or resin-bonded sand, plaster, graphite, gray iron, low-carbon steel, and hot-work die steel. Gray iron is the most commonly used, particularly for large molds in aluminum and magnesium casting. Inserts are also used for various parts of the mold.11.6 Name and describe the characteristics of the types of sand-molding machines.The oldest known method of molding, which is still used for simple castings, is to compactthe sand by hand hammering (tamping) or ramming it around the pattern. For most operations,however, the sand mixture is compacted around the pattern by molding machines11.7 What is the difference between sand- and shell-mold casting?Casting processes are generally classified as expendable-mold or permanent-moldcasting. The most common expendable-mold processes are sand, shell-mold, plastermold,ceramic-mold, and investment casting. Common permanent-mold processesinclude slush casting, pressure casting, die casting, and centrifugal casting.11.8 What are composite molds? Why are they used?Composite molds increase the strength of the mold, improve the dimensional accuracy and surface finish of castings, and may help reduce overall costs and processing time.11.9 Describe the features of plaster-mold casting.The ceramic-mold casting process, also called cope-and-drag investment casting, is similar to the plaster-mold process, with the exception that it uses refractory mold materials suitable for high-temperature applications.QUALITATIVE PROBLEMS11.24 Why is it that die casting can produce the smallest parts?The die-casting process, developed in the early 1900s, is a further example of permanentmold casting. The molten metal is forced into the die cavity at pressures ranging from 0.7 MPa-700 MPa (0.1 ksi-100 ksi). The European termpressure-die casting, or simply die casting, which is described in this section, is not to be confused with the term pressure castingQUANTITATIVE PROBLEMS11.57 Calculate the production rate of single-crystal silicon from the two methods illustrated in Figure 1 1.3 1.a. Conventional casting of turbine bladesb. Directionally solidified blades.c. Single-crystal blades13REVIEW QUESTIONS13.1 What is the difference between a plate and a sheet?Plates, which are generally regarded as having a thickness greater than 6 mm (114 in.),are used for structural applications, such as machine structures, ship hulls, boilers, bridges, andnuclear vessels. Plates can be as much as 0.3 m (12 in.) thick for the supports for large boilers,150 mm (6 in.) thick for reactor vessels, and 100-125 mm (4-5 in.) thick for battleships and tanks.Sheets are generally less than 6 mm thick; they are provided to manufacturing facilitiesas flat pieces or as strip in coils for further processing into various products. They areused for automobile and aircraft bodies, appliances, food and beverages containers, andkitchen and office equipment.13.2 Define (a) roll gap, (b) neutral point, (c) draft.Thus, the higher the friction and the larger the roll radius, the greater the maximum possibledraft (and reduction in thickness) becomes. This situation is similar to the use of large ,tires (high R) and rough treads (high p) on farm tractors and on off-road earth-movingequipment, which permit the vehicles to travel over rough terrain without skidding.13.3 What factors contribute to spreading in flat rolling?Traditionally, the initial material form for rolling is an ingot. As stated in Section 5.4,however, this practice is now being rapidly replaced by that of continuous casting and rolling,at much higher efficiency and a lower cost.13.4 Explain the types of deflection that rolls undergo.Rolls undergo certain geometric changes. Just as astraight beam deflects under a transverse load, roll forces tend to bend the rolls elasticallyduring rolling13.5 Describe the difference between a bloom, a slab, and a billet.The initial breaking down of an ingot or of a continuously cast slab is done by hot rolling,Billets are usually square, with a cross-sectional area smaller than blooms; they arelater rolled into various shapes, such as round rods and bars, by the use of shaped rolls. Hotrolledround rods are used as the starting material for rod and wire drawing (Section 15.10);they are called wire rods.13.6 Why is roller leveling necessary?Rods and tubes that are not sufficiently straight (or are supplied as coil) can be straightenedby passing them through an arrangement of rolls placed at different axes .The rolls subject the product to a series of bending and unbending operations, a process similar to roller leveling.QUALITATIVE PROBLEMS13.18 How does applying tensions affect flat-rolling practice?Pack rolling is a flat-rolling operation in which two or more layers of mctal are rolledtogether; this process improves productivity. Aluminun foil, for exarnple, is pack rolled ill two layers. QUANTITATIVE PROBLEMS13.32 Using simple geometric relationships and the inclined-plane principle for friction, prove Eq. (1 3.1).This is due to the frictional constraint at the surfaces along the arc of contact between the roll and the strip. This situation produces residual stresses that are opposite of those in the case of small-diameter rolls.16REVIEW QUESTIONS16.1 How does sheet-metal forming differ from rolling, forging, and extrusion?Compared to those made by casting and by forging, sheet-metal parts offer the advantagesof light weight and versatile shape.16.2 What causes burrs? How can they be reduced or eliminated?Polyurethanes are used widely,because of their resistance to abrasion, their resistance to cutting by burrs or by sharp edges on the sheet metal, and their long fatigue life.16.3 Explain the difference between punching and blanking.A different operation is performed at the same station with each stroke of a series of punches16.4 List the various operations performed by die cutting. What types of applications do these processes have in manufacturing?A number of alternative methods have been considered for the manufacturing of thispart and of similar axisymmetric parts for this jet engine. In the end, the most economicalmethod was found to be the one shown in Fig. 16.4216.5 Describe the difference between compound, progressive, and transfer dies.In a transfer die setup, the sheet metal undergoes different operations at different stations, which are arranged along a straight line or a circular path. After each step, the part is transferred to the next station for further operations.16.6 Name the various methods by which sheet-metal blanks can be cut from a large sheet.After a blank is cut from a larger sheet, it is formed into various shapes. Basically, all sheet forming processes employ various dies and tooling to stretch and bend the sheet. Before we consider these processes, however, certain characteristics of sheet metals must be reviewed, because of their important effects on the overall operation16.7 List the characteristics of sheet metals that are important in sheet-forming operations. Explain why they are important.Although sheet-forming operations rarely involve simple uniaxial stretching like that in a tension test, observations concerning tensile testing can be useful for understanding the behavior of sheet metals. Recall, from Section 2.2, that a specimen subjected to tension first undergoes uniform elongation; only when the load exceeds the ultimate tensile strength does the specimen begin to neck16.8 What is the significance of anisotropy?An important factor that influences sheet-metal forming is aniuotrop (directionality) of thesheet. Recall that anisotropy is acquired during the therrno mechanical processing of the sheet,and that there are two types of anisotropy16.9 Describe the features of forming-limit diagrams (FLD).The sheet is marked with a grid pattern of circles, typically 2.5 to 5 mni (0.1to 0.2 in.) in diameter, uslng electrochemical or photoprinting techniques. The blank is then stretched over a punch, and the deformation of the circles is observed and measured in regions where failure (rzec.kiizg and tezring)h as occurred.16.10 List the properties of materials that influence spring back. Explain why they do.In bending, this recovery is called springback; you can easily observe it by bending then releasing apiece of sheet metal or wire.16.11 Make a list of common bending operations, giving one application for each.Several bending operations commonly used in industry are described in this section.QUALITATIVE PROBLEMS16.24 Describe the cutting process that takes place when a pair of scissors cuts through aluminum foil.One side of aluminum foil is matte, the other side shiny: The foil-to-foil side hasFIGURE 13.6 Changes in the grain structure of cast or of large-grain wroughtmetals during hot rolling. Hot rolling is an effective way to reduce grinsi ze inmetals, for improved strength and ductility. Cast structures of ingots or continuouscastings are converted to a wrought structure by hot workingQUANTITATIVE PROBLEMS16.63 Why is the bending force P proportional to the square of the sheet thickness, as seen in Section 16.5.4?If the clearance is too small, the blank may be simply pierced or sheared by the punch. The comer radii of the punch and of the die are also important. If they are too small, they can cause fracture at the corners; if they are too large, the cup wall may wrinkle (in a phenomenon calledpuckering).27REVIEW QUESTIONS27.1 Explain fusion as it relates to welding operations.Fusion welding is defined as melting logelher :uid coalescing materials by means of heat. The thcrlnal energy required for these welding opertllions is usu:lly supplied by chcriicotrr lc lcc,lric.crml eans.27.2 Describe the reactions that take place in an oxyfuel gas torch. What is the level of temperatures generated?Furthermore, temperature gradients in thc workpiccc cause distortions by differential thermal expansion and contraction. When a portion of the heat generated is conducted into the workpiece, it expands the part being ground, making it difficult to control dimensional accuracy.27.3 Explain the features of neutral, reducing, and oxidizing flames. Why is a reducing flame so called!1At a ratio of 1 : 1, that is, when there is no excess oxygen, it is considered to be a neutral flame.With a greater oxygen supply, it becomes an oxidizing flame.2The flame with pure hydrogen gas is colorless, so it is difficult to adjust the flame by eyesight. Most other gases, such as natural gas, propane, and butane, are not suitable lor oxyfucl welding, becaiset heir heat output is low or because the fame is oxidizing.27.4 Why is an oxidizing flame desirable in welding copper alloys?Only in the welding of copper and copperbased alloys is an oxidizing flame desirable, because in those cases a thin protective laycr of sligf orms over the molten metal.27.5 Describe the procedure to be followed in an oxyfuel gas welding operation.They arc lvailablca s rod or wire made of metals compatible with those to he welded. These consumable filler rods may be bare, or they may be coated with flux.27.6 Explain the basic principles of arc-welding processes.In tr1.c. rtltlirdgc,v clopcd in the mid-1 800s, the heat required is obtained from electrical energy.The process involves either a con.sumublor a toncon.srneulebcletr odc) (rod or wire).27.7 Why is shielded metal-arc welding a commonly used process? Why is it also called stick welding?The electric arc is generated by touching the tip of a coated electrode against the workpiece and then withdrawing it quickly to a distance suffi