5.材料与加工

Metal Materials (金属材料) 1 Metals are among the most widely used materials. They are useful to us because of their physical and mechanical properties, which are very different from those of wood, concrete, or plastic. For instance, metals are strong enough to withstand high loads at high temperatures, yet flexible enough to be worked easily. 2 Properties of Metals： The many kinds of metals used in schools and industry have different properties. Some metals are very hard. This means they resist scratching. Others are strong, yet tough and ductile. Ductile means capable of being drawn out or hammered thin. Metals are also good conductors of electricity and heat. Because of the many properties of the various kinds of metals, almost every product need can be met. The following chart describes some of the most important and useful properties. 3 The properties of metals are due to their structure; that is, the special way their atoms are grouped together to make up a solid material. Pure metals have simple structures. Alloys have more complex structures. Metal Properties Property Description Hardness Resists penetration, wear, cutting, and scratching Tensile strength Resists stretching Ductility Easy to form without tearing or rupturing Toughness Resists shock and impact; not brittle Elasticity Resists bending Compressive strength Resists squeezing Fatigue strength Resists fracturing 4 Alloys： Most metals used in product manufacturing are alloys, not pure metals. Pure metals are seldom used except in the laboratory. Alloys are composed of two or more metals and small amounts of nonmetallic elements. By carefully alloying, new metals with desirable properties are created. Some of the results are surprising. For example, nickel can be combined with copper to produce an alloy that is stronger than either nickel or copper. A great number of alloys can and have been made because of the ease with which metal atoms combine with other atoms. 5 One of the most important alloys used in industry is steel, which is an alloy of iron and carbon. The carbon content of a metal is shown as a percentage or as “points”. One percent (1.00%) carbon is called 100 carbon, 100 point carbon, or point 100 carbon. You can tell the difference among steels by grinding a piece and studying the sparks. High-carbon steel gives off a bomblike cluster of sparks, while low-carbon steel gives off a long, spread-out pattern. 6 Metallurgy is the science of making metals and alloys for practical use. For thousands of years it was an art where results came from hard experience and little understanding. Now it is a science using the basic principles of the microstructure (microscopic structure) of metals and alloys and how that structure affects their behavior and properties. Scientists continue to experiment with metals, to create the new and improve the old. 7 Kind of Metals Our supply of metals comes from the earth in the form of metal ores. That is, the metals are combined with other materials, such as rock. The ores are refined and then made into forms usable by the manufacturing industry. Many kinds of metal steel, copper, nickel, lead, aluminum, gold, silver, and others are used to make the products we need. 8 The following paragraphs describe metals you will probably find in the shop and how they are refined. Pay special attention to the properties and common uses of each so that you will be able to select wisely for any projects you make. Ferrous metals are mostly iron in content. Nonferrous metal contain little or no iron. 9 Ferrous Metals The main materials in this group are iron and steel. Iron is one of the basic elements used in making cast iron, wrought iron, and steel, The great iron ore producing areas in this country are in Minnesota, Michigan, and Wisconsin. The steel making centers are in Indiana, Ohio, and Pennsylvania. 10 Steel is one of our most important metals, It is tough, durable, plentiful, and is used in every industry. Well over 500,000 people in this country are involved in the mining, refining, and fabricating of steel. 11 The first important step of the steel making process is to take out the iron from the ore. This is done in a blast furnace . Coke, iron ore, and limestone make up the charge for the furnace. The coke supplies the heat ( about 3500 degree F ) , and the limestone serves as a flux; that is , it combines with the impurities to draw them away from the iron. The iron is then made into steel by one of three processes; open-hearth furnace, electric furnace, or basic-oxygen furnace. 12 In each of these three systems, raw iron, which is fairly soft, is made into steel by adding other metals and chemicals to it to form alloys. The alloys are varied to make special steels for drawing into pipe, for casting, for machining, and for many other uses. After the steel is made, it is poured into ingot molds. Later it is milled into bars, sheets, plates, and other forms to be used by industries. 13 Instead of reheating ingots and rolling them into blooms, slabs, or billets, these three shapes of steel can also be made by a new process called continuous casting. Molten steel is poured continuously into a water-cooled mold that is open at the top and bottom. A starting bar temporarily closes the bottom. The steel gradually cools and begins to become solid in the mold. Then the starting bar is slowly pulled downward, drawing the steel with it. The rate at which molten steel is poured in at the top is matched with the rate at which the solid steel is pulled out at the bottom. In this way, a long, continuous piece is formed. It can then be cut into lengths as desired. Different shapes of molds are used, depending upon whether blooms, slabs, or billets are being made. 14 Some steel are hot rolled (squeezed between rollers which red hot). Such metals have a bluish scale on the surface and are called hot-rolled steel (HRS). Cold-rolled steels (CRS) are smoother and have a clean shiny surface finish. 15 Nonferrous Metals： These metals contain little or no iron and are soft and easily worked. They are good conductors of heat and electricity and are generally more durable than iron and steel. 16 Copper is reddish brown and can be bought as sheets, tubes, or rods in a variety of sizes. Copper is perhaps best known for its uses in the electrical industry—it is second only to silver as a conductor of electricity. It is also used for working utensils, for roofing, and as an alloy for other metals. It is easily worked into bows, but hammering makes it harder. (This is known as work harding.) To soften the copper, it must be heated red hot and plunged into water. Copper tarnishes easily and does not machine easily. Brass is an alloy of copper (90 percent) and zinc (10 percent). It is yellow-gold and more brittle than copper. Brass comes in forms similar to copper. Bronze resembles brass in both color and working properties. It is an alloy of about 90 percent copper and 10 percent tin, with small amounts of nickel or aluminum. Pewter is also an alloy containing about 10 percent copper and 90 percent tin. It is an excellent material for making bowls and vases because it is so easily formed. 17 Sterling silver is a kind of lovely, warm and lustrous metal, and is used for jewelry and fine tableware. An article marked “sterling” must contain not less than 0.925 parts of silver with 0.075 parts of copper or some other alloying metal added for hardness. 18 Aluminum is silver-white, is remarkably light, and is easily worked. It work-hardens quickly and must be softened as follows: Cover the metal with chalk, heat until the chalk turns brown, and allow the metal to air-cool. It is available in sheets, tubes, rods, and bars. Aluminum is very light, corrosion resistant, and an excellent conductor of electricity. Although it is soft, aluminum can be made strong by adding copper or zinc. Aluminum alloys are used for bridges, airplanes, house siding, kitchen utensils, and many other products. Today about 27 percent of aluminum is recycled. Using recycled aluminum to make new products is less costly than refining ore, and it helps reduce the production of waste. 19 Heat-Treating： In the forging process, the metal become soft when you heat it and hard but brittle (easily broken) when you work it. In order to make chisels and other tools usable, they must be heated and cooled through a specific temperature sequence. This process of heating and cooling metal to change its properties is called heat treatment, and is usually done in a heat-treating furnace. Here, for example, are the steps in heat-treating a screwdriver made of tool steel. 20 First, anneal the workpiece by heating until cherry red, then cooling slowly in air. This process softens the metal and relieves the strains of forging. Next, harden by reheating about ¾” of the screwdriver tip until it is cherry red. This is called the critical temperature. Plunge the metal quickly into water, moving it with a circular motion. Finally, temper the metal by applying heat about 1” above the tip, When the temper color (light purple in this case) reaches the tip, plunge the tool into water. Tempering reduces the brittleness of the piece. 21 Mild steel is heat-treated by a process called case hardening. This is done by adding a special carbon hardening powder to the surface of a metal workpiece, and then hardening this outer case. 22 To case harden metal, heat the workpiece to a bright red. Remove any scale with a wire brush. Dip, roll, or sprinkle the powder on the wokpiece.. The powder will melt and adhere to the surface, forming a shell around the work. Reheat to bright red, hold at this temperature for a few minutes, and then quench in clean, cold water. This will give the workpiece a completely hard outer surface, or case. Five Basic Machining Techniques (五种基本机械加工技术) 1 The five basic techniques of machining metal include drilling and boring, turning, planing, milling, and grinding. Variations of the five basic techniques are employed to meet special situations. 2 Drilling consists of cutting a round hole by means of rotating drill, on the other hand , involves the finishing of a hole already drilled or cored by means of a rotating, offset, single-point tool. On some boring machines, the tool is stationary and the work revolves; on others, the reverse is true. 3 The lathe, as the turning machine is commonly called, is the father of all machine tools. The piece of metal to be machined is rotated and the cutting tool is advanced against it. We will discuss the structure and functions of lathe in later paragraphs of this article. 4 Planing metal with a machine tool is a process similar to planing wood with a hand plane. The essential difference lies in the fact that the cutting tool remains in a fixed position while the work is moved back and forth beneath it. Planers are usually large pieces of equipment; sometimes large enough to handle the machining of surfaces 15 to 20 feet wide and twice as long. A shaper differs from a planer in that the workpiece is held stationary and the cutting tool travels back and forth. 5 Milling consists of machining a piece of metal by bringing it into contact with a rotating cutting tool which has multiple cutting-edges. There are many types of milling machines designed for various kind of work. Some of the shapes produced by milling machines are extremely simple, like the slots and flat surfaces produced by circular saws. Other shapes are more complex and may consist of a variety of combinations of flat and curved surfaces, depending on the shape given to the cutting-edges of the tool and on the travel path of the tool. 6 Grinding consists of shaping a piece of work by bringing it into contact with a rotating abrasive wheel. The process is often used for the final finishing to close dimensions of a part that has been heat-treated to make it very hard. This is because grinding can correct distortions that may have resulted from heat treatment. In recent years, grinding has also found increased application in heavy-duty metal removal operations. 7 The Lathe：The lathe is one of the most useful and versatile machines in the workshop, and is capable of carrying out a wide variety of machining operations. The main components of the lathe are the headstock and tailstock at opposite ends of a bed, and a tool-post between them which holds the cutting tool. The tool-post stands on a cross-slide which enables it to move sidewards across the saddle or carriage as well as along it, depending on the kind of job it is doing. The ordinary centre lathe can accommodate only one tool at a time on the tool-post, but a turret lathe is capable of holding five or more tools on the revolving turret. The lathe bed must be very solid to prevent the machine from bending or twisting under stress. 8 The headstock incorporates the driving and gear mechanism, and a spindle which holds the workpiece and causes it to rotate at a speed which depends largely on the diameter of the workpiece. A bar of large diameter should naturally rotate more slowly than a very thin bar; the cutting speed of the tool is what matters. Tapered centers in the hollow nose of the spindle and of the tailstock hold the work firmly between them. 9 A feed-shaft from the headstock drives the tool-post alone the saddle, either forwards or backwards, at a fixed and uniform speed. This enables the operator to make accurate cuts and to give the work a good finish. Gears between the spindle and the feed-shaft control the speed of rotation of the shaft, and therefore the forward or backward movement of the tool-post. The gear which the operator will select depends on the type of metal which he is cutting and the amount of metal he has to cut off. For a deep or roughing cut the forwards movement of the tool should be less than for a finishing cut. 10 Centers are not suitable for every job on the lathe. The operator can replace them by various types of chucks, which hold the work between jaws, or by a front-plate, depending on the shape of the work and the particular cutting operation. He will use a chuck, for example, to hold a short piece of work, or work for drilling, boring or screw-cutting. 11 A transverse movement of the tool-post across the saddle enables the tool to cut across the face of the workpiece and give it a flat surface. For screw-cutting, the operator engages the lead-screw, a long screwed shaft which runs along in front of bed and which rotates with the spindle. The lead-screw drives the tool-post forwards along the carriage at the correct speed, and this ensures that the threads on the screw are of exactly the right pitch. The operator can select different gear speeds, and this will alter the ratio of spindle and lead-screw speeds and therefore alter the pitch of the threads. A reversing lever on the headstock enables him to reverse the movement of the carriage and so bring the tool back to its original position. Laser Machining (激光加工) 1 Laser, acronym for light amplification by stimulated emission of radiation. Lasers are devices that amplify light and produce coherent light beams, ranging from infrared to ultraviolet. A light beam is coherent when its waves, or photons, propagate in step with one another. Laser light, therefore, can be made extremely intense, highly directional, and very pure in color (frequency). 2 Based on the laser medium used, lasers are generally classified as solid state, gas, semiconductor, or liquid. The most common solid laser media are rods of ruby crystals and neodymium-doped glasses and crystals. The ends of the rod are fashioned into two parallel surfaces coated with a highly reflecting nonmetallic film. Solid-state lasers offer the highest power output. They are usually operated in a pulsed manner to generate a burst of light over a short time. 3 The laser medium of a gas laser can be a pure gas, a mixture of gases, or even metal vapor and is usually contained in a cylindrical glass or quartz tube. Two mirrors are located outside the ends of the tube to form the laser cavity. Gas lasers are pumped by ultraviolet light, electron beams, electric current, or chemical reactions. 4 The semiconductor laser usually consists of a junction between layers of semiconductors with different electrical conducting properties. The laser cavity is confined to the junction region by means of two reflective boundaries. Common uses for semiconductor lasers include compact audio digital disk (CD) players and laser printers. 5 The most common liquid laser media are inorganic dyes contained in glass vessels. They are pumped by intense flash lamps in a pulse mode or by a gas laser in the CW mode. Tunable dye lasers are a type for which frequency can be adjusted with the help of a prism inside the laser cavity. 6 The use of lasers is restricted only by imagination. Lasers have become valuable tools in industry, scientific research, communication, medicine, the military, and the arts. Laser machining is based on principles that were only recently discovered. The process of laser machining depends on the interaction of an intense, highly directional coherent monochromatic beam of light with a workpiece, from which material is removed by vaporization. 7 Powerful laser beams can be focused on a small spot with enormous power density. Consequently, the focused beams can readily heat, melt, or vaporize material in a precise manner. Lasers have been used, for example, to drill holes in diamonds, to shape machine tools, to trim microelectronics, to heat-treat semiconductor chips, to cut fashion patterns, to synthesize new material, and to attempt to induce controlled nuclear fusion. 8 Welding is the process in which two or more pieces of metal are joined together by the application of heat, pressure, or a combination of both. The welding processes most commonly employed today include gas welding, arc welding, and resistance welding. Other new joining processes include thermite welding, laser welding, and electron-beam welding. 9 The use of lasers for welding has grown during the second half of the 20th century. This method produce high-quality welded products at a rapid rate. Laser welding has valuable applications in the automotive and aerospace industries. 10 Laser-beam machining (LBM) is accomplished by precisely manipulating a beam of coherent light to vaporize unwanted material. LBM is particularly suited to making accurately placed holes. The LBM process can make holes in refractory metals and ceramics and in very thin materials without warping the workpiece. Extremely fine wires can also be welded using LBM equipment.