Metallurgy and Manufacturing - Coursework Example

Manufacture Objectives 1) to analyse and formulate the machining process plan to complete the machining of the component based on CNC machining; 2) for each machining process/operation: to identify the resources, such as machine, tool, fixture, and etc.; 3) for each machining process/operation: describe/sketch the work holding strategy; 4) for each machining operation: to identify the manufacturing parameters where appropriate; 5) to write the NC program of CNC machining of the component using ISO G-code, which should be able to run in the CNC machines in the manufacturing workshop . d) During UCLan weeks 31-33, each student is expected to demonstrate/present his/her work Introduction Metals owe their importance to their unique mechanical properties, the combination of high strength with the ability to change shape plastically ductility and malleability). This plasticity enables them to be shaped, e.g. into motor car bodies, tin cans and girders, by processes of mechanical working such as pressing, drawing, rolling and forging. Even more important, this same plasticity gives strong metals their extraordinary toughness, the ability to endure all the knocks and shocks of long rough service without breaking or crumbling away. METALLURGY AND MANUFACTURING Metallurgy today is an applied science. Its fascination lies in the challenge of using science to give mankind the best engineering materials that the laws of nature and the Earth's natural resources will allow. The applied science of metallurgy links the science of metals to the metal industries. This link is only maintained and strengthened by deliberate care and attention, for there is always a tendency for the scientific and industrial sides to drift apart. It is as natural for the research man to commit himself totally to the electrical characteristics of these materials are extremely sensitive to the presence of minute concentrations of impurity atoms, which concentrations may be controlled over very small spatial regions. The semiconductors have made possible the advent of integrated circuitry that has totally revolutionized the electronics and computer industries. ENGINEERING REQUIREMENTS OF MATERIALS Engineering requirements of a material mean as what is expected of from the material so that the same can be successfully used for making engineering components such a crankshaft, spanner, etc. When an engineer thinks of deciding and fabricating an engineering part, he goes in search of that material which possesses such properties as will permit the component part to perform its functions successfully while in use. For example, one may select high speed steel for making a milling cutter or a power hack-saw blade. The main engineering requirements of materials fall under three categories, Fabrication requirements Service requirements Economic requirements Fabrication requirements mean that the material should be able to get shaped (e.g., cast, forged, formed, machined, sintered etc.) and joined (e.g., welded, brazed, etc.) easily. Fabrication requirements relate themselves with material's machinability, ductility, castability, heat-treatability, weldability, etc. Service requirements imply that the material selected for the purpose must stand up to service demands, e.g., proper strength, wear resistance, corrosion resistance, etc. Economic requirements demand that the engineering part should be made with minimum overall cost. Minimum overall cost may be achieved by proper selection of both technical and marketing variables. Properties of Engineering Materials Property of a material (or Material Property) is a factor that influences qualitatively or quantitatively the response of a given material to imposed stimuli and constraints, e.g., forces, temperatures, etc. Properties render a material suitable or unsuitable for particular use in industry. The material property is independent of the dimension or shape of the material, e.g., tensile strength of annealed, fine grained pure aluminium will be around 4.8 x 107 N/m2 irrespective of the dimensions of the specimen tested. In principle all material properties have a statistical behaviour. Different material properties are: Mechanical properties Thermal properties Electrical properties Magnetic properties Chemical properties Optical properties Physical properties Technological properties. MECHANICAL PROPERTIES Mechanical properties include those characteristics of material that describe its behaviour under the action of external forces. The knowledge of mechanical properties of materials is very essential in order to construct a mechanically sound structure such as a bridge on the river. Mechanical properties can be determined by conducting experimental tests on the material specimen. Mechanical properties determine the behaviour of engineering materials under applied forces and loads. The response of the materials to applied forces will depend on the type of bonding, the structural arrangement of atoms or molecules and the type and number of imperfections, which are always present in solids except in rare circumstances. For this reason, mechanical properties are very sensitive to manufacturing processes and operations, which may result in highly variable characteristics even in materials of the same chemical composition. — Various mechanical properties are : Elasticity A liquid or gas adapts itself to the shape of its container, but a solid has a shape of its own, which it tends to preserve. Loading a solid will change its dimensions, but the resulting deformation will disappear upon unloading. This tendency of a deformed solid to seek its original dimensions upon unloading is ascribed to a property called elasticity. The recovery from the distorting effects of the loads may be instantaneous or gradual, complete or partial. A solid is called perfectly elastic if this recovery is instantaneous and complete; it is said to exhibit delayed elasticity or inelastic effects, respectively, if the recovery is gradual or incomplete. Accurate measurements reveal some delayed elasticity and inelastic effects in all solids. Giving a precise definition of elasticity and setting forth the rudiments of the theory of elasticity require a discussion of the concepts of strain, stress and modulus of elasticity. Stress Plasticity Plasticity is that property of a material by virtue of which it may be permanently deformed when it has been subjected to an externally applied force great enough to exceed the elastic limit. The subject of plasticity is of great importance to an engineer for it is this property that, in most cases, enables him to shape (e.g., roll, forge) metals in the solid state. The minimum stress that should cause permanent deformation can be computed from a knowledge of the bond strength. The result of such computations gives values that are from 100 to 1000 times the stress required to initiate plastic deformation of crystal as determined by testing. For most materials, the plastic deformation follows the elastic deformation. Referring to stress-strain curve a material obeys the law of elastic solids for stresses below the yield stress and this is followed by the plastic deformation. The mechanism of plastic deformation is essentially different in crystalline materials and amorphous materials. Crystalline materials undergo plastic deformation as the result of slip along definite crystallographic planes whereas, in amorphous materials, plastic deformation occurs when individual molecules or groups of molecules slide past one another. Toughness Toughness is the ability of the material to absorb energy during plastic deformation up to fracture. Toughness refers to the ability of a material to withstand bending or the application of shear stresses without fracture. By this definition, copper is extremely tough but cast iron is not. Specimen geometry as well as the manner of load application are important in toughness determinations. For dynamic loading conditions and when a notch (or point of stress concentration) is present, notch toughness is assessed by using an impact test. Furthermore, toughness is a property indicative of a material's resistance to fracture when a crack is present. For the static situation, toughness may be ascertained from the results of a tensile stress-strain test. Toughness of a material, then, is indicated by the total area under the material's tensile stress-strain curve up to the point of fracture. Resilience Resilience is closely related to toughness. Resilience is the capacity of a material to absorb energy when it is elastically deformed and then upon unloading, to have this energy recovered. The associated property is the modulus of resilience, which is the strain energy per unit volume required to stress a material from an unloaded state up to the point of yielding. Resilience is usually measured by determining the rebound of a pendulum or ball after a single impact. It represents the ratio of energy given up on recovery from deformation to energy required to produce deformation. Bearing Materials Bearings support moving parts, such as shafts and spindles, of a machine or mechanism. Bearings may be classed as (i) Rolling contact (i.e., Ball and roller) bearings. («) Plain bearings. Rolling contact bearings are almost invariably made of steel that can be hardened after machining. Both plain carbon and alloy (Ni, Cr, Mo) steels are employed for different applications. For making plain bearings, an extremely wide range of materials is available and will be discussed below. PROPERTIES OF BEARING MATERIALS A bearing material should: (i) Possess low coefficient of friction. (if) Provide hard, wear resistant surface with a tough core. (iii) Have high compressive strength, (j'v) Have high fatigue strength, (v) Be able to bear shocks and vibrations, (vi) Possess high thermal conductivity to dissipate heat generated due to friction between the bearing and the rotating shaft. (vii) Possess adequate plasticity under bearing load. (viii) Possess adequate strength at high temperatures. (ix) Be such that it can be easily fabricated. (x) Possess resistance to corrosion. (xi) Be such that it does not cause excessive wear of the shaft rotating in it, i.e., bearing material should be softer than the shaft material. (Xii) Possess ant seizure characteristics. (xiii) Be having small pieces of a comparatively hard metal embedded in a soft metal. (xiv) Maintain a continuous film of oil between shaft and bearing in order to avoid metal to metal contact. (xv) Possess ability to embed in itself any dirt, etc., present in lubricating oil. (xvi) Should be cheap and easily available. TYPES OF BEARING MATERIALS They are Lead or tin-based alloys. Cadmium-based alloys. Aluminium based alloys. Silver-based alloys. Copper-based alloys. Sintered bearing materials. Non-metallic bearing materials. Lead or tin based alloys (Babbitt metals) They may be divided as The high tin alloys with more than 80% tin and little or no lead. The high lead alloys with about 80% lead and 1-12% tin. The alloys with intermediate percentages of tin and lead In addition to lead and tin, these bearing alloys contain antimony and copper also. Typical compositions of A lead based alloy A tin based alloy Pb 75% Sn 88% Sbl5% Sb 8% Snl0% Cu 4% Lead base alloys are softer and brittle than the tin base alloys. Lead base alloys are cheaper than tin base alloys. Tin base alloys have a low coefficient of friction as compared to lead base alloys. Lead base alloys are suitable for light and medium loads, whereas tin base alloys are preferred for higher loads and speeds. Whereas tin base alloys find applications in high speed engines, steam turbines, lead base alloys are used in rail road freight cars. Solidus temperature of Tin base alloys — Approx. 222°C Solidus temperature of Lead base alloys —Approx. 240°C Besides, both these alloys possess Good ability to embed dirt Good conformability to journal Good corrosion resistance Very good seizure resistance, etc. White metals are tin base or lead base bearing alloys and are usually referred to as babbitts. Sintered bearing materials Copper powder with 8 to 10% tin and sometimes with 1 to 3% graphite is used for making porous sintered bronze bearings. Besides using bronze powder, iron powder has also been tried for making sintered iron bearings. Such bearings possess greater strength. Non-metallic bearing materials They are Teflon (poly tetrafluoroethylene) It has coefficient of friction < 0.04, without lubrication. It has good stability at high temperatures. It is chemically inert to water and many chemicals and solvents. Fillers like glass and graphite increase the resistance to deformation. Mach inability The ease of machining depends upon (i) the design of tools, (ii) method of lubrication, and (iii) the microstructure and properties of the metal. — Mach inability is an important property of a metal used in manufacturing operations. It may be measured as the quantity of chips that can be removed in a given time, as the weight of chips per hour. Since the behavior of tool is important also, one may choose to determine mach inability in terms of useful life of the tool used or in terms of energy absorbed in machining one kg of chips. Taylor' measured the cutting qualities of a tool by measuring the fastest speed of turning that permitted the tool to hold its edge for 20 minutes. If surface finish appears to be of prime importance, one may like to determine mach inability in terms of surface finish of the work piece. One may, thus, conclude that mach inability involves several properties of a material, each of varying importance. Hence, mach inability of a metal may be considered very good if the greatest amount of material can be removed in the shortest time for each grind of a given tool, while obtaining satisfactory surface finish, with the ultimate objective being low overall cost. Good mach inability is associated with: the removal of material with moderate forces the formation of rather small chips medium degree of tool abrasion good surface finish. All machinable metals are compared to a basic standard, and the comparison yields a percentage of rating which indicates the ease of cutting each metal. The standard metal used for the 100% mach inability rating is steel, coded by the American Iron and Steel Institute (AISI) Index as B 1112 steel. It is free machining steel. When compared to this steel, different metals have their mach inability rating as follows: Metals Mach inability Rating (%>) Aluminium 300-2000 Ni-steels 40-50 C-steels 40-60 Cast Iron 50-80 Thermal expansion When thermal energy is added to a material, a change in its dimensions occurs. For example, if a 10 cm long rod of mild steel is heated (and it is free to expand) it increases in length. This phenomenon is thermal expansion and the property of a material responsible for this is known as coefficient of thermal expansion. The coefficient of (linear) thermal expansion is the amount of expansion in a unit length of a solid material as a result of a temperature rise of 1°. — Coefficient of thermal expansion, a = 1 di I ' Dt ...(in)(vi) Copper, nickel, aluminum and austenitic alloys retain their much of tensile ductility and resistance to shock at low temperatures in spite of the increase in strength, (v/i) F.C.C. metals and alloys retain their ductility substantially unimpaired up to - 24°C. (viii) A tendency for B.C.C. metals (e.g., steels) to (become sensitive to multi axial stresses under shock load conditions and this is manifested by the sharp decline in the value of the absorbed energy in the notch impact test thereby indicating that the materials) behave in a brittle manner, at lower temperatures. THERMAL PROPERTIES By thermal property is meant the response of a material to the application of heat. As a solid absorbs energy in the form of heat its temperature rises and its dimensions increase. The energy may be transported to cooler regions of the specimen if temperature gradients exist and ultimately, the specimen may melt. Thermal shock resistance Thermal shock defines the conditions of a body when it is subjected to sudden and severe changes in temperature caused either by a change in external environment or by internal heat generation. The ability of a body to withstand such temperature changes without failure is called thermal shock resistance. A ductile material will withstand severe thermal shock much better than brittle materials of comparable strength and thermal properties. In ductile materials any excessive thermal stress developed can be dissipated as the result of plastic deformation whereas, in brittle materials, the stress at a point of stress concentration is usually the governing stress tending toward failure. Thus, in ductile materials, even a very severe thermal shock may not result in fracture, but it can cause distortion or excessive deformation. However, low ductility and notch sensitivity will enhance thermal crack formation. - Thermal fatigue also may occur in systems subjected to cycles of many sudden changes of temperature. Repeated cycles of rapid heating or drastic cooling tend to reduce the capacity of the will be either larger or smaller in diameter as compared to the solvent atom. For example, steel contains iron as solvent and carbon as solute atoms, having large difference in their diameters. Since, solvent and solute atoms have different sizes, when solute is added to solvent, distortion in lattice takes place. If the solute atom is larger than the solvent atoms, compressive strain fields are set up, and if it is smaller, tensile fields. In both the cases, the stress field of a moving dislocation interacts with the stress field of the solute atom, thereby increasing the stress required to move the dislocation through the crystal. This impedes dislocation motion. The more the difference between atomic sizes of solvent and solute atoms, the higher is the stress field around solute atoms. This provides more resistance to the motion of dislocations and hence increases the tensile strength and hardness of the material. If the amount of solute or the number of solute atoms is more, greater will be the local distortion in the lattice and hence more will be the resistance to the moving dislocations. This will increase the hardness and strength of the material. References Aldridge, D.R., “Developing the Shared Energy Savings Contract,” Heating/Piping/ Air-Conditioning (September 1995), pp 61-63. Angrist, S.W., Direct Energy Conversion (The Allyn and Bacon, Inc., Boston, MA, 1971). Aphornratana, S., “Thermodynamic Analysis of Absorption Refrigeration Cycles Using the Second Law of Thermodynamics Method,” International Journal of Refrigeration, vol 18, No. 4 (1995), pp 244-252. Buist, R.J., “Effective Applications of Thermoelectric Heat Pumps,” Proc. 14th Intersociety Energy Conversion Engineering Conf. (Am. Chem Soc., Washington, DC, 5-10 August 1979), vol 2, pp 1850-1853. Cooper, W.S., “Operating Experience With Water Loop Heat Pump Systems,” ASHRAE Trans vol 100, No. 1 (1994), pp 1569-1576. Cory, J., “Advancements in Automotive Stirling Engine Development,” Proc. 20th Intersociety Energy Conversion Engineering Conference, SAE P-164 (1985), 3.203-3.211. 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