Materials Selection and Testing - Assignment Example

Task 1 Identify the most suitable process for manufacturing the components identified as A,B,C and D. With the aid of evidence from a range of sources and using sketches and diagrams give a detailed report of the methods that could have been used to manufacture the components and why they are suitable, ensure you comment on the effect the process has on the microstructure of the component. (i) Component A Component A is a piston. The piston is manufactured by gravity die casting. Figure 1 The manufacturing involves use of steel molds with the process being semi-automated or fully automated as can be seen in Figure 2 and figure 3. The process of melt preparation will usually depend on the alloys in use. The die-mold and molding poring temperature are of great interest so as to ensure quality of casting.   Figure 2: Figure 3 After casting the piston, there is heat treatment so as to improve the mechanical and the physical properties of the cast. The heat treatment process involves controlled heating and cooling in which process there is no change in shape of the casting. Through the heat treatment there is improvement in strength and hardness in addition to improving the machinability of the casting. The heat treatment takes place in properly designed furnaces. Solution heat treatment- this involve heating the casting to a high temperature for required time , then the cast is quenched rapidly through immersion in water of glycol solution, the objective being increasing the strength of the casting. The other heat treatment process is artificial aging where the cast piston is to be heated for specified time period, more than the solution treatment. With the heating process being extended over time, there is refinement of grain structure, resulting to high strength properties of the object. Artificial aging in essence speeds up the aging process of the cast which is supposed to occur naturally over a number of years (ii) Component B The manufacturing process suitable for manufacturing the con rod ( figure 4) is closed die drop forging. This is sometimes referred to as “near-net-shape” or “impression die” forging, closed-die drop-forging offers numerous technical advantages, saves on material usage and carries an upfront tooling investment that’s not nearly as expensive as you may think. Drop forgings are available in a wide range of materials. Size ranges from as little as a few grams up to many tonnes. Figure 4 Improved Strength to Weight Ratio In the closed die drop forging process, metal bar or billet is heated before being placed in the die then hammered until the metal completely fills the die cavity as can be seen in figure 5. During this process of plastic deformation, the material’s grain structure becomes compressed and aligned to the component shape which imparts greatly increased directional strength with reduced stress concentrations in corners and fillets. Components manufactured this way are stronger than their equivalent machined-from-solid or cast parts. Figure 5 Structural Integrity Forging a component greatly reduces the possibility of metallurgical defects such as porosity or alloy segregation as found in some castings. This leads to reduced scrap, a uniform response to heat treatment and predictable component performance in the field. There is virtually no possibility of porosity being introduced during the forging process. Even this can be checked with a low cost ultrasonic test after manufacture. The possibility of small surface cracks can be managed with a simple crack detection procedure towards the end of the process. Post Forging Machining Parts can be machined post forging with no loss of quality, because there are no voids or porosity in the fished article. Forging is often combined with machining for improved dimensional accuracy. This can also be achieved by post forge coining or sizing. (iii) Component C :Turbine blade The component c (figure 6) is manufactured by investment casting. In this process we have a master mould which is used to product moulds. Figure 6 Having a mould that has the turbine blade pattern, the next step is to produce the wax pattern. The wax pattern is produced by pouring wax into the mould and swishing it around until the point when we have an even coating in the range of 3mm covering the inner surface of the mould with a repeat of the same up to a point when the final desired thickness is achieved. The other method of producing a pattern is by filling the entire mould with molten wax and then allowing it to cool so as to produce a solid object. The later is the method that is used in production of turbine blade. With the wax pattern successfully in place the next step is the production of ceramic mould. This is achieved through a repetition of a series of 3 steps: coating, stuccoing and hardening up to the point when we have a desired thickness. In coating we have the wax pattern dipped into slurry of fine then drained so that we have a uniform surface coating being created. In the first step of this process fine materials also referred to as prime coat are to be used so that we have the fine details of the mould being preserved. In stuccoing we have the application of coarse ceramic particles through the dipping of the pattern in a fluidized bed, by the pattern being placed in a rainfall sander of application of the materials by hand. The hardening process is undertaken as a curing process of a coating. There is a repeat of the process up to a point when the desired thickness of coating, normally 5 -15mm is achieved. Having achieved the desired thickness the moulds are left to dry completely, a process that will required about 16 to 48 hrs. Some of the materials that can be used in producing the investment are zircon, silica, alumina and aluminate silicates. Zirconian may be a preference because it will not react with molten metal. With the ceramic investment dried next will be dewax process. This involves the fully cured moulds being turned upside down and then being placed in a furnace so as to melt the wax out. There should be precaution taken to ensure that the investment does not break as a result of expansion of the wax as it melts out. This is ensured by having quick melting of the wax. Upon removal of the wax the investment will be subjected to burnout preheating where the mold is heated to between 870 °C and 1095 °C so as to remove any temperature and residual wax. This heating may also serve as the preheating that is required before poring, which is the next step. It may also be necessary that the mould is left to cool so that it can be tested and preheating done separately. Preheating is necessary so as to allow the metal staying liquid much longer, allowing it to fill all the ceramic mould details thus increasing dimension accuracy. After the mold being preheated next is pouring. This involves the mold being placed up side up in a tub filled with sand. Now the molten metal is poured by gravity with possibility of forced application through application of positive air pressure. Finally we have divesting where the cast is removed from the mold. The cast is to be cleaned up, the sprue cut off. Heat treatment is undertaken for hardening and stresses releaf. (iv) Component D A cold chisel 9Figure 7) is generally forged out of tool steel or H.C.S. It is to be normalized after forging and the cutting edge is to be ground to the correct angle. Hardening and tempering is done by single heating method. Figure 7 In this method the chisel is heated up to 2” from the cutting edge in clean fire to a temperature of 760C (cherry red).Then the tool is removed from the source of heat. Scales are quickly removed by means of a wire brush. Half of the heated portion is quenched in clear water, holding the chisel vertically and moving it up and down to prevent water line cracks. The tool is then removed from water, cleaned and the cutting edge is polished quickly by using emery paper or a smooth file or by rubbing on the floor. Now wait for the tempering colour (dark purple 290C) to appear at the cutting edge. When it appears, the whole tool is quenched in water. The degree of hardness obtained, is tested by cutting a piece of mild steel by the heat treated chisel. Screw drivers, punches, drifts, scribers and all other tools whose cutting edges are to be hard and the remaining portion to be tough are hardened and tempered in the same way. Task 2 Carry out a macro and microscopic examination of the component identified as E. In about 500 words and with the aid of photographs and sketches describe the effect the forming process and welded joint shown in the image identified as E has upon the microstructure of the material, include in your explanation a clearly labelled sketch of the cross-section identifying the changes in structure. Figure 8 shows a schematic diagram of the bracket while figure 9 is the picture of the actual bracket. Figure 8 Figure 9 The figure 10 shows a general macro-structure that that is to be observed across welded joint. We have three zones : fusion zone (FZ) that melts and then resolidifies when the welding process is undertaken with columnar grains. Heat affected zone (HAZ) where there is no melting but we have microstructural changes and tghen we the base metal (BM) where the metal structure remain intact. For the three metals the BM will retain its current structure. The micro –structure in the FZ and HAZ are expected to be as shown in Figure 12 and figure 13 respectively. In both cases we have martensitic microstructure even though in the former have columner grains. This are related to the cooling and thermal regimes associated with the zones during the welding process. Figure 10 Mild Steel Hot Rolled (parent material). Figure 11 Figure 12 Figure 13 Mild Steel Hot Rolled (parent material). Figure 14 0.4% carbon steel normalised Figure 15 A research done on mild steel to find out the effect of coarse initial grain size and heat on microstructure. The heat input for welding was sett at 0.5, 1 and 2 kJ/mm. It was found that lower heat input could result in maximum hardness as a result of formation of martensite. Observation of initial coarse grain (ICG) was that there was maximum hardness for ICG where the heat inputs of 0.5 and 1 kJ/mm where this is associated with high carbon , martensite and banite. The pattern of changes in hardness with level of heat is as shown in figure 16. Figure 16 Task 3 Task 3 Carry out tensile tests on a series of polymer samples, analyse the results and explain the effect the properties of the samples has on the results. Carry a simple workshop heat treatment test on 3 samples of plain carbon steel, explain results. HDPE and LDPE From the figures 17 and Figure 18 it can be seen that the high density polyether (HDPE) has higher strength compared to the low density polyether (LDPE) as can be seen from the table 1 where the former has UTS=89.333 and the later UTS=56.5. LDPE is seen to have a higher elongation of 100 while HDPE elongation is 87.1. Een though the elocation are close HDPE elongation is happening at higher a stress level close to UTS but in HDPE elongation happens when thestress in the material has totally reduced meaning that it is at a point when the material has failed and can no longer sustain any load. This shous that LDPE has better characteristics when it comes to ultimate tensile strength design. Table 1: Summary of material characteristics Material UTS Elongation 1. HDPE 89.333 87.1 2. LDPE 56.5 100 3. Poly styrene 495.333 40.4 4. Toughened poystyrene 1692 6.25 5. Nylon 2044.667 273.5 6. Nylon GF 4248 10.7 High density polyether: Figure 17 Low density polyether: Figure 18 Nylon Nylon has proved to be widely used plastic because of having extreme strength, being wear resistant and also because of its self lubricating properties. The material is also characterized by being resilient, non marring, has high impact resistance, with continuous operating temperature of about 180º F, and it is also light weight. Figure 19 shows the characteristics of Nylon 66 while figure 8 shows the characteristic of Glass filled Nylon 66. It can be seen that the nylon 66 has lower UTS of 2044.667 compared to that of Glass filled Nylon 66 (Figure 20) which is much higher at 4248. But reiforcement of thwe Nylon 66 by glass filling increases the strength but but makes it much rigid where it can be seen that the elongation of Nylon 66 is much high with a value of 273.5 while the rainforced Nylon 66 GF has elongation of 10.7. Nylon 66 Figure 19 Nylon 66 glass filled: Figure 20 shows a polystyrene curve which is characterised by having very steep initial steep rise to a maximum value which then drops slightly upon yielding. After the drop in strength after yielding the strength of material is seen to increase linearly with plastic deformation up to a point when it breaks. The UYS of this material is 495.33N/mm2 while the elongation is 40.4mm Figure 21 on the other hand shows the curve of toughened polystyrene otherwise known as general purpose polystyrene. This is seen as being rigid and dimensionally stable but is less ductile. The toughened polystyrene is seen to have a less steep initial rise curve. The Curve rises steadily up to the yield point where the stress much higher compared to that seen in polystyrene. The UYS of 1692NN/mm2 is much higher compared to that observed in polystyrene. On the other hand the hardened polyststyrene is much less ductile with alongation of 6.25mm. The toughness in the material is as a resuklt of an increase in rubber content. Polystyrene Figure 20 Toughened polystyrene Figure 21 References Gordon, J.E.(1976), The New Science of Strong Materials, or Why You Don’t Fall Through the Floor, Princeton University Press. Hertzberg, R.W.(1976), Deformation and Fracture Mechanics of Engineering Materials, Wiley, New York,. Knott, J.F. (1973). Fundamentals of Fracture Mechanics, John Wiley – Halsted Press, New York. Strawley, J.E.(1965), and W.F. Brown, Fracture Toughness Testing, ASTM STP 381, 133, 1965. . Read More

Components manufactured this way are stronger than their equivalent machined-from-solid or cast parts. Figure 5 Structural Integrity Forging a component greatly reduces the possibility of metallurgical defects such as porosity or alloy segregation as found in some castings. This leads to reduced scrap, a uniform response to heat treatment and predictable component performance in the field. There is virtually no possibility of porosity being introduced during the forging process. Even this can be checked with a low cost ultrasonic test after manufacture.

The possibility of small surface cracks can be managed with a simple crack detection procedure towards the end of the process. Post Forging Machining Parts can be machined post forging with no loss of quality, because there are no voids or porosity in the fished article. Forging is often combined with machining for improved dimensional accuracy. This can also be achieved by post forge coining or sizing. (iii) Component C :Turbine blade The component c (figure 6) is manufactured by investment casting.

In this process we have a master mould which is used to product moulds. Figure 6 Having a mould that has the turbine blade pattern, the next step is to produce the wax pattern. The wax pattern is produced by pouring wax into the mould and swishing it around until the point when we have an even coating in the range of 3mm covering the inner surface of the mould with a repeat of the same up to a point when the final desired thickness is achieved. The other method of producing a pattern is by filling the entire mould with molten wax and then allowing it to cool so as to produce a solid object.

The later is the method that is used in production of turbine blade. With the wax pattern successfully in place the next step is the production of ceramic mould. This is achieved through a repetition of a series of 3 steps: coating, stuccoing and hardening up to the point when we have a desired thickness. In coating we have the wax pattern dipped into slurry of fine then drained so that we have a uniform surface coating being created. In the first step of this process fine materials also referred to as prime coat are to be used so that we have the fine details of the mould being preserved.

In stuccoing we have the application of coarse ceramic particles through the dipping of the pattern in a fluidized bed, by the pattern being placed in a rainfall sander of application of the materials by hand. The hardening process is undertaken as a curing process of a coating. There is a repeat of the process up to a point when the desired thickness of coating, normally 5 -15mm is achieved. Having achieved the desired thickness the moulds are left to dry completely, a process that will required about 16 to 48 hrs.

Some of the materials that can be used in producing the investment are zircon, silica, alumina and aluminate silicates. Zirconian may be a preference because it will not react with molten metal. With the ceramic investment dried next will be dewax process. This involves the fully cured moulds being turned upside down and then being placed in a furnace so as to melt the wax out. There should be precaution taken to ensure that the investment does not break as a result of expansion of the wax as it melts out.

This is ensured by having quick melting of the wax. Upon removal of the wax the investment will be subjected to burnout preheating where the mold is heated to between 870 °C and 1095 °C so as to remove any temperature and residual wax. This heating may also serve as the preheating that is required before poring, which is the next step. It may also be necessary that the mould is left to cool so that it can be tested and preheating done separately. Preheating is necessary so as to allow the metal staying liquid much longer, allowing it to fill all the ceramic mould details thus increasing dimension accuracy.

After the mold being preheated next is pouring. This involves the mold being placed up side up in a tub filled with sand.

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