Materials Selection And Testing - Report Example

Material Selection Student Number: Date: Summary The report finds that the aluminium piston is manufactured by gravity die casting. The con rod is manufactured by sand casting, while the turbine blade is manufactured using investment casting. The chisel is manufactured by steel forging process, tempering and then quenching in oil. Fabrication of component E affects the grain structure of the welded part such that, the welded region is characterized by larger grains compared to the parent material. For the polymers, addition of glass fibre in nylon 66 increases the tensile strength of the material. A similar observation is also made with toughened polystyrene. Introduction Engineering materials are typically divided into metals and alloys, ceramics and glasses, polymers, and composites. Each material has its own unique physical, mechanical and chemical properties. Mechanical processes used in the manufacture of components using these materials, such as mechanical working of metals and metal alloys, powder processing of ceramics and metals, adhesive welding, extrusion of polymers and formation of polymer sheets etc. affect the structure and properties of the materials. Processing of metals and metal alloys When a metal undergoes solidification, grains are formed. Depending on the rate of cooling and subsequent treatments, the grains may be small, large or long (Lin, 2015). Grain growth occurs when the metal is left to cool for a long period of time. The direction of grain growth and grain size can be altered by deforming the metal or metal alloy in the hot or cold state and this affects the material’s mechanical properties such as strength, hardness and ductility (Lin, 2015). Heat treatment processes affect the grain size and structure and thus, the material properties. Rapid cooling promotes large grains, while slow cooling promotes large crystals. There are various methods used to produce metal and metal alloy products by manipulating the shape through plastic deformation. This methods include: hot working, cold working, hot rolling, cold rolling, forging, drawing, spinning and extrusion. Processing of Ceramic and Glass Materials To make products from ceramic, clay is formed into the required shape and then heated in a furnace until all moisture is lost. The temperature of the furnace is then raised to about 800oC which makes the particles to fuse without melting. The products are then passed through a kiln where they are preheated, sintered and cooled slowly to prevent cracking (Shackelford & Doremus, 2008). Glass is produced by melting minerals such as silica and shaping them using other method. The principles of change in structure in ceramics and glass is similar to those for metals and metal alloys. Thermal and thermochemical treatments change the structure in nominally amorphous ceramic materials comparable to metals and metal alloys. For example, oven tops made from glass-ceramic are thermally treated to form a microstructure with a low coefficient of thermal expansion. Ceramics have problems in change of structure due to their high inherent hardness, directionality of molecular bonding, limited slip systems and slow equilibration (Shackelford & Doremus, 2008). These limitations result in hard and soft glide planes and low ductility. Despite the limitations, tough, thermally treated ceramics such as zirconia have been developed and increasingly being used as structural ceramics. Processing of Polymers Thermoplastics are melted and extruded through a die of the required shape (see figure 1). The temperature and pressure of the polymer must be within certain limits to allow for correct processing (Gerdeen, et al., 2005). Processing methods for polymers include: extrusion and moulding. Figure 1: Extrusion process Composites Composites are made of two or more materials to achieve the best properties (Advani & Hsiao, 2012). They are classified as particle composites and fibre composites. In this report, we study the effect of manufacturing processes on the grain structure of the parent material. The components labeled A-D have been provided and the report provides a detailed explanation of the methods used in the manufacture of the components and why they are suitable. For component E, a microscopic examination of the grain structure has been shown to illustrate structural changes that occur in the material after welding. We also carried out tensile test on six samples of polymer materials and reported the results. Report analysis and Discussion Task 1 Most, if not all of these components can be made by liquid casting and moulding. Casting is the process of making metal components by pouring a molten liquid metal into a mould with the desired shape and allowing it to cool and solidify. The product produced has a similar shape as the mould and undergoes machining process for completion or further processing such as drawing and rolling. There is rapid cooling at the surface of the mould and slower cooling towards the core of the mould. This difference in cooling between the outer region and the core produces different grain structure, thus casting may be difficult on the outside (Kundu, et al., 2010). Rapid cooling results in fine crystal grains. Figure 2: A: Aluminium piston; B: Con rod; C: Stainless steel turbine blade; D: Cold chisel; E: Fabricated bracket. Different methods of casting that could have been used to manufacture the components A-D include: sand casting, centrifugal casting, die casting (pressure casting and gravity casting), investment casting and powder techniques. Sand casting – Sand casting is used with metals such as cast steel, cast iron and brass. Using a wooden blank, the shape of the component is made in the sand. Risers are made in the sand to allow the escape of gases so that the molten metal takes up the shrinkage space (see figure 3). This is to prevent the formation of air cavities and other defects. Sand casting is very expensive and is not ideal for production of components in large quantities. Sand casting is suitable for the production of the parts of fabricated bracket and the con rod. Casting can be performed on both ferrous and non-ferrous materials to produce complex components. The casted parts in figure 2 E are joined by welding to form a permanent bond. Figure 3: Sand casting Centrifugal Casting – This method of casting is similar to die casting. In this method, several moulds are connected to a single feed point. Centrifugal casting is suitable for the production of tubes, reams and gear blanks. Die casting – Die casting is more efficient and economical manufacturing process that is used to produce dimensionally accurate complex shapes. The molten metal is fed into a metal mould by pressure or by gravity. Initially, producing the moulds is very expensive, but it achieves a longer time of production. Production of many components is achieved by having multiple moulds connected to the feed point. There is rapid cooling which produces a good surface finish. Aluminium, zinc, copper and magnesium have a high degree of fluidity, which makes it suitable to use this method in manufacturing the aluminium piston (Sahoo & Sahu, 2014). Investment casting – This process involves making wax shapes in a metal mould. The shape is lined with a ceramic material. After the wax is melted, a ceramic mould resembling the component is formed (Sahoo & Sahu, 2014). The molten metal is poured in the mould and the ceramic is broken to remove the cast. This method is suitable for manufacturing components from metals with high melting temperature, e.g. the stainless steel turbine blades. High temperatures of such metals would destroy ordinary die casting moulds. Investment casting produces excellent dimensional tolerance. Thus, investment casting would be appropriate for the production of turbine blade (Lin, 2015). Chisel is made from tempered steel, a material with high melting point and therefore, this method could also be used to make this component. Heat Treatment Process Heat treatment is a process that is used to control the microstructure of materials, specifically metals such as aluminium, iron and steel. This imparts properties that that are beneficial to the working life of a component, such as temperature resistance, strength, ductility and surface hardness (Lin, 2015). Heat treatment processes include tempering, case hardening, ageing and solution treatment, annealing, quenching and normalizing. In this report, we will discuss how metal hardening affects the structure, properties and behavior of the parent steel material from which the chisel is made from. Metal Hardening Hardening is a heat treatment process that involves heating of steel and keeping it at a suitable temperature until all pearlite transforms to austenite. The heated steel is then quenched. Metal quenching is a process in which the heated steel is quenched in oil or water or any other suitable coolant to achieve full hardness. The material is hardened by rapid cooling when immersed in coolant. To achieve final material hardness, toughness and dimensional stability, the quenched material is tempered, aged, or stress relieved. The temperature at which austentizing occurs depends on the amount of carbon in the steel material. It is essential that the material is heated for longer times to ensure that the core of the material is transformed into austenite. The structure of hardened steel part is affected by the cooling rate, particularly within a temperature range of 650 to 550oC (Lin, 2015). At this temperature austenite transforms more rapidly into pearlite, cementite, or ferrite microstructure. Martensite microstructure is formed at temperatures between 300-200oC. This causes increase in volume of the material, thus developing high internal stresses as well as strains. Hardening of steel makes the material to achieve maximum hardness and tenacity while reducing grain size and ductility (Kundu, et al., 2010). Other metal treatment Processes Forming – Metal forming includes a wide range of manufacturing processes in which the material is plastically deformed into the desired shape by applying a mechanical force that exceeds the material’s yield strength. Typically, metal forming is performed at three basing temperature ranges: Cold working (room temperature), warm working (above room temperature but below recrystallization temperature) and hot working (temperature above recrystallization). Forming processes include: rolling, forging, extrusion, drawing, spinning, shearing and bending. Cold working causes internal stresses which deform metal crystals – producing long crystals (Ahmad, et al., 2014). Cold working changes material properties dramatically. For example, carbon steels become stronger and harder after cold working. Hot working produces an oxide skin on the surface material which makes the material more hardened and durable. Hot working recrystallizes the metal, closing up porosity and breaking up inclusions in the material. The process destroys old weaker grain structures to produce isotropic grain structure in the material. Drop Forging – The chisel is made by forging process. Forging is a material manufacturing process in which the material is shaped using compressive forces. The forging process can be performed at low temperatures (cold forging) or at high temperatures (hot forging). In drop forging, the material to be formed is heated first, before forcing it into a die with contours. The force applied can be as high as 2000 tons. Drop forging can be classified into open die and closed die forging. This method of material processing can be used in the manufacture of chisel. Figure 4: A basic configuration of drop forging process Drop forging is performed on a hammer and anvil. The moving (upper) die is fixed on the ram, while the lower die is fixed on the anvil (see figure 4). The compressive energy applied is transferred to the work piece through the ram and the upper die. The forging process controls the resultant structure and properties of the parent material. The impact force applies a stress on the material within a very short time (Woodworth, 2015). The mechanical impact force is concentrated on the surface layers and the rapid withdrawal of the stress results in metal relaxation before the effect of the force penetrates to the center of the material. This results in inhomogeneous structure – with the outer layers exhibiting a typical hot-worked structure while the center of the material is still as cast. Increasing the impact force to achieve a deeper penetration leads to internal cracking of the material (Lin, 2015). A more homogenous structure is achieved by press forging which results in total penetration. Task 2 Cold Rolling as a forming process of steel Rolling is a cold working process in which a material is pushed through a gap in between two heavy power drivel rolls (see figure 5) to produce a continuous shape and a sheet metal. In cold rolling, the metal material is pushed between the two rolls and compressed at room temperature (or temperature below the crystallization temperature). Cold rolling introduces strain in the material which determines its final hardness and other properties (Lin, 2015). Cold rolling dissociates the ferrite and pearlite microstructure in low carbon steels. The cold rolling process changes the aspect ratio of grains along the direction of rolling producing a banded morphology with strain energy that is used to recrystallize the material. The process deforms the grain structure – the grains become elongated in the direction of rolling, increasing the grain boundary (Ahmad, et al., 2014). The attainable strength also depends on the grain size of steel. As more grains are deformed and dislocated, the material becomes more brittle and less malleable. The defects introduced in the crystal structure of the material creates a hard microstructure that prevents slip. Figure 5: Rolling Welding The component E is a fabricated bracket (see figure 6) with two parts that are joined by welding. The fabrication of the bracket “E” has some effects on the grain structure of the parent material. Effects of welding on the grain structure of steels Welding joints in carbon steels are characterized by larger sizes of grains around the fusion line. The grains are oriented in the direction of heat flow. Weld Parent Figure 6: Welded section of a fabricated bracket The heat affected zone (HAZ) contains Widmanstatten ferrite, with small colonies of pearlite (see figure 7a & 6b) (Boumerzoug, et al., 2010). Welding may produce some defects caused by abnormal stirring of the heat affected zone, making the component to be prone to failure. Figure 7(a): microscopic structure of HAZ steel weld Figure 7 (b): Microstructure of welded steel showing different grains Solid state transformations such as phase transitions, grain growth, recrystallization, tempering, and annealing occur in the HAZ of steel welds. The HAZ region with coarse grains is adjacent to the weld fusion region and is characterized by larger grains than those in the base metal (Boumerzoug, et al., 2010). The center of the weld metal differs from the other zones. This is because of pseudo-grains and inhomogeneity of the microstructure due to faster cooling rates. The center of the weld primarily contains ferrite and colonies of pearlite. Task 3 Tensile tests are performed to measure the amount of force required to break a polymer sample and the extent to which the sample can be stretched before reaching a breaking point. The stretching is done using a machine, such as Instron. The machine clamps each end of the sample specimen. Figure 8 below shows the geometry of the tensile testing machine and how the sample specimen is fixed on the machine for testing. Figure 8: Geometry of tensile testing machine for polymers When the machine is turned on, the two fixed heads stretches the sample in opposite directions while measuring the force applied and extension of the sample. The sample is subjected to increasing tensile force until it deforms or breaks. The tensile test can be done on any polymeric material that can be machined into a sample specimen. Such tests are used to produce stress-strain diagrams that are used in the determination of tensile modulus (Advani & Hsiao, 2012). The results of tensile test are useful in specifying optimal materials, providing quality control checks for materials, and design of parts to withstand the applied forces. The figures below show the results obtained when samples of polymers were subjected to tensile test. Figure 9 (a): Tensile test for HDPE Max. Applied force before breaking = 589.33 N Change in length = 8.5 mm Figure 9(b): Tensile test for LDPE Max. Applied force before breaking = 79.33 N Change in length = 7.1 mm Figure 9(c): Tensile test for Nylon 66 Glass filled Max. Applied force before breaking = 4244.67 N Change in length = 10.15 mm Figure 9(d): Tensile test for Nylon 66 Max. Applied force before breaking = 1696.0 N Change in length = 17 mm Figure 9(e): Tensile test for Polystyrene Max. Applied force before breaking = 532.0 N Change in length = 2.5 mm Figure 9(f): Tensile test for toughened Polystyrene Max. Applied force before breaking = 1692.0 N Change in length = 6.2 mm The table below summarizes the results of tensile test from the graphs in figure 9(a) – 9(f). Table 1: Tensile test results for polymer specimen samples Materials Ultimate tensile force (N) Elongation (mm) HDPE 589.33 8.5 LDPE 79.33 7.1 Nylon 66 (Glass filled) 4244.67 10.15 Nylon 66 1696.0 17 Polystyrene 532.0 2.5 Toughened Polystyrene 1692.0 6.2 From the results above, we can compare the mechanical properties of the polymers. The ultimate tensile force measures the ability of a material to resist breaking. Nylon 66 (Glass filled) exhibits the largest tensile force (4244.67N) of all the polymer samples, while LDPE exhibits the lowest tensile force (79.33N). Toughened polystyrene exhibits more tensile force (1692.0) than ordinary polystyrene (532.0). In terms of elongation, Nylon 66 exhibits the highest elongation (17.0mm) of all the polymers, while polystyrene and toughened polystyrene exhibit the lowest levels of elongation. Measuring tensile strength of a material is crucial for materials that are going to be under tension in a structure. Polymers for structural use need to have good tensile strength. Conclusions Die casting is the best process for manufacturing of the aluminium piston. This is because the process is efficient and economical, and can produce dimensionally accurate shapes. Furthermore, aluminium material has a high degree of fluidity which makes die casting a suitable method for manufacturing the aluminium piston. For the Turbine blade, investment casting is the preferred method of manufacturing the components because the metals used to make these components have high melting temperatures. Sand casting is suitable for the manufacture of the con rod because sand casting can produce complex shapes from ferrous and non-ferrous materials. The chisel is produced through forging process because of the process can shape hard materials into the required shape using an impact force. Welding of steels results in larger grain sizes around the fusion line at the HAZ. The grains become oriented in the direction of heat flow and are smaller compared to the grains of the parent material. From the results obtained in polymer tensile test, we can conclude that addition of glass fibre to nylon 66 strengthens the material, which also exhibits the highest tensile strength of all the polymer samples tested. Toughened polystyrene is also stronger than ordinary polystyrene. LDPE has the lowest tensile strength of all the samples. In terms of elongation nylon 66 exhibited the highest elongation while polystyrene was observed to exhibit the lowest elongation. References Read More

Thermal and thermochemical treatments change the structure in nominally amorphous ceramic materials comparable to metals and metal alloys. For example, oven tops made from glass-ceramic are thermally treated to form a microstructure with a low coefficient of thermal expansion. Ceramics have problems in change of structure due to their high inherent hardness, directionality of molecular bonding, limited slip systems and slow equilibration (Shackelford & Doremus, 2008). These limitations result in hard and soft glide planes and low ductility.

Despite the limitations, tough, thermally treated ceramics such as zirconia have been developed and increasingly being used as structural ceramics. Processing of Polymers Thermoplastics are melted and extruded through a die of the required shape (see figure 1). The temperature and pressure of the polymer must be within certain limits to allow for correct processing (Gerdeen, et al., 2005). Processing methods for polymers include: extrusion and moulding. Figure 1: Extrusion process Composites Composites are made of two or more materials to achieve the best properties (Advani & Hsiao, 2012).

They are classified as particle composites and fibre composites. In this report, we study the effect of manufacturing processes on the grain structure of the parent material. The components labeled A-D have been provided and the report provides a detailed explanation of the methods used in the manufacture of the components and why they are suitable. For component E, a microscopic examination of the grain structure has been shown to illustrate structural changes that occur in the material after welding.

We also carried out tensile test on six samples of polymer materials and reported the results. Report analysis and Discussion Task 1 Most, if not all of these components can be made by liquid casting and moulding. Casting is the process of making metal components by pouring a molten liquid metal into a mould with the desired shape and allowing it to cool and solidify. The product produced has a similar shape as the mould and undergoes machining process for completion or further processing such as drawing and rolling.

There is rapid cooling at the surface of the mould and slower cooling towards the core of the mould. This difference in cooling between the outer region and the core produces different grain structure, thus casting may be difficult on the outside (Kundu, et al., 2010). Rapid cooling results in fine crystal grains. Figure 2: A: Aluminium piston; B: Con rod; C: Stainless steel turbine blade; D: Cold chisel; E: Fabricated bracket. Different methods of casting that could have been used to manufacture the components A-D include: sand casting, centrifugal casting, die casting (pressure casting and gravity casting), investment casting and powder techniques.

Sand casting – Sand casting is used with metals such as cast steel, cast iron and brass. Using a wooden blank, the shape of the component is made in the sand. Risers are made in the sand to allow the escape of gases so that the molten metal takes up the shrinkage space (see figure 3). This is to prevent the formation of air cavities and other defects. Sand casting is very expensive and is not ideal for production of components in large quantities. Sand casting is suitable for the production of the parts of fabricated bracket and the con rod.

Casting can be performed on both ferrous and non-ferrous materials to produce complex components. The casted parts in figure 2 E are joined by welding to form a permanent bond. Figure 3: Sand casting Centrifugal Casting – This method of casting is similar to die casting. In this method, several moulds are connected to a single feed point. Centrifugal casting is suitable for the production of tubes, reams and gear blanks. Die casting – Die casting is more efficient and economical manufacturing process that is used to produce dimensionally accurate complex shapes.

Read More