CHAPTER 2 PRODUCTION OF POROUS CASTINGS

Transcription

1 34 CHAPTER 2 PRODUCTION OF POROUS CASTINGS 2.1 CASTING Casting is the most common practice often used for making complex shapes that would be otherwise difficult or uneconomical to manufacture by other manufacturing methods. The process usually involves pouring of molten metal into a mould that contains hollow cavity of the desired shape, and then allowed to solidify. The solidified part is known as a casting, which is knocked out of the mould to complete the process. Then the removed casting is taken for fettling process to remove Runners and Risers. Finally the required quality of casting is obtained after the cleaning process. According to Banhart and Baumeister (1998) casting of metal and alloys around a filler material has attracted lot of interest in recent times. The following are the three steps: ( i ) Preparation of space-holder. ( ii ) By using organic or inorganic cores. ( iii) Infiltration of the filler material with metal and removal of space holders. It is a process of pouring the molten metal into a mould cavity, which contains cores of desired shape, size and then allowed to solidify. After

2 35 solidification the cores were removed and cavity of desired shape and size is obtained. Casting of metals and alloys around a filler material has recently attracted a lot of interest, because it is potentially a very economic way to create cellular structures of a wide range of metals and porosities. The filler material can either consists of low density materials that remain in the material or it consists of compact materials that can be removed after solidification. The method practiced here is casting metal around granules as this method produces an interconnected cellular (open cell) structures were metal is introduced into the casting mould as shown in Figure 2.1. Figure 2.1 Casting around granules 2.2 PRODUCTION OF POROUS GUNMETAL CASTINGS In this research work four porous gunmetal castings were produced. The steps involved in production of porous castings are shown in Figure 2.2.

3 36 PRODUCTION OF POROUS GUNMETAL CASTINGS MELTING OF METAL PATTERN AND MOULD BOX MAKING PREPARATION OF SAND BALLS MOULD MAKING FILLING THE MOULD WITH SAND BALLS POURING POROUS CASTING KNOCK OUT REMOVAL OF GATES AND RISERS CLEANING Figure 2.2 Layout for production of gunmetal casting Pattern Making Pattern making is the first step in a casting process. Skilled pattern maker builds a pattern of the object from the design provided using wood. The metal to be cast contract during solidification and this may be nonuniform due to uneven cooling. Therefore to compensate this reduction in size

4 37 the pattern was slightly larger than the finished product, a difference known as contraction allowance. For easy removal of pattern from the mould and to avoid damages during removal of pattern from the mould, pattern was slightly tapered. This allowance is known as draft allowance. Also during removal of pattern, the pattern is slightly shaken. This makes the size of the mould slightly larger. To compensate the above oversize an allowance known as Rapping or Shaking allowance considered and the pattern is under sized to compensate the above. Contract level of each metal and alloy differs from one another, hence scaled rules are different for each metals. In this research work, a wooden pattern as shown in Figure 2.3 of size 150mm X 150mm X 65mm was used to produce a mould, using green sand with 5% Bentonite and 3.5% moisture. A multi-part moulding wooden box of size 640mm X 640mm X 150mm was prepared to receive the pattern. The moulding box made of two parts, the bottom part known as drag and the upper part known as cope. The parting line or the parting surface is in line or surface that separates the cope and drag. Figure 2.3 Wooden pattern (gunmetal)

5 Preparation of Sand Balls Sand balls (cores) were used as space-holder filler. Sand balls were prepared manually by using core box with a mixture consisting of silica sand, Bentonite, clay and sodium silicate as additive material. The cavity in the sand is formed by using a pattern that was typically made out of wood. Percentage of Bentonite in sand mixture is an important factor in this research for producing porous castings because the porosity of the castings largely depends on the strength of the sand balls. Hence the sand balls should be strong enough to create hollow cavity in castings, this can be achieved only when the sand balls made with adequate amount of Bentonite which holds the sand together by providing bonding action. Bentonite composition used for making the sand balls given in Table 2.1. Table 2.1 Bentonite composition for gunmetal samples S.No. Sand ball size, in mm Percentage of Bentonite by weight % % % % To the above composition sufficient quantity of water was added. From the mixture, round balls of sizes ranging from 15mm to 30mm were made as shown in Figure 2.4 to Figure 2.7. These balls were dried and kept ready for use in molten gunmetal casting.

6 39 Figure 2.4 The picture of sand balls of size 15mm (gunmetal) Figure 2.5 The picture of sand balls of size 20mm (gunmetal)

7 40 Figure 2.6 The picture of sand balls of size 25mm (gunmetal) Figure 2.7 The picture of sand balls of size 30mm (gunmetal)

8 Mould Making There are four basic steps in mould making process. 1. Pattern placed in sand in the moulding box to create mould. 2. Incorporate the pattern and sand in the gating system. 3. Remove the pattern. 4. Fill the mould cavity with sand balls. The type of sand used for moulding was Green sand. Green sand is a mixture of silica sand, clay, moisture and other additives. The prepared sand mixture was then compressed around the pattern forming a mould cavity. The gating system (sprue and gates) serves as the path by which molten metal flows into the pattern cavity and feeds the shrinkage which develops during casting solidification. Proper design of gating system is critical in meeting the three important requirements. Proper gating includes runners and risers: (i) Prevents short flow paths and fast flow prevents casting misruns due to premature solidification. (ii) Provide molten metal for feeding shrinkage during solidification. (iii) Removal of gas and steam. The arrangement of mould cavity with gating, risering system and sand balls is as shown in Figure 2.8.

9 42 Figure 2.8 Mould showing the positioning of sand balls (gunmetal) Moulding boxes are made in segments that may be latched to each other and to end closures. For a simple object flat on one side the lower portion of the box closed at the bottom, will be filled with prepared green sand or casting sand. The sand was placed through a vibratory process called ramming. The pattern is placed on the sand and additional sand was filled around the pattern and rammed carefully. Excess sand was removed. After ramming, a cover plate is placed on the box and the box turned and unlatched. The two halves of the box, namely drag and the cope were parted separately. Pattern with its sprue was removed carefully without damaging the mould cavity. The two halves of the box are closed finally. This forms a green mould which was dried to receive the hot metal. If the mould is not dried adequately, a steam explosion can occur that can throw the molten metal. The sand balls of required size were filled into the prepared mould

10 43 cavity. The mould cavity and the sand balls were preheated to a temperature of 200 C Melting of Gunmetal Alloys of copper with tin, zinc and lead have been used for at least 2000 years due to their ease of casting and good strength and corrosion resistance. The use for cannons in mediaeval times led to the term gunmetal. Charge materials used in this research work for melting are: i) Copper ii) iii) iv) Tin Zinc Lead Cuprit (neutral or reducing fluxes) was used as flux material. 1 kg of cuprit required for 100 kg of metal. Cuprit form a protective blanket over the metal during melting to prevent contamination of the melt from the furnace atmosphere and to protect alloying elements, especially zinc, from oxidation, thereby suppressing zinc fume and the formation of showers of zinc oxide in the air. In this research work, the melting was carried out using oil-fired crucible furnace with capacity of 180 kgs shown in Figure 2.9. The charge was carefully selected to avoid impurities. Before charging the crucible and crucible lid were cleaned. The crucible was preheated before charging. The flux placed in the bottom of the crucible prior to charging.

11 44 Figure 2.9 Oil-fired crucible furnace The crucible was charged with required composition to full and ignited. When the charge in the furnace melts, the level of metal in the in the crucible lowers. Metal was charged additionally through the opening found on the lid. At 1100 C metal completely melts and was ready for pouring. Skimming was done to remove slag from the molten metal. Phosphorous (deoxidant) added to the molten metal, which combine with all oxygen present to form a fluid slag. After skimming the molten metal was poured in to the mould using a ladle. Figure 2.10 shows the gunmetal undergoing melting in oil fired crucible furnace.

12 45 Figure 2.10 Melting of gunmetal Melting point of gunmetal is around 999 o C and at specific gravity of 8.72x 10-6 kgmm -3. Table 2.1 shows the composition of gunmetal casting used for this research work. Table 2.2 Composition of gunmetal. Sample Copper Cu Element weight in % Tin Sn Zinc Zn Lead Pb Experiment Experiment Experiment Experiment

13 Pouring of Gunmetal The soundness of castings depends on how the metal enters a mould and solidifies. Proper care was taken in pouring the metal into the mould cavity. A ladle was used to transfer molten metal furnace to the moulds. In lip-pouring ladles are skimmed clean before pouring. Lesser the holding time reduces the oxidation of metal. When the finished mould is closed and ready for pouring, a weight was placed on the top surface to stop the pressure of the molten metal lifting the top of the mould and running out. Molten metal about 999 C is poured into the mould from a ladle. At this temperature the metal at fluid form and will fill at the small detail in the mould. As the metal cools around 855 C it begins to solidify. As the metal changes from liquid to solid the volume of the metal reduces, so more liquid metal is drawn from feeders (risers) to compensate. Shrinkage porosity may occur if insufficient metal available to fill the voids. To overcome the above, proper feeding arrangements are designed Knockout After the molten metal has been poured into the mould, it is allowed to cool and solidify. When the casting has solidified, it is removed from (the sand) the moulding box. This operation is called as knockout. First the drag and the cope were separated. Castings were removed manually by striking the sand with metal bar and pulled out of the sand with the help of a hook bar. Proper care was taken to avoid any damage to the casting during knockout.

14 Removal of Gates and Risers After removal of casting from the mould, the runners, gates and risers were removed. Gates and risers were removed by chipping hammers. Chipping hammer is used for copper alloy castings Cleaning Cleaning is an important operation which helps casting in giving the casting a good appearance after the same has been removed from the sand mould. Cleaning involves removal of following items from the casting: (i) Cleaning of exterior and interior of casting surface (including the removal of core sand). (ii) The removal of fins, wires and protuberances at gate and riser location (Trimming). (iii) Final surface cleaning, giving the casting its outward appearance (Finishing). In this research work, cleaning was done by shot blasting. Sand balls responsible for formation of porosity in the casting have to be removed clearly. Shot blasting provides higher rate of output than other process. Water jet cleaning was employed for cleaning and removal of core sand (sand balls). Only after removal of sand balls, formation of pores was analysed. 2.3 CHARACTERISATION OF POROUS GUNMETAL CASTINGS To characterise the nature of pores formed, all the four porous gunmetal castings were experimented. The density, percentage of porosity, cut-section analysis and radiography tests were conducted to confirm the

15 48 pores formed all through the castings. The hardness and compression tests were carried out to examine the mechanical properties of porous gunmetal castings. Figure 2.11 shows the characterisation of gunmetal foam castings. Porous Castings Characterisation Visual Exami nation Radiogr aphy Test Density Measure ment Porosity Measure ment Cut- Section Analysis Compressi on Test Hardness Test Figure 2.11 Characterisation of porous gunmetal castings Visual Examination Visual examination commonly defined as the examination of the material, component or product for the condition or non-conformance using light and the eyes. Visual examination typically means non-destructive testing using only raw human senses by not using any specialized equipments. Also visual inspection determines the surface problems or discontinuities over the surface. In this research work, the casting samples produced by using 15mm, 20mm, 25mm and 30mm sand balls were inspected (after cleaning and removal of sand balls) to examine the formation of pores and nature of pores

16 49 formed all through the casting samples. All the six faces of the castings were manually inspected Radiography Test Radiographic testing is a non-destructive testing method of inspecting materials for hidden flaws using the ability of short wavelength electromagnetic radiation (high energy photons) to penetrate various materials by either an X-ray tube(radiographic machine), or gamma ray source. In this research work gamma rays are passed through the test piece and the intensity of transmitted rays was recorded on a photographic film. Film is positioned behind the casting sections being radiographed. The distance from source to casting, section thickness, exposure, and the time was properly selected to give satisfactory results. Since most defects transmit the short-wavelength light better than the sound metal. The film is darkened more where the defects are in the line of source beam. Possible imperfections are indicated as density changes in the film in the same manner as an X-ray shown in the broken bones. Porous gunmetal samples were subjected to radiographic inspection for analysing the pores formed in the metal, the dark region (as like defects in perfect castings) of the film represents the more penetrable part of the object than the light rights which were more opaque. The dark regions represent the pores formed in the casting. The following are the parameters used: Source = Iridium 192 Film used = D7 Penerameter/ASTM = F40/50 Technique = SWSI

17 Density Measurement The density or mass density of a material is defined as its weight per unit volume. In some cases density is also defined as mass per unit volume. This quantity is more properly called as specific weight. Different materials usually have different densities. So density is an important concept regarding buoyancy purity and packaging. Less dense fluids float on more dense fluids, if they do not mix. In some cases density is expressed as the dimensionless quantities specific gravity or relative density. The foams are characterised in terms of their density, since the mechanical properties of the metallic foams largely depend on the density. The densities of the samples were determined by weighing the sample using a digital balance and by measuring their dimensions. Multiplying the mean value of the measured dimensions, the volume is obtained. The density is calculated using the formula shown in Equation 2.1. Weight of porous casting Density (2.1) Volume of producedsample Porosity Measurement Porosity is a measure of the void space in a material, and is a fraction of the volume of voids over the total volume. Percent porosity is a rough measure of the open volume equal to 100% minus the part density. The total volumes of interconnected and isolated porosity are normally found out. In this research work, the density of non-porous sample initially taken as the bulk casting density. Experimentation was done with porous models. Experiment conducted by varying the sizes from 15mm to 30mm. Achieved density are calculated using Equation 2.1 and in the density of produced porous castings. Soong-keun Hyun et al (2004) proposed that the

18 51 strength of the foam depends mostly on the base material and the relative density of the foam. Percentage porosity was calculated using the formula shown in Equation 2.2. Bulk castingdensity - Achieved density % porosity X100 (2.2) Bulk casting density Visual Examination of Cut-section Cut-section analysis is the most effective method to characterise the nature and distribution of pores throughout the castings. It provides basic information about the formation of pores inside the casting. It also provides information about the distribution of pores. If pores are interconnected then it is an open-cell foam casting. If the pores are not interconnected then it leads to closed cell foam casting. In this research work, all the foam castings with dimensions 150mm x 150mm x 65mm produced by casting technique were cut in to four equal halves to examine the distribution of pore structures and to confirm the formation of interconnectivity of pores. Air jet was used to clean the pores. The molten metal fills the voids in between the cores filled in the die and nucleation starts at the surface of the cores used. So, the shape and size of the pores depends directly on the size and geometry of the cores Compression Test The compression test of metallic foams is considered as one of the most applicable test for the characterisation of their metallic stability. It is the method for determining the behaviour of materials under crushing loads. The evaluation of the mechanical behavior of a sample under conditions of compression can be performed to provide basic material property data that is critical for component design and service performance assessment. The

19 52 requirements for compression strength values and the methods for testing these properties are specified in various standards for wide variety of materials. Testing was performed on machined material samples or on fullsize or scale models of actual components. A compression test is a method for determining the behaviour of materials under a compressive load. In this research work tests were performed using a universal testing machine made by Alfred J Amsler & Co, Schaffhausen, Switzerland as shown in Figure The cut-sections with dimensions 75mm x 75mm x 65mm were placed between two plates, and then applying a force to the specimen by moving the crossheads together. During the test, the specimens were compressed, and deformation versus the applied load was recorded. Figure 2.12 Compression testing machine

20 Hardness Test The Vickers hardness test method consists of indenting the test material with a diamond indenter, in the form of a pyramid with a square base and an angle of 136 degrees between opposite faces subjected to a test force between 1gf to 100 kgf. The full load is normally applied for 10 to 15 seconds. In this research work 10 kgf load for 10 seconds was applied. The advantages of the Vickers hardness test are that extremely accurate readings can be taken, and just one type of indenter is used for all types of metals and surface treatments. The Vickers method is capable of testing the softest and hardest of materials, under varying loads. The Vickers testing machine is shown in Figure Figure 2.13 Vickers Hardness tester The specifications of Zwick hardness tester are given below: Specification Intender : Hardness Tester with objective lens and micro load attachment : Diamond Objective lens : 100x, 200x, 400 x and 600x Load : 0.03 kg to 30kg.

21 PRODUCTION OF POROUS STAINLESS STEEL CASTINGS In this research work five stainless steel porous castings were produced. Following steps are involved in production of porous castings as shown in Figure PRODUCTION OF POROUS STAINLESS STEEL CASTINGS MELTING OF METAL PATTERN AND MOULD BOX MAKING PREPARATION OF SAND BALLS MOULD MAKING FILLING THE MOULD WITH SAND BALLS POURING POROUS CASTING KNOCK OUT REMOVAL OF GATES AND RISERS CLEANING HEAT TREATMENT ACID PICKLING AND WATER J ET CLEANING Figure 2.14 Layout for production of porous stainless steel casting

22 Pattern Making Pattern making is the first step in the development of porous castings. A pattern maker builds a pattern of the object from the design provided by using wood. While designing the pattern, required allowance are taken in to consideration by following the scaled rules. Here a wooden pattern of size 250mmx250mmx60mm as shown in Figure 2.15 was used to produce the mould cavity with carbon dioxide process by using a multi-part moulding steel box of size 500mmx600mmx150mm. Figure 2.15 Wooden pattern (stainless steel) Preparation of Sand Balls In this research work sand balls were prepared by mixing 6.5 percent of bentonite with silica sand by weight. To this correctly measured quantity of mixture, sufficient quantity of water was added. From the prepared mixture three sizes of sand balls with 7.5mm, 10mm and 15mm were made manually as shown in Figure 2.16 to Figure Sand balls were dried properly and kept ready for use in preparation of porous stainless steel castings.

23 56 Figure 2.16 The picture of sand balls of size 7.5mm (stainless steel) Figure 2.17 The picture of sand balls of size 10mm (stainless steel)

24 57 Figure 2.18 The picture of sand balls of size 15mm (stainless steel) Mould Making In this research work carbon dioxide process was used to make the mould. In carbon dioxide process clean dry silica sand was first mixed with liquid sodium silicate in a muller. Moulding boxes are made in segments that were latched to each other for complete closure. Pattern was placed inside the mould box, prepared sand mixture filled and compressed around. To harden the mould carbon dioxide gas was passed into the mould. The sodium silicate present in the sand reacts with carbon dioxide gas and forms a hard substance called silica gel. The silica gel is a hard substance like cement and helps in binding of sand grains. The method of introducing the gas must be simple, rapid and uniform throughout the sand body. Normally the time taken to harden a medium size body is about 20 to 30 seconds.

25 58 After the hardening process the boxes are unlatched and the two halves were parted separately. Pattern with its sprue are removed carefully without damaging the mould cavity. Any defect introduced by the removal of the pattern were corrected and cleaned. Finally the mould and sand balls were preheated to a temperature of 150 C. Prepared sand balls were filled completely in to the mould cavity as shown in Figure Figure 2.19 Mould cavity filled with sand balls (stainless steel) Melting of Stainless Steel Pure iron is too soft to be used for the purpose of structure, but by addition of small quantities of other elements (chromium, nickel, silicon, manganese etc) greatly increases the mechanical strength. This is because the different atom sizes of other elements interrupts the orderly arrangement of atoms in the lattice prevents them from sliding over each other. Alloying elements have the capability to block slip planes. Here in this research work stainless steel (CF8) was used for production of porous castings. CF8 grade is corrosion resistant iron base

26 59 alloy. The alloy find application in architectural hardware, food and beverage processing equipments, pumps, valves, manifolds, transfer piping, impellers, propellers, filter screens, agitators and scrubber components. Table 2.3 shows the charge materials used for melting. Table 2.3 Charge materials for stainless steel Chromium Nickel Silicon Manganese Carbon Carbon Sulphur In this research work CF8 stainless steel was melted using medium frequency tilting type electric furnace of 150 kg capacity as shown in Figure Figure 2.20 Stainless steel melting furnace Raw material used for melting metal was SS304 scrap. SS304 scrap was added in to the furnace and melted. Sample check was done when the

27 60 melt in the furnace reaches 80% capacity of the furnace. At this point of time virgin alloys like nickel, chromium, Ferro-silicon and low carbon Ferromanganese were added at correct proportions. Finally sample check was done to confirm the composition. When the temperature reaches 1575 C, pearlite ore was added to the melt for removal of slag before tapping. Table 2.4 shows the composition of materials with iron in stainless steel castings used in this research work. Table 2.4 Composition of stainless steel samples Item Chromium NickelSilicon Manganese Molybde num Carbon SulphurPhosphorus % % % % % % % % Exp Exp Exp Exp Exp Pouring of Metal After removing the slag portion from the melt, molten metal was poured in to the ladle at temperature of 1590 C. 0.1% Ferro-Titanium, 0.1% Ferro-Silico-Zirconium and 0.15% Calcium-Silicide was added as deoxidizers in the ladle. The ladle was taken to the mould and the molten metal was poured in to the cavity through the runner at temperature of 1540 C. Molten metal rises through the riser and confirms the filling of the cavity. Riser is provided to know the filling of metal and to provide additional metal during solidification to compensate for the solidification shrinkage.

28 Knockout Adequate time was given to the molten metal for solidification. Solidification time purely depends on the shape and size of the metal. In this research work fourteen hours was given to the metal to solidify. First the drag and cope were separated and the knock out process was carried out manually, by striking the sand with metal rod. Proper care was taken to avoid any damage to the castings Removal of Gates and Risers Gates and risers are pouring and feeding arrangements for castings, till the metal solidifies. After removing the casting from the mould after solidification, the runners, gates and risers are to be removed to give required shape to the casting. Gates and risers were removed with the help of electric arc cutting Cleaning Cleaning is an essential process to give casting good appearance. In this research work both exterior and interior cleaning is essential. The sand balls responsible for formation of porosity were removed so as to effect porosity to the developed castings. Shot blasting was carried out to clean the surface of the casting and removal of sand balls from the castings to analyse the nature of pores. Shot blasting provides higher rate of output than other process and removes the core to the depth of interconnectivity. Grinding work carried out to remove the projections and fins Heat Treatment Heat treatment is a metal working process used to alter the physical, sometimes chemical properties of a material. Heat treatment involves the use

29 62 of heating or chilling, normally to extreme temperatures to achieve desired results such as hardening or softening of a material. Heat treatment applies only to the process where heating and cooling are done for the specific purpose of altering the properties intentionally. In this research work heat treatment was carried out to improve corrosion resistance and mechanical properties. Castings heated to a temperature of 1080 C in an electric heat treatment furnace and soaked for three hours and then quenched in water. By heat treatment process internal stresses occurred, get relieved and the original mechanical properties of metal established Acid Pickling Stainless steel materials are naturally self-passivate whenever a clean surface is exposed to an environment that can provide enough oxygen to form the chromium rich oxide surface layer. Chromium in stainless steel is primarily responsible for self-passivation mechanism. Stainless steel cannot be considered corrosion resistant under all service conditions. Depending on the type of steel there will be certain conditions where the passive state is broken down and prevented from reforming. In this state the surface becomes active resulting in corrosion. Pickling is the removal of thin layer of metal from the surface of the stainless steel. Mixture of nitric and hydrofluoric acids are usually used for pickling stainless steels. Pickling is the process used to remove the heated tinted layers from the surface of stainless steel fabrications, where the steels surface level has been reduced. Pickling makes the layer passive to oxidation (Roger Crookes 2007).

30 63 In this research work porous stainless steel castings were placed in the bath containing nitric acid, hydrofluoric acid and water in the ratio of 1:2:4. Castings were kept for 30 minutes in the solution. After 30 minutes the castings were taken out and cleaned with water jet. 2.5 CHARACTERISATION OF POROUS STAINLESS STEEL CASTINGS Five samples of porous stainless steel castings were produced. In one of the porous casting, acid pickling was not carried out for examination. To characterise the nature of pores formed pores stainless steel castings were experimented. The castings were visually examined for formation of pores. The density and percentage of porosity were measured. Radiography tests were conducted to confirm the formation of pores all through the castings. Figure 2.21 shows the characterisation of stainless steel castings. Figure Characterisation of porous stainless steel castings

31 Visual Examination Visual examination typically means non-destructive testing only by raw human senses by not using any specialized equipments. Visual examination also reflects the surface problems and discontinuities over the surface. Visual inspection is commonly used for the examination of the material component or product for the conditions using light and eyes. In this research work five samples of stainless steel were produced with three sand ball sizes, namely 7.5mm, 10mm and 15mm. The porous samples after shot blasting, heat treatment and acid pickling process were manually inspected for pore formation and nature of pores on all surfaces of the porous castings Radiography Test In this research work all the five porous stainless steel castings were subjected to radiographic inspection to confirm the formation of pores. Pores are the open volume within a metal matrix or network. The dark region on the film represents the formation of pores, because the light penetration was more and opaque. The following are the parameters used. Source = Iridium 192 Film used = D7 Penerameter/ASTM = F40/50 Technique = SWSI

32 Density Measurement Density is a physical property of matter, as each element and compound has a unique density associated with it. Density defined in qualitative manner as a measure of relative heaviness of the objects with a constant volume. Density may also refer to how closely packed or crowded the material appears to be. Density is a measure of the mass of the substance in a standard unit of volume. Foams are characterised in terms of their density, since mechanical properties of foams depend on density. Density of the samples can be determined by weighing the porous samples with a digital balance and by measuring the dimensions. Volume is obtained by multiplying the mean value of the measured dimensions. The density can be calculated by using the formula from Equation Porosity Measurement Percentage of porosity is a rough measure of the open volume equal to 100% minus the part density. The total open volumes of the interconnected and isolated porosity are normally included in this value. Distribution is critical factors only when describing the open volume available. For calculating the porosity of samples in this research work, the density of non-porous sample was taken as the bulk casting density. Experiments were conducted by varying the sand ball sizes from 7.5mm to 15mm and porous castings developed. Percentage of porosity was calculated using the formula shown in Equation 2.2 for all the five stainless steel samples.

33 PRODUCTION OF POROUS ALUMINIUM CASTINGS In this research five aluminium castings were produced by permanent mould casting process. Following steps were involved in the production of process castings as shown in Figure PRODUCTION OF POROUS ALUMINIUM CASTINGS MELTING OF METAL DIE MAKING PREPARATION OF SAND BALLS FILLING THE MOULD WITH SAND BALLS POURING POROUS CASTING SEPARATION OF DIE AND CASTING REMOVAL OF GATES AND RISERS CLEANING Figure 2.22 Layout for production of aluminium casting

34 Die Making Permanent mould casting (Gravity die casting) is a versatile process for producing engineered metal parts by pouring molten metal into reusable metal moulds. The moulds are called dies. The die was designed to produce complex shapes with a high degree of accuracy and repeatability. Parts were defined with smooth textured surfaces and are suitable for variety of attractive finishes. Mostly this process is suitable for non-ferrous metals like aluminium. This process is suitable for large quantity of small to medium sized castings. Benefits of permanent mould castings: 1. Excellent dimensional accuracy. 2. Smooth surface finish. 3. Thinner walls can be cast as compared to sand castings. 4. Reduces or eliminates secondary operations. 5. Rapid production rate. In this research work, the die was made using cast iron plates of 25mm thickness with dimensions 250mmx250mmx60mm. The die had a runner at the centre and four risers on all the four corners of the as shown in Figure The risers at the four corners of the die cavity were provided to ensure complete filling of the molten metal and to provide extra metal during solidification.

35 68 Figure 2.23 Cast iron die Preparation of Sand Balls In this research work four sizes of sand balls were prepared. Sand balls with 10mm, 15mm, 20mm and 25mm were prepared by mixing Bentonite with silica by weight as shown in Table 2.5. Table 2.5 Bentonite composition for aluminium samples S.No. Sand ball size, in mm Percentage of Bentonite by weight % % % % To this correctly measured quantity of mixture sufficient water was added. From this mixture round balls of sizes 10mm, 15mm, 20mm and

36 69 25mm were prepared as shown in Figure 2.24 to Figure These balls were dried and kept ready for use in aluminium castings. Figure 2.24 The picture of sand balls of size 10mm (aluminium) Figure 2.25 The picture of sand balls of size 15mm (aluminium)

37 70 Figure 2.26 The picture of sand balls of size 20mm (aluminium) Figure 2.27 The picture of sand balls of size 25mm (aluminium)

38 Filling of Sand Balls For the development of porous castings, the selected sizes of dried sand balls were filled in the die completely. The shape and size of the pores required directly depend on the shape and size of the sand balls used for the experiment. Volume of cores used was kept maximum to have maximum percentage of porosity, so that minimum amount of liquid metal was required to fill the voids Melting of Aluminium Here in this research work LM6 aluminium alloy was used for production of porous castings. LM6 alloy exhibits excellent resistance to corrosion under both ordinary atmospheric and marine conditions. LM6 alloy was melted using electric resistance furnace of 50 kg capacity as shown in Figure Figure 2.28 Aluminium melting furnace

39 72 Raw material used for melting the metal was LM6 Ingots. LM6 ingots was added into the furnace and melted. When the metal melts and reaches a temperature of 710º C sodium chloride flux was added to the melt to remove the slag portion. After removing the slag to achieve high integrity from atmosphere, melt was degassed by adding chloroethane base. Table 2.6 shows the material composition in percentage with aluminium used in this research work. Table 2.6 Composition of materials with aluminium Item %Cu %Mg %Si %Fe %Mn %Ni %Zn %Pb Exp Exp Exp Exp Exp Pouring of Metal After removal of slag and degassing, the molten metal was poured into the ladle at a temperature of 690º C. The ladle was taken to the die and the molten metal was poured into the cavity through the runner. Molten metal rises through the risers and confirms the filling of the cavity. Riser also provides additional metal during solidification to compensate for the solidification shrinkage. The metal starts freezing at 575º C and solidifies.

40 Separation of Die and Poured Castings Adequate time was given for solidification of the metal. Castings along with gating and risering were removed from the die after the molten metal gets solidified Removal of Gates and Risers After removing the casting from the die, the runner and risers were removed with help of wooden hammer Cleaning Cleaning is an important process to give casting good appearance. In this research work in addition to exterior cleaning, the sand balls responsible for formation of porosity were also to be removed. Shot blasting was carried out to remove sand balls as well as to clean surface of the castings. Finally grinding was carried out to remove the fins and projections. 2.7 CHARACTERISATION OF POROUS ALUMINIUM CASTINGS Five samples of porous aluminium castings were developed. All the porous castings were visually examined for the formation of pores. The density and the percentage of porosity were measured. Radiography tests also were conducted to confirm the formation of pores all through the castings. Figure 2.29 shows the characterisation of aluminium castings.

41 74 Figure Characterisation of porous aluminium castings Visual Examination Visual examination is commonly used for the examination of the material for conditions using light and eyes. Before examination, the castings are cleaned completely with shot blasting and air jet cleaning. In this research work five aluminium samples were produced using sand ball sizes ranging from 10mm to 25mm. Experiment 1 was conducted with sand ball size of 10mm, experiment 2 and experiment 3 with sand ball size of 15mm, experiment 4 was conducted with sand ball size of 20mm and experiment 5 was conducted with sand ball size of 25mm Radiography Test The porous aluminium samples were subjected to radiographic inspection for analyzing the formation of pores in the developed porous aluminium castings. Following are the parameters used in radiographic technique.

42 75 Source = Iridium 192 Film used = D7 Penerameter/ASTM = F40/50 Technique = SWSI The dark regions of the film represent penetrable part of the object (void) than the light regions which were opaque (metal) Density Measurement Foams are characterised in terms of their density, since the mechanical properties of the foam largely depend on density. Density defined in qualitative manner as a measure of relative heaviness of the objects with a constant volume. Density of the samples can be determined by weighing the pores samples with a digital balance and by measuring the dimensions. Volume is obtained by multiplying the mean value of the measured dimensions. The density can be calculated by using the Equation Porosity Measurement Percent porosity is a rough measure of the open volume equal to 100% minus the part density. The total volume of interconnected and isolated porosity is normally included in this value. Initially bulk density was taken as the density of non-porous sample and the achieved density arrived by Equation 2.1. the Equation 2.2. Percent porosity of the five alminium samples can be found using