Application Of Silicon Carbide In Abrasive Water Jet Machining
18Application of Silicon Carbide in AbrasiveWater Jet MachiningAhsan Ali Khan and Mohammad Yeakub AliInternational Islamic University MalaysiaMalaysia1. IntroductionSilicon carbide (SiC) is a compound consisting of silicon and carbon. It is also known ascarborundum. SiC is used as an abrasive material after it was mass produced in 1893. Thecredit of mass production of SiC goes to Edward Goodrich Acheson. Now SiC is used notonly as an abrasive, but it is also extensively used in making cutting tools, structuralmaterial, automotive parts, electrical systems, nuclear fuel parts, jewelries, etc.AWJM is a well-established non-traditional machining technique used for cuttingdifficult-to machine materials. Nowadays, this process is being widely used for machiningof hard materials like ceramics, ceramic composites, fiber-reinforced composites andtitanium alloys where conventional machining fails to machine economically. The fact isthat in AWJM no heat is developed and it has important implications where heat-affectedzones are to be avoided. AWJM can cut everything what traditional machining can cut, aswell as what traditional machining cannot cut such as too hard material (e.g. carbides),too soft material (e.g. rubber) and brittle material (e.g. glass, ceramics, etc.). The basiccutting tool used in water jet machining is highly pressurized water that is passedthrough a very small orifice, producing a very powerful tool that can cut almost anymaterial. Depending on the materials, thickness of cut can range up to 25 mm and higher(Kalpakjian & Schmid, 2010). A water jet system consists of three components which arethe water preparation system, pressure generation system and the cutting head andmotion system.As far as technology development is concerned, three types of water jet machining havebeen found and used. The first type is a typical water jet machining which was used in themiddle of 18th century. The first attempt was in Russia in 1930s to cut hard rock using thepressurized water jet. The typical water jet machining used only water as the cutting toolwhich allows only cutting limited materials. The second type is AWJM as the improvementto the original water jet machining technique. Addition of abrasive to water enhances thecapability of machining by many times. AWJM is an appropriate and cost effectivetechnique for a number of uses and materials. Third type of AWJM includes cutting ofdifficult-to-machine materials, milling and 3-D-shaping, turning, piercing, drilling,polishing etc. These operations can be performed just by using plain water jet machining.However, due to special considerations such like the type of material or shape complexity ofthe part to be produced, the addition of the abrasive material is required.www.intechopen.com
432Silicon Carbide – Materials, Processing and Applications in Electronic DevicesThe use of the AWJM for machining or finishing purposes is based on the principle oferosion of the material upon which the jet is incident. The primary purpose of the abrasivematerial within the jet stream is to develop enough forces to erode the work material.However, the jet also accelerates the abrasive material to a high speed so that the kineticenergy of the abrasives is high enough to erode the work material. The secondary purposeof the water is to carry away both the abrasive material and the eroded material from themachining zone and clear the work area. AWJM gives a clean cut without any damage of thecut surface.2. Application of AWJMGenerally, water jets are used for (Momber W. & Kovacevic, R., 1998)1. industrial cleaning2. surface preparation3. paint, enamel and coating stripping4. concrete hydrodemolition5. rock fragmentation6. solid stabilization7. decontamination8. demolition9. metal recycling10. manufacturing operationsIn the area of manufacturing, the water jet-technique is used mainly for material cutting byplain water jets (e.g., plastics, thin metal sheets, textiles, foam, very hard materials likecarbides, very soft materials like rubber, etc.). Sometimes burrs are formed due to machiningof metals by conventional techniques. Those burrs can be removed by plain water jetmachining. Some parts work under dynamic load and fatigue failure is the most commontype of failure for those parts. Fatigue strength of those parts can be improves by peeningthe surface with a high pressure water jet. Fibrous materials like Kevlar cannot be machinedby conventional machining techniques because of pullouts of the fibers. But AWJM can beemployed to machine those materials without any pullout of fibers. AWJM can also be usedfor milling 3-D shapes. During abrasive water jet milling the surfaces not to be machined ismasked before machining and only the areas to be machined are exposed to the jet head.Turning and grooving can also be performed on a lathe using an abrasive water jet. Piercing,drilling and trepanning are other cutting operations performed by AWJM. Water jetmachining is a very common technique used to polish and improve work surfacesmoothness.The performance of AWJM depends on some key factors. The hardness of the abrasive is animportant factor. Harder the abrasive, faster and more efficient will be the machiningprocess. Machining efficiency of abrasives also depends on their structure. Grain shape isanother factor in evaluation of an abrasive material for abrasive water-jet process. Shape ofabrasives is characterized by their relative proportions of length, width and thickness.During AWJM machining rate to a large extend depends on the size of the grains. Largergrains have higher kinetic energy and their cutting ability is also higher. But though thematerial removal rate of smaller grains is smaller, they are used for finishing works. Grainsize distribution and average grain size also play role in the performance of AWJM.AWJM has many advantages over other machining techniques. They are:www.intechopen.com
Application of Silicon Carbide in Abrasive Water Jet Machining1.2.3.4.5.433Almost all types of materials can be machined by AWJM irrespective of their hardness,softness or brittleness. Almost all types of metals, plastics, fibrous materials, glass,ceramics, rubbers, etc. can be machines by this technique.The surface machined by AWJM is smooth and usually they don’t need any subsequentmachining operation. Abrasives of very small size should be used to produce a smoothsurface.AWJM is performed at room temperature. For that reason there is no problem of heataffected zone like other machining techniques. There is no structural change, no phasetransformation, no oxidation or no decarburization of the machined surface.The technique is environment friendly. Abrasives like SiC, garnet, alumina, silica sand,olivine together with water are environmental friendly. They don’t emit any toxic vaporor unpleasant odor.A major problem in conventional machining like milling, drilling, etc. is burr forming.But AWJM doesn’t produce any burr. Rather the technique is used for deburring.3. Elements of AWJMIn AWJM abrasives are added to water. The performance of AWJM to a great extenddepends on the properties of abrasives. The geometry of cut is a key indicator of AWJM.3.1 Water abrasive water jet machineThe main element of the abrasive water jet system is the abrasive jet. Water is pressurizedup to 400 MPa and expelled through a nozzle to form a high-velocity jet. In AWJMabrasives are added to water using a specially shaped abrasive-jet nozzle from separatefeed ports. As the momentum of water is transferred to the abrasives, their velocitiesincrease rapidly. It results a focused, high-velocity stream of abrasives that exits thenozzle and performs the cutting action of the work surface. A schematic diagram ofAWJM is presented in Fig.1Fig. 1. Abrasive water jet machining (Source: Kalpakjian & Schmid, 2010)Normal water is filtered and passed to the intensifier. The intensifier acts as an amplifier asit converts the energy from the low-pressure hydraulic fluid into ultra-high pressure water.The hydraulic system provides fluid power to a reciprocating piston in the intensifier centersection to amplify the water pressure. Using a control switch and a valve water ispressurized to the nozzle. Abrasive is added to water in the nozzle head (Fig 2) and thewww.intechopen.com
434Silicon Carbide – Materials, Processing and Applications in Electronic Devicesmixture comes out of the nozzle with a very high energy and pressure. In AWJM water ispressurized up to 55,000 pounds per square inch (psi) and then is forced to come outthrough a small orifice (round or square) at a speed of 2500 feet (762meters) per second,which is about two and half times the speed of sound. As water exits the nozzle at a highspeed, the abrasive material is injected into the jet stream or sucked into the stream by aphenomenon known as the ‘Venturi’ effect. The main purpose of addition of abrasives is toenhance the jet length and improve the cutting ability of the jet. It was found by Chacko etal., 2003 that an addition of polymer to the water jet increases the jet penetration depth.There are two types of AWJM; the slurry and entrainment. The only thing that differentiatesthem is the way the abrasives are added to the water. In the slurry system, the abrasive ismixed with the water before the water being pressurized. The mix is then pressurized andpassed to the end of the nozzle. This method causes extensive wear to the elements or partsof the water jet head due to the friction of the abrasives. In an entrainment system a pipe isconnected to the water inlet. When the high-velocity pressurized water passes through thepipe, a vacuum is created causing the abrasive to be sucked into the water stream.Fig. 2. Abrasive water jet cutting head.3.2 AbrasivesSiC is known for its very high hardness and abrasion resistance. It is dark gray in color; itshardness and modulus of elasticity are 2,800 knoop (kg/mm2) and 476 GPa respectively.This material is produced according to specific technology to imitate the natural abrasive. Itis heat resistant, and decomposes when heated to about 2,700 C. Very pure silicon carbide iswhite or colorless; crystals of it are used in semiconductors for high-temperatureapplications. Its small coefficient of expansion, which decreases with increasingtemperature, high hardness and sharp crystal edges make it a very good abrasive. It isprimarily used for grinding nonferrous materials such as brass, copper, bronze andaluminum. Other applications of SiC include grinding of glass, wood, rubber and plastics.Recently SiC is gaining popularity in as an excellent abrasive for AWJM.But a survey shows that 90% of the AWJM is done using garnet (Mort, 1995). In industries80 mesh garnet is a popular abrasive. It is possible to cut slightly faster rate with harderabrasives. However the harder abrasives also cause the mixing tube on the nozzle to wearwww.intechopen.com
Application of Silicon Carbide in Abrasive Water Jet Machining435rapidly. It is worth mentioning that not all garnets are the same. There are wide variationsin purity, hardness, sharpness, etc, that can also affect the cutting speed and operating cost.Garnet is a natural type of abrasive. Garnet has three basic structural components. They areAlmandine (Fe3, Al2 (SiO4)3), Pyrope (Mg3Al2(SiO4)3) and Spessartite (Mn3Al2(SiO4)3). Garnetalso contains impurities like SiO2, Al2O3, FeO, MnO, MgO, and CaO. The hardness of garnetabrasive particles of Almandine, Pyrope and Spessartite are 7-7.5 Mohs, 7.5 Mohs and 7-7.5Mohs respectively. Aluminum Oxide (Al2O3) is another popular abrasive used in AWJM. Itis also known as alumina. Its melting point is about 2,000 C and specific gravity is about 4.0.It is insoluble in water and organic liquids and slightly soluble in strong acids and alkalis.Alumina is available in two crystalline forms. Alpha alumina is composed of colorlesshexagonal crystals. Gamma alumina is composed of minute colorless cubic crystals withspecific gravity about 3.6 that are transformed to the alpha form at high temperatures.Alumina powder is formed by crushing crystalline Alumina. It is white when pure.Alumina is extremely tough and is wedge shaped. It is used for high-speed penetration intough materials without excessive shedding or fracturing of the grains. It is mainly used forgrinding high tensile strength materials like carbon steels, alloy steels, tough bronze andhard woods. Other abrasives used in AWJM are olivine, slag, silica sand, etc.3.3 Geometry of cutThe surface of cut is not vertical. It is characterized by a taper. Based on the width of cut atthe top and the bottom it is calculated as follows:TR ( b a) / 2For a divergent (V-shaped) slot b is larger than a as shown in Fig. 3.Fig. 3. Usual geometry cutHochheng & Chang, 1994 and Momber et al., 1996 investigated the top width of cut duringAWJM of ceramics using magnesia and bauxite abrasives and found that the top width ofcut decreases with the work feed. But the top width of cut increases with increase inpressure and SOD. However, the slot may be of different shapes as shown in Fig. 4.www.intechopen.com
436Silicon Carbide – Materials, Processing and Applications in Electronic DevicesV-shapedBarrel TaperReverse TaperRhomboid or Trapezoidal TaperFig. 4. Different shapes of the taper producedV shaped taper is produced a result of the jet spending more time over an area to erodethe top of the material more than the bottom. Also, "splash back" as the abrasives arebounced back from the material will tend to erode the sides. This is the most commontype of taper. Reverse taper tends to happen during AWJM of soft materials where thematerial is rapidly eroded or when work feed rate is very slowly. Because as the jet streamexpands farther away from the nozzle, it removes more material from the bottom thanfrom the top. Barrel taper is produced where the middle is wider than the top or thebottom. Barrel taper is usually produced during machining of very thick materials.Rhomboid taper is actually normal V-shaped taper that has been tilted. It is producedwhen the nozzle is not perpendicular to the work surface.Along the vertical surface the quality of the surface is not uniform. It can be divided intothree zones (Fig. 5). At the top there is a small initial damaged zone height hIDZ. After thatzone there is a smoother zone of height hSC. At the bottom of the surface there is a wavysurface of height hRC.A typical waviness of the cut slot is shown in Fig. 6. Because the cutting tool is basically abeam of water, it acts as a "floppy tool". The jet lags between where it first enters thematerial and where it exits. Bottom of the jet lags behind the cutting head.www.intechopen.com
Application of Silicon Carbide in Abrasive Water Jet MachiningFig. 5. Surface cut by AWJM (Source: Momber & Kovacevic, 1998)Fig. 6. Waviness of the cut surface (Source: Waterjet machining tolerances, 2011,http://waterjets.org)www.intechopen.com437
438Silicon Carbide – Materials, Processing and Applications in Electronic Devices4. Machining of carbides by SiC4.1 Influence of jet pressure on work surface roughnessExperiments were conducted to investigate the influence of pressure on surface roughness.During the experiments the jet pressure were varied from 35 kbar to 50 kbar. Work feed rate,abrasive flow rate and depth of cut were kept contstant at 1.36 mm/min, 135 g/min and3.18 mm respectively. The results show that increasing in pressure will decrease theroughness of work materials. In short surface finish becomes smoother and better. It ispresented in Fig. 7. From Fig. 8 is evident that surface roughness drastically increases at ahigher depth from the top surface. Fig. 9 shows the deterioration of surface smoothnessalong the depth of the cut 002.0001.0000.00035404550pressure(kbar)Fig. 7. Effect of pressure on surface . 8. Surface roughness at different depths from the top surface of carbidewww.intechopen.com
Application of Silicon Carbide in Abrasive Water Jet MachiningPressure: 35kbarPressure: 40kbarPressure: 45kbarPressure: 50kbarFig. 9. Surfaces of the machined insert carbide tools at constant flow rate (135g/min)www.intechopen.com439
440Silicon Carbide – Materials, Processing and Applications in Electronic Devices4.2 Influence of abrasive flow rate on surface roughnessThis section presents the results of the experiments conducted to investigate the influence ofabrasive flow rate on surface finish. In these experiments abrasive flow rate was varied from135 g/min to 175 g/min. Work feed rate and jet pressure were kept constant at 1.36mm/min and 45 kbar respectively. It can be observed from Fig. 10 that abrasive flow rateplay a vital role in water jet cutting. Surface roughness values are presented at the top,middle and at the bottom of the cut surface. The surface roughness decreases due to largeramount per minute of abrasives used. Means the surface of the cutting profile becomessmoother with higher flow rate. In Fig. 11 surface roughness at different depths from the topsurface of carbide is presented. Increasing the flow rate will reduce surface roughness.Surfaces cut at different abrasive flow rate are presented in Fig. 000.000135145155165175flowrate(g/min)Fig. 10. Effect of abrasive mass flow rate on surface roughness during machining epth(mm)Fig. 11. Surface roughness at different depths from the top surface of carbidewww.intechopen.com
Application of Silicon Carbide in Abrasive Water Jet MachiningAbrasive mass flow rate: 135 g/minAbrasive mass flow rate: 145 g/minAbrasive mass flow rate: 155 g/minAbrasive mass flow rate: 165 g/minAbrasive mass flow rate: 175 g/minFig. 12. Photographs of the machined carbides by varying the flow ratewww.intechopen.com441
442Silicon Carbide – Materials, Processing and Applications in Electronic Devices5. ContaminationIn AWJM material removal occurs through erosion and results from the interaction betweenan abrasive water jet and the work-piece materials. However, there are certain drawbacks inthis technology. One of them is the contamination of the surfaces generated during themachining process by fractured abrasives. This contamination may generate seriousproblems with the further treatment of these surfaces, such as grinding, welding and/orcoating. Other surface properties such as fatigue resistance will also influence negatively bysurface contaminants.Although particle embedment is a shortcoming of this technology, it can be minimized bycontrolling the machining parameters. However, it was reported that if the nozzle isoscillated during AWJM, then contamination reduces to a great extent (Chen et al., 2002 andSiores et al., 2006). The present study presents the influence of machining parameters onsurface contamination of mild steel. Some investigations performed for the blasting of steelsurfaces by air-driven solid particles found that the higher the blasting angle, the higher thecontamination. Other than that, there is also a study to reduce the contamination by usingthe oscillation nozzle. The major aim of the present study is to investigate abrasivecontamination on the surface of the mild steel during the AWJM. Furthermore, theparameters of the AWJM which are the feed rate, flow rate of the abrasive and the pressurewere varied during the AWJ cutting process in order to study their level of contaminationswith the changes of parameters. Then a quantitative microstructure analysis using DigitalCamera Microscope was performed to investigate abrasive contamination at the cuttingsurfaces. In this microscope the number of embedded SiC abrasives can be counted whenthe image of the surf
Application of Silicon Carbide in Abrasive Water Jet Machining Ahsan Ali Khan and Mohammad Yeakub Ali International Islamic University Malaysia Malaysia 1. Introduction Silicon carbide (SiC) is a compound consisting of silicon and carbon. It is also known as carborundum. SiC is used as an abrasive ma terial after it was mass produced in 1893. The credit of mass production of SiC goes to Ed .
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