Recognition of surface roughness by Digital color image processing

Feasibility of scCO2 based metal working fluids

CHAPTER 1INTRODUCTIONThe Public concern about the environment is more prevalent than it has been since the birth of the environmental movement in the 1960s. Driven largely by wide-spread fear over the dangers of global warming, the eminence of water shortages, and the scarcity of easily accessible petroleum reserves, the recent surge in public interest reflects a clear departure from attitudes observed in the previous three decades. Where the focus previously had been on reducing emissions or remediation of waste sites, the interest now is on creating environmentally sustainable systems that will prevent impacts from current and future generations. Here we have a proposed technology that would use Carbon Di Oxide CO2, a waste from other industrial processes, as an environmentally friendly and technically superior way to deliver lubricants in metals manufacturing operations. The water-based coolants used today present numerous occupational and environmental health impacts that inhibit the long-term sustainability of manufacturing processes. Most of these impacts are effectively eliminated if lubricants can be delivered in minimum quantities through sprays of carbon dioxide. Here we explores the following technical questions associated with CO2-based metalworking fluids: Do they perform as well as conventional coolants? If so, why ? And are the overall life cycle impacts of CO2-based fluids lower than water-based? The results will pave the way for a possible reinvention of metalworking fluids that, if implemented, could lead to significant reductions in the environmental impacts associated with manufacturing processes. 1.1 BACKGROUNDThe environmental movements are driven by the clear impacts associated with the unwanted emissions into local and regional environments. Workers got sick on the job and toxic waste went untreated into local ecosystems. With widespread commitment and federal money, treatment plants multiplied, contaminated land cleared, and discharge laws were passed to protect air, water, and land emissions. Most manufacturing processes that cut or form metal parts, e.g., automotive and aerospace, use metalworking fluids (MWFs) to cool and lubricate the cutting zone. Metal-on-metal contact results in shortened tool life or low product quality if MWFs are not used. The widespread use of these fluids in manufacturing has led to concern over the occupational and environmental health impacts they can produce. In an effort to reduce the impacts, some researchers and manufacturers have investigated the recycling of metalworking fluids to maintain their quality and reduce disposal frequencies. Other work has been conducted on the use of vegetable-based components to formulate fluids that are renewable.A significant amount of interest has been placed recently on the development of dry or near-dry machining operations. In minimum quantity lubrication (MQL) operations, a small amount of MWF is delivered in a spray of air and no fluid is recycled, eliminating much of the infrastructure typically associated with conventional water-based MWFs.1.2 OBJECTIVEThe oil and water mixtures that comprise most MWFs are a serious environmental and occupational health problem. Microorganisms thrive in the fluids, aerosols from the fluids cause decreased air quality in the workplace and water picks up metals and carries other organic constituents, making them a hazardous waste problem when they reach the end of their useable life. MWFs also require a significant input of energy to maintain and circulate through a large manufacturing facility. Many of the additives that are included in traditional MWFs, such as anti-corrosion agents, biocides, and deformers, are toxic and pose a waste treatment problem.In addition to the health impacts of fluids, the oil and water mixtures are inadequate lubricants in some state-of-the-art machining operations. High performance alloys and light metals are often extremely difficult and expensive to machine. In these cases, more effective cooling and better lubricant delivery than that afforded by conventional MWFs is necessary. Micromachining, such as that performed in many bio-medical applications, is a growing area that requires new methods of lubrication.Because of the numerous technical and health limitations associated with MWFs, they are an expensive part of typical machining operations. So a need for a new metal working fluid which is cost effective and provides better cooling effect compared to aquatious or petroleum MWFs arises. Here we have a proposed technology that would use Carbon Di Oxide CO2, a waste from other industrial processes, as an environmentally friendly and technically superior way to deliver lubricants in metals manufacturing operations.

1.3 OVERVIEWThe Chapter 2 provides a literature review that begins with an introduction to MWFs, their function and formulation. An overview of the chemical characteristics that make it a viable solvent is presented along with a discussion of the spray applications of CO2 for particle formation and the chemical thermodynamics needed to describe and model lubricant solubility in CO2.The Chapter 3 explains about super critical Carbon Di Oxide, its thermodynamic properties, solubility, cooling potential, production of super critical Carbon Di Oxide. The chapter 4 deals with the feasibilty of scCO2 based MWFs in micromilling. Mechanical micro manufacturing is receiving increasing attention due to its ability to create truly three-dimensional (3D) features in multiple length scales and its compatibility with abroad range of engineering materials. The chapter 5 explains the tapping torque efficiency test which is used to compare the performance of different metal working fluids. In chapter 6 we discuss the results obtained from the experiments ,we discuss about specific cutting energy, burr formation and surface roughness, tool wear, tapping torque etc. and in chapter 7 we concludes that it is feasible to use scCO2 as a metal working fluid based upon the results obtained from the experiments.

CHAPTER 2SUPER CRITICAL CARBON DI OXIDE (scCO2)A Supercritical fluid is defined state of a compound, mixture or element above its critical pressure (Pc) and critical temperature (Tc). Critical temperature (Tc) is maximum temperature at which a gas can be converted into a liquid by an increase in pressure. Critical pressure (Pc) is the minimum pressure which would suffice to liquefy a substance at its critical temperature. Above the critical pressure, increasing the temperature will not cause a fluid to vaporize to give a two-phase system. Critical point is the characteristic temperature (Tc) and pressure (Pc) above which a gas cannot be liquefied.Carbon dioxide above its critical temperature and pressure (Tc= 31.1C and Pc= 72.8 atm) is called super critical Carbon Di Oxide (scCO2). The scCO2 effectively dissolves many lubricating oils and forms chilled micro particles of lubricant as it expands out of a nozzle. The scCO2 approach is mechanically much simpler than oil-in-air MQL systems since the solubility of the lubricant in scCO2 eliminates elaborate mixing strategies required to aerosolize the oil in air.The scCO2 is a tunable substitute for organic and aqueous solvents and can dissolve many common MWF lubricants at pressures and temperatures above the critical point of CO2 (73 atm, 31C). The rapid expansion of supercritical solutions for coating and spraying applications has been well-documented. These rapidly expanding solutions of scCO2 can reach temperatures below 80C with a uniform coating of the solubilized material forming on the spray target. Under supercritical conditions, CO2 has compressibility and viscosity of a gas phase while having high density and solvency of a liquid phase. This makes scCO2 a convenient MWF carrier because it will dissolve lubricants under pressure and carry them to the process without the need for mixing. As the scCO2 expands, temperatures drop and provide superior cooling to todays oil-in-air MQL systems.

Fig.2.1 Phase diagram of CO2Figure 3.1 illustrates the supercritical region for CO2 as a function of temperature and pressure. Creating a scCO2 MWF is a straight forward process that starts by compressing CO2 above 73 atm (1100 psi) at just over room temperature, and bubbling the gas through a pool of process lubricant (e.g., bio-based oil or petroleum oil) as shown in Figure 2.2. The process lubricant may not be needed in certain applications. Flexible high pressure tubing can be used to deliver the super critical fluid from the pressure vessel to the cutting zone. A valve actuates the release of the scCO2-based MWF to the machining process. Since the pressure of the scCO2 drops as the fluid flows through the nozzle, the spray cools to cryogenic temperatures forming dry ice while cold liquid oil particles form as they precipitate out of the scCO2solution. These high-speed sprays allow good penetration of the cold oil into the cutting process. The sprays also clear chips effectively, leaving the work area free of debris and oily waste.

Fig 2.2 scCO2 delivery system (A. cylinder of food-grade CO2, B. cooling unit, C. pump, D. one way valve, E. high pressure vessel, F. heating element, G. soybean oil, H. nozzle, I. spray of scCO2-based MWF, V1 and V2 are valves)In the supercritical state, the solubility of oil in CO2 is finely tunable via the system temperature, pressure, and nozzle geometry. For instance, increasing pressure from 73 atm to 350 atm (11005000 psi) further increases oil solubility, while the sprays are frozen, leading to a larger number of colder droplets penetrating the process. This means, unlike water-based systems, that cooling potential and lubricating potential are not mutually exclusive in scCO2 systems. In micro emulsions, higher oil concentrations lead to less cooling and more lubricity while higher water concentrations lead to more cooling and less lubricity. In contrast, increasing pressure beyond the minimal conditions for the supercritical state increases both cooling and lubricity simultaneously. One example of a scCO2 MWF delivery system is provided in Figure 2.2. In this system food-grade CO2 is compressed from 48 atm (700 psi) to super critical pressures above 73 atm (1100 psi) using a compressor. The CO2 is bubbled into a 1 liter high-pressure vessel containing lubricating oil (e.g., soybean oil). The outlet from the vessel removes supercritical fluid phase CO2 oil and delivers it through a nozzle aimed at the working zone. The system can be mounted on a small cart with the exception of the CO2 tank and the whole system can be easily retrofitted so existing machine tools.2.1 OIL SOLUBILITY

Fig 2.3 Oil solubilityWe have examined several characteristics of scCO2 MWFs such as solubility of MWF lubricants in scCO2 and the heat removal capacity of scCO2 sprays. For instance, we have shown that different amounts of soybean oil can be carried to the manufacturing process simply by adjusting the pressure as shown in Figure 2.3. Some oils are also more soluble than others. In fact petroleum oils are much more soluble in scCO2 than soybean oil as shown in Figure 2.3. However, this may not lead to higher performance and to the extent that higher oil consumption rates are unnecessary, using petroleum vs. bio-based oil can lead to the wasting of oil. Lower concentrations of vegetable-based oil are more effective than higher concentrations of oil in scCO2 systems.

2.2 COOLING POTENTIAL

Fig 2.4: Cooling potentialThe cooling potential of an scCO2 system is a function of distance of the nozzle to the target, the scCO2 pressure, the off-axis position of the nozzle relative to the target, the length to diameter ratio of the nozzle, and the temperature of the scCO2 inside the pressure vessel. The relationship of these factors to heat removal capacity is shown in Figure 3.4. The data indicate that operating conditions for scCO2-based MWF can be selected to produce higher cooling potential by increasing the pressure. It should be noted that the scCO2 cooling potential may not be greater than MWF floods depending on operating conditions. This is despite the much colder temperature of the spray and is due to the lower molecular density of the rapidly expanding scCO2 spray relative to water. In a flood of liquid, many more water particles are available to remove heat than CO2 dry ice particle. Because of their lower molecular density, gases can be delivered to the cutting zone at much higher pressures than liquids without disrupting the cutting process and therefore can gain access to interstitial spaces to remove heat at locations that may be inaccessible to aqueous MWF sprays.

CHAPTER 3FEASIBILTY OF SCCO2 BASED MWF IN MICROMILLING3.1 INTRODUCTIONMechanical micro manufacturing is receiving increasing attention due to its ability to create truly three-dimensional (3D) features in multiple length scales and its compatibility with abroad range of engineering materials. As such, mechanical micro-manufacturing processes provide capabilities that are complementary to lithographic processes used in micro electro mechanical systems fabrication in terms of materials, geometries, and dimensions in terms of materials, geometries, and dimensions. In particular, micromachining, which uses micro-scale milling and drilling tools within high precision machining environments, has the potential to become an effective and wide-spread technique for creating 3D structures and devices.While having similar process kinematics, the mechanics of material removal in micromachining differs significantly from macro scale machining. This difference in process mechanics brings fundamental changes to the forces, surface and dimensional quality, and tool wear characteristics experienced during micromachining. One approach to improve micro machinability is to use metal-working fluids. MWFs are proven to improve machinability and part finish at the macro scale by providing lubrication and cooling, as well as by facilitating chip evacuation. Machining in a cryogenic or low temperature MWF environment has been shown to improve surface finish and machinability at the macro scale.Supercritical Carbon Di Oxide (scCO2) MWF is one such low temperature MWF which has been shown to improve tool life and surface finish, particularly at high material removal or deformation rates, on materials ranging from carbon steels to titanium and compacted graphite iron in macro-scale machining and forming applications3.2 EXPERIMENTAL METHODMicromachining experiments were performed on the high-precision miniature machine tool (MMT) shown in Fig.4.1. The MMT includes a 160,000 rpm air turbine, air bearing miniature ultra high-speed spindle equipped with a 3.125mm (1/8in.) precision collet. The feed motions are provided by three-axis slides (Aerotech VRALS130-XYZ) with a 10 nm resolution and a maximum linear (feed) speed of 250 mm/s. To facilitate the measurement of micromachining forces, the workpiece is mounted on a dynamometer (Kistler 9256C1), which in turn is attached to the three-axis slides. A stereo microscope with 95times magnification is used to view the workpiece surface during the initial tool approach. Fig3.1: High precision machine toolfig 3.2: Rapidly expanding scCO2The scCO2 MWF system employed for this study is shown in Figs 4.2 and 4.3. Commercially available food grade CO2 is compressed above its critical pressure (7.32 MPa) and sent to a high pressure mixing chamber, where it passes through vegetable oil present at the bottom of the chamber. The resulting solution of scCO2 and vegetable oil is maintained at a specified temperature above the critical temperature of CO2(31.1C) using a temperature sensor and heating coil apparatus controlled by a central programmable logic controller (PLC) unit. A 150lm orifice nozzle is used to rapidly expand the solution of lubricant in scCO2 as a high velocity spray directed to the tool/workpiece interface. Under these delivery conditions, the flow rate of oil is about 22 ml/h and the flow rate of is about 19 kg/h.

Fig 3.3: scCO2 delivery system3.3 DESIGN OF EXPERIMENTAISI 304 grade Austenitic stain-less steel and oxygen free high conductivity (OFHC) 101 grade copper were chosen as workpiece materials. AISI 304 is a difficult material to machine due to high tool-wear rates, high cutting-zone temperatures, work-hardening, and built up edge formation. Cu-101 is a ductile material that creates challenges in achieving sufficient surface finish and form accuracy. Micromilling of these materials was performed using surface roughness, burr formation, average specific cutting energy, and tool wear as metrics of machinability.Table 4.1: Design of experiments

A two level half-factorial design of experiments was conducted with three factors: feed per tooth (fz), axial depth of cut (ap), and cutting velocity (vc). As shown in Table 4.1, all four machining conditions were employed with scCO2 MWF and dry cutting conditions on both AISI 304 (experiments 18) and OFHC Cu-101 (experiments 916). For each experimental condition, seven channels (25 mm long) per condition were slot-milled with each channel separated by 0.7 mm. Channels on both materials were cut using 254lm diameter two-fluted tungsten carbide micro-end mills with a helix angle of 30 deg. A fresh tool was used for each experiment. The eight experiments for each material were distributed over three workpieces. Prior to each experiment, the surface of the workpiece was cleaned using a 500m diameter micro end mill.scCO2 MWF was expanded from a pressure of 10.3 MPa and temperature of 34C.

3.4 MATRICS AND MEASUREMENTSBurr formation was analyzed qualitatively using a burr chart created from scanning electron microscope (SEM) images of the micro milled channels. For stainless steel, only the images from the first channel were used for each machining condition to eliminate the effect of tool wear. For copper, given low wear rates, the images from either the first or the second channel were used.A 3D optical surface profile meter (Zygo NewView 7300) with sub-nanometer out-of-plane resolution and 2.2m in-plane resolution (with the 10objective) was used for measuring surface roughness. The roughness values (Ra) from four areas sized 0.8 mm0.2 mm on the bottom surface of the first channel were measured. Both the average and the standard deviation of Ra values were then calculated.Cutting forces were measured using a three-axis dynamometer( Kistler 9256C1, 2 mN noise threshold with linearity and hysteresis less than 0.5% of the measurement range), a charge amplifier Kistler Type 5010 Charge Amplifier), and a data acquisition system (NI PXI-6115).The effect of scCO2 MWF on micromilling may be obtained by determining the average specific cutting energy for each experiment considering the kinematics of the milling process. The specific cutting energy, also referred to as specific cutting force or specific cutting power in the literature, is calculated by dividing the average peak-to-valley tangential cutting force for each revolution by the average (uncut) chip area obtained by multiplying the average feed (f/2) with axial depth of cut. Averaging the cut-ting forces per revolution rather than per tooth pass eliminates the effect of tool-tip run out on the forces. To remove the effect of tool wear, only the forces from the first channel in each experiment were used. These forces were averaged over 500 revolutions to determine both the averages and the standard deviations. Reported error bars for surface roughness and specific cutting energy represent 95% confidence for these measurements, though they do not include variation between channels.

Fig 3.4: Cutting forces for Cu-101 when machining with scCO2MWF at 3lm/tooth chip load, 40lm axial depth of cut, and100 m/min cutting speedTool wear was assessed based on the reduction of the tool diameter with increased length of cut, as evidenced from the observed changes in the channel widths. The wear analysis was completed only for Cu-101 due to the observed chipping of the tools for certain cases during machining of AISI 304. In order to facilitate better measurement of channel widths, replicas of the channels were fabricated through elastomer molding. Optical profilometer measurements of the molded features were then obtained, and the measured profiles were post processed to determine the average channel widths. The channel widths were defined as the parallel distance between feature side-walls in a plane that is 1lm below their top surfaces.

CHAPTER 4TAPPING TORQUE EFFICIENCY TESTINGTapping torque values are measured as the taps cut the threads in predrilled holes in a metal specimen. This step is repeated with different MWFs. The test result is expressed as simple torque or as percentage of efficiency. The average tapping torque efficiency and 95% confidence interval values calculated for the less than30 (> 30) independent and randomized trials performed for each MWF.

Fig 4.1: Tapping efficiency test (a)aquatious MWF b) scCO2+soybean oil

CHAPTER 5RESULTS AND DISCUSSIONS5.1 SPECIFIC CUTTING ENERGY

Fig 5.1: ASCE a) for AISI 304 stainless steel and (b) and Cu-101The average specific cutting energies for all experiments are provided in Fig.5.1. It is observed that the specific cutting energy in condition 1 (low chip load and low axial depth of cut) is significantly higher than the other three conditions in both dry and scCO2 MWF conditions in both materials, which is most likely due to the size effect that becomes prominent when the ratio of uncut chip thickness to tool nose radius drops below 1. Specific cutting energy is reduced for condition 1 when using scCO2MWF compared with dry machining, and is statistically indistinguishable for the other conditions. The reduction in specific cutting energy from the use of scCO2 MWF could be a consequence of a local increase in material hardness and a reduction in minimum chip thickness resulting from exposure to the low-temperature spray.

5.2 TOOL WEARFigures 5.2 (a) (b) show the percentage reduction in channel width and SEM images of cutting tool nose respectively, when machining copper under condition 2 for both dry and scCO2 MWF cases. No discernable tool wear is observed with scCO2MWF, whereas an average wear rate of 3.39lm/channel is observed for dry machining in condition. Condition 4 (not shown in fig5.2) showed an average wear rate of 6.02lm/channel

Fig 5.2: (a) Wear progression and (b) SEM images of tool nose wear in dry and scCO2assisted machining of Cu-101The reduction in tool wear observed under scCO2 application may be an outcome of improved lubrication, increased heat removal, increase in material hardness and machinability, or a combination of these factors. For conditions 1 and 3, the tool wear was seen to be comparable for dry and scCO2 MWF cases. It should be noted that width data for the last two channels of dry machining in condition 2 (Fig.5.2(a)) could not be obtained due to excessive burr formation, and SEM images of cutting tools for condition 4 could not be obtained due to tool breakage during handling.5.3 BURR FORMATION AND SURFACE ROUGHNESS

Fig 5.3: Burr formation on AISI 304 and Cu-101 with and without scCO2MWF for all machining conditions examined Figure 5.3 shows a chart of top surface burrs for both stainless steel and copper. Machining parameters (feed, axial depth of cut and cutting speed) for each condition are indicated in parentheses next to the condition number, and their values for each material are shown in Table 4.1. In general, scCO2 MWF assisted machining gives lower burr formation and thus cleaner channels as compared to dry machining. The most significant improvement was seen in conditions 1 and 3 for stainless steel (low feed conditions), and in condition 4 for copper (high feed and highest material removal rate).In conditions 1 and 4 for stainless steel with scCO2 MWF, the tool chipped, thus yielding two distinct depths of cut as seen in Fig.5.3. Further work will be needed to determine, whether the chipping was related to the method of applying scCO2 MWF. The average surface roughness values are shown in Fig.8.Compared to dry machining, scCO2 MWF roughness values are lower by up to 69% in stainless steel and up to 33% in copper. In all cases, the observed surface roughness with scCO2 MWF was either equal to or lowers than under dry conditions. Improvements in surface finish from lower roughness or reduced burr formation may be attributed to the same factors that are responsible for lower specific cutting energy and tool wear under application of scCO2 MWF.5.4 TAPPING TORQUE EFFICIENCY TEST

Fig5.4: Tapping torque efficiency for straight oil, water-, and scCO2-based MWFsThe data reveal that soybean oil is a better lubricant than mineral oil in the tap-ping process, either in straight oil or emulsified form, as previously observed. In addition, the data support the well-documented and intuitive fact that straight oils perform better in tapping processes than semi-synthetic emulsions. It is noteworthy that scCO2 (without oil) had a statistically indistinguishable performance relative to the mineral oil micro emulsion (0.75% w/w oil-in-water). In over ten years of testing MWF formulation using the tapping torque test, a performance of 125% relative to the reference fluid has never before been observed. It was observed that the soybean oil + scCO2 system performs on average approximately 10% better than straight soybean oil, 20% better than the soybean oil semi-synthetic emulsion, and 30% better than straight scCO2. This unprecedented result confirms that the combination of soybean oil and scCO2 performs better than either can alone. The performance of straight soybean oil can be improved by using scCO2 for enhanced delivery of chilled, high velocity, oil particles. Using scCO2 also delivers more efficient quantities of oil to the cutting zone, representing less than 20% the amount of oil delivered during a typical tapping operation.

Fig 5.5: Magnified Images of Chip Surfaces Cut from 1018 Cold Rolled Steel During Tapping Using (a) scCO2 Alone; (b) Mineral Oil in H2O; (c) Straight Mineral Oil; (d)Soybean Oil in H2O; (e) Soybean Oil Alone; and (f) Soybean Oil in scCO2

CHAPTER 6

CONCLUSIONWe investigated the feasibility of using super critical carbon dioxide (scCO2) based metalworking fluids (MWFs) in the micromilling of stainless steel and copper and in tapping operation. Tapping torque efficiency test reveals that a mixture of scCO2 and soybean oil gives maximum tapping torque efficiency Burr formation and surface roughness analyses showed that scCO2 MWFs are effective in improving part quality under most cutting conditions. Average specific cutting energies were also typically observed to be lower when using scCO2 MWF relative to dry conditions. In addition, it was observed that the application of scCO2 MWF reduces tool wear for high feed rate conditions during machining of Cu 101. Based on these results it can be concluded that scCO2 MWFs can be a productive asset to micromachining, and that further research is warranted into its performance relative to other MWFs, as well as into the relative importance of cooling versus lubrication mechanisms in achieving better micro machinability.

REFERENCES1. Feasibility of Supercritical Carbon Dioxide Based Metalworking Fluids in Micro milling S. D. Spear Department of Mechanical Engineering, MI 48109-2122B. A. Gozen B. Bediz O. B. Ozdoganlar Department of Mechanical Engineering, Carnegie Mellon University,PA 15213-3815 asme 2013.2. Current advances in sustainable Metalworking Fluids research by Steven J. Skerlos Department of Mechanical Engineering, University of Michigan, USA. asme 20123. carbon dioxide based metalworking fluids by Andres F. Clarens Environmental Engineering and Natural Resources and Environment)in The University of Michigan asme 2008.4. Vegetable Oil-Based Metal Working Fluids-A Review Vaibhav Koushik A.V, Narendra Shetty. S & Ramprasad.C Dept. of Mechanical Engineering, Atria Institute of Technology5. Super Critical Carbon Dioxide Metal Working Fluid Delivery System (Steve E. Hecker, Daniel R. Leader, Daniel S. Merz, and David P. Powers ME450, Team 26 Winter 2008 sSection Instructor: Steve S kerlos Department of Mechanical Engineering University of Michigan Ann Arbor, MI 48109-2125 ).6. Application of vegetable oil-based metalworking fluids in machining ferrousmetalsA review ( S.A. Lawal , I.A. Choudhurya Y. Nukmana) Department of Engineering Design and Manufacture, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia7. INTEGRATED SUPERCRITICAL FLUID EXTRACTION AND BIOPROCESSING (Owen Catchpole, Stephen Tallon, Peter Dyer, Fernando Montanes, Teresa Moreno, Erika Vagi, Wayne Eltringham and Jagan Billakanti ) Integrated Bioactive Technologies, Industrial Research Limited, Lower Hutt, New Zealand

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