In order to reduce delamination induced by conventional drilling of CFRPs, non- conventional drilling processes, e.g. vibration assisted drilling, laser machining, water jet or abrasive water jet machining and ultrasonic drilling, have been applied to CFRP components [4, 9, 11, 126]. In vibration-assisted drilling, workpiece material is removed by pulsed and intermittent cutting process due to vibration of the drill in axial direction as opposed to continuous cutting in conventional drilling process, resulting in lower thrust force and lower possibility of delamination damage [4, 9, 11, 126]. In ultrasonic drilling, ultrasonic vibration of the tool in axial direction causes localised impact between workpiece material and
66 chipping, resulting in good surface finish of drilled hole. In laser machining, a focused spot of high-energy beam (108 W/cm2) with 0.1-1.0 mm diameter is used to remove material by melting, vaporization or chemical degradation through the thickness of workpiece material [4, 11, 126]. Laser machining offers a benefit of delamination elimination since there is no mechanical contact between the drilling tool and the workpiece [4, 55, 126]. However, thermal damage on the machined surface of the hole is a problem associated with laser machining due to difference in thermal conductivity of carbon fibres and polymeric matrix [4, 11, 55, 126]. Water jet or abrasive water jet machining also removes material by localised impact, hence resulting in improved quality of the machined surface [4, 126]. Although these non-conventional drilling processes have shown potentials for improving drilling performance associated with CFRPs, conventional drilling processes are still the most widely used routing hole-making process for CFRP components in industry [3, 4, 7-9, 11, 47, 55].
3.6
Conclusion
Among the hole-making processes in industry, conventional drilling processes using a drilling tool is still important and commonly applied for routine hole-making processes for CFRP components. However, damage induced by conventional drilling usually occurs due to high level of anisotropy in CFRPs. Delamination is considered the damage of most concern due its severe effect to the structural integrity and mechanical properties of CFRP components. For this reason, there has been much research work reported on approaches for reducing or eliminating delamination induced by conventional drilling of CFRPs. From the literature, there have been many attempts to reduce delamination by optimising machining parameters, using improved material and geometry of the tool and using conventional drilling with modified process. However, there has been little published work on the application of cutting fluids and cryogenic cooling to reduce delamination induced by conventional drilling of CFRPs. This indicates a research gap for the application of cutting fluids and cryogenic cooling to improve drilling performance of CFRPs. Due to an improvement in metal machining by cryogenic cooling, which has been reported in other published work, plus a trend for moving towards an eco-friendly machining process, application of cryogenic cooling has shown potential to be a preferable option for improving performance of drilling process of CFRPs.
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4
An Overview of the Application of Cryogenic
Cooling in Machining Processes
4.1
Introduction
As discussed in Chapter 3, there has been little published research on the application of cutting fluids to improve performance when drilling CFRPs. The conventional water-based emulsion cutting fluids are generally avoided when drilling CFRPs in industry to prevent material degradation due to moisture absorption and to reduce the cost of cleaning although they have been used for the purpose of dust suppression and for controlling cutting temperature in some cases [10, 13, 32]. In addition, there has been pressure on industries to minimise or eliminate the use of water-based emulsion cutting fluids in machining processes due to the cost implications plus health and environmental problems caused by the use of such conventional cutting fluids [28-31]. The cryogenic cooling by using cryogen medium in liquid or gaseous form has been claimed to be more environmental friendly cooling and lubricating method for machining processes and has shown improvement in the performance of metal machining processes [28-31, 36]. Therefore, cryogenic machining has the potential to be applied in the conventional drilling process for CFRPs to improve drilling performance without causing material degradation problems due to moisture absorption and additional cost of cleaning the parts since the cryogen medium such as liquid nitrogen (LN2) or solid or
gaseous carbon dioxide (CO2) will evaporate to the atmosphere after absorbing the cutting
heat [31, 34, 36]. Although there has been little published research work on the use of cryogenic cooling in the drilling of CFRPs, it is important to understand the application of this technique and its effect on the machining processes. For this reason, an overview of the application of cryogenic cooling will be presented in this chapter. Although various cryogen mediums exist such as nitrogen, carbon dioxide, helium and oxygen, the overview of the cryogenic cooling presented in this chapter will focus the cryogenic cooling by the use of nitrogen and carbon dioxide, which are the common medium used in the cryogenic cooling for machining work [31, 34, 36].
4.2
Reasons for the Application of Cryogenic Cooling in Machining
Processes
Before considering the application techniques for cryogenic cooling and their effect on the performance of machining processes, the reasons why cryogenic cooling is used for improving machinability are worth considering. In the machining process of metallic materials, the cutting mechanism involves extensive amount of plastic deformation of the workpiece material in the primary shear zone, area (a) in Figure 4.1, with a typical shear
68 strain of γ = 10-50 [72]. According to Trent and Wright [72] and Knight and Boothroyd [132], 99% of the energy which is used to cause this deformation is converted into heat because of the large amount of plastic deformation. In addition to the heat being generated in the primary and secondary shear zones, heat is also generated as a result of plastic deformation in the tertiary shear zone, area (c) in Figure 4.1, where the flank face of the tool makes contact with the newly formed surface of the workpiece. Most of the heat generated in the primary shear zone will be carried away with the chips, while some will be transmitted into the workpiece through conduction. Combined with the conduction of the heat generated in the tertiary shear zone, the heat in the primary shear zone which is conducted in to the workpiece increases the temperature of the workpiece, which needs to be controlled to prevent dimensional accuracy error due to thermal expansion of the workpiece. The heat generated in the secondary shear zone on the underside of the chip is of most concern and most difficult to remove. Since seizure between the chip and rake face of the tool usually occurs in the secondary shear zone, workpiece material in the secondary shear zone is subjected to a continuous and larger shear strain relatively to that in the primary shear zone, which was previously mentioned. Due to a continuous and larger shear strain, higher temperature is generated in the secondary shear zone. Consequently, this is where the maximum temperature of the cutting process is generated. The temperature in the secondary shear zone as high as 1150°C have been measured during the machining of AISA/SAE 4140 at 925 m/min [133]. Although some of the heat in the secondary shear zone can be carried away through conduction into the chip, most of the heat is accumulated at the interface between the rake face of the tool and the underside of the chip because of the continuous plastic deformation of the workpiece material in the secondary shear zone where seizure between the chip and rake face of the tool occurs. Since the underside of the chip is seized to rake face of the tool, the heat generated in the secondary shear zone in conducted into the cutting tool very efficiently resulting in an increase in the cutting tool temperature for which the maximum temperature of the rake face of the tool is the same as that in the secondary shear zone where seizure occurs. This results in lower hardness and strength of the tool which can significantly affect tool life, quality of the machined surface and hence limit productivity of the machining process since cutting temperature increases with the increased cutting speed [28, 31, 72].
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Figure 4.1: Quick-stop section showing deformation in the primary shear zone (area (a)), secondary shear zone (area (b)) and tertiary shear zone (area (c)) when machining 60/40 brass at 120 m/min (adapted from [72])
In addition to the effort to develop tool materials with higher elevated temperature strength and hardness, cutting fluids are used in machining process to remove heat generated in the cutting zone, which will reduce the cutting temperature, as well as reduce the friction at the tool/chip interface [28, 31, 72]. However, as previously mentioned, there has been a trend to minimise or eliminate the use of conventional water-based emulsion cutting fluids in machining processes in industry due to their economical, health and environmental problems [28-31]. The cost associated with the use of conventional cutting fluids, especially the maintenance and treatment/disposal cost, can be high and it is reported to be approximately 16% of the total manufacturing cost [134]. The additives and chemicals, which are added to conventional cutting fluids to obtain stability in the performance of the cutting fluids, as well as the heavy metals being carried with the cutting fluids during machining process, can cause environmental problems such as toxicity and contamination in ecological areas if the cutting fluids are not treated properly before disposed [29-31]. In addition, toxicity of the chemicals in conventional cutting fluids can cause health problems to machine operators such as skin cancer and respiratory system problems when they are in contact with the cutting fluids and inhale the mist of cutting fluids in the machining area [29- 31]. As a consequence, there has been a trend to moving towards a more environmental- friendly machining process by minimising or eliminating the use of conventional cutting fluids.
Due to a low temperature of liquid cryogen mediums, cryogenic cooling using liquid nitrogen (-196°C) and liquid carbon dioxide (-80°C when expands to room temperature and air pressure [30]) have potential to replace conventional cutting fluids in removing the heat
70 application of a high-pressure jet in the cutting zone, while being environmental friendly and sustainable [30, 31, 34, 36]. It is reported that changing from using conventional cutting fluids to cryogenic cooling significantly reduces solid waste, water usage, global warming potential and aquatic toxicity, while eliminating the machine operators’ health concerns due to exposure to the chemicals in conventional cutting fluids [29, 33]. Although the use of cryogenic cooling by LN2 would increase the energy used for production of LN2 cutting
fluid, a higher productivity of the machining process obtained when using cryogenic cooling can compensate with this increase of energy usage [33]. Since a major proportion of air is nitrogen gas (78%), which is an inert gas, it is harmless when it evaporates in the machining area or when it is released to the atmosphere as long as the level of the gas is below the critical level which can kill machine operators by suffocation [33, 34, 36]. Although carbon dioxide (CO2) is a greenhouse gas, the use of CO2 in cryogenic machining is not considered
a generator of greenhouse gases into the atmosphere because CO2 can be obtained as a by-
product from the chemical industry and power plants [30]. Since CO2 can be obtained as a
by-product from the chemical industry, power plants and a natural CO2 gas well [3, 29, 30,
35, 44], it is suggested that the use of cryogenic cooling by CO2 would reduce the energy
and waste associated with the production of cutting fluids, including the cryogenic distillation of LN2.
Although there has been a trend to avoid using conventional cutting fluids and move towards cryogenic machining, which is claimed to be cheaper and more environmental friendly, it is suggested that conventional cutting fluids will still be applied in a majority of industrial machining processes such as in high speed machining processes for many years in the future. Changing from using a conventional cutting fluid to cryogenic cooling will require additional cost and time for installing new cutting fluid systems. In addition, most of the machinists in industry are experienced in machining with a conventional cutting fluid, so they will require training for using a new machining system with cryogenic cooling. If a modern cutting fluid, which is developed to reduce harmful chemical and to increase service life, is maintained, treated and disposed properly, problems to environment and machine operators’ health will not be an issue. Unfortunately, cost associated with the use of an effective maintenance, treatment and disposal systems will still be concerned when applying conventional cutting fluids. However, this increased cost might be compensated if a significant improvement in tool life and productivity of machining processes could be achieved.
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