Performance evaluation of cryogenic cooling in reaming titanium alloy

Removal of materials in metal cutting operations through drilling and reaming of hard materials is a difficult process. Wear of the tool and high cutting zone temperature have big effect on it. In this study, experiments have been carried out in a reaming operation on titanium alloy material under flood and cryogenic LN₂ cooling separately. Cutting speed, feed rate, and hole depth (constant) are the three input variable parameters. Torque (M_t), thrust force (F_t), cutting temperature (T), quality of the hole, surface roughness (R_a), and chip morphology are the output parameters. In both cooling conditions, each of the nine experiments based on orthogonal array (OA) L₉ were conducted under both cooling conditions. Based on the results obtained, cutting temperature was reduced by 12–21%, thrust force reduction is 17–32%, and torque reduction is 7–30% in cryogenic LN₂ cooling. Surface roughness is increased by 4–15% and hole quality (circularity and cylindricity) parameters are affected in cryogenic LN₂ cooling with respect to flood cooling. Better chip breaking was found in both flood and cryogenic LN₂ cooling. No drastic changes were observed in microstructure under both cooling conditions.     

Cryogenic Drilling of Ti–6Al–4V Alloy Under Liquid Nitrogen Cooling

This article is focused on experimental study of the effects of cryogenic liquid nitrogen (LN₂) coolant during drilling of Ti–6Al–4V alloy material with three different levels of cutting speed (V_c) and feed rate (f) at a constant depth. Cutting temperature (T), thrust force (F_z), torque (M_z), surface roughness (R_a), and hole quality are the output responses investigated by using cryogenic LN₂ coolant compared with a wet coolant. Tool wear and chip morphology were examined with the changes in cryogenic LN₂ coolant. It is found that cryogenic LN₂ coolant results in lowering cutting zone temperature which helps more removal of heat from the cutting zone. Lower thrust forces and surface roughness were observed due to less friction and better chip breaking in cryogenic LN₂ condition. Also better chipping results in improvement in hole quality, viz., circularity and cylindricity in cryogenic LN₂ condition. Less serration and uniform segmentation results in better chip morphology and no damage to the cutting inserts resulted in improved tool life in cryogenic LN₂ condition. The main application of cryogenic LN₂ coolant in the cutting zone provides better lubrication and is more effective than wet coolant. The effects of this investigation show that cryogenic LN₂ coolant is an alternative approach for a wet coolant in the drilling process.     

Experimental Investigations on Cryogenic Cooling in the Drilling of Titanium Alloy

In this study, a drilling experiment was conducted on titanium ASTM B265 Grade 2 material using PVD coated carbide inserts. Two types of coolants (Wet and LN₂) were used. The variables in the experiment were feed rate (f) and cutting speed (V_c). The depth of the drilling was constant. Cutting temperature (T), thrust force (F_t), surface roughness (R_a), and the hole quality (circularity, cylindricity, and perpendicularity) were analyzed. The tool wear and chip morphology were studied. The result of the experiment indicates that there is 6–59% reduction in cutting temperature when LN₂ is used, high thrust force values were recorded for LN₂ coolant condition, surface roughness (R_a) values were higher for LN₂ coolants. Hole quality is not favorable in LN₂ coolant supply.     

Experimental investigations on cryogenic cooling by liquid nitrogen in the end milling of hardened steel

Milling of hardened steel generates excessive heat during the chip formation process, which increases the temperature of cutting tool and accelerates tool wear. Application of conventional cutting fluid in milling process may not effectively control the heat generation also it has inherent health and environmental problems. To minimize health hazard and environmental problems caused by using conventional cutting fluid, a cryogenic cooling set up is developed to cool tool–chip interface using liquid nitrogen (LN₂). This paper presents results on the effect of LN₂ as a coolant on machinability of hardened AISI H13 tool steel for varying cutting speed in the range of 75–125 m/min during end milling with PVD TiAlN coated carbide inserts at a constant feed rate. The results show that machining with LN₂ lowers cutting temperature, tool flank wear, surface roughness and cutting forces as compared with dry and wet machining. With LN₂ cooling, it has been found that the cutting temperature was reduced by 57–60% and 37–42%; the tool flank wear was reduced by 29–34% and 10–12%; the surface roughness was decreased by 33–40% and 25–29% compared to dry and wet machining. The cutting forces also decreased moderately compared to dry and wet machining. This can be attributed to the fact that LN₂ machining provides better cooling and lubrication through substantial reduction in the cutting zone temperature.     

Experimental studies on the cryogenic machining of biodegradable ZK60 Mg alloy using micro-textured tools

Reducing the contact area between the cutting tool rake surface and chip promotes the machining performance of the work material and increases the tool life. Magnesium alloys are ductile-lightweight materials that form continuous chips during machining. The present investigation discusses the orthogonal turning of ZK60 magnesium alloy with linearly textured cutting inserts under both dry and liquid nitrogen (LN₂) cooling conditions. Linear grooves that are parallel and perpendicular to chip flow direction were created using Nd-YAG laser on the tungsten carbide cutting inserts. The effect of texturing combined with the application of LN₂ cooling is studied by evaluating the machining temperature and forces, microhardness, surface roughness and tool wear. Textured tools considerably minimize the liaison area of the chip with the rake plane compared to non-textured tools, which resulted in favorable effects in machinability. In case of cryogenic machining, textured tools substantially minimize the friction by the coupled effect of micro-pool lubrication and the formation of thin-film lubrication between the tool–chip/tool–work interfaces. Parallel-textured tools aided with cryogenic cooling exhibit superior performance during machining among the different types of tools employed in the present investigation.     

Effect of cryogenic CO2 and LN2 coolants in milling of aluminum alloy

The research work was carried out on the end milling of Al 6082-T6 alloy with cryogenic CO₂, LN₂ and wet conditions. The highest axial force (F_z), normal force (F_y) and feed force (F_x) values were recorded on cryogenic LN₂ machining. Use of cryogenic LN₂ helped to reduce the cutting temperatures up to 38.29% and 32.8% when compared with wet and cryogenic CO₂ conditions, respectively. The conventional fluid coolant offered a better surface roughness value (R_a) over cryogenic coolants at a feed rate of 0.015 and 0.02 mm/tooth and cutting speed of 100 m/min. The workpiece surface quality degraded in cryogenic machining conditions during the slot end milling operation of aluminum alloy under the given machining parameters.     

Improving the machining performance characteristics of the µEDM drilling process by the online cryogenic cooling approach

Productivity and surface quality would significantly affect the performance of the micro electrical discharge machining process (µEDM). Thus, the machining performance would be enhanced by improving the material removal rate (MRR) and surface quality. In this investigation, cryogenic LN₂ cooling was introduced to the conventional µEDM setup for developing an innovative process of cryogenically cooled µEDM process (CµEDM). The favorable outcomes of this process were estimated by selecting discharge current (I_p) and pulse on duration (T_on) for determining the effects of the machining performance including MRR and surface integrity. Surface quality was also analyzed by microstructural analysis and a scanning electron microscope (SEM) for evaluating the effects of the cryogenically cooled µEDM process. The experimental result shows 54–62% improvement in MRR and 22–36% improvement in average roughness values. Hence, it is suggested that cryogenically cooled µEDM facilitates improvement in productivity and surface quality.     

Environment Friendly Machining of Ni–Cr–Co Based Super Alloy using Different Sustainable Techniques

In order to eradicate the use of mineral based cutting fluid, the machining of Ni–Cr–Co based Nimonic 90 alloy was conducted using environment friendly sustainable techniques. In this work, uncoated tungsten carbide inserts were employed for the machining under dry (untreated and cryogenically treated), MQL, and cryogenic cutting modes. The influence of all these techniques was examined by considering tool wear, surface finish, chip contact length, chip thickness, and chip morphology. It was found that the cryogenically treated tools outperformed the untreated tools at 40 m/min. At cutting speed of 80 m/min, MQL and direct cooling with liquid nitrogen brought down the flank wear by 50% in comparison to dry machining. Similarly at higher cutting speed, MQL and cryogenic cooling techniques provided the significant improvement in terms of nose wear, crater wear area, and chip thickness value. However, both dry and MQL modes outperformed the cryogenic cooling machining in terms of surface roughness value at all the cutting speeds. Overall cryotreated tools was able to provide satisfactory results at lower speed (40 m/min). Whereas both MQL and cryogenic cooling methods provided the significantly improved results at higher cutting speeds (60 and 80 m/min) over dry machining.     

A comparison between wet and cryogenic drilling of CFRP/Ti stacks

This paper compares conventional and cryogenic cooling in the deep hole drilling of carbon fiber reinforced polymer (CFRP)/Ti stacks. Various parameters are taken into account to find if the use of cryogenic coolant is justified by the improvement of the final results. Both the thrust and the torque were acquired continuously during the machining operations and compared both in average and distribution. The use of a cryogenic coolant brings a reduction in thrust force and torque without any sensible drawback. Overall the results prove that cryogenic drilling is a suitable technology for CFRP/Ti stack drill.     

Cryogenic design of liquid-nitrogen circulation system for long-length HTS cables with altitude variation

Liquid-nitrogen (LN₂) circulation systems with altitude variation are investigated and designed for application to practical long-length HTS cables. This study is motivated by the KEPCO’s immediate plan to install new HTS cables in existing utility tunnels including inclined and vertical sections. The distribution of pressure and temperature along the LN₂ circulation loop is examined for various geographic conditions, taking into account the gravitational effect of altitude variation. The cryogenic cooling requirements are defined in terms of the pressure and temperature of LN₂, and a design process is graphically demonstrated on phase diagram. It is concluded that the LN₂ flow rate along the cable with altitude variation should be carefully determined with the constraints on pressure and temperature, and the proposed graphical method is useful in the design.     

A compact cryogenic pump

A centrifugal cryogenic pump has been designed at Argonne National Laboratory to circulate liquid nitrogen (LN₂) in a closed circuit allowing the recovery of excess fluid. The pump can circulate LN₂ at rates of 2–10 L/min, into a head of 0.5–3 m. Over four years of laboratory use the pump has proven capable of operating continuously for 50–100 days without maintenance.     

Cryogenic turning of the Ti–6Al–4V alloy with modified cutting tool inserts

Productivity in the machining of titanium alloys is adversely affected by rapid tool wear as a consequence of high cutting zone temperature. Conventional cutting fluids are ineffective in controlling the cutting temperature in the cutting zone. In this research work, an attempt has been made to investigate the effect of liquid nitrogen when it is applied to the rake surface, and the main and auxiliary flank surfaces through holes made in the cutting tool insert during the turning of the Ti–6Al–4V alloy. The cryogenic results of the cutting temperature, cutting forces, surface roughness and tool wear of the modified cutting tool insert have been compared with those of wet machining. It has been observed that in the cryogenic cooling method, the cutting temperature was reduced by 61–66% and the surface roughness was reduced to a maximum of 36% over wet machining. The cutting force was decreased by 35–42% and the flank wear was reduced by 27–39% in cryogenic cooling over that of wet machining. Cryogenic cooling enabled a substantial reduction in the geometry of tool wear through the control of the tool wear mechanisms. The application of liquid nitrogen to the heat generation zones through holes made in the cutting tool insert was considered to be more effective over conventional machining.     

Experimental study of laminar natural convection heat transfer from a vertical cylinder to slush nitrogen under constant heat flux conditions

Natural convection heat transfer from a vertical cylinder immersed in slush and subcooled liquid nitrogen and subjected to constant heat fluxes was investigated in order to determine the relative merits of slush nitrogen (SlN₂) for immersion cooling. A glass dewar was used as a test vessel in which a cylindrical heater was mounted vertically, and heat transfer measurements were carried out for SlN₂ and subcooled liquid nitrogen (LN₂) in the laminar flow range. The results revealed advantages of SlN₂ over subcooled LN₂ in natural convection cooling. The local temperatures of the heated surface surrounded by solid nitrogen particles are measured to increase at much slower rates than in subcooled LN₂, which is due to the latent heat of fusion of solid nitrogen. Even after the solid nitrogen particles surrounding the heater are apparently depleted, the average heat transfer coefficients for SlN₂ are still found to be greater than those for LN₂ with the improvement in heat transfer being larger for lower Grashof number regime. Our analysis also indicates that solid nitrogen particles in close proximity to heated surface do not discourage local convection due to the porous nature of SlN₂, making the heat transfer in SlN₂ more effective than in the case of solid–liquid phase change of nitrogen involving melting and conduction processes.     

Investigation of cryogenic level sensors for LN2 and LOX

Investigations of two different types of cryogenic level sensors (capacitance and High Temperature Superconductor (HTS) for level measurement of liquid nitrogen (LN₂) and liquid oxygen (LOX) are presented here. They were tested for an active length of 400 mm in LOX and LN₂. A discrete diode array level sensor was used as a primary standard for calibrating these sensors. Comparative studies on linearity, sensitivity and other parameters at the operating temperatures are presented.     

Experimental investigation of dry,wet and cryogenic boring of AA 7075 alloy

In this research work, an attempt has been carried out to examine (investigate) and study the dry, wet and cryogenic boring of AA 7075 alloy, which is predominantly used in transport applications in defense (aeronautical parts), oceanic and automaker industries. To ensure direct supply of the coolant, and real-time measurement of cutting temperature a modified boring bar is used (modification is carried out using EDM to accommodate placement of a thermocouple to obtain real-time measurement of temperature readings during the boring cycle). It is observed that during cryogenic boring of AA 7075 alloy there is a considerable reduction in the cutting force (F_c), cutting temperature (T_c) and surface roughness (R_a) by 56.16%, 84.70%, 58.98% compared to dry boring and 48.43%, 80.70%, 34.70% compared to wet boring, respectively. Decrease in F_c and T_c leads to a reduction in high stresses at localized points during machining and in turn curtail wear in workpiece and tool. Lubrication provided by cryogenic fluids also plays a sizable role in reduction of F_c and T_c. Reduction in lower F_c and T_c has a glaring effect on the surface characteristics of the hole produced during the boring process. Tool wear is reduced in cryogenic boring by 36.96% and 17.57% compared to dry and wet boring, respectively. Taguchi and ANOVA was carried out which helped in determining feed as an important parameter with respect to F_c and R_a during boring of AA 7075 under dry, wet and cryogenic conditions whereas speed as an important parameter in determining T_c in dry and wet conditions and feed for T_c in cryogenic boring condition. TOPSIS analysis highlighted speed of 770 rpm and feed of 0.055 mm/min as the most closest to ideal solution for all three different cutting conditions. Surface morphology study after boring of AA 7075 highlighted better surface characteristics in cryogenic bored surface compared to dry and wet boring. Roughness measured in AFM for tool used in boring highlighted a decrease in 86.79% and 66.01% in cryogenic boring in juxtaposition with dry and wet boring, respectively. A surge in compressive residual stress is observed in cryogenic bored surface by 10.41% and 3.5% in juxtaposition with dry and wet boring, respectively, highlighting an abatement in tensile residual stress and better workpiece integrity as compared to dry and wet boring conditions.     

Cryogenic system with the sub-cooled liquid nitrogen for cooling HTS power cable

A 10 m long, three-phase AC high-temperature superconducting (HTS) power cable had been fabricated and tested in China August 2003. The sub-cooled liquid nitrogen (LN₂) was used to cool the HTS cable. The sub-cooled LN₂ circulation was built by means of a centrifugal pump through a heat exchanger in the sub-cooler, the three-phase HTS cable cryostats and a LN₂ gas-liquid separator. The LN₂ was cooled down to 65 K by means of decompressing, and the maximum cooling capacity was about 3.3 kW and the amount of consumed LN₂ was about 72 L/h at 1500 A. Cryogenic system design, test and some experimental results would be presented in this paper.     

Numerical study of cryogenic micro-slush particle production using a two-fluid nozzle

The fundamental characteristics of the atomization behavior of micro-slush nitrogen (SN₂) jet flow through a two-fluid nozzle was numerically investigated and visualized by a new type of integrated simulation technique. Computational fluid dynamics (CFD) analysis is focused on the production mechanism of micro-slush nitrogen particles in a two-fluid nozzle and on the consecutive atomizing spray flow characteristics of the micro-slush jet. Based on the numerically predicted nozzle atomization performance, a new type of superadiabatic two-fluid ejector nozzle is developed. This nozzle is capable of generating and atomizing micro-slush nitrogen by means of liquid-gas impingement of a pressurized subcooled liquid nitrogen (LN₂) flow and a low-temperature, high-speed gaseous helium (GHe) flow. The application of micro-slush as a refrigerant for long-distance high-temperature superconducting cables (HTS) is anticipated, and its production technology is expected to result in an extensive improvement in the effective cooling performance of superconducting systems. Computation indicates that the cryogenic micro-slush atomization rate and the multiphase spraying flow characteristics are affected by rapid LN₂-GHe mixing and turbulence perturbation upstream of the two-fluid nozzle, hydrodynamic instabilities at the gas-liquid interface, and shear stress between the liquid core and periphery of the LN₂ jet. Calculation of the effect of micro-slush atomization on the jet thermal field revealed that high-speed mixing of LN₂-GHe swirling flow extensively enhances the heat transfer between the LN₂-phase and the GHe-phase. Furthermore, the performance of the micro-slush production nozzle was experimentally investigated by particle image velocimetry (PIV), which confirmed that the measurement results were in reasonable agreement with the numerical results.     

Design of high-efficiency Joule-Thomson cycles for high-temperature superconductor power cable cooling

Liquid nitrogen (LN₂) is commonly used as the coolant of a high temperature superconductor (HTS) power cable. The LN₂ is continuously cooled by a subcooler to maintain an appropriate operating temperature of the cable. This paper proposes two Joule-Thomson (JT) refrigeration cycles for subcooling the LN₂ coolant by using nitrogen itself as the working fluid. Additionally, an innovative HTS cooling cycle, of which the cable coolant and the refrigerant are unified and supplied from the same source, is suggested and analyzed in detail. Among these cycles, the highest COP is obtained in the JT cycle with a vacuum pump (Cycle A) which is 0.115 at 78 K, and the Carnot efficiency is 32.8%. The integrated HTS cooling cycle (Cycle C) can reach the maximum COP of 0.087, and the Carnot efficiency of 24.8%. Although Cycle C has a relatively low cycle efficiency when compared to that of the separated refrigeration cycle, it can be a good alternative in engineering applications, because the assembled hardware has few machinery components in a more compact configuration than the other cycles.     

Study of drilling of composite material and aluminium stack

Drilling of CFRP/Al or CFRP/Ti, or Al/CFRP/Ti is a challenge to manufacturing engineers. Drilling of CFRP is manageable but the minute drill hits the Al or Ti, those hot and continuous chips destroy the hole. Few studies are carried out in this aspect; CFRP/Al is not reported till date. It is important to select right process parameters when drilling multimaterial stack since each material in the stack requires a different set of machining parameters. Hence, Drilling trials have been carried out in carbon-fibre reinforced plastics (CFRP)/aluminium (Grade 2024) stack without coolant, with plain carbide (K20) drills of various diameters to choose optimum process parameters. The parametric influences on thrust force, torque as well as surface finish were experimentally evaluated. The experimental results show that the quality of holes can be improved by proper selection of cutting parameters. This is substantiated by monitoring thrust force, torque, surface finish, circularity and hole diameter. For the CFRP, the circularity is found to be around 6 μm at low feed rates, when the feed is increased the circularity increases to 25 μm. The wear tests carried out show that, during first 30 holes, thrust force in CFRP undergoes a more important increase (90%) than thrust force of aluminium (6%).     

Design and development of a helium injection system to improve external leakage detection during liquid nitrogen immersion tests

The testing of assemblies for use in cryogenic systems commonly includes evaluation at or near operating (therefore cryogenic) temperature. Typical assemblies include valves and pumps for use in liquid oxygen-liquid hydrogen rocket engines. One frequently specified method of cryogenic external leakage testing requires the assembly, pressurized with gaseous helium (GHe), be immersed in a bath of liquid nitrogen (LN₂) and allowed to thermally stabilize. Component interfaces are then visually inspected for leakage (bubbles). Unfortunately the liquid nitrogen will be boiling under normal, bench-top, test conditions. This boiling tends to mask even significant leakage.One little known and perhaps under-utilized property of helium is the seemingly counter-intuitive thermodynamic property that when ambient temperature helium is bubbled through boiling LN₂ at a temperature of −195.8 °C, the temperature of the liquid nitrogen will reduce.This paper reports on the design and testing of a novel proof-of-concept helium injection control system confirming that it is possible to reduce the temperature of an LN₂ bath below boiling point through the controlled injection of ambient temperature gaseous helium and then to efficiently maintain a reduced helium flow rate to maintain a stabilized liquid temperature, enabling clear visual observation of components immersed within the LN₂. Helium saturation testing is performed and injection system sizing is discussed.