Chilled Air System and Size Effect in Micro-milling of Nickel−Titanium Shape Memory Alloys
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Although Nickel-Titanium Shape Memory Alloys (NiTi SMAs) are used in a variety of applications due to their shape memory and superelasticity properties, their features of high ductility, temperature sensitivity, and strong work hardening render these materials difficult to machine. The viability of a new approach in improving the machinability through temperature control using chilled air system application was investigated. Differential scanning calorimetry was used to characterise material response to thermal loads. Microstructure phase identification was evaluated with X-ray diffraction. Micro-milling tests were performed using chilled air system and benchmarked to dry cutting and the use of minimum quantity lubricant (MQL). To augment lubrication, chilled air was also applied concurrently with MQL. Results indicated that the application of chilled air reduced cutting temperature and minimised burr height, while their simultaneous application with MQL further improved the machinability. Further investigation was conducted to explore the influence of the ploughing mechanism on machining performance and product quality. The results pointed to higher feed per tooth producing better outcomes. This paper puts forward a new hypothesis that the machinability could be improved by inhibiting or locking in phase transformation through temperature control, and optimising chip thickness, one of the principal parameters of size effect.
KeywordsMicro-machining Nickel-titanium alloys Chilled air Minimum quantity lubricant Shape memory alloy Size effect Sustainability
List of Symbols
Depth of cut
Austenite phase transformation finish temperature
Austenite phase transformation start temperature
Force in x-direction
Force in y-direction
Force in z-direction
Feed per tooth
Martensite phase transformation finish temperature
Martensite phase transformation start temperature
Cutting edge radius
Undeformed chip thickness to tool edge radius ratio
1.1 Shape Memory Alloys
Shape memory alloys (SMAs) are materials which exhibit the two very exceptional properties of pseudoelasticity and shape memory effect (SME). The pseudoelasticity or superelasticity factor in the SMA enables the material to revert to its original form when mechanical loading is applied without any thermal activation. The SME feature subjects the material to a memorisation process (returns to original shape) between two transformation phases, either by temperature or magnetic field [1, 2]. These properties have made SMAs the material of choice in both medical and industrial engineering industries for a diverse range of applications such as (1) the manufacturing of eyeglass frames, surgical stents, orthodontic archwires, and active catheters, and (2) the production of fasteners, sealing and coupling, aerospace actuators (magnetic), sensors and microelectromechanical systems (MEMS), cellular phone antennas, fuel injectors, and small helicopter rotors . These applications could be divided into four primary function categories i.e. free recovery, constrained recovery, actuator, and superelasticity .
Among all SMAs, Nickel Titanium-based (NiTi) alloys are the most frequently used due to their additional advantages of strong shape memory effect, superelasticity, biocompatibility, and corrosion resistance . Unconventional cutting processes such as electrical discharge machining, waterjet machining, and laser machining have proven to be better alternative processes compared to conventional cutting in the manufacture of final products . To date, machining of these SMAs by milling has not been exhaustively investigated despite the process being a potentially viable alternative production process. Due to the various degree-of-freedoms in the milling process, complex structures could be produced, an element that is important in facilitating new application areas for NiTi shape memory alloys.
1.2 Challenges in Micro-machining.
‘Micromechanical machining to create miniature devices and components with features that range from tens of micrometres to a few millimetres in size’ ;
‘Machining of dimensions between l and 999 m’ ;
‘Mechanical cutting of features with tool engagement less than 1 mm with geometrically defined cutting edges’ .
In this research, the definition of micro-milling is based on the premise that when 1–999 µm diameter end mills are used, the undeformed chip thickness would be comparable to the cutting edge radius or material grain size. This presents a size effect challenge for machining and could be considered a micro-machining domain.
Premature tool breakage and unpredictable tool life have been reported as major issues in this type of microscale cutting . In terms of phase, differential response of phases to cutting influences cutting force and burr formation on grain boundaries found in multiphase materials . Conventional flood coolants also might not be appropriate for micro-machining due to the flow pressure of the lubricants potentially deflecting the cutting tool .
1.3 Minimum Chip Thickness
Material removal rate in micro-machining is significantly lower compared to conventional macro-scale machining. The challenge however is in its size effect which pushes the material to spring back for ductile phases , and ploughing effect when undeformed chip thickness is comparable or lower than cutting edge radius . When the undeformed chip thickness is lower than the minimum thickness, no chip is generated in machining and no cutting occurs with the material only compressed. Chips start to form when the undeformed chip thickness is equivalent to the minimum thickness. Some elastic deformation would still occur leading to less material removed from the desired depth. A chip is formed and removed only when undeformed chip thickness is significantly higher than the minimum thickness. Ratio selection is therefore critical in micro-milling as it defines the ploughing effect material removal and at the same time influences chip thickness, effective rake angle, and specific cutting energy .
1.4 Machinability of NiTi SMAs
The machining of NiTi shape memory alloys often produces very poor workpiece quality due to the high ductility, temperature sensitivity, and strong work hardening features of the materials. Tool wear is also excessively high  even when cutting parameters have been optimised . This higher tool wear leads to higher friction and directly contributes to higher cutting temperature . When the temperature exceeds the austenite temperature, the yield strength will in turn increase sharply .
The high ductility nature of the material also leads to other serious issues such as poor chip breaking and a high propensity of burrs. The material is only deposited onto the slot side and not separated from the workpiece . Machining strategy also plays an integral role with down milling strategy creating narrower burrs compared to up milling . Burr formation unfortunately cannot be completely avoided. The non-removal of burrs poses two major risks: (1) they could lead to injury especially for workers in the assembly line, and (2) burrs stuck in certain parts of the component could become loose at any point in time and subsequently cause serious damage during operation .
Yet, another critical challenge is phase transformation. When NiTi is heated, it begins to transform from martensite to the austenite phase, which is relatively hard and has a considerably higher Young’s modulus. As martensite, NiTi has very low yield strength and could be easily transformed into a new shape by application of external force and thermal load . Kaynak et al. report that machining of martensite NiTi leads to force reduction as compared to preheated work material. Heating the sample changes the phase and renders the material more difficult to machine. Biermann et al. however maintain machining in martensitic phase as more challenging than austenite due to the elements of high ductility and tensile strength which generate high force and high tool wear. Surface quality in austenitic machining is comparatively more acceptable than that produced by the martensitic structure.
In terms of mechanical load, the phase transformation temperature of the workpiece increases when stress is applied to the material . This is consistent with the findings by Fuentes et al. which reported that the austenite finish temperatures of wire increased in a linear relationship to the loads applied. Based on the available studies on machining nickel alloys, the assumption that could be made is the microstructure phase is influenced by operating temperatures and differentiates the machinability of the material. Mechanical loads could also shift the critical temperatures for microstructure.
1.5 Research Motivation
Cutting fluids in general have been the conventional choice to manage the machinability challenges caused by temperature or friction increase and chip management [23, 24]. However there is growing concern that cutting fluids could pose significant health, safety, and environmental hazards [25, 26]. An estimated 80% of all occupational diseases of operators are reportedly linked to skin contact with cutting fluids . The associated costs of such fluids are not limited to their purchase and preparation alone but also maintenance and disposal costs due to many types of cutting fluid not being biodegradable hence necessitating expensive pre-disposal treatments [26, 28].
Recent improvement strategies have included applying different types of cutting fluids in the machining process. Effective unconventional methods such as dry cutting, minimum quantity lubrication (MQL), cryogenic cooling, chilled air, and the use of solid lubricants have been found to be viable substitutes of cutting fluids to optimise machining performance and minimise risks . Many of the researches on NiTi have focused on dry cutting, minimum quantity lubricant, and cryogenic cooling although there is limited or no scientific enquiry to date on the effectiveness of the chilled air system in improving the machinability of the alloys.
The hypothesis for this study is the premise that effective cooling delivered by chilled air systems could reduce heat generated at tool or workpiece interface, maintain the workpiece material in its easier to cut martensitic phase, and improve the machinability of NiTi SMAs. In addition, since ploughing mechanism could influence the transition from twinned to detwinned martensite, the effect of work hardening could be minimised through the appropriate selection of feed per tooth rate. Experiments to test this hypothesis are detailed in the following section.
2 Experimental Details
2.1 Work Material Characterisation
The workpiece material selected for this study was NiTi. Improving its machinability at the micro scale would enable cost-effective fabrication of micro components such as mini actuators, sensors, micro controllers, micro medical devices, and implantable applications . To link the material thermal cycle to the machining strategy, the first stage was to characterise the material to establish its phase at room temperature and the critical temperature at which it would transform. Differential Scanning Calorimetry (DSC) was used to evaluate the material response to thermal loads.
For the microstructure observation, the as-received sample was first mounted on carbon conductive black resin and subsequently ground with three different abrasive paper grits (600, 1200, and 2400). It was then polished with 6, 3 and 1 microns diamond slurry monocrystalline paste to obtain a mirror-like surface free from any scratches. The sample was later etched using a solution of 3.2% hydrofluoric acid (HF), 14.6% nitric acid (HNO3), and 82.2% de-ionised water for 10 s . Essential safety precautions were observed since HF and HNO3 exposure could potentially lead to serious injuries, skin burn, eye damage and even fatality.
2.2 Micro-end Mill Cutting Tools
Prior to the cutting tests, all micro tools were inspected using Quanta 200 Scanning Electron Microscope (SEM) to check their geometry and cutting edge radius feature. This was to ensure there were no unacceptable tool conditions such as multi-cutting edges, chipping, burr, and other defects. The average cutting edge radii of micro tools was measured to be .99 µm.
2.3 Experimental Setup and Machining Conditions
Micro-milling tests were conducted on a NiTi alloy block 70 mm length by 20 mm width and 18 mm height in dimension. The composition of alloy matrix of nickel to titanium was confirmed by Energy Dispersive X-ray Analysis (EDXA) measurement to be 55 to 45 wt% respectively. The tests were undertaken on a Mikron HSM 400 high-speed machining centre. Cutting forces were measured with a Kistler mini-dynamometer type 9256C and cutting temperature analysed using a FLIR Thermo Vision™ A40 thermal camera.
Four different modes of cooling/lubricant systems—dry, chilled air, minimum quantity lubricant, and chilled air applied concurrently with minimum quantity lubricant—were examined in this experiment. A constant cutting velocity of 35 m/min (23,000 rpm), table feed rate of 184 mm/min, feed per tooth of 4 µm, and depth of cut of 30 µm were used as had been established from pilot cutting tests. Ten consecutive passes of 15 mm length each were milled with 40% of tool diameter step over. The material removal rate was 1.1 mm3/min. This was considered reasonable compared to previous studies with 3.15 mm3/min and 0.25 mm3/min [1, 16]. Each test was repeated three times with new cutting tools.
2.4 Thermal Measurement
A ThermoVision A40 infrared camera, a non-contact temperature measurement featuring high-resolution, real-time imaging and remote monitoring solution, was used to measure workpiece temperature under different cutting conditions. The location of the analysed area was at the end of the slot milled. The distance between the analysed area and camera was 15 cm. The measured surface was earlier painted with matte black paint with emissivity Ɛ = 0.94.
2.5 Cutting Force Measurement
A three-component Kistler Mini-Dynamometer Type 9256C was used in this experiment to measure feed (X), step-over and depth force. The instrument consisted four 3-component force sensors mounted under high preload between the cover plate and two lateral base plates. The very low threshold enabled the measurement of small forces. This type of dynamometer suitable for this experiment due to its rust resistant and rust proof features against the ingress of spray water and cutting fluid. Data acquisition system for force measurement was measured by the Kistler DAQ system Type 5697A1 and the signal amplified using Kistler Type 5070A charge amplifier.
2.6 Surface Roughness, Nano-hardness, Burr size, and Tool Wear
A Keyence VK-X200K 3D Laser Scanning Microscope which measures and analyses 3D surfaces was used to measure burr size and surface roughness. This instrument provides non-contact nanometer-level profile, roughness, and film thickness data on any material. High accuracy 0.5 nm linear scale module was built in to improve the Z-resolution that enabled better detection of surface features. Scanning electron microscope (SEM) images were also used with Image J software to measure tool wear. The hardness of the sub surface machined zone was recorded using a nanoindenter MTS Nano Indenter XP at 400 nm indentation depth. Systematic uncertainties were minimised by first calibrating the equipment to be used. Random uncertainty was addressed by performing each measurement at least five times.
3 Results and Discussion
3.1 Thermal Analysis
From the results, it is apparent that the existence of chilled air significantly contributed to lowering the cutting temperature compared to dry or MQL conditions. Based on this outcome, it could be deduced that convective heat transfer from the chilled air flow was effective in removing heat generated during the cutting process compared to the use of dry and MQL alone. Besides the cold air, the chilled air velocity flow also acted as a medium to remove chips and metal fines on the workpiece and cutting tools.
3.2 XRD Analysis
X-ray diffraction Bruker D8 Discover instrument was used in the investigation of the phase composition of the machined materials. Cobalt (Co) radiation was utilised for scanning and Bruker corundum reference standard employed to calibrate the height of the sample holder using the x-ray beam half cut method. This same process was applied to ensure the experimental sample remained at the correct height with respect to the x-ray beam. A brief pre-scan was conducted to select the 2 theta range with respect to approximate patterns found for NiTi type materials from the International Centre for Diffraction Data (ICDD) database. The diffraction angles were between 40° to 100°, Cobalt radiation 35kV, and current 40 mA. The diffraction was performed along the milled slot.
Micro-XRD results for different cutting conditions
ICDD PDF No.
Chilled air + MQL
3.3 Microstructure Analysis
All samples were etched for 10 s by dipping them into an etchant of 3.2% hydrofluoric acid (HF), 14.6% nitric acid (HNO3), and 82.2% de-ionised water as reported earlier. ASTM average grain size number was taken as an indicator to measure the grain. Microstructure analysis was undertaken only on the machined slot end sides, below the slot floor. The measurement lines were drawn 20, 40 and 60 µm from the slot floor into the material bulk. The linear intercept method was used for measuring the grain sizes.
Average grain size number of the subsurface machined area in relation to cutting condition was analysed where a higher value would indicate more grains being intercepted in a particular test line. There was however, no distinct difference between the results, except under chilled air applied concurrently with MQL conditions whereby the grain was larger than as-received, dry, and MQL. Under chilled air, the results were closer to the as-received material compared to other conditions. In terms of grain size, the average size for as-received, dry, chilled air, MQL, and combined chilled air and MQL were measured as 24, 25, 24, 23 and 36 µm respectively. There was no clear basis for this increase in grain size for chilled air combined with MQL although the forces were also considerably lower.
3.4 Cutting Force
3.5 Tool Wear
3.6 Surface Roughness
3.8 Burr Formation
However, results were varied in terms of burr height with chilled air demonstrating the lowest height while MQL formed the highest burrs. This was strongly influenced by the appearance of the burr shape whereby burrs under the chilled air system were softer and easier to bend compared to the stiffer and harder variations under MQL.
3.9 Size Effect in Machining of Nickel-Titanium Shape Memory Alloys
Chilled air applied concurrently with MQL
Ratio of maximum undeformed chip thickness to tool edge radius, rr
.4, 1.0 and 3.0
Table federate, vf (mm/min)
Feed per tooth, fz (µm)
Spindle speed, N (rpm)
Depth of cut, ae (µm)
In micro machining, the material removal rate is substantially lower compared to conventional macro-scale machining. Normally, undeformed chip thickness would be equal or lower in size to the cutting edge radius. In this research, the ratio of undeformed chip thickness to cutting edge radius, rr, was observed based on cutting force, specific cutting force, surface roughness, burr formation, and tool wear to determine the implication of cutting variables on process performance. The values of the rr ratio were .4, 1.0 and 3.0.
3.9.1 Cutting Force and Specific Cutting Force
3.9.2 Surface Roughness
3.9.3 Burr Formation
3.9.4 Tool Wear
This research has shown that for temperature-sensitive NiTi alloy investigated, the application of chilled air is able to reduce heat generated during the cutting process and prevent the transition from martensite to the austenite phase. Since the martensite phase has better machinability than austenite for this alloy, the use of chilled air ensures that the material remains in the easier-to-cut phase. Whether chilled air would be suitable to prevent phase transitions in NiTi depends largely on the phase transformation temperature. For the material in this study, this was evaluated as 37 °C (martensite start), 21 °C (martensite finish), 47 °C (austenite start), and 67 °C (austenite finish). In this instance, the application of chilled air was a cheaper and easier option to avoid phase transition (austenite phase) especially in microscale cutting.
Weinert and Petzolt (2008) in their study reported that the martensite start, martensite finish, austenite start, and austenite finish temperatures for their workpiece were − 37 °C, − 103 °C, − 46 °C, and 13°C respectively. In this case the use of chilled air (with outlet temperature of − 15°C) is inferred to be inadequate to maintain the material under martensite phase and easier-to-cut phase. Cryogenic cutting becomes a more suitable solution. A scientific study using multiple criteria, comparing the effects of cryogenic cooling, and chilled is an open area for further research.
Shape memory alloys are difficult to mechanically machine due to their high ductility, temperature sensitivity, and strong work hardening properties. Controlling cutting temperature is a potentially viable strategy to address this machinability issue. Chilled air systems are expected to be easier to deploy in industry compared to cryogenic machining systems. In this project, the impact of chilled air systems on the machinability of NiTi shape memory alloys was investigated and benchmarked to dry machining and machining with minimum quantity lubricant (MQL).
To compensate for the lack of lubrication in the chilled air system, chilled air was applied concurrently with minimum quantity lubricant. There is an acute lack of literature on the machinability of NiTi alloys in the presence of chilled air systems. The investigation in this research was further extended to explore the effects of undeformed chip thickness to tool edge radius. The key findings of this research work are as follows:
• This paper provides evidence that differential scanning calorimetry could evaluate the critical phase transformation temperatures for shape memory alloys whereby this information could then be used to determine the maximum critical cutting temperature given the difference in machinability for the material phases. Ideally, the cutting process should be undertaken when the material is in easier to cut condition; below the temperature at which the NiTi alloy starts to transform from martensite to austenite.
• Simultaneous use of chilled air and minimum quantity lubricant (MQL) showed significant potential in improving micro scale machinability of NiTi, lowering cutting temperature, reducing cutting force and flank wear, and significantly reducing burr width.
• The ratio of undeformed chip thickness to cutting edge radius could be used as a relevant parameter in the micro-milling of NiTi since it influences cutting force, surface roughness, burr formation, and tool wear.
• The specific cutting force increased considerably when the uncut chip thickness was small due to the effect of the ploughing process when it became dominant.
• The finest surface roughness was attained when the undeformed chip thickness was larger than the tool radius. At this juncture, conventional shearing was the dominant cutting process.
• Burr size was found to be lower with an increase in the chip load. Lower-sized burr formation occurred when undeformed chip thickness was larger than edge radius.
• Based on the results in the size effect experiment, cutting in the ploughing mode consistently demonstrated undesirable results by increasing the specific cutting force, surface roughness, and burr size while inducing coating delamination.
• This paper contributes to machining strategy of the nickel titanium alloys (that start to transform to austenite above room temperature and at 47 °C for the case presented), based on simultaneously use of chilled air and minimum quantity lubricant augmented by finishing in a shear mode cutting regime. Other forms of more severe cooling such as cryogenic machining, can be considered for materials that have significantly lower temperatures for the start of transformation to austenite.
The first author gratefully acknowledges the financial support of the government of Malaysia and University Malaysia Perlis.
- 6.Chae, J., Park, S.S., & Freiheit, T. (2006). Investigation of micro-cutting operations. International Journal of Machine Tools and Manufacture, 46(3–4), 313–332.Google Scholar
- 14.Weinert, K. and Petzoldt, V. (2008) ”Machining NiTi micro-parts by micro-milling”. Materials Science and Engineering: A, 481–482, 672–675.Google Scholar
- 27.HSE, Medical aspects of occupational skin disease (Vol. 24, pp. 1–8).Google Scholar
- 31.Methods, S.T. (2010. “Standard test methods for determining average grain size 1”, 96, pp. 1–26.Google Scholar
- 34.Duerig, T. W., Melton, K. N., & Stockel, D. (1990). Engineering aspect of shape memory alloys. London: Butterworth-Heinemenn Ltd.Google Scholar
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