International Journal of Machine Tools and Manufacture
Minimum quantity lubrication drilling of aluminium–silicon alloys in water using diamond-like carbon coated drills
Introduction
Metal removal fluids decrease friction between the cutting tool and the workpiece material, preventing tool wear and, in the case of aluminium, reducing adhesion to the tool. Current environmental and health concerns, however, require manufacturers to reduce the volume of their waste streams [1]. The dry machining process (i.e., machining without the use of metal removal fluids) satisfies the aforementioned circumstances for steel and other ferrous materials [2], [3], but the dry machining of aluminium and especially the dry drilling of cast Al–Si alloys has proven difficult due to aluminium's adhesion to the drill. The chips that adhere to the drill, particularly to the high-speed steel (HSS) drills create obstacles to chip evacuation through the drill flutes. Such chip clogging often results in rapid drill failure. The challenge is to minimize the adhesion of aluminium to the drill, which can be achieved to a certain degree with the use of carbon-based coatings, primarily diamond-like carbon (DLC) coatings [2], [4], [5], [6], [7], [8], [9]. Meanwhile, an environmentally friendly machining technique that feeds minute quantities of lubricant, i.e., 5–50 ml/h [10] to the cutting edge of the tool—a method known as minimum quantity lubrication (MQL)—is promising and could be used in conjunction with the DLC coated drills. This work investigates the application of aluminium adhesion mitigating DLC coatings in the MQL drilling of Al–Si alloys. The effects of the water MQL process were considered not only because water provides an environmentally sustainable MQL fluid, but also because DLC coatings revealed favourable tribological properties under high humidity. It is useful to review the existing literature on the friction and wear of DLC coatings in water environments.
Traditional hard coatings based on nitrides, including TiN, TiCN and CrN, demonstrate poor adhesion mitigating performance against aluminium alloys, generating high coefficient of friction (COF) and an unacceptably high amount of aluminium transfer during pin-on-disc tests [2], [3]. Carbon-based tool coatings, on the other hand, particularly DLC coatings, display improved performance during tribological tests, with a low COF and minimum aluminium adhesion under ambient testing conditions compared to other hard coatings [2], [4], [5], [6], [7]. In practice, when DLC coatings are exposed to ambient and humid environments their performance becomes sensitive to the hydrogen content of the coatings [4], [5], [6]. Ronkainen et al. [11] compared the tribological performance of hydrogenated DLC (H-DLC with 40% H) and non-hydrogenated DLC (NH-DLC with <1% H) in water. The tribological experiments consisted of reciprocating sliding tests against alumina balls under a normal load of 5 N. The H-DLC coating performed poorly under water lubricated conditions, resulting in rapid wear. The COF of the H-DLC film was higher than that of the NH-DLC film, which was as high as 0.6 at the beginning of the test but then decreased until it reached a low value of 0.05. Stallard et al. [12] studied the tribological behaviour of two commercial carbon-based coatings (NH-DLC—designated as Graphit-iC and H-DLC—designated as Dymon-iC) by testing them in air, water and oil. Each coating was tested using a pin-on-disc arrangement at 10, 40 and 80 N against a WC–6%Co ball. The NH-DLC coating showed an initial COF of 0.08 in ambient air, which was higher than that of the H-DLC with a COF of 0.04 at high load of 80 N. The NH-DLC coating exhibited a slightly lower COF of 0.07 when tested in water, and wear rates were low (2.3×10−8 mm3/Nm), but the H-DLC coatings failed rapidly when tested in water. Suzuki et al. [13] studied the tribological properties of H-DLC films using a reciprocating friction tester in a water environment against a martensitic stainless steel (AISI 440C) ball. They found that the COF and the specific wear rate of H-DLC films were 0.07 and 10−8 mm3/N m, respectively. On the other hand, the COF and wear rate in ambient air were 0.03 and 10−7 mm3/N m. In water testing environment, the ball surface was covered with more transferred materials than it was in air. Konca et al. [4] studied the effects of testing atmosphere on the tribological behaviour of the NH-DLC against the same 319 aluminium–silicon alloy that is used in this work and measured high COFs (0.46, 0.47, and 0.44) and high wear rates (2.48×10−4, 3.60×10−4 and 6.64×10−5 mm3/m) for the NH-DLC coatings in vacuum, nitrogen and dry air (0% RH) at 1 N load. The lowest COF (0.08) and wear rate (4.38×10−7 mm3/m), however, were observed in air with an 85% RH. It was generally thought that the presence of hydrogen in water promotes the formation of C–H bonds, which helps to passivate the carbon bonds on DLC film surfaces [4], [5], [6], [7], [8], [9], [10].
A relatively small number of studies have been reported on the application of MQL when machining aluminium alloys, and most of them use uncoated steel or carbide drills. Kishawy et al. [14] compared the effects of flooded lubrication and MQL on tool wear, chip morphology, surface quality and cutting forces during milling of an A356 aluminium–silicon alloy (6.69% Si, 0.44% Mg, 0.02% Cu) using an uncoated carbide tool. For the MQL, a synthetic phosphate ester (30 ml/h) with “extreme pressure additives” was used. The MQL milling displayed reduced friction forces (142 N) when compared to flooded lubrication (146 N) at a speed of 5000 m/min and a feed rate of 0.2 mm/tooth. Kelly and Cotterell [15] studied the effects of MQL used during the drilling of an aluminium–magnesium alloy (4.50% Mg, 0.70% Mn) with uncoated HSS. The authors compared feed force, torque and surface roughness for flooded (mineral soluble oil, flow rate of 5.2 l/min), MQL mist (vegetable oil, 20 ml/h) and dry conditions. Reductions in torque and feed force were obtained for all methods of coolant application with an increase in cutting speed (25–105 m/min) and feed rate (240–500 mm/min). In terms of torque responses, the mist application (2.2 N m) performed better than the flooded (2.4 N m) and dry conditions (3.8 N m). The mist application was also superior in feed force responses (400 N) at higher speeds and feed rates than the other cooling conditions (flood −450 N, dry −675 N). Braga et al. [16] studied the drilling performance of the uncoated and diamond coated carbide drills under both MQL (10 ml/h of mineral oil) and a flood of soluble oil (one part of oil for 25 parts of water) in the drilling of A356 aluminium–silicon alloys. Their evaluations revealed that the performance of the drilling in terms of feed forces when using MQL (maximum feed force 1.26 kN) was analogous to that found using a high amount of flooded soluble oil (maximum feed force 1.25 kN). The power consumed when using flooded soluble oil (maximum consumed power 0.86 kW) was slightly higher than that when using MQL (maximum consumed power 0.80 kW) for uncoated carbide drills.
Consequently, according to the studies reviewed above, the MQL performed better than both dry and flooded coolants when cutting aluminium alloys. In contrast, Bardetsky et al. [17] observed that flooded coolant (40 l/min) outperformed MQL (10 ml/h) during the high speed milling of 319 Al alloys. They used cemented carbide inserts (Coromant grade H13A) and a synthetic phosphate ester liquid (BM2000) as the MQL lubricant. The cutting speed, axial depth of cut and feed rate were 4,290 m/min, 2.0 mm and 0.08 mm/tooth, respectively. A lower resultant cutting force (300 N) was observed in the case of the flooded cooling system compared to MQL (350 N). The highest resultant cutting force (450 N) was observed during dry machining, due to significant aluminium adhesion. The authors stated that in flooded conditions, the formation of a thick boundary layer of lubricant reduced the direct contact between the tool and workpiece material.
In summary, published reports agree (except Ref. [17]) that MQL performs better than flooded lubrication during aluminium machining. The development of an effective MQL drilling process ultimately depends on the (i) type of MQL agent; (ii) composition of the aluminium alloy being machined and; (iii) tribological properties of the drill surfaces. The survey also indicates that DLC's may be beneficial as tool coatings. In this respect, reliable measurement methods that can distinguish between the drilling performances of DLC coated drills with different compositions in MQL machining must be developed. This approach will help to select the optimum MQL–DLC coating conditions for aluminium drilling.
This study evaluates the cutting performance of uncoated and DLC coated HSS drills during the MQL drilling of 319 Al using distilled water. We have examined the dry drilling performance of DLC coatings in a recent publication [18]. We have used the same methodology here to analyse the MQL response of the DLC coatings against 319 Al, which is an Al–Si alloy with good castability and machinability (using conventional metal removal fluids). This alloy was selected due to its extensive applications in vehicle components requiring light weight and reasonably good strength. These applications include automotive components such as crankcases, engine blocks and heads in internal combustion engines, where drilling is one of the most vital machining process. The purpose of using lightweight Al–Si alloys in vehicles (rather than conventional cast iron) is to improve fuel economy and reduce environmental emissions, and thus it is likely that the use of Al–Si alloys will expand in the future. Consequently, advancement of environmentally sustainable alternatives to the flooded drilling of Al–Si alloys is critical to North America's lightweight automobile manufacturing strategy.
The drilling performance was assessed by measuring the torque and thrust force generated during drilling. This approach allows us to compare dry drilling with MQL drilling as well as comparing H-DLC and NH-DLC tool coatings with the uncoated HSS drills. The resulting torque and thrust force measurements were complemented by quantitative metallographic analyses of the tested drill surfaces to establish correlations between measured parameters and aluminium adhesion to the tool surface. The torque responses of uncoated HSS drills, and those coated with DLC tested under flooded drilling condition have been used to benchmark the improvements that can be achieved using MQL drilling.
Section snippets
Workpiece material: 319 Al alloy
The workpiece material was a sand cast 319 Al alloy tested in rectangular blocks of 30×15×2.54 cm3 in as-cast condition, supplied by General Motors Co. (Michigan, USA). 319 Al is a hypoeutectic aluminium alloy containing (in wt%) 6.0% Si, 3.5% Cu, 0.26% Fe, 0.1% Mg, 0.01% Mn, 0.01% Ni, 0.08% Ti and the balance Al. The bulk hardness of the 319 Al was 72.40 HR-15T, measured as Rockwell Superficial Hardness using a 1/16 in. (1.59 mm) diameter ball and a 15 kg load. The matrix microhardness of the 319
Flooded versus dry drilling of 319 Al
The initial step of the experimental methodology consisted of measuring torques and thrust forces produced during drilling of 319 Al under (i) flooded and (ii) dry drilling conditions. Flooded and dry drilling represent the two extreme conditions and thus are used as the reference points in the assessment of the improvements that could be achieved using the DLC coated tools under the MQL drilling condition.
Fig. 5 compares the average torques measured during the drilling of 319 Al under
Conclusions
The key observations of this work can be summarized as follows:
- (1)
Drilling a 319 grade Al–Si alloy workpiece using 30 ml/h water spray (H2O-MQL) required lower torque and thrust forces compared to dry drilling. Both the uncoated HSS and DLC coated drills showed lower torque and thrust force (in terms of both maximum and average values) during H2O-MQL drilling. The average torque response obtained using H2O-MQL was comparable to that measured for conventional flooded cooling.
- (2)
During H2O-MQL drilling,
Acknowledgements
The authors would like to thank Natural Sciences and Engineering Research Council of Canada (NSERC) and General Motors of Canada Ltd. for their financial support.
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