Tribological behavior of Al–6.5%, –12%, –18.5% Si alloys during machining using CVD diamond and DLC coated tools

    Tribological behaviour of Al- 6.5%, - 12%, - 18.5% Si alloys during machining using CVD diamond and DLC coated tools S. Bhowmick, A. Banerji, A.T. Alpas PII: DOI: Reference:

S0257-8972(15)00525-3 doi: 10.1016/j.surfcoat.2015.08.073 SCT 20548

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

2 May 2015 28 July 2015 23 August 2015

Please cite this article as: S. Bhowmick, A. Banerji, A.T. Alpas, Tribological behaviour of Al- 6.5%, - 12%, - 18.5% Si alloys during machining using CVD diamond and DLC coated tools, Surface & Coatings Technology (2015), doi: 10.1016/j.surfcoat.2015.08.073

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Tribological behaviour of Al- 6.5%, -12%, -18.5% Si alloys during machining using CVD diamond and DLC coated tools

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S. Bhowmick, A. Banerji and A. T. Alpas*

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Department of Mechanical, Automotive and Materials Engineering, University of Windsor 401 Sunset Avenue, Windsor, Ontario, Canada N9B 3P4

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Abstract

Tribological behaviour of cast Al-Si alloys, including samples from hypo-eutectic (Al-

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6.5% Si), eutectic (Al-12% Si) and hyper-eutectic (Al-18.5% Si) compositions that were

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subjected to dry drilling experiments were studied. Tools materials tested consisted of hydrogenated diamond-like carbon (H-DLC), CVD diamond coated and uncoated WC-Co drills.

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Tool failure mechanisms were identified and correlated with the drilling torques. The drilling

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torque vs. number of holes curves typically exhibited three stages, each identified with a characteristic slope (m). A failure criterion was established, such that when m ≥1.0×10-2 N-m the

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onset of tool failure in dry drilling of Al-Si alloys could be predicted. For hypo-eutectic and eutectic Al-Si alloys drilled using uncoated WC-Co the failure criterion was satisfied rapidly (<70 holes) and extensive aluminum adhesion to the drill occurred. The use of H-DLC and CVD diamond coated drills reduced aluminum adhesion and built-up edge formation and maintained m ≤ 1.0×10-2 N-m throughout the drilling tests range. Drilling of hyper-eutectic Al-Si alloys with the H-DLC coated drills led to shortened tool lives characterized by flank wear as the primary Si particles removed the H-DLC coating and increased friction and wear. CVD diamond coated drills produced low tool wear and maintained low steady state m values. It was concluded that during dry drilling of Al-Si alloys, where aluminum adhesion was a performance limiting factor, H-DLC coatings could replace CVD diamond tools. Keywords: Al-Si alloys; Drilling; CVD diamond; Diamond-like Carbon, Friction, Adhesion.

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ACCEPTED MANUSCRIPT 1. Introduction Al-Si alloys castings, both hypo- and hyper-eutectic grades are widely used in automotive

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and aerospace industries. These alloys are difficult to machine regardless of their Si content but

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for different reasons. During machining of low Si content alloys, aluminum adhesion to the tool

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materials such as HSS and WC-Co is the main problem—especially during dry drilling and tapping whereas machining of high Si content alloys often causes flank wear of the tool. Thus,

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for Al-Si alloy machining, both aluminum adhesion and tool wear are known to cause premature tool failure [1-3]. Past studies have shown that conventional wear resistant tool coatings that are

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effective for machining of ferrous materials are not nearly as efficient when used on Al-Si alloys as they fail to prevent aluminum adhesion. Metal nitrides based hard coatings, including TiN,

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TiCN, and TiAlN, are among those that have demonstrated unsatisfactory tribological performance (high friction and adhesion) against aluminum alloys under dry sliding contact [4].

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Research work aiming at improving machining performance of the pro-eutectic and near eutectic Al-Si alloys should therefore consider the adhesion mitigating properties of the tool coatings as the primary selection criterion. It was shown that diamond and diamond-like carbon (DLC) coatings significantly diminish the adhesive transfer of aluminum to the tools during dry drilling and tapping operations [5-9]. Yet the frictional behaviour of the DLC coatings, both hydrogenated (H-DLC) with about 40 at % H and non-hydrogenated grades (NH-DLC) with less than 2 at % H have certain limitations, namely their properties are sensitive to changes in atmospheric humidity and temperature. The coefficient of friction increases abruptly at above 200 ˚C for H-DLC and 100 ˚C for NH-DLC [10-12]. For this reason, polycrystalline diamond (PCD) is often the preferred tool material [13-15] for machining applications where high temperatures are generated. The diamond tool coatings deposited by cathodic vapour deposition

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ACCEPTED MANUSCRIPT (CVD) method combine high hardness with wear resistance and possess satisfactory adhesion mitigating properties and thus offer state of the art manufacturing tool material for Al-Si

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machining [16, 17].

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Dry machining of metals is desirable due to its environmental benefits, involving little or no usage of chemically hazardous cutting fluids, but is yet to be applied to large scale

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manufacturing. Dry machining on the other hand generates the most demanding machining

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conditions. This is an advantage, however, for laboratory scale tribological studies undertaken for the purpose of delineating performance limitations of the tool coatings. These studies may

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provide useful baseline information for potentially robust and cost effective coatings for Al-Si machining processes. A review of the existing literature on the dry machining of Al-Si alloys

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using DLC coatings, PCD and CVD diamond coated tools is provided in Sections 1.1 and 1.2.

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1.1 Dry Machining of Al-Si alloys using PCD and CVD diamond coated tools

Heath [13] compared cutting performance of PCD tools with the uncoated WC-Co in cutting of hypereutectic Al-20% Si alloy. An increase in tool life by 500 min was observed for PCD tools while the uncoated WC-Co tool lasted for only 50 min. Oles et al. [14] tested CVD diamond and PCD tools in turning of Al-18% Si with both materials showing similar tool lives. However, the surface roughness produced by the CVD diamond tools (0.84 µm) was slightly higher than that generated by the PCD tools (0.62 µm). Shen [15] compared the machining performance of CVD diamond coated inserts to the uncoated WC-Co inserts in turning of Al18.5% Si. While the uncoated WC-Co tools failed after 7 min the tool life was extended to 70 min with the use of CVD diamond coated tools which also required two times lower cutting force (0.8 kN) compared with the uncoated WC-Co. Liang et al. [16] by conducting turning

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ACCEPTED MANUSCRIPT experiments on Al-18.5% Si observed that smoother surfaces with an average roughness, Ra of 0.6 µm were obtained for the PCD inserts compared to Ra of 0.85 µm generated by the uncoated

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WC-Co, which was attributed to the higher wear of the WC-Co tool inserts. Gomez et al. [17]

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reported that during turning of Al-18.5% Si the use of CVD diamond coatings with a CrN/Cr interlayer prolonged the tool life by 50%. Ng et al. [18] studied face milling of a hypo-eutectic

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alloy Al-6.5% Si and noted a high cutting force of 900 N for the uncoated WC-Co tool and

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significant flank wear that also produced a high surface roughness. A 25% reduction in the cutting force was achieved with the use of a PCD tool. Roy et al. [19] studied turning of a

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eutectic Al-12% Si alloy where two folds reduction in the cutting force was observed for the CVD diamond coated tools compared to the uncoated WC-Co tools (70 N) whose surfaces

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exhibited extensive aluminum adhesion.

In summary, both PCD and CVD diamond coatings were observed to reduce flank wear

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during machining of the hyper-eutectic Al-Si alloys. For the eutectic and hypo-eutectic Al-Si alloys, these coatings also showed better performance than the uncoated tools by reducing Al adhesion. However, manufacturing and finishing costs of the PCD tool inserts and CVD diamond coating are high with CVD diamond coated tools available at $200 per drill and PCD tools available at $3000 per drill ($3.00/hole), although they are used for machining of critical automotive components made of Al-Si such as the engine block and valve bodies. DLC coatings have also shown low friction and good Al adhesion mitigating properties in tribological experiments [8-12]. It is due to this reason and because of their low cost (at approximately $15 per drill) DLC coated tools have the potential to be used in machining Al-Si castings. Previous studies that considered the cutting performances of DLC coatings in aluminum machining are reviewed in Section 1.2.

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ACCEPTED MANUSCRIPT 1.2 Dry machining of Al-Si alloys using DLC coated tools

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DLC (with 10-20 at.% H) coated tools reduced the thrust force to 40 N compared to uncoated WC-Co (50 N) in the turning experiments conducted on Al-16% Si by dos Santos et al. [20].

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Dasch et al. [21] tested different carbon based coatings and compared with PCD during drilling

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of Al-6.5% Si. NH-DLC coating that generated power consumption as high as 8 HP did not

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provide much improvement over the HSS drills (10 HP). However, using a blend coated (NHDLC/H-DLC) drill a low 4 HP of power consumption was recorded and performed better than

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the H-DLC coated tools. A correlation was found between the amount of aluminum adhesion and the high power required to drill as high aluminum adhesion resulted in chip clogging. Wain et al.

400 holes) was observed when using this coating compared to the

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prolonged tool life (

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[22] tested the H-DLC coated drills during the drilling of Al-6.5% Si and reported that a

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uncoated HSS (40 holes). The improvement in tool life was consistent with the small built-up edge (BUE) formed when drilling with the H-DLC coated drill [21]. Bhowmick and Alpas [23] measured the torques and thrust forces generated when H- and NH-DLC coated HSS drills were used in drilling of Al-6.5% Si. H-DLC coated drill produced consistently lower average torque (2.11 N-m) when compared to NH-DLC coated (2.63 N-m) and uncoated (4.11 N-m) HSS drills with smaller torque spikes (occasional peaks on the torque-time plots) that are indicative of low aluminum adhesion to the coating surface. The metallographic observations confirmed that HDLC coated drills almost eliminated metal transfer to the drill flutes and diminished BUE formation on the drill’s flank face and cutting edge. The tapping of Al-6.5% Si was studied by Bhowmick et al. [24] using H-DLC coated taps that were again shown to produce lower average torque (1.01 N-m) and a smaller amount of Al transfer to the taps compared to tapping using uncoated HSS tools (3.18 N-m). 5

ACCEPTED MANUSCRIPT In summary, DLC coatings were found to improve tool lives of HSS drills and the H-DLC coatings provided slight advantage over NH-DLC coatings in reducing the cutting forces during

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dry cutting of aluminum alloys. Studies on DLC coated tools, however, are limited to Al-Si

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alloys with low Si concentrations (<12% Si). Meanwhile it becomes clear from the survey in Section 1.1.that the coatings’ performances may vary with the Si content and also the tool life

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limiting damage mechanisms change when DLC, PCD and CVD diamond coatings were used, as

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summarised in Table 1. This work evaluates the tribological performance of H-DLC coated tools in drilling of Al-Si alloys with both low and high Si percentages in comparison with CVD

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diamond and PCD coatings using a common experimental methodology. The experimental methodology incorporates the following stages: (i) measurement of the drilling torques until tool

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failure or to a predetermined number of drilling cycles; (ii) determination of drilling induced temperature and examination of its role on tool life; (iii) delineation of the role of the coefficient

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of friction COF on the drilling torque and temperature; and (iv) characterization of aluminum transfer to the tool contact surface and flank wear using metallographic techniques, which are then used to rationalize the tool failure mechanisms.

2. Experimental Details 2.1. Properties and Microstructure of Al-Si Alloys The Al–Si alloys studied consisted of two sand cast alloys, one with 6.5 wt.% Si (319 Al) and the other with 12.0 wt.% Si, and a die cast alloy with 18.5 wt.% Si (390 Al). Table 2 lists the weight percentage of constituent elements in each alloy. Alloys received in an as cast condition were solution treated and aged to different times to provide the starting samples with the same

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ACCEPTED MANUSCRIPT bulk hardness. All three alloys, Al-6.5% Si, Al-12.0% Si and Al-18.5% Si were subjected a solution treatment that consisted of heating each alloy to 490 ˚C for 8 h following which the

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solution treated samples were quenched in warm water (65 ˚C) and artificially aged. Al-6.5% Si was aged for 5 h at 200 ˚C, Al-12.0% Si was aged for 5 h at 175 ˚C and Al-18.5% Si alloy was

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aged for 4 h at 180 ˚C. The bulk hardness values of the age hardened alloys were 52 ± 3 HRB for

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Al-6.5% Si, 54 ± 4 HRB for Al-12% Si and 56 ± 4 HRB for Al-18.5% Si on the Rockwell-B

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scale. The drilling tests were conducted immediately after the heat treatment. A commercially pure 1100 Al with a hardness of 32 ± 2 BHN was tested as a reference.

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Figs. 1 (a-c) show the optical micrographs of Al-6.5% Si, Al-12% Si and Al-18.5% Si after the heat treatment where eutectic silicon particles can be seen for each alloy along with the

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primary silicon particles in the microstructure of Al-18% Si. The intermetallic phases identified (by EDS and XRD) as Al5Cu2Mg8Si6, Al15(Fe,Mn)3Si2 and CuAl2 are marked on the

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micrographs.

2.2. Cutting Tools and Tool Coatings WC-Co twist drills (6.35 ± 0.01 mm diameter) with hardness of 78 ± 4 HRC were used for the drilling experiments. One set of drills were coated by a hydrogenated diamond-like carbon (H-DLC) coating with a thickness of 1.50 ± 0.25 µm deposited using plasma enhanced chemical vapour deposition (PECVD) process. H-DLC coating had an amorphous structure and incorporated

30 at.% H [12]. The hardness of the coating, measured using a Hysitron TI 900

Triboindenter equipped with a Berkovich nano-indenter, was 1428 HV (14 ± 3 GPa). A second set of WC-Co drills were coated by diamond using a hot filament CVD system in a chamber containing hydrogen and methane as precursor gases to a thickness of 2.00 ± 0.50 µm. 7

ACCEPTED MANUSCRIPT The chamber pressure was maintained at ∼40 Torr and the substrate temperature was ∼850 ˚C. The CVD diamond coatings, deposited in this way, had a hardness of 9177 HV (80 ± 2 GPa) and

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consisted of a polycrystalline grain structure with an average size of 5.00 ± 0.65 µm.

2.3. Drilling Tests, Tool Life and Torque Measurements

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Dry drilling tests were performed in a CNC drill press using a low cutting speed of

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2500 rpm (50 m/min) at a feed rate of 0.25 mm/rev. Each (blind) hole drilled was 19 mm in depth, which was ~3 times larger than the drill diameter. The tool life was evaluated by

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determining the total number of holes drilled until drill failure occurred. The drill failure was defined either as jamming of the drill due to large scale Al adhesion--as in the case of drilling of

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most Al-Si alloys using uncoated WC-Co drills or as the flank wear of the tool upon reaching 200 µm--as in the case of drilling of Al-18.5% Si using WC-Co and H-DLC coated drills. For

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other cases drilling tests were continued until 150 holes were drilled in succession without the tool failure. The holes were drilled on Al-Si blocks in lines with a horizontal center-to-center spacing of 10 mm.

Drilling tests were conducted without the use of metal removal fluid i.e. under the dry drilling condition. As dry drilling maximizes the transfer of aluminum to the tools and promotes tool failure, it allows failure mechanisms of uncoated, H-DLC and CVD diamond coatings to be examined. The torque forces generated during dry drilling were measured using a non-contact magneto-static torque sensor installed on the drill holder as a function of the time. The duration of each drilling cycle, i.e., the time elapsed between the initial contact and the complete retraction of the drill bit was 4.25 s. The torque sensitivity of the sensor was 1×10-4 N-m. Three

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ACCEPTED MANUSCRIPT tests were conducted using each of the three different coated and uncoated drills on the Al-Si

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blocks in order to determine the tool lives.

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2.4. Pin- on- Disk Tests

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The magnitudes of torques generated during drilling holes on each type of Al-Si alloy are

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affected by the friction at the tool-workpiece interface. Sliding friction tests were conducted using a pin-on-disk-type tribometer to measure the coefficient of friction (COF) values of Al-Si

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alloys. Pins with a 4 mm radii, machined from the same alloys used as the workpieces in drilling tests, were placed in dry sliding contact against samples made of uncoated WC-Co, H-DLC-

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coated HSS and CVD diamond tool surfaces in ambient air for sliding cycles up to 5.00 × 103,

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using a linear speed of 0.12 ms-1 and normal load of 5.0 N. The friction sensitivity of the sensor

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was 1×10-3.

During machining the tool-workpiece interface temperature increases accelerating coating wear and/or Al adhesion to the tool surface. The tribological properties H-DLC and CVD diamond coatings intended as drill coatings for Al-Si castings should remain stable at elevated temperatures and dissipate in the heat generated rapidly. For this purpose, the pin-on-disk tests were conducted at 100 ˚C, 200 ˚C and 300 ˚C to determine the changes in frictional properties at elevated temperatures. The amount of aluminum adhesion at the cutting edge and the flank wear of tools were determined using a scanning electron microscope (SEM, JEOL 5400 SEM).

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ACCEPTED MANUSCRIPT 3. Results and Analyses 3.1. Drilling Torque and Tool Life

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Drilling of 1100 Al samples using an uncoated WC-Co drill led to drill failure at the 28th

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hole as a result of aluminum adhesion to the tool, which manifested as large torque spike (12 N-

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m) at the last drilling cycle shown in Fig. 2 (a). The tool lives of WC-Co drills were also very short when drilling Al-6.5% Si (Fig. 2b) and Al-12% Si (Fig. 2c); the drill adhered to the Al-

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6.5% Si block at the 71st hole and to Al-12% Si at the 42nd hole. In Figs. 2 (b, c) where the

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drilling torques generated at the last 25 holes (125th-150th) are shown, several torque spikes can be observed as a result of adhesion, but the tool was able to release itself each time until it

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became totally jammed. The highest torque observed was at 11.83 N-m for Al-6.5% Si and

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12.03 N-m for Al-12% Si at the last drilling cycle as indicated in Figs. 2 (b-c). When drilling Al-

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18.5% Si the WC-Co drills failed at the 2nd hole producing a large torque of 12.06 N-m (Fig. 2d). The use of H-DLC coated drills generated low and uniform torques during drilling of 1100 Al and Al-(6.5-12)% Si. As Fig. 2 (e) shows, with the H-DLC coated drills the intended 150 holes could be drilled without evidence for tool-life impeding adhesion of Al to the tool surface. For Al-6.5% Si and Al-12% Si, 150 holes were also drilled easily using the H-DLC coated drills. However, the torques response became less uniform with an increase in the Si content, and 48% of 150 holes exhibited torque spikes during drilling of Al-12% Si compared to 6% for Al-6.5% Si (Fig. 2 [f, g]). Drilling of Al-18.5% Si with the H-DLC coated tools resulted in catastrophic failure (Fig. 2h) similar to the uncoated WC-Co (Fig. 2d). No significant difference could be observed between CVD diamond coated and H-DLC coated drills in drilling of the alloys with Si content less than 12%. Low and uniform torques observed in drilling of 1100 Al and Al-6.5% Si using CVD diamond coated drills are shown in 10

ACCEPTED MANUSCRIPT Figs. 2 (i, j). Occasional torque spikes were observed in drilling of Al-12% Si using CVD diamond coated drills (Fig. 2 (k)) for which 24% of the holes exhibited torque spikes, which is

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lesser than when H-DLC was used (Fig. 2f). The most notable difference arises when drilling of

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Al-18.5% Si with CVD diamond coated drills is compared with H-DLC coated drills. The use of CVD diamond coated drills allowed the tool life to increase and drilling of the intended number

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of holes of 150 (Fig. 2l). Further analyses of the torque curves were found to be instructive and

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3.2. Analyses of Average Torque Curves

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the results of these analyses are reported in Section 3.2.

3.2.1. Dry Drilling Using Uncoated WC-Co Drills

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The average drilling torque (in N-m) for each hole was calculated from the difference in

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torque between the onset of chip clogging and the drill’s retraction [25] and the change of torque

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values from the 1st hole to the last hole are plotted. Fig. 3 a shows the variation of the average torques between drilling of the1st hole to the last hole for tests done using WC-Co drills. An increase in the average torque until failure was observed for all Al-Si alloys. The variations of the average torque with the number of holes can be more accurately examined when the slope of these curves are considered. In general three stages in the slopes could be identified. Namely, an initial slope (m1), a steady state slope (m2) and a high slope (m3) were determined for most testing conditions. Figs. 3[b-e] show the m values obtained for the alloys tested against uncoated WC-Co. In the initial stage the m1 values are high, but decrease to a low slope of m2 that could be considered as a quasi-steady state stage. As the drilling cycles increases the chips start to adhere to the drills more frequently, resulting in a high slope, m3. The occurrence of the third stage is the precursor to tool failure (Figs. 3 [b-d]). For clarity, the following distinction was made between m1 and m3: an initial high slope that continuously 11

ACCEPTED MANUSCRIPT increased without possibility of attaining steady state was always designated as m 3; an initial high slope that changed to a low steady state m2 was designated as m1–as in the case of uncoated

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drills case m1 and m2 was followed by a transition to a high and unstable m3 stage.

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As an example, for the first 10 holes for Al-6.5% Si, m1 of 2.85 × 10-2 N-m was evident as

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shown in Fig. 3 (c). A low slope of m2 = 6.0 × 10-3 N-m was then attained between 15th and 45th holes and then the slope increased to a high m3 of 6.0 × 10-2 N-m. The initial slope observed for

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1100 Al was m1= 4.2 × 10-3 N-m and for Al-12% Si m1= 8.4 × 10-2 N-m Fig. 3 (d). The steady state slopes, m2 = 6.0 × 10-3 N-m for 1100 Al, and m2 = 2.0 × 10-3 N-m for Al-12% Si were

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similar. An order of magnitude increase in the slope namely to m3 = 7.3 × 10-2 N-m was noted for 1100 Al and to m3 = 6.1 × 10-2 N-m for Al-12% Si. In case of drilling Al-18.5% Si, only one

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as drilling starts.

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short stage of high slope of m3= 0.04×102 N-m (Fig. 3e) and shows catastrophic failure as soon

Based on the average slope change analyses presented above, a novel tool failure criterion can be proposed as the transition from m2 to m3 can be regarded to coincide with the onset of tool failure. This transition corresponded to 28 holes for 1100 Al, 71 holes for Al-6.5% Si and 42 holes for Al-12% Si for dry drilling. For the materials and testing conditions considered here when the steady state m value (m2)

1.0 × 10-2 N-m the drill material is considered to be safe. It

is noted that the steady state torque slope failure criterion presented here can be further developed and applied to the other machining systems. The practical advantage of this criterion is that the tool can be removed when the slope change occurs without allowing excessive tool and workpiece damage to occur.

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ACCEPTED MANUSCRIPT 3.2.2. Dry Drilling Using H-DLC Coated Drills

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The drilling in 1100 Al and Al-(6.5-12)% Si using H-DLC coted drills showed prolonged tool life compared to the uncoated WC-Co as stated in Section 3.1. The average torque values for

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1100 Al and Al-Si alloys are shown in Fig. 4a, which could quantified by stating that steady

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state slopes for 1100 Al (m2 = 1.0 × 10-3 N-m) and Al-6.5% Si, (m2 = 3.0 × 10-3 N-m) were similar (Fig. 4 [b, c]). The initial slope become apparent when drilling of Al-12% Si (Fig. 4d)

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where the first 50 holes exhibited an m1 of 2.1 × 10-2 N-m followed by an m2 =2.0 × 10-3 N-m,

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similar to the alloys with lower Si percentages. Therefore, for eutectic and pro-eutectic Al-Si alloys drilled by H-DLC coated drills m remained smaller than 1.0 × 10-2 N-m such that, using

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the criterion proposed in Section 3.2.2., no drill failure was imminent. In contrast, drilling Al-

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18.5% Si using H-DLC coated drills exhibited a high slope (m3) of 0.02×102 N-m (Fig. 4e),

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without reaching a steady state, consistent with rapid tool failure. 3.2.3. Dry Drilling Using CVD Diamond Coated Drills

Drilling using CVD diamond coated drills generated similar average torque values for all alloys including Al-18.5% Si (Fig. 5a). A steady state slope of m2 = 1.0 × 10-3 N-m for 1100 Al, 3.0 × 10-3 N-m for Al-6.5% Si and 5.0 × 10-3 N-m for Al-12% Si were observed, Figs. 5 (b-d). In case of Al-18.5% Si, an initial slope, m1 = 2.1 × 10-2 N-m up to 50th cycle and an m2 = 1.0 × 10-3 N-m were recorded (Fig. 5 [e]). 3.2.4. Flooded Drilling Using Uncoated WC-Co Drills It is instructive to compare the results of torque slope analyse obtained for dry drilling of AlSi alloys with those obtained under flooded drilling. During flooded drilling using uncoated WCCo lower average torque values (Fig.6a) resulted as expected, and the corresponding steady state 13

ACCEPTED MANUSCRIPT slopes were m2=1.5×10-3 N-m for 1100 Al, m2=0.5×10-3 N-m for Al-6.5% Si, m2=0.6×10-3 N-m for Al-12% Si and m2=2.4×10-3 N-m for Al-18.5% Si (Fig.6 [b-e]). Although the steady state

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slope values observed in case of flooded machining using uncoated WC-Co drills were lower

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than that observed for DLC coated drills, in both cases drill failure could be avoided as long as steady state m values persisted. The three stages of drilling slopes and the corresponding m

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3.3. Temperature Increase During Drilling

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values for the Al-Si alloy and drill material combinations are summarised in Table 3.

The increase in torque reported in Section 3.1 was accompanied by an increase in the tool

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and workpiece temperature. The temperature of the last drilled hole in the workpiece was

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measured by inserting a chromel-alumel type thermocouple (sensitivity ±1°C) inside the

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workpiece at the mid-point of the drilled hole and 0.5 mm away from the surface of the hole as shown in the inset of Fig. 7 (a) . As the uncoated WC-Co drill penetrated to the 1100 Al workpiece the temperature increased rapidly and reached at 210 ºC when the drill reached to a depth of 19 mm. In drilling of Al-6.5% Si, the temperature increase was faster with a maximum of 300 ºC whereas for of Al-12% Si an even faster temperature increase was recorded reaching at 420 ˚C. A similarly high temperature of 410 ˚C was reached in drilling of Al-18.5% Si before the tool was stuck half way during the drilling process. Using H-DLC coated drills reductions in workpiece temperatures were evident and constant temperature plateaux were observed for all alloys tested except for the Al-8.5% Si (Fig. 7 (b)), for which the maximum temperature was 410 ˚C similar to the uncoated WC-Co. The temperatures profiles obtained using CVD diamond coated drills in 1100 Al and Al-(6.5-18.5)% Si are shown in Fig. 7 (c). Similar to the H-DLC coated drills, the temperatures reached low constant values for all alloys tested. The maximum 14

ACCEPTED MANUSCRIPT temperature reached for Al-18.5% Si was 225 ˚C and this was consistent with the low torque and long tool life.

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A non-contact infrared thermometer (sensitivity ±1°C) was used to measure the

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maximum tool-workpiece temperature during drilling of the first 50 holes, as described in [25].

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This method measures the temperature over a larger area and the values measured are generally lower than the thermocouple measurements but it is convenient for measuring temperatures of a

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large number of holes. Figs. 8 (a -c) show that with the uncoated WC-Co drills the workpiece temperatures increased as the number of holes increased. The temperature reached 275 ˚C for

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1100 Al at the onset of tool failure, 237 ˚C for Al-12% Si and 279 ˚C for Al-18.5% Si (Fig. 8 a). The aluminum alloy with lowest Si content (Al-6.5% Si) had the longest tool live and slower

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accumulation of heat. At the point of 50th hole the temperature reached to 126 ˚C. During the course of drilling using H-DLC coated drills, the maximum temperature did not exceed 52 ˚C for

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1100 Al, 62 ˚C for Al-6.5% Si and 73 ˚C for Al-12% Si (Fig. 8b). However, the H-DLC coated drills generated a high temperature during drilling of Al-18.5% Si (Fig. 8b) as the temperature increased to 280 ˚C while drilling the 2nd hole. The CVD diamond coated drills generated consistently low temperatures in drilling for all the four alloys. The maximum temperature was 56 ˚C for 1100 Al, 50 ˚C for Al-6.5% Si, 75 ˚C for Al-12% Si and 110 ˚C for Al-18.5% Si and the increase in the temperatures were gradual (Fig. 8c). The temperature curves were analyzed in a way similar to the torque curves. In general, three different stages of slopes were observed in the temperature vs. number of holes curves. These were designated as t1 (initial rise in temperature with the number of holes), t2 (steady state temperature) and t3 (rapid rise in temperature without reaching a steady state). Not all stages were apparent for all workpiece material/tool combinations and the slope values depended on the 15

ACCEPTED MANUSCRIPT alloy and tool combination as summarised in Table 4. The drilling of 1100 Al and Al-(6.5-12)% Si using uncoated WC-Co showed all stages of t1, t2 and t3. On the other hand, for 1100 Al and

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Al-6.5% Si drilled using H-DLC only the t2 stage was apparent. A single stage of t3 was

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observed for Al-18.5% Si when uncoated WC-Co and H-DLC drills were employed--consistent with a single high m3 stage of torque (Fig 6). CVD diamond coated drills provided low t values

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for all test samples, which correlated well with the low m observed. A correlation was

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established between the quasi-steady state slope of average torque (m2) and the slope of drilling temperature (t2), as shown in Fig. 9. A linear relationship can be found which can be expressed

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.

4. Discussion

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as

The drilling performances of WC-Co, H-DLC coated and CVD diamond coated drills against Al-Si workpieces representative of pre-eutectic, eutectic and hypereutectic compositions were assessed by measuring and analysing torque, temperatures generated at the tool-chip interfaces. This section discusses the friction, adhesion and wear mechanisms as a function of the Si percentage in the Al-Si workpieces. As the drilling induced temperature increased, the softened aluminum transferred to the tool, and adhered to the cutting edge. During dry drilling using uncoated WC-Co the adhered aluminum layer covered the entire drill tip regardless of the Si percentage of the alloys. The built-up edge (BUE) formed in this way was generally as thick as 500-600 µm and altered the

16

ACCEPTED MANUSCRIPT interfacial contact between the tool and workpiece, increasing the torque [26, 27]. Adhesion of aluminum to the tool was responsible and formation of torque spikes and for drill failure for pro

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eutectic and eutectic alloys. Smaller BUEs correlated with low steady state slope, m2, values.

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Metallographic analyses were conducted to determine the amount of aluminum adhered to the drill flute using an image analysis program that estimated the percentage of the drill surface area

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covered by the adhered aluminum. Back scattered electron (BSE) SEM images taken from drill

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flutes from locations shown in inset of Fig. 10 (a) differentiated adhered aluminum from the substrate. According to Fig. 10 (b)–where the reported values are the averages obtained from five

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images from each drill surface—nearly 100% area of uncoated WC-Co drill flutes was covered by adhered aluminum and this was independent of the Si percentage of the alloys. H-DLC and

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CVD diamond coated tools both reduced the amount of transferred aluminum to about 40% area coverage for Al-12% Si and 20% for Al-6.5% Si-- indicative of similar aluminum adhesion

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mitigating properties of both coatings. On increasing the Si content of the Al-Si alloy the coating was subjected to abrasive wear by Si particles. Consequently coated tool became less effective for mitigating aluminum adhesion as parts of WC-Co substrate become exposed. During drilling of Al-18.5% Si, 70% of the surface of H-DLC (and about 45% of CVD diamond) coated drill was covered by aluminum. Adhesion was no longer the main mechanism for drill failure when Al-18.5% Si alloy was considered as the wear of the drill became a significant factor. Flank wear of the drill bit was the main mechanism responsible for tool failure in drilling of Al-18.5% Si irrespective of the type of tool material used. This type of wear occurred as a result of abrasion by the large and angular primary silicon particles that are shown in Fig. 1 (c). The progression of flank wear was determined using a stereo microscope after every 15 holes. The flank wear was the most severe on the uncoated WC-Co (Fig. 11) with a depth reaching at

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ACCEPTED MANUSCRIPT 800 µm prior to failure. Lesser but still significant wear (500 µm) was detected on H-DLC coated tools. For CVD diamond coated, the flank wear was limited to a depth of 20 µm. This

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suggests that the during drilling of Al-18.5% Si, the DLC coatings were removed by abrasion

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during contact against the primary Si particles resulting in high torque while the CVD diamond

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was not destroyed resulting in lower torque values.

To rationalize the interfacial mechanisms of heat dissipation and adhesion it is important to

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establish the role coefficient of friction (COF) of the drill coatings as a function of Si content of

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the alloys tested. This section discusses the role of COF values, determined by conducting pinon-disk tribological experiments, on material transfer and adhesion mechanisms observed during

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drilling experiments when different coatings are considered.

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As noted in Section 3, the H-DLC and CVD diamond coated drills generated low and steady state torques when tested against Al-Si alloys with Si up to12% and maintained relatively low

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interface temperatures. The tribological behaviour of these alloys examined by performing pinon-disk tests that recorded two distinct friction periods, namely an initial running-in (µR), and a low friction smooth steady state period (µs) as a function of sliding contact cycles, as summarized in Table 5 (a, b). The running-in period corresponds to the initial contact between two surfaces and can represent the fresh contact during the drill-workpiece interaction [28-30]. Accordingly, the average of µR plotted as a function of Si % (Figs. 12 [a, b]) indicated an increase in the µR with increasing the Si content of the alloy tested at different temperatures. However this increase in µR was not a strong function of Si% and did not exceed 0.22 even for Al-18.5% Si in temperature range of 25 ˚C to 200 ˚C when tests were conducted against the HDLC. The H-DLC coating performance deteriorated at above 300 ˚C and resulted in high COF. The H-DLC coating was entirely removed from the sliding track when tested at 300 ˚C

18

ACCEPTED MANUSCRIPT regardless of the alloy composition. H-DLC coated drills could be safely used for machining of commercial purity Al and Al-(6.5-12)% Si as the temperature generated during dry drilling of

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these alloys did not exceed 200 ˚C (Fig. 8b). The CVD diamond coating maintained a low µR (and µS) behaviour at 300 ˚C (Fig. 12b). Corresponding to the low maximum drilling temperature

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(265 ˚C ) recorded during dry drilling of Al-18.5% Si, CVD diamond showed a low COF (µR=

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0.31) against Al-18.5% Si at 300 ˚C, indicating that the CVD diamond coated tools can

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withstand the extreme drilling conditions where other coatings fail. Fig. 13 shows the relationship between µR and the temperature increase in drilling operations, where for µ R<0.35

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the coatings are "safe" to use in machining Al-Si alloys while µR>0.35 marks the onset of tool failure (marked as ―unsafe‖). It is noted that CVD-diamond coating falls entirely in the safe

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region while the H-DLC is marked as safe until above 200 ˚C. Finally, the drill failure mechanisms are summarized in Fig. 14 using an average torque vs.

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silicon content diagram which indicates an increase in the torque with increasing the silicon content. When the dominant drill failure mechanism was adhesion of aluminum to the tool, as in case of Al-Si alloys with up to 12% Si, the H-DLC coatings reduced the torque effectively and proved advantageous over the CVD diamond coated drills considering equal performance in terms of adhesion mitigation at a fraction of the cost of CVD diamond. When the Si content increased and the Al-Si alloys became abrasive towards the coating, as for hypereutectic compositions, CVD diamond which sustains low flank wear damage should be preferred.

5. Summary and Conclusions

1. The drilling torque varied with the number of holes drilled and typically exhibited three stages, each identified with a characteristic slope, m. For dry drilling, a tool failure 19

ACCEPTED MANUSCRIPT criterion was established as m ≥1.0×10-2 N-m, which distinguished between the different drill coatings. H-DLC coated drills prolonged the tool life when drilling Al-Si alloys with

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Si ≤ 12%. During drilling of Al-18.5% Si the tool failure criterion was met rapidly for HDLC and WC-Co coated tools, but not for CVD diamond coated tools.

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2. The tool failure mechanism for Al-Si alloys with Si ≤ 12% consisted of Al transfer and

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adhesion while for alloys with higher Si content flank wear was the dominant failure

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mechanism. CVD diamond coated drills reduced flank wear more effectively than other tool coatings.

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3. H-DLC coated drills can substitute the use of CVD diamond coated tools in machining Al-Si alloys for which the tool life is limited by material transfer due to their adhesion-

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Acknowledgments

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mitigating properties and cost advantage over the CVD diamond.

Auto 21 Innovation through Research Excellence Centre is gratefully acknowledged for the financial support.

List of References [1] W.S. Miller, L. Zhuang, J. Bottema, A.J. Wittebrood, P. De Smet, A. Haszler, A. Vieregge, Recent development in aluminium alloys for the automotive industry, Mater. Sci. Eng. A 280 (2000) 37–49. [2] J.R. Davis, Friction and wear of internal combustion engine parts, Friction, Lubrication, and Wear Technology, ASM Handbook, vol. 18, 10th ed., ASM International, Materials Park, OH, 1992, pp. 553–562. [3] E. Konca, Y.-T. Cheng, A.M. Weiner, J.M. Dasch, A.T. Alpas, Elevated temperature tribological behavior of non-hydrogenated diamond-like carbon coatings against 319 aluminum alloy, Surf. Coat. Technol. 200 (2006) 3996 – 4005. 20

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[4] E. Konca, Y.-T. Cheng, A.M. Weiner, J.M. Dasch, A. Erdemir, A.T. Alpas, Transfer of 319 Al alloy to titanium diboride and titanium nitride based (TiAlN, TiCN, TiN) coatings: effects of sliding speed, temperature and environment, Surf. Coat. Technol. 200 (2005) 2260– 2270.

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[5] K. Saijo, M. Yagi, K. Shibuki, S. Takatsu, The improvement of the adhesion strength of diamond films, Surf. Coat. Technol. 43-44 (1990) 30-40.

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[6] H. Yoshikawa, A. Nishiyama, CVD diamond coated insert for machining high silicon aluminum alloys, Diamond Relat. Mater. 8 (1999) 1527–1530.

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[7] G. Castro, F.A. Almeida, F.J. Oliveira, A.J.S. Fernandes, J. Sacramento, R.F. Silva, Dry machining of silicon–aluminum alloys with CVD diamond brazed and directly coated Si3N4 ceramic tools, Vacuum 82 (2008) 1407–1410.

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[8] F.G. Sen, Y. Qi, A.T. Alpas, Material transfer mechanisms between aluminum and fluorinated carbon interfaces, Acta Mater 59 (2011) 2601–2614.

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[9] F.G. Sen, X. Meng-Burany, M.J. Lukitsch, Y. Qi, A.T. Alpas, Low friction and environmentally stable diamond-like carbon (DLC) coatings incorporating silicon, oxygen and fluorine sliding against aluminum, Surf. Coat. Technol. 215 (2013) 340–349.

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[10] Y. Qi, E. Konca, A.T. Alpas, Atmospheric effects on the adhesion and friction between non-hydrogenated diamond-like carbon (DLC) coating and aluminum – A first principles investigation, Surf. Sci. 600 (2006) 2955–2965. [11] A. Banerji, S. Bhowmick, A.T. Alpas, High temperature tribological behavior of W containing diamond-like carbon (DLC) coating against titanium alloys, Surf. Coat. Technol. 241 (2014) 93-104. [12] S. Bhowmick, A. Banerji, A.T. Alpas, The high temperature tribological behaviour of Si, O containing hydrogenated diamond-like carbon (a-C:H/a-Si:O) coating against an aluminum alloy, Wear 330 (2015) 261-271. [13] P. J. Heath, Developments in applications of PCD tooling, Journal of Materials Processing Technology 116 (2001) 31-38. [14] E.J. Oles, A. Inspektor, C.E. Bauer, The new diamond-coated carbide cutting tools. Diamond Relat. Mater. 5 (1996) 617–624. [15] C.H. Shen, The importance of diamond coated tools for agile manufacturing and dry machining, Surface & Coating Technology 86-87 (1996) 672-677.

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ACCEPTED MANUSCRIPT [16] Q. Liang, Y.K. Vohra, R. Thompson, High speed continuous and interrupted dry turning of A390 aluminum/silicon alloy using nanostructured diamond coated WC–6 wt.% cobalt tool inserts by MPCVD, Diamond Relat. Mater.17 (2008) 2041–2047.

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[17] H. Gomez, D. Durham, X. Xiao, M. Lukitsch, P. Lu, K. Chou, A. Sachdev, A. Kumar, Adhesion analysis and dry machining performance of CVD diamond coatings deposited on surface modified WC–Co turning inserts, J. Mater. Process. Technol. 212 (2012) 523– 533.

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[18] E.G. Ng, D. Szablewski, M. Dumitrescu, M.A. Elbestawi, J.H. Sokolowski, High speed face milling of a aluminum silicon alloy casting, CIRP Ann. Manuf. Technol., 53 (2004) 69-72.

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[19] P. Roy, S.K. Sarangi, A. Ghosh, A.K. Chattopadhyay, Machinability study of pure aluminium and Al–12% Si alloys against uncoated and coated carbide inserts, Int J Refract Met H 27 (2009) 535–544.

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[20] G.R. dos Santos, D.D. da Costa b, F.L. Amorim, R.D. Torres, Characterization of DLC thin film and evaluation of machining forces using coated inserts in turning of Al–Si alloys, Surf. Coat. Technol. 202 (2007) 1029–1033.

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[21] J.M. Dasch, C.C. Anga, C.A. Wong, Y.-T. Cheng, A.M. Weiner, L.C. Lev, E. Konca, A comparison of five categories of carbon-based tool coatings for dry drilling of aluminum, Surf. Coat. Technol. 200 (2006) 2970 – 2977. [22] N. Wain, N.R. Thomas, S. Hickman, J. Wallbank, D.G. Teer, Performance of low-friction coatings in the dry drilling of automotive Al–Si alloys, Surf. Coat. Technol., 200 (2005) 1885-1892. [23] S. Bhowmick, A.T. Alpas, The performance of hydrogenated and non-hydrogenated diamond-like carbon tool coatings during the dry drilling of 319 Al, Int J Mach Tool Manu, 48 (2008) 802–814. [24] S. Bhowmick, M.J. Lukitsch, A.T. Alpas, Tapping of Al–Si alloys with diamond-like carbon coated tools and minimum quantity lubrication, J. Mater. Process. Technol. 210 (2010) 2142–2153. [25] S. Bhowmick, A.T. Alpas, Minimum quantity lubrication drilling of aluminum–silicon alloys in water using diamond like carbon coated drills. Int J Mach Tool Manu 48 (2008) 1429-1443. [26] V.C. Venkatesh, W. Xue, A study of the built- up edge in drilling with indexable coated carbide inserts, J. Mater. Process. Technol. 58 (1996) 379-384.

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ACCEPTED MANUSCRIPT [27] S. Bhowmick, A.T. Alpas, The role of diamond-like carbon coated drills on minimum quantity lubrication drilling of magnesium alloys, Surf. Coat. Technol. 205 (2011) 5302– 5311.

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[28] S. Bhowmick, M.J. Lukitsch, A.T. Alpas, Dry and minimum quantity lubrication drilling of cast magnesium alloy (AM60), Int J Mach Tool Manu 50 (2010) 444–457.

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[29] S. Bhowmick, A. Banerji, A.T. Alpas, Tribological behavior and machining performance of non-hydrogenated diamond-like carbon coating tested against Ti–6Al–4V: Effect of surface passivation by ethanol, Surf. Coat. Technol. 260 (2014) 290-302.

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[30] S. Bhowmick, A.T. Alpas, The performance of diamond-like carbon coated drills in thermally assisted drilling of Ti-6Al-4V, J. Manuf. Sci. Eng. 135 (2013) 061019-1-15.

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Figure Captions

Figure 1: Optical micrograph showing the microstructure of (a) Al-6.5% Si; (b) Al-12% Si and

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(c) Al-18.5% Si. The main intermetallic constituents are Al5Cu2Mg8Si6 particles, CuAl2 and Al15

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(Fe, Mn)3Si2 with a script morphology.

Figure 2: Variations in the torque profiles generated during the drilling of each hole with time. (a) torque data representing the entire drilling cycle in drilling in 1100 Al using uncoated WCCo. (b) the last 25 holes (47th – 71th) under drilling in Al-6.5% Si using uncoated WC-Co. (c) the last 25 holes (19th -44th) drilled in drilling in Al-12% Si using uncoated WC-Co. (d) torque of the entire drilling cycles in drilling Al-18.5% Si using uncoated WC-Co. Torque responses for the last 25 (126th –150th) holes drilled using H-DLC coated HSS in (e) 1100 Al; (f) Al-6.5% Si and (g) Al-12% Si. (h) Torque responses for the entire drilling cycle in drilling in Al-18.5% Si using H-DLC coated HSS. Torque responses for the last 25 (126th –150th) holes drilled using diamond coated WC-Co in (i) 1100 Al; (j) Al-6.5% Si; (k) Al-12% Si and (l) Al-18.5% Si. Tool life is indicating in bracket.

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ACCEPTED MANUSCRIPT Figure 3: (a) The variations of average torque with the number of holes during drilling in 1100 Al, Al-6.5% Si, Al-12% Si and Al-18.5% Si using uncoated WC-Co tools. The figure shows

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reduced data for clarity. The error bars indicate the standard deviation of torque data for each

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hole. (b-e) the different stages of slopes of average torques variations.

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Figure 4: (a) The variations of average torque with the number of holes during drilling in 1100

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Al, Al-6.5% Si, Al-12% Si and Al-18.5% Si using H-DLC coated HSS tools. The figure shows reduced data for clarity. The error bars indicate the standard deviation of torque data for each

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hole. (b-e) the different stages of slopes of average torques variations.

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Figure 5: (a) The variations of average torque with the number of holes during drilling in 1100 Al, Al-6.5% Si, Al-12% Si and Al-18.5% Si using CVD diamond coated WC-Co tools. The

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figure shows reduced data for clarity. The error bars indicate the standard deviation of torque data for each hole. (b-e) the different stages of slopes of average torques variations.

Figure 6: (a) The variations of average torque with the number of holes during drilling in 1100 Al, Al-6.5% Si, Al-12% Si and Al-18.5% Si using uncoated WC-Co tools in flooded drilling conditions. The figure shows reduced data for clarity. The error bars indicate the standard deviation of torque data for each hole. (b-e) the different stages of slopes of average torques variations. Figure 7: The variations of temperature generated during drilling in 1100 Al, Al-6.5% Si, Al12% Si and Al-18.5% Si using (a) uncoated WC-Co; (b) H-DLC coated HSS and (c) CVD

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ACCEPTED MANUSCRIPT diamond coated WC-Co. The schematic of the thermocouple set-up for temperature measurements is shown in inset.

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Figure 8: The variations of maximum temperature with the number of holes measured by infra-

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red thermometer for (a) uncoated WC-Co; (b) H-DLC coated HSS and (c) diamond coated WCCo.

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Figure 9: Steady state slope of average torque plotted against the steady state slope of

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temperature.

Figure 10: (a) Back scattered electron image of a section of the drill flute of the H-DLC coated

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HSS drills tested in Al-6.5% Si. A schematic diagram of drills and locations of SEM images taken on the drill flute is shown in inset of (a). (b) A plot of the area percentage of the drill flute

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surface covered by the transferred aluminum with the silicon content after drilling using uncoated WC-Co, H-DLC coated HSS and diamond coated WC-Co.

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Figure 11: Comparison of tool wear progression with the number of holes in drilling in Al18.5% Si using uncoated WC-Co, H-DLC coated HSS and diamond coated WC-Co. The succession of tool wear of diamond coated tools after 15 and 150 holes are shown in inset. Figure 12: The variations of average running-in coefficient of friction with the silicon content at different test temperatures for (a) H-DLC and (b) CVD Diamond. Figure 13: The variations of average running-in COF with the temperature that were generated in drilling operations. Figure 14: Drill failure mechanism map for 1100 Al and Al-(6.5-18.5)% Si. The drills failed by the adhesion of aluminum to the drills for 1100 Al and Al-(6.5-12)% Si. The drill failed by the extensive tool wear in drilling in Al-18.5% Si.

25

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Turning

Al-18% Si

Turning

Al-12% Si

Turning

Al-6.5% Si Al-16% Si Al-6.5% Si Al-6.5% Si Al-6.5% Si

Milling Turning Drilling Drilling Tapping

[13] [14]

70

Flank wear

[15]

-

-

Flank wear

[16]

-

>5

Flank wear

[17]

70 N

-

Adhesion

[18]

225 N 40 N 7 N-m 6.51 N-m 1.01 N-m

> 8.25 > 7.50 > 10

Adhesion Flank wear Adhesion Adhesion Adhesion

[19] [20] [21] [23] [24]

800 N

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Al-18% Si

Flank wear Flank wear

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Turning

500 10.2

-

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Al-18% Si

References

Tool Life (min)

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Cutting Turning

Failure Mechanism

Cutting Force/ Torque

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Al-20% Si Al-18% Si

Tools/ Coatings PCD PCD CVD Diamond PCD CVD Diamond CVD Diamond PVD H-DLC H-DLC H-DLC H-DLC

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Machining Processes

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Materials

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Table 1: Summary of literature survey describing materials compositions, machining processes, tool coatings, cutting torque and force, tool life and failure mechanisms of different tools.

Table 2: Chemical compositions in wt.% of the as-cast Al-Si alloys. Alloy designation Al-6.5% Si Al-12% Si Al- 18.5% Si

Chemical compositions in wt.% Si 6.50 12.70 18.40

Cu 3.50 2.97 4.00

Fe 1.00 0.26 0.23

Mg 0.10 0.09 0.57

Mn 0.50 0.42 0.07

Ni 0.35 1.00 0.02

26

Pb <0.01

Sr <0.002

Sn <0.01

Ti 0.25 0.12 0.05

Zn 1.00 0.01 0.10

Al Balance Balance Balance

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Al-12% Si

6.0×10-3

7.3×10-2

-3

6.0×10

-3

6.0×10

-2

2.0×10

-3

6.1×10

-2

28×10

8.4×10 -

-

0.04×10

-

1.0×10-3

-

3.0×10

-3

2.0×10

-3

2.1×10

2

-2

-

-

-

0.02×10

Diamond coated WC-Co m1 (Nm) m2 (Nm) m3 (Nm) -

1.0×10-3

-

-

3.0×10

-3

-

5.0×10

-3

-

1.0×10

-3

-

2

2.0×10

-2

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Al-18.5% Si

-2

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Al-6.5% Si

42×10-3

H-DLC coated HSS m1 (Nm) m2 (Nm) m3 (Nm)

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1100 Al

Uncoated WC-Co m1 (Nm) m2 (Nm) m3 (Nm)

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Alloy designation

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Table 3: Slopes of average torque curves of the tools and materials tested. m1, m2 and m3 represent initial, steady state and rapid rate of increase in average torque with the number of holes.

Al-6.5% Si Al-12% Si Al-18.5% Si

0.11×102

6.0×10-2

0.04×10

2

5.1×10

-2

0.10×10

2

6.0×10

-2

-

0.09×102

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1100 Al

Uncoated WC-Co t1 (˚C) t2 (˚C) t3 (˚C)

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Alloy designation

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Table 4: Slopes of maximum temperatures generated using uncoated WC-Co, H-DLC coated HSS and diamond coated WC-Co tools in drilling of 1100 Al and Al-(6.5-18.5)% Si. t1, t2 and t3 represent initial, steady state and rapid rate of increase in maximum temperature with the number of holes.

-

0.02×10

2

0.10×10

2

1.36×10

2

H-DLC coated HSS t1 (˚C) t2 (˚C) t3 (˚C) -

1.0×10-3

-

-3

34.1×10 -

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3.0×10 -3

30.0×10 -

-2

Diamond coated WC-Co t1 (˚C) t2 (˚C) t3 (˚C)

0.50×10

-

8.0×10-2

-

-

7.1×10

-2

-

4.1×10

-2

-

6.0×10

-2

0.01×102

2

0.02×10

2

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Table 5 (a, b): Running-in and steady state coefficient of friction of (a) H-DLC and (b) CVD diamond against 1100 Al and Al-(6.5-18.5)% Si at different test temperatures.

25 ˚C Running In Steady State 0.01±0.01 0.01±0.01 0.02±0.01 0.05±0.01 0.07±0.01 0.04±0.03 0.08±0.05 0.04±0.04

CVD Diamond 100 ˚C 200 ˚C Running In Steady State Running In Steady State 0.14±0.07 0.03±0.03 0.06±0.01 0.07±0.02 0.14±0.01 0.10±0.01 0.13±0.04 0.04±0.01 0.15±0.02 0.09±0.01 0.14±0.02 0.10±0.03 0.24±0.06 0.11±0.01 0.20±0.02 0.12±0.01

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H-DLC 100 ˚C 200 ˚C Running In Steady State Running In Steady State 0.12±0.03 0.07±0.01 0.07±0.02 0.04±0.01 0.19±0.02 0.12±0.01 0.12±0.01 0.10±0.01 0.22±0.02 0.16±0.02 0.13±0.03 0.07±0.01 0.17±0.03 0.26±0.02 0.35±0.20 0.15±0.03

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1100 Al Al-7%Si Al-12%Si Al-18%Si

25 ˚C Running In Steady State 0.19±0.02 0.19±0.01 0.19±0.01 0.23±0.01 0.22±0.03 0.16±0.01 0.23±0.01 0.19±0.01

(b)

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1100 Al Al-7%Si Al-12%Si Al-18%Si

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300 ˚C Running In Steady State 0.42±0.03 0.67± 0.07 0.33 ±0.04 0.71±0.07 0.65 ±0.15 0.73±0.09 0.91±0.06 0.75±0.09

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300 ˚C Running In Steady State 0.09±0.01 0.06±0.01 0.09±0.02 0.07±0.01 0.22±0.04 0.09±0.03 0.28±0.04 0.19±0.03

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Research Highlights: H-DLC coatings improved tool life during drilling of low Si grade Al-Si alloys

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A failure criterion (m2 ≥1.0×10-2 N-m) was established to predict tool failure

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Tribological tests indicated low COF for H-DLC coating up to 200 ˚C

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CVD-Diamond coating showed low torque and low COF for all Al-Si alloys

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H-DLC coated drill can substitute CVD-diamond for machining of Al-(0-12%)Si alloys

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