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Accepted for oral presentation at International Symposium on Green Manufacturing and Applications (ISGMA 2016)

1. Introduction

The negative effects of cutting fluids on manufacturing cost, human health, and environment have raised alarming signal to the machining industries [1-2]. One of the challenges of manufacturing industry today is how to reduce of metal working fluids. With increasing awareness of healthy environmental conditions, vegetable oils have become alternative lubricants for industrial applications. It is also supported by the properties of the vegetable oils that have good lubrication, high kinematic viscosity, non toxic, renewable ability, and good biodegradable ability [3]. Various studies have been carried out on stainless steel machining in order to evaluate vegetable oil based cutting fluids such as rapeseed oil [4], coconut oil [5], sunflower oil

and canola oil [6], palm oil [7] and castor oil [8]. Some of the alternatives which are under investigated are using

vegetable oil based-minimum quantity lubricant (MQL-VO). Among the vegetable oils, palm oil is used more often than other vegetable oil. After crude palm oil refining, the oil may be separated by thermo mechanical means (involving cooling, crystallization, and filtering) into liquid (refined palm olein) and solid phases (palm stearin) [9].

The introduction of MQL-VO has been shown to work well in short-term tests over a range of processes. MQL is beneficial in terms of tool life in deep-hole drilling [10], surface roughness [11], lower process costs and produced dry chips [12]. It was shown that MQL can be regarded as an alternative to flood cutting from the viewpoints of toll performance, cost, health, safety and environment [13] and

MQL Drilling of Austenitic Stainless Steel using Re-fined Palm Olein: Effect of Coating Tool on Surface

Roughness and Tool Wear

Safian Sharif1,,#, Ahmad Zubair Sultan2, and Denni Kurniawan1

1Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, Skudai, 81310, Johor, Malaysia 2Politeknik Negeri Ujung Pandang, Tamalanrea, 90245, Makassar, South Sulawesi, Indonesia

# Corresponding Author / [email protected], TEL: +607-5534850, FAX: +60-7-5566159

KEYWORDS: Machinability; Stainless steel; MQL; Drilling; Coated carbide; Tool Wear; Surface roughness

This present study investigates the effect of tool coating during minimum quantity of lubricant (MQL) drilling of austenitic stainless steel using refined palm olein (RPO) as cutting fluid. Two types of tool coating, which are TiAlN and TiSiN, on tungsten carbide drill with diameter 4±0.01 mm, point angle of 130° and helix angle of 30° were used in this study to machine AISI 316L stainless steel workpiece with dimension was 102 mm x 60 mm x 10 mm and microhardness 179.5 HV. Drilling tests are conducted on DECKEL MAHO DMC835V CNC machining centre with cutting speed of 12 m/min and feed rate of 0.025 mm/rev. The MQL system in this trial delivered using Economizer I with 5.5 bar air supplied, the spray output was 27 ml/h, adjusted 20 degrees and located 35 mm away from the cutting tool. Tool wear measured continuously during experimental process using Raxvision tool microscope connected to iSolution image analyzer software. Tool overhang was set at 30 mm. Surface roughness (Ra) is measured with an Accretech Handysurf tester. The cut–off and sampling lengths for each measurement are taken as 0.8 and 4 mm, respectively. For each hole, the surface roughness is measured parallel to the drilled axis at four radial positions at 0°, 90°, 180° and 270° using a surface tester. The surface roughness is an average taken from twelve measurements from three times repeated at each position. Tool life of the drill and surface roughness of the drilled hole were the machining responses investigated. It was found that the MQL-RPO drilling using TiSiN coated carbide tool produced better result in terms of tool life (reaching 7.54 minutes) compared to using TiAlN coated tool (of only 4.19 minutes). On the other hand, related to surface roughness, the best result was obtained by TiAlN coated tool. Through two-factor anova with replication on 1st hole, it was found that wear of the tool affect the surface roughness significantly while different types of coating has no significant effect on tool wear and surface roughness. In addition, there is no interaction between types of coating and wear of the tool that influence the surface roughness.

International Symposium on Green Manufacturing and Applications (ISGMA 2016)

MQL can improve plant conditions, conserve ecology environment, reduce producing cost and improve producing quality [14]. However, in terms of diameter error, testing indicates that there is no difference, between MQL and the flooded system, and regarding to circularity error and cylindricity error, in average MQL has the worst performance compare to dry and flooded cooling [15]. It is likely that long-term capability and robustness of MQL technique remain still unanswered and more material specific issues may require additional testing [2].

Belluco and De Chiffre [4] conducted flooded drilling 316L using HSS-co tools and found that average tool life ranging from 23, 39, 40, 49, 53 and 63 minutes with mixture of mineral oil-rapeseed oil-ester oil, mixture of rapeseed oil-ester oil-meadow foam oil, mixture of vegetable oil-rapeseed oil-ester oil respectively. Using tool life and cutting forces as performance criteria, vegetable oil based cutting fluids outperformed the mineral oil based. Tool life was increased by 177% while thrust force was reduced by 7%. The presence of sulphur and phosphorus-based additives in the vegetable oil based fluids helped prevent adhesion. In other research, Belluco and De Chiffre [8] testing of vegetable oil based metal working fluid through different operations. HSS-E tool used to make holes on 316L workpiece. Cutting forces testing and provide result that vegetable oil outperformed the commercial mineral oil. Performance evaluation of cutting fluids can be measured by cutting force, while efficiency evaluation can be determined by cutting torque.

During drilling AISI 304, Kuram et al., [8] applied vegetable oil based as metal working fluids formulated of crude sunflower (CS) and refined sunflower oil (RS) compare to commercial mineral cut-ting fluid (CM) as reference. Three types of metal working fluids i.e.: mixture of CS1+ 20% Tween85 (viscosity of 1.7 cp), mixture of S1+ 20% Tween20 (viscosity of 1.9 cp) and mixture of S2 + 20% Tween20 + 15% Tween85 (viscosity of 1.3 cp). Based on the experi-ment result, the lowest roughness value of 1.01μm and highest of 2.26μm were achieved at using RS2 and CM, respectively. Compared to RS1, RS2 gave better roughness value for all machining condi-tions, which might be related to difference in viscosity. Viscosity af-fects the flow of cutting fluid. So, cutting fluid with low viscosity ex-pectedly can reach the tool-workpiece interface more effectively, making chips to be flushed away from the cutting zone and prevent-ing a finished drilled hole surface from becoming scratched [16].

Ozcelik et al., [17] compared two different refined sunflower oil (RS) when drilling AISI 304 stainless steel. Metal working fluid formulated using mixture of: RS1+20% Tween85 (viscosity of 1.5 cp) and RS2+20% Tween85+9% Peg400 (viscosity of 1.1 cp). Mineral metal working fluid (MO) and semi synthetic cutting fluid (SS) used as reference [17]. Related to surface roughness, minimum value of 1.36 μm were obtained by using RS1 mixture, followed by RS2 mixture with 1.43 μm, MO with 1.48 μm, and being maximum of 1.92 μm by SS. RS1 produced better surface roughness compared to RS2 although the former has higher viscosity. This can be attributed to the lubrication ability, in which cutting fluid with low viscosity has

poor lubricating capability [16]. This result hinted that there is a critical cutting fluid viscosity value that can give the best surface roughness out of this AISI 304 workpiece. In other research, Ozcelik et al. [18] explore the influence of vegetable-based cutting fluids on the HSS-E wear when drilling AISI 304. Vegetable based metal working fluids developed from crude sunflower oil and refined sunflower and canola oil. Experimental results show that due to its higher lubricant properties, refined canola oil based cutting fluid gives the better performance compares to the others.

Related to research of difficult-to-machine material such as austenitic stainless steel, there are not many studies being conducted. Further research required on cooling conditions, tool materials, cutting parameters and tool geometries [19] in order to explore the productivity of this material machining. The properties of austenitic stainless steel have unfavourable impact on the machining process, with the result that Machining of austenitic stainless steel is considered difficult due its unfavourable properties when subjected to machining. Because of the low heat conductivity of austenitic stainless steel, generated heat cannot be transferred into the workpiece and chips effectively. As result heat concentration at the tool cutting edge occur [19], in machining, these characteristics cause the formation of built-up edges when carbide tools are used [20]. This is commonly occur when cutting with cemented carbides, giving rapid tool failure and poor tool life [21]. At the same time it is necessary to meet the surface integrity requirements, where tool wear can lead to residual stress occurred and poor surface roughness in the drilled hole surface [22].

The main objective of this research study is to investigate the machinability of AISI 316L under drilling with Refined Palm Olein based Minimum Quantity Lubricant (MQL-RPO) technique using various carbide tools including TiAlN and TiSiN coated carbide.

2. Drilling Experiments

Austenitic stainless steel AISI 316L was used as the workpiece materials in the present study. The dimension of the workpiece material was 102 mm x 60 mm x 10 mm with microhardness 179.5 HV. Specimen prepared using milling and surface grinding in order to meet above dimension and to prepare surface references for both drilling trials and measurement process. The specimen was clamped on precision jig as shown in Fig. 1, before through hole drilled on the specimen.


Fig. 1 Experimental SetupThe workpiece material used known as one of the hard to

machine material and mainly used in the production of pharmaceutical and photographic equipment, chemical/ pharmaceuticals equipment, paper and textile processing equipment, food preparation equipment particularly in chloride environments and medical implants such as pins, screws and orthopaedic implants like total hip and knee replacements.

The chemical compositions of workpiece materials were determined with EDS (Energy Dispersive Spectroscopy) analysis and given in Table 1

Table 1 Chemical compositions of 316L austenitic stainless steel (by %volume)

Fe Cr N Ni Mo Mn Si S C Pbal 16.5 .1 10.2

32.6 2.0 .6 .0

3.03 .03

Drilling tests are conducted on DECKEL MAHODMC835V CNC machining centre with cutting speed 12 m/min and feed rate 0.025 mm/rev using coated carbide drill bits with diameter 4±0.01 mm, point angle 130° and helix angle 30°. Refined palm olein (RPO) used as minimum quantity lubricant (MQL) fluids with the characteristics given in Table 2. The MQL system in this trial delivered using Economizer I systems which are completely self contained, positive displacement continuous spray systems were used. With 5.5 bar air supplied, the spray output was 27 ml/h, adjusted 20 degrees and located 35 mm away from the cutting tool. A new drill tool is used in each trial to ensure the same initial conditions of each test. Tool wear measured continuously during experimental process using Raxvision tool microscope connected to iSolution image analyzer software. Tool wear measured at d/6 mm located to chisel edge [23] after particular drilling intervals. Tool overhang was set at 30 mm. Surface roughness (Ra) is measured with an Accretech Handysurf tester. The cut–off and sampling lengths for each measurement are taken as 0.8 and 4 mm, respectively. For each hole, the surface roughness is measured parallel to the drilled axis at four radial positions at 0°, 90°, 180° and 270° using a surface tester. The surface roughness is an average taken from twelve measurements from three times repeated at each position.

Table 2 Properties of the MQL Fluid

MQL Fluids RPO Test Method

Appearance Yellow VisualDensity at 15 C (kg/L) 0.9095 ASTM D1298-85(90)

Viscosity 40°C (mm2/s) 53.18 ASTM D445-94

Viscosity 100 °C (mm2/s) 10.36 ASTM D445-94

Viscosity Index 188 ASTM D2270-93Pour Point (°C) 6 ASTM D97-93Flash Point (°C) 320 ASTM D92-90

3. Results and Discussion

3.1 Experiments Result

Tool wear and surface roughness obtained under RPO-MQL drilling using different tool coating present in Table 3. New tool represented by first hole while worn out tool represented by finish hole where tool reached tool life criteria.

Table 3 Tool wear and surface roughness of first and last hole during RPO-MQL drilling.

Tool

Coating

Replication

Tool WearSurface

Roughness

New

tool

Worn

Tool

New

tool

Worn

Tool

TiAlN

Coated

Carbide

1 0.0335 0.0785 2.86 1.87

2 0.0175 0.0855 2.51 1.57

3 0.0335 0.0955 2.37 1.13

TiSiN

Coated

Carbide

1 0.0185 0.091 2.86 1.72

2 0.0265 0.110 2.59 2.18

3 0.0275 0.0945 2.97 2.27

Tool wear and surface roughness of 3 times replications in aver-age are depict in Fig. 2.


(a)

(b)Fig. 2 (a) Tool wear and (b) surface roughness under RPO-MQL

drilling.

3.2 Statistical Analysis and Discussion

Analysis made by using a two-factor anova with replication to determine whether there is a significant effect of different tool coating and different tool conditions simultaneously on tool wear response. Hypothesis null (H0) is no tool wear difference among all trials while alternative hypothesis (H1) stated that differences occur among all trials. Anova test result presented in Table 4.

Table 4 Anova result for tool wear data

Related to tool coating, since F (0.673881) < Fcritic (5.317655), it is mean that H0 fail to reject and concludes that different tool coating has no significant effect on tool wear or in other words, TiAlN coated

and TiSiN coated tools give no significant variation on tool wear. But due to wear progress during prolong drilling process, TiSiN coated tool gives longer tool life than by TiAlN coated tool.

Interaction between coolant condition and tool coating not exist and not influence the tool wear, since F (2.695525) < Fcritic (5.317655) where Ho accepted.

Fig. 3 Tool wear affected by increasing drilling distance at various

tool coating under same MQL fluids.

Flank wear progress due to increases hole number drilled under RPO-MQL drilling shown in Fig. 3. Obviously during MQL-RPO drilling, the tool lives of the TiSiN coated tool were found to be higher than that of TiAlN coated tools. Similar results was reported by Wang et al. during high speed milling of hardened steel material [24]. These were mainly due to superiority of the hardness and oxidation temperature of the TiSiN coating. From this Fig. also observable that TiAlN coated drill bit worn very rapidly and only able to drill ten holes before tool failure or approximately 55 % compared to the tool life of TiSiN coated. The tool failure modes included flank wear, micro chipping and flaking. Similar pattern was obvious at initial wear stage for both tools where tool wear increases rapidly until 3rd holes. During stable wear stages of the tool, flank wear increased almost linearly. Both tools were failure before reached the tool life criteria.

The same analysis and similar hypothesis applied for surface roughness response to provides test result using anova two factors with replication presented in Table 5.

Table 5 Anova result for surface roughness data


In terms of tool conditions, since F (29.89964) > Fcritic

(5.317655), it is mean that H0 rejected and concludes that surface roughness for different tool conditions are statistically different or in other words, new and worn tools give significant different on surface roughness.

Related to tool coating, since F (5.290992) < Fcritic (5.317655), it is mean that H0 fail to reject and concludes that different tool coating has no significant effect on surface roughness or in other words, TiAlN coated and TiSiN coated tools give no significant different on surface roughness. But due to wear progress, with increases number of holes, TiAlN coated tool gives lower roughness value (Fig. 4) compare to the TiSiN coated tool, despite the fact that tool life presented by uncoated and TiSiN coated tools better than TiAlN (Fig. 3).

Interaction between tool condition and tool coating on surface roughness not found to be significant since F (0.861476) < Fcritic (5.317655) where H0 accepted.

Fig. 4 Surface roughness affected by increasing drilling distance at various tool coating under same MQL fluids.

Fig. 4 displays the progress of surface roughness with increasing drilled hole under RPO-MQL drilling using different tool coating. Obviously the surface roughness values for TiAlN significantly lower than uncoated and TiSiN coated tool were between 1.25 to 2.60 μm and decrease with drilling distance. Lower surface roughness probably can be addressed to the smaller friction coefficient of TiAlN coated tool (0.55) than the TiSiN coated tool (0.9) as coefficient of friction directly determine the force required to produce movement [25] and higher cutting force tend to triggered higher vibration. And also can be addressed to the formation of the micro thin oxide layer of Al2O3 as a result of the reaction between the TiAlN and the oxygen at the cutting zone [26]. The oxide layer acts as a solid lubrication thus reducing the friction hence lowering the surface roughness.

3.3 Microstructure alterations.

Fig. 4 shows the subsurface micro microstructure of the hole surface when drilling with different cooling condition. It was

observed that a thin layer of plastic flow and microstructure deformation occurred towards the drilling direction. This deformation layer is due to the presence of high dislocation density. The transition in microstructure is probably due to high pressure of mechanical forces acting on the cutting tool during the drilling process on the workpiece. In addition, deformation can occur due to high cutting temperatures that accompany plastic deformation which produce a soft region (by thermal softening) underneath the machined surface.

Fig. 5 Microstructures of the hole surface of the 1st hole when drilling using RPO-MQL (a) TiSiN coated carbide and (b) TiAlN coated carbide.

With regards to RPO-MQL drilling, using the same cutting condition under TiAlN coated carbide, the depth of affected layer and grain refinement on the surface and subsurface of drilled hole sample was found 17 µm in average. When using TiSiN coated carbide, average subsurface deformation depth was found as 19 µm. This may occur due to the mechanical forces acting on the cutting tool during drilling process on the workpiece. In addition, deformation can occur due to high cutting temperatures which produce a soft region underneath the machined surface.

TiAlN coated carbide produced less plastic deformation compared to the TiSiN, probably due to formation of the micro thin oxide layer of Al2O3 as a result of the reaction between the TiAlN and the oxygen as the oxide layer acts as a solid lubrication thus also reducing the friction during cutting and the extrusion process and hence reducing workpiece temperature. As deformation temperature is lower, temperature induced annihilation partially occurs and hence after process is completed lowest affected layer visible. This phenomenon is confirmed by the microstructure images (Fig. 5) and measured microhardness (Fig. 6) that show good agreement.


3.4 Microhardness Variations.

Fig. 6 shows the microhardness distribution beneath the surface at different coolant conditions. In general, similar trend is observed that the subsurface close to the machined surface shown high values of microhardness and these values decrease with an increase in the depth until they stabilize and reach the hardness value of the bulk material. The recorded highest hardness value at outer surface of the drilled hole was 250 HV for TiSiN while for TiAlN was 210 HV.

Fig. 6 Variation in microhardness value for different Tool Coatings

Compared to the bulk material, the heat affected zone (HAZ) had higher microhardness than the bulk because of hardening effect due to high cutting temperature during drilling operations and also associated with low thermal conductivity of austenitic stainless steel. Similar result has been reported by Liu et al [27] when they performed drilling of Nitinol.

4. Conclusions

The conclusions of the investigation can be summarized as follows:

In general, under different tool coating, it was found that the RPO-MQL drilling using TiSiN coated tool produced the better result in terms of tool life compare to TiAlN coated tool. In the other hand, related to surface roughness, best result obtained by TiAlN coated tool, followed by TiSiN coated tool.

Result showed that RPO-MQL using TiSiN coated tool provided longer tool life (7.54 minutes) than by RPO-MQL with TiAlN (4.19 minutes). Lower surface roughness probably can be addressed to the smaller friction coefficient of TiAlN coated tool than the TiSiN coated tool as coefficient of friction directly determine the force required to produce movement and higher cutting force tend to triggered higher vibration. And also can be addressed to the formation of the micro thin oxide layer of Al2O3 as a result of the reaction between the TiAlN and the oxygen at the cutting zone. The oxide layer acts as a solid lubrication thus reducing the friction hence lowering the surface roughness.

Based on anova analysis using first and last hole data for 3 replications, it was revealed that tool conditions affect the surface

roughness and tool wear significantly or in other words, new and worn tool have significant effect on flank wear of the tool and surface roughness of the machined surface.

Based on anova analysis using first and last hole data for 3 replications, it was concluded that different tool coating has no significant effect on tool wear and surface roughness or in other words, TiAlN coated and TiSiN coated tools give no significant variation on tool wear and surface roughness.

Based on anova analysis using first and last hole data for 3 replications, it was concluded that no interaction between tool condition and tool coating that influence the tool wear or surface roughness.

ACKNOWLEDGEMENTFinancial support from the Ministry of Higher Education,

Malaysia and Universiti Teknologi Malaysia through Research University Grant scheme (no. 06H89) is gratefully acknowledged.

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