*3.3.2. Chip thickness analysis*

*3.3.1. Cutting force and temperature analysis*

54 Lubrication - Tribology, Lubricants and Additives

**Figure 5.** Cutting force and temperature results.

**Figure 5** shows the results of cutting force, *Fc*

**Figure 4.** Tapping torque and torque efficiency value of all lubricant samples.

measured in the Z-axis and the maximum cut-

ting temperature results after the orthogonal cutting operations. It is shown that SE produced the highest cutting force and cutting temperature at ca. 612 N and 308°C respectively. SE generated poor lubrication condition on the cutting zone as compared to the other lubricant samples. MJO + PIL1% produces the greatest reduction of cutting force (2% reduction) as well as the cutting temperature (10% reduction) compared to the SE which corresponds to the good lubrication ability of the PIL additive contained in the base oil, MJO. The addition of 0.05 wt. % hBN solid particles also improved the lubrication ability of the MJO base oil. It is anticipated that the different type of lubricant used with the addition of the same additive did

The average chip thickness after the machining processes is exhibited in **Figure 6**. During the material removal process, the chip is formed due to the elastic, elastic-plastic, and plastic deformation processes of the workpiece material. It is mainly influenced by the heat generation under high stresses and temperature arisen due to the high deformation resistance between the cutting insert and the workpiece material being cut [48]. The chip thickness is one of the parameters that affected the chip formation mechanisms with the shearing angle between the uncut chip thickness and the cutting forces required during the material removal process [39].

An effective surface lubrication on the cutting zone has helped reduce the chip thickness produced after the machining operations by reducing the thermal stresses that occurred on the sliding surfaces. As presented in the previous subsection, the reduced friction due to high lubrication effect of MJO + PIL1% compared to the SE has successfully decreased 20% of the chip size which indicates the reduced tensile strain on the outer surface of the chip during bending. The specific thermal effects were reduced due to the adequate lubricant being sprayed and penetrated the sliding interfaces [49]. In addition, the fast and strong electrostatic interactions between the lubricant and additive molecules with the metal substrates had formed the tenacious lubricant film that reduced the contact area at the shear zone, thus resulting in the reduction of frictional force [20, 30, 38]. Furthermore, this phenomenon also contributed to thinner chips being cut with large shear angle and low cutting energy. These results were

comparable to the findings reported by Somers et al. [35]. They identified that the more polar IL additive has improved the properties of the polar vegetable oil. They also indicated that the ILs can form low-shear layers of anions and cations when adsorbed onto the metal surface and they can also break down to form a protective tribolayer by reacting with the exposed metal. The more polar ILs induced physical and chemical interactions with the metallic surface by adsorption at the sliding contact, thus contributing to the reduction of friction.

*3.3.4. Evaluation of tool-chip contact length*

**Figure 8.** Results of the measured tool-chip contact length.

The tool-chip contact region was analyzed underneath an optical microscope and the average contact length was measured and presented in **Figure 8**. The interaction between the metal chip and the tool rake face produces two contact regions of sticking and sliding friction during the orthogonal cutting process [43, 48]. The contact length and cutting forces are greatly influenced by the cutting lubricants applied on the cutting zone. The total contact length, *Lc*

Tribological Interaction of Bio-Based Metalworking Fluids in Machining Process

http://dx.doi.org/10.5772/intechopen.72511

57

tional to the product of the chip thickness and the effective friction coefficient. Here, the lubrication effect by using different MQL lubricant mixtures had improved the tool/chip contact

and propor-

has been reported to depend most strongly on the deformed chip thickness *hc*

**Figure 7.** Specific energy of the orthogonal cutting process lubricated with all lubricant samples.

### *3.3.3. Evaluation of specific cutting energy*

The calculated specific cutting energy following Eq. (2) of the orthogonal lathe machining is presented in **Figure 7**. Specific cutting energy is correlated with the energy during plastic deformation and friction [43, 48]. It indicates the amount of energy required to perform plastic deformation and overcome friction in the machining process [39]. Specific cutting energy decreases with temperature as the shear stress of the material in the shear plane decreases with the reduction of the chip thickness compression ratio [48].

It is clearly seen from **Figure 7** that the SE poses the highest cutting energy which correlates with the production of high shear stresses in the shear plane. The shear angle is also a predominant factor controlling the distribution of stresses together with the chip compression ratio and the rake angle. The lubrication characteristics of the lubricant mixtures have considerably reduced the shear stresses between metal surfaces when they were in relative motions by providing adequate lubrication film with high load carrying capacity, thus reducing the requirement of the cutting energy [43]. As mentioned earlier, the formation of tribofilm by the active end of the lubricant molecules and the additives on the metal surfaces has directly enhanced the lubrication properties on the contact zone. The complex and branched chain of TMP triester and the additive molecules caused a superior lubricity behavior and a high thermal stability which offered better tribological behavior regarding friction reduction and lower the generated thermal stresses.

Tribological Interaction of Bio-Based Metalworking Fluids in Machining Process http://dx.doi.org/10.5772/intechopen.72511 57

**Figure 7.** Specific energy of the orthogonal cutting process lubricated with all lubricant samples.

#### *3.3.4. Evaluation of tool-chip contact length*

comparable to the findings reported by Somers et al. [35]. They identified that the more polar IL additive has improved the properties of the polar vegetable oil. They also indicated that the ILs can form low-shear layers of anions and cations when adsorbed onto the metal surface and they can also break down to form a protective tribolayer by reacting with the exposed metal. The more polar ILs induced physical and chemical interactions with the metallic surface by

The calculated specific cutting energy following Eq. (2) of the orthogonal lathe machining is presented in **Figure 7**. Specific cutting energy is correlated with the energy during plastic deformation and friction [43, 48]. It indicates the amount of energy required to perform plastic deformation and overcome friction in the machining process [39]. Specific cutting energy decreases with temperature as the shear stress of the material in the shear plane decreases

It is clearly seen from **Figure 7** that the SE poses the highest cutting energy which correlates with the production of high shear stresses in the shear plane. The shear angle is also a predominant factor controlling the distribution of stresses together with the chip compression ratio and the rake angle. The lubrication characteristics of the lubricant mixtures have considerably reduced the shear stresses between metal surfaces when they were in relative motions by providing adequate lubrication film with high load carrying capacity, thus reducing the requirement of the cutting energy [43]. As mentioned earlier, the formation of tribofilm by the active end of the lubricant molecules and the additives on the metal surfaces has directly enhanced the lubrication properties on the contact zone. The complex and branched chain of TMP triester and the additive molecules caused a superior lubricity behavior and a high thermal stability which offered better tribological behavior regarding friction reduction and lower the generated thermal stresses.

adsorption at the sliding contact, thus contributing to the reduction of friction.

with the reduction of the chip thickness compression ratio [48].

*3.3.3. Evaluation of specific cutting energy*

**Figure 6.** Result of the measured chip thickness.

56 Lubrication - Tribology, Lubricants and Additives

The tool-chip contact region was analyzed underneath an optical microscope and the average contact length was measured and presented in **Figure 8**. The interaction between the metal chip and the tool rake face produces two contact regions of sticking and sliding friction during the orthogonal cutting process [43, 48]. The contact length and cutting forces are greatly influenced by the cutting lubricants applied on the cutting zone. The total contact length, *Lc* has been reported to depend most strongly on the deformed chip thickness *hc* and proportional to the product of the chip thickness and the effective friction coefficient. Here, the lubrication effect by using different MQL lubricant mixtures had improved the tool/chip contact

**Figure 8.** Results of the measured tool-chip contact length.

length compared to the surface lubricated with SE. It was clearly seen that the various types of the metalworking fluids clearly affected the tool-chip contact length. The poor lubrication effect of the SE has resulted in the high frictional sticking contact with the workpiece material and produces high shear and strain stress on the cutting edge, which leads to the high frictional stress and longer contact length [39, 49].

The lubrication characteristics of metalworking fluids considerably reduced the friction between surfaces when they were in relative motions, thus reducing the requirement of cutting energy [48]. SE contributed to poor lubrication behavior where oxidization at high operation temperature could easily take place [18]. MRPO + hBN0.05% shows a decrement of 0.4% in specific cutting energy under the given cutting condition, while MJO + PIL1% shows superior performance with 2% improvement compared to the SE. The presence of solid additive of boron nitride particles in the vegetable base oils did help to increase the base oil performance, however, the higher adsorption rate of PIL than the hBN particles are seen to impose better lubrication effect. The poor lubrication film of the SE and MRPO + hBN0.05% was contributed by a low formation of protective layers on the contacting surfaces and also corresponds to the decreased of spray penetration into the cutting zone which leads to the direct contact of metal asperities that produces high friction between the tool and workpiece surfaces. In conclusion, the low energy required posed by the lubricant mixtures clearly proved the ability of these lubricant additives to be used as new formulations for an advanced renewable bio-based MWF from Jatropha and palm olein oils. They may become an attractive alternative to the world dominating mineral-based MWFs.

Tribological Interaction of Bio-Based Metalworking Fluids in Machining Process

http://dx.doi.org/10.5772/intechopen.72511

59

From the experimental data analyzed in this work, the following conclusions are obtained:

• The presence of polyol ester and fatty acids in the newly refined biodegradable lubricants from Jatropha and palm olein oils plays a significant importance to the tribological behavior on the metal sliding pairs in terms of wear and friction reduction. The presence of alkyl groups affects the good tribological behavior posed by the vegetable-based oils, MJO & MRPO. • The MJO & MRPO lubricants proved to be more effective in enhancing their machining performances during the tapping torque tests and the orthogonal cutting experiments with the addition of a small quantity of an oil-miscible ionic liquid and hBN solid particles as

• Good synergistic effect on the tribological and machining performance of the lubricant mixtures was shown by enhanced machining performances with improved physical properties and lubrication effects. All lubricant mixtures have shown reduced tapping torque, improved tapping efficiency, low cutting force and cutting temperature, reduced specific cutting energy and tool-chip contact length compared to the conventional synthetic ester, SE. • The addition of phosphonium-based ionic liquid (1 wt. % PIL) into the MJO and MRPO is found to impose better lubrication ability than the addition of hBN solid particles (0.05 wt. %)

• The low amount of the additives used in the MJO & MRPO has great potential to be used as

• 'Greener' manufacturing activities by using renewable sources and low amount of biocompatible additives resulted in good energy efficiency and cleaner environment. In terms of sustainable machining operation, MJO + PIL1% & MJO + hBN0.05% are found to be a good alternative as reference for replacing the industrial dominating mineral oil-based lubricants.

lubricant additives in the bio-based cutting fluids for metalworking applications.

in improving the tribological and machining performances of the base oils.

**4. Conclusions**

lubricant additives.

The lubricant forms an intrinsically hydraulic wedge between the chip and the rake face of the tool insert, which may prevent seizure between the tool-chip interfaces and thus greatly lower the cutting force [48]. The addition of additives has successfully reduced the total stresses on the sticking and sliding regions during the chip formation processes [26, 36]. The tribofilm formed on the sliding surfaces acts as a wear-protected film and the lubricant spray mist penetrates to the tool cutting edge. It reduces the friction stress and subsequently shortens the tool/chip contact length and improved chip control [43, 48]. Sticking or seizure occurs at the tool edge interface and the chip then slides beyond the sticking region. The occurrence of material transfer or adhesion on the rake face indicates the material wear mechanism and the lubrication effects of different lubricant samples during the material removal process. The surface topography of the sliding region is presented in **Figure 9**.

**Figure 9.** Optical and SEM images of the selected surface morphology on the cutting tools rake face at 50×, 75×, and 200× magnifications respectively; (a, a', a") SE; (b, b', b") MJO + PIL1% and (c, c', c") MRPO + hBN0.05.

The lubrication characteristics of metalworking fluids considerably reduced the friction between surfaces when they were in relative motions, thus reducing the requirement of cutting energy [48]. SE contributed to poor lubrication behavior where oxidization at high operation temperature could easily take place [18]. MRPO + hBN0.05% shows a decrement of 0.4% in specific cutting energy under the given cutting condition, while MJO + PIL1% shows superior performance with 2% improvement compared to the SE. The presence of solid additive of boron nitride particles in the vegetable base oils did help to increase the base oil performance, however, the higher adsorption rate of PIL than the hBN particles are seen to impose better lubrication effect. The poor lubrication film of the SE and MRPO + hBN0.05% was contributed by a low formation of protective layers on the contacting surfaces and also corresponds to the decreased of spray penetration into the cutting zone which leads to the direct contact of metal asperities that produces high friction between the tool and workpiece surfaces. In conclusion, the low energy required posed by the lubricant mixtures clearly proved the ability of these lubricant additives to be used as new formulations for an advanced renewable bio-based MWF from Jatropha and palm olein oils. They may become an attractive alternative to the world dominating mineral-based MWFs.
