A Review on Solid Lubricant Based Machining of Difficult-to-Machine
Materials
Molatelo MAUPIa Kapil GUPTAa,1
aDept. of Mechanical and Industrial Engineering Technology, University of Johannesburg, Johannesburg-South Africa
Abstract. Nowadays, solid lubricants are being considered as sustainable alternate to the conventional cutting fluids. Solid lubricants such as graphite, molybdenum disulfide, and boric acid etc. play vital role to achieve sustainability, productivity, and surface quality in machining. This article presents an introduction to solid lubricants and lubrication technique, highlights the use of solid lubricants in the machining of difficult-to-machine materials, and reviews some previous articles focused on solid lubricant based machining. The article aims to facilitate the researchers working in the field to conduct further research and development.
Keywords Cutting fluid; lubrication; machinability; surface integrity; sustainability,
1. Introduction
Titanium, inconel, stainless steels, and composites are considered as difficult-to- machine (DTM) materials due to certain inherent properties [1]. These materials possess poor machinability that results in high machining cost, poor work surface quality, and high consumption of energy and resources [1, 2]. Metalworking cutting fluids such as straight cutting oils, water emulsifiable oils, synthetic and semi synthetic fluids etc. have been used in machining industry for cooling and lubrication purposes. Although they may provide good cooling and lubrication, but they are highly toxic, environmentally unfriendly, and unsafe [1-4]. They cause skin and allergic problems to the workers, harmful particulate emissions, and recycling problems. Considering the aforementioned limitations of conventional cutting fluids, there has been a series of innovative efforts in the field to develop sustainable lubricants and lubrication techniques. The solid lubricants are one of them. Development, application and effects of solid lubricants have been reviewed and presented in this paper.
Varieties of solid lubricants are gaining popularity due to their special properties to act as cushion between the workpiece material and the tool for utmost cooling and lubrication. Safe, clean and efficient machining advocates the use of solid lubricants [5].
The types of solid lubricants include waxes, paste, soaps, graphite, molybdenum disulphide, tungsten disulphide, boric acid and mixture of various substrates.
Solid Lubricants have the capability to reduce the friction between the work piece and the tool without the assistance of a liquid medium and minimize the temperature and thereby tool wear significantly. It also improves the surface finish and integrity of the
1 Corresponding Author, 7225, John Orr Building-DFC, University of Johannesburg, Doornfontein-2028, Johannesburg (RSA); Tel: +27-011-5599081; Fax: +27-011-5596943; Email: [email protected]
workpiece material. Excellent lubricity; high biodegradability; corrosion resistance; cost effectiveness; reusability and recyclability are the significant benefits of solid lubricants [5-8].
2. Mechanism and Working Principle
The existence of high temperature during machining of DTM materials is due to heat generation in the machining zone i.e. between tool chip interface [1-2]. It causes severe tool failure and deterioration in work surface quality. The conventional lubricants and lubrication system are not successful to overcome this problem as well as highly toxic and harmful. Solid lubricants possess capability to control the machining zone temperature effectively. The fundamental function of the solid lubricants is to generate and maintain a thin lubrication film in between surfaces coming into contact i.e. tool and workpiece during machining operations and prevent heat generation [5, 6]. It greatly helps to minimize tool wear, improve surface finish, and enhance work surface integrity.
Solid lubricants are supplied to the machining zone by various methods. High Pressure Minimum Quantity Solid Lubricant (HP-MQSL) jet assisted machining is one of them [8]. HP-MQSL system as shown in Fig. 1 consists of mainly four components:
pump and piping, nozzle, compressor and mixing chamber. Solid lubricant mixture with the cutting oil preferably green lubricant is supplied to the machining zone at moderated flow rate and high velocity for effective lubrication. Some of the solid lubricants such as boric acid and molybdenum disulfide are supplied under the influence of compressed air using special indigenously developed setup and arrangements [6, 7].
Figure 1: Solid Lubrication: supply of solid lubricants with MQL system, (Source: From [8], published under CC BY 4.0 license, available from: https://doi.org/10.1016/j.procir.2016.01.045).
Significant enhancement in the machinability of DTM materials has been achieved by solid lubricant based machining techniques and discussed in the next section.
3. Review of Past Literature
A review of the literature available on solid lubricant based machining reveals significant enhancement in machinability of difficult-to-machine materials in terms of reduction in tool wear and cutting forces and improvement in surface finish and integrity.
Overall, it identifies solid lubricants as sustainable alternate to conventional lubricants.
Table 1 summarizes the important past work on machining of DTM materials conducted by researchers using solid lubricants. A discussion on reviewed literature is as follows.
Reddy and Rao [5] conducted an important investigation on solid lubricant based end milling of AISI 1045. They investigated the effect of graphite and molybdenum disulphide powder of 2 microns average particle size on cutting force, surface roughness, chip thickness, and specific energy consumption. It was observed that due to the presence of free electron molybdenum disulphide possesses more adhesion tendency than graphite.
Reduction in the shear strength of steel due to the formation of thin lubrication film of graphite and molybdenum disulphide made machining easier and substantially reduced the cutting forces. The maximum reduction observed in specific energy consumption is 28 % while using molybdenum disulphide compared to wet machining. Lowest chip thickness that refers to highest process efficiency was found in case of molybdenum disulphide assisted machining. Both solid lubricants outperformed wet machining and achieved higher surface finish.
Table 1: A Summary of past literature on solid lubricant based machining Researcher and
Year Material
and machining
Type of solid
lubricant used Findings
Reddy and Rao (2006) [5]
AISI 1045 Steel End milling
Graphite and molybdenum disulphide
-Solid lubricants over performed wet cutting fluids
-Significant reduction in tool-chip friction, surface roughness, and specific energy consumption
Singh and Rao
(2008) [6] Bearing steel
Turning Molybdenum
disulphide -Significant improvement in surface quality
-Better than dry turning Damera and Pasam
(2008) [7]
EN8 Steel Turning
Boric acid -Machining with Boric acid is advantageous over wet and dry machining -Sustainable machining with reduction in cutting forces and tool wear Gunda et al. (2016)
[8] STAVAX ESR
(hard steel) Turning
Mixture of
MoS2 and
SAE40 oil
-High velocity solid lubricant jet assisted machining
-Improved chip breakability -Improved surface finish Moura (2015) [9] Ti06Al-4V
Turning Graphite and
molybdenum disulphide
-Significant reduction in flank wear -Molybdenum disulphide performed better than graphite
-Prolonged tool life Paturi et al. (2016)
[10]
Inconel 718 Turning
Tungsten disulphide
-Improvement in surface finish better than MQL conditions
Mahadi et al.
(2014) [11]
AISI 431 Steel Turning
Mixture of Boric acid with vegetable oil
Improvement in surface quality better than conventional lubricant
Singh and Rao [6] developed a lubrication system to supply desired quantity of fine graphite powder during turning of bearing steel. Graphite particles of 2 microns size and at a flow rate of 2 gram/min resulted in improved surface quality.
Damera and Pasam [7] also developed a set-up to supply boric acid while machining EN8 steel. A detailed investigation was conducted to analyze the effect of machining parameters on cutting force, tool wear, temperature, and surface roughness. The results of solid lubricant based machining were also compared with wet and dry cutting. They reported effective lubrication performance of boric acid that provided effective separation between tool and chip surfaces to minimize friction force and thereby cutting force. A significant reduction in temperature and therefore tool wear was observed in solid lubrication machining compared to wet and dry cutting.
A high-pressure minimum quantity solid lubrication system was developed by Gunda et al. [8]. Investigation was conducted on effect of MoS2 and SAE 40 oil mixture on machinability of STAVAX ESR steel. They achieved tool life enhancement and surface finish improvement through reduction in abrasion, built-up edge formation and chipping.
Results were compared with those obtained by dry, wet, and plain MQL conditions.
Figure 2 presents a comparison of surface roughness results of various cooling/lubrication conditions.
Fig 2 Surface roughness comparison for various cutting conditions by Gunda et al. [8], (Source: From [8], published under CC BY 4.0 license, available from: https://doi.org/10.1016/j.procir.2016.01.045).
Moura et al. [9] experimented machining of Titanium (Ti-6Al-4V) alloy with graphite and molybdenum disulphide as lubricants under MQL conditions. They used TiAlN coated carbide inserts. Titanium high speed machining with MOS2 was identified as the best option for prolonged tool life and improved surface quality.
Another important study where tungsten disulphide (WS2) with MQL was used to machine Inconel 718 by Paturi et al. [10] shows much improvement in surface finish of work material compared to MQL alone. The reason behind is the reduction in tool-chip friction and heat generation due to the cushion like arrangement of the layered structure of WS2 on both tool and work surfaces. This study identifies potential of WS2 to be a sustainable alternate of harmful conventional cutting fluids.
Environmentally friendly and non-toxic mixture of palm kernel oil with boric acid was used by Mahadi et al. [11] to enhance the machinability of AISI 431 steel under MQL
environment. At same cutting parameters, lubricant with boric acid significantly reduced surface roughness compared to lubricant without boric acid.
Conclusion
This paper reviews solid lubricants and lubrication based machining of difficult-to- machine materials. Solid lubricants not only possess excellent cooling and lubrication properties, but also highly sustainable. Graphite, boric acid and Molybdenum disulphide solid lubricants are by far the most popular solid lubricants. It is concluded that solid lubricants are capable to enhance the machinability of difficult to machine materials and are at the forefront of sustainable machining. Future research on flushing and tool cleaning while using solid lubrication is required to be done. The paper hopes to facilitate researchers and encourages future research and development in order to establish the field further.
References
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