Effects of minimum quantity lubrication on turning AISI 9310 alloy steel using vegetable oil-based cutting fluid

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Abstract

This paper presents the effects of minimum quantity lubrication (MQL) by vegetable oil-based cutting fluid on the turning performance of low alloy steel AISI 9310 as compared to completely dry and wet machining in terms of chip–tool interface temperature, chip formation mode, tool wear and surface roughness. The minimum quantity lubrication was provided with a spray of air and vegetable oil. MQL machining was performed much superior compared to the dry and wet machining due to substantial reduction in cutting zone temperature enabling favorable chip formation and chip–tool interaction. It was also seen from the results that the substantial reduction in tool wears resulted in enhanced the tool life and surface finish. Furthermore, MQL provides environment friendliness (maintaining neat, clean and dry working area, avoiding inconvenience and health hazards due to heat, smoke, fumes, gases, etc. and preventing pollution of the surroundings) and improves the machinability characteristics.

Introduction

Currently, there is a wide-scale evaluation of the use of metalworking fluids (MWFs) in machining. Industries are looking for ways to reduce the amount of lubricants in metal removing operations due to the ecological, economical and most importantly occupational pressure. From a study, Kalhofer (1997) revealed that respiration and skin problems were the main side affects of MWF. However, Greaves et al. (1997) studied the types of occupational risks associated with MWFs, which became airborne and formed aerosol during machining and showed that these risks were numerous and widespread. It is, therefore, important to find a way to manufacture products using the sustainable methods and processes that minimize the use of MWFs in machining operations. In addition, it is essential to determine the optimal cutting conditions and parameters, while maintaining long tool life, acceptable surface finish and good part accuracy to achieve ecological and coolantless objective. Erdel (1999) demonstrated successfully a method in the minimization of MWFs called near dry machining (NDM). NDM uses very small amounts of MWF in a flow of compressed air that can be approximately 10,000 times less than overhead conventional flood cooling. Besides, the costs of acquisition, care and disposal of MWFs are two times higher and have to be taken into account when the economics involved in machining operations are compared. The increasing cost associated with the use and disposal of MWFs can be up to 17% per part for automotive components. The total manufacturing cost has to be considered even if the costs associated with cutting tools increase with decrease in use of MWF. However, Brockhoff and Walter (1998) stated that the total manufacturing cost would be lower as compared to the cost of traditional overhead flood cooling using large amounts of water-miscible MWFs.

Besides, according to the National Institute of Occupational, Safety and Health (NIOSH, 1983), it is estimated that 1.2 million workers are potentially exposed to the hazardous/chronic toxicology effects of MWF. Workers can be exposed to MWF in a variety of ways. Bennett and Bennett (1987) found that a source of significant exposure to MWF was inhalation of aerosols. NIOSH recommended no respiratory protection for MWF concentrations of 0.5 mg/m3 or less. However, Greaves et al. (1997) showed that the chronic bronchitis, asthma, chest symptoms and airway irritation were linked to aerosol exposures of MWF as low as 0.41 mg/m3. Additionally, NIOSH (1997) reported that the effectiveness of OEM (original equipment manufacturer) mist enclosures was not built-in, which was a violation of the guidelines for exhaust ventilation of machining operations listed in ANSI technical report. Effectiveness of these enclosures was also reported to be less than 10% in eliminating unwanted mist during wet machining. Moreover, the goal of the new international global standard ISO14001:1996 is to support environmental protection and prevention of pollution in balance with socio-economic needs. Organizations that consider the implementation of appropriate and economically viable technologies need only to achieve those environmental objectives incorporated in the standard. The international standard may affect all organizations that aim to supply or manufacture parts on a global or domestic scale. Klocke and Eisenblätter (1997) demonstrated the interest of dry machining and eventually met with success in the field of environmentally friendly manufacturing. However, these can be sometimes less effective when higher machining efficiency, better surface finish quality and severer cutting conditions are required. In these circumstances, semi-dry operations utilizing very small amounts of cutting lubricants are expected to become a powerful tool and, in fact, they already play a significant role in a number of practical applications. Minimum quantity lubrication (MQL) refers to the use of only a minute amount of cutting fluids typically at a flow rate of 50–500 ml/h. Sometimes this concept of minimum quantity lubrication is referred to as near dry lubrication or micro-lubrication. According to MaClure et al. (2001), the concept of MQL has also been suggested since a decade ago as a means of addressing the issues of environmental intrusiveness and occupational hazards associated with the airborne cutting fluid particles on factory shop floors. The minimization of cutting fluid also leads to economical benefits by way of saving lubricant costs and cycle time for cleaning workpiece, tool, and machine. However, there has been little investigation of the cutting fluids to be used in MQL machining. Stäbler et al. (2003) suggested the types of fluids not applicable for the minimum quantity lubrication were water mixed cooling lubricants and their concentrates, lubricants with organic chlorine or zinc containing additives, lubricants that have to be marked according to the decree on hazardous materials, and products basing on mineral base oils in the cooling lubricant 3 ppm (parts per million) benzpyrene. From performance, cost, health, safety and environment points of view, Krahenbuhl (2005), therefore, considered vegetable oils as viable alternative to petroleum-based metalworking cutting fluids. The important factors for selecting the vegetable oils as a feasible choice are:

  • (i)

    molecules, being long, heavy, and dipolar in nature, create a dense homogeneous and strong lubricating film that gives the vegetable oil a greater capacity to absorb pressure.

  • (ii)

    lubricating film layer provided by vegetable oils, being intrinsically strong and lubricious, improves workpiece quality and overall process productivity reducing friction and heat generation.

  • (iii)

    higher flash point yields opportunities for increased rates of metal removal because of reduced smoke formation and fire hazard.

  • (iv)

    higher boiling point and greater molecular weight of vegetable oil result in considerably less loss from vaporization and misting.

  • (v)

    vegetable oils are nontoxic to the environment and biologically inert and do not produce significant organic disease and toxic effect.

  • (vi)

    moreover, ACGIH (2001) reported no sign and symptom of acute and chronic exposure to vegetable oil mist in human.

Significant progress has been made in dry and semi-dry machining recently and MQL machining in particular has been accepted as a successful semi-dry application due to its environmentally friendly characteristics. Some good results have also been obtained using this technique. Dhar et al. (2006) employed MQL machining technique in turning AISI 4340 steel with uncoated carbide tool (SNMM 120408). During experimentation, process parameters such as cutting velocity, feed rate and depth of cut were kept constant at 110 m/min, 0.16 mm/rev and 1.5 mm respectively. Water-soluble cutting fluid was supplied at flow rate of 60 ml/h and mixed with compressed air prior to being impinged on the cutting zone at a high speed. Under same cutting conditions, MQL caused a significant reduction in tool wear and surface roughness as compared to dry and wet turning. Lugscheider et al. (1997) used MQL machining technique in the reaming process of gray cast iron (GG25) and aluminum alloy (AISI 12) with coated carbide tools. The authors concluded that MQL caused a reduction in tool wear as compared to the completely dry process and, consequently, resulted in an improvement in surface quality of the holes. Derflinger et al. (1996), on the other hand, describes the drilling of aluminum–silicon alloys as a process where dry cutting is impossible due to the high ductility of the workpiece material. Without cooling and lubrication, the chip sticks to the tool and breaks it in a very short time during cutting.

Machado and Wallbank (1997) conducted experiments on turning medium carbon steel (AISI 1040) using a venturi to mix compressed air (the air pressure was of 2.3 bar) with small quantities of a liquid lubricant, water or soluble oil (the mean flow rate was in between 3 and 5 ml/min). The mixture was directed onto the rake face of a carbide tool against the chip flow direction. The application of a mixture of air and soluble oil was able to reduce the consumption of cutting fluid, but it promoted a mist in the environment with problems of odors, bacteria and fungi growth of the overhead flooding system. For this reason, the mixture of air and water was preferred. However, even if the obtained results were encouraging, the system needed yet some development to achieve the required effects in terms of cutting forces, temperature, tool life and surface finish.

Varadarajan et al. (2002) developed alternative test equipment for injecting the fluid and used it with success in hard turning for which a large supply of cutting fluid is the normal practice. The test equipment consisted of a fuel pump generally used for diesel fuel injection in truck engines coupled to a variable electric drive. A high-speed electrical mixing chamber facilitated thorough emulsification. The test equipment permitted the independent variation of the injection pressure, the frequency of injection and the rate of injection. The investigations performed by the authors revealed that a coolant-rich (60%) lubricant fluid with minimal additives was the ideal formulation. During hard turning of an AISI 4340 hardened steel of 46HRC (460 HV), the optimum levels for the fluid delivery parameters were a flow rate of 2 ml/min, a pressure of 20 MPa and a high pulsing rate of 600 pulses/min. In comparison, for the same cutting conditions, with dry cutting and wet cutting, the minimum quantity of cutting fluid method led to lower cutting forces, temperatures, better surface finish, longer tool life. In addition, it was observed that tightly coiled chips were formed during wet turning and during minimal application, while long snarled chips were prevalent during dry turning. It must be noted that during minimal application, the rate of fluid was only 0.05% of that used during wet turning. The major part of the fluid used during minimal quantity application was evaporated; the remnant was carried out by work and chips and was too low in volume to cause contamination of the environment.

Klocke and Eisenblätter (1997) dealt with the drilling tests using minimum cooling lubrication systems, which were based on atomizing the lubricant directly to the cutting zone. Small quantities of lubricant, in the order of 10–50 ml/h, were mixed with compressed air for an external feeding through a nozzle and for internal feeding through the spindle and tool. Internal feed systems with their ability to deliver the mixture very close to the drill–workpiece contact point may achieve very good results in terms of surface finish and tool life.

Lahres et al. (1999) presented the dry machining of synchronizing cones for automotive application. The work material was austenitic 22Mn6 steel. In the first step of their study, dry machining was compared to machining with coolant and minimal lubricant system. The used minimal lubricant system worked with special oil, which had food-grade quality. The volume flow rates of air and oil were about 50 l/min and 20 ml/h respectively and hence, the produced chips were dry after leaving the contact zone of the cutting process. At this oil volume flow, a single chip can carry a maximum of 1 ml. Therefore, the chips could be declared as being almost dry and passed for metallic recycling without further treatment. The results exhibited an advantage for the minimal lubricant technique and for the dry machining. Wakabayashi et al. (1998), by model experiments, suggested that ester supplied onto a rake face of a tool decomposed to carboxylic acid and alcohol and its carboxylic acid formed a chemisorbed film with lubricity. Itoigawa et al. (2005), however, found that in actual conditions with high machining load, existence of this kind of boundary film was uncertain.

The review of the literature suggests that the concept of MQL presents itself as a possible solution for machining in achieving slow tool wears while maintaining the cutting forces/power at reasonable levels, if the MQL parameters can be strategically tuned. The main objective of the present work was to experimentally investigate the roles of minimum quantity lubrication by vegetable oil-based cutting fluid on chip–tool interface temperature, chip color and shape, chip reduction coefficient, tool wear and surface roughness in turning alloy steel (AISI 9310) by the industrially used uncoated carbide tool (SNMG 120408 TTS) at different cutting velocities and feeds combinations as compared to wet and dry machining.

Section snippets

Experimental procedure and selection of turning conditions

Experiments were carried out by plain turning a 100 mm diameter and 710 mm long rod of AISI 9310 alloy steel of common use in a powerful and rigid lathe (France, 15hp) at different cutting velocities and feeds under dry, wet and MQL by vegetable oil conditions. These experimental investigations were conducted with a view to explore the role of MQL on the machinability characteristics of that work material mainly in terms of cutting temperature, chip formation, tool wear and surface roughness. The

Effects of MQL on cutting temperature

The machining temperature at the cutting zone is an important index of machinability and needs to be controlled as far as possible. Cutting temperature increases with the increase in specific energy consumption and MRR. During machining any ductile materials, heat is generated at the primary deformation zone due to shear and plastic deformation, whereas secondary deformation and sliding cause heat generation at chip–tool interface. Furthermore, rubbing produces heat at work–tool interfaces. All

Conclusions

Based on the results of the present experimental investigation, the following conclusions can be drawn:

  • i.

    MQL provided significant improvements expectedly, though in varying degree, in respect of chip formation modes, tool wear and surface finish throughout the range of Vc and S0 undertaken mainly due to reduction in the average chip–tool interface temperature. Wet cooling by soluble oil could not control the cutting temperature appreciably and its effectiveness decreased further with the increase

Acknowledgements

This research work has been funded by Directorate of Advisory Extension and Research Services (DAERS), Committee for Advanced Studies & Research (CASR), BUET, Dhaka, Bangladesh, sanction DEARS/CASR/R-01/2005/D-1052(39) dated 03/05/2005. The authors are also grateful to the Department of Industrial and Production Engineering, BUET for providing the facilities to carryout the experiment. The help extended by the Department of Materials and Metallurgical Engineering, BUET for obtaining the

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