Machining of 6061 aluminium alloy with MQL, dry and flooded lubricant conditions.pdf

Available online at www.sciencedirect.com Materials Letters 62 (2008) 276 – 278 www.elsevier.com/locate/matlet...

Available online at www.sciencedirect.com Materials Letters 62 (2008) 276 – 278 www.elsevier.com/locate/matlet Machining of 6061 aluminium alloy with MQL, dry and flooded lubricant conditions P.S. Sreejith ⁎ Department of Mechanical Engineering, Cochin University of Science and Technology, Kochi — 682 022, Kerala, India Received 15 January 2007; accepted 8 May 2007 Available online 13 May 2007 Abstract This paper reports on the effect of different lubricant environments when 6061 aluminium alloy is machined with diamond-coated carbide tools. The effect of dry machining, minimum quantity of lubricant (MQL), and flooded coolant conditions was analyzed with respect to the cutting forces, surface roughness of the machined work-piece and tool wear. The three types of coolant environments are compared. It is found that MQL condition will be a very good alternative to flooded coolant/lubricant conditions. Therefore, it appears that if MQL properly employed can replace the flooded coolant/lubricant environment which is presently employed in most of the cutting/machining applications, thereby not only the machining will be environmental friendly but also will improve the machinability characteristics. © 2007 Elsevier B.V. All rights reserved. Keywords: Force; Machining; MQL; Surface roughness; Turning; Tool wear 1. Introduction many times more than the labor and overhead costs. Hence the implementation of machining without coolants (dry machining) Cutting fluids are employed in machining operations to will bring down the manufacturing costs. In dry machining, improve the tribological processes, which occur when two higher order friction between tool and work, and between tool and surfaces, the tool and work-piece make contact. The cutting fluid chip can lead to high temperatures in the machining zone. This improves the tool life, surface conditions of the work-piece and high temperature at the machining zone will ultimately cause the process as a whole. It also helps in carrying away the heat and dimensional inaccuracies for the work-piece and tool wear debris produced during machining. On the other hand, the problems. Therefore for pursuing dry machining, the disadvan- cutting fluids have many detrimental effects. Many of the fluids, tages associated with it have to be compensated. which are used to lubricate metal forming and machining, The minimal quantity lubrication (MQL) can be practised contain environmentally harmful or potentially damaging instead of dry machining. A cutting fluid for MQL should be chemical constituents. These fluids are difficult to dispose and selected not only on the basis of primary characteristics (cutting expensive to recycle and can cause skin and lung disease to the performance) but also of its secondary characteristics, such as operators and air pollution. biodegradability, oxidation stability, and storage stability. Those Because of the negative effects associated with the cutting processes, in which the friction and adhesion play a dominant fluids and also the stringent environmental policies, a lot of role, generally require the usage of minimal quantities of fluid. research has been recently directed towards minimizing the use of MQL refers to the use of cutting fluids of only a minute quantity, cutting fluids or to totally avoid them [1–4]. Elimination on the which are about three to four orders of magnitudes lower than use of cutting fluids, if possible can be a significant incentive. The that used in flooded lubricating conditions. The concept of costs connected with the use of cutting fluids are estimated to be MQL is sometimes referred to as “near dry lubrication” or “micro lubrication”. ⁎ Corresponding author. Tel.: +914842607676, +919447812820. There are reports, which indicate that MQL in an end-milling E-mail address: [email protected]. process is very much effective [8,9]. This is considered to be 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.05.019  P.S. Sreejith / Materials Letters 62 (2008) 276–278 277 because lubricant can reach the tool face more easily in milling operations compared with other cutting operations. MQL with rapeseed oil has only a small lubricating effect in light loaded machining conditions. This was because the boundary film formed on the tool surface is not strong enough to sustain low friction and to avoid adhesion of work material; but MQL with water droplets showed good lubrication performance during the same cutting conditions. This paper reports on the effect of coolant/lubricant environ- ment on the tool wear, cutting forces and surface roughness Fig. 2. Variation cutting force with cutting speed. produced during machining of 6061 aluminium alloys at different cutting speeds. Tool wear was measured during each machining test using a 2. Experimentation toolmakers microscope. The surface roughness of the machined work-piece was measured using a Hommelwerke® T1000 Turning operations were performed on 6061 aluminium profilometer. The arithmetic mean deviation Ra with a sampling alloys. Cutting speeds up to 400 m/min were employed. The length of 0.8 mm was applied as the surface finish evaluation main objective of the present study was to analyze the effect of criterion. A Kistler 9121 three component piezoelectric quartz the coolant environment on tool wear, cutting forces and surface crystal dynamometer was used for on-line cutting force measure- quality of the work-piece during turning operation. ments. The numerical values of the cutting forces were conti- The tests were performed using diamond-coated carbide nuously monitored and recorded throughout the tests by using a inserts under three different coolant environments of dry cutting, three-channel charge amplifier (model 5019) with data acquisition. MQL and flooded coolant conditions. MQL was applied at two rates of 50 ml/h and 100 ml/h. Gravity fed MQL system with 3. Results and discussion commercial oil BP Microtrend 231L was used for experimenta- tion. The inserts confirmed to ISO designation CNGA 120408 3.1. Tool wear T01020 WG (positive rake angle 15°, clearance angle 7°, nose During machining, at all cutting environments, work material radius 0.8 mm). Tool holder type with ISO designation STG adhered to the edges of the tool; but the quantity of the adhered material CL2020K16 was used. varied with the type of coolant environment. As the speed of machining Machining tests were performed using a high rigidity lathe increased from 50 to 400 m/min, the adhesion between the tool and the Kingsbury50 CNC, 25 hp, 5000 rpm. Preliminary experiments chip also increased correspondingly. This could be due to the increase were conducted to determine the machining parameters and in thermal softening of the chip as the temperature increased with the coolant quantities. From these experiments the depth of cut and increase in cutting speed. The adhesion of the work material to the tool feed rate were fixed at 1.0 mm and 0.15 mm/rev respectively. was observed to be having the highest rate during dry cutting. The material adhesion was seen all over the tool surfaces like flank, rake Fig. 1. Progress of flank wear. Fig. 3. Typical variation of surface roughness. 278 P.S. Sreejith / Materials Letters 62 (2008) 276–278 and clearance surfaces especially when the speed of machining was adhered on to the work surface increasing the surface roughness of the increased from 250 to 400 m/min. The quantity of the adhered material machined surface. These defects would have given rise to a high Ra reduced considerably with flooded coolant compared to the dry cutting value at high cutting speeds. operation. During MQL machining, the amount of material adhered At all the cutting speeds, it was observed that the surface roughness was seen to be more compared with flooded coolant and less compared could be improved by the application of coolant. The improvement in to dry machining. As the quantity of the lubricant was increased from surface finish can be attributed to the reduction in the material transfer 50 ml/h to 100 ml/h during MQL, there was not any considerable onto the machined surface. At a higher speed of 400 m/min, it was clear reduction in the adhered material. The larger amount of adhered from the graphs that the quantity of the coolant was not a deciding material during MQL conditions may be due to the tool geometry. By factor for surface roughness but there has to be MQL conditions which reducing the nose radius of the tool, and geometrical modifications, the can reduce the surface roughness. amount of adhered material can be brought down. Investigations have to be carried out in this direction by changing the tool geometry for MQL conditions. 4. Conclusions Fig. 1 shows the change in flank wear VB with machining distance. The flank wear was shown for 2 different cutting speeds of 50 m/min and 6061 aluminium alloy has been machined under different 400 m/min. It was found that increasing the cutting speed from 50 to conditions of dry, MQL and flooded coolant/lubricant using 400 m/min resulted in a significant increase in the flank wear. There was diamond-coated carbide inserts. The process of machining was not much difference in flank were at MQL conditions of 50 ml/h and successful. Since MQL conditions can be applied to the 100 ml/h as seen from the figure. The wear land width is seen to be almost same with MQL and flooded coolant application. This suggests that the machining, it seems that this process has got economic coolant application has very little influence on flank tool wear. But the advantage. It was seen that the application of coolant does not coolant has a significant effect on the amount of material adhesion on the necessarily reduce tool wear since at MQL conditions the tool tool. wear was found to be lower, but the amount of coolant determines the material adhesion on the tool surface. The tools 3.2. Cutting forces used for machining experienced nose wear and flank wear with deformation of the coating. The cutting forces were found to be Fig. 2 shows the relationship between the resultant cutting forces dependent on the coolant system. For improving the quality of and cutting speeds measured under various machining environments. the work-piece surface, coolant is necessary. As expected, the resultant cutting force was the highest under dry MQL technology is still new, but it was seen that MQL cutting conditions. The higher cutting forces were due to the effect of conditions during the machining operation were able to produce adhesion of the work material on the tool. The cutting forces were lower when the tool was sharp during the initial stages of machining results comparable with that of flood lubricant conditions. and was seen to increase as adhesion on the tool progresses. The However, several issues including tool wear, tool geometry, resultant force was seen to be the lowest with flooded coolant system. type and constituents of the coolant, machine reliability etc. This is because due to the flooded cooling, the adhesion on the tool is have to be studied in detail in order that this method (MQL) can lowest. This lower adhesion produces lower frictional force. MQL be universally accepted and used. machining also reduces the frictional forces like flooded conditions, but for getting a lower resultant force like flooded system, a further References investigation on the constituents of the coolant has to be carried out. R.B. Aronson, Why dry machining? Manuf. Eng. (1995) 33–36. 3.3. Machined surface quality F. Klocke, G. 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