Performance of minimum quantity lubrication using paraffin based nanofluids during hard turning of AISI420 stainless steel

Abstract

Cooling fluids have been used successfully in machining processes to suppress tool wear, enhance surface quality, and reduce the cutting forces and temperature. However, cooling fluids have negative effects on the cost of handling and the environment. Furthermore, under Minimum Quantity Lubrication (MQL) conditions, the usage of mineral oil-based cooling fluid for hard turning is found to be unsatisfactory. The aim of this study was to evaluate the performance of titanium aluminium nitride (TiAlN) coated carbide cutting tool (KC5010) when turning martensitic stainless steel (AISI 420) with hardness of 48±1 HRC under MQL conditions. The cooling fluid flow rate and air pressure were employed at 50 ml/h and 5 bar, respectively. In the first Phase, seven experiments were conducted at medium cutting speed and medium feed (135 m/min, 0.20 mm/rev) (MM), under MQL using different cooling fluids. The cooling fluids used were paraffin oil and nanofluids consisting of the mixtures of paraffin oil with iron oxide (γ-Fe2O3) nanoparticles as well as with nano graphene (xGnP). The concentration of nanofluids were 0.40% (1.6g), 0.80% (3.2g), and 1.20% (6.4g) by weight of γ-Fe2O3 and xGnP. The sizes of γ-Fe2O3 and xGnP nanoparticles were ≤10 nm, and these were separately added to 400g of paraffin oil. Among others, the evaluation was in terms of tool life, surface roughness, cutting forces and vibration. Based on the result of Phase 1, different cutting parameters were investigated in Phase 2, where MQL using cooling fluid consisting of a mixture of paraffin oil and 0.80% wt γ-Fe2O3 was selected for further investigation. This concentration of nanofluid performed best in terms of tool life, surface roughness, vibration and cutting forces when compared to the other cooling fluids investigated in Phase 1. The tool life improvement was 47.5% and 46.8% compared to paraffin oil and xGnP nanofluid, respectively. Surface roughness was enhanced by 83.3% and 44.4% compared to paraffin oil and xGnP nanofluid conditions, respectively. The vibration level decreased by 33.5% when using 0.80% wt γ-Fe2O3 compared to 0.40% wt xGnP, and cutting forces (feed force) were reduced by 4.6% and 46.2% compared to paraffin oil and xGnP nanofluid conditions, respectively. The cutting conditions investigated in Phase 2 were low (L), medium (M) and high (H) combinations of cutting speeds (100, 135 and 170 m/min) and feed rates (0.16, 0.20 and 0.24 mm/rev). This involved a further eight experimental conditions (i.e. LL, LM, LH, ML, MH, HL, HH, and HM). The eight experimental conditions, when combined with the MM condition of Phase 1 results in a two-factor, three-level full factorial design with two center points. In Phase 2, better surface roughness and lower cutting forces were obtained at LH. However, the longest tool life and the highest material removal rate were obtained at a low cutting speed and feed rate. A combination of adhesion wear and abrasion wear was the dominant wear mechanisms. Catastrophic failure occurred at high cutting speed and feed rate, resulting in the shortest tool life among all experiments. The flank and crater wears were dominant at low and medium cutting speeds and feed rates. Continuous chips were observed at low speed and saw tooth chip at high feed rate. Empirical models for the various machining responses were developed and these were used to determine the optimum process parameters within the limits investigated.