Indian Journal of Engineering & Materials Sciences Vol. 26, June-August 2019, pp. 235-253 Performance assessment of air-water and TiO 2 nanofluid mist spray cooling during turning hardened AISI D2 steel Ramanuj Kumar*, Ashok Kumar Sahoo, Purna Chandra Mishra & Rabin Kumar Das School of Mechanical Engineering, KIIT University, Bhubaneswar 751 024, India Received 24 September 2018; Accepted 17 July 2019 Spray cooling has greater potential to dissipate heat from the heating source. Application of spray cooling in hard turning (hardness ≥ 55 HRC) is a newer concept and rarely investigated. In present work, the hard turning of AISI D2 steel under air-water mist spray impingement cooling (SIC) environment has been carried using multilayer (TiN (bottom) / TiCN (middle) / Al 2 O 3 (top)) coated carbide tool. Taguchi L 16 (4 5 i.e. 5 factor such as cutting speed, feed, depth of cut, air pressure and water pressure and their 4 levels) orthogonal array-design of experiments have been chosen for the experimentations. The detailed investigation on machining responses like flank wear (VBc), surface roughness (Ra), chip-tool-interface temperature (T), chip morphology, chip reduction coefficient (CRC) and restricted chip-tool contact length (RCL) have been carried out. Tool-wear phenomena like micro-chipping, chipping, abrasion and severe abrasion have been majorly observed on tool-tip and highly affected by cutting speed. The higher magnitude of Ra (1.304 μm, 1.332 μm, 1.344 μm, and 1.420 μm) has been noticed with highest feed rate (0.16 mm/rev) machining condition. The chip-tool interface temperature under SIC surrounding ranges from 49.7  C to 156.2  C, which have been found very low for hard turning concern. Widely popular multi-response technique namely grey relational analysis (GRA) has been implemented to get the optimal combination of input variables. Further, stepwise preparation methodology of nano TiO 2 powder and de-ionized water based TiO 2 nanofluid (0.01% weight concentration) has been discussed briefly. The average size of TiO 2 particle has been found as 10-20 nm. Further, tool life under two different (air-water and air-TiO 2 nanofluid) spraying environments using optimal cutting condition has been evaluated and compared. The tool life under air-TiO 2 nanofluid is found to be 119 minutes which is about 70 % more than the tool life (70 minutes) obtained under air-water spray cooling. From ANN modeling, mean absolute error (MAE) for response Ra, T and VBc have been found to be 2.1 %, 3.1 % and 1.9 %, respectively, which indicated the well fit of models. Keywords: Hard turning, Spray impingement cooling, Flank wear, Surface roughness, Chip-tool interface temperature, Tool life, TiO 2 nano fluid, ANN modeling 1 Introduction Machining of difficult-to-cut metal alloys under dry environment generates enormous heat at the cutting zone. Excessive heat at the cutting zone results in the reduction in tool life, generation of unwanted residual stress on work-specimen, white layer formation, alteration in microstructure and hinder heat dissipation. Therefore, to address this problem diverse cooling techniques were employed in turning of heat-treated steel such as wet/flooded cooling, near dry or minimum quantity lubrication (MQL) cooling, cryogenic cooling, high-pressure cooling, and nano-fluid cooling 1 . All of these cooling methods utilize a different variant of cutting fluids which facilitates the reduction in frictions between work-tool and tool-chip interfaces which attribute the reduction in temperature. One of the primary benefits of cutting fluid in machining operation is the capability of heat dissipation which helps in reduction of temperature as well as surface roughness. In recent years, a novel cooling alternative known as spray impingement cooling (SIC) has been utilized to facilitate lubrication/cooling in machining of hard- to-cut material 2 . In spray impingement cooling, an air- water mixture is sprayed and atomized before reached to the cutting zone however, only residual fraction of coolant effortlessly penetrate into the machining zone through capillary action and decreases the friction as well as heat thus enhanced surface quality. Two-phase spray cooling was suggested for dissipation of heat- flux from the heating source 3,4 . There are several terms like nozzle-tip to surface distance, spray droplet size; rate of liquid flow and air to liquid loading directly influenced the effectiveness of spray cooling in industry 5 . According to Ravikumar et al. 6 , optimum cooling competence (2.76 MW/m 2 at ——————— *Corresponding author (E-mail: ramanujkumar22@gmail.com)