Performance Assessment of Solid Lubricants and Minimum Quantity Lubrication in Machining

Abstract

Sustainability has attained a lot of importance in today's manufacturing industries. The sustainable manufacturing approach extends efforts towards the optimum use of resources without compromising quality and the associated value of workpiece. Machining is viewed as an important process able to produce precise dimensions and finish required on the workpiece. Productivity and part quality have been important concerns for industries. The heat generated while machining adversely affects the quality of the product in terms of dimensional accuracy and surface finish. Hence, the effective control over heat dissipation is important to ensure a good surface quality of workpiece. For many years, coolants, popularly known as metalworking fluids have continued to be used for the heat removal until the effects of these fluids are realized government regulation regarding the disposal of these fluids, environmental protection, and the need for development of low cost, clean, and green process leads the manufacturing industry to take initiative towards the development of sustainable manufacturing processes. In this context, there is a clear need to explore the potential for using solid lubricants within machining combined with minimum quantity lubrication in order to establish a stable and reliable machining process. This work has attempted to fill some of the gaps in contemporary research in improving the machining performance with the application of minimum quantity lubrication and solid lubricants. AISI 52100 steel material is selected for machinability studies, as it is widely used in the automotive industry for the production of axle, roller bearings, ball bearings, spindle, forming and molding dies, etc. Turning is the main machining process for the production of these parts. PVD carbide insert with TiAlN coating is utilized to perform turning experiments. The insert with TiAlN coating offers higher strength, wear, and abrasion resistance. Calcium fluoride (CaF2) is selected as a solid lubricant for experiments under MQSL environment. CaF2 with cubic lattice structure exhibits good adhesion property which promotes the formation of lubricating films, preventing direct metal to metal contact. The present work correlates the interrelationship of various turning parameters, namely, cutting speed, feed, and depth of cut on the surface roughness and power consumption. Response surface methodology (RSM) has been used to develop the mathematical model that can be used to predict the surface roughness and power consumption. Confirmation experiments have been conducted at various test conditions to show that the developed model could predict the value of surface roughness and power consumption accurately. The developed statistical model further utilized to find out the optimum parameters for surface roughness using a Genetic Algorithm. 9 orthogonal array with minimum quantity solid lubrication machining. Optimum turning parameters namely cutting speed, feed, and depth of cut have been determined by grey relational grade obtained using the grey relational analysis for multi-performance characteristics, namely, surface roughness, chiptool interface temperature, power consumption, and tool wear. The confirmation tests have been conducted to validate the optimum process parameters obtained. Experimental investigations have been carried out to analyze the effect of solid lubricant mixed with cutting fluid under minimum quantity lubrication. Experiments have been performed to evaluate the effects of the different flow rate of lubricant mixture, different particle size and concentrations of CaF2 as a solid lubricant mixed with base oil during machining. The results of the study provided an insight that the machining performance cannot be improved by supplying more amount of cutting oil in MQL and MQSL. Hence, there is an important point about the optimum flow rate of the lubricant mixture used for lubrication. The better performance of MQSL (300 ml/hr) for selected machining conditions proving it to be an effective alternative to the use of conventional cutting fluids in machining. In the present work, four machining environments are compared, namely dry, conventional flood cooling, MQL, and MQSL with 10 μm, 30 μm, and 50 μm as the average particle size of CaF2. The improvement in process performance shows the ability of solid lubricant with the smaller size to maintain effective lubricant film between tool and workpiece interface during machining and thus reduced the friction effects. CaF2 with bigger particle size presented larger flank wear of the tool resulting in increased surface roughness. Also, the enhanced process performance is observed in the form of better surface finish and reduced tool wear with the application of CaF2 with MQL. In order to check the effect of the various concentrations of solid lubricant, experiments have been performed by adding 10%, 20% and 30% calcium fluoride (CaF2) by weight with base oil. Longer tool life and thus longer machining time is obtained in case of machining with MQSL added with 20% concentration of CaF2 which may be due to enhanced lubrication and cooling with reduced friction resulted in lower cutting temperature during machining. As expected better surface finish is obtained in machining with solid lubricant with 20% concentrations of CaF2. With selected flow rate, concentration and particle size of solid lubricant, four cutting environments have been compared, namely dry, MQL, flood cooling, and MQSL. It is seen that MQSL and flood cooling has performed better by lowering tool flank wear as compared to dry and MQL. SEM analysis of inserts has been carried out in order to compare the wear mechanism in all machining conditions. In the present work, investigations on tool life with different coatings have been done with selected cutting speeds and results are compared to dry, MQL and MQSL machining. Improved tool life almost by 20% can be seen with MQL and MQSL cutting condition, resulted due to enhanced cooling and lubricating action as compared to dry machining. Higher tool life of almost up-to 20% obtained for PVD multilayer (TiAlN/TiN) tool as compared to the CVD bilayer (TiCN/Al2O3) can be attributed to higher adhesion strength and hardness of the coating layer to the substrate. An attempt has also been made to evaluate the tribological properties of CaF2 with different particle size and weight ratios in the base oil. CaF2 is selected as additives 2 is taken as 10%, 20% and 30% by weight ratio in the base oil. The comparison of experimental 0% and 20% concentration by forming a continuous thin film of lubricant and reduced friction coefficient. In summary, this work has thus provided a better understanding to enhance the performance of machining operation with the application of solid lubricants and minimum quantity lubrication. Also, the improvement in productivity of process through the optimization of the process, and the economic and environmentally compatible lubrication approach for the machining operation.