Experimental Research On Turning Process And Surface Integrity With Minimum Quantity Lubrication

Minimum Quantity Lubrication ( MQL ) is a newly developed lubricating technology in which only a minimal amount (usually less than 100ml/h) of lubrication is sprayed toward the cutting zone in form of oil mist. This new cooling/lubricating method has been regarded as an efficient means to solve the environmental, health and economic problems existing in traditional flood cooling method. Surface integrity is an important aspect of physical property that determines performance of machined components in terms of wear, chemical resistance and fatigue life. Surface integrity is composed of residual stress, surface finish, micro-hardness and white layer. For now, the machining of bearing steel mainly adopts dry cutting. The influence of MQL on surface integrity is limited to surface finish, while research on residual stress, which is more important to surface integrity, is not so far explored.To explore the effect of MQL on machining process and on surface integrity of machined components, the present paper undertook an experimental method to compare performance of MQL and dry cutting when turning AISI 52100 with a PCBN tool. Parameters of MQL, such as location of nozzle, air pressure and volume of lubricants, are chosen according to relevant literatures. Previous theoretical models for prediction of cutting temperatures are combined and improved by considering material thermal properties as functions of temperature. An experiment plan was arranged and implemented based on research purpose. Experimental results of cutting forces, chip morphology, residual stresses, white layer and micro-hardness were measured and analyzed to figure out the influence of MQL on surface integrity of machined components of AISI 52100.Cutting forces of AISI 52100 decreases as cutting speed rises. Components of cutting forces are all higher in MQL cutting than in dry cutting. As the volume of lubricant increases, lubrication effect replaces cooling effect as the dominant factor. Cutting forces increase with feed rate in both MQL and dry cutting. However, cutting forces increase a bit slower in MQL, indicating that the effect of MQL becomes significant as feed rate increases. With the increase of lubricant volume, cutting forces in MQL decrease. Meanwhile, fluctuation of cutting forces decreases as volume of lubricant elevates.Chips are continuous at low cutting speed. As cutting speed elevates, the chips break more frequently. It is indicated from the chip color that MQL is effective in suppressing cutting temperature by reducing friction coefficient.It is indicated from changes of micro hardness that micro hardness is dominated by cutting temperature near the surface but by cutting forces in subsurface. At higher temperatures, the surface materials may be softened by heat treatment, however, micro hardness in the subsurface is enhanced by cutting forces. It is also indicated that values of micro hardness at the surface are higher under MQL turning, while in subsurface, hard turning and MQL result in similar micro hardness.The metallographic structures of the machined surface indicate that hard turning tend to result in thermal damage and white layer in comparison to MQL, and white layers are thicker under hard turning. As elevation of temperature result in thicker white layer, thus MQL is able to suppress elevation of temperature. This fact demonstrates from another perspective the mechanism that MQL results in lower cutting forces and cutting temperatures by reducing friction coefficient.Residual stresses increase with higher turning speeds and feed rates. Changes in cutting forces and cutting temperature indicate that residual stresses would increase to some extent when MQL is adopted.