Investigating the influence of film formation on the machining performance in an atomization-based cutting fluid spray system

Titanium alloys are difficult to machine materials due to their poor thermal conductivity, high affinity to tool materials at cutting temperatures and production of thin chips during machining. Heat generated in the tool-chip contact region is not readily dissipated, leading to a shortened tool life. Various cooling methods have been studied for titanium machining including high pressure coolant application, minimum quantity lubrication and recently, the Atomization-based cutting fluid (ACF) delivery system to improve upon the cooling and lubrication provided by conventional flood cooling. In the ACF system, the impingement of atomized droplets of the cutting fluid on the tool surface results in the formation of a thin liquid film that penetrates the narrow tool-chip contact region and improves tool life. The characteristics of the thin film such as the film thickness and velocity influence the performance of the ACF system and are dependent on a large number of parameters including cutting fluid properties, spray parameters, i.e., gas pressure, fluid flow rate, spray distance and spray angle, and the cutting tool surface geometry. Numerous experimental studies have been done to evaluate the effectiveness of the ACF system in titanium machining. However, these efforts fail to bring forth the mechanism of film formation and penetration of the film in the tool-chip interface. The purpose of this thesis is to study the effect of cutting tool surface geometry, the ACF spray parameters and the physical properties of the cutting fluid on the characteristics of the thin film formed in an atomization-based cutting fluid (ACF) delivery system. A computational model is developed using three sub-models that are used to predict the carrier gas flow, droplet trajectories and the film formation, respectively. The model is validated through film thickness measurements using a laser displacement sensor. Turning inserts with chip-breaking grooves along with a conventional flat insert are used to study the effect of cutting tool surface geometry on the model-predicted film characteristics, including film thickness and velocity. Carrier gas pressure and cutting fluid flow rate are also varied to study the effect of ACF spray parameters on the film characteristics. Machining experiments are also conducted to investigate the effect of film characteristics on the machining performance in terms of tool wear, which show that the tool wear is minimum at a particular film thickness value and large film velocity value for a particular cutting fluid using the ACF system. The effect of surface tension and viscosity on the film characteristics is also studied and it is observed through machining experiments that the composition of a cutting fluid is an important consideration in improving machining performance in addition to the film characteristics