Investigation into the Machinability of Titanium Alloys under Cryogenic Conditions

Abstract

Manufacturing has been an important activity throughout the history of human civilization. Manufacturing industries use raw materials and labor to convert natural resources into finished products through the use of technology. The manufacturing sector accounts for around 45% of global energy consumption. Manufacturing processes should therefore be productive as well as sustainable. Machining is a subtractive manufacturing process that removes excess material through the use of a cutting tool in order to give the bulk engineering material a functional geometry. It constitutes about 15% of all manufacturing processes. Machining processes are simple, efficient, versatile and economic for bulk production. These processes represent a major chunk of activities run to design and manufacture parts in various industries. A number of metals, alloys and ceramics are processed through machining. Titanium alloys are generally the preferred choice when it comes to applications where corrosion resistance, high strength to weight ratio and good fatigue properties are required. In addition, titanium based alloys also exhibit biocompatibility. These alloys are extensively used in medical, marine and aerospace industries. On the other hand, their low thermal conductivity and high temperature strength reduce tool life and increases energy consumption during the metal cutting processes. Several research studies have focused on productivity, sustainability and quality aspects of machining of titanium alloys as these are important aspects of manufacturing research. Nevertheless, systematic parametric analysis for machining titanium alloys under various cooling conditions, especially cryogenic conditions presents a research gap. Also, since the available process maps were developed under dry conditions, there is a need for development of tool wear and energy maps, being vital output responses, under cryogenic conditions. This research focuses on investigating the machinability of Ti-6Al-4V alloy over a range of machining parameters under varying cutting environments. Dry, wet and cryogenic conditions were selected as cutting environments for comparative analysis. Different machining input parameters were taken into consideration for analysis of key responses including tool wear, specific cutting energy, surface roughness and material removal rate. Process maps were developed by plotting tool wear rate and specific cutting energy against cutting speed and feed. These maps can be effectively used on the shop floor to select specific machining parameters for desired output response. Initially tool wear progression was analyzed under dry and cryogenic conditions which formed the basis for further research. Comparative tool wear analysis was conducted at low, moderate and high tool wear regions. It was observed that cryogenic machining improved tool life owing to its cooling capacity. Next, investigation into the machinability of Ti-6Al-4V was conducted by statistical analysis and multi objective optimization under varying machining conditions. Feed rate, cutting speed and depth of cut were taken as the input parameters under dry, wet and cryogenic conditions. Statistical analysis results identified cutting speed as the key input parameter for tool wear rate and specific cutting energy consumption in terms of contribution ratio. On the other hand, feed had the highest contribution ratio for surface roughness. Multi objective optimization was carried out to optimize the machining output. Each machining run was ranked using grey relational analysis. Analysis of regression model of multi objective function identified feed as the most effective input parameter followed by machining environment. Process maps, developed using cryogenic media, demarcated wear and energy charts into regions of low, moderate and high tool wear and energy zones. A high wear zone found amidst lower zone region was marked as avoidance zone. Tool wear map regions of low, moderate, high and avoidance zone were analyzed by plotting tool chip contact length. As compared with the wear map for turning of titanium alloys under dry conditions, the avoidance zone for this wear map shifted towards high feed region. EDS elemental analysis was also carried out to determine and analyze the wear mechanisms during the machining of Ti-6Al-4V. These maps were also characterized using chip morphology by chip compression ratio and shear angle. These maps can prove to be highly useful in making the manufacturing system productive and sustainable at the same time.