Experimental Invesitgation on the Effects of Burnishing Process for Turned Titanium Alloy (Ti-6AL- 4V)

Abstract

Surface modifications and surface treatments play a very important role in increasing the service life of several critical parts of equipments that are employed in manufacturing and structural applications. Latest technologies utilize sophisticated surface modification methods like laser treatment and coatings to increase service life. But the biggest drawback of these technologies is the prohibitively high cost involved and hence is economically not viable for simple to moderate but vital applications like aerospace, automobiles, biomedical, gas turbines and machine parts. In this context an effective, efficient and economical option are the mechanical surface enhancement techniques that have been employed successfully over the years to increase the wear resistance, fatigue life and corrosion resistance of metallic components. These techniques improve surface finish, increase hardness and impart compressive residual stresses on component surfaces to counter the damage caused by the machining process. This is particularly the case in difficult-to-cut materials, namely titanium and nickel based alloys. Burnishing is a mechanical surface smoothing and surface enhancement technique, which can eliminate the damaging effects of machining processes like surface roughness, crack and tensile residual stress and improve the surface integrity of components. Development of a simplified burnishing process with parameters optimized to achieve fair levels of multiple responses is a need of the hour for industry and is attempted in the present work. The present research aims to study the effects of burnishing process on turned titanium alloy (Ti-6Al-4V) using Taguchi technique, response surface methodology and finite element method. In the first phase of study, the analysis of surface roughness and hardness of titanium alloy, which is turned with polycrystalline diamond tool under different turning parameters, different lubricating modes was carried out. The optimum conditions of machining for the best finish i.e., minimum quantity lubrication mode, high cutting speed, low feed rate, high nose radius with low depth of cut were established using Taguchi method. The second order surface roughness model based on response surface methodology (RSM) was developed using the experimental data obtainedfrom Taguchi’s orthogonal array. The predicted values from the RSM model agreed with the experimentally obtained results. In the second phase, the surface integrity of the above turned titanium specimens subjected to burnishing was investigated. The effect of burnishing parameters like burnishing force, feed, speed and number of passes was studied. The surface integrity factors investigated were surface roughness, hardness, residual stresses and micrography.The optimum conditions of burnishing i.e., a combination of burnishing speed in the medium range, small feed, low burnishing force with three passes was helpful for reduced surface roughness. However, burnishing with medium speed and feed, higher force and more number of tool passes improved the surface hardness. Greater improvements in surface finish (77%) and visible improvement in hardness (17%) were observed when compared with turned surfaces. A finite element method (FEM), model was developed to determine the residual stresses due to burnishing using the software LS-Pre Post. A comparison of residual stresses obtained by the FEM model and the experimental observations was made and validated. A novel and improved burnishing tool was proposed and burnishing trials were conducted with this tool and the improvement was measured. A comparison of the performance of ball burnishing and roller burnishing was also carried out. Finally the improvement in wear resistance of ball burnished titanium was investigated using pinon-disk wear testing equipment. Taguchi optimization in our investigation, showed greater improvements in specific wear rate (52% reduction) and coefficient of friction (64% reduction), when compared to turned surfaces. Thus, the study contributes a simple burnishing process which can be implemented by a small/medium enterprise to achieve better industrial components with improved surface hardness, surface finish and wear resistance.