Machinability Assessment of Commercially Pure Titanium (CP-Ti) Grade 2 during Turning Operation

Abstract

Titanium alloys are most widely used in aerospace, military, marine, medical and bio-medical industries because of their unique properties such as high corrosion resistance, biocompatibility, superior strength-to-weight ratio and ability to withstand at elevated temperatures. In spite of the above mentioned capabilities these alloys are classified as “Difficult-to-machine” type materials. This is due to lower thermal conductivity and high chemical affinity of these materials. Poor thermal conductivity and increased chemical reactivity during titanium machining causes rapid tool wear. This might be contributed to the high heat generated at primary deformation zone. This situation necessitates proper selection of the machining parameter as well as cutting tool material before machining the workpiece. Although, a large number of cutting tool materials are now commercially available in the market, carbide inserts were observed to be the most suitable cutting tool material for titanium machining. In addition to that, it was also noticed that the performance of these cutting tool materials could be further enhanced by applying cryogenic treatment.
Therefore, in the initial stage of the research work, some of the key machinability characteristics of commercially pure titanium (CP-Ti) grade 2 which was not adequately addressed so far, were studied and reported. Characterization of the selected cutting tool material was done before and after cryogenic treatment. Further, the benefits of using cryo-treated tools were tested by performing a comparative analysis of various turning responses such as chip compression ratio, coefficient of friction, cutting force, tool wear, surface roughness and chip morphology. The analysis was further extended to examine the influence of cutting speed and cooling method on the machinability of the selected work material.
In the next stage, a generalized regression neural network (GRNN) model was developed for the prediction of various machining responses. Further, a cutting force model (Kienzle’s force model) was also explored. The feasibility along with the application potential of both the aforesaid models were tested and validated through experimentally measured data sets. Additionally, three distinct multi-criteria decision making (MCDM) approaches viz. Technique for order of preference by similar to ideal solution (TOPSIS), Vlsekriterijumska optimizacija l KOm-promisno Resenje (VIKOR) and Fuzzy-TOPSIS were recommended with an objective to attain optimal parametric combination in order to maximize the productivity and to minimize the cost of production without compromising the quality.
In the final stage of the research work, a three dimensional finite element model was developed based on Lagrangian criterion. Simulation of the turning operation was performed using DEFORM 3D software in order to approximate the responses viz. feed force (Fx), radial force (Fy), tangential force (Fz), flank wear (Vb) and machining temperature (Tm). Usui’s tool wear model was used to predict the flank wear. Morphology of the free and back surfaces of the chips was examined under a field emission electron microscope (FESEM). Turning experiments were carried out on a heavy duty lathe equipped with a 3D dynamometer. Secondly, a quadratic model was acquired for all the aforementioned quality characteristics using response surface methodology (RSM). Analysis of variance (ANOVA) test was performed to confirm the adequacy of the developed quadratic model. The results obtained from simulation and quadratic model were compared with the experimental data sets. Results of the experimental as well as numerical analysis indicated that, the machinability of the work material (CP-Ti grade 2) could be significantly improved by proper selection of cutting tool material, cooling approach and cutting parameters along with their ranges. Cryogenically treated inserts were found to be more favorable because it demonstrated superior resistance to flank and crater wear during machining of CP-Ti grade 2.