Abstract: This study presents the effects of thermal-assisted machining on the suppression of vibrations during end milling of titanium alloys Ti-6Al-4V using PCD inserts. The effects of heating temperature on suppression of vibrations were investigated. Induction coil heating was utilized to generate heat close to the cutting zone prior to cutting. One room temperature and three preheated conditions with same cutting speed, feed and axial depth of cut were chosen in order to investigate the effects of preheating temperature on amplitude of the vibrations. Vibration signals were recorded using an accelerometer type transducer and a Dual Channel Portable Signal Analyzer. The recorded signals were analyzed using the PULSE Labshop software. The reductions of the peak values were systematically calculated and the effects of preheating temperature on the amplitudes were plotted in block diagram. The effect of thermal-assisted machining on the chip formations was also investigated.
INTRODUCTION
Titanium alloys (e.g., Ti-5Al-2.5Sn (alpha), Ti-13V-11Cr-3AL (beta) or Ti-6Al-4V (alpha- beta)) offer high toughness, high strength-to-weight ratio, excellent corrosion and creep resistance, biocompatibility and are used mainly in aerospace, gas turbine, rocket, nuclear, chemical vessels and increasingly in biomedical applications (Ulutan and Ozel, 2011). Due to high toughness and work hardening behavior of these alloys, machining is generally extremely difficult. Many researchers have reported in literature addressing the issues related to machining of titanium alloys such as rapid tool wear, poor surface finish, high vibrations and undesirable alteration of machined surfaces as workpiece easily forms a work-hardened layer in response to the machining induced strain loading on the sub-surface. Furthermore, the low thermal conductivity of these alloys often leads to increased temperatures at the cutting edge and results in adhesion of workpiece material to the cutting edge and presence of hard abrasive particles in alloys structure creating accelerated tool wear.
The main problem in machining of titanium alloys is that their tendency to generate chatter during machining due to high tool-work contact length in conjunction with small uncut chip thickness. Many metal cutting scientists consider the formation of chips with serrated teeth as the primary cause of chatter. Amin (1983) established that chatter in turning was a result of resonance caused by mutual interaction of the vibrations caused by the formation of serrated chip elements and the natural vibrations of the system components, like spindle, tool holder, etc.
Many factors contribute to the formation of chatter, among which are the geometry of the cutting tool and cutting conditions. Tool geometry plays an important role in the formation of chip. The type of chip with its typical secondary and sometimes primary serration leads to the generation chatter in the machine-tool fixture-work system. The principal and auxiliary cutting edge angles and nose radius play a significant role in this process. It is desirable to have a very high nose radius to improve surface finish but high nose radius leads to the formation of very thin chips and the tendency of machine tools to chatter.
Some researches offer alternative methods in enhancement of machinability of titanium alloys (such as longer tool life, higher surface finish, lower cutting force and lower vibrations). They focus on the benefits of thermally assisted machining in improving the machinability of such alloys. It is due to the fact that the flow stress and strain hardening rate of materials normally decrease with increasing temperature due to thermal softening (Sun et al., 2010). Thermally enhanced machining is the process that uses an external heat source to heat and soften the workpiece. As a result, the yield strength, hardness and strain hardening of the work-piece reduce and deformation behaviour of the hard-to-machine materials (especially ceramics) changes from brittle to ductile. This enables the difficult-to-machine materials to be machined more easily and with low machine power consumption which leads to increase in material removal rate and productivity (Chryssolouris et al., 1997). Amin and Talantov (1986) tried furnace heating of the workpiece of titanium alloy to reduce chatter and improve machinability but furnace heating and subsequent mounting of the hot job on the machine is not a practical method to be employed in real life situation. Amin and Abdelgadir (2003) proposed the application of preheating of the work material using induction heating to improve machinability during end milling of medium carbon steel. Induction coil-assisted machining was utilized during end milling of AISI D2 hardened steel using PCBN tool inserts (Lajis et al., 2011). They found that the benefits of preheated machining on tool life and volume of metal removed were much higher than those compared to room temperature conditions. The effect of high frequency induction heating was studied in end milling of titanium alloy Ti-6Al-4V using uncoated tungsten carbide inserts (Amin et al., 2009). They found that induction coil heating helped in substantially increasing tool life. Experiment with preheating at 650°C gave benefit in increasing tool life by 3.25 times compared to the experiment at room temperature. High frequency induction heating was proved as a suitable technique for preheated machining. Gas flame heating was used to improve the machinability of austenitic manganese steel (Ozler et al., 2001). They considered heating temperature as one the variables in line with other cutting parameters namely, cutting speed, feed and depth of cut while hot turning of austenitic manganese steel under liquid petroleum gas flame. The effects of surface temperature and other cutting parameters on tool life were studied rigorously and an expression for tool life prediction using factorial regression method was developed. Wang et al. (2002) performed LAM using YAG continuous solid laser on Al2O3 particle reinforced aluminum matrix composite (Al2O3p/Al). The result of their study showed that in machining of Al2O3p/Al composite the cutting force was reduced by 30-50%, tool wear was reduced by 20-30% and machined surface quality was improved in laser assisted machining as compared with conventional cutting.
The main objective of this study is to investigate the effect of thermally-assisted machining by high frequency induction coil heating on suppression of vibrations during end milling of titanium alloy using Polycrystalline Diamonds (PCD) inserts.
MATERIALS AND METHODS
Workpiece material: The workpiece material used in the experiments was alpha-beta titanium alloy Ti-6Al-4V. The microstructure of this workpiece is shown in Fig. 1. The microstructure consists of both coaxial and columnar alpha phase and inter-granular beta phase. The chemical composition and the mechanical properties of the alloys are shown in Table 1 and 2, respectively.
| Table 1: | The composition of titanium alloy Ti-6Al-4V |
| Table 2: | The mechanical properties of titanium alloy Ti-6Al-4V |
| Fig. 1: | Microstructure of Ti-6Al-4V with equiaxed and columnar alpha grains (Light) with intergranular beta phase (Dark), Etchant: 10% HF, 5% HNO3, 85% H2O |
Experimental works: End milling tests were conducted on Vertical Machining Centre (VMC ZPS, Model: MLR 542 with full immersion cutting. Titanium alloy Ti-6Al-4V bar was used as the workpiece. Machining was performed with a 20 mm diameter end-mill tool holder (R390-020B20-11M) fitted with one insert. Polycrystalline Diamond (PCD) inserts were used in the experiments. All of the experiments were run at room temperature and with preheating. High frequency induction heating was utilized to run the preheated machining. Selected cutting conditions for the experimentation are shown in Table 3.
| Table 3: | Cutting condition for experimental work |
RESULTS AND DISCUSSION
Thermal-assisted machining has significant effect on suppression of vibration in end milling with PCD inserts as can be seen from the FFT diagram of acceleration amplitudes vs. frequency in Fig. 2. It is obvious that four peaks are mainly found in the FFT diagram and independently used for analyzing.
The acceleration amplitude of vibration and the reductions of acceleration amplitude as the effect of thermal-assisted machining on the work material in end milling of Ti-6Al-4V are presented in Table 4 and 5, respectively.
| Table 4: | Acceleration amplitudes of vibration |
| Fig. 2(a-d): | FFT output of end milling with PCD inserts at (a) Room temperature and preheated conditions at (b) 315°C, (c) 450°C and (d) 650°C, Cutting speed: 127 m min-1, Axial DOC: 1 mm, Feed: 0.088 mm tooth-1) |
| Fig. 3: | The Effects of preheating temperature on reducing maximum acceleration amplitude of vibration, Cutting Speed: 127 m min-1, Axial DOC: 1 mm, Feed: 0.088 mm tooth-1 |
| Fig. 4: | Chip shrinkage coefficient vs. preheating temperature |
| Table 5: | Reduction of acceleration amplitudes |
Compared to room temperature, acceleration amplitudes declined in the range of 23.5-57.1%, 41.2-68.8% and 52.9-85.7%, respectively for preheating temperature of 315, 450 and 650°C. So, it is apparent to diminish vibration in end milling of titanium alloy, thermally-assisted machining can play a vital role. Apart from many other benefits, reduced vibration/chatter during cutting substantially reduces the bouncing effects on the tool tips and consequently, reduces the tool wear rates. Fig. 3 shows the effects on preheating temperature on the reduction of acceleration amplitude during preheated machining with PCD inserts. It may be observed that preheating considerably reduces the maximum acceleration amplitude during cutting.
| Fig. 5(a-c): | Cross section of chips produced in various runs at temperature (a) Room temperature, (b) 450°C and (c) 650°C for the same cutting condition |
The effects of preheating temperature on chip shrinkage coefficients are presented in Fig. 4. With the increment of temperature, the length of chips tends to increase which reduces the chip shrinkage coefficient leading to the formation of stable and thinner chips, as shown in Fig. 5.
| Fig. 6: | Peak to valley ratio vs. preheating temperature |
| Fig. 7: | Chip serration frequency vs. preheating temperature |
With the increment of temperature both the chip shrinkage coefficient and the primary chip serration frequency are found to be reduced. The peaks to valley ratio of the chips are found to be lower with preheating, as shown in Fig. 6. which has also been demonstrated in the plot in Fig. 7, which states chip serration frequency as a function of temperature. It indicates that preheating helps in reducing serration of the chip and leads to the formation of almost continuous chip. This helps in reducing chatter during preheated machining and eventually facilitating lower tool wear.
CONCLUSION
Form the present study, the following conclusion have been made:
| • | Suppression of vibration/chatter is exposed as another benefit of applying high frequency induction heating in end milling of titanium alloy Ti-6Al-4V. For instance, reduced acceleration amplitude of vibration ranging from 53 to 86% was detected during preheated machining at 650°C with PCD inserts. These reductions substantially lower the bouncing effects acting on the cutting tools and thus reduce the tool wear rates. A reduction of vibration/chatter also contributes to a decreased surface roughness, i.e., higher surface finish |
| • | It is observed that an increase in preheating temperature results in reduction of chip shrinkage coefficient and primary chip serration frequency. Furthermore, thermal-assisted machining also contributes to longer, thinner and continuous chip which is attributable to lower vibration and cutting force |
ACKNOWLEDGMENTS
The authors wish to thank the Ministry of Science, Technology and Innovation (MOSTI) Malaysia for their financial support to the above project through the e-Science Fund Project (Project No. 03-01-08-SF0001) and the Research Management Centre IIUM for overall management of the project.