To achieve greater productivity, titanium alloy requires cutting
at higher speeds (above 100 m min
Titanium alloys have well-known applications that include automotive,
aerospace as well as biomedical implants owing to their excellent
properties, higher strength to density ratio and remarkable corrosion
resistance (Arrazola et al., 2009; Ezugwu et al., 2003). Unalloyed
titanium that has
Comparison of room temperature properties of aerospace alloys (Jaffery et al., 2013; Jaffery and Mativenga, 2012; Hughes et al., 2006).
In recent years, special attention has been paid by several researchers to overcome major difficulties in cutting titanium alloy due to growing production demands. These efforts include the improvement of cutting tools using coating materials (Jaffery and Mativenga, 2012; Li et al., 2018a; Kuram, 2018; Kumar et al., 2018), identification of the most favorable process parameters for wear minimization (Li et al., 2018b; Revuru et al., 2018), optimization of multiple responses (Gupta et al., 2018) and improving the existing tool design as well as use of cryogenic machining (Shokrani et al., 2018). One of the prime challenges also includes a reduction in energy demands during machining titanium alloy. Material characteristics and the process parameters used in cutting operation influences the energy consumed during machining (Balogun and Mativenga, 2014). Therefore, researchers have focused on different strategies to minimize the energy consumption during machining that includes the selection of optimum process parameters or the development of effective numerical models for machine tools (Warsi et al., 2018a, b).
Machining superalloy like titanium and nickel alloy continued as a topic of
interest for industries and researchers all together. Since titanium alloy
sustain their strength at elevated temperatures due to which temperature at
the chip-tool interface can reach up to 1000
Strategies targeted for energy minimization have also gained a lot of interests in recent years and several studies have been reported for energy analysis in various machining processes. Some notable contributions for energy modeling and cost reduction as well as sustainable manufacturing strategies for machining can be found in following literature (Mativenga et al., 2011; Behrendt et al., 2012; Kara and Li, 2011). The Specific consumption energy (SCE), which is the energy consumed in removing a unit volume of the material, can also be controlled by machinist through careful selection of the process parameters as it depends on the machinability of material and the cutting conditions during operation (Balogun et al., 2015). As the energy consumption during machining hard materials such as titanium is also affected by the tool wear, this research focuses on the effect of the tool flank wear progression on the SCE during dry turning Ti-6Al-4V in different cutting situations. Wear map published by Jaffery and Mativenga (2009) was used to identify cutting parameters in three important zones low, moderate and high tool wear zone. These conditions were used to evaluate the effect of increasing tool wear on the SCE in turning Ti-6AL-4V titanium alloy.
Titanium alloy machining has not been investigated in term of energy consumption like other engineering materials. In case of soft materials like aluminum, it has been reported that the tool wear is negligible and hence the machining process does not affect the SCE consumed during the actual cutting process (Warsi et al., 2018b). For the case of titanium alloy, the tool wear severely affects the energy consumed for the actual cutting operation. Therefore, current research is carried out to know the impact of flank wear growth on specific cutting energy in turning carried out for titanium alloy (Ti-6AL-4V), as the energy consumed is affected by the tool blunting and wear continuously. As a final point, knowing the energy demands of difficult to cut materials like titanium require such study as it is helpful to identify the right combination of feed and speed that reduces the SCE and promote economic machining. The energy measured at the beginning of the process and the end of useful tool life will be different due to wear growth, which will also produce variations in SCE as well as the surface roughness of the part. The study is meaningful as it will also provide a strong basis for producing alternative tool material as well as increasing the machining efficiency. This work presented here has identified the correlation between wear and SCE in turning Ti-6AL-4V at different machining conditions.
Titanium Ti-6Al-4V bar was used in this study as the workpiece material
having length 300 mm and diameter 70 mm. The mechanical properties and
chemical composition of the workpiece material are given in Table 1
(Hughes et al., 2006; Jaffery et al., 2016) and Table 2. Uncoated CCMW 09
T3 04 H13 cutting inserts (produced by SANDVIK) without chip breaker having
0 rake angle were used to perform turning experiments. Computer numerical
control (CNC) Turning Center ML-300 manufactured by YIDA Precision Machinery
Co., Ltd, having spindle power 15 kW and total power of 26 KW was used to
carry out dry turning. The maximum spindle speed of 3400 rev min
Composition (wt %) of Ti-6Al-4V alloy used in experiments.
The cutting power during machining was measured using clamp-on meter
Yokogawa CW 240. This power analyzer is capable of measuring power, voltage,
current and power factor of the CNC machine and can record it for an
interval of up to 0.1 s. The measurements were done in two-cycle;
recording air cut power (
The recorded actual and air cut power to calculate cutting power at
cutting condition (
Flank wear measurement using an optical microscope.
Wear map adopted from Jaffery and Mativenga (2009) with cutting conditions for study highlighted.
Air cut energy was recorded when the machine tool was not involved in cutting and the machine is ready to perform cutting operation with all its components electrically energized. Whereas, actual cutting energy recorded when the tool workpiece was engaged in cutting operation. The methodology has also been used by previous researchers (Warsi et al., 2018b; Li and Kara, 2011) for estimating SCE.
Wear of the inserts was analyzed and measured according to ISO standard
3685; standard for tool life testing (ISO, 1993). The rejection
criteria for cutting tools during the experiments remained as localized
flank wear
The experimental setup used for turning operation and the SCE measurement.
Energy zones during machining titanium (Ti-6Al-4V) alloy at speed,
Average flank wear progression vs. cutting length.
Process parameters selected from the maps.
SEM Micrographs of thr inserts showing wear at different length of cuts.
The cutting conditions were selected from wear map published previously by
Jaffery and Mativenga (2009), including cutting
condition from the high, moderate and low wear region as highlighted in
Fig. 3. These machining conditions are presented in Table 3. The selected
cutting conditions from the map are well in agreement with the tool
manufacturer recommended conditions (
Variation in the SCE due to increase in the wear at different
cutting lengths for (low wear region conditions);
SEM image showing the adhesion of material on rake face at low
speed (
The energy recorded was analyzed for each experimental condition and three important zones on the cutting power-time graph were observed as shown in Fig. 5. The first zone corresponds to the start of the machining cycle where tool-workpiece interaction occurs and thus produces variation in the power values recorded due to chattering at the start of cutting (Tool-work-piece interaction zone). The second zone corresponds to the stable energy zone where the tool- workpiece interaction occurs smoothly without fluctuation in power recorded (stable zone). The third zone represents the region in which the increased power fluctuation is observed as the tool wear starts progressing along the flank face till the end of useful tool life. In order to achieve greater results, machining in this zone must be avoided as the power demands increase exponentially in the wear progression zone. The study of energy consumption in the wear progression zone is very necessary as it affects the power demands required during machining as well as the integrity of the final produced work part. The results have been presented for the first time (turning Ti-6Al-4V alloy) to the knowledge of the author, therefore, machining at such severe condition must be carefully carried out to achieve sustainable manufacturing while maximizing productivity.
Variation in the SCE with the increase in the tool wear at
different cutting lengths for (moderate wear region conditions);
Variation in the SCE with the increase in the tool wear at
different cutting lengths for (high wear region conditions);
For each machining condition, the tool state and wear were monitored after a
specified length of cut and analyzed using an optical microscope. Similarly,
the SCE was also measured for each step of the experimental condition
performed. The tool flank wear progression observed for eight selected
machining conditions is presented in Fig. 6, indicating the evolution of
the tool flank wear progression (
The progressive tool wear was analyzed for all conditions by considering the variation in progressive SCE in dry turning Ti-6Al-4V alloy. The results for the tool wear progression for the condition in low wear zone (low speed) was observed to produce less variation in the SCE. At low speed and feed rates, the MRR is low which results in less worn out tool and thus produces little variation in the SCE consumed as shown in Fig. 7a, b. In case of uncoated carbide tools at low cutting speed, a layer of adhesive material is formed on the tool face which protects it from wear out (Fan et al., 2016). This is also confirmed by the SEM image of the tool when used for low-speed cutting Fig. 8.
Temperature variation with increasing the cutting speeds (Fan et al., 2016).
Figure 9a, b the conditions were selected from moderate wear zone and thus cutting operations resulted in moderate progression of the flank wear. However, the change in the SCE values was noticeable which could be attributed to the higher wear rate due to adhesion wear. The wear and SCE in condition 8 show a high values as the feed rate is higher than condition 3.
The change in the wear and SCE resulted from cutting conditions in the high
tool wear zone (high-speed range) are presented in Fig. 10a–d.
Condition 1 and 2 show the highest rate for both wear and energy among the
selected conditions this is because of shear localization and strong
adhesion of the material near edge resulting in localized temperature zone
at high speed (Dudzinski et al., 2002). This increase in
flank wear during cutting with an increase in velocity and feed has overly
been reported by earlier researchers (Venugopal et al., 2007b; Kaplan et
al., 2018). The higher wear rate and hence the variation in SCE in condition
2 (
When the speed is high the tool is subjected to high thermal stresses which result in poor cutting performance of the cutting edge. The increase in temperature with cutting speed is shown in Fig. 11 (adopted from Fan et al., 2016).
The wear progression and SCE with machining time at two cutting condition
are shown in Fig. 12. The tool life observed is significantly high in
Condition 6 (
The variation in flank wear and SCE with an increase in cutting speed (Cond. 1 and Cond. 6).
The SCE has a direct correlation with the flank wear, this can be seen from Fig. 13. There is a significant increase in the values of SCE as the wear increases beyond the acceptable limit (0.3 mm) and indicates a need to change the tool. Therefore, this work carries great importance to use SCE as a method for relating the tool wear and administering the change of tooling when needed. The tool flank wear can also be modeled in terms of SCE as shown in the Fig. 13. Using these simplified relationships machinist can predict the wear and/or SCE, thereby, optimizing the energy consumption demand during machining operations. The high-speed machining requires further investigation using the different coated tool as well as also provides the basis for the development of new tool material. There is also a case of using different cutting environments (wet, cryogenic and MQL) to study the tool wear progression and its influence on SCE.
SCE at different flank wear in turning Ti-6Al-4V;
The research work presented here relates the tool wear and the SCE at high, moderate and low wear condition. Tool wear is a major reason for the variation in SCE, as the machining process progresses. The wear is accelerated due to the increase in speed and the feed rate, noticeably reducing the life of the tool. When the cutting speed is higher, a high SCE was observed which is related to an abrupt increase in the tool flank wear because of elevated temperature at the tool and chip edge. However, for lower cutting speed the SCE was low and thus tools can be used for longer cuts that lead to higher machining time. To accomplish sustainable machining goal with efficient resources, processes must be carried out at low SCE values which will also ensure low wear of the tool at that point. Additionally, SCE and wear can be related to a machining condition using simplified relationships. The results stated here can also be used in selecting machining conditions which help to improve the tool life and in minimizing the SCE utilization in dry turning of titanium using H13 inserts. The authors are currently investigating the phenomenon of tool flank wear and energy using different coolants as the high-speed machining of such alloy must be investigated for improved results.
No data sets were used in this article.
MY and MK designed experiments, AK helped in developing methodology and graphs plotting. MY and ZK performed experiments supervised by SHIJ on the CNC machine and characterization of tools. LA and RA reviewed drafted paper, suggested amendments and guidance during revisions.
The authors declare that they have no conflict of interest.
This paper was edited by Xichun Luo and reviewed by two anonymous referees.