Investigation of High-Speed Milling and High Efficiency Milling of Ti6Al4V

— Ti6Al4V a Titanium alloy, is one of the prominent materials used in spacecraft components and it is a difficult to machine material. In this paper and experimental investigation is carried out to compare High Efficiency Milling (HEM) and High-Speed Milling (HSM) of Ti6Al4V. The MRR, surface roughness and tool wear were determined from the investigation. From the work it was found out that the MRR for HEM was less than that of HSM, while surface roughness variation was less and no significant tool wear was observed for HEM strategy.


I. INTRODUCTION
Titanium alloys constitute one of the most important materials in spacecraft because of its high specific strength and good corrosion resistance. Several spacecraft mechanism components are made of Titanium alloy Ti6Al4V. As machining is the most apt manufacturing strategy in realizing the spacecraft titanium parts, owing to the high dimensional and geometric tolerances called for, it is majorly used for the realization of the parts. Of various machining operations, major bulk of the workpiece material is removed using milling operations.
As Titanium is very difficult to machine material due to its low thermal conductivity and chemical reactivity, only limited cutting tool materials can be used. Also due to the poor machinability, cutting tools experience tool wear leading to reduced tool life. Hence, it calls for exploration of advanced machining technologies like High Speed & High Efficiency Machining etc. King, R. I (1985) discussed the history of High Speed Machining initially proposed by Salomon and its relevance to aircraft structures made of Aluminium. Ippolito, R et al. (1988) conducted the High-Speed turning tests for steel with ceramic tool to study the effects of machining parameters on surface finish, tool life, chip formation. Schulz, H., & Moriwaki, T. (1992) reviewed the key developments in high-speed machining and related fields like cutting tools and machine tools and mentioned more than fifty percent reduction in time is achievable. Highspeed machining of Aluminium aircraft structures, titanium fan blades and hardened steel dies was presented in Tlusty, J.  2016) developed an average surface roughness (Ra) model for milling of Ti-6Al-4V alloy using Carbide inserts tooling. Responsive Surface Methodology (RSM) is used to arrive at a relation between Ra and machining parameters. Their findings showed that the depth of cut is the most influencing parameter on surface roughness in high-speed machining range of Ti-6Al-4V while cutting speed and feed rate does not have a notable effect.

III. HIGH EFFICIENCY MACHINING
High-speed machining involves high cutting speeds and low feeds per tooth, leading to extremely short times of contact between workpiece and tool, very high frequencies of contact and high cutting temperatures [1]- [17]. The HSM calls for totally different tool design concentrating mainly on the insert type tools, wherein only the limited height of the tool is utilized for the machining. To efficiently utilize the entire tool length, new machining strategy was developed known as High Efficient Machining (HEM), which calls for different tool design, machine tool architecture and machining strategies. From [1]- [23] it was observed that, very limited work pertaining to comparative study of High-speed machining versus High Efficient Machining of aerospace components, has been done. Since Ti6Al4V alloy is the most commonly used Titanium alloy in spacecraft components and it is a difficult to machine material, exploration of advanced machining strategies especially milling is required. To the best of the authors knowledge and from the literature survey, no work was done to study the High Speed Milling (HSM) and High Efficiency Milling (HEM) of Ti6Al4V for spacecraft components. Hence it is proposed in this work to carry out the experimental investigation of the same for Ti6Al4V alloy.

IV. METHODOLOGY a. Work Material
Since Titanium components are difficult to machine and they are found in various spacecraft subassemblies, Ti6Al4V alloy was selected for the experiment as it is the most widely used alloy in spacecraft components. The composition and properties of Ti6Al4V are given in Table  1 & Table 2 and HEM is carried out on Ti6Al4V. The composition and properties of Ti6Al4V are given in Table  1 & Table 2.   Thermal Conductivity (W/(m-K)) 17 Linear Thermal Expansion Coefficient (10 -6 K -1 ) 9

b. Geometry of test Part
A block of 110mm length, 110mm width and 37.5mm height was used for carrying out milling experiments. The features which are generally encountered in the spacecraft components were considered while arriving at the internal topology of the sample piece. The CAD model of the sample piece is given in Fig 1.

c. Machine tool and Cutting Tools
All experiments were conducted on DMC 650V vertical CNC Milling machine with a maximum spindle speed of 20,000 rpm. A CERATIZIT Indexable cutter with 20mm diameter with TiN-TiB2 (Titanium Nitride -Titanium Boride Coated Inserts) were used for HSM experiment and CERATIZIT TiSiN coated solid carbide end mill cutter with 20mm diameter was specially used for HEM experiment. These tools were used mainly for roughing operations.

d. Experimental Procedure
The experiment was conducted by performing the CNC Milling operation on the workpiece material to achieve the final component as per the CAD model in Fig 1. The toolpaths for HSM and HEM were generated in UG NX and POWERMILL software respectively. Sample toolpath for HSM is given in Fig 3. The cutting speed range for HSM were presented in Schulz, H., &Moriwaki, T. (1992) and same were used to calculate cutting speed for the experiment. The cutting parameters employed in this investigation for HSM were listed as follows: Cutting speed Vc=113 m/min (correspondingly, the spindle speed N was 1800 rpm), feed fz=0.093 mm/tooth (correspondingly, the feed rate for three flute cutter was 502 mm/min), axial depth of cut ap=0.5mm and radial depth of cut ae=6mm (30% of 20mm diameter cutter).
The methodology to select the cutting parameters for HEM are elucidated in [23] and same were considered for fixing the HEM cutting parameters. The cutting parameters for HEM were listed as follows: Cutting speed Vc=25.13 m/min (correspondingly, the spindle speed N was 400rpm), feed fz=0.0625 mm/tooth (correspondingly, the feed rate for two flute cutter was 100 mm/min), axial depth of cut ap=20 mm and radial depth of cut ae=1mm (5% of 20mm diameter cutter).
Before arriving at the cutting parameters several trials were done on sample workpieces and finally above mentioned cutting parameters were finalized. The milling was carried out up to 20mm depth as per CAD model. The MRR for actual machining operation were determined for both HSM and HEM and results of same are illustrated in Fig 6. It is inferred from the graph that, MRR for HEM is around 7% less than that of HSM. This may be because of the very less radial depth of cut, leading to longer toolpath.

b. Surface Roughness (SR)
The surface roughness was measured on both walls and floor, and maximum Ra value is reported in the work and results are given in Fig 7. From the results it was observed that SR value on floor for HEM was less than that of HSM while SR values on walls was considerably high for HSM than HEM. This may be due to gradual wear of the tool. in Fig 8 and Fig 9 respectively. For HSM flank wear of 0.677mm on lengthwise and 0.562mm on width of the flank was observed. However, for HEM no flank wear was observed but loss of coating was observed for both length and width of flak wear. The increased axial depth of cut with less speeds and feeds compared to HSM may be the cause of coating loss on tool and absence of tool wear.

c. Chip Morphology
In HSM, the chips are shorter and curly, as the cutting edge in contact with the metal during machining is short, due to the low depth of cut. In HEM, as cutting edge of the tool is utilized to its optimum cutting length, the length of the chips is more, due to more depth of cut.

VI. CONCLUSION
It can be inferred from the experiment conducted that, there is slight decrease in MRR for HEM than HSM. However, HEM was instrumental in generating better surface finish and lesser tool wear compared to HSM. It can be seen that for Ti6Al4V, HEM seems to be more promising than HSM. However, owing to several factors, the trade-off between HSM and HEM while machining spacecraft components would be a suitable option, depending upon the component than choosing only one method.