Drillability of titanium alloy 6246

Darsin, Mahros
Pasang, Timotius
Chen, Zhan
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Doctor of Philosophy
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Auckland University of Technology

Titanium alloys are relatively new materials that have been used extensively, especially in aerospace applications. The main properties are: a high strength to weight ratio (three times that of steel), an ability to maintain strength at elevated temperatures, excellent corrosion resistance and biocompatibility. The latter makes these alloys suitable to be used for body implants, such as for teeth and bones. However, titanium alloys are not easy to machine. They tend to behave in a springy manner due to a low modulus (Young) ratio. Their low heat conductivity makes the heat that emerges from machining mostly absorbed by the tool. Consequently, the tool loses its ability to cut further. One titanium alloy, named as Ti-6Al-2Sn-4Zr-6Mo (or Ti-6246), which has better corrosion resistance than the most widely used Ti-6Al-4V, faces the same problem when coming into the machining workshop. Meanwhile, only limited research has been conducted on the Ti-6246 alloy relating to its machinability, such as turning, milling and slab milling. Some non-conventional machining processes have been tried, such as Electrical Discharge Drilling, and Wire-EDM. There are more advantages in attempting hybrid machining by using Ultrasonic Assisted Turning (UAT) though. However, to my knowledge, no paper has been published on drilling the Ti-6246 alloy. Drilling is a fairly simple process. However, the geometry of the drill bit or drill insert is very complicated. During drilling, a drill insert employs a different cutting speed from zero in the centre of the chisel to the maximum at the outer side (the rubbing side). This research will concentrate on the drillability of the Ti-6246 alloy. Drillability is derived from the machinability of the drilling process. It means an attempt to find the best parameters which result in the minimum force employed for drilling, with the longest tool life, whereby the minimum surface roughness and excellent roundness can be achieved. To reach the goals an extensive and systematic set of experiments will be carried out, while simultaneously the phenomena behind the process must be understood. Therefore, an understanding of microstructure and mechanical properties during the process will be studied in depth by utilising a comprehensive analysis of tools, forces, material and chips all together. The Taguchi L-18 method was used for designing the experiments. This was followed by an ANOVA analysis of data using Minitab 17 software, where the optimum characteristic of drillability has been achieved. However, combinations of different machining parameters and environments have been suggested due to the different criterion of the characteristics of drillability. As displayed by the chip formation the optimum result was achieved when keeping the Ti-6246 alloy in its received condition and applying a cutting speed of 50 m/min, and a feed rate 0.15 mm/rev for drilling 30 mm with a coolant. Consequently, in accordance with tool deterioration criterion, machining with a coolant, at 30 mm deep, a cutting speed of 50 m/min and a feed rate of 0.15 mm/min simultaneously, this would result in the least tool deterioration. Further, the optimal condition for drilling forces would be achieved when drilling with a cutting speed of 27 m/min, a feed rate of 0.08 m/rev on depth of only 10 mm without coolant, while the material should be HT1 treated. Lastly, the optimum surface roughness between 0.3113 µm to 0.5235 µm was achieved when drilling with these parameters: Cltoff, HTAR, h10, Vc35, Fr0.08.

Titanium alloy 6246 , Drillability , Microstructure , SEM , Taguchi method , ANOVA , Chips
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