Economical Comparison of Cryogenic vs. Traditional Turning of Ti-6Al-4V: A Case Study

Abstract

Due to their poor machinability, titanium alloys need to be worked at low cutting speeds to prevent a fast tool failure caused by the very high temperatures that are reached at the tool-chip interface. As demonstrated by previous works by the authors, an improvement of productivity for Titanium alloys can be obtained by adopting cryogenic cooling during the machining operations.The present work shows the features of a toolholder specifically designed for cryogenic adduction in turning operations, following Hong’s design guidelines. The paper compares tool life results between traditional and cryogenic rough turning by adopting Grade 5 titanium as the working material. Rough turning is economically more relevant to the machining industry, especially in the aerospace field where generally a large quantity of rough material has to be removed due to the very high buy-to-fly ratio of aerospace components. A full factorial experimental plan was performed basing on typical rough turning parameters. Machining outputs such as forces, roughness, temperatures, friction coefficients were calculated in order to define statistical differences between cryogenic cooling method using the special toolholder and traditional oil water emulsion cooling system.Furthermore, thanks to tool life results the Taylor’s law for cryogenic and traditional cases was calculated and an hypothetical production scenario for Ti6Al4V parts was analysed. An analytical model to calculate production costs and time was built for both cooling methods. A 4-turrets turning centre was considered and cooling methods and costs per hour of machine tool were taken into account in a cost model. The results show that the benefits in terms of tool life offered by liquid nitrogen cooling allows to improve productivity by adopting higher optimal cutting parameters. This improvement, coupled to an increase of tool life, is very significant and allows not only to reduce time of production but also to cover the major costs of liquid nitrogen and have a slight reduction of machining total costs.

Additive manufacturing and postprocessing of Ti-6Al-4V for superior mechanical properties

“…14 -16 These discrepancies can be partially attributed to the different compositions of Ti-6Al-4V used. 5 The borderline cooling rate for the diffusion-controlled β → α transformation is below about 20°C/s. 14 -16 Above 20°C/s, either the composition-invariant β → α m massive transformation occurs, where diffusional activity occurs mainly at the advancing interfaces, 25 or the composition-invariant diffusionless β → α ′ martensitic transformation occurs.…”

Influence of single and dual cryogenic jets on machinability characteristics in turning of Ti-6Al-4V

“…11 At the same time, benefits like clean and neat machining environment, removal of produced chips from cutting zone due to high pressure of cryogenic jet, no smoke or fume, and no oil or liquid cutting fluid spattered over the surfaces are attained by the application of cryogenic coolant. 12 In addition, the cost-related issue has been reported by Tirelli et al, 13 where it has been shown that cryogenic cooling incurs reduced cost than traditional wet cooling and improves tool life which eventually increases the machining productivity. For these reasons, cryogenic cooling by LN 2 has presented itself as a tenable solution to conventional cutting fluids.…”

Solid-State Phase Transformations in Thermally Treated Ti–6Al–4V Alloy Fabricated via Laser Powder Bed Fusion

“…Titanium and its alloys are the most representative example, being advanced structural materials, characterized by outstanding properties such as high strength-to-weight ratios, fracture toughness, corrosion resistance (immunity to seawater), and biocompatibility, but also affected by major technological drawbacks in terms of machinability, such as the extreme reactivity and the low thermal conductivity [3,4]. Only low-speed machining of forged titanium has been successfully employed over history, resulting in more than half of the raw materials wasted as scrap [5,6].…”