Evaluating the Machinability of Titanium Alloys, Timetal 407 and Ti-6Al-4V

An industry focussed research project has been undertaken to evaluate the machinability of an emerging titanium alloy, Timetal 407 and offer a fundamental understanding for the chip control issues. This research identified two key stages in the life cycle of a titanium alloy where consideration of the machinability would provide economic opportunities for the aerospace manufacturing sector. First in the development of emerging alloys and secondly in optimising the cutting parameters during the machining of aerospace components.
To assess machinability in the developmental stage of an emerging alloy, a series of small scale experiments were utilised to assess the machinability outputs for Timetal 407 and Ti-6Al-4V, with the aim of drawing conclusions about each of the alloys limitations and strengths in regards to key machinability characteristics; tool wear, chip control, subsurface deformation and cutting forces. The key to these tests was to remain low cost, offering a quick turnaround of results relative to high speed machining, in order to filter out a large number of alloy chemistries and microstructural combinations.
For the second stage, machinability maps for Timetal 407 and Ti-6Al-4V were developed using data from high speed machining operations which determined for a range of cutting parameters (cutting speed and feed rate) where machinability outputs were acceptable and Material Removal Rate (MRR) was maximised. The machinability map again incorporated the key machinability characteristics: tool wear, chip control and subsurface deformation as primary points of consideration on the map, with cutting force as a secondary consideration. The map also identified a region of cutting parameters where improved chip management of Timetal 407 could be achieved. Using the high speed machining and small scale machinability experiments, mechanical twins and thermoplastic instability were identified as key determinants for chip control.
The outcomes of this research were an ability to characterise machinability using a small scale experimental framework, as well as fulfilling the industry requirements through improving chip management. The improvement of chip management had a number of concurrent benefits to manufacturing operations of large structural gas turbine engine components through reducing cutting time by 43% between Ti-6Al-4V and Timetal 407, whilst also reducing cleaning downtime between parts and reducing machine maintenance to fix issues with chip swarf conveyors. The risk of component rework due to tool breakage or surface finish defects have been reduced, offering potential savings of 300 minutes per part for ‘on machine’ rework, or 480 minutes per part for ‘hand dressing’ by an operator. Due to these operational improvements, the flow of parts through manufacturing facilities has been improved, reducing inventory stock and work in progress.