Machining Science

Low “buy-to-fly” ratios of titanium and nickel-based superalloys in aerospace make machining science a key strategic research area aiming at improving machining speeds and increasing tool life while decreasing workpiece surface damage.

With organisations such as the Air Transport Action Group (with members including Rolls Royce, Airbus and GE) issuing targets such as a 1.5% reduction in emissions per annum between now and 2020, means an increasing demand for aircraft efficiency.

Targets such as these have meant there has never been a higher demand for Titanium and Nickel Superalloy components within aerospace design. Although novel processes are in development looking at producing near net shape components, conventional material removal processes are just as relevant in modern manufacturing.

With strong links to the Advanced Manufacturing Research Centre (AMRC) machining science is a key area of research for the STAR group. Companies such as Safran (formally Messier Bugatti Dowty), Sandvik Coromant and Rolls Royce provide funding for specific fundamental research. Titanium and Nickel Superalloy materials have been categorised as difficult to machine materials due to the high mechanical resilience provided by the chemistry of modern alloys such as Ti-5553 and RR1000 Ni Superalloys.

The increasing use of these alloys has led to drive towards a holistic approach to conventional machining research covering low to intermediate TRL levels aiming to capture a lab scale scientific understanding of large scale industrial machining operations. From the characterisation of material deformation during machining, chemical and process variation to process optimisation on industrial scale components, the STAR group is actively working toward a better understanding of the continuously evolving machining process.

The STAR machining research team consists of post-graduate and post-doctoral researchers from a wide variety of fields including Materials science, mechanical engineering and chemistry. The groups Capabilities and previous activities include Microstructural characterisation, mechanical property evaluation, lab scale orthogonal cutting trials and full scale machinability testing.

Design

The plateau being reached for current alloys has led to concerted efforts by industry to develop new competing alloys. The aim is to develop materials with comparable or improved mechanical properties, but which possess characteristics that permit improvements in machining conditions, through increased tool life or cutting speeds. The importance of this is the significant cost involved in the machining of high performance components, where machining costs typically account for 40%, but as high as 60%, of a components overall cost.

An example of this improvement is the development of Ti-54M®, developed by TIMET UK Ltd., which posses comparable mechanical properties to Ti-6Al-4V but possesses a 10% improved in cutting speed. Understanding the reason for this improvement in machining performance can lead to the development of the next generation of high performance titanium alloys, which posses superior machining performance.

Further developments in the area of small scale testing, in the form of the plain strain machining (PSM) test have occurred. Due to the significant contribution of machining to component cost it is significantly advantageous to understand the likely machining performance of new alloys during the initial alloy characterisation stage, as opposed to currently when machining performance is only considered once significant time and resources have been committed to the production of a full billet. The PSM test requires 0.1% of the material requirements of a industrial machining trial and offers >95% in cost saving.

Development

During machining of titanium alloys, the intimate contact between the workpiece and tool at temperatures above 800°C provide a high thermodynamic driving force for diffusion of tool material atoms across the tool-workpiece interface. This theory is also valid for diffusion of workpiece atoms into the tool.

In titanium alloy machining; tool wear has been shown to be a combined effect of abrasion, plastic deformation, adhesion, and chemical reaction between the workpiece and cutting tool. It has been observed that mechanical wear dominates at low cutting speeds while chemical reactivity between the tool and workpiece plays a more critical role at higher speeds.

At higher cutting speeds, the relative contribution of chemical wear to the total wear increases exponentially, since the solubility and diffusivity follow an Arrhenius type of relationship with temperature.

Despite new coating technologies currently being developed to aid titanium machining, customers are still using the same 1930s technology of uncoated straight grade carbide tools for longer tool life and enhanced workpiece surface integrity. This same tool is used for all titanium alloys irrespective of phase morphology or chemistry.

This is despite the fact that different classes of titanium alloys exhibit markedly different machining characteristics. Many researchers recognise the important role of chemical wear during high speed machining operations but fail to investigate these reaction mechanisms prior to costly machining trials.

More recently, some researchers have attempted to replicate the machining environment with a variety of different diffusion couple methods. However, there is currently no recognised standard diffusion couple test which is reliable, informative and indicative of an industrial machining trial.

This research investigates a novel diffusion bonding method which, when coupled with thermodynamic modelling can provide a strong indication of the complex reaction mechanisms at the tool-workpiece interface.

This work will help the machining community to understand tool wear mechanisms at new levels of detail. Furthermore, it could play a role in enhancing tool grade development for increased efficiency in titanium machining.

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