A286 and INCONEL 718 are materials that, due to their high Ni content and heat resistance, are very difficult to machine, and this results in the high cost of parts manufactured with these materials. The results of this process will open the way for more cost-effective production processes for aerospace parts. TiAl is a good candidate material for future aerospace applications, due to its low weight and good resistance at high temperatures. However, its low machinability (10% of that of Ni alloys) makes the production costs very high for many applications. A reduction of the TiAl intermetallic machining cost will open up the possibility of the design and application of new parts.
The partners of this project wanted to develop new machining tools, nanocomposite coatings and machining processes to address the following industrial objectives:
- tool life increase;
- reduction of production costs;
- increase of machining productivity (more advanced cutting parameters);
- optimal coolant utilisation, and
- improvement of finishing quality.
All these objectives were related to the machining processes of heat resistant alloys used in aerospace applications. The selection of materials will be focused on the following:
- Fe-Ni alloys: A286;
- Ni based materials: INCONEL 718, IN 100;
- Intermetallics: g-TiAl;
- Reduction by more than 50% of the process costs where the tools have been coated and new cutting technologies have been applied;
- Increase by more than 100% in machining efficiency of milling Fe-Ni, Ni alloys and g-TiAl;
- Increase by more than 100% in machining efficiency of turning Fe-Ni, Ni alloys and g-TiAl;
- Increase by more than 50% in machining efficiency of drilling Fe-Ni, Ni alloys and g-TiAl.
The term 'machining efficiency' is directly related to production costs, and takes into account parameters such as tool life, machining speed and tool cost.
The project objectives were addressed by the development of new tool geometries, hard and low-friction nanocomposite coatings produced by physical vapour deposition (PVD) methods and new machining processes (high pressure cooling).
At a first stage, the machining shops and end-users collaborated with the R&D centres, and the tools and coating producers, in defining which machining processes and tools were used to evaluate the new developments. Three demonstrators (one from each material family) were defined, as they were used at the end of the project to evaluate the performance of the new tools, coatings and machining processes. With the defined characteristics of the tools and their problems, the possibility of addressing new tool designs were evaluated, taking into account that these tools will be coated.
New nanocomposite coatings were developed, considering the special requirements specified by the end-users and machining shops. There were mainly three lines of the research. The system AlTiSiN was be optimised to cope with the demands of hardness and friction. An approach was also made to other compositions, where the Ti was substituted by other elements that had shown better friction behaviour, or where low-friction phases were added.
The developed nanocomposite coatings were tested in the laboratory facilities. The coatings were applied on standard and newly developed tools, and the machining parameters that show the best behaviour of the tool from an efficiency point of view were investigated. The tools were also analysed to determine the failure modes, and this analysis acted as input for coating optimisation. At this point, optical microscopes, scanning electron microscopes (SEM), profilometry and roughness measurement devices, and metallography facilities were used. Furthermore, advanced machining processes, like high pressure cooling machining, were tested on the coated tools to increase even more efficiency (tool lifetime, machining speed, etc). The tests started with the Fe-Ni and the Ni alloys, and further addressed the machining process for the g-TiAl intermetallic.
The third leg of the testing table consisted of real production tests performed at the machining shops. The results of these tests provided feedback and helped the optimisation of both coatings and machining parameters. The results were evaluated in the production of real parts (demonstrators), one of each from the material families selected.
The main achievements of this project have been:
- Improved understanding of the nanocomposite coatings fundamentals mainly focussed on the effect of impurities (e.g. Oxygen) in the coating properties and the importance of having 1 monolayer of interfacial Si3N4.
- Development of new nanocomposite coatings that improve machining processes:
- Machining processes are complex, and during this project the optimization had to be focussed on many aspects, and the right combination of all of them resulted in the optimum cutting procedures:
- Tool design: tool geometry.
- Tool substrate: a compromise of a hard carbide able to machine difficult to cut materials but with a good toughness value to avoid cracks and brittle tool failure.
- Surface preparation: This improves the tool cutting performance by optimizing edge geometry and reducing stress.
- Coating: That increases surface hardness, reduces friction or improves thermal performance of the tools.
- This has resulted in the development of new high performance tools:
- Rough mill for Inconel and A286 alloys
- Rough and finishing mills for IN100
- Drill for Inconel
- Tool life of drills for Inconel has been increased over 50% by using high pressure cooling, with pressures up to 120 bar.
- Increase of tool life by changing the wear mechanism from chipping (catastrophic) to continuous wear. This makes tool life much more predictable and decreases the risk of unexpected failure (which can cause severe damage in very expensive components).
The benefits are related to the increase in productivity, i.e. reduction of machining time with an additional contribution in tool consumption saving. This will be achieved by the new developments allowing for more advanced machining conditions (tougher and quicker) and controlled tool wear development.