Intermetallics inspire Metron Additive Engineering’s business direction
Aerospace adoption of titanium aluminide has opened up the field for novel techniques in manufacturing. Metron CEO, Dimitris Katsanis, spoke to Materials World about his work with intermetallics.
Design and manufacturing company Metron Additive Engineering, Ilkeston, UK, is following in the footsteps of GE in the use of titanium aluminide (TiAl) as a lightweight and heat-resistant material. GE has used the intermetallic material in low-pressure turbine blades on its GEnx engine, which powers the Boeing 787 and Boeing 747-8 aircraft.
The material’s brittle nature at room temperature can make it difficult to work with, but this changed with the electron beam melting process (EBM), which made it easier to manage. Inspired by the material’s success in aerospace applications, Metron CEO and Founder, Dimitris Katsanis, has been exploring how to get the most out of the material, in the hope of bringing parts to market by the end of 2019.
Taking on intermetallics
Since the early 1990s, Katsanis has been working in manufacturing and established Metron in 1996. The company has designed bikes for the British cycling team since 2002, and is currently working on designs for the 2020 Tokyo Olympics.
But despite success in design consultancy, Katsanis was becoming aware of the benefits additive manufacturing (AM) could bring to the aerospace industry and in 2015 decided to pursue the development of new materials that would support the growth of this technology. At this point, GE had already found that using TiAl in its engines and switching from casting to EBM resulted in a much lower scrap rate, and therefore cost saving.
‘When you have an aero engine manufacturer switch from one manufacturing method to another, that was what told me there was something in that process. So that was very much the trigger,’ he said. Katsanis invested in an ARCAM EBM machine. Using this, the metal powder is subjected to a high-power electon beam that selectively melts the matter to create solid parts. Metron has since made more than 3,500 parts using the machine with a success rate of more than 95%. The company mostly works with titanium to manufacture high-temperature parts.
TiAl is about 48% aluminium, 48% titanium, 2% niobium and 2% chromium, and the intermetallic has some unusual properties. ‘Aluminium has a melting point of 650°C, titanium is about 1,700°C,’ Katsanis explained. ‘When you put the two together, the melting temperature of that is about 1,450°C. Despite the lower melting temperature, TiAl has a higher maximum use temperature of about 800°C whereas normal titanium can go to about 400°C, which is a bit unexpected.’
Metron Metallurgist, Ashfaq Mohammad, said the strength of intermetallic alloys at high temperatures tends to be as good as at room temperature. ‘This is made possible by the unique arrangement of titanium and aluminium atoms within the crystal structure of TiAl. Because there are no foreign aluminium atoms to pin down the atoms in pure titanium, the metal starts to “flow” as the temperature rises, which means the metal becomes softer.’
TiAl has a lower density than the go-to material Inconel – the oxidation and corrosion-resistant alloys commonly found in high-temperature parts. ‘TiAl can give you a performance very close to Inconel but at about half the density,’ he said, adding that, when used to manufacture components in high-performance equipment, such as in an aircraft or a machine that functions at a high RPM, low density properties are vital for both weight and energy savings.
However, while TiAl has a maximum operating temperature 200°C lower than Inconel, which is 1,000°C, it is still a viable option for aerospace. Jet engines operate at very high temperatures, but not every area runs so high, therefore the components in these sections need not withstand the same maximum temperature. For example, the low-pressure side of the turbine is a prime area for incorporating TiAl parts, to save weight and cost, as well as other applications such as exhaust engine valves for high-performance car engines and perhaps even rocket engine parts for satellite launch vehicles. TiAl’s reduction in density compared to Inconel is what Katsanis mostly appreciates.
As previously mentioned, TiAl is not an easy material to work with at room temperature. ‘It is difficult to machine but to form it is almost impossible. To cast it, the scrap rate is very high because when the material melts, it is not free-flowing. So common manufacturing processes are struggling to achieve results,’ Katsanis said.
To overcome the technical difficulties, Katsanis adopted the EBM process and found a great deal of success. ‘In the laser beam process, you have a layer of powder on a plate and the laser beam melts the area of the material you want in order to create a part, and then you create another layer and another,’ he said.
‘With the electron beam, a layer of powder is put down, but the whole surface area of the material is pre-heated. This causes some of the material to sinter and it raises the temperature of the working area. In the case of the titanium, it raises the temperature to around 700°C, but in the case of TiAl it is around 1,000°C.’
When using a laser process, the part needs to be taken out of the machine still on the plate and put into a furnace to carry out the heat treatment due to the high degree of residual stresses on the material. This is not the case with EBM. Katsanis explained, ‘As you are making the part, you are also annealing the material, so when you take the part out of the machine you have almost no residual stresses. The benefit of this is you do not have to do heat treatment afterwards.
‘This is because when you melt the material, it cools down very quickly – within milliseconds it will drop in temperature to around 200°C. In the case of using EBM, you have a constant high temperature of around where you are processing it, which makes it a much slower cooling down process. This is beneficial for TiAl because of the brittleness at or near room temperature.
‘If you tried to do this on a normal laser machine, the temperature difference would create enormous internal stresses and the material would crack, which has been proven. With EBM, we are annealing as we go along, and because of the high-temperature environment, we can work more easily with more brittle materials because the cooling rate is slower so the component does not crack as it cools down.’ Katsanis said that, even considering the slower cooling time, because the electron beam gun is more powerful than most lasers and forms thicker layers with each pass, the production time is still faster.
EBM is proving a cost-effective way to manufacture TiAl intermetallic parts that are precision-made and incur less time, energy and labour, due to cutting down stages including heat treatment and tooling. Current limitations include the amount of metal powder held in the machine at any one time, the speed at which the beam moves to heat the material, the proximity of one melt path to the next, the temperature of the pre-heat and how long it takes each layer to set. However, the technique is still evolving and Metron is working on expanding the factors restricting mass manufacturing of components, to help expand its adoption in the aerospace as well as the automotive industry.