3D printing: helping to shape the future of gas turbines

If there is a trend in technology that will revolutionise how we design gas turbines, it is advanced manufacturing," believes Nicolas Vortmeyer, Head of Technology and Innovation for Fossil Power Generation at Siemens, according to a recent article in Siemens' house magazine Living Energy. "This is something totally new, a real breakthrough...I strongly believe we're entering a new era of turbomachinery-based power generation."

Not so long ago 60% (LHV) combined cycle efficiency was seen as next big milestone, the "four minute mile", for gas turbine technology. But Siemens has achieved getting on for 61% with its all-air-cooled H class gas turbine technology, and now the talk is of 63% and beyond within the next ten years or so.

Advanced manufacturing technology, in particular additive manufacturing (aka 3D printing), is likely to play a key role.

In additive manufacturing 3D shapes are created by building up layers of material by, eg laser sintering - allowing levels of intricacy and precision to be achieved that are unattainable by traditional manufacturing methods such as machining. This can be applied to the complex air cooling paths in gas turbine blades, for example, providing valuable efficiency gains.

Selective laser melting

The process of selective laser melting (an example of additive manufacturing) is already being used by Siemens in commercial applications. Basically, a computer model of the component is used to render it as very thin sections. These sections are used as a template by a laser to melt (or "sinter") a 30-50 micron thick layer of powdered metal, so that it corresponds to the section. The process is repeated to create the component layer by layer.

It "allows us to realise geometries that can hardly be manufactured right now", says Nicholas Vortmeyer, opening up "new frontiers for heat transfer and structural integrity."

Within the Siemens industrial turbines business, development of selective laser melting/sintering techniques started in 2007 with the purchase of additive manufacturing technology and equipment from EOS, which, within a short time frame, proved able to adapt one of its machines to meet Siemens' specific needs.

An early successful application was in the manufacture of the 37 MW SGT-750 gas turbine, where it was employed to realise the advanced burner swirl concept. In fact the manufacturing of this component was only possible by selective laser melting. A very complex multi-element component was manufactured as one single component and the results achieved clearly confirmed the benefits of selective laser melting for the design, prototyping and manufacture of new advanced burner swirls.

But it is not only in the manufacture of new components that selective laser melting has proved its worth, it also has huge potential in the repair of components, enabling repair to be entertained as an option where previously replacement would have been recommended.

One example is the successful application of selective laser melting to the repair of burner tips for the 47 MW SGT-800 gas turbine. "Selective laser melting allows the new tip to be '3D printed' onto the old burner. This greatly simplifies and speeds up the repair, by a factor of about ten," says Vladimir Navrotsky, head of Technology & Innovation at Siemens Energy Service Oil & Gas and Industrial applications. "Also, the repaired components can be upgraded to the latest burner design."

Since 2013 burners both manufactured and repaired using selective laser melting have been successfully employed in commercially operating gas turbines.

One challenge for selective laser melting/sintering is providing assurance that the properties of the sintered materials are adequate for the operating conditions envisaged. Currently only a limited number of materials (eg, steel, aluminum, titanium) are used in the selective laser melting process. Its application to Ni-based materials is still under development.

For some laser sintered materials the properties have been found to be somewhere between castings and forgings.
Looking to the future, Willibald Fischer, head of product management for Siemens gas turbines, envisages the possibility that service teams could "show up at remote plants with their own 3D selective laser melting tools." Once it has been determined what repair is needed they "will be able to manufacture the part on-site, rather than getting it shipped in."

Tomo-Lithographic Molding

Another promising advanced manufacturing technology being enthusiastically embraced by Siemens is a process called TOMO - Tomo-Lithographic Molding - originally developed by Mikro Systems, Inc of Charlottesville, VA, USA.

In the TOMO process a computer model of the component to be manufactured is again "sliced" into very thin (down to around 25 microns or so) sections, or "toma" (the "tomographic" bit of the process), and these slices are used to create photomasks, which in turn are employed to etch (or "chemically machine") metal foils (the "lithographic" part). The layers are laminated together to create a master pattern. The combination of dissimilar "toma" enables 3D cavities to be created of unprecedented precision. The master pattern is used to produce a mould, which can be employed to cast a ceramic material in the required shape.

Siemens, which has a licence agreement with Mikro Systems, has established a facility in Charlottesville for the commercial production of airfoil ceramic cores for gas turbine blades and vanes using this technology. Such components employ complex internal cooling passages. The ceramic cores are used during the investment casting process to produce these internal passages. Modifying ceramic cores has up to now typically been a time consuming and expensive process, making product improvements a significant challenge. The TOMO process helps achieve a faster time to market and lower-cost development process for the complex cores needed to implement advanced cooling features.

The advancements are expected to improve the cooling capability of gas turbine blades by employing cooling passage configurations that were previously not feasible or not cost effective to manufacture, allowing blading to tolerate higher temperatures and use less cooling air, thus enabling higher levels of engine performance and efficiency.

With support from the US Department of Energy and the American Recovery and Reinvestment Act (ARRA), Mikro Systems was funded via Small Business Innovation Research (SBIR) grants and Mikro Systems and Siemens have partnered to develop applications of TOMO technology to a range of gas turbine components.

Game changing technology?

Randy Zwirn, president and CEO of Siemens Energy, Inc. and CEO of the Siemens Energy Service Division believes TOMO gives Siemens "game changing enabling technology" to better its world record of nearly 61% efficiency. He also believes that through retrofitting, TOMO technology can be applied for improving the efficiency of power plants already in operation.

Basically, "this manufacturing technology opens the airfoil design space in ways thought not to be feasible only a few years ago," says Thierry Toupin, CEO of the products business unit of the Siemens Fossil Power Generation Division.

During 2013 and early 2014 Siemens has been focusing its efforts on commercialising the TOMO technology, with development and growth of personnel at the Charlottesville facility.

Siemens says significant progress has been made in the areas of materials performance, process stability and quality management.
Cores are currently being shipped to foundries to support component testing in actual engines and qualification of production parts.
Siemens reports that the Charlottesville factory is currently producing five different core designs using "innovations enabled by the TOMO tooling system", while Mikro Systems continues to provide support in terms of both product refinement and further development of tooling systems.

Mikro Systems says its growth strategy is to apply its technology to a wide range of gas turbine applications, including commercial and military aviation engines, and next-generation turbines for use in natural gas fuelled combined cycle and integrated gasification combined cycle (IGCC) plants.

The TOMO technology is contributing to Siemens' ARRA-funded/DoE-managed project to develop hydrogen-turbines for coal-based IGCC power generation.

Reducing time-to-market

In addition to enabling the realisation of gas turbine blade designs that were previously considered impossible to manufacture, advanced manufacturing technology can also help reduce time-to-market for future design enhancements through reduced tooling costs, reduced production lead times, and more efficient manufacturing processes.

A particular benefit is the potential it creates for rapid prototyping. In the past, it took years to develop a prototype part and test it in a gas turbine. Now, designers have the possibility of generating an idea and testing it much more quickly. Nicolas Vortmeyer notes: "A lot of the effort of developing gas turbines involves trial and error. If we can quickly manufacture our own components, we can try out our ideas really fast."

Willibald Fischer believes rapid prototyping enabled through processes such as selective laser melting and TOMO technology could mean that trials of prototype castings and cores for hot gas parts can be undertaken at least 50% faster than at present.