Additive manufacture Well thought-out cooling, consistent lightweight construction, and a difficult-to-machine working material – a case for additive manufacture. With the SLM process, a monolithic engine was finished in a few days.
In conventional construction, cooling channels are milled in a workpiece and afterwards, in numerous working steps, closed over again. If one wishes to manufacture an engine with complex structures, this means that the entire work lasts at least half a year and is therefore also very cost-intensive. In addition, the specifications in the aerospace industry are extremely demanding. Not only is lightweight construction necessary throughout, but the materials must also stand up to particularly high stress.
Selective laser melting (SLM) offers numerous possibilities in metal-based additive manufacture of components. It is possible, for example, to make them directly with interior structures and with consistent lightweight construction. A further advantage lies in the integration of a number of parts into one component. This functional integration and the reduced need for follow-up processing result in significant cost savings in the manufacturing process. With a rocket engine as an example, the firm Cellcore has now shown how the advantages of the SLM process can be exploited optimally in aerospace. In collaboration with SLM Solutions, a monolithic workpiece was created with the material IN718.
The motor manufactured by Cellcore and SLM Solutions consists of a thrust chamber, the core element of a liquid-fuelled engine, with combustion chamber wall, fuel inlet and also an injection head with an inlet for the oxidator. In the combustion chamber, a chemical reaction takes place and releases a gas. Because of the heat developed, this gas expands and is ejected with enormous force. This backwards reaction provides the thrust necessary to power the rocket. During the combustion process, very high temperatures arise in the chamber, so the walls have to be cooled so as not to burn through. To this purpose, liquid fuel, kerosene or hydrogen, is led upwards through cooling channels in the combustion chamber wall before being introduced into the combustion chamber through the injection head. There the fuel is mixed with the oxidator and is burnt with the help of a sparking plug.
Filigree structural cooling for higher efficiency
By using an additive manufacturing process from SLM Solutions, it proved possible to reduce the manufacturing time to five working days, which led to substantial savings in time and cost. The monolithic rocket motor produced with this SLM technology consists of injector and thrust chamber and combines integral design, that is, the uniting of numerous individual parts into one component, with multifunctional lightweight construction.
The core element is the structure developed by Cellcore and producible only with the help of SLM technology. It guarantees strength and is suitable for dissipating heat. The characteristics of this structural cooling substantially surpass those of conventional approaches, such as right-angled, concentrically-running cooling channels. Structural cooling offers an optimum ratio between strength and the mass used, as well as reducing flow resistance and simultaneously increasing the effective surface. It is therefore not only more efficient, but also integrates additional functions. The lattice structure furthermore leads to a reduction in weight compared to traditionally manufactured components.
SLM processes nickel-chrome alloy
SLM Solutions supported Cellcore in setting up the complex component so that it was optimally prepared for the SLM process. Part of this was the development of specific parameters for the component geometry, including downskin optimisation and optimal component orientation in the machine space. In addition, areas critical for construction were identified in order to rule out mistakes, thus guaranteeing the success of the construction job.
In order to meet the high requirements for materials in the aerospace industry, the engine was made from the nickel-chrome alloy IN718. IN718 is a precipitation-hardenable working material which offers outstanding tensile, fatigue, creep and fracture strengths up to a temperature of 700 °C. This makes IN718 an important alloy for aircraft and gas turbine components as well as for a wide variety of high-temperature applications, such as rocket motors. In conventional processing, this material is hard to machine and leads to severe tool wear.
The engine was printed on the SLM 280 machine from SLM Solutions. The outer dimensions of the machine are 280 mm × 280 mm × 365 mm and it works with a patented multi-themed technology. The powder is handled using a PSV power supply unit, which eliminates manual refilling with powder from individual powder bottles. With an ultrasonic sieve, integrated into the PSV, the available powder is sieved shortly before starting the process, so no oversized particles or foreign bodies can get into the construction process. The transporter powder between the PSV and the SLM machine takes place completely automatically by means of gas flow conveying.
After successfully completing construction, the first thing is to unpack the component and to release it from the base plate. Furthermore, the supporting structures are removed and follow-up work is done on the component. “The concept study shows the enormous potential of additive manufacturing, whether in the aerospace or other sectors,” Andreas Krüger, CEO of Cellcore, is convinced. “We are very impressed by the possibilities of this technology from SLM Solutions. In particular, the great faithfulness to detail and the quality of the extremely filigree multifunctional cooling structure was very persuasive.” With SLM technology, it is possible to save expensive and time-consuming manufacturing steps and simplify the engine structure, thus saving substantial costs. For in conventional manufacture, the injection elements are produced individually, given follow-up treatment, and bolted in separately.
Simplified follow-up work despite complex structures
In addition, grooves for oxygen are milled in the housing and these in turn have to be provided with a cover so that the gas does not escape. The next step of bolted mounting, in turn, takes place in a clean room.
Numerous working steps which need their own follow-up work, or at least assembly steps, can be combined in additive manufacturing and thus reduced. Follow-up work for the component, despite complex structure, is minimised due to additive manufacturing. Similarly, severe tool wear is avoided.
After this engine, it is still not all over at Cellcore: “We are currently working out how to apply this principle in various other projects,” says Krüger. “For example, enormous added value can be achieved for engine and turbine components, or for tempered tools.”
* Kristal Kilgore is Marketing Director at SLM Solutions in 23560 Lübeck, tel. (00 49-4 51) 40 60-30 00, info@ slm-solutions.com