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Laser melting with metals is increasingly gaining in importance in aircraft manufacturing. The reasons for this are typical concerns in the industry however: quicker throughput times, more cost-effective components and heretofore unimaginable freedom of design. Two new key words, "lightweight" and "bionics" point to an emerging trend: additive manufacturing provides the basis for a new process that is changing the way engineers think about design. In terms of aircraft design, future components will be able to absorb specific lines of force yet still be able to fulfill the demands of lightweight construction methods. At the same time, sustainability and resource conservation make a contribution to overall improvements in cost structures.
The bracket connector used in the Airbus A350 XWB was honored as a finalist in the running for the "2014 German Industry Innovation Award." In the eyes of the jury, this cross-industry project is revolutionizing the way structural aircraft components are made and lightweight construction is implemented in civil aircraft. Previously this component was a milled part made of aluminum (Al); now it is a printed part made of titanium (Ti) with a weight reduction of greater than 30%.
New design approaches for aircraft structural elements
The arguments for the laser melting of metals in aircraft construction are geometric freedom and weight reduction. The "lightweight construction" approach is intended to help airlines operate their aircraft more economically. For retaining elements (brackets), the achievable weight reduction results in a tendency towards lower fuel consumption or the potential to increase the load capacity of aircraft. A new aircraft design requires thousands of Flight Test Installation (FTI) brackets, which are produced in very small unit quantities. Additive layer manufacturing allows designers to come up with new structures. The additive components are in fact more than 30% lighter than conventional cast or machined parts. In addition, the CAD data are the direct basis for an additive construction job. The omission of tools reduces the costs and shortens the time until the component is available for use by up to 75%. Since tools are not required in the process, it's now possible at an early stage to produce functional samples of components that are similar to series produced components. This is done without upfront costs for tools. This means that sources of error can be identified in the early stages of the design process, which allows for optimization of processes within the project as a whole. Peter Sander, Head of Emerging Technologies & Concepts, Airbus, Hamburg: "Previously we budgeted around six months to develop a component – now, it's down to one month."
"Green technology" conserves resources
Milling of aircraft parts results in up to 95% recyclable waste. With laser melting, the user receives components with "near-final contours," and the process produces only around 5% waste. "In aircraft manufacturing, we work with the "buy to fly" ratio, and 90% is a fantastic figure. Of course, this value is also reflected in the positive energy balance," said Prof. Dr.-Ing. Claus Emmelmann, CEO, Laser Zentrum Nord GmbH, Hamburg. This makes the process especially attractive when valuable and expensive aircraft materials, such as titanium, are being used. A tool-less manufacturing strategy saves time and improves the cost structure. Targeted energy consumption and conservation of resources are key features of the laser melting process. Frank Herzog, CEO & President, Concept Laser GmbH, Lichtenfels: "LaserCUSING is a green technology and improves the often discussed environmental footprint of production."
Aircraft construction as an engine of change
Generally speaking, laser melting results in a positive effect on manufacturing costs for small to medium-sized unit quantities. Peter Sander: "Batch size considerations are more essential in aircraft construction than in volume manufacturing in order to achieve economies of scale." For instance, the comparatively higher relative investment costs for casting molds are eliminated, as well as any costs for tools that may be required. In addition, laser additive manufacturing offers greater design freedom than conventional manufacturing strategies. This way undercuts and interior channels, e.g. for cooling, can be produced. In aviation, aircraft manufacturers are already thinking of cooled elements for electronics or intelligent, hydraulic components. Prof. Dr.-Ing. Emmelmann: "I see great potential in particular for structural components with dimensions of up to one meter, as well as for engine components." However, joining methods for increasing component size right up to the limits of physics are not hard to imagine. The real highlight remains: previously unimaginable geometries can be combined with functionalities for the first time. The flow of forces in the component can already be determined very accurately in the CAD design. In general, laser melting technology is capable of developing safety-related components that are even better, lighter and more durable than the components available today. Moreover, the material properties are slightly different. Prof. Dr.-Ing. Emmelmann: "Materials produced using laser additive manufacturing have greater rigidity while at the same time, less ductility; this can be enhanced with the right heat treatment, however."
Spare parts supply 2.0: Timely, decentralized and "on demand"
Spare parts constitute a new playing field for "additive aeronauts". In the future it will be possible to manufacture spare parts in "on demand" in decentralized locations and without the need for tools. In the event of a component failure, the spare part can be produced directly where it is needed. Decentralized production networks may be formed and global and regional strategies are possible. This minimizes transport distances and above all, delivery times. As a consequence, maintenance-related downtimes and inspection times for aircraft are reduced. In the near future it will be possible to significantly reduce the large spare parts depots with rarely used parts that are currently essential given the long life cycles of today's aircraft. A reduced capital commitment increases flexibility and especially the time needed to obtain safety-related components. This is especially attractive given the cost pressures in the aviation industry.
Bionics in component or product design
Laser melting with metals allows extremely fine, even bone-like, i.e. porous structures to be produced. "Future aircraft parts will therefore have a "bionic" look", Prof. Dr.-Ing. Emmelmann believes. Over millions of years Nature has produced optimized functional and lightweight construction principles which minimize the amount of resources required in clever ways. Airbus is currently analyzing solutions found in nature with regard to their applicability. By relying on "intelligent exposure strategies" of the laser, it can apply layers to a component in a strategic manner in order to produce custom properties in terms of structure, rigidity and surface quality. Peter Sander: "The first prototypes show the great potential of a bionically-motivated approach involving all relevant safety requirements. The process is expected to launch something of a paradigm shift in design and production."
Fatigue strength as a parameter
"The current limitations of the technology exist because of the compromises that have to be with regard to surface quality, which, however, are comparable with those of cast components," says Prof. Dr.-Ing. Emmelmann. These phenomena cause a significant reduction in the fatigue strength of titanium for example. Precisely this parameter is essential for structural components in aircraft manufacturing, which are exposed to high stress. Here you have to consider the high loads to which aircraft are exposed in their extremely long life cycle (>30 years). Nevertheless, downstream surface treatments, such as those using microblasting, can significantly increase fatigue strength when combined with proper heat treatment. Prof. Dr.-Ing. Emmelmann: "As a result, the values of a rolled material can be achieved."
Quality as a significant parameter
For aircraft manufacturers, monitoring during the component's construction phase is one of the most important aspects of the industrial application. Peter Sander: "In practice, the "Inline Process Monitoring" provided by the QMmeltpool QM Module from Concept Laser means the process is monitored over a very small area of 1x1 mm² using a camera and photo diode. The process is then documented." The QM modules such as QMmeltpool, QMcoating, QMatmosphere, QMpowder and QMlaser are the fundamental instruments for active quality assurance while the component is being manufactured. They measure laser output, the melt bath, the layer structure of the metal powder and monitor and document the entire manufacturing process seamlessly. An additional mark in terms of quality assurance is the capability to work in a closed system, which guarantees that the process remains free of particles and contamination. All disruptive influences that could negatively influence the process should be eliminated this way. Frank Herzog: "These days it's accurate to call this a regulated manufacturing process that provides repetition accuracy and process reliability." Prof. Dr.-Ing. Emmelmann emphasizes by saying: "The QS software now enables us to monitor and document key data, such as laser parameters, melt pool parameters, as well as the composition of the inert gas atmosphere." Disruptions due to contamination can be eliminated. In an on-going research project we are developing our own quality assurance concept, which is based partly on optical coherence tomography."