Innovative Materials and Manufacturing Techniques in Aviation
The Aviation Team is mainly composed by materials and mechanical engineers, who share the ambition to build a competence center for the Austrian and European aircraft industry in the area of advanced lightweight materials and manufacturing techniques. The group tackles scientific and technological challenges in order to develop new high performance metallic and composite structures, produced both by additive manufacturing (AM) processes and joining methods. The main goal is to develop advanced lightweight metal-composite hybrid structures which can be used in several engineering applications.
The development of new fabrication and joining techniques for lightweight alloys has become a matter of ensuring success for structural components not only in aircraft but also in automotive and space industries. The development of new, strong, high corrosion resistant and lightweight structures, such as those based on aluminum, magnesium and titanium, as well as of advanced polymer-based composite materials, such as Fiber Reinforced Thermoplastics (FRT), has changed the current paradigm in the design of lightweight constructions. As a result, advanced engineered composites are increasingly combined with lightweight metals, performance wise, aiming to increase the weight-to-strength structural performance of components integrating transportation vehicles to reduce fuel consumption and gas emission.
The main scientific drivers are the challenges associated with the mitigation of detrimental effects on material property, which can result from the thermal or thermo-mechanical processing by AM and joining methods. Moreover, other scientific topics involve the understanding of the complexity associated with joining advanced polymers and composites to lightweight alloys (given that both material types present dissimilar mechanical, physical and chemical properties), and the existing intricate relationships between interface bonding mechanisms, microstructure and mechanical performance of metal-polymer hybrid joints.
Techniques & Equipment
Ultrasonic Joining (U-Joining)
Ultrasonic joining (patent US 9,925,717 B2 and EP 3 078 480 B1) is a new direct assembly technique in which ultrasonic energy is applied to join unreinforced or reinforced thermoplastics to surface-structured metallic parts (for instance produced by additive manufacturing processes or by metal injection molding). Ultrasonic vibration and pressure create frictional heat at the interface between metal and composite, softening the latter and allowing the reinforcement (structured on the surface of the metallic part) to be inserted through the composite thickness. As a result, a metal-composite hybrid joint with improved out-of-plane strength and damage tolerance is achieved.
The AddJoining concept (patent application DE 10 2016 121 267 A1) uses a new approach to produce complex hybrid parts, by combining the principles of joining and polymer additive layer manufacturing (ALM) to produce layered metal-polymer hybrid structures. AddJoining has potential to overcome some of the main limitations related to production time of state-of-the-art manual lamination techniques, allowing the production of future composite-metal layered structures with high-specific strength and tight dimensional and damage tolerances.
FricRiveting (patents US 7,575,149 B2 and EP 1 790 462 B1) is an innovative joining technique for polymer, polymer composite and polymer-metal hybrid structures. In this process, polymeric parts are joined by a metallic rivet; the joining is achieved by mechanical interference and adhesion between the metallic and polymeric joining partners. During the process, in its basic configuration, a rotating cylindrical metallic rivet is inserted into a polymeric part. Due to the local increase of temperature, a molten polymeric layer is formed around the tip of the rotating rivet. The local temperature increases, leading to the plasticizing of the inserted tip of the rivet. While the rotation is being decelerated the axial pressure is increased, then the so called forging pressure is applied. As a result, the plasticized tip of the rivet is further deformed – i.e., the original rivet tip diameter is increased - being anchored inside the polymeric part. This joining technology is adequate to produce overlap riveted joints between metal-polymer, metal-composite and composite-composite connections in aircraft and automotive structures.
RFSSW is an alternative welding technology (patents US 6,722,556 B2 and EP 1 690 628 B1) for producing similar, dissimilar and hybrid overlap joints between metal-metal, thermoplastic polymer-polymer and composite-composite materials. Since fiber-reinforced thermoplastic composites are difficult to weld or bond by traditional joining processes, there is an open niche to research and develop alternative joining technologies. The main improvement of RFSSW over the Friction Stir Spot Welding (patented by The Welding Institute – TWI, England) is the absence of a keyhole feature in the spot seam; this usually leads to higher joint strengths due to the reduction of the geometrical notch effect promoted by the presence of a keyhole.
FSpJ (patents US 8,567,032 B2 and EP 2 329 905 B1) is a recent and further development to the RFSSW. This novel joining technology allows the manufacturing of spot joints between metals and thermoplastic composites, without filler materials or adhesive. In this technology, frictional heat is generated between the metallic workpiece and a non-consumable rotating tool, plunging into the metal. The metal is locally deformed and slightly inserted into the thermoplastic at the interface because of the generated heat and the applied axial force by the tool, creating mechanical interlocking between the metal and thermoplastic. Moreover, a thin layer of the thermoplastic is melted at the interface with the metal and reconsolidated during cooling, generating adhesion between the two materials. Therefore, strong overlap spot joints are formed in seconds instead of minutes and hours as found in adhesive bonding.
Friction surfacing is a solid-state metal deposition process, which can be applied to address a number of material problems such as corrosion, wear and fatigue, allowing the combination of metallurgical incompatible materials. The list of materials that can be used in this process includes aluminum alloys, magnesium alloys, tool steels and stainless steels. The process involves rotating a solid consumable rod with one of its ends pressed against a substrate material such as a plate. In doing so, heat is generated at the tip of the rod, producing a plasticized layer of material. By moving the substrate laterally relative to the consumable rod, it is possible to deposit the plasticized material onto it. There is no melting of the substrate material and therefore no dilution of the its material into the deposit. The composition of the deposit is the same as that of the consumable, and it is inherently homogenous and has good mechanical strength and adherence. The process has been further extended to deposit metal matrix composites (MMCs). For this purpose, hard particles are inserted into one or more holes or slots machined in the consumable bar. Consequently, upon the process the consumable bar turns into the matrix with hard particles dispersed within. The process is being currently addressed as an additive layer manufacturing technique.
Wire Based Electron Beam Additive Manufacturing (w-EBAM)
w-EBAM is a wire-based Directed Energy Deposition (DED) process using electron beam as energy source. It is a relatively new Solid Freeform Fabrication method - a variant of additive manufacturing - which is able to produce, layer by layer, near net-shaped parts directly from a computer aided design without the use of mold or additional tooling. In a nutshell, the process works by melting a wire into a molten pool, which is created and sustained by a focused electron beam in a high vacuum chamber. Depending upon the equipment used either the wire-deposition head or the substrate are moved to allow layers upon layers to be deposited; the shape complexity increases from the former to the latter modes of deposition, once substrate translation offers more flexibility than a mobile deposition-head. This technology may be mainly used to support fabrication and repair of large structures, such as spacecraft primary structures and replacement components. Given its versatility, this cutting-edge technology has gained importance within the DED processes, with increasing potential for industrial applications, namely as a candidate technology for fully replacing or complementing forging in certain applications.