While additive manufacturing (AM) processes have come a long way in producing a variety of shapes and sizes in printed components using an array of materials, an ideal process for producing large monolithic titanium components with user-customized control has remained a challenge. This means the aerospace industry has been lacking an optimized AM process for plate material for components such as wing spars, bulkheads, and frames.
UK-based GKN Aerospace and the U.S. Energy Department’s Oak Ridge National Laboratory (ORNL) recently signed a five-year research agreement to address the technology. The partnership will focus on improving the deposition process of additive manufacturing for aerospace to achieve volume production of large titanium components using a process called laser metal deposition with wire, or LMD-w. LMD-w involves using a robot-manipulated laser to melt the surface of a titanium substrate, creating a localized pool of molten titanium into which titanium wire is fed to form a bead. Also using robotics, the melt pool is manipulated along a 3-D path to create a net-shaped (or near-net-shaped) aerospace preform component bead by bead onto the substrate in layers.
The process is highly efficient for aerospace compared to current subtractive manufacturing methods, which waste an enormous amount of expensive raw materials. GKN’s LMD-w process is already flying on several aero and space applications and remains under ongoing development for new products at the company’s Additive Manufacturing Centre of Excellence in Trollhattan, Sweden.
According to Rob Sharman, GKN’s Global Head of Additive Manufacturing, the LMD-w process has several advantages over other metal deposition processes. For starters, it leverages widely tunable laser energy and a wire feed rate that allows users to choose the deposition rate and material properties. This way, they can optimize build strategies and tailor the mechanical performance of the parts to the application.
GKN Aerospace's AM cell located at Oak Ridge National Labs. Photo credit: GKN Aerospace
“In addition, the wire feedstock used in the LMD-w process is completely consumed in the melt pool, resulting in a higher material efficiency than powder-based deposition processes, which typically have powder that is not completely consumed in the melt pool,” Sharman told Design News. “The laser heat source provides other advantages over electron beam or plasma heat sources as it is capable of depositing finer features, does not require a vacuum chamber, and does not vaporize critical alloying elements during deposition.”
The LMD-w process is intended to be a complement to existing manufacturing processes for the production of large-scale titanium aircraft structures (rolling, forging, extruding, forming, and machining). In most cases, it won’t fully replace traditional methods.
“LMD-w is another tool in the design toolbox; it creates the opportunity to design a new manufacturing sequence using a combination of manufacturing processes that reduces cost, material usage, work in progress, and cycle time,” said Sharman. “Currently, the biggest cost savings for aerostructure components are those with high buy-to-fly ratios that require lengthy machining operations to remove material from hard-to-machine areas with high aspect ratio features (e.g., deep pockets).”
The ability to customize the build process from incoming material to final part is particularly noteworthy with LMD-w technology. Users can select a part build strategy (including geometry of deposition and incoming material, path planning, deposition rate, etc.) and combine it with a finishing strategy that achieves the final part requirements for surface finish, mechanical performance and more. This opens up possibilities for a multitude of potential manufacturing concepts: the key is to find the most cost-effective strategy that achieves the technical requirements.
To date, GKN’s LMD-w process has already been used on components of advanced rocket engine sub-systems for the European Space Agency’s Ariane 6 launch vehicle: specifically, an advanced 2.5 mm diameter nozzle for the Ariane program’s Vulcain 2.1 engine. Large-scale use of laser welding and laser metal deposition for key structural features resulted in 90 percent reduction of component parts, taking it down from approximately 1,000 parts to 100 parts, according to GKN.
The next step is to scale the LMD-w technology up. The research that will result from the partnership with Oak Ridge National Laboratories is underway at the Department of Energy’s Manufacturing Demonstration Facility at ORNL to industrialize GKN’s technology for larger volume and larger scale AM projects. Specifically, the goal is to create a prototype machine that will manufacture complex medium- and large-scale aircraft structures in titanium. The second focus will be on a process called electron beam melting (EBM) that can produce precise, complex, small- to medium-sized components. A metal powder is melted with an electron beam, again building up the component layer by layer. The partnership will support work already in progress to make the process ready for introduction into full-scale, high-volume aerospace production.
“The partnership leverages ORNL's core capabilities in materials science, high performance computing and material characterization,” Sharman told Design News. “ORNL’s core scientific capabilities build on a growing ecosystem in metal additive manufacturing at the US Department of Energy’s Manufacturing Demonstration Facility, leading to improved efficiency of materials and energy usage for aerospace applications.”
Once the AM process and its novel deposition technology are mastered, it could unlock new materials and design potential for future aircraft designs. GKN is currently exploring options to combine multiple advanced materials and manufacturing processes to design parts and systems with cost and performance not achievable with today’s traditional processes, according to the company.