Manufacturing of novel metal alloys
The Institute for Applied Laser, Photonics and Surface Technologies ALPS of Bern University of Applied Sciences (Berner Fachhochschule BFH) has been developing a laser-based process to produce novel metal alloys in seconds. They can be super hard, acid-resistant or ductile. Such alloys can be used e.g. in aerospace industry. The technique that can form those alloys is called laser direct metal deposition or LDMD.
Laser direct metal deposition basically is a cladding process. A laser beam and a powder beam are guided coaxially by a nozzle to the surface of a workpiece such that the powder as well as the surface of the workpiece are molten. A bulk layer of the molten powder metal is generated on the workpiece. The process can be utilized for single-layer depositions as well as for multi-layer applications to generate complex shapes in the sense of Additive Manufacturing, in which objects are created in layers from a 3D-model.
Laser as the key to success
In 3D LDMD, composition and structure of the built-up part can directly be adapted by varying the process parameters. In particular, the alloy can be adjusted by mixing different elemental metal powders in the powder nozzle.
The possibility to create novel alloys by mixing elemental metal powders and adjusting their properties during the process opens doors to new applications. In combination with rapid heating and cooling typical for laser processes, LDMD is able to create new alloys with outstanding properties.
Furthermore, changing the ratio of the elemental metal powders during the process allows for adjusting the material properties locally, e.g. microstructure, hardness or ductility can be modified according to the local requirements. And thanks to high laser power and concentrated energy input this technique works for refractory metals like tungsten and molybdenum as well.
Two metal alloys like nickel aluminides (NiAl, NiAl3 and Ni3Al) with different local properties have successfully been created by means of LDMD.
At present parts in gas turbines e.g. stators, rotors and blades are typically made from so-called Nickelsuperalloys by casting, forging or milling. These methods have been developed and optimized for decades and provide parts with acceptable properties even at high temperature or under harsh conditions.
But these parts have one major weakness: local changes in properties need to be applied in additional post-treatments like hardening or coating and they can only be applied on the surface. Locally adapted properties as well as chemical and corrosion resistance are supposed to further increase the resistivity and lifetime. During the casting process heating and cooling of the material happen very slowly compared to LDMD. In general, the material stays in an equilibrium state, which then defines the material properties. Post-process thermal treatments can be applied to modify e.g. the hardness of the metal. On the contrary, laser processes are characterized, inter alia, by rapid heating and cooling rates, which allow for generating alloys in a non-equilibrium state. Combined with the possibility to vary the ratio of the elemental metal powders to be mixed, tailormade local material properties can be created.
To explore the full potential of this promising technique, simulations must be adapted and performed. Currently only few non-equilibrium data for metals exist which can be used as input. But the macroscopic properties of the part derived from the requirements can be defined to feed the simulation. As a result, optimized structures combined with locally adjusted material properties will make their contribution to weight reduction and energy saving.
- Dr. Hossein Najafi
- Dr. Valerio Romano
- Matthias Steck