The use of metal foams allows additively manufactured components to be reinforced in specific areas while maintaining a low weight. For this, researchers at the IW have developed an aluminium alloy that has to meet opposing requirements. On the one hand, its melting range must be sufficiently wide for the production of metal foams. Furthermore, it must be formable to be processed into wires using methods such as extrusion. The wires must also be weldable so that they can be used in the Wire and Arc Additive Manufacturing (WAAM) process.
Production of metal powder from the aluminium alloy by inert gas atomization
Since the alloy must be in powder form for the production of the foamable materials, a metal powder is produced from the developed aluminium alloy. Jannes Mevert, a doctoral student at the IW, has worked with undergraduate students to develop a corresponding process chain with suitable analytics.
The institute’s own inert gas atomization plant, which is located in the SCALE research building, is used for powder production. In the underlying Electrode Induction Melting Inert Gas Atomization (EIGA) process, a suspended, rotating rod electrode is inductively melted at its lower end by a conical copper coil, causing the melt to flow vertically through a ring nozzle. A high-pressure stream of inert gas is directed through this ring nozzle onto the molten jet, which is atomized into fine metal droplets that solidify in flight and form spherical particles (see Figure 1). The quality of the powder particles is analysed using laser diffraction and electron microscopy.
Powder metallurgical production of aluminium foams
Using the metal powder, the powder metallurgical production of foamable semi-finished products takes place, which can later be expanded by specific heat treatments. For this purpose, the aluminium powder is mixed with the blowing agent titanium hydride (TiH2), which has proven itself for aluminium foams due to its decomposition kinetics.
The basic investigations initially involved an evaluation of the expansion behaviour as a function of the temperature-time exposure during heat treatment. For this purpose, the mixture of metal powder and blowing agent is compacted into pellets at approximately 800 kN. When the semi-finished product is heated to temperatures at least above the solidus temperature of the metal, i.e., around 580 °C, it begins to melt and thus transitions to a viscous state. At the same time, the blowing agent begins to decompose, releasing hydrogen, which is present in the form of gas bubbles within the melting compact. The gas bubbles cause the semi-finished product to expand, forming a highly porous internal structure (see Figure 2).
Subsequent rapid cooling preserves the porous structure of the metal. If the semi-finished product is heated for too long and/or the temperature is too high, the unstable foam collapses, resulting in a process window for maximum expansion and porosity as well as minimum foam density. One of the main tasks of doctoral student Jannes Mevert is to investigate these relationships and coordinate the required foam properties with the subsequent manufacturing processes of the other subprojects in the Transregio.
Process-integrated foaming in the additive manufacturing process
The next focus of the project work is on wire development. The planned work therefore primarily involves the production of cylindrical semi-finished products from the foamable material, which are to be extruded into wire-shaped semi-finished products using the IW’s 2.5 MN extrusion press.
These foamable wires are to be used for processing with wire-based additive manufacturing processes such as WAAM. The wires are to be foamed by the process heat introduced during welding or by a secondary heat source in order to specifically adjust the porosity in the component to be manufactured additively and thus locally grade the properties of the material.
This would allow the material properties of lightweight components to be adapted and optimized in a completely new and diverse way with a high degree of design freedom, depending on the application.
