Materials engineers perfectly know the potential of scandium as an alloying ingredient in aluminium alloys. At the end of the 50s, Soviet scientists developed the first scandium-containing alloys, aeronautic engineers learned what scandium could do to aluminium and implemented it on MiG-21 jet fighters, the most-produced supersonic jet aircraft in aviation history, and subsequently on MiG-29. Then, decades later, engineers at Airbus came up with a second-generation alloy called Scalmalloy®: this aluminium-magnesium-scandium (Al-Mg-Sc) alloy has been developed and patented by German-based company APWorks as a high-strength aluminium alloy for powder-bed metal 3D printing processes (SLM, DMLS).
Scalmalloy exhibits good properties for stiffness, exceptionally high strength to weight ratio, excellent weldability and ductility, low density to minimise parasitic mass from non-structural features.
At room temperature, Scalmalloy has close to the same specific strength as titanium Ti6Al4V and significantly higher strength than aluminium AlSi10Mg. This means that in designs where the required strength is limiting the design, Scalmalloy will be lighter than AlSi10Mg at nearly the same mass as Ti6Al4V. The specific stiffness advantage is present but not as strong, meaning that, for applications where stiffness is key, Scalmalloy will be lighter than AlSi10Mg, but still heavier than Ti6Al4V.
Scalmalloy has the same easy machinability as any other Al-alloy, much better than titanium alloys which are known to be very difficult during machining, requiring careful and rigid clamping in combination with well-adjusted machining parameters to avoid undesired part damage.
Compared to other aluminium alloys for additive manufacturing, Scalmalloy offers a unique level of corrosion resistance and a highly stable microstructure up to 250°C, which makes it well-suited for a wide range of high-performance applications.
The high-cooling rates of melt pools during SLM establishes the thermodynamic conditions for a fine-grained crack-free aluminium structure saturated with fine precipitates of the ceramic phase Al3-Sc. The precipitation allows tensile and fatigue strength of Scalmalloy to exceed those of AlSi10Mg by 60-70%.
The mechanical properties of Scalmalloy are enhanced after ageing treated. The rapid solidification and cooling rates during laser melting processes can place more Sc and Zr into solution and thereby enable more Al3 (Sc, Zr) strengthening precipitates to form after the post ageing treatment. As to the enhancement of tensile ductility of the heat-treated Scalmalloy, the existence of residual stress needs to be considered. During the SLM process, a great thermal gradient will be created due to the repetitive rapid heating and cooling, which induce the residual stress, thus resulting in a high density of dislocations in the Al matrix. Therefore, the residual stresses were relaxed through ageing treatment which benefits the tensile ductility.
With high strength and low density, Scalmalloy is particularly suited to the following applications:
- Aircraft construction
- Special engineering