As mentioned in the paper, this is a typical "medium-entropy" alloy.
Pure metals are soft, because all their atoms have the same size, so the atom layers can slide over the others.
Mixing metals with different sizes increases the strength, because now the atom layers are no longer smooth, but they have bumps, which prevent sliding.
It can be shown that mixing many different kinds of atoms, taking from each about the same quantity, can provide very good mechanical properties, because the bumps in the atom layer will be frequent and they will have varied sizes and a random distribution, which will prevent any alignment between bumps, which could facilitate the sliding of the layers. Think about how to design an anti-sliding shoe sole. Random bumps of random size would give the best result.
The so-called "high entropy" alloys contain at least 5 different metals, with about the same quantity from each.
However the alloys that contain almost equal quantities of each component are very expensive. In order to make a cheap alloy, one must have one or at most two components in a much larger quantity than the others, so that the abundant components can be chosen from the few cheap metals, e.g. iron, manganese or aluminum, while the other components, which are added in small quantities, can be chosen from expensive metals, like nickel and cobalt.
For this reason, the better "high-entropy" alloys are normally replaced by the cheaper "medium-entropy" alloys, which use 5 metals, like the "high-entropy", but which are used in quite different quantities, with larger quantities from the cheaper metals, if possible.
The use of "high-entropy" alloys and "medium-entropy" alloys has begun only relatively recently. They are used to replace the cheaper classic alloys only when they offer a decisive advantage that can justify their higher cost.
This case is one such example. From the classic aluminum alloys, some of the weaker alloys, like AlSi or AlMgSi can be easily 3D printed, but they have low strength. The classic high-strength aluminum alloys cannot be 3D printed. Therefore this was a clear case when a newer kind of alloy must be tried, if high strength is desired. They have experimented with certain kinds of medium-entropy aluminum alloys, to keep the cost acceptable (and also in this case the high content of aluminum keeps the density low and the conductivity high, which are frequently the reasons for choosing an aluminum alloy), and the results were good.
Nevertheless, this alloy is likely to be several times more expensive than AlSi or AlMgSi, so it will be used only when its high strength is necessary.
This is not a medium entropy alloy, it's a standard alloy in terms of the ratio of components, which forms medium entropy intermetallic precipitates which gives the alloy it's properties. Intermetallic MEA is an odd term I'm not really familiar with and would want to look into more, but is a little suspicious. Furthermore, while MEAs (3-4 equal primary components) and HEAs (5+ equal components) do have good mechanical properties, I'd be wary of the atomic size argument, last time I've been involved in it, that argument has increasingly been questioned, as the atomic size of the elements in question are generally pretty similar.
I get the sense that stronger alloys are more "brittle" and harder to do things like welding, as they'll crack instead of yielding from all the thermal stresses. This is probably the same sort of thing with laser melting and 3D printing: solidification under high thermal gradients. It seems this material is not only high-strength but also ductile enough to gracefully handle the thermal stresses.
It's more complex than that. A lot of the material properties depend on both the cooling and the tempering in aluminum alloys.
The phase diagrams for these types of alloys look wild (you often want to achieve
a certain material phase during cooling to "lock" in to get certain characteristics), and it can be difficult to ensure that the smaller metals participate during cooling. Also difficult to dissipate these slightly during tempering, typically to increase ductility.
This is probably why 3d printing hasn't been done in earnest, you can't design something within tight tolerances with unknown material properties.
3D printing of metals is being done in earnest, although the industry prefers the term Additive Manufacturing. Metal powder bed fusion is a stable, reliable process that is being successfully used commercially. It's generally confined to high-value applications that require extreme geometric complexity, but it can be invaluable in industries like aerospace, motorsport and medical. The range of viable materials is still somewhat limited, but covers a good range from titanium and aluminium alloys through to tool steels and heat-resistant super-alloys.
So you need to control the solidification process to plot a course through the phase diagram, spending the right amount of time in each region, and ending up in a good place. And this alloy has a phase diagram that is compatible with a method of 3d printing.
I think this is also why welding aluminum alloys was such a pain before the introduction of friction stir welding, which doesn't melt the metal. FSW was invented surprisingly late, in the 1990s.
Pure metals are soft, because all their atoms have the same size, so the atom layers can slide over the others.
Mixing metals with different sizes increases the strength, because now the atom layers are no longer smooth, but they have bumps, which prevent sliding.
It can be shown that mixing many different kinds of atoms, taking from each about the same quantity, can provide very good mechanical properties, because the bumps in the atom layer will be frequent and they will have varied sizes and a random distribution, which will prevent any alignment between bumps, which could facilitate the sliding of the layers. Think about how to design an anti-sliding shoe sole. Random bumps of random size would give the best result.
The so-called "high entropy" alloys contain at least 5 different metals, with about the same quantity from each.
However the alloys that contain almost equal quantities of each component are very expensive. In order to make a cheap alloy, one must have one or at most two components in a much larger quantity than the others, so that the abundant components can be chosen from the few cheap metals, e.g. iron, manganese or aluminum, while the other components, which are added in small quantities, can be chosen from expensive metals, like nickel and cobalt.
For this reason, the better "high-entropy" alloys are normally replaced by the cheaper "medium-entropy" alloys, which use 5 metals, like the "high-entropy", but which are used in quite different quantities, with larger quantities from the cheaper metals, if possible.
The use of "high-entropy" alloys and "medium-entropy" alloys has begun only relatively recently. They are used to replace the cheaper classic alloys only when they offer a decisive advantage that can justify their higher cost.
This case is one such example. From the classic aluminum alloys, some of the weaker alloys, like AlSi or AlMgSi can be easily 3D printed, but they have low strength. The classic high-strength aluminum alloys cannot be 3D printed. Therefore this was a clear case when a newer kind of alloy must be tried, if high strength is desired. They have experimented with certain kinds of medium-entropy aluminum alloys, to keep the cost acceptable (and also in this case the high content of aluminum keeps the density low and the conductivity high, which are frequently the reasons for choosing an aluminum alloy), and the results were good.
Nevertheless, this alloy is likely to be several times more expensive than AlSi or AlMgSi, so it will be used only when its high strength is necessary.