Those things are so large that even if they’re made out of foam or paper, they’re probably going to be too big to be a sensible shape, compared to thinner smaller alternatives.

Depleted uranium and titanium. The US army uses depleted uranium in their tanks, and should not be radio-active.

Regardless of weight, they would not be very combat effective, compared to other swords.
First of all, the blades, if you can call them that, are so thick, that they will be very bad at cutting.
Because they are so wide, they will not be as good at stabbing, either.
And because they are so bulky, they have terrible weight distribution, and will be not at all be nimble.

@ragingloli I don’t mind if they’re bad at cutting and stabbing since my characters won’t even be using them for such things. The whole point of these weapons is to break shield formations and cause severe bodily damage to armored opponents (kinda like a mace).

Since we are starting from a clean slate let’s redesign the sword.
We want it very strong and light so use modern materials and beam theory to develop the ideal shape and cross section. For a given mass cross section a tubular beam is much stronger than a solid core.
Make it hollow on the inside but use that space to have a spring loaded extension that adds length when swung hard but retracts when used for close quarters combat.
Select the metal laminate or powdered metal to provide the characteristics needed to defeat the opponent’s armor. Ductility, toughness, hardness, and yield strength are all factors.

I’d start with skeletonized titanium alloy sword with hardened edges. Titanium is considered hard and brittle so it should be reinforced with a beam-like structure.

Well, if you want to break shield formations, you will need mass. Lots of mass.
That is why they had field artillery, mortars, ballistae and trebuchets, or used cavalry to harrass them, or archer barrages to soften them up.
And since your characters are not superhuman, those swords will not be enough.
If you want to defeat armour, you either need substantial blunt force trauma (mass, so a mace or warhammer will be the preferred weapon), or be able to puncture the armour (mass concentrated to a single point, like a war pick or polearm, such as pole axe, or bec de corbin), or be able to circumvent the armour by targeting the gaps in between the armour plates, so you will want a light, nimble weapon, that is thin enough to get through not just the gaps, but also be able to puncture the chain mail, that is usually worn underneath the plate.

I would look to existing weapons from the east and west for scale. There’s a reason why they had the geometries they did. These things evolved to be as effective as possible for combat (nobody wants to walk into an actual battle wielding something that looks impressive but is ineffective). If we’re talking big-ass swords examples in real life would be the zweihander and odachi.

This video goes into more depth about how big weapons in a fantasy setting by people with superhuman strength still has problems, especially if the wielders themselves aren’t increasing in mass.

It will need to have a hollow but structurally reinforced center with a hard outer cover. Much like the bones of a bird. Such a thing cannot be machined but we can 3D print it. Ideally, it’s one contiguous piece of carbon steel. The cutting edges would be a little thicker to enable resharpening. Inside the hollow structure we fill with an amount of mercury calibrated to the user’s strength profile. It certainly won’t hit like a sword it’s size but it’ll at least look cool. It would be very effective against fleshy bits of an opponent, not against armor though.

Melding known sword technology with fantasy manufacturing you could make your sword with a metal laminate having 1) a high carbon or tool steel (https://knifeinformer.com/discovering-the-best-knife-steel/) in the middle as a cutting edge, sandwiched between 2) two layers of a tough steel to make it harder to break, and 3) manufacture it using a 3D metal printing process to have a lightweight core [hollow structural grid of titanium] to reduce weight without sacrificing stiffness.