Hmm... Qoon, it's been suggested, but it's just not doable right now. Making items out of several metals would partially solve this, but It could be incredibly complex to make even a single item if you have to specify everything about it. Everything can be solved with menus, though
Yeah, I'm not suggesting that you be required to specify that the thing be /filled/ with the lead? But since weapons breaking isn't part of combat - just getting the weight of the lead bar to figure into the weight of the final weapon should do the trick. If lead is the only material you use for this, for example, the recipe isn't THAT hard: Filled Mace = Lead Bar + Coke Bar + Requested Metal?
Something like this can be done. But it requires a workaround.
Create a custom reaction: 1xLead + 1xAdamantine -> 1xUtopium and give Utopium all the material properties of Adamantine except for the weight which is that of lead. And you have a reasonably good approximation of a lead-filled adamantine shell.
I've been considering multi-material weapons and something similar to the above hack as a short-term and/or testing measure.
My thoughts on what I'd ideally like is that three materials should cover almost all of the common combinations adequately:
1) Face / Edge / Shell: On blunt weapons, the primary striking surface; on piercing weapons, the primary point and immediate reinforcement; on cutting weapons the cutting edge; something like an metal-banded or nail-studded quarterstaff might also use this for the bands / studs. Depending on design, probably 5% to 20% of the mass of the business end. Almost the sole contributor of max edge and hardness; provides some toughness and mass; for special creature weapons the part that contacts the creature most directly (either for cases where a creature is unusually weak to a material, or where a creature tends to damage weapons not having some property).
2) Core: On blunt weapons, the main body or filling of the business end; on piercing weapons, the backing material behind the point; on cutting weapons the core of the blade (and also the tang probably). Depending on design, the remaining 80% to 95% of the mass of the business end. For most non-polearm weapons the primary contributor to mass, also provides some toughness.
3) Haft / handle / shaft / grip: Provides two important functions: a place to hold the weapon so that strength may be transferred to it, and serves as a lever arm to allow speed / strength tradeoffs. In some weapons, a comparatively trivial fraction of the mass compared to the business end; in others (particularly polearms and whips) the majority of the weapon.
The below examples also sort of illustrate another idea I've had, which is to add two improvements available to many materials: "hardened" and "spring". "Hardened" materials have had some combination of micro-alloying, surface coatings, heat treatment, quench treatment, tempering, etc. applied to increase their max edge, hardness, and rigidity... but usually at the cost of reducing their toughness and flexibility. In other words, they can hold a sharper edge but are more brittle. "Spring" materials have similar sorts of treatment, but aimed in the other direction; they are tougher and more flexible, but less good at holding an edge. A possible third option is "Softened", which isn't used much except for training weapons and a few special cases; it increases the flexibility greatly at the cost of just about everything else; at the DF level, this is probably best treated as part of "spring".
Historically, sometimes the extra treatment(s) were primarily at the material-refinement stage, but I think more often at the weapon-production stage. There are two obvious options: one which is fairly easy to mod would be to simply create additional types of bars, so for instance instead of just "steel bar" there would be "mild steel bar", "hardened steel bar", and "spring steel bar". The existing production chain would produce a "mild steel bar", then there would be a new order to produce a "hardened steel bar" or "spring steel bar" from a "mild steel bar" by using time and fuel / magma. The other option would be to treat this as a finishing option, perhaps similar to "Encrust": "Temper", which would post-hoc improve an already created weapon. This might be more realistic some of the time, but I think is more hassle in the DF setting and harder to mod in.
Some possible examples, drawing from history and common fiction:
Steel warhammer: hardened steel face, steel core, hardwood haft
Weighted Maul: steel face, lead core, mild steel haft
Macuahuitl ("obsidian shortsword"): obsidian face, hardwood core, hardwood haft
Katana: hardened steel face, spring steel core, hardwood haft
Iron-banded quarterstaff: iron face, hardwood core, hardwood haft
Bronze Scourge: bronze face, leather core, leather haft
Steel Pick: hardened steel face, steel core, hardwood haft
Superior Steel Pick: hardened steel face, spring steel core, spring steel haft
Neanderthal Spear: hardened hardwood face, hardwood core, hardwood haft
Stone-Age Spear: flint face, flint core, hardwood haft
Pilum: hardened iron face, softened (or spring) iron core, hardwood haft
Stiletto: hardened steel face, steel core, steel haft
Hazmat Hammer: hardened beryllium bronze face, lead core, spring aluminium bronze haft
Ultimate Hammer: adamantine face, platinum core, spring steel haft
The same face / core / haft system could be extended to crafts and armors with only a bit more work, perhaps renamed "surface / core / handle". A "Quimbaya-style semi-ceremonial breastplate of treated tumbaga" might work out in DF terms to "rose gold surface, black bronze core, leather handle".
Now, how to test these? In the short term, modding in a bunch of new bars (and possibly logs) to handle particularly interesting or useful cases seems to be the direct and immediate answer. For instance, one common combination might be called "good steel", which would take 2 steel and 1 wood input, and produce 3 bars of "good steel" something with the output stats equivalent to hardened steel for cutting, intermediate steel for toughness, and slightly less than steel for weight. A "superior steel" reaction might then take 3 steel bars input to produce an early super-metal with the edge properties of hardened steel and the toughness properties of spring steel. A "weighted steel" reaction might be 2 steel and a lead, with most properties dominated by the steel but significantly heavier from the lead, and so on.