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One caution if you go the alloy route and it matters to you: most alloys do not have a melting point, they have a melting range.

They start to soften at one temperature and are fully liquid at a higher one, with a mushy zone between. For a temperature reference that ambiguity may be unacceptable — you would see gradual slumping rather than a clean transition.

The exception is a eutectic composition, which melts sharply at a single temperature like a pure substance does. If you want a crisp indication, look specifically for a eutectic in your range rather than any alloy that happens to be listed with the right number.

The gap is real for pure elements and not real at all for alloys, and that distinction is the answer.

Among common metals, the pure melting points genuinely cluster either side of your window — the low-melting ones are well below and the structural ones are well above. There are elements in the range but they are not things you have lying around.

Alloys fill the gap completely, because alloying almost always lowers the melting point relative to the higher-melting component, and the amount depends continuously on composition. Aluminium alloys, magnesium alloys and various brasses and bronzes cover the 500 to 600 band densely.

So the question to ask is not "which material melts at 550" but "which alloy composition melts at 550", and there the answer is that several do and the composition is a dial rather than a fixed value.

The flux example is the one that would have answered this immediately if I had thought of it. I was searching for a solubility figure when the everyday industrial evidence is that the two phases separate cleanly enough to be a purification method.