"调制汞合金" can be translated as "amalgamate (the process of preparing/mixing) mercury alloy" or more specifically and naturally in dental contexts (since mercury alloy is commonly used in dentistry) as "prepare dental amalgam" (if referring to the dental filling material).
If emphasizing the action of mixing or preparing the alloy itself, a more general phrase could be "mix/prepare a mercury alloy".
For example:
"The dentist is preparing dental amalgam for the filling." （牙医正在为补牙准备汞合金。）
"We need to mix a mercury alloy with the right proportions." （我们需要按正确比例调制汞合金。）