Solid metals do not fuse in a vacuum because fusion, or the process of two separate solid metals combining to form a single solid, requires the presence of heat and, in most cases, pressure. In a vacuum, there is an absence of air or any other gas, and thus, there is no pressure or medium to transmit heat effectively.
Fusion typically occurs when the atoms or molecules of two separate metals come into close contact and form metallic bonds. This process requires the atoms to be heated to a temperature where their kinetic energy is high enough to overcome the forces holding them in their solid state. When the metals are in contact, the atoms can diffuse and rearrange themselves, leading to the formation of a solid alloy.
In the presence of air or another gas, heat can be transferred from a heat source to the metals through conduction or convection. The gas molecules act as a medium that carries the heat energy and facilitates the transfer. The pressure exerted by the gas also plays a role in bringing the metals into close contact, increasing the likelihood of atomic diffusion and fusion.
In a vacuum, there is no gas to transfer heat, and the absence of pressure reduces the chances of close contact and diffusion between metal atoms. Without the necessary heat and pressure, fusion cannot occur.
However, it's important to note that certain metals have low melting points and can undergo sublimation directly from solid to gas phase under reduced pressure. This process is called vapor deposition and is different from fusion.