brazing
Brazing joins metal with a filler metal that melts and flows into the joint, while the parts being joined stay completely solid. It is a bit like gluing two things together, except the glue is a molten metal that freezes into a strong bond. The defining rule is temperature: the filler melts above 450 degrees C but below the melting point of the base metals, so it becomes liquid and wets the surfaces without ever melting the parts themselves. Bicycle frames, copper plumbing, carbide tool tips, and heat exchangers are commonly brazed.
The trick that makes brazing work is capillary action. You leave a very thin, clean gap between the parts, heat everything to the brazing temperature, and touch the filler (often a silver, copper, or brass alloy) to the joint. The molten filler is pulled into the narrow gap by capillary force, the same effect that draws water up between two glass plates, and it wets and metallurgically bonds to both surfaces. A flux cleans away oxide so the filler can wet properly. When it freezes, it holds the two solid parts together.
Brazing shines where you must join dissimilar metals, thin sections, or complex assemblies without melting and distorting the parts, and it makes joints far stronger than soldering. The honest limits are that joint strength is governed by the filler and the gap, not the strong base metal, so a brazed joint is usually weaker than the parts it joins; surfaces must be clean and the gap tightly controlled; and service temperature is capped by the filler's melting point. Braze it right and it is strong; braze a dirty or wide gap and it is unreliable.
A tungsten-carbide cutting tip is brazed onto a steel tool shank with a silver-alloy filler: heated past 450 degrees C, the filler wicks into the thin gap by capillary action and bonds the two without melting the carbide or the steel.
Only the filler melts; capillary action pulls it into a thin gap, which is why brazing needs clean surfaces and tight clearances.
A brazed joint is only as strong as the filler and the gap, so it is usually weaker than the base metals it joins; brazing does not melt the parts, which is both its advantage and its strength limit.