weld, solder
煇 centers on joining metal by applying intense heat: welding and soldering. Both meanings share the core idea of fusing materials with fire.
煇 combines 火 (fire) with 軍, which likely contributes the sound. The fire component points to heat-based processes like welding.
Fire 火 on the left and 軍 on the right: picture a military workshop where fire is used to weld and solder metal armor and weapons.
For キ, imagine using a key-shaped soldering iron to weld metal: key -> キ, and the fire 火 heats the joint.