To connect two pieces of metal together, commonly you weld them. This is versatile and relatively inexpensive if you have a high-power electrical supply, but it tends to stress and distort the metal, also creating a heat-affected zone with a different crystal structure, and it is challenging for some combinations of metals.
But many metals can be electrodeposited fairly easily, notably copper and nickel. What if you instead electrodeposited metal at the junction of two existing metal pieces? You could use an insulated-shaft tool with electrolyte flowing through it, similar to the kind used for EDM small hole drilling (and presumably also ECM drilling), but electrolytically depositing metal and progressively withdrawing from the hole rather than being fed into it. This could permit “welding” “without heat” (< 100°) and thus without distorting the metal. The material deposition rate would surely be much smaller than with traditional welding, but if it can be deposited in precisely the right place, you need much less material.
As with ECM, tool oscillation is a potentially valuable approach for increasing flow of electrolyte through the process gap.
Deep eutectic systems and molten salts offer a similar approach for metals that can’t be electrodeposited from an aqueous solution. This requires far lower temperature and is consequently far less hazardous than welding, for example, magnesium by traditional methods.
Such “welding” might also be able to compensate for its low material deposition rate by simultaneously depositing material evenly over a very long seam, for example through electrolyte held in blotter paper. Without inserting the electrode into the seam, good penetration would be difficult or impossible to achieve, so you need to bevel the edges so that you can electrodeposit at the bottom of a V-groove.
A place where this technique would really shine would be in replacing “spray welding” for building up worn parts with new metal, at which point it’s pretty much just ordinary electroforming.