I bought a spray welding rig recently, but I haven't tried to use it yet. Metalspray was an alternative to welding crankshafts when my father was building race engines back in the early 1950s, and some pretty hot engines got that treatment. The shaft is turned undersize until it's round, and V grooves are cut into the surface. Then the shaft is heated with a special acetylene torch and powdered metal alloy with a binder is sprayed onto the grooved shaft. Then the shaft is reground back to standard size. The process is used when rebuilding jet turbine engines, so if it's done properly it's safe and reliable.
Another method of building up a shaft is by "submerged arc welding". In that process, powdered flux is poured over the shaft as it rotates and a specialized MIG welder is used to build up the diameter. Then the shaft is reground to the desired size. Both of the above procedures are fairly complicated and somewhat expensive, but they're cost-effective when scarce or high value parts are involved. Here's a video of the submerged artc process. The machine shop in Nashville where I ran the connecting rod reconditioning department did a lot of crankshaft welding.
www.youtube.com/watch?v=UYGzVzpWrw4Jerry
