Simply forging a piece doesn't make it stronger. A stock removal knife isn't less strong than a forged knife if both are tempered properly and the forged knife is FORGED properly. In fact forging can make some objects stronger ( think lifting hooks ) when forged by an experienced smith who truly knows how metal moves and how to heat it properly.
That said : When referring to welding, if you mean forge welding ? Forge welding done correctly is as strong as the parent material and should pose no weakness issues.