Any welding on a fork can effect the fork properties negatively.
The general rule is that there should be no welding on the top surface of the blade or the front face of the upright (as they are in tension). Any deviation from this rule must always be discussed with Engineering so that the appropriate safety margins can be applied.
There are a number of methods where applications can be adjusted to avoid welding in critical areas. For example this block is welded at the side rather than the front.
DO NOT WELD WITHIN A MINIMUM OF 13mm (1/2") FROM THE SURFACES DESCRIBED ABOVE.