The most common cause of undercut and spatter is

• A. low welding amperage
• B. high welding amperage
• C. incorrect shielding gas
• D. too high travel speed

Answer: (A)

Heat input controls how the base metal melts and how the filler bead fills the joint. When amperage is too low, there isn’t enough heat to properly fuse the metal and deposit filler along the toe, so a groove forms along the edge—an undercut. The weak, unstable arc from low current also causes irregular metal transfer, producing droplets that spatter rather than smooth deposition. That combination is why low amperage is commonly linked to both undercut and spatter. If amperage were higher, you’d typically see excessive penetration or burn-through; incorrect shielding gas tends to cause porosity or contamination; travel speed effects can cause undercut too, but spatter is less characteristic of high speed alone.