Problem Solving - Weld Discoloration, Spatter And Black Smut

Aluminum Welding Guide

Problem Solving – Weld Discoloration, Spatter And Black Smut

4xxx series filler metals produce less weld discoloration, spatter and smut than 5xxx series filler metals. The Magnesium in 5xxx alloys vaporizes in the arc and condenses as a black powder next to the weld bead. The Mg in 5xxx alloys has a lower vapor pressure than either silicon or aluminum when melted in the arc. This lower vapor pressure of Mg increases vaporization and causes some disintegration of the transfering droplet as separation from the tip of the electrode occurs. Small spatter and vaporized Mg are thrown outside of the arc plasma column. Increased black smut and spatter are encountered next to the weld bead with 5xxx filler metal alloys.

Hobart manufactures all of its products to minimize weld discoloration, spatter, and smut from contamination.

Introduction of oxygen into the shielding gas envelope via air, moisture, and contaminants will increase the burning (oxidation) of the filler metal producing discoloration, spatter, and smut.

To Minimize Weld Discoloration, Spatter and Black Smut:

Use a 4xxx filler alloy vs 5xxx.
Air contains both oxygen and moisture which causes weld discoloration and smut buildup.
To minimize air in the shielding gas do the following:
- Decrease gun angle (the typical forehand angle is 10°-15° from perpendicular).
- Increase gas cup size.
- Hold gas cup closer to base metal.
- Shield arc from drafts.
- Clean spatter buildup from gas cup.
- Check gun and hoses for water leaks.
- Shut the welding gun’s cooling water off when the gun is not in use.
  - Keep electrode covered while on the welding machine or in storage to minimize oxidation, moisture condensation, and other contamination.
  - Degrease base metal with the correct solvents.
  - Solvent clean and stainless steel brush or etch and stainless steel brush the base metal weld joint prior to fit-up to remove water stains and heavy oxides. Stainless steel brushing should only be done after solvent cleaning. Stainless steel brushing prior to solvent cleaning traps hydrocarbons in the base metal surface.