AWS Certified Welding Inspector — FundamentalsWelding ProcessesMedium
A CWI is inspecting a Laser Beam Weld (LBW) on a thin-gauge aluminum alloy. The inspector observes significant porosity within the weld bead. Which of the following is a common cause of porosity in LBW of aluminum alloys?
- AHigh travel speed
- BInadequate shielding gas coverage
- COxide films on the base material
- DExcessive laser power density
Show answer & explanationAnswer & explanation
Correct answer: C. Oxide films on the base material
Aluminum alloys readily form a tenacious oxide film (Al2O3) with a much higher melting point than the base metal. During LBW, this oxide film can become entrapped in the weld pool, preventing proper outgassing of dissolved hydrogen and other gases, leading to significant porosity.
Why the other options are wrong
- A. High travel speed can lead to insufficient penetration or undercut, but it's not a primary cause of porosity in aluminum LBW.
- B. Inadequate shielding gas coverage primarily leads to surface oxidation and discoloration, not typically internal porosity in LBW.
- D. Excessive laser power density can lead to keyhole instability or spatter, but not typically the primary cause of widespread porosity in aluminum LBW.
LBW Porosity in Aluminum
Porosity in Laser Beam Welds of aluminum alloys is commonly caused by the presence of tenacious oxide films and dissolved hydrogen, which become entrapped during solidification.
- Aluminum readily forms a high-melting point Al2O3 oxide film.
- Oxide films can trap hydrogen and other gases in the weld pool.
- Hydrogen solubility decreases significantly upon solidification.
- Requires careful surface preparation (cleaning, mechanical removal).
- Shielding gas choice can influence hydrogen pick-up.
Memory trick: Oxides and hydrogen, a bubbly fusion.