Laser welding is a high energy-density welding method characterized by concentrated heat input, narrow weld seams, and low deformation, and it is widely used in metal processing industries. During welding, if the weld area is exposed to oxygen or oxidizing gases, oxidation may occur, resulting in black, yellow, blue or oxide-colored weld surfaces, which negatively affect mechanical performance and appearance quality. This article analyzes the main causes of weld oxidation from the perspectives of material characteristics, environmental factors, gas shielding, and process parameters.
1. Basic Mechanism of Weld Oxidation
Weld oxidation occurs when high-temperature metal reacts with oxygen or oxidizing gases. The temperature of the laser welding zone can reach 1000℃–3000℃, and the molten pool and heat-affected zone remain in a thermally activated state, which increases the tendency of oxygen to penetrate and form oxide layers. The common oxidation reactions include:
Metal + O₂ → Metal oxide
Preferential oxidation of alloying elements (e.g., Cr, Mn, Ti, Al)
Therefore, the degree of oxidation is closely related to temperature, material composition, and oxygen concentration.
2. Main Causes of Weld Oxidation
(1) Insufficient Gas Shielding
This is the most common cause of oxidation during laser welding and typically involves:
Insufficient gas flow, resulting in incomplete oxygen isolation
Improper nozzle angle causing incomplete coverage
Inappropriate gas types (e.g., nitrogen being less inert than argon)
Gas diffuser or shielding structure inadequacies
The general anti-oxidation effectiveness of shielding gases is approximately:
Argon ＞ Helium ＞ Nitrogen ＞ No shielding
If shielding is insufficient, the molten pool becomes exposed to air during cooling, which increases oxidation.
(2) High Oxygen Concentration in the Welding Environment
If the welding environment contains excessive oxygen due to poor ventilation or low gas purity, shielding effectiveness decreases. Common conditions include:
Low-purity shielding gas with excessive oxygen content
Ambient airflow introducing oxygen into the weld zone
Gas line leakage introducing air
For example, if industrial argon purity is lower than 99.99%, weld oxidation can significantly increase.
(3) Materials with High Oxidation Tendency
Different materials exhibit different chemical reactivity. Metals containing the following elements are more prone to oxidation:
Cr, Mn, Si, Al, Ti and other active elements
High-manganese steel, stainless steel, titanium alloys, and magnesium alloys
Examples:
Stainless steel contains chromium, which forms chromium oxide above 600℃, leading to discoloration.
Titanium alloys react with oxygen above 400℃, causing weld discoloration and embrittlement.
Material characteristics therefore play a critical role in oxidation behavior.
(4) Excessive Laser Heat Input
Heat input determines molten pool temperature and duration. Excessive heat input causes:
Higher molten pool temperatures and intensified oxidation
Longer cooling times, increasing oxidation exposure windows
The resulting weld discoloration often follows a sequence such as:
Gold → Blue → Purple → Gray/Black
Darker colors generally indicate higher oxidation levels.
(5) Low Welding Speed
Welding speed is inversely proportional to molten pool exposure time:
Slow speed → Longer exposure → Increased oxidation
Fast speed → Shorter exposure → Reduced oxidation
Oxidation discoloration is especially common in thin-sheet welding at low travel speeds.
(6) Lack of Post-Weld Shielding
Some materials require post-weld shielding during cooling; otherwise, oxidation may occur while the material remains hot. Typical cases include:
Titanium alloys, requiring trailing gas shielding
Stainless steel tubing, requiring internal shielding (argon back purging)
Insufficient post shielding results in visible thermal discoloration and oxide layers.
3. Effects of Weld Oxidation
Weld oxidation affects not only appearance but also may cause:
Reduced corrosion resistance (chromium oxidation decreases stainless steel passivation)
Reduced ductility and toughness (e.g., titanium embrittlement)
Increased crack sensitivity
Reduced fatigue performance
Increased post-processing requirements (acid pickling or mechanical removal)
Therefore, industries with high weld quality requirements must control oxidation levels.
Weld oxidation during laser welding is mainly caused by insufficient shielding, material oxidation tendency, high environmental oxygen levels, high heat input, and improper welding speeds. To reduce oxidation, gas shielding strategies, process parameters, equipment design, and environmental conditions must be optimized in combination.