22 possible causes for porosity in welding

Weld metal porosity is a common issue encountered in welding processes, often arising from various factors. According to sources, it can be caused by environmental issues, contamination, or improper welding techniques. Understanding these potential causes is crucial to achieving high-quality welds.

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Porosity is defined as the presence of trapped gas, leading to unwanted contamination in the weld metal. Shielding gases may not always fully envelop the weld pool, allowing atmospheric air to interfere with proper fusion. As a result, defects appear as rounded holes known as spherical porosity (see Figure 1. If the holes elongate, they may form what's referred to as wormholes or piping.

Although porosity has acceptable levels in certain codes, it can lead to weld rejection depending on the standard applied. Fortunately, porosity can be prevented in approximately 90 percent of cases. By identifying the potential causes, welders can effectively convert reject parts into acceptable welds.

Common Causes of Porosity in Welding

Let’s delve into the potential culprits of porosity in welding, ranked from the most common to the least:

1. Insufficient gas in the cylinder. A frequent occurrence that welders should monitor.
2. Drafts or air disturbances disrupt shielding gas flow. Even small fans located up to 25 feet away can impact processes like shielded metal arc welding (SMAW) and flux-cored arc welding (FCAW). Welders must also consider drafts from machinery or open doors, especially if exceeding 4 to 5 miles per hour.
3. Moisture presence can cause significant issues. This may range from simple water to condensation, particularly when temperatures fall below 50 degrees F. Preheating metal to around 200 to 220 degrees F can eliminate moisture.
4. Clogged or restricted gas metal arc welding (GMAW) nozzles, often due to weld spatter, disrupt gas delivery. Welders should inspect nozzle openings before welding.
5. The nozzle's distance from the weld puddle can reduce gas volume, allowing atmospheric air to dilute shielding gas.
6. An improper angle of the GMAW gun can cause an unintended suction of air. A 5 to 15-degree angle perpendicular to the joint is preferred.
7. Contaminants like paint and oil release gases at welding temperatures, particularly with solid-wire GMAW and gas tungsten arc welding (GTAW).
8. Welding over mill scale and rust can emit decomposition gases that compromise weld quality.
9. Galvanization, involving zinc, presents challenges as zinc vaporizes quickly at welding temperatures.
10. Moisture absorption in welding electrodes and flux can lead to porosity. Proper storage methods are essential.
11. Excessive gas flow can pull in outside air and create turbulence in the weld zone. Adhering to recommended flow rates helps prevent this.
12. A damaged gas hose may not properly deliver shielding gas. Long hoses are particularly susceptible to kinking.
13. Incorrect use of antispatter compounds can introduce gases when exposed to welding heat.
14. Contaminated filler metals, including those stained by dirt or grease, can release gases during welding.
15. Dirty GMAW gun liners can contaminate the weld pool with debris from the workshop.
16. Welding at an outside corner joint may expose the weld to atmospheric interference.
17. Open root joints risk trapping air in liquid metal.
18. Contaminated welding gas poses a threat to quality. Shops should ensure gas certification meets required standards.
19. Gas hoses previously used for other applications may harbor contaminants, affecting weld quality.
20. Damaged O-rings on welding equipment can introduce air into the process.
21. Hoses with cuts or burns could create shielding gas leakage.
22. Defective gas solenoids may contribute to porosity-causing conditions.

Best Practices to Prevent Porosity in Welding

To minimize porosity, welders should remember these procedural guidelines:

• Ensure adequate shielding gas coverage at the start of a weld in tight corners. A burst of shielding gas may not suffice.
• Purge the gas line after breaks to avoid shield gas-free starts.

High-strength, low-alloy steels can trap hydrogen during solidification, causing long-term concerns such as cracking. Identifying the factors contributing to weld porosity is essential for maintaining quality in welding operations.

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Check for system leaks regularly by pressurizing the setup and monitoring the regulator dial. A stable dial indicates readiness, whereas a declining number suggests a leak that must be investigated.

The characteristics of the porosity often provide clues to its source. For detailed insights into the issue, refer to AWS B1.11, Guide for the Visual Examination of Welds.

In the context of metal additive manufacturing, porosity refers to the level of solidity achieved in produced parts, specifically the presence of cavities or holes between layers.

Some metal additive manufacturing methods involve lasers melting metal powder, adhering it to previously melted layers, which ideally results in high-density parts. However, incomplete melts can lead to porosity due to gas pockets within the powder.

To ensure dense parts, manufacturers typically recommend materials they have tested, with settings tailored to achieve optimal results. Additionally, post-processing treatments like hot isostatic pressing help eliminate any remaining cavities.

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