Welding is a highly effective technique for joining separate pieces of metal to create a single component, and is commonly used in sheet metal fabrication processes to finalize part production. Despite its benefits, there is a risk of welding defects that can result in weld failure.
Welding defects are frequently observed in sheet metal welding, often resulting from incorrect welding patterns or methods. These defects can compromise the joint’s strength, causing the product to fail altogether. Thus, it is crucial to identify the underlying causes of these defects and take preventive measures to avoid them.
The following article provides an overview of welding defects, their origins, and preventative measures. Additionally, this guide explores techniques for detecting hidden defects, aiding in the identification of different defects and discontinuities. Read on to learn more about this topic.
What Are Welding Defects?
Welding defects refer to any imperfections or irregularities that form in a weldment, compromising its intended use or aesthetic appearance. According to ISO 6520, any irregularities that compromise the weld are classified as weld defects, while flaws that do not compromise the weld are classified as weld discontinuities. Acceptable limits for these discontinuities are outlined under ISO 5817 and 10042.
The size, shape, and extent of welding defects can vary depending on the metal structure and the welding process used. While the primary causes are often attributed to the wrong choice of welding method or incorrect welding patterns, there are several other factors that can contribute to specific flaws in a weldment.
Welding defects can appear both on the inside and outside of the metal, causing joint weakness and affecting the appearance of the weldment. While some flaws may still fall within acceptable limits, others can result in the rejection of the product. As a result, it is crucial to take measures to prevent weld failure.
Types of Welding Defects
Defects caused by welding can be categorized according to their location in the metal. They may be external or internal.
External Welding Defects
External welding defects are defects that are visible on the surface of the metal weldment. They are superficial and can be detected through visual inspection or by using methods such as Magnetic Particle Inspection (MPI) or Dye Liquid Penetrants (DPI). Examples of external weld defects include cracks, undercuts, overlaps, porosity, spatter, and more.
Internal Welding Defects
Internal welding defects refer to flaws or irregularities within the metal material that are not visible from the surface of the weldment. These defects can be challenging to detect using visual inspection or non-destructive tests. However, methods like Ultrasonic Testing and Radiographic Testing can be used to identify them. Examples of internal welding defects include incomplete penetration, slag inclusions, and incomplete fusion.
Common Types of Weld Defects
- Crack
One of the most undesirable welding defects is cracks, which are formed due to local rupture caused by stress and cooling effects. They are particularly significant because their shape creates a considerable stress concentration at the tip, making the weldment vulnerable to fracture. Welding cracks can take different forms, sizes, and shapes, including:
- Longitudinal
- Transverse
- Crater
- Radiating
- Branching
Depending on the temperature they occur, cracks can be:
Hot Cracks
Hot cracks are a type of welding defect that typically occur during the solidification and crystallization of the weld joint, which involves temperatures above 10,000 degrees Celsius. These cracks can be categorized as either solidification or liquation cracks. Solidification cracks usually form due to high impurity or carbon content in the metal or disruptions in heat flow. In contrast, liquation cracks occur when heating temperatures increase, causing constituents with low melting points to liquefy.
Cold Cracks
Cold cracks are defects that occur after the weld metal solidifies and can develop several days after welding. These cracks typically run parallel to the fusion boundary and may grow away from it due to residual tensile stress. Cold cracks are caused by various factors such as low preheating, high stresses, low temperatures, high hydrogen content, and susceptible material structure.
Causes Of Weld Crack
- Poor ductility or contamination of the base metals
- Combining high welding speed with low current
- High residual stress solidification from shrinkage
- Lack of preheating before starting welding
- High content of sulfur and carbon in base metals
- Using hydrogen as shielding gas for welding ferrous metals
Remedies for Weld Crack
- Select appropriate metal materials and ensure their surfaces are clean before welding.
- Use the correct welding speed and current for the job.
- Preheat the base metal and reduce the cooling speed of the joint to minimize residual stress.
- Use the appropriate sulfur and carbon mixture for the base metal.
- Reduce the gap between weld joints to minimize the risk of cracking.
2. Undercut
Undercut defects are characterized by irregular grooves in the base metal that take the shape of notches. They are caused by the melting of the base metal away from the weld zone and are categorized by their length, depth, and sharpness. These defects run parallel to the weldment and can lead to a loss in thickness, making the weld joint more susceptible to fatigue. The types of undercuts include:
- Root undercut: found at the root of the weld and can be due to an incorrect choice of welding current or speed.
- Toe undercut: found at the toes of the weld and can be caused by an incorrect welding angle or electrode manipulation.
- Sidewall undercut: found at the sides of the weld and can be due to the incorrect size of the welding electrode or a misaligned torch.
Causes of Undercut
- Using too high voltage or too fast weld speed that causes melting at the top edge
- High arc voltage during welding
- Wrong electrode angle or the electrode is too large for the weld joint
- Using the wrong filler metal that doesn’t match the base metal
- Incorrect selection of shielding gas for the welding process
Remedies for Undercut
- Adjust the welding parameters such as travel speed and power input.
- Reduce the arc voltage or arc length. The voltage should be between 15 to 30 volts and the welding arc length should not exceed the electrode core diameter.
- Maintain an electrode angle between 30 to 45 degrees on the standing leg.
- Use the appropriate filler metal for the base metal being welded.
- Ensure the proper selection of shielding gas based on the metal type and thickness.
- Weld in flat positions to minimize the effects of gravity on the molten metal.
3. Porosity
Porosity, also referred to as wormhole welds, is a welding defect that happens when air or gas bubbles are trapped in the weld. During the welding process, gases like hydrogen, carbon dioxide, and steam are produced. These gases get trapped in the weld bead and create a spongy appearance when viewed in cross-section.
The trapped gases can be distributed uniformly throughout the weld or confined to a specific location. These bubbles can weaken the weld metal, making it more prone to fatigue and damage. The following are the types of porosity defects that may occur based on their formation:
Porosity, also referred to as wormhole welds, is a welding defect that happens when air or gas bubbles are trapped in the weld. During the welding process, gases like hydrogen, carbon dioxide, and steam are produced. These gases get trapped in the weld bead and create a spongy appearance when viewed in cross-section.
The trapped gases can be distributed uniformly throughout the weld or confined to a specific location. These bubbles can weaken the weld metal, making it more prone to fatigue and damage. The following are the types of porosity defects that may occur based on their formation:
- Gas Porosity: Small spherical-shaped cavity generated due to trapped gases. It can take different forms such as surface pores, elongated cavities, and linear porosity.
- Worm Holes: Elongated or tubular cavities formed during solidification of trapped gases. They can appear as single holes or a group of holes throughout the weld surface.
- Surface Porosity: A type of porosity that breaks the surface of the weld metal.
Causes of Porosity
- Inadequate coating of electrode or use of corroded electrode
- Presence of grease, oil, water, rust, or hydrocarbon on the weld surface
- Using incorrect shielding gas
- Too high arc voltage or gas flow. The voltage should typically be between 15 to 30 volts
- Poor surface treatment of base metal
Remedies for Porosity
- Use the appropriate shielding gas based on the type of metal and thickness.
- Maintain the recommended arc voltage and current, and avoid fluctuations during welding.
- Ensure proper storage of electrodes to prevent corrosion.
- Conduct proper pre-weld cleaning of the base metal to remove contaminants.
- Monitor and control the weld pool size and prevent overheating.
4. Spatter
Spatters are a type of welding defect characterized by small particles of metal that are expelled from the welding arc during various welding processes such as arc welding, tack welding, or gas welding. Although spatters occur less frequently during MIG welding, they can still happen. These metal particles can stick to the surface of the base metal and the length of the weld bead, as well as the joint design.
Accumulated spatters within the nozzle can dislodge and impair the weld bead, as well as pose a safety hazard for welders due to the sharpness of spatter projections.
Causes of Spatter
- Incorrect voltage or amperage settings, such as too low voltage or too high amperage
- Using the wrong shielding gas
- Rigid electrode working angle during welding
- Using a wet electrode or an electrode with too long of an arc length
- Contamination of the metal surface, such as grease, oil, or rust
Remedies for Spatter
- Adjust the welding parameters by increasing the voltage and reducing the amperage.
- Choose the appropriate shielding gas for the type of welding being done.
- Adjust the electrode angle to a more flexible position and decrease the arc length.
- Clean the metal surface before welding to prevent contamination.
5. Overlap
A weld overlap is a welding defect in which the filler material at the weld’s toe does not bond with the underlying metal. This occurs when the weld pool flows excessively and extends beyond the toe, resulting in an angle of less than 90 degrees between the weld metal and the base metal.
Causes of Overlap
- Improper welding technique
- Varying electrode angle during welding
- Using electrodes that are too large for the workpiece
- High welding current or excessive heat input.
Remedies for Overlap
- Use the appropriate welding technique and adjust the arc length.
- Maintain a consistent electrode angle throughout the weld.
- Avoid using larger-sized electrodes that might generate more heat.
- Consider welding in flat positions, if possible.
- Decrease the heat input or welding current.
6. Lack of Fusion
Incomplete fusion, also known as lack of fusion, is a welding defect that occurs due to inaccurate welding, resulting in unfilled gaps. This can be caused by several factors, including:
The lack of fusion may occur in three forms:
- Between the parent metal and the weld metal at the weld’s root.
- Between the parent metal and weld metal at the sidewall weld, known as lack of sidewall fusion.
- Between adjacent layers of weld metal during multi-run welding, known as lack of inter-run fusion.
Incomplete fusion can also be visible on the outer surface of a weld when there is improper fusion between the outer sidewall and the parent metal.
Causes of Lack of fusion
- Insufficient heat input
- Metal surface contamination
- Inappropriate electrode diameter for the material thickness
- Excessive travel speed
- Large weld pools advancing beyond the arc
Remedies for Lack of fusion
- Optimize the heat input to a suitable range for the material and welding technique.
- Thoroughly clean the welding area and the metal surface before welding.
- Use the correct electrode diameter according to the material thickness.
- Control the travel speed to ensure proper fusion.
- Use an appropriate weld pool size that does not exceed the arc’s capabilities.
7. Lack of Penetration
Penetration in welding refers to the depth from the top surface of the base metal to the deepest point of the weld. Incomplete penetration is a welding defect that occurs when the metal groove is too narrow, and the weld metal does not spread or reach the bottom of the joint. This results in reduced joint strength and can lead to weld failure.
Causes of Lack of Penetration
- Improper joint alignment
- Excessive space between the weld
- Too fast movement of the welding bead resulting in inadequate metal deposition
- Low amperage setting preventing sufficient melting of the metal
- Incorrect electrode positioning
Remedies for Lack of Penetration
- Ensure proper joint preparation and alignment.
- Increase weld deposition.
- Adjust amperage settings to provide adequate heat input for the base metal.
- Decrease arc travel speed to allow for proper melting and penetration.
- Position the electrode correctly to allow for proper weld placement.
8. Burn Through
A burn-through is a welding defect that occurs when too much heat is applied during the welding process, causing holes to form through the metal. This defect is typically found in thin metal sheets with less than 1/4-inch thickness, although it can also occur in thicker metals if welding settings are too high or if the torch movement is too slow.
Causes of Burn Through
- Incorrect or excessive welder settings for the thickness of the metal being welded
- Large gaps between metal pieces
- Slow torch movement during welding
- Using incorrect wire sizes for the type and thickness of metal being welded
Remedies for Burn Through
- Adjust the current or welder setting to avoid using too high heat.
- Avoid having excessive gaps between metal plates to prevent overheating.
- Optimize the travel speed to ensure it is not too slow, depending on the welding technique used. For instance, MIG welding may require a travel speed between 14 to 19 inches per minute, while orbital welding equipment may require a travel speed of 4 to 10 IPM.
- Avoid using large bevel angles that can lead to excessive heat input.
- Use tight wire sizes to regulate the heat input.
- Ensure adequate metal clamping and hold-down to prevent any movement during welding.
9. Slag Inclusion
Slag inclusion is a common visible welding defect that occurs due to the use of flux in welding processes like stick welding, flux-cored arc welding, and submerged arc welding. The flux is a solid material used as a shielding agent during welding, which can sometimes melt in the weld zone or on the surface of the weld, resulting in the presence of a vitreous material called slag.
Causes of Slag Inclusion
- Inadequate cleaning of base metal or weld joint
- Too fast welding speed
- Failure to clean the weld pass before starting a new one
- Improper welding angle
- Weld pool cools down too quickly
- Welding current is too low.
Remedies of Slag Inclusion
- Increase the current density to melt the slag effectively.
- Reduce rapid cooling by preheating the base metal or using specialized heat control techniques.
- Adjust the electrode angle for proper penetration and to avoid trapping slag in the weld.
- Remove any slag from the previous bead before starting a new one.
- Adjust the welding speed to allow proper fusion of the metal and avoid trapping slag in the weld.
10. Spatter
Spatter is a common welding defect where small particles from the weld stick to the surrounding surface, especially in gas metal arc welding. While it cannot be entirely eliminated, it can be minimized through the following methods:
Causes of Spatter
- High running amperage
- Low voltage setting
- Steep work angle of the electrode
- Contaminated surface
- Long arc
- Incorrect polarity
- Erratic wire feeding
Remedies of Spatter
- Clean the surface thoroughly before welding to prevent contamination.
- Reduce the arc length to minimize spatter.
- Adjust the weld current to an appropriate level to reduce spatter.
- Increase the electrode angle to reduce spatter.
- Ensure that the correct polarity is used to avoid spatter.
- Check for and fix any feeding issues in the welding process to minimize spatter.
Conclusion
Welding is a complex process that involves choosing the right technique and ensuring proper implementation to prevent welding defects. These defects can negatively impact the quality of your product and business performance. It is essential to understand the types of welding defects, their causes, and how to prevent them.
To meet your manufacturing goals, it is crucial to invest in the right tools and choose a reliable metal welding partner. RapidDirect provides high-quality sheet metal welding services with robust facilities and a strict quality inspection process. Our team of skilled engineers and technicians uses the right materials and techniques to manufacture parts that meet your requirements. Contact us today to get started on your project.