A seam weld may appear complete from the outside, yet the product may still fail an airtightness test. In other cases, welding quality may be acceptable at the beginning of production but gradually deteriorate, resulting in localized leakage, peeling, or insufficient joint strength. These are common resistance seam welding problems, and they are often difficult to identify through visual inspection alone.
Important Principle
A continuous surface weld mark does not necessarily mean that the internal weld is continuous, and it does not guarantee an airtight joint.
Resistance seam welding uses rotating wheel electrodes to apply both pressure and welding current to the workpiece. The weld seam is normally formed by a series of overlapping weld nuggets. Under certain conditions, it may also form a more continuous fused zone. Only when weld nugget size, position, and overlap remain consistent can a reliable airtight or liquid-tight joint be produced.
What Is a “Weak Weld” in Seam Welding?
The term “weak weld” is commonly used on the shop floor, but it is not a precise technical definition. In seam welding, it may refer to one or more of the following conditions:
- Insufficient weld nugget size
- Low joint strength
- Partial fusion between the workpieces
- Inadequate overlap between weld nuggets
- Unwelded or discontinuous areas within the seam
- Visible wheel marks without sufficient bonding at the interface
A weak seam weld does not always cause the parts to separate immediately. However, for products such as fuel tanks, water tanks, heat exchangers, silencers, shock absorber components, and other sealed containers, even a very small unwelded area may result in air or liquid leakage.
Do Not Simply Increase Welding Current When Leakage Occurs
When a leakage problem occurs, one of the most common reactions is to increase the welding current or reduce the welding speed. This may provide temporary improvement, but it does not always solve the actual problem.
The heat generated during resistance welding is influenced by welding current, electrical resistance, and current duration. Electrode force also changes the contact condition and resistance between the workpieces. Excessive electrode force may reduce contact resistance and result in insufficient heating. Insufficient force may cause unstable contact, localized overheating, arcing, or metal expulsion.
Seam welding quality must therefore be evaluated through the combined effect of:
- Welding current
- Welding time
- Wheel speed
- Electrode force
- Workpiece condition
- Equipment stability
It should never be judged by one parameter alone.
The following are the most common causes of seam weld leakage and weak welds.
Seven Common Causes of Seam Weld Leakage and Weak Welds
1. Insufficient or Unstable Welding Heat
If the actual welding current is too low, the current duration is too short, or the wheel speed is too high, the interface between the workpieces may not generate a sufficiently large weld nugget. Common causes include:
- Welding current set too low
- Wheel speed too high
- Insufficient welding energy per unit length
- Pulse-on time too short
- Cooling time or pulse interval too long
- Insufficient welding transformer or power-source capacity
- Excessive impedance in the secondary circuit
- Loose electrical connections
- Supply voltage fluctuation
- Differences between controller settings and actual current output
For this reason, troubleshooting should include checking the actual welding current waveform or monitored current value, rather than relying only on the percentage or set value displayed on the controller.
2. Insufficient Weld Nugget Overlap
Pulse seam welding does not normally produce one completely uninterrupted weld bead. Instead, it creates a sequence of individual weld nuggets that overlap one another and form a continuous sealing path. Resistance seam welding may use either continuous current or intermittent current, depending on the product, material, and required production speed.
If the wheel speed is too high, the pulse frequency is too low, or the cooling interval is too long, gaps may remain between adjacent weld nuggets. These gaps may be too small to detect visually but may still allow gas or liquid to pass through.
The weld pitch in pulse seam welding can be conceptually expressed as:
Weld pitch = welding travel speed × one complete weld-and-cool cycle time
Therefore, even when the welding current remains unchanged, increasing wheel speed or increasing the pulse interval will reduce weld nugget overlap.
The process should not be evaluated only by the controller’s weld time and cool time settings. The following must be considered together:
- Wheel speed
- Weld nugget length
- Weld pitch
- Actual overlap percentage
3. Electrode Wheel Wear, Contamination, or Profile Change
The electrode wheel does more than conduct welding current. It also:
- Applies force to the workpiece
- Defines the welding area
- Controls current density
- Guides the weld path
During production, the electrode wheel may develop the following problems:
- The wheel face becomes wider or flatter
- Grooves form on the wheel edge
- Coating, oxide, or workpiece material adheres to the wheel
- The wheel develops lateral runout
- The upper and lower wheels become misaligned
- Clearance develops in the wheel shaft, bearing, or guide mechanism
As the electrode contact area becomes larger, current density decreases even when the welding current setting remains unchanged. This may gradually reduce weld nugget size.
Electrode wheel geometry, alignment, cleanliness, and dressing condition all affect weld consistency.
When welding quality is acceptable immediately after replacing or dressing the electrode wheel but leakage begins after extended production, electrode wear, pickup, and cooling should be checked before increasing the welding current.
4. Insufficient Cooling of the Electrode Wheel or Welding Transformer
Seam welding is a continuous, high-duty resistance welding process. Compared with ordinary spot welding, the electrode wheels and welding transformer are generally exposed to higher thermal loads. Insufficient cooling may cause:
- Continuous temperature rise of the electrode wheels
- Reduced electrode hardness
- Accelerated deformation of the wheel profile
- Workpiece overheating
- Electrode sticking or material pickup
- Excessive transformer temperature rise
- Reduced welding output
- Different results between cold-start and hot-production conditions
The following cooling conditions should therefore be verified:
- Cooling water flow rate
- Water temperature
- Water quality
- Inlet and outlet direction
- Blockage or restriction in individual cooling circuits
Water-cooled electrodes, transformers, and power components are commonly used in resistance welding systems to control thermal loading during production.
5. Unstable Workpiece Fit-Up, Dimensions, or Positioning
Stable machine output does not guarantee stable welding quality if the workpiece condition varies. Differences in workpiece dimensions, overlap, roundness, or fixture positioning can change both the actual electrode force and the current path through the joint. Common problems include:
- Insufficient overlap width
- Poor workpiece roundness
- Poor flatness
- Local gaps at the joint interface
- Inconsistent bending, stamping, or rolling dimensions
- Burrs causing premature local contact
- Insufficient fixture clamping
- Weld path not centered on the overlap
- Workpiece eccentricity during rotation
- Vertical workpiece runout
Burrs, incorrect contact positions, and poor fit-up may also create current shunting paths. As a result, part of the welding current may bypass the intended joint interface.
6. Oil, Oxide, or Inconsistent Coating on the Workpiece Surface
Resistance welding relies on electrical resistance at the workpiece interface to generate heat. Surface condition therefore has a direct effect on heat generation and distribution.
The following may cause unstable contact resistance:
- Oil contamination
- Rust
- Oxide scale
- Paint
- Stamping residue
- Inconsistent coating thickness
The result may not simply be “no weld.” Instead, some areas may receive insufficient heat while other areas overheat or produce expulsion.
Coated materials such as galvanized steel may also transfer coating material onto the copper-alloy electrode surface. This can accelerate electrode contamination and change the wheel profile.
For coated materials, the following should be evaluated separately:
- Welding schedule
- Electrode material
- Electrode wheel profile
- Dressing frequency
- Cleaning frequency
7. Insufficient Overlap at the Weld Start, End, or Intersection
One of the most common leakage locations in circumferential seam welding is the point where the weld starts and ends. Possible causes include:
- Welding current begins before full electrode force is reached
- The workpiece has not reached stable speed before welding begins
- Current is stopped too early at the end of the weld
- Start-to-end overlap is insufficient
- Current upslope or downslope is not properly controlled
- Current shunting occurs at the weld overlap area
- Reheating at the overlap causes excessive thinning or burn-through
Circumferential seam welding should not simply stop after one complete revolution. The start and end positions normally require a suitable overlap distance based on the weld pitch and product requirements. This overlap must be sufficient to prevent leakage, but it must also be controlled to avoid overheating or burn-through.
Recommended Troubleshooting Procedure for Seam Weld Leakage
Step 1: Identify the Actual Leak Location
It is not enough to know that a product leaks. The exact leak position should be identified and marked on the workpiece.
Common leak-testing methods include:
- Water immersion bubble test
- Soap solution test
- Pressure decay test
- Tracer gas test
- Helium leak testing
Water immersion and soap solution tests are useful for locating the leak point. Pressure decay testing is suitable for determining whether the complete part leaks, but the result may be affected by workpiece temperature and internal volume changes. For very low allowable leak rates, helium or another tracer gas may be considered.
The following test conditions should be defined according to the actual product application and customer specification:
- Test pressure
- Holding time
- Allowable leak rate
- Test medium
- Product temperature
Step 2: Determine Whether the Leak Location Follows a Pattern
The position and timing of the leak often provide important diagnostic information:
| Leakage Pattern | Priority Inspection Area |
|---|---|
| Every part leaks at the same location | Product design, fixture positioning, overlap dimension, weld start and end |
| Leak locations appear randomly | Surface contamination, current fluctuation, force instability, electrode pickup |
| Leakage begins after extended production | Electrode wear, insufficient cooling, thermal drift |
| Leakage occurs only at the weld start or end | Start-end overlap, force timing, current upslope and downslope |
| Expulsion or burn-through occurs but the part still leaks | Insufficient force, poor fit-up, local overheating, poor wheel condition |
| Weld appearance is normal but the airtightness test fails | Insufficient nugget overlap or internal lack of fusion |
Step 3: Check the Workpiece and Mechanical System Before Changing Parameters
The following should be inspected in sequence:
- Confirm that the workpieces fit together correctly
- Verify the overlap width and joint position
- Confirm that the upper and lower electrode wheels are aligned
- Check the electrode wheels for runout, wear, or contamination
- Verify that the actual electrode force is stable
- Confirm that actual wheel speed matches the set value
- Check cooling water flow and temperature
- Verify that the actual welding current is stable
When workpiece fit-up, machine alignment, or electrode condition is poor, changing the controller settings may only hide the problem temporarily.
Step 4: Use Destructive Testing to Evaluate the Weld Nuggets
An airtightness test can confirm that leakage exists, but it does not fully explain why the weld failed. During machine trials or process development, the following methods may be used:
- Peel test
- Chisel test
- Tensile test
- Shear test
- Weld cross-section preparation
- Macrographic examination
- Nugget width measurement
- Fusion depth evaluation
- Nugget overlap evaluation
Destructive testing helps determine whether the problem is caused by:
- Undersized weld nuggets
- Insufficient overlap
- Weld-position deviation
- Incomplete fusion
- Excessive workpiece thinning
- Cracking
Step 5: Adjust One Major Variable at a Time
A baseline welding condition should first be established. Then adjust one parameter at a time, such as:
- Welding current
- Wheel speed
- Weld time
- Cool time
- Electrode force
Changing several parameters simultaneously may produce an acceptable sample, but it becomes difficult to determine which change caused the improvement. It also makes it harder to establish a stable production process window.
Choosing a Seam Welding Machine: Do Not Look Only at Maximum Welding Current
Producing one acceptable sample and maintaining stable quality during long-term production are two different requirements. For continuous weld seams that must be airtight or liquid-tight, the following factors should be evaluated when selecting a seam welding machine.
1. Define the Actual Weld Requirement
The first step is to determine whether the product requires:
- Structural strength
- Airtightness
- Liquid tightness
- Continuous surface appearance
- Roll spot welding at a fixed pitch
- A specified allowable leak rate
- A specified test pressure
- A specified production speed
- A specified daily output
When airtightness is required, machine design and process validation should not rely only on weld appearance or mechanical strength.
2. Confirm the Machine’s Continuous-Duty Capacity
In addition to maximum welding current and rated kVA, the following should be considered:
- Welding transformer duty cycle
- Total weld length per part
- Number of parts per minute
- Electrode wheel cooling capacity
- Transformer cooling capacity
- Secondary circuit design
- Temperature rise during continuous production
- Output consistency between cold and hot operating conditions
A machine may have a high maximum current rating but still be unsuitable for long seams or high-volume production if its cooling capacity or duty cycle is insufficient.
3. Evaluate Welding Power Supply and Control Accuracy
Possible power-source options include:
- Single-phase AC
- Three-phase rectified DC
- Medium-frequency direct current, or MFDC
- Other inverter-based systems
The power source should be selected according to:
- Material
- Material thickness
- Surface coating
- Required wheel speed
- Required welding current
- Electrical supply conditions
- Control response
- Monitoring and compensation requirements
MFDC and other inverter-controlled systems can normally provide more precise current control and more process data. However, not every product requires MFDC. The most important consideration is whether the machine capacity, control method, and mechanical structure match the actual application.
4. Wheel Speed and Electrode Force Must Remain Stable
Continuous seam quality is highly sensitive to speed variation. If the drive system produces low-speed crawling, speed fluctuation, or excessive backlash, the heat input per unit length will vary even when the welding current remains stable.
The force system must also maintain stable and appropriate electrode force despite:
- Workpiece dimensional variation
- Thermal expansion
- Joint thickness variation
- Workpiece deformation during welding
5. Electrode Wheel Design and Dressing Method
The electrode wheel should be selected according to the material and product design. Important considerations include:
- Electrode wheel material
- Wheel face width
- Wheel edge profile
- Internal or external cooling
- Electrode wheel diameter
- Dressing method
- Replacement procedure
- Maintenance accessibility
The electrode wheel is part of the welding process, not merely a consumable part. An unsuitable electrode wheel may create a very narrow acceptable welding window.
6. Fixture and Workpiece Positioning Accuracy
A seam welding fixture must ensure that every workpiece enters the weld area in the same position and orientation.
For circumferential seam welding, the fixture design should consider:
- Workpiece roundness
- Rotation center
- Clamping deformation
- Weld start and end position
- Workpiece runout
- Joint alignment
For longitudinal seam welding, the fixture should control:
- Joint straightness
- Feeding direction
- Overlap width
- Workpiece flatness
- Weld-path alignment
7. Process Monitoring and Quality Traceability
Depending on product risk and quality requirements, the following parameters may be monitored:
- Actual welding current
- Electrode voltage
- Welding time
- Wheel speed
- Electrode force
- Cooling water flow
- Dynamic resistance
- Welding energy
- Electrode usage count
- Electrode dressing interval
Advanced welding controllers may use current, voltage, resistance, and energy data to detect production drift or abnormal welding conditions. However, welding data within the allowable range does not guarantee that the product is leak-free. For sealed products, welding-process monitoring and leak testing should be treated as complementary quality-control methods.
Conclusion: Stable Continuous Seam Quality Depends on the Entire Process
Seam weld leakage and weak welds are rarely caused by only one welding parameter. They normally result from the combined effect of:
- Material condition
- Surface condition
- Workpiece fit-up
- Electrode wheel condition
- Welding current
- Wheel speed
- Electrode force
- Weld nugget overlap
- Machine cooling
- Fixture accuracy
When a leakage problem occurs, the solution should not be limited to repeatedly increasing welding current. The correct approach is to:
- Identify the leakage pattern.
- Check the workpiece and mechanical condition.
- Verify actual welding output.
- Perform destructive weld evaluation.
- Establish a stable process window through controlled testing.
When planning a new seam welding machine, the following information should be provided to the machine manufacturer:
- Workpiece material
- Material thickness
- Surface coating
- Joint design
- Overlap dimensions
- Weld shape
- Total weld length
- Required production speed
- Required production volume
- Leak-test pressure
- Holding time
- Allowable leak rate
- Existing defective samples
- Actual leak locations
- Weld strength requirements
- Surface appearance requirements
Dahching Electric can evaluate suitable equipment solutions according to the product geometry, weld requirement, and production conditions.
Possible solutions include:
- Circumferential seam welding
- Longitudinal seam welding
- Continuous seam welding
- Pulse seam welding
- Roll spot welding
Depending on project requirements, the machine may also be integrated with:
- Welding-process monitoring
- Cooling-system monitoring
- Automatic loading and unloading
- Airtightness testing
- Production data recording
- Quality traceability
Frequently Asked Questions About Seam Weld Leakage and Weld Quality
Q1: Can I solve seam weld leakage simply by increasing the welding current?
Increasing welding current may be included as part of process testing, but it should not be treated as the only solution. If the leakage is caused by poor workpiece fit-up, electrode wear, unstable wheel speed, or insufficient weld nugget overlap, increasing current may cause:
- Expulsion
- Excessive indentation
- Electrode sticking
- Workpiece thinning
- Burn-through
Q2: Does continuous seam welding always require continuous current?
No. A continuous sealed seam may be produced by overlapping pulse weld nuggets or by using continuous current. The suitable mode depends on:
- Material
- Material thickness
- Welding speed
- Surface appearance
- Machine thermal load
- Required leak tightness
Q3: Why does the seam look normal but still fail the airtightness test?
The surface wheel marks mainly show where the electrode contacted the workpiece. They do not necessarily show the actual weld nugget size, fusion condition, or overlap at the internal joint interface. Small unwelded areas, inadequate start-end overlap, internal cracks, or incomplete fusion may only be detected during leak testing.
Q4: Why does the machine weld correctly at startup but begin leaking after extended production?
Common causes include:
- Electrode wheel temperature rise
- Electrode wheel wear
- Coating pickup
- Insufficient cooling
- Welding transformer temperature rise
- Thermal drift
- Gradual workpiece dimensional variation
The actual welding current, wheel speed, electrode force, and cooling condition should be compared between cold-start and hot-production conditions.
Q5: What is the most important specification for an airtight seam welding machine?
Maximum welding current and rated kVA are not sufficient by themselves. The following must also be evaluated:
- Duty cycle
- Wheel-speed stability
- Electrode-force control
- Electrode wheel system
- Machine-frame rigidity
- Cooling capacity
- Fixture accuracy
- Welding-process monitoring
- Start-end overlap control
- Long-term production stability
If you are facing seam weld leakage or weak weld issues, or planning an airtight seam welding machine, please contact Dahching. We will evaluate the most suitable solution based on your product and process requirements.