Robot Laser Welding: Common Defects and Quality Standards

Appearance quality of laser welding seam in robot welding

The laser welding joints are classified into levels I, II, and III according to the regulations of GJB481. Level I and Level II joints are specified in the process documents, while those not specified are considered Level III joints. For different welding joints, the requirements for the appearance quality of laser welding seams are as follows:

(1) Weld Seam Width

The width of the same weld seam should be uniform, and the ratio of the maximum front width to the minimum front width should not exceed 1.2. The ratio of the maximum front width to the minimum front width at the shrinkage of the annular weld seam should not exceed 1.4. The weld seam width is illustrated in Figure 3-53.

Figure 3-53 Weld Bead Width

The weld seam width should meet the specified requirements, as shown in Table 3-28.

Table 3-28 Weld Seam Width

Thickness of base metal t/mmWidth on the front side /mmWidth on the back side /mm
1.0≤<2.0≥1.5t or 2.4, whichever is smaller≥0.4
2.0≤t<5.0≥1.2t≥0.6

(2) Seam Height

The schematic diagram of seam height is shown in Figure 3-54.

Figure 3-54: Seam Height
Figure 3-54: Seam Height

For level I and level II joints, the front and back seam heights should comply with the requirements in Table 3-29 (excluding joints machined after welding). No specific requirements are set for level III joints.

(3) Misalignment

The schematic diagram of seam misalignment is shown in Figure 3-55.

Solution: For proper alignment of the welded parts, adjustment of welding fixtures and improvement of the accuracy of part assembly can eliminate misalignment defects.

Figure 3-55 Misalignment
Figure 3-55 Misalignment

Table 3-29: Requirements for Front and Back Seam Heights

Thickness of base metal t/mmHeight of the front bead protrusion /mmHeight of the back bead protrusion /mm
1.0≤t<2.0Take the smaller value of either less than 30% or 0.4≤1.0
2.0≤t<4.0Take the smaller value of either less than 25% or 0.8≤1.5
4.0≤t<5.0Take the smaller value of either less than 20% or 1.0≤2.0

The permissible misalignment for Class I and Class II joints can be found in Table 3-30; no specifications are provided for Class III joints.

Table 3-30 Permissible Misalignment for Butt Joints

Weld Seam GradeMisalignment/mm
I≤10%t
II≤15%t
Note: t represents the thickness of the base metal.

(4) The illustration of incomplete fusion is shown in Figure 3-56.

Figure 3-56 Incomplete Fusion

The cause of the depression on the surface of the weld metal is due to poor positioning of the welding spot during welding, with the spot center being close to the lower part and deviating from the center of the weld, resulting in partial melting of the base metal.

Solution: Adjust the light/wire match.

The permissible values for incomplete fusion in Class I and Class II joints can be found in Table 3-31; no specifications are provided for Class III joints.

Table 3-31 Permissible Values for Incomplete Fusion

Joint TypeJoint GradeIncomplete Fusion
Weld Length/mmMaximum Depth of a Single Defect/mmAccumulated Length of a Single Defect over any 100mm Length of Weld/mm
Flat butt joint, Groove butt jointI10%t15%t≤15
II15%t20%t≤30
Note: t represents the thickness of the base metal.

(5) Undercut

The schematic diagram of undercut in the weld is shown in Figure 3-57.

Figure 3-57 Undercut

Solution: During butt welding, the laser beam should be as perpendicular to the workpiece surface as possible. Excessive deviation in the angle of the laser beam can lead to poor wetting between the molten metal and the unmelted metal, causing undercut. Reducing the welding speed can decrease the likelihood of undercut.

For Class I and Class II joints, undercut with a fillet radius R less than 1.5mm is not allowed, and no specific requirement is specified for Class III joints. The allowable depth of undercut should comply with the regulations in Table 3-32.

Table 3-32 Allowable Values for Undercut

Joint ClassMaximum Depth of Undercut (mm)Accumulated Length of Undercut on any 100mm Long Weld Seam (mm)
I8%t≤10
II10%t≤20
Note: t represents the thickness of the base metal

(6) Surface Depression

Surface depression is a prominent phenomenon in laser welding. The central depression on the surface of the weld seam is caused by the recoil force generated by metal vaporization, which pushes the liquid metal toward the weld point surface.

During the cooling process, the rapidly solidifying accumulated metal on the surface fails to completely fill in, resulting in the central depression. The primary cause of central depression is material loss due to rapid evaporation and spattering of the metal, as depicted in Figure 3-58.

Figure 3-58 Surface depression in laser weld seam
Figure 3-58 Surface depression in laser weld seam

Solution: Slow welding speed, excessive laser power, or the use of negative focal length can lead to a large amount of material melting and evaporating from the workpiece surface, causing surface depression, especially in cases of laser autogenous welding without filler. Increasing the welding speed, reducing laser power, and adjusting the focal length can mitigate surface depression.

The allowable depth of depression should comply with the regulations in Table 3-33.

Table 3-33 Allowable Values for Surface Depression

Joint ClassUndercut Depth/mm
Ih ≤ 0.1, and not greater than 0.5
IIh ≤ 0.2, and not greater than 0.5
IIIh ≤ 0.3t, and not greater than 1
Note: t represents the thickness of the base metal; h denotes the undercut depth.

(7) Weld Bead

When there is a significant change in the weld path, such as welding at an angle, it is easy to encounter weld beads or uneven formation at the corners, as shown in Figure 3-59.

Figure 3-59 Weld Bead

Countermeasure: Optimize welding parameters, adjust the teaching points and laser incidence angle at the corners to ensure smooth rotation of the laser at the corners.

(8) Spatter

After laser welding, many metal particles may appear on the surface of some workpieces or materials, affecting both the appearance and the functionality.

The generation of spatter is due to the presence of impurities or surface coatings on the workpiece surface, which interact with the laser, causing disturbance in the material’s metal vapor and plasma, making the entire welding process unstable, leading to the overflow of partially molten metal particles. Spattering on the back of the weld seam is more common when welding thin sheets, as illustrated in Figure 3-60.

Figure 3-60 Laser Weld Surface Spatter
Figure 3-60 Laser Weld Surface Spatter

Solution: Frontal spatter on the workpiece is generally caused by surface impurities. Thoroughly cleaning and washing the surface to be welded before welding to remove oxides, impurities, oil, rust, and other contaminants will significantly reduce spatter during laser welding.

Back spatter on the workpiece is more common in thin plate laser welding. Reducing laser power or increasing welding speed and defocusing can prevent the laser from penetrating the back of the test piece, thus reducing spatter.

(9) Interruption or Uneven Thickness of Weld Seam

During laser welding, an unstable wire feed or discontinuous light emission can cause interruptions or uneven thickness in the weld seam, as shown in Figure 3-61.

Figure 3-61 Laser Weld Interruption
Figure 3-61 Laser Weld Interruption

(10) Weld Seam Build-Up

Excessive filling material in the weld seam during filling welding results in a too high weld seam. This is caused by excessive wire feed speed or too slow welding speed.

Solution: Increase welding speed or decrease wire feed speed, or reduce laser power.

(11) Weld Offset

The weld metal does not solidify at the center of the joint structure. This is due to inaccurate positioning during welding or misalignment of the light and wire during filling welding.

Solution: Adjust the laser incidence position, adjust the position of the light and wire during filling welding, and the position of the light, wire, and weld seam.

Internal Quality of Robot Laser Welding Seam

(1) Cracks

The schematic diagram of cracks is shown in Figure 3-62, and cracks are not allowed inside the robot laser welding seam joint.

During the laser welding process of carbon steel materials, due to the relatively small amount of heat input from the laser, the welding deformation and stress generated during welding are also relatively small, so cracks generally do not occur. However, for laser welding of stainless steel materials, the semi-melted zone contains a large amount of austenite in the solid-state structure.

The rapid cooling speed of the laser welding seam is sufficient to transform the austenite into martensite, which increases the tendency for stress to cause crack formation. Cracks often originate from low-melting-point eutectics. The elements S, P, and B in the laser welding seam increase the tendency for crack formation, and if the welding parameters are improperly selected, cracks may also occur.

Solution: For deep penetration welding, using optimized pulse waveform to control the cooling speed of metal solidification process and reduce internal stress is an effective method to suppress crack formation.

In addition, adjusting the fixture and reducing the transverse restraint of the weld seam will also reduce the probability of longitudinal crack formation.

There is a greater probability of crack formation at the end of the welding, so adjusting the laser parameters and, at the end of the welding, not immediately turning off the laser, but extending the laser emission time to maintain a certain amount of melting at the end position, can also reduce the tendency for cracks.

(2) Pores

Due to the rapid cooling speed of the laser welding process, the metal around the keyhole does not have sufficient time to fill back, resulting in shrinkage pores. Surface pores are pores exposed on the surface of the weld seam, which are generally rare; most pores appear inside the weld seam.

When the metal solidifies, the solubility of hydrogen in the liquid metal decreases, and when the rapidly evaporating metal vapor in the keyhole condenses rapidly in the molten pool, slightly smaller internal pores are formed, as shown in Figure 3-63.

Figure 3-62: Cracks
Figure 3-62: Cracks
Figure 3-63: Pores in the laser welding seam
Figure 3-63: Pores in the laser welding seam

Solution: Properly reducing the welding speed, increasing the time for gas to escape from the molten pool, and increasing the filling time of the metal around the keyhole can reduce the cooling speed, which is a means to solve surface pores or shrinkage holes.

Single pores and chain-like pores that may exist inside Class I and Class II joints should comply with the requirements in Table 3-34 and Table 3-35, and should not have sharp-angled pores. No requirements are specified for Class III joints. When the pore size D is not greater than 0.2mm, it is not considered a defect.

Table 3-34: Requirements for individual pores in joints

Class of JointMaximum allowable pore size D/mmPore spacing /mmThe cumulative length of pores within any 100mm long weld within /mm
ITake the smaller value of 0.5t or 2.0mm≥3DTake the smaller value of 3t or 12
IITake the smaller value of 0.75t or 2.5mm≥2DTake the smaller value of 5t or 20
Note: t is the thickness of the base material.

(3) Inclusions

X-ray visible inclusions are not allowed within the welded joint.

(4) Incomplete Penetration

The schematic diagram of incomplete penetration is shown in Figure 3-64. Incomplete penetration is not allowed in Class I and Class II joints. For Class III joints, incomplete penetration is allowed along the full length of the weld, provided that the usage requirements are met.

Table 3-35: Requirements for Chain-Like Porosity in Welded Joints

Sealing RequirementsMaximum allowable size D of a single porosity within the chain/mmSpacing between individual porosities within the chain/mmThe number of chain porosities within a 100mm length of the weld shall not exceedThe length of distribution of chain porosities shall not exceed/mm
None0.3t or the smaller value between 1.0≥DTwo18
Present0.3t or the smaller value between 0.5≥DOne10
Note: Nail tip pores are not considered chain porosities; t denotes the thickness of the base material.

Solution: Incomplete penetration is mainly caused by insufficient laser power, and excessive welding speed can also result in incomplete penetration. Increasing the laser power, reducing the welding speed, increasing the pulse energy during pulse welding, and reducing the number of pulses can enhance the laser power, thereby increasing the heat input for metal melting and increasing the penetration depth.

Additionally, under unchanged parameters, using negative focal length can also increase the penetration depth.

(5) Lack of Fusion

The schematic diagram of lack of fusion is shown in Figure 3-65. Lack of fusion is not allowed in Class I and Class II joints. For Class III joints, lack of fusion is allowed along the full length of the weld, provided that the usage requirements are met.

Figure 3-64: Incomplete Penetration
Figure 3-64: Incomplete Penetration
Figure 3-65: Lack of Fusion

Robot Laser Weld Seam Inspection

Refer to Table 3-36 for the items and requirements of robot laser weld seam inspection.

Table 3-36 Robot Laser Weld Seam Inspection Items and Requirements

Inspection ItemRequirementsInspection Tools
Weld Seam LengthUnless specified on the drawing, the actual length of the weld should have an additional 1.5mm at the start and end of the effective length.Vernier caliper, fillet weld gauge (applicable for fillet welds)
Weld Seam WidthCompliance with drawing requirements.Vernier caliper, weld seam gauge
Penetration DepthUnless specified on the drawing, the fusion rate should exceed 30% (or 0.8~1.2mm).After sectioning the weld, measure the weld cross-section dimensions using a vernier caliper or an electronic microscope
Weld Seam Peel TestSecure the weldment in a dedicated fixture, apply external force to the base metal using a notching tool or hammer until the weld fractures, and observe the tearing condition.Visual inspection using a 5x magnifying glass
Weld Seam Tensile and Torsion TestTensile testing machine, torque wrench

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