Key Factors Affecting the Quality of Laser Welding

There are many factors affecting the quality of laser welding, some of which are highly variable and quite unstable. How to correctly set and control these parameters to ensure their appropriate range in the high-speed continuous laser welding process is crucial for the reliability and stability of weld formation. This is an important issue relating to the practicality and industrialization of laser welding technology.

The main factors affecting the quality of laser welding into three aspects: welding equipment, workpiece condition, and welding parameters. Taking the example of plate butt single-side welding with double-side formation.

A general analysis of these three aspects will be done below.

Main Factors Affecting the Quality of Laser Welding:

(1) Equipment

  • Output power and stability
  • Beam mode
  • Focal length of the focusing lens
  • Quality and stability of optical components

(2) Parts Status

  • Plate thickness
  • Material
  • Gap between seams
  • Seam straightness
  • Misalignment

(3) Welding Parameters

  • Laser power
  • Welding speed
  • Focus position
  • Type and flow rate of protective gas

Welding Equipment

Composition of Welding Equipment

The laser welding equipment mainly consists of a laser, a light guide and focusing system, a welding machine, and a control system.

(1) Laser

Lasers used for high-power laser welding are mainly CO2 lasers and YAG lasers. Recently, high-power fiber lasers and disc lasers (disk laser) have received considerable attention as new types of lasers. The characteristics of CO2 lasers and YAC lasers are compared in Table 2-1.

Table 2-1 Comparison of Characteristics between CO2 Lasers and YAG Lasers

Type of LaserWavelength/μmMetal Absorption RatioBeam QualityBeam QualityFiber TransmissionTransmissive Optical Components
CO2 Laser10.6LowGoodExcellentNot PossibleMade with special optical materials (like ZnSn, GaAs, etc.), which are relatively expensive.
YAG Laser1.06Relatively HighInferiorSubparPossibleConstructed from standard optical glass, which is comparatively affordable.

The most important performance of a laser is its output power and beam quality (beam mode). Considering these two aspects, the CO2 laser has a significant advantage: it has a good beam mode, small spot size after focusing, and high output power. Most applications in production are in the range of 1.5~15kW, but the largest laser in the world has reached 50kW. YAC lasers are commonly used below 4kW and can reach a maximum of 10kW.

Its beam quality is worse than CO2 laser, but its wavelength is shorter, only 1/10 of CO2 laser, which is beneficial for metal surface absorption, especially for welding materials like aluminum alloy with high reflectivity to infrared. It can be transmitted by fiber, which greatly simplifies the light guide system and is more suitable for three-dimensional laser welding and cutting.

Fiber lasers have both the high beam quality of CO2 lasers and the short wavelength of YAC lasers that can be transmitted by fiber. Moreover, they have high reliability, long life, small size, and high conversion efficiency, and will be increasingly widely used.

(2) Light Guide and Focusing System

The light guide and focusing system consist of a circular polarizer, a beam expander, a reflector or fiber, and a focusing lens, which implement the functions of changing the beam polarization state and direction, transmitting the beam, and focusing.

Focusing lenses are divided into focusing lenses and reflecting focusing mirrors. The focusing performance of the lens is good, and focusing lenses are often used for laser welding below 2kW. As the laser power increases, the thermal effect and thermal failure tendency of this lens made of transmission material increase sharply, so reflecting focusing mirrors should be used for high-power welding above 2kW.

Reflective focusing mirrors are made of metals with high reflectivity to lasers. The mirror surface is stable, thermal distortion is small, the failure value is high, but the focusing performance is not as good as that of the lens, and the relative position accuracy of the focusing mirror surface and the human-emitted laser is very high, which is difficult to adjust and easily causes focusing spot astigmatism.

The focal length of the focusing lens is an important parameter that affects the focusing effect and welding process performance, generally selected between 127mm (5in) and 200mm (7.9in).

A small focal length can obtain a small focusing spot diameter and high spot radiant exposure, but if it is too small, it is easy to be contaminated and fly damaged during the welding process, and once the mirror surface is contaminated, its absorption of the laser will significantly increase, reducing the laser power reaching the workpiece, and may cause lens rupture.

(3) Laser Welding Machine

Its function is to achieve relative motion between the beam and the workpiece, complete laser welding, and it can be divided into dedicated welding machines and general welding machines. The latter often adopts a numerical control system, including two-dimensional, three-dimensional welding machines with Cartesian coordinates or joint-type laser welding robots.

Factors Affecting Welding Quality in Welding Equipment

The primary requirements for laser quality are the beam mode, output power, and their stability. The beam mode is a crucial indicator of beam quality – the lower the order of the beam mode, the better the beam focusing performance, the smaller the beam spot, the higher the irradiance under the same laser power, and the greater the weld penetration to width ratio.

Generally, the fundamental mode (TEM00) or low-order modes are required; otherwise, it’s difficult to meet the demands of high-quality laser welding. Although there are some difficulties in using domestically produced high-power lasers for laser welding due to beam quality and output power stability, foreign lasers have achieved high beam quality and output power stability, which usually aren’t factors affecting laser welding quality.

The condition of the optical components has a crucial impact on the quality of laser welding.

Under high-power laser operation, optical components, especially lenses, will degrade, reducing the transmittance ratio; this can generate a thermal lens effect, causing the focal length to change during the welding process (the lens heats up and expands, altering the refractive index and shortening the focal length); surface contamination can also increase transmission loss. Therefore, the quality, maintenance, and operational status monitoring of optical components are critical in ensuring welding quality.

Workpiece Condition

Laser welding requires high edge processing and assembly accuracy of the workpiece, strict alignment of the beam spot with the weld, and the original assembly accuracy and beam spot alignment should not change due to thermal deformation during welding. This is because the beam spot is small, the weld is narrow, and no filler metal is generally added.

If the assembly is not strict and the gap is too large, the beam will pass through the gap without melting the parent material, or cause obvious undercutting and sinking; if the deviation of the beam spot from the seam is slightly larger, it may cause incomplete fusion or incomplete penetration.

For example, when butt welding a 2-3mm thick plate, the gap and beam spot seam deviation should not be greater than 0.1mm, and misalignment should not exceed 0.2mm.

When the weld is long, the preparation before welding is very difficult, ordinary shears cannot meet the requirements, and it must be machined again, or high-precision shears or laser cutting should be used to prepare the bevel. It is also necessary to design suitable precision jigs according to the specific workpiece situation to ensure the necessary assembly accuracy and apply a certain clamping force.

Welding Parameters

Main Welding Parameters

The most important welding parameters are laser power, welding speed, and beam focus position. Given the equipment conditions and workpiece condition, these three parameters will determine the laser welding mode (conduction welding or deep penetration welding) and weld formation (penetration depth and width), as well as the stability of the welding process and weld formation.

These issues will be systematically discussed in the next section.

The type and flow of shielding gas affect the protection of the weld pool and the effect of suppressing photonic plasma, which in turn affects weld quality.

Analysis of Monitorability of Each Welding Parameter

Among the four welding parameters, welding speed and shielding gas flow are easy to monitor and maintain stability, while laser power and focus position are parameters that may fluctuate during the welding process and are difficult to control.

Although the laser power output from the laser has high stability and is easy to monitor, due to the loss of the guiding and focusing system, the laser power reaching the workpiece will change. This loss is related to the quality of the optical components, their usage time, and surface contamination, making it difficult to monitor and a source of uncertainty in welding quality.

The beam focus position is a factor in the welding parameters that greatly affects weld quality but is also the most difficult to monitor and control. At present, in production, the focus position needs to be manually adjusted and determined through repeated process experiments to achieve the ideal penetration depth.

However, during the welding process, due to the flying optical path, workpiece deformation, thermal lens effect, or three-dimensional welding of spatial curves, the focus position may change and possibly exceed the allowable range.

For the above two situations, on the one hand, high-quality, high-stability optical components should be used, daily maintenance should be paid attention to, contamination should be prevented, and cleanliness should be maintained; on the other hand, it is necessary to develop real-time monitoring and control methods for the laser welding process to optimize parameters, monitor changes in the laser power and focus position reaching the workpiece, realize closed-loop control, and improve the reliability and stability of laser welding quality.

This is one of the directions for the development of laser welding technology.

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