Laser Technology

1. Laser Welding Principle

Laser welding can be achieved using continuous or pulsed laser beams. The principle of laser welding can be divided into heat conduction welding and laser deep penetration welding. Power density less than 10⁴~10⁵ W/cm² is heat conduction welding, characterized by shallow penetration and slow welding speed. When the power density is greater than 10⁵~10⁷ W/cm², the metal surface is heated, creating "cavities" and forming deep penetration welding, which is characterized by fast welding speed and a large depth-to-width ratio.

The principle of heat conduction laser welding is as follows: laser radiation heats the surface to be processed, and the surface heat diffuses inward through heat conduction. By controlling laser parameters such as the width, energy, peak power, and repetition frequency of the laser pulse, the workpiece melts, forming a specific molten pool.

Laser welding machines used for gear welding and metallurgical thin plate welding mainly involve laser deep penetration welding. The principle of laser deep penetration welding will be discussed in detail below.

Laser deep penetration welding typically uses a continuous laser beam to join materials. Its metallurgical physics is very similar to electron beam welding, with the energy conversion mechanism achieved through a "keyhole" structure. Under sufficiently high power density laser irradiation, the material evaporates and forms a keyhole. This vapor-filled keyhole acts like a blackbody, absorbing almost all the energy of the incident beam. The equilibrium temperature inside the keyhole reaches approximately 2500 °C. Heat is transferred from the outer wall of this high-temperature keyhole, melting the metal surrounding it. The keyhole is filled with high-temperature vapor generated by the continuous evaporation of the wall material under beam irradiation. The keyhole walls surround molten metal, and the liquid metal surrounds solid material (in most conventional welding processes and laser conduction welding, energy is first deposited on the workpiece surface and then transferred to the interior). The liquid flow and surface tension outside the keyhole walls maintain a dynamic equilibrium with the continuously generated vapor pressure inside the keyhole. As the laser beam continuously enters the keyhole, the material outside the keyhole continues to flow. As the laser beam moves, the keyhole remains in a stable flow state. In other words, the pinhole and the molten metal surrounding it move forward at the same speed as the guide beam. The molten metal fills the gaps left after the pinhole moves away and then solidifies, thus forming a weld. All of this happens so quickly that welding speeds can easily reach several meters per minute.

2. Key Process Parameters of Laser Deep Penetration Welding

Laser Power

Laser welding involves a laser energy density threshold. Below this threshold, the penetration depth is shallow; once reached or exceeded, the penetration depth increases significantly. Plasma is generated only when the laser power density on the workpiece exceeds this threshold (material-dependent), marking the commencement of stable deep penetration welding. If the laser power is below this threshold, only surface melting occurs on the workpiece, meaning welding proceeds in a stable heat conduction mode. When the laser power density is near the critical condition for keyhole formation, deep penetration welding and conduction welding alternate, resulting in an unstable welding process with large fluctuations in penetration depth. In laser deep penetration welding, the laser power simultaneously controls the penetration depth and welding speed. The weld penetration depth is directly related to the beam power density and is a function of the incident beam power and the beam focal spot. Generally, for a laser beam of a certain diameter, the penetration depth increases with increasing beam power.

Beam Focal Spot

The beam spot size is one of the most important variables in laser welding because it determines the power density. However, measuring it for high-power lasers is challenging, despite the existence of many indirect measurement techniques.

The diffraction-limited spot size of the laser beam can be calculated based on optical diffraction theory. However, due to aberrations in the focusing lens, the actual spot size is larger than the calculated value. The simplest practical measurement method is the isothermal profilometry method, which involves charring a thick piece of paper and penetrating a polypropylene plate before measuring the focal spot and the diameter of the perforation. This method requires practical measurement to master the laser power and the duration of beam contact.

Material Absorption Value

The absorption of laser light by a material depends on several important properties, such as absorptivity, reflectivity, thermal conductivity, melting temperature, and evaporation temperature, with absorptivity being the most important.

Factors affecting the absorptivity of a material to a laser beam include two aspects: First, the material's resistivity. Measurements of the absorptivity of polished surfaces show that the absorptivity is proportional to the square root of the resistivity, which in turn varies with temperature. Second, the surface condition (or smoothness) of the material has a significant impact on the beam absorptivity, thus significantly affecting the welding effect.

The output wavelength of a CO2 laser is typically 10.6 μm. Non-metallic materials such as ceramics, glass, rubber, and plastics have high absorption rates at room temperature, while metallic materials absorb it poorly at room temperature, with absorption only increasing sharply once the material melts or even vaporizes. Surface coatings or oxide films are effective methods to improve the material's absorption of the laser beam.

Welding Speed

Welding speed significantly affects weld penetration. Increasing the speed results in shallower penetration, while excessively low speeds lead to over-melting and burn-through. Therefore, for a specific material with a given laser power and thickness, there is a suitable range of welding speeds, within which the maximum penetration can be achieved. Figure 10-2 shows the relationship between welding speed and penetration for 1018 steel.

Shielding Gas

Inert gases are commonly used to protect the molten pool during laser welding. While surface oxidation may not be a concern for certain materials, helium, argon, and nitrogen are commonly used in most applications to prevent oxidation of the workpiece during welding.

Helium is poorly ionized (but has high ionization energy), allowing the laser beam to pass through smoothly and reach the workpiece surface unimpeded. This is the most effective shielding gas used in laser welding, but it is relatively expensive.

Argon is cheaper and has a higher density, resulting in good protection. However, it is easily ionized by high-temperature metal plasma, which shields part of the beam from reaching the workpiece, reducing the effective laser power and impairing welding speed and penetration. Welds protected with argon have smoother surfaces than those protected with helium.

Nitrogen is the cheapest shielding gas, but it is not suitable for welding certain types of stainless steel, mainly due to metallurgical issues such as absorption, which can sometimes create porosity in the joint area.

A second function of shielding gases is to protect the focusing lens from metal vapor contamination and molten droplet sputtering. This is especially important in high-power laser welding, where the ejected material becomes very powerful.

A third function of shielding gases is their effectiveness in dispersing the plasma generated by high-power laser welding. Metal vapor absorbs the laser beam and ionizes into a plasma cloud. The protective gas surrounding the metal vapor also ionizes due to heating. If there is too much plasma, the laser beam is consumed to some extent by the plasma. Plasma exists as a secondary energy source on the working surface, resulting in shallower weld penetration and a wider weld pool. The electron recombination rate is increased by increasing collisions between electrons, ions, and neutral atoms, thereby reducing the electron density in the plasma. The lighter the neutral atoms, the higher the collision frequency and recombination rate; on the other hand, only a protective gas with high ionization energy can prevent an increase in electron density due to the ionization of the gas itself.

The size of the plasma cloud varies depending on the shielding gas used, with helium having the smallest size, followed by nitrogen, and argon having the largest. A larger plasma cloud results in a shallower weld penetration. This difference is primarily due to the varying degrees of ionization of the gas molecules, and also to the differences in metal vapor diffusion caused by the different densities of the shielding gases.

Helium has the lowest ionization and density, allowing it to quickly displace rising metal vapor from the molten metal pool. Therefore, using helium as a shielding gas maximally suppresses plasma, thereby increasing weld penetration and welding speed; its light weight also allows it to escape easily, reducing the likelihood of porosity. However, based on our actual welding results, argon shielding has proven to be quite effective.

The impact of the plasma cloud on weld penetration is most pronounced at low welding speeds. Its effect diminishes as the welding speed increases.

The shielding gas is ejected at a certain pressure through a nozzle and reaches the workpiece surface. The hydrodynamic shape of the nozzle and the diameter of the outlet are crucial. The shielding gas must be sufficiently large to cover the welding surface, but the nozzle size must be limited to effectively protect the lens and prevent metal vapor contamination or metal spatter damage. The flow rate must also be controlled; otherwise, the laminar flow of the shielding gas will become turbulent, atmospheric entrainment will enter the molten pool, and porosity will ultimately form.

To improve the shielding effect, an additional lateral blowing method can be used, where the shielding gas is injected directly into the pinhole of the deep penetration weld through a smaller diameter nozzle at a certain angle. The shielding gas not only suppresses the plasma cloud on the workpiece surface but also influences the plasma within the pinhole and the formation of the pinhole, further increasing the penetration depth and achieving a weld with an ideal depth-to-width ratio. However, this method requires precise control of the gas flow rate and direction; otherwise, turbulence can easily occur, damaging the molten pool and making the welding process unstable.

Lens Focal Length

During welding, the laser is typically focused, usually using lenses with focal lengths of 63~254mm (2.5”~10”). The size of the focused spot is directly proportional to the focal length; the shorter the focal length, the smaller the spot. However, the focal length also affects the depth of focus, meaning the depth of focus increases proportionally with the focal length. Therefore, a shorter focal length can increase power density, but due to the shallow depth of focus, the distance between the lens and the workpiece must be precisely maintained, and the penetration depth is also limited. Due to the influence of spatter and laser mode generated during welding, the shortest focal length actually used in welding is often 126mm (5”). When the joint is large or when it is necessary to increase the weld size by increasing the spot size, a lens with a focal length of 254mm (10”) can be selected. In this case, to achieve the deep penetration keyhole effect, a higher laser output power (power density) is required.

When laser power exceeds 2kW, especially for 10.6μm CO2 laser beams, due to the use of special optical materials in the optical system, reflection focusing is often used to avoid optical damage to the focusing lens. Polished copper mirrors are typically used as reflectors. Because of their effective cooling properties, they are often recommended for focusing high-power laser beams.

Focus Position

During welding, the focus position is crucial for maintaining sufficient power density. Changes in the relative position of the focus to the workpiece surface directly affect the weld width and depth. Figure 2-6 shows the effect of focus position on the penetration depth and weld width of 1018 steel.

In most laser welding applications, the focus is typically positioned approximately one-quarter of the way below the workpiece surface to achieve the desired penetration depth.

Laser Beam Position

When laser welding different materials, the laser beam position controls the final weld quality, especially in butt joints where it is more sensitive than in lap joints. For example, when welding a hardened steel gear to a low-carbon steel drum, proper laser beam position control will result in a weld primarily composed of low-carbon components, which exhibits better crack resistance. In some applications, the geometry of the workpiece to be welded requires the laser beam to be deflected at an angle. When the deflection angle between the beam axis and the joint plane is within 100 degrees, the workpiece's absorption of laser energy will not be affected.

Laser power increase and decrease control at the start and end points of welding

During laser deep penetration welding, regardless of the weld depth, the pinhole phenomenon always exists. When the welding process is terminated and the power switch is turned off, a pit will appear at the end of the weld. Additionally, when the laser weld layer covers the original weld, excessive absorption of the laser beam can occur, leading to overheating or porosity in the weldment.

To prevent these phenomena, the power start and end points can be programmed to make the power start and end times adjustable. That is, the starting power is electronically increased from zero to the set power value within a short time, and the welding time is adjusted. Finally, at the end of welding, the power is gradually reduced from the set power to zero.

3. Characteristics, advantages, and disadvantages of laser deep penetration welding

Characteristics of laser deep penetration welding

1) High aspect ratio. 1) **Deep and narrow weld:** Because the molten metal forms around the cylindrical high-temperature steam cavity and extends towards the workpiece, the weld becomes deep and narrow.

2) **Minimum heat input:** Due to the extremely high temperature inside the aperture, the melting process occurs very rapidly, resulting in very low heat input to the workpiece, minimizing heat deformation and the heat-affected zone.

3) **High density:** The aperture filled with high-temperature steam facilitates stirring of the weld pool and gas escape, leading to a porosity-free, fully penetrated weld. The high cooling rate after welding further refines the weld microstructure.

4) **Strong weld:** The intense heat source and the full absorption of non-metallic components reduce impurity content and alter the size and distribution of inclusions in the weld pool. The welding process requires no electrodes or filler wire, resulting in less contamination in the molten zone, making the weld strength and toughness at least equal to or even exceeding that of the base metal.

5) **Precise control:** Because the focused spot is very small, the weld can be precisely positioned. Laser output has no "inertia," allowing for rapid stops and restarts at high speeds. CNC beam movement technology enables the welding of complex workpieces. 6) Non-contact atmospheric welding process. Because the energy comes from a photon beam, there is no physical contact with the workpiece, so no external force is applied to the workpiece. Furthermore, magnetism and air have no effect on the laser.

Advantages of Laser Deep Penetration Welding

1) Due to the much higher power density of focused lasers compared to conventional methods, welding speed is fast, the heat-affected zone and deformation are small, and it can weld difficult-to-weld materials such as titanium.

2) Because the beam is easy to transmit and control, and there is no need for frequent changes of welding torches and nozzles, and no vacuuming required for electron beam welding, downtime is significantly reduced, resulting in high load factor and production efficiency.

3) Due to the purification effect and high cooling rate, the weld has high strength, toughness, and overall performance.

4) Due to the low average heat input, processing accuracy is high, reducing reprocessing costs; in addition, laser welding operating costs are also lower, thus reducing workpiece processing costs.

5) Beam intensity and precise positioning can be effectively controlled, making automated operation easy.

Disadvantages of Laser Deep Penetration Welding

1) Limited welding depth.

2) High requirements for workpiece assembly.

3) High initial investment in laser systems.