SHINHOP laser

To weld your product well, in addition to understanding the performance of laser welding equipment, you also need to master certain welding process knowledge to better utilize the performance of the welding equipment and improve the level of welding process technology. Whether you are a process engineer or a novice user, it is best to understand the basic knowledge of laser welding process before using the laser welding machine.
I. Principle of Laser Welding:
Laser welding can be achieved using continuous or pulsed laser beams. The principle of laser welding can be divided into conduction welding and laser deep penetration welding. When the power density is less than 10^4~10^5 W/cm², it is conduction welding, where the penetration is shallow and the welding speed is slow; when the power density is greater than 10^5~10^7 W/cm², the metal surface is heated to form a "keyhole", resulting in deep penetration welding, characterized by fast welding speed and a large depth-to-width ratio.
The principle of conduction laser welding is as follows:
Laser radiation heats the surface to be processed, and the heat diffuses inward through conduction. By controlling laser parameters such as pulse width, energy, peak power, and repetition frequency, the workpiece melts to form a specific molten pool. The laser welding machine used for gear welding and metallurgical thin plate welding mainly involves laser deep penetration welding.

The following focuses on the principle of laser deep penetration welding.
Laser deep penetration welding generally uses a continuous laser beam to complete the connection of materials. Its metallurgical physical process is very similar to electron beam welding, where the energy conversion mechanism is completed through a "keyhole" structure. Under sufficiently high power density laser irradiation, the material evaporates and forms a small hole. This small hole filled with vapor acts like a black body, absorbing almost all of the incident beam energy, with the equilibrium temperature inside the cavity reaching about 2500°C. The heat is transferred from the high-temperature cavity wall to melt the surrounding metal. The small hole is filled with high-temperature vapor generated by the continuous evaporation of the wall material under beam irradiation, and the molten metal surrounds the solid material (while in most conventional welding processes and laser conduction welding, energy is first deposited on the surface of the workpiece and then transferred to the interior).
The liquid flow outside the hole and the surface tension of the wall layer maintain a dynamic balance with the continuous vapor pressure generated inside the cavity. The beam continuously enters the small hole, and the material outside the small hole flows continuously. As the beam moves, the small hole remains in a stable flowing state.
This means that the small hole and the molten metal surrounding the hole wall move forward with the speed of the leading beam, filling the gap left after the molten metal moves away and subsequently solidifying, thus forming the weld seam. All of this happens so quickly that the welding speed can easily reach several meters per minute.
II. Main Process Parameters of Laser Deep Penetration Welding:
1. Laser Power.
There is a threshold for laser energy density in laser welding. Below this value, the penetration is very shallow. Once this value is reached or exceeded, the penetration will increase significantly. Only when the laser power density on the workpiece exceeds the threshold (which is material-dependent) will plasma be generated, marking the stable deep penetration welding process. If the laser power is below this threshold, only surface melting occurs on the workpiece, meaning the welding proceeds as stable conduction welding. 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 significant fluctuations in penetration. During laser deep penetration welding, the laser power simultaneously controls the penetration depth and welding speed. The penetration depth of the weld is directly related to the beam power density and is a function of the incident beam power and the beam focus size. Generally, for a laser beam of a certain diameter, the penetration increases with the increase in beam power.
2. Beam Focus Size.
The size of the beam spot is one of the most important variables in laser welding, as it determines the power density. However, measuring it for high-power lasers is a challenge, although many indirect measurement techniques exist.
　　The diffraction-limited spot size of the beam focus can be calculated based on the theory of light diffraction, but due to the aberration of the focusing lens, the actual spot is larger than the calculated value. The simplest practical measurement method is the isothermal contour method, which involves burning and penetrating a polypropylene sheet with thick paper and then measuring the spot and hole diameters. This method requires measuring practice to master the appropriate laser power and the time of beam action.

3. Material Absorption Value.
The absorption of laser by materials depends on several important properties of the material, such as absorption rate, reflectivity, thermal conductivity, melting temperature, and evaporation temperature, among which the absorption rate is the most important.
Factors affecting the absorption rate of materials to laser beams include two aspects: first, the resistivity of the material. Measurements of the absorption rate of polished surfaces of materials have found that the absorption rate is proportional to the square root of the resistivity, which varies with temperature; secondly, the surface condition (or smoothness) of the material has a significant impact on the absorption rate of the beam, thus affecting the welding effect.
　　The output wavelength of CO2 lasers is typically 10.6 μm. Non-metallic materials such as ceramics, glass, rubber, and plastics exhibit a high absorption rate at room temperature, whereas metals show poor absorption at this temperature, which only increases significantly once the material melts or vaporizes. Employing surface coatings or generating oxide films on the surface is an effective method to enhance the material's absorption of the laser beam.
4. Welding Speed.
Welding speed has a significant impact on penetration depth. Increasing the speed will make the penetration shallower, but if the speed is too low, it will lead to excessive melting of the material and burn through the workpiece. Therefore, for a specific laser power and a certain thickness of a specific material, there is an appropriate range of welding speeds, and within this range, the corresponding speed value can achieve maximum penetration.
5. Protective Gas.
In the laser welding process, inert gases are commonly used to protect the molten pool. When welding certain materials where surface oxidation is not a concern, protection may not be considered. However, for most applications, gases such as helium, argon, and nitrogen are often used for protection, preventing oxidation of the workpiece during the welding process.
Helium is not easily ionized (it has a high ionization energy), allowing the laser to pass through smoothly, with the beam energy reaching the workpiece surface without obstruction. This makes it the most effective protective gas used in laser welding, although it is relatively expensive.
Argon is relatively cheap and has a higher density, providing better protection. However, it is easily ionized by high-temperature metal plasma, which results in some of the beam being shielded from reaching the workpiece, reducing the effective laser power for welding and impairing welding speed and penetration. The surface of the welded piece protected by argon is smoother than that protected by helium.
Nitrogen is the cheapest protective gas, but it is not suitable for welding certain types of stainless steel, mainly due to metallurgical issues such as absorption, which can sometimes lead to porosity in the overlap area.
The second function of using protective gas is to protect the focusing lens from contamination by metal vapor and splatter from liquid droplets. This is especially necessary during high-power laser welding, as the ejected materials become very forceful, making lens protection even more critical.
The third function of protective gas is to effectively disperse the plasma shield generated during high-power laser welding. Metal vapor absorbs the laser beam and ionizes into a plasma cloud, and the protective gas surrounding the metal vapor will also ionize due to heating. If there is too much plasma, the laser beam is partially consumed by the plasma.
Plasma exists as a second form of energy at the work surface, causing shallower penetration and a wider surface of the molten pool. The electron recombination rate can be increased by increasing the collisions between electrons, ions, and neutral atoms, thereby reducing the electron density in the plasma. The lighter the neutral atom, the higher the collision frequency and recombination rate; on the other hand, only protective gases with high ionization energy will not increase electron density due to the ionization of the gas itself.
As can be seen from the table, the size of the plasma cloud varies with the protective gas used, with helium being the smallest, followed by nitrogen, and argon resulting in the largest. The larger the plasma size, the shallower the penetration. This difference is primarily due to the varying degrees of ionization of gas molecules and also due to the differences in density of the protective gases affecting the diffusion of metal vapor.

Helium has the least ionization and the lowest density, allowing it to quickly expel the rising metal vapor generated from the molten pool. Therefore, using helium as a protective gas can maximally suppress plasma, thereby increasing penetration and improving welding speed; due to its light weight, it can escape easily and is less likely to cause porosity. Of course, from our actual welding results, the effect of using argon for protection is also quite good.
The effect of the plasma cloud on penetration is most pronounced at low welding speeds. As the welding speed increases, its effect diminishes.
Protective gas is ejected through the nozzle at a certain pressure to reach the workpiece surface. The fluid dynamics shape of the nozzle and the size of the outlet diameter are very important. It must be large enough to drive the ejected protective gas to cover the welding surface, but to effectively protect the lens and prevent contamination or damage from metal vapor, the nozzle size must also be limited. The flow rate must also be controlled; otherwise, the laminar flow of the protective gas will turn into turbulent flow, drawing in the atmosphere into the molten pool and ultimately forming porosity.
To enhance the protective effect, an additional lateral blowing method can be used, which involves directing the protective gas at a certain angle into the deep molten welding hole through a smaller diameter nozzle. The protective gas not only suppresses the plasma cloud on the workpiece surface but also affects the plasma inside the hole and the formation of the small hole, further increasing penetration and achieving a more ideal weld with a good depth-to-width ratio. However, this method requires precise control of the gas flow rate and direction; otherwise, it can easily create turbulence that disrupts the molten pool, making the welding process difficult to stabilize.
6. Focal Length.
During welding, a focusing method is usually used to converge the laser, typically selecting lenses with focal lengths of 63~254mm (2.5”~10”). The size of the focused spot is proportional to the focal length; the shorter the focal length, the smaller the spot. However, the length of the focal length also affects the depth of focus, which increases synchronously with the focal length. Therefore, a short focal length can increase power density, but due to the small depth of focus, the distance between the lens and the workpiece must be precisely maintained, and the penetration is also not large.
Due to the influence of spatter generated during the welding process and the laser mode, the shortest depth of focus used in actual welding is mostly around a focal length of 126mm (5”). When the joint is larger or when it is necessary to increase the weld size by enlarging the spot size, a lens with a focal length of 254mm (10”) can be selected. In this case, to achieve the effect of deep molten small holes, higher laser output power (power density) is required.
　　When the laser power exceeds 2kW, particularly for a 10.6μm CO2 laser beam, special optical materials are used in the optical system. To prevent the risk of optical damage to the focusing lens, reflective focusing methods are frequently employed, typically utilizing polished copper mirrors as reflectors. Due to their effective cooling properties, these mirrors are often recommended for focusing high-power laser beams.
7. Focal Position.
During welding, to maintain sufficient power density, the position of the focal point is crucial. The relative position of the focal point to the workpiece surface directly affects the width and depth of the weld.
In most laser welding applications, the position of the focal point is usually set about 1/4 of the required penetration depth below the workpiece surface.
8. Laser Beam Position.
When performing laser welding on different materials, the position of the laser beam controls the final quality of the weld, especially in the case of butt joints, which are more sensitive to this than lap joints. For example, when welding a quenched steel gear to a low-carbon steel drum, correctly controlling the position of the laser beam will favor the formation of a weld primarily composed of low-carbon components, which has better crack resistance.
In some applications, the geometric shape 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 10 degrees, the workpiece's absorption of laser energy will not be affected.
9. Control of gradual increase and decrease of laser power at the start and end points of welding. During laser deep penetration welding, regardless of the depth of the weld, the phenomenon of small holes 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 welding layer covers the original weld, excessive absorption of the laser beam occurs, leading to overheating of the workpiece or the formation of pores.
　　To prevent the above phenomena, a program can be developed for the power start and stop points, making the start and stop times adjustable. The starting power can be increased from zero to the set power value within a short time using electronic methods, and the welding time can be adjusted. Finally, at the end of the welding process, the power is gradually reduced from the set power to zero.

3. Characteristics and advantages and disadvantages of laser deep penetration welding:
1. Characteristics of laser deep penetration welding
1) High depth-to-width ratio. Because the molten metal surrounds the cylindrical high-temperature vapor cavity and extends towards the workpiece, the weld becomes deep and narrow.
2) Minimal heat input. Because the temperature inside the small hole is very high, the melting process occurs extremely quickly, resulting in very low heat input to the workpiece, with minimal thermal deformation and heat-affected zone.
3) High density. Because the small hole filled with high-temperature vapor facilitates the stirring of the welding pool and the escape of gases, resulting in a porosity-free weld. The high cooling rate after welding also helps to refine the weld structure.
4) Strong welds. Due to the intense heat source and sufficient absorption of non-metallic components, the impurity content is reduced, and the size and distribution of inclusions in the molten pool are altered. The welding process does not require electrodes or filler wires, resulting in less contamination of the molten area, making the strength and toughness of the weld at least equal to or even exceeding that of the base metal.
5) Precise control. Because the focused light spot is very small, the weld can be precisely positioned. The laser output has no "inertia," allowing for rapid stops and restarts at high speeds, and CNC beam movement technology can weld complex workpieces.
6) Non-contact atmospheric welding process. Because the energy comes from the photon beam and there is no physical contact with the workpiece, no external forces are applied to the workpiece. Additionally, magnetic fields and air have no effect on the laser. 2. Advantages of laser deep penetration welding
1) Due to the much higher power density of focused laser compared to conventional methods, welding speed is fast, and the heat-affected zone and deformation are minimal, allowing for welding of difficult materials such as titanium.
2) Because the beam is easy to transmit and control, there is no need to frequently change welding guns or nozzles, and there is no vacuum required for electron beam welding, significantly reducing downtime for auxiliary operations, resulting in high load factors and production efficiency.
3) Due to the purifying effect and high cooling rate, the strength, toughness, and overall performance of the weld are high.
4) Due to low average heat input and high processing accuracy, reprocessing costs can be reduced; additionally, the operating costs of laser welding are also lower, thereby reducing the processing costs of the workpiece.
5) Effective control over beam intensity and fine positioning makes it easy to achieve automated operations.
3. Disadvantages of laser deep penetration welding
1) Limited welding depth.
2) High assembly requirements for workpieces.
3) High initial investment for laser systems.