1. Laser welding process parameters
1.1 Power density
Power density is a crucial parameter in laser processing. Higher power density can rapidly heat the surface layer to its boiling point within microseconds, resulting in significant vaporization. As a result, high power density is advantageous for material removal processes such as cutting, notching and drilling.
On the other hand, a lower power density takes a few milliseconds to reach the surface temperature boiling point. This allows the bottom layer to reach the melting point before the surface layer vaporizes, making it easier to create a strong fusion weld. Therefore, the power density for conduction laser welding is typically in the range of 10^4 to 10^6 W/cm².
1.2 Laser pulse waveform
The waveform of the laser pulse is a crucial factor in laser welding, especially in sheet metal welding.
When a high-intensity laser beam strikes the surface of the material, 60% to 98% of the laser energy at the metal surface is reflected and lost. This reflectivity depends on the surface temperature and varies accordingly.
The reflectivity of the metal fluctuates considerably during a laser pulse.
1.3 Laser pulse width
Pulse width is an important parameter in pulsed laser welding. It is not only distinct from material removal and melting, but also a crucial factor that determines the cost and volume of processing equipment.
1.4 Effect of defocus amount on welding quality
Laser welding typically requires a certain degree of defocus due to the high power density of the center of the laser focus spot, which can easily cause evaporation and pinholes. On the other hand, the power density distribution is relatively uniform in the plane away from the laser focus.
There are two blur modes available: positive and negative blur. Positive blur occurs when the focal plane is above the workpiece, while negative blur occurs when it is below.
According to the theory of geometric optics, the power density in corresponding planes is approximately the same when the positive and negative separations are equal. However, in reality, the shape of the weld pool is different.
Negative defocus can result in greater penetration, which is related to the molten pool formation process. Experimental results suggest that the material begins to melt within 50 to 200 us after being heated by the laser, forming metal in liquid phase and vaporization and commercial pressure vapor, which emits dazzling white light at very high speed.
At the same time, the high vapor concentration causes the liquid metal to move to the edge of the weld pool, creating a depression in the center of the weld pool.
When negative defocus is used, the internal power density of the material is greater than that of the surface, facilitating the production of stronger melting and vaporization. This allows light energy to be transferred deeper into the material, resulting in greater penetration. Therefore, negative defocus should be used for greater penetration, while positive defocus should be used when welding thin materials in practical applications.
two . Laser welding technology
1) Sheet to sheet welding
It includes butt welding, end welding, center penetration fusion welding and center piercing fusion welding.
2) Wire-to-wire welding
It includes wire-to-wire butt welding, cross welding, parallel back welding and T-type welding.
3) Welding of wire and block element
Laser welding can be used to connect the wire and the block element successfully, and the size of the block element can be arbitrary.
Attention should be paid to the geometric dimension of the wire element when welding.
4) Welding different metals
To weld various types of metals, it is necessary to determine their weldability and the range of weldable parameters.
It is important to note that laser welding can only be performed between certain material combinations.
Although laser brazing may not be appropriate for connecting certain components, lasers can be used as a heat source for both welding and brazing, which also offers the benefits of laser welding.
There are several welding methods available, and laser welding is mainly used for printed circuit board (PCB) welding, especially for wafer assembly technology.
3 . The advantages of laser welding
- Local heating reduces the risk of thermal damage to the element and results in a small heat-affected zone, allowing welding close to the thermal element.
- Non-contact heating can melt bandwidth without the need for auxiliary tools. This allows processing on double-sided printed circuit boards after installing the double-sided components.
- The stable nature of repeated operation coupled with minimal flux pollution in welding tools makes laser brazing a favorable option. Furthermore, laser irradiation time and output power are easily controlled, resulting in high laser brazing yield.
- The laser beam can be easily split using optical elements such as half lenses, mirrors, prisms and scanning mirrors. This allows simultaneous symmetrical welding of multiple points.
- Laser brazing mainly uses a laser with a wavelength of 1.06 um as a heat source, which can be transmitted through optical fiber. This allows the processing of parts that are difficult to weld using conventional methods, providing greater flexibility.
- The laser beam is well focused and easily automated for multistation devices.
4 . Laser Deep Penetration Welding
4.1 Metallurgical process and technology theory
The metallurgical process of laser deep penetration welding is similar to electron beam welding in that they both rely on the “small hole” structure to complete the energy conversion.
When the power density is high enough, the material evaporates, creating a small hole. This hole is filled with vapor and acts like a black body, absorbing almost all the energy from the incident light. The equilibrium temperature inside the hole cavity is about 25,000 degrees.
Heat is transferred from the outer wall of the high-temperature cavity to melt the surrounding metal. The hole is continuously filled with high-temperature vapor generated by the evaporation of the wall material under the irradiation of the light beam.
The four walls of the hole are surrounded by molten metal, which in turn is surrounded by solid material. The liquid metal outside the hole flows and is kept in dynamic equilibrium with the continuous vapor pressure inside the hole cavity.
As the beam moves, the hole remains stable. This means that the keyhole and the molten metal around the keyhole advance with the speed of the main beam. The molten metal fills the gap left by the moving keyhole and condenses, forming the weld.
4.2 Influencing factors
The factors influencing laser deep penetration welding are laser power, laser beam diameter, material absorptivity, welding speed, shielding gas, lens focal length, focus position, the position of the laser beam and the control of the increase and decrease of laser power at the beginning and end. of welding.
4.3 Characteristics of laser deep penetration welding
1) High aspect ratio
As molten metal forms around the cylindrical cavity of high-temperature steam and extends into the workpiece, the weld becomes deep and narrow.
2) Minimum heat input
Due to the high temperature of the source cavity, the fast speed of the melting process, and the low heat input to the workpiece, the thermal deformation and heat-affected zone are very small.
3) High density
Because the small hole filled with high temperature steam favors the agitation of the weld pool and the escape of gas, resulting in the formation of non-porous penetration welding.
The high cooling rate after welding facilitates the refinement of the weld microstructure.
4) Strengthen the weld.
5) Precise control.
6) It is a non-contact atmospheric welding process.
4.4 Advantages of laser deep penetration welding
- Welding speed is faster with a focused laser beam due to its higher power density compared to conventional methods. Additionally, it can weld refractory materials such as titanium and quartz with smaller heat-affected zones and less deformation.
- Easy transmission and control of the laser beam eliminates the need for frequent torch and nozzle changes, leading to reduced downtime and increased load factor and production efficiency.
- Purification and high cooling rates contribute to weld seam strength and overall performance.
- The low heat input and high machining precision of laser welding reduce reprocessing costs, making it an economical solution.
- Laser welding allows for easy automation and effective control of beam intensity and precise positioning.
4.5 Laser deep penetration welding equipment
In general, carbon steel has good laser welding effects, and the welding quality mainly depends on the impurity content.
As with other welding processes, sulfur and phosphorus are factors that can affect sensitivity to welding cracks.
To obtain satisfactory welding quality, preheating is required when the carbon content exceeds 0.25%.
When welding steels with different carbon content, it is recommended to slightly tilt the welding torch to the side with low carbon materials to ensure the quality of the joint.
Due to its high sulfur and phosphorus content, low carbon edge steel is not suitable for laser welding.
Due to the low impurity content, the welding effect of low carbon dead steel is excellent.
Medium and high carbon steels and common alloy steels can also be laser welded effectively. However, preheating and post-weld treatment are necessary to eliminate stresses and prevent crack formation.
5 . Laser welding of steel materials
5.1 Laser welding of carbon steel and common alloy steel
In general, carbon steel performs well in laser welding, and the welding quality is influenced by the impurity content.
Similar to other welding techniques, sulfur and phosphorus are the main factors that can cause cracking in welding.
When the carbon content exceeds 0.25%, preheating is necessary to achieve desirable welding quality.
When welding steels with different carbon content, tilting the welding torch to the side with lower carbon content can ensure the quality of the joint.
Laser welding is not recommended for low-carbon edged steels due to their high sulfur and phosphorus content.
Low carbon dead steel provides excellent welding results due to its low impurity content.
Medium and high carbon steels, as well as common alloy steels, can be effectively laser welded, but preheating and post-welding treatment are required to eliminate stresses and prevent crack formation.
5.2 Laser welding of stainless steel
In general, laser welding of stainless steel is easier to obtain high-quality joints than conventional welding. This is because the small heat-affected zone from high speed welding makes sensitization less problematic.
Compared to carbon steel, stainless steel, with its lower thermal conductivity, allows for easier deep penetration and narrow welds.
5.3 Laser welding between different metals
The high cooling rate and small heat-affected zone of laser welding create favorable conditions for the compatibility of materials with different structures after melting many different metals.
It has been proven that the following metals can be successfully welded: stainless steel and low carbon steel, 416 stainless steel and 310 stainless steel, 347 stainless steel and hastelloy nickel alloy, nickel electrode and cold forged steel, and bimetallic strips with different happy nickel.
