Power density is a key parameter in laser processing.Higher power density can quickly heat the surface layer to its boiling point in a few microseconds, resulting in significant evaporation.Therefore, high power density is conducive to material removal processes, such as cutting, engraving, and drilling. On the other hand, the lower power density takes a few milliseconds to reach the boiling point of the surface temperature. This allows the bottom layer to reach the melting point before the surface layer evaporates, making it easier to form a firm weld.Therefore, the power density of conducted laser welding is usually in the range of 10^4 to 10^6 W/cm2.

1.2 Laser pulse width：

Pulse width is an important parameter in pulsed laser welding. It is not only different from material removal and melting, but also a key factor in determining the cost and volume of processing equipment.

1.3 The influence of defocus on welding quality：

Laser welding usually requires a certain degree of defocus, because the power density in the center of the laser focus spot is high, which can easily cause evaporation and holes. On the other hand, the power density distribution is relatively uniform on the plane away from the laser focus.

There are two defocus modes to choose from: positive defocus and negative defocus. Positive defocus occurs when the focal plane is above the workpiece, and negative defocus occurs below the workpiece.

Negative defocus will lead to a greater melting depth, which is related to the formation process of the bath. The experimental results show that the material begins to melt within 50 to 200us after being heated by the laser, forming a liquid phase metal and vaporizing and commercial pressure steam, and emitting dazzling white light at an extremely high speed.

At the same time, the high concentration of vapor causes the liquid metal to move towards the edge of the bath, forming a depression in the center of the bath.

When negative defocus is used, the internal power density of the material is higher than the surface power density, which is more likely to produce stronger melting and vaporization. This allows light energy to be transferred to a deeper part of the material, resulting in greater penetration. Therefore, negative defocus should be used for larger melting depths, while positive defocus should be used when welding thin materials in practical applications.

2. Laser welding technology：

1) Board-to-board welding：

It includes butt welding, end welding, center permeable welding and center perforated welding.

2) Wire-to-wire welding：

It includes wire-to-wire butt welding, cross welding, parallel lap welding and T-welding.

3) Welding of wire and block components：

Laser welding can be used to successfully connect the wire to the bulk element, and the size of the bulk element can be arbitrary. Attention should be paid to the geometry of the line element when welding.

4) Welding of different metals：

For welding various types of metals, it is necessary to determine their weldability and range of weldability parameters.

It should be noted that laser welding can only be carried out between certain material combinations.

Although laser brazing may not be suitable for connecting certain components, lasers can be used as a heat source for soft brazing and brazing, which also has the advantages of laser welding.

There are a variety of welding methods to choose from. Laser welding is mainly used for printed circuit board (PCB) welding, especially wafer assembly technology.

3. Advantages of laser welding：

Local heating reduces the risk of thermal damage to the components and creates a small heat-affected zone that allows welding near the thermal components.

Non-contact heating can melt water without auxiliary tools. This allows the processing of the double-sided printed circuit board after installing the double-sided components.

The stability of repeated operations, coupled with the minimum pollution of flux to welding tools, makes laser brazing a favorable choice. In addition, the laser irradiation time and output power are easy to control, and the laser brazing yield is high.

Optical components such as semi-lenses, mirrors, prisms, and scanning mirrors can be used to easily separate the laser beam. This allows multiple points to be welded symmetrically at the same time.

Laser brazing mainly uses a laser with a wavelength of 1.06um as a heat source, which can be transmitted through optical fiber. This makes it possible to process parts that are difficult to weld by traditional methods, providing greater flexibility.

The laser beam has good focusing properties, and it is easy to automate multi-station equipment.

4. Laser deep melt welding：

4.1 Metallurgical technology and process theory：

The metallurgical process of laser deep melt welding is similar to electron beam welding, which relies on a “small hole” structure to complete the energy conversion.

When the power density is high enough, the material evaporates, forming a small hole. This hole is full of steam, like a black body, absorbing almost all the energy of incident light. The equilibrium temperature in the hole cavity is about 25,000 degrees.

Heat is transmitted from the outer wall of the high-temperature cavity, melting the surrounding metal. The hole is constantly filled with high-temperature steam generated by the evaporation of the wall material under the beam of light.

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 maintains a dynamic balance with the continuous vapor pressure in the hole cavity.

When the beam of light moves, the hole remains stable.This means that the small hole and the molten metal around the hole move forward at the speed of the guided beam. The molten metal fills the gap left by the moving hole and condenses to form a weld.

4.2 Influencing factors：

The factors that affect laser deep fusion welding are laser power, laser beam diameter, material absorption rate, welding speed, protective gas, lens focal length, focus position, laser beam position, and laser power start and stop increase or decrease control.welding.

4.3 Characteristics of laser deep melt welding：

1) High aspect ratio: As the molten metal forms around the cylindrical high-temperature steam chamber and extends to the workpiece, the weld becomes deeper and narrower.

2) Minimum heat input: Due to the high temperature of the source cavity, the melting process is fast, the heat input of the workpiece is small, and the thermal deformation and heat-affected zone are small.

3) High density: Because the small holes filled with high-temperature steam are conducive to the agitation of the bath and the escape of gas, thereby forming a non-porous melt-through welding.

The cooling speed is fast after welding, and the weld organization is easy to refine.

4) Strengthen the welds.

5) Precise control.

6) Non-contact, atmospheric welding process.

4.4 Advantages of laser deep melt welding：

Compared with traditional methods, the power density of the focused laser beam is higher, so the welding speed is faster. In addition, it can weld refractory materials such as titanium and quartz with smaller heat-affected zones and smaller deformation.

The laser beam is easy to transmit and control, eliminating the need for frequent replacement of torches and nozzles, thereby reducing downtime and improving load factor and production efficiency.

Purification and high cooling rate help improve weld strength and overall performance.

The low heat input and high machining accuracy of laser welding reduce the reprocessing cost, making it a cost-effective solution.

Laser welding can be easily automated and effectively control beam intensity and precise positioning.

4.5 Laser deep melt welding equipment：

Generally speaking, carbon steel laser welding has a good effect, and the welding quality mainly depends on the impurity content.

Like other welding processes, sulfur and phosphorus are factors that affect the sensitivity of welding cracks.

In order to achieve satisfactory welding quality, preheating is required when the carbon content exceeds 0.25%.

When welding steel with different carbon content, it is recommended to tilt the welding torch slightly to the side of the low-carbon material to ensure the quality of the joint.

Due to the high sulfur and phosphorus content, low-carbon boiling steel is not suitable for laser welding.

Due to the low impurity content, the welding effect of low-carbon sedative steel is excellent.

Medium and high carbon steel and ordinary alloy steel can also be effectively laser welded. However, preheating and post-welding treatment are necessary to eliminate stress and prevent crack formation.

5. Laser welding of steel：

5.1 Laser welding of carbon steel and ordinary alloy steel：

Generally speaking, carbon steel has good laser welding performance, and the welding quality is affected by the impurity content.

Similar to other welding techniques, sulfur and phosphorus are the key factors that cause welding cracks.

When the carbon content exceeds 0.25%, preheating must be carried out to achieve the ideal welding quality.

When welding steel with different carbon content, tilt the welding torch to the side with low carbon content to ensure the quality of the joint.

Due to the high sulfur and phosphorus content, laser welding is not recommended for low-carbon boiling steel.

Low-carbon carbon steel exhibits excellent welding effect due to its low impurity content.

Both medium and high carbon steels and ordinary alloy steels can be effectively laser welded, but preheating and post-welding treatment are required to eliminate stress and prevent the formation of cracks.

5.2 Laser welding of stainless steel：

Generally speaking, laser welding of stainless steel is easier to obtain high-quality joints than traditional welding. This is because the small heat-affected zone with high welding speed makes sensitization no longer a problem.

Compared with carbon steel, stainless steel has lower thermal conductivity and is easier to achieve deep melt welding and narrow welds.

5.3 Laser welding between different metals：

Laser welding has a fast cooling speed and a small heat-affected zone, which creates favorable conditions for the compatibility of different tissue materials after a variety of different metals are melted.

Analysis of Laser Welding Technology: Process and Technology

Table of contents：

1. Laser welding process parameters：

1.1 Power density：