Laser welding is a highly efficient and precise welding method using a high energy density laser beam as a heat source. Laser welding is one of the important aspects of the application of laser material processing technology. As one of the two basic modes of laser welding (the other being heat conduction welding), laser deep-fusion welding is becoming more and more widely used.

Deep fusion, or is called deep penetration welding. It is commonly used to weld thicker materials with high laser power. In deep fusion welding, the laser beam is focused together to create a high power density on the work piece. In fact, the focused area of the laser beam vaporizes the metal, creating a blind hole in the molten metal pool (i.e., a deep melt hole). The metal vapor pressure blocks the surrounding molten metal so that the blind hole remains open during the welding process. The laser power is absorbed by the melt mainly at the vapor-melt boundary and at the wall of the deep melt hole. The focused laser beam and the deep molten hole move continuously along the weld trajectory. The weld material melts in front of the deep molten hole and re-consolidates behind it to form the weld seam.

The factors that affect the effectiveness of laser deep fusion welding include：

1、Material itself

The absorption of laser light by the material to be welded determines the efficiency of laser welding. There are two factors that affect the absorption rate of laser light by the material: 1) the material resistance coefficient. After measuring the absorption rate of polished surfaces of different materials, it was found that the absorption rate of laser light by the material is proportional to the square root of the resistance coefficient, which in turn varies with temperature; 2) the surface state of the material has a more important effect on the absorption rate of the beam and thus has a significant effect on the welding effect.

2、Laser power density

The prerequisite for deep fusion welding is a focused laser spot with a sufficiently high power density, which therefore has a decisive influence on the weld seam formation. The laser power controls both the depth of penetration and the welding speed. For a certain diameter laser beam, when the laser power is increased, the depth of penetration is deepened and the welding speed is accelerated. There is a critical value of laser power to achieve a certain weld depth. When this critical value is reached, the melt pool boils violently, and when it is exceeded, the melt depth decreases sharply. In addition, due to the force of metal vapor, the melt pool will form small holes, and small holes is the key to deep melt welding. The focal spot power density is not only proportional to the laser power, but also related to the laser beam and focusing optical path parameters.

3. Welding speed

    During deep fusion welding, the welding speed is inversely proportional to the depth of melt. If the welding speed is increased while keeping the laser power constant, the heat input will decrease and the melt depth will be reduced. Therefore, an appropriate reduction in welding speed can increase the melt depth, but too low speed can lead to excessive melting of the material and the phenomenon of workpiece weld through. Therefore, for a specific laser power and a specific thickness and type of material, there is a suitable welding speed range to obtain the upper limit of the melt depth.

4、Focus point position

Deep fusion welding, in order to maintain sufficient power density, the location of the focal point is critical. Only if the focal point is located at a suitable position within the workpiece surface, the resulting weld can form a parallel section and obtain the upper limit of melt depth.

5、Protective gas

The role of the protective gas is twofold: 1) to exclude air from the local area of the weld and protect the working surface from oxidation; 2) to inhibit the generation of plasma clouds during high power laser welding.

6、Workpiece joint clearance

Workpiece splice gap, assembly gap is directly related to the welding workpiece melt depth, weld width. In deep fusion welding, such as joint gap more than the spot size, it is impossible to weld; joint gap is too small, sometimes in the process will produce overlapping joint plate, fusion difficulties and other undesirable effects; joint gap is too large, it is easier to weld through; slow welding can make up for some of the weld defects due to excessive gap, while narrowing of weld seam during high-speed welding means more stringent requirements for assembly.

Nowadays, the development of laser industry is more and more extensive, and more and more people know about laser welding. Laser welding has advantages that are incomparable to traditional welding, but of course, when using it, you still need to choose the welding method according to your situation. Nowadays, laser welding machines have gradually entered the market, replacing the traditional welding methods.