Laser welding process method

Laser welding is an efficient and precise welding method using high energy density laser beam as heat source.
Laser welding is one of the important aspects of laser material processing technology application.
The following briefly introduces several technological methods.

1. Plate to plate welding
It includes four processing methods:
Butt welding;
End welding;
Center penetration welding;
Fusion welding of central perforation.
2. Line-to-line welding
It includes four processing methods:
Line-to-line butt welding;
Cross welding;
Parallel lap welding;
3. Welding of metal wire and block parts
Using laser welding, wires can be successfully connected to lumped elements, where lumped elements can be of any size. Attention should be paid to the geometry of welding wire elements when welding.

4. Welding of different metals
Welding of different types of metals must solve the range of weldability and weldability parameters.
Laser welding between different materials is only suitable for some material combinations.
Laser brazing is not suitable for the connection of some parts, but laser can be used as a heat source for soft and hard brazing, which also has the advantages of laser brazing.
Brazing can be used in a variety of ways, among which laser soldering is mainly used for the welding of printed circuit boards, especially for the assembly technology of chip components.

Factors affecting the quality of laser welding
Laser welding is a process in which the workpiece is irradiated by high-energy laser beam, so that the working temperature rises sharply, and the workpiece melts and rejoins to form a permanent joint.
Laser welding has better shear strength and tear strength.
There are many factors that affect the quality of laser welding. Some of them are very unstable and have considerable instability. How to set and control these parameters correctly to keep them in the correct range of high-speed continuous laser welding process to ensure welding quality. The reliability and stability of weld formation is an important issue related to the practicality and industrialization of laser welding technology.

The main factors that affect the quality of laser welding are welding equipment, workpiece condition and process parameters.
1.Welding equipment
The most important quality requirements of laser are beam mode, output power and its stability.
The lower the beam pattern order, the better the beam focusing performance, the smaller the spot, the higher the power density under the same laser power, and the greater the weld depth and width.
Generally, basic mode (TEM00) or low-order mode is required, otherwise it is difficult to meet the requirements of high-quality laser welding.
At present, China’s lasers are still difficult to be used in laser welding in terms of beam quality and power output stability.
From the foreign situation, the stability of laser beam quality and output power has been quite high, and it will not become a problem of laser welding.
The biggest factor affecting the welding quality of the optical system is the focusing mirror, and the focal length used is generally between 127mm(5in) and 200mm(7.9in). A small focal length is beneficial to reduce the waist spot diameter of the focusing beam, but it is easy to be polluted and damaged by splashing when it is too small.
The shorter the wavelength, the higher the absorption.
Generally, materials with good electrical conductivity have high reflectivity.
The reflectivity of YAG laser is 96% for silver, 92% for aluminum, 90% for copper and 60% for iron.
The higher the temperature, the higher the absorbance, which is linear. Generally, coating phosphate, carbon black and graphite on the surface can improve the absorption rate.

2. Workpiece condition
Laser welding requires that the edge precision of the workpiece to be processed and assembled is high, and the solder joints and welds are strictly aligned. During the welding process, the original assembly precision and point alignment of the workpiece are not distorted due to welding heat.
This is because the laser spot is small and the weld is narrow, and generally no filler metal is added. If the assembly gap is too large, the beam can pass through the gap without melting the base material, or cause obvious light punching and depression. If the point-to-seam deviation is slightly large, it may lead to incomplete fusion or incomplete penetration.
Therefore, in general, the assembly clearance and spot seam deviation of plate butt joint should not be greater than 0.1mm, and the offset should not be greater than 0.2 mm..
In actual production, sometimes laser welding technology can’t be used because these requirements can’t be met.
In order to obtain good welding effect, the allowable butt gap and lap gap should be controlled within 10% of the thickness of the thin plate. Successful laser welding requires close contact between the welded substrates. This requires careful tightening of parts to achieve the best results.

3. Welding parameters
(1) Influence on laser welding mode and weld forming stability, the most important welding parameter is the power density of laser spot, and its influence on welding mode and weld forming stability is as follows.

The power density of laser spot is stable heat conduction welding, unstable mode welding and stable deep penetration welding from small to large.
The power density of laser spot is mainly determined by laser power and beam focal position under certain beam mode and focal length of focusing mirror.
The laser power density is proportional to the laser power.
And the influence of focus position has an optimal value.
When the focus of the beam is at a certain position below the surface of the workpiece (in the range of 1 ~ 2 mm, depending on the thickness and parameters), the most ideal weld can be obtained. Deviating from this optimal focus position, the light spot on the workpiece surface becomes larger, which leads to the decrease of power density, and to a certain extent, it will cause the change of welding process form.
The influence of welding speed on the form of welding process and stable parts is not as significant as that of laser power and focus position. Only when the welding speed is too high, the stable deep penetration welding process cannot be maintained because the heat input is too small.
In actual welding, stable deep penetration welding or stable heat conduction welding should be selected according to the requirements of welding parts for penetration depth, and modal instability welding should be absolutely avoided.
(2) In the range of deep penetration welding, the influence of welding parameters on penetration.

In a stable range of deep penetration welding, the higher the laser power, the greater the penetration, which is about 0.7 times.
Moreover, the higher the welding speed, the shallower the penetration.
Under certain laser power and welding speed, the focal point is in the best position when the penetration depth is maximum. If it deviates from this position, the penetration will be reduced, and even it will become the mode of unstable welding or stable heat conduction welding.

(3) The role of protective gas

The main functions of protective gas are:
Protect the workpiece from oxidation during welding.
Protect the focusing lens from metal vapor pollution and liquid droplet sputtering.
Disperse the plasma produced by high power laser welding.
Cool the workpiece to reduce the heat affected zone.
The shielding gas is usually argon or helium, or nitrogen if the apparent mass is not high.
The trend of plasma generation is obviously different: helium, because of its high ionization and rapid thermal conductivity, has a lower trend of plasma generation than argon under the same conditions, thus allowing a greater melting depth.
In a certain range, with the increase of shielding gas flow, the trend of plasma suppression increases, thus increasing the melt depth, but it tends to be smooth when it increases to a certain range.
(4) Monitoring analysis of each parameter.

Among the four welding parameters, welding speed and shielding gas flow rate are easy to monitor and keep stable, while laser power and focus position are parameters that may fluctuate and are difficult to monitor during welding.
Although the laser power output by the laser is highly stable and easy to monitor, the laser power reaching the workpiece will change due to the loss of the light guide and focusing system, which is related to the quality, time use and surface pollution of the optical workpiece, so it is difficult to monitor and become an uncertain factor of welding quality.
The focal position of the beam is one of the most difficult factors to monitor and control among the welding parameters that affect the weld quality.
At present, in production, manual adjustment and repeated process tests are needed to determine the appropriate focus position to obtain the required melt depth.
However, due to the deformation of the workpiece, thermal lens effect or multi-dimensional space curve during welding, the focus position may change and may exceed the allowable range.

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