In the field of laser welding, whether as the user of customers or the equipment of suppliers, once a laser welding project is put in front of us, almost the primary consideration is to use pulse welding or continuous welding.
Because this problem involves many factors such as cost, process, processing efficiency, light source and later equipment transformation and upgrading, we have to be cautious.
This paper briefly expounds the principle, light source, beam characteristics, characteristics and application scope of their respective welding processes for reference.
Continuous welding of the galvanized sheet by the semiconductor laser
Pulsed welding of stainless steel plate with Nd 3 +: YAG laser
1. Relevant principles
There are many ways to classify lasers.
For example, according to the wavelength classification, lasers can be divided into infrared, visible and ultraviolet;
According to the working substance of laser, there are CO2 laser, fiber laser, Nd3 +: YAG solid-state laser, Nd3 +: YAG disc laser (unique to Ttrumpf), direct semiconductor laser, dye laser, etc., while the continuous and pulse are determined according to the working mode of the laser.
Generally speaking, the laser beam will output once when it goes back and forth in the resonant cavity, and multiple outputs will be formed when it goes back and forth (also known as high-frequency oscillation).
When the output frequency is high to a critical value, we call it continuous light output, otherwise it is called pulse light output, and the corresponding lasers are continuous laser and pulse laser.
However, there is no unified and clear standard for the division of the two in the academic circles and the industry.
The common saying is that when the repeated light frequency is 102Hz and below, it is pulse (hereinafter referred to as pulse), 102 ~ 103Hz is quasi continuous (hereinafter referred to as QCW), 103 ~ 106Hz is continuous (hereinafter referred to as CW), and 106 ~ 109Hz or even higher is supercontinuum.
For example, in the familiar field of metal cutting, the typical frequency parameter of IPG, Raycus and other fiber lasers is 5000Hz, while the typical frequency of early Nd3 +: YAG solid-state laser cutting machine is 300Hz.
In terms of their power parameters, CW lasers generally only give power, while pulse lasers will give single pulse power, average power, pulse width and frequency.
The specific cost-effective method is:
Average power = Single pulse power × Pulse width × Frequency
2. Laser light source
In the field of metal laser welding, generally speaking, the lasers used in pulse welding are basically Nd3 +: YAG solid-state lasers, while fiber lasers are used in most cases of continuous welding.
In recent years, with the continuous maturity and stability of industrial-grade direct semiconductor lasers, their applications in continuous welding are gradually increasing.
However, with the slow development of laser technology, the boundary between the two began to become less clear.
Nd3 +: YAG pulse laser is characterized by pulse light output with low light output frequency. A laser with small power can emit ultra-high pulse energy.
For example, a 500W pulse laser can have a single pulse power of 12KW or even higher.
In this way, the welding penetration of pulse laser with the same power is greater than that of fiber (continuous) laser.
CW laser is characterized by ultra-high output frequency and relatively stable and low single pulse energy.
There is a simple analogy to illustrate the difference between them.
For the two working modes, pulse welding is similar to pile driving by pile driver.
Although the speed is slow, each impact has great weight, while continuous welding is more like electric hammer hammering nails. Each time the force is very small, but the hammering speed is very fast.
3. Beam characteristics
As we all know, the beam emitted by a CW (fiber) laser is a typical Gaussian beam, that is, the power density at the optical center is very high, and the power density decreases rapidly along the optical center.
The light intensity of the pulsed laser is flat-top distribution, that is, the energy is approximately evenly distributed on the surface perpendicular to the beam.
Of course, the beam distribution of direct semiconductor laser is also similar to the flat top distribution, which will not be described in detail here.
Energy distribution of Gaussian beams
Energy distribution of flat-topped beam
4. Welding process
The light output frequency of continuous welding is very high.
If good welding protection and appropriate welding parameters are adopted, a uniform and smooth weld can be obtained, which basically does not need grinding or polishing.
Due to the low light frequency of pulse welding, clear and intermittent knocking sound can be heard in the work, and a flat fish scale weld is obtained, which is similar to argon arc welding, or full single spot welding spots will be formed as required.
In the process of processing, continuous welding only needs to select several parameters such as appropriate welding track, running speed and power, which is relatively simple, while pulse welding needs to comprehensively consider many parameters such as pulse width, light output frequency, single pulse power, running speed and pulse waveform, which is relatively complex.
Pulse welding spot (welding spot)
Continuous welding seam
In addition, the energy density of the fiber laser beam at the optical center is very high, and under the current technical means, the laser beam can be coupled to the energy transmission fiber with an extremely fine core diameter, so the medium and high-power light laser is very suitable for deep penetration welding to obtain the weld with great depth width ratio.
The flat-top distribution of pulsed laser beam has great advantages for heat conduction welding represented by thin-plate splicing welding.
Because of its high stability, low power consumption, high efficiency, extremely high beam quality and energy density, continuous (fiber) laser has continuously occupied the market originally belonging to CO2 laser and Nd3 +: YAG solid-state laser in the welding field in recent years, and the market share will not continue to expand in the future.
The Nd3 +: YAG solid-state laser has a long development time and a huge market.
It will have great prospects in some special fields in the future.
As mentioned above, at present, continuous welding is mostly used for deep penetration welding in the market.
For example, optical fiber (continuous) laser is basically used for welding in the field of auto parts. With the continuous development of optical fiber laser technology and direct semiconductor laser technology, continuous welding will be applied to more fields.
In addition, with its high efficiency and stability, continuous welding processing is well in line with the general trend of the manufacturing industry from manual and semi-automatic production mode to intelligent and automatic production mode.
The market of pulse welding will continue to shrink in the future, especially at this stage, fiber lasers have developed quasi-continuous lasers that can compete with them.
However, we have also seen that Nd3 +: YAG solid-state laser has the characteristics of low cost, simple use and easy maintenance, and pulse welding is required for many special materials and special welding requirements, which also makes pulse welding exist for a long time in the future.
How to choose two welding methods is a problem that needs to be comprehensively considered. Factors such as price, use cost, process requirements and production efficiency should be taken into account.
Pulse welding has low efficiency and high cost, but low cost and high single pulse energy;
Continuous welding has high efficiency, low operating cost but high price.
They have their own advantages and disadvantages, so they have different application ranges.