Interpretation of common process parameters of laser welding

Laser welding is one of the important aspects of the application of laser processing technology, which is also the most attention-seeking and most promising welding technology in the 21st century. Compared with traditional welding methods, laser welding has many advantages, including higher welding quality and faster efficiency. At present, laser welding technology has been widely used in manufacturing, powder metallurgy, automobile industry, electronics industry, biomedicine, and other fields.

Principle of laser welding

Laser welding belongs to molten welding, using a laser beam as the welding heat source. The welding principle is that excites the active medium through a specific method to make it oscillate back and forth in the resonant cavity and then convert it into a beam of stimulated radiation. When the beam and the workpiece contact each other, its energy is absorbed by the workpiece, and welding can be performed when the temperature reaches the melting point of the material.

Laser welding principle

According to the mechanism of welding molten pool formation, laser welding has two basic welding mechanisms: heat conduction welding and deep penetration (small hole) welding. The heat generated during heat conduction welding is diffused into the workpiece through heat transfer so that the surface of the weld is melted without vaporization. It is often used in the welding of low-speed thin-walled components. Deep penetration welding vaporizes the material and forms a large amount of plasma. Due to the large heat, small holes will appear at the front of the molten pool. Deep penetration welding can completely penetrate the workpiece and has large input energy and fast welding speed. It is the most widely used laser welding mode at present.

Main process parameters of laser welding

There are many process parameters that affect the quality of laser weldings, such as power density, laser pulse waveform, defocusing amount, welding speed, and auxiliary blowing protective gas.

1. Laser power density

Power density is one of the most critical parameters in laser processing. With higher power densities, the surface layer can be heated to the boiling point in the microsecond time range, resulting in a large amount of vaporization. Therefore, high power density is very beneficial for material removal processes such as punching, cutting, and engraving. For lower power density, it takes several milliseconds for the surface temperature to reach the boiling point. Before the surface vaporizes, the bottom layer reaches the melting point, which is easy to form a good fusion weld. Therefore, in thermal conduction laser welding, the power density range is 104-106W/cm2.

2. Laser pulse waveform

The laser pulse waveform is not only an important parameter to distinguish material removal from material melting, but also a key parameter to determine the volume and cost of processing equipment. When the high-intensity laser beam hits the surface of the material, 60-90% of the laser energy will be reflected and lost on the surface of the material, especially gold, silver, copper, aluminum, titanium, and other materials with strong reflection and fast heat transfer. During a laser pulse signal, the reflectivity of the metal changes with time. When the surface temperature of the material increases to the melting point, the reflectivity decreases rapidly, and when the surface is in a molten state, the reflection stabilizes at a certain value.

Laser welding pulse waveform of different materials

3. Laser pulse width

Pulse width is an important parameter of pulsed laser welding. The pulse width is determined by the penetration depth and the heat-affected zone. The longer the pulse width, the larger the heat-affected zone, and the penetration depth increases with the power of 1/2  of the pulse width. However, the increase of the pulse width will reduce the peak power, so increasing the pulse width is generally used for heat conduction welding. The weld size formed is wide and shallow, especially suitable for lap welding of thin and thick plates. However, lower peak power results in excess heat input, and each material has an optimal pulse width that maximizes penetration.

4. Defocusing the amount

Laser welding usually requires a certain amount of defocus, because the power density in the center of the spot at the laser focus is too high, and it is easy to evaporate into a hole. The power density distribution is relatively uniform across the planes away from the laser focus.

There are two ways to defocus:

Positive and negative defocus. The focal plane above the workpiece is positive defocus, otherwise it is negative defocus. According to the geometric optics theory, when the distance between the positive and negative defocus planes and the welding plane is equal, the power density on the corresponding plane is approximately the same, but the shape of the molten pool obtained is actually different. When the defocus is negative, a larger penetration depth can be obtained, which is related to the formation process of the molten pool.

5. Welding speed

The welding speed determines the welding surface quality, penetration, heat affected zone, etc. The speed of welding speed will affect the heat input per unit time. If the welding speed is too slow, the heat input will be too large, causing the workpiece to burn through. If the welding speed is too fast, the heat input will be too small, causing the workpiece to be welded. The method of reducing the welding speed is usually used to improve the penetration.

6. Auxiliary blowing protective gas

Auxiliary blowing shielding gas is an essential process in high-power laser welding. On the one hand, it is to prevent the contamination of the focusing mirror due to sputtering of metal materials; Gases such as helium, argon, nitrogen, etc. are often used in the laser welding process to protect the molten pool to protect the workpiece from oxidation during the welding process. Factors such as the type of shielding gas, the size of the airflow, and the blowing angle have a great impact on the welding results. Different blowing methods will also have a certain impact on the welding quality.

Helium is not easy to ionize (higher ionization energy), which allows the laser to pass smoothly, and the beam energy reaches the surface of the workpiece without hindrance. This is the most effective shielding gas to use in laser welding, but it is more expensive.

Argon is cheaper and denser, so the protective effect is better. However, it is susceptible to high-temperature metal plasma ionization, which shields part of the beam from the workpiece, reduces the effective laser power for welding, and also damages the welding speed and penetration. The surface of the weldment shielded with argon is smoother than when shielded with helium.

Nitrogen is the cheapest shielding gas, but it is not suitable for welding certain types of stainless steel, mainly due to metallurgical issues such as absorption and sometimes porosity in the lap area.

Summary

As a new type of welding technology, laser welding has the characteristics of high energy density, high speed, high precision, deep penetration, and strong adaptability. Laser welding technology is bound to play a more important role in the field of material processing.

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Christophe Rude
Christophe Rude
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