With the popularity of handheld laser welders in industry, people want to know more about laser welding. This article describes two different laser welding modes and the factors that have an impact on the effectiveness of laser welding.
Laser welding can be achieved with either a continuous or pulsed laser beam, and can be classified as heat transfer welding or laser deep fusion welding based on the principle of laser welding. The following is a description of these two laser welding modes.
Heat conduction welding
Heat conduction welding involves the diffusion of heat into the workpiece by heat transfer. The workpiece is melted and a specific melt pool is formed by controlling the laser parameters such as the width, energy, peak power and repetition frequency of the laser pulse. This laser welding mode produces melting phenomenon only on the surface of the weld, the inside of the workpiece is not completely melted through and basically no vaporization is produced. The shallow melt depth and slow welding speed after welding are mostly used for welding of thin-walled metal materials at low speed.
Laser deep fusion welding
Laser deep fusion welding not only completely penetrates the material, but also vaporizes the material, forming a large amount of plasma. Due to the high heat, a keyhole will appear at the front of the melt pool. Deep fusion welding is the most widely used laser welding mode because of its high input energy, high welding speed and large depth-to-width ratio. Laser welding machines for gear welding and metallurgical sheet welding mainly involve laser deep fusion welding.
Different process parameters have different effects on the laser welding effect. Three factors that have an impact on the laser welding effect are described here.
Laser Power
There is a laser energy density threshold in laser welding, below which the depth of melt is shallow, and once this value is reached or exceeded, the depth of melt increases substantially. Only when the laser power density on the workpiece exceeds the threshold value (material dependent), plasma is generated, which marks the stabilization of deep fusion welding.
If the laser power is below this threshold, only surface melting of the workpiece occurs, i.e. the welding proceeds in a stable heat transfer type. And when the laser power density is near the critical conditions for small hole formation, deep fusion welding and conduction welding alternate and become unstable welding processes, resulting in large fluctuations in melt depth. When laser deep fusion welding, the laser power controls both the depth of penetration and the welding speed. In general, for a certain diameter laser beam, the depth of melt increases as the beam power increases.
Welding speed
Welding speed has a large impact on the depth of melt. Increasing the welding speed will make the depth of melt shallower, but too low a speed will lead to excessive melting of the material and welding through the workpiece. Therefore, for a certain laser power and a certain thickness of a specific material should have a suitable welding speed range, and in which the corresponding speed value can be obtained when the maximum depth of melt.
Protective gas
The laser welding process often uses inert gases to protect the molten pool, which can be disregarded when certain materials can be welded regardless of surface oxidation, but for most applications helium, argon and nitrogen are often used to protect the workpiece from oxidation during the welding process.
Helium is not easily ionized, allowing the laser to pass through and the beam energy to reach the surface of the workpiece unimpeded. This is the most effective shielding gas used in laser welding, but it is more expensive. Argon is cheaper and more dense, so it protects better. However, it is susceptible to high temperature metal plasma ionization and as a result shields part of the beam from reaching the workpiece, reducing the effective laser power for welding and also impairing the welding speed and depth of melt. Nitrogen is the cheapest type of shielding gas, but it is not suitable for certain types of stainless steel welding. This is mainly due to metallurgical problems, such as absorption, which sometimes creates porosity in the lap zone.
The second function of using shielding gas is to protect the focusing lens inside the laser welding gun from metal vapor contamination and sputtering of liquid molten droplets. This is especially necessary in high power laser welding, where the ejecta become very powerful. The third function of the shielding gas is to disperse the plasma shielding produced by high power laser welding.
