Basic Knowledge And Tips For Laser Welding

Laser welding is a field of metal processing technology that uses lasers, and is widely used in various industrial fields such as automobiles, precision equipment, electrical appliances, aerospace, and medical care. This time, I will select laser welding, which is particularly used in the field of sheet metal processing, from the many applications of laser welding and introduce its basic matters.

What is Laser Welding? (Features)

First of all, "laser" is a word coined by taking the first letters of the English term "Light Amplification by Stimulated Emission of Radiation".

When you first think about the meaning, it may seem a bit complicated, but in fact, as long as you understand it as "artificial light", you will be right. Compared with ordinary light such as sunlight and lighting that we see every day, laser, as artificial light, has the characteristics of constant "wavelength", "phase" and "direction". By adjusting and using these three elements collectively called "coherence", lasers can be used for various processing. Laser welding is a processing method that focuses laser light as artificial light and irradiates the object to partially melt and solidify the metal to join the metal. When laser welding is introduced in the field of sheet metal processing, it has advantages such as easy suppression of thermal deformation, easy management of welding conditions, and less noticeable welds compared to arc welding, which is a previous processing method.

Principle of laser welding

In laser welding, a laser oscillator is used to generate laser light, which becomes a heat source, and then it is amplified and transmitted using optical fibers. The light is first transmitted to the vicinity of the workpiece. A laser processing head is required at this stage. A lens is provided inside the laser processing head to focus the transmitted laser light into a state suitable for processing. By

focusing the light with the help of a lens, the light energy can be concentrated on a smaller area, thereby obtaining a higher energy for the molten metal. In order to prevent oxidation of the molten metal, welding is usually performed while blowing a shielding gas such as argon or nitrogen.

Types of laser welding

Let's take a look at the types of laser welding suitable for manual welding of sheet metal processing. Portable laser welding machines using YAG lasers were the first to become popular in Japan in the 1990s. Since then, the Japanese sheet metal industry has long believed that "laser welding machine = YAG laser", but after the fiber laser welding machine was launched in the mid-2010s, fiber lasers have become the mainstream of portable laser welding machines in recent years. In addition, there are portable welding devices made using disk lasers.

Manual welding using a fiber laser welding machine

· YAG laser welding

YAG is a crystal called yttrium aluminum garnet. YAG lasers generate laser light by irradiating strong light to YAG crystals. YAG lasers have a wavelength of 1064nm that is easily absorbed by metals, so they can melt metals with less energy, which is suitable for laser welding. On the other hand, in order to generate laser light, it is necessary to flash a flash lamp, and because of the high heat, a refrigerator is required to cool the oscillator to the welding torch, so the power consumption is large, and the energy used for processing is less than the electricity used, so there are also cases where the weld is not penetrated. The maintenance cost burden of consumables such as cooling water and lamps is also large, which can be said to be a major disadvantage in use.

· Fiber laser welding

Fiber laser is a laser that uses optical fiber to amplify and transmit the generated excitation light, and has a wavelength of 1070nm that is easily absorbed by metals. Among many lasers, it has a particularly high energy density and has the characteristics of easy convergence of the beam, and its major advantage is that it can achieve deep melting of metals. Compared with YAG laser, it has many advantages such as deep melting, low running cost, and almost no trouble and cost for adjustment and maintenance, and its popularity has accelerated in recent years. Although fiber laser has the characteristics of high power and high efficiency, if the power is too high in manual welding of sheet metal, it will cause danger to the operator, so the power is usually limited to about 1kW when making products. When pursuing higher power and greater penetration depth, mechanical welding and robot welding can be studied.

· Disk laser welding

Disk laser is a laser that uses a disk-shaped YAG crystal to amplify the excitation light generated and then transmit it using an optical fiber. Germany's KuaiTong company successfully achieved high power and stabilization, so it has become widely popular as an industrial laser. In recent years, its possibilities have been re-examined and the development of new uses is also actively underway. There is no single device in Japan that can use disk laser for manual welding, but users who own KuaiTong's laser cutting machine can optionally introduce a welding torch for manual welding.

Advantages and Disadvantages of Laser Welding

Laser welding used in sheet metal processing has the following advantages over TIG welding, which has been widely used in the past:

· Smaller thermal deformation even for thin plates

TIG welding has a shallower weld penetration and a larger heat-affected layer, so the weld line energy to the metal is large, and the product will have a larger thermal deformation. The treatment of the generated thermal deformation depends largely on the craftsman's skills, which is the most difficult part of TIG welding. On the other hand, laser welding uses a focused laser to concentrate high energy on a smaller focus to melt the metal, so the weld penetration width is narrower and the heat-affected layer is smaller, so thermal deformation is less likely to occur. In addition, in the "pulse oscillation" of frequently turning the laser on and off, melting and solidification are repeated several to dozens of times per second, so deformation can be suppressed.

· Ensure sufficient strength of the weld

Laser welding has a thinner weld bead, and there is sometimes a concern that the strength is insufficient, but the weld penetration is deeper, so the internal strength is sufficient compared to the appearance. In laser welding, the amount of alloy that temporarily melts and solidifies is small, so it does not break. In strength tests conducted at industrial test sites, it was confirmed that it has a strength that exceeds that of TIG welding.

· Shortened processing time

Laser welding has low welding line energy and is less likely to cause deformation, so the time required to eliminate deformation can be greatly reduced. In addition, laser welding is basically welding the base material, so the process of grinding the weld protrusion can be omitted. Moreover, as long as the processing conditions are met, there is almost no weld burn, so it is possible to reduce the electrolytic grinding process. The processing time that was previously difficult to deal with is reduced, which promotes improved production efficiency and reduced manufacturing costs.

· Easy to manage and operate welding conditions

There are also models in which the welding conditions can be registered as methods and then called up. It can be said that the easy management and reproduction of welding conditions are one of its major advantages. In this case, as long as the conditions are given by the skilled person, the unskilled person can easily reproduce the processing conditions of the skilled person. Therefore, if unskilled people can also be responsible for thin plate welding, the skilled person can focus on the work with high added value, which helps to improve the overall production efficiency of the welding process.

Disadvantages

· Poor gap processing ability

Laser welding has the property of focusing the laser energy on a very small focus of about φ0.1 to φ0.6mm to melt the metal. Due to this property, it has the weakness that it cannot weld if there is a gap. In the case of a model with a spot diameter of 0.1mm, if there is a gap of 0.1mm, the laser will pass through the middle and welding will not be possible, so measures such as improving the accuracy of the bending process and setting jigs are required.

· Not good at overlay welding

Laser welding is good at welding the base material, but not good at overlay welding. In addition to the possibility that the laser energy may not reach the base material at the point where the electrode is melted, it is also difficult to accurately align the welding point/electrode/focus, and sufficient strength may not be ensured. When overlay welding is indicated in the drawing, it is necessary to study whether laser welding should be used. On the contrary, if you want to use laser welding to ensure strength and beauty, it is best not to write an instruction for overlay welding.

· Safety measures need to be taken

If the laser is used incorrectly, it may cause a serious accident, so all laser products have safety-related specifications in the JIS standard "Laser Product Safety Standard". All manual welding devices using lasers are classified as "Class 4", the most dangerous, and manufacturers also take various safety measures. Please be careful to follow the specifications and operating instructions to use the equipment correctly, such as setting up a laser management area, using a welding helmet/goggles dedicated to laser welding, using a portable welding torch with a safety device, and properly managing the keys to the equipment.

Laser welding processing example Let's take a look at the actual processing example of laser welding.

(Test model: portable torch type fiber laser welding machine) First, we will introduce the processing example of butt welding of SUS304 with a thickness of 0.5mm and a thickness of 1.0mm. For thin plates that laser welding is good at, low deformation welding can be achieved. Beautiful finishing can be achieved without removing deformation and burn marks after welding.

Below is a tube welded to a 1.0mm thick SUS304 plate. The weld bead can be seen on the inner side of the tube. The base metal was welded without using a welding rod. The plate was tilted during the shooting to make the weld bead easier to see, but there was almost no deformation on the SUS plate side. There was no welding burn mark on the inner side either.

The last sample is a box welded with a SUS304 plate thickness of 1.2mm. The workpiece is held by a fixture, and the laser is used for positioning welding before the actual welding is carried out. The product is completed with low deformation, eliminating the trouble of finishing.

Tips for laser welding

Laser welding can achieve both welding strength and beauty at the same time. It is a joining method with many advantages, such as low deformation welding of thin plates and easy condition management. However, it also has disadvantages such as poor gap processing ability and poor cladding. Tips for making good use of laser welding: Please pay attention to the following points.

·     Improve the accuracy of the bending process, set up a jig, and weld after creating a seamless state

·     If you ask an experienced person to provide processing conditions and register the method in advance, it will be easy for an unskilled person to reproduce the processing

·     If you keep the drawings based on the previous processing methods such as TIG welding unchanged and simply change the processing method to laser welding, it will be difficult to complete the processing smoothly. Sometimes you can consider changing the design.

·     If you want to maximize the advantages of laser welding such as strength and beauty, it is recommended to design based on laser welding from the design stage.

Summary

The biggest advantage of laser welding is that it can weld thin plates with low deformation. In addition, unskilled people can weld, so it is possible to greatly improve the welding process that used to rely on the skills of craftsmen. As long as you can correctly understand its disadvantages and deal with them appropriately, the rest are only advantages. Please make the most of the advantages of laser welding.