Laser Welding Sheet Metal: A Beginner’s Guide

Introduction

Welding is a primary process in sheet metal fabrication, characterized by high labor intensity and harsh working conditions. As such, high proficiency is essential.

Automation in welding and the development of innovative joining methods have been consistent focal points for professionals in welding technology.

A pivotal aspect of welding automation is the control of welding quality and efficiency. During the process, challenges such as arc and weld seam alignment, uniformity of component gaps, weld penetration, and control of welding distortion must be addressed.

With the rapid advancements in laser welding technology, it has taken a significant leap forward and has matured in its application across various sectors, including household appliances, high-tech electronics, automotive manufacturing, high-speed train production, and precision machining.

The benefits of laser welding can be understood by comparing it with traditional arc welding. This post will delve into the process of laser welding and explore how to achieve better results.

To evaluate the quality of a laser weld, the depth-to-width ratio and surface morphology are taken into consideration. This post will examine the process parameters that impact these indices.

Experiments of laser welding were conducted on stainless steel, aluminum, and carbon steel plates. The results provide practical insights that can be utilized in welding production.

Laser welding is a cutting-edge production technique that utilizes a laser with high energy density as the heat source for welding. It is widely used in the sheet metal manufacturing industry due to its advantages, including high energy density, fast welding speed, environmental friendliness, minimal plate deformation, and more.

Laser welding, based on the characteristics of the weld seam formation, can be divided into conduction welding and deep penetration welding. Conduction welding utilizes low laser power, resulting in longer melt pool formation times and shallow melt depths.

It’s primarily used for welding small components.

In contrast, deep penetration welding has a high power density, where the metal in the laser irradiation area melts quickly.

This melting is accompanied by intense vaporization, achieving weld seams with significant depth and a width-to-depth ratio of up to 10:1.

Thin sheet components can be joined using various welding methods, including laser welding, brazing, atomic hydrogen welding, resistance welding, plasma arc welding, and electron beam welding.

When comparing laser welding to other common welding techniques, it offers significant advantages in terms of the heat-affected zone, thermal deformation, weld seam quality, the necessity of filler material, and the welding environment.

The comparison between laser welding and other welding methods can be found in Table 1.

Table 1 Comparison between Laser Welding and Other Welding Methods

Characteristics of Laser Welding

Laser welding utilizes a laser that channels a high-energy laser beam into a fiber optic. After transmission, it is collimated into parallel light using a collimating lens and then focused onto the workpiece.

This results in an extremely high-energy heat source that melts the material at the joint. The molten metal then rapidly cools to form a high-quality weld. The appearance of a laser-welded sheet metal piece is shown in Figure below.

Easy Operation:

Laser welding machines are straightforward to use. The operation is simple, easy to learn, and user-friendly. The required expertise level for operators is relatively low, translating to savings in labor costs.

High Flexibility:

Laser welders can weld from any angle and are adept at accessing hard-to-reach areas. They can handle complex welding components and irregularly shaped large pieces, offering unparalleled flexibility in welding from any orientation.

Enhanced Safety:

The high-safety welding nozzle activates only upon contact with metal, featuring a touch switch with body temperature sensing. Specific safety standards must be adhered to when operating the specialized laser generator, including wearing protective eyewear to minimize potential eye damage.

Superior Laser Beam Quality:

Once the laser is focused, it achieves a high power density. With high power and low-mode laser focusing, the resulting spot diameter is tiny, significantly promoting automation in thin sheet welding.

Fast Welding Speed with Deep Penetration and Minimal Distortion:

Due to the high power density of laser welding, tiny pores form in the metal during the process. The laser energy travels deep into the material through these pores with minimal lateral spread. The depth of material fusion is considerable, and the welding speed is rapid, covering a large area in a short time.

Reduced Labor Costs:

Owing to the minimal heat input during laser welding, post-weld distortion is minor. This results in a visually appealing weld finish, leading to reduced post-weld processing, which in turn significantly cuts down or even eliminates labor costs associated with smoothing and leveling.

Capability to Weld Difficult Materials:

Laser welding is not only suitable for joining a variety of dissimilar metals but also for welding metals and alloys like titanium, nickel, zinc, copper, aluminum, chromium, gold, silver, steel, and cutting alloys. It caters well to the developmental needs of new materials in household appliances.

Particularly Suited for Welding Thin Sheets and Non-coated Aesthetic Components:

Given its high aspect ratio in welding, low heat input, minimal heat-affected zone, and reduced distortion, laser welding is especially apt for welding thin sheets, non-coated aesthetic components, precision parts, and thermally sensitive components. This can further minimize post-weld corrections and secondary processing.

Comparison between laser welding and traditional arc welding

Introduction to arc welding

Traditional arc welding can be broadly categorized into several types, including electrode arc welding, tungsten inert gas (TIG) welding, metal inert gas (MIG) welding, and submerged arc welding.

Electrode arc welding involves striking an arc between the electrode and the workpiece, generating heat that melts the metal at the point of contact between the electrode and the workpiece. This creates a pool of molten metal. The electrode is then moved in a specific direction, creating new pools of molten metal and solidifying previous pools, resulting in the formation of a weld.

A diagram of the welding process is depicted in Figure 1.

Fig. 1 Schematic diagram of electrode arc welding

Tungsten Inert Gas (TIG) welding uses a tungsten electrode as the discharge electrode, which does not get consumed during the process. The welding area is protected by an inert gas, typically argon, and the heat generated by the arc is used to melt both the base metal and welding material. The result is a smooth welding surface with little to no spatter.

Gas Metal Arc Welding (GMAW) is a process where an arc is generated between the welding wire and the base metal, causing the wire and the base metal to melt. The melted material solidifies, forming a weld.

Although arc welding still holds a dominant position in the welding industry, the increasing demands for high-quality and efficient welding have led to limitations in its application in some high-end sheet metal manufacturing fields. Some of the disadvantages of traditional arc welding include:

• The need for high-purity argon as the shielding gas, which can be expensive.
• A high level of skill and certification is required for proper operation, making it inaccessible to many.
• High-density current and heat generated during the welding process can negatively impact the welding of metal plates with low melting points.
• Low welding efficiency and slow speed.
• The welding surface is rough, requiring additional grinding for a smooth surface.
• Limited welding strength.

Advantages of laser welding

Mainstream laser welding techniques include self-fusion welding, swing welding, wire filling welding, galvanometer welding, and composite welding that combine various welding methods.

Table 1 highlights the advantages of laser welding technology compared to traditional arc welding.

For high-end sheet metal manufacturing industries that require high value-added products with consistent quality, small weld gaps, and high efficiency, laser welding is a top choice.

Table 1 Comparison of characteristics between laser welding and arc welding

Laser welding process parameters

Laser welding seam index

The requirements for the welding effect of sheet metal parts vary depending on customer needs. These requirements are primarily reflected in the following indicators:

• Surface morphology (e.g. concave, flat, or convex)
• The depth-to-width ratio of the weld pool
• Absence of defects such as pores, cracks, impurities, and undercuts

The surface morphology of the weld can be altered by adjusting factors such as welding power, defocus, and splicing mode. The depth-to-width ratio of the weld pool is an important factor for determining the strength of the weld.

For customers who have strength requirements for their welding products, a series of steps must be taken, including wire cutting, inlaying, grinding and polishing, corrosion testing, and microscopic metallographic analysis. This process reflects the hardness of the weld, which is closely related to the depth-to-width ratio. The tensile strength index of the weld can also be determined through a tensile strength test. Figure 2 shows the metallographic analysis of penetration ratio.

Fig. 2 metallographic analysis of penetration ratio

In certain working environments, weldments may have defects such as pores, cracks, impurities, and undercuts, which can pose serious safety risks. For instance, some products require strict standards for airtightness and water tightness.

Figure 3 depicts a comparison between normal welds and welds with defects.

Fig. 3 weld seam diagram

Influencing factors of laser welding

Several factors directly impact laser welding, including welding temperature, melting point of the welding materials, laser absorption rate of the welding materials, and thermal influence.

In terms of the welding process, factors such as material properties, laser power, welding speed, focus position, shielding gas, and weld gap must be taken into consideration.

The laser absorptivity of the welding materials affects the quality of the weld. Materials such as aluminum and copper have higher laser absorptivity, while carbon steel and stainless steel have lower laser absorptivity. Welding materials with high absorptivity typically require more energy to melt and form a stable weld pool.

Laser power is the energy source for laser welding and plays a critical role in determining the welding effect. The larger the laser power, the better the welding effect. However, too much laser power can lead to instability in the weld pool and reduced depth. Therefore, choosing the appropriate laser power value is crucial.

There is an inverse relationship between welding speed and penetration. Faster welding speeds result in lower energy input, while slower speeds can cause overheating, particularly in heat-sensitive materials such as aluminum.

The position of the focus directly affects the penetration and width of the weld. When the focus is located on the surface of the welding material, it is referred to as zero focus. When the focus is above or below the welding material, it is called eccentric focus. The zero focus spot is the smallest and has the highest energy density, while off-focus welding has a lower power density but a larger light spot, making it suitable for welding workpieces with a larger range.

The type and method of shielding gas also affect the welding process. The function of shielding gas is not only to prevent oxidation during welding but also to suppress the plasma cloud generated during laser welding. The choice of shielding gas can impact the appearance and color of the weld surface.

The weld gap of the workpiece to be welded is related to the penetration, width, and morphology of the weld. A weld gap that is too large can result in difficulty fusing and combining, as well as expose the laser and potentially damage the tooling or workpiece. Increasing the light spot or swing can improve the welding, but the improvement is limited.

Welding test analysis

The welding test was conducted using a Yaskawa GP25 robot, Prima laser, ospri welding joint (core diameter 100μm, focal length 300mm), and WSX wire feeder. The welding effect was tested on 1.5mm Q235 carbon steel plate, SS304 stainless steel, and 3-series aluminum alloy plate.

Based on experience, the following reference can be provided for the testing process:

For the test welding of 1mm thin plate, a starting power of 1kW and a welding speed of 30mm/s can be used. The reference power can be calculated as P=A·X, where A is a constant coefficient (A≥0) and X is the plate thickness. As the plate thickness increases, the constant coefficient A gradually decreases and is also influenced by the welding method.

Analysis of welding process of carbon steel plate

See Table 2 for the swing welding process parameters of Q235 carbon steel plate with thickness of 1.5mm.

Table 2 swing welding process parameters of Q235 carbon steel plate

The test data shows that when swing welding carbon steel plates, the laser power should be increased with the increase of welding speed while ensuring the swing range remains unchanged. If the swing speed is too slow, the weld will be uneven.

In general, less energy is required for carbon steel self-fusion welding compared to carbon steel self-fusion swing welding, and less energy is required for carbon steel self-fusion swing welding compared to carbon steel swing wire filler welding. The energy required is mainly controlled by power and speed, with a higher power and faster speed requiring more energy.

Ideally, to balance quality and efficiency, the welding speed should be increased as much as possible. However, too fast welding may cause instability and be limited by laser power and material properties. Therefore, a balance between power and speed is typically sought.

Analysis of aluminum plate welding process

In the test, the core diameter of the optical fiber selected was 100μm. For welding high reflective and heat-absorbing materials such as aluminum and copper, a higher power density is needed for melting. In this case, zero focus welding is necessary.

Zero focus welding allows for maximum power density with minimum power, making it ideal for welding small parts and melting the metal to form a molten pool. Table 3 provides the welding process parameters for different materials.

Table 3 Comparison of welding process parameters of different materials

The test data indicates that, with other parameters remaining constant, the ideal welding effect for 3-series aluminum alloy requires a slower welding speed compared to Q235 carbon steel, as more heat is needed.

Analysis of stainless steel white blowing process

Table 4 shows the comparison of whitening process parameters for stainless steel welding seams with a thickness of 1.5mm. The comparison of the welding effect can be seen in Figure 4.

The welding parameters of the three welds in Figure 4 (from left to right) correspond to the serial numbers 1, 2, and 3 in Table 4, respectively.

Fig. 4 Comparison of welding effect

Table 4 Comparison of process parameters of stainless steel welding seam whitening

In order to obtain a whitened surface on stainless steel, it is necessary to rapidly cool and crystallize the metal in a shielding gas atmosphere after laser melting. If the power is too high, a lot of heat will be retained in the metal plate, causing slow cooling and an increased risk of oxidation and discoloration. If the power is too low, the metal may not fully melt.

If the speed is too fast, the blowing tooling may not be sufficient, affecting the blowing effect. If the speed is too slow, there will be excessive heat accumulation. To achieve a whitened surface, it is important to find a balance between power, speed, and blowing.

If it is not possible to achieve a whitened surface in one attempt, it may be possible to do so by welding one layer at a slightly higher power and then reducing the power for a second layer.

Conclusion

During the process of laser welding, to ensure high-quality welding results, it is important to consider a range of factors including material properties, laser power, welding speed, focus position, shielding gas, and weld gap.

For commonly used materials such as carbon steel, stainless steel, and aluminum plates, the initial test parameters mentioned earlier can be used as a reference, and then adjusted according to the specific characteristics of the material and customer requirements to achieve the desired welding effect.