Causes (principles) of spatter generation during laser welding and required measures
Spatter prevention measures are essential for high-quality laser welding. Here, we explain the causes of spatter generation during laser welding and the required measures.
Finally, we introduce our proposed "Blue-IR Hybrid Scanning Head" and "beam profile control using DOEs."
Spatter prevention measures are essential for high-quality laser welding. Here, we explain the causes of spatter generation during laser welding and the required measures.
Finally, we introduce our proposed "Blue-IR Hybrid Scanning Head" and "beam profile control using DOEs."
Table of Contents
1. What Is Spatter for Laser Welding?
1-1. What Is Spatter?
1-2. Issues Due to the Influence of Spatter
2. Causes of Spatter Generation during Laser Welding (Principles)
2-1. Classification of Laser Welding Processes
2-2. Causes and Mechanisms of Spatter Generation
3. Spatter Prevention Measures during Laser Welding
3-1. First Generation (Laser Parameter Control: * Output, Focus Diameter, and Speed)
3-2. Second Generation (Beam Scanning: * Wobbling Movement)
3-3. Third Generation (Beam Profile Control)
3-4. Fourth Generation (Utilizing Short Wavelength Lasers * Blue and Green)
4. Our Proposal for Spatter Prevention Measures
4-1. Proposal for Beam Profile Control Centered on Using DOEs
4-2. Proposal of the Blue Laser Scanning Head (LW + Blue)
4-3. Proposal of the Blue-IR Hybrid Scanning Head (LW + Hybrid)
1. What Is Spatter during Laser Welding?
1-1. What Is Spatter?
Spatter generation during laser welding is one of the phenomena that occur during welding processes. Spatter refers to the phenomenon in which small particles are ejected vigorously from molten metal (the molten pool). These particles are said to range in size from a few microns to several hundred microns.
* Spatter-like phenomena include "fumes" and "plumes." These consist of a mixture of metal vapor and gases generated during laser welding, which can cause harmful effects as smoke or soot. These particles are typically less than a few microns in size.
1-2. Issues Due to the Influence of Spatter
Spatter generated during laser welding causes two major issues.
- Negative impacts on products
- Decreased weld quality (appearance defects, reduced weld strength)
- Spatter adhesion to prohibited areas (increased defective products, increased post-processing) - Negative impacts on equipment and processes
- Spatter adhesion to optical components (increased frequency of protective glass replacement)
- Increased cleaning frequency (increased maintenance time)
2. Causes of Spatter Generation during Laser Welding (Principles)
Before explaining the causes (mechanisms) of spatter generation in laser welding, first I will provide the prerequisite knowledge about the laser welding process (keyhole welding and heat conduction welding).
Before explaining the causes (mechanisms) of spatter generation in laser welding, first I will provide the prerequisite knowledge about the laser welding process (keyhole welding and heat conduction welding).
2-1. Classification of Laser Welding Processes
Laser welding processes can be classified into keyhole welding and heat conduction welding.
The disadvantage of heat conduction welding is its shallow penetration depth of 0.2 mm or less, which limits the products to which it can be applied. As a result, many products have been processed using keyhole laser welding in recent years. Therefore, spatter prevention measures are important during keyhole welding.
Observing the behavior of keyholes is important during keyhole welding. However, since keyholes are difficult to observe, please refer to the simulation video.
It shows how a keyhole is formed by laser irradiation and how the keyhole (small hole) is quickly filled.
2-2. Causes and Mechanisms of Spatter Generation
The causes of spatter generation can be broadly divided into two factors, as shown in the diagram on the left.
Next, we will explain the mechanism of spatter generation.
Looking at the laser welding processing point microscopically, we can observe the interaction between the molten metal (also called the molten pool) and the keyhole at the processing point. When a high-energy laser is applied, the area near the formed keyhole becomes extremely hot, and simultaneously, plasma vapor is ejected while the molten metal behaves turbulently. This turbulence causes the metal to be ejected with great force, generating spatter.
In other words, the key points to prevent spatter generation are "how to control the interaction between the molten metal and the keyhole" and "how to stabilize the molten metal and the keyhole."
3. Spatter Prevention Measures for Laser Welding
Spatter prevention measures have evolved thanks to advances in core technologies, including laser oscillators and optical components. This section explains spatter prevention measures, divided into the first through fourth generations.
Spatter prevention measures have evolved thanks to advances in core technologies, including laser oscillators and optical components. This section explains spatter prevention measures, divided into the first through fourth generations.
3-1. First Generation (Laser Parameter Control: * Output, Focus Diameter, and Speed)
Laser parameter control is a conventional method that has been used for a long time; however, it remains the foundation of spatter prevention measures.
- Laser output: This involves a wide range of control parameters, including setting the optimal laser output for the target metal material and shape, as well as slope control of the laser output.
- Focal spot diameter: Controllable factors include setting the optimal focal spot diameter and defocusing to adjust the focal point position.
- Welding speed: It is also an important factor in controlling the interaction between the molten metal and the keyhole.
3-2. Second Generation (Beam Scanning: * Wobbling Movement)
Beam scanning is a spatter prevention measure that wobbles the focused laser beam at high speed. Wobbling is often achieved using a galvano mirror, typically with circular motion. (There are also some unusual techniques, including the use of sine wave or figure-eight movements.)
The mechanism for reducing spatter generation depends on the materials to be welded (particularly aluminum and copper); however, a higher welding speed stabilizes the molten metal and keyhole, making it an effective spatter prevention measure. (At first glance, a slower welding speed may seem more effective for spatter prevention. However, a faster welding speed is better. This is because a slower welding speed keeps the laser beam focused on one spot of the molten metal longer, causing the molten metal to bump and resulting in large keyhole turbulence, which makes spatter generation more likely.)
3-3. Third Generation (Beam Profile Control)
Beam profile control is a spatter prevention method that modifies the beam profile of the irradiated laser. With conventional oscillators and optical systems, laser beam profiles have typically been controlled to form a Gaussian or top-hat intensity curve. However, this alone does not provide effective spatter prevention; therefore, in recent years, techniques that actively modify the beam profile shape have become generally used.
In particular, the ring-mode beam profile has become a common trend in spatter prevention measures for laser welding. This beam profile features a Gaussian-shaped central (core) beam surrounded by a ring-shaped top-hat beam.
The mechanism by which the ring-mode beam profile reduces spatter is as follows:
- The keyhole entrance diameter is enlarged, which suppresses the ejection of plasma vapor.
- The molten pool is enlarged, which suppresses molten metal turbulence.
- Cleaning, preheating, and slow cooling effects are expected, which stabilize the behavior of the molten metal and keyhole.
3-4. Fourth Generation (Utilizing Short-wavelength Lasers * Blue and Green)
In recent years, the rapid advancement of automotive electrification has led to increased demand for copper laser welding. To prevent copper spatter, an effective method is the use of short-wavelength lasers.
This is because the optical absorption rate of copper is less than a few percent when using conventional near-infrared light with a wavelength of 1070 nm, whereas it improves to 40–50% when using short-wavelength lasers with wavelengths of 420–515 nm, potentially reducing spatter.
The mechanism behind spatter reduction is as follows: While low optical absorption tends to cause unstable behavior, such as bumping of the molten metal, high optical absorption allows the metal to melt stably, making spatter generation less likely.
However, it should be noted that when using short-wavelength lasers as-is, the welding process is limited to heat conduction welding, which restricts the penetration depth to 0.2 mm or less. To address this problem, combinations of "short-wavelength lasers" and "third-generation beam profile control"* have been proposed in recent years, attracting attention in the laser welding industry.
* Examples include hybrid lasers combining Blue and IR lasers, and ring-mode short-wavelength lasers.
4. Our Proposals for Spatter Prevention Measures
Spatter prevention measures have been explained up to this point. Finally, our proposal will be presented.
We are focusing on spatter prevention measures based on third-generation beam profile control and fourth-generation short-wavelength approaches. The following presents three use cases.
Spatter prevention measures have been explained up to this point. Finally, our proposal will be presented.
We are focusing on spatter prevention measures based on third-generation beam profile control and fourth-generation short-wavelength approaches. The following presents three use cases.
4-1. Proposal for Beam Profile Control Using DOEs
Third-generation spatter prevention measures: Beam profile control uses two methods to form the desired beam profile, employing an oscillator and an optical system. The oscillator method is generally easy to handle and widely used. However, when this method has limitations in spatter prevention, the optical system method is proposed.
There are several approaches to implementing the optical method, and we propose the optimal approach for each project. We particularly specialize in beam profile control using an optical element called a DOE (diffractive optical element).
As shown in the images on the left, a DOE has a lens with fine asperities on its surface and uses the diffraction optical phenomenon to form the desired beam profile. Using this technology, it is possible to reduce the amount of spatter by approximately 75 to 90%.
Please watch this video demonstrating the actual reduction in spatter achieved through beam profile control using a DOE (diffractive optical element).
4-2. Proposal of the Blue Laser Scanning Head (LW + Blue)
Fourth-generation spatter prevention measures: This is an example of the use of short-wavelength lasers. The Blue Laser Scanning Head can also be equipped with an optional DOE. (Beam profile control functionality can be added.)
4-3. Proposal of the Blue-IR Hybrid Scanning Head (LW + Hybrid)
Fourth-generation spatter prevention measures: This is an example of the use of short-wavelength lasers. It shows a case using the Blue-IR Hybrid Scanning Head.
When Blue and IR lasers pass through the same optical system, chromatic aberration occurs, causing the Blue and IR light to deviate from each other at the focal point. However, our scanner features chromatic aberration correction, eliminating concerns about misalignment at the focal point.
Please watch this video demonstrating high-quality welding achieved with the Blue-IR Hybrid Scanning Head using its chromatic aberration correction function.