Handheld laser welding is still relatively new in the world of fabrication. Even as adoption increases, there is a persistent myth that continues to give some welders pause: “laser welds are weak”.
And there’s little reason to think otherwise based on just pictures and videos. After all, laser welds have narrow seam widths that look downright small compared to the bulky beads of traditional arc welds.
However, appearances are often deceiving. The truth is that laser welds are plenty strong, often outperforming comparable arc welds in strength tests. But the factors that determine their strength differ somewhat from those that determine the strength of arc welds.
Let’s settle the question of laser weld strength by exploring some key concepts and reviewing strength-test data that provides convincing proof.
Don’t Judge a Weld by its Cover
When understanding the differences between arc welding and laser welding, it is critical to recognize that the appearance of a weld bead can be misleading. While arc welds produce a wide, rounded bead, laser welds create a narrower, neater seam. But what’s not immediately apparent is the weld’s depth of penetration.
This drawing highlights the relative differences in weld geometry between arc welds and laser welds using a fillet weld as an example.
Traditional arc welds create joints that owe their strength to a combination of bead size and contact area with the joint. While penetration matters – with how much depending on the joint in question – covering a wide area while avoiding excess convexity or concavity is critical to creating a strong arc weld.
By contrast, laser welding concentrates more energy into a smaller spot. This focused heat travels deeply into the material to achieve significantly higher penetration, especially relative to the width of the weld. This deep penetration is the primary source of a laser weld’s considerable strength.
Getting Technical: The Weld Throat
To understand a weld’s strength, it’s essential to look at its cross-section. Whether examining an arc weld or a laser weld, the most essential variable determining its strength is its throat.
A deep penetration handheld laser welding cross section. Credit: Handheld Laser Institute.
The throat is responsible for a joint’s resistance to forces like shear and tension. By achieving deeper material penetration, laser welds rely primarily on internal throat dimensions whereas arc welds rely primarily on external throat dimensions.
Other Factors Contributing to Laser Weld Strength
Deep penetration and the resulting weld throat make up the lion’s share of a laser weld’s strength. But several other factors contribute as well.
Reduced Heat Input
As previously mentioned, a laser’s energy is highly concentrated, which leads to a much smaller heat-affected zone (HAZ) compared to traditional welding methods. Combined with the faster travel speeds typically achieved with the process, laser welding introduces significantly less heat into the target material.
Besides reducing distortion, a reduced HAZ minimizes localized stresses that can lead to weaker welds.
Improved Metallurgical Properties
The intense, focused heat of laser welding is followed by rapid cooling, ultimately changing the target material’s microstructures. While metallurgical changes are not unique to laser welding, they do differ from those typically caused by arc welding.
The exact outcome is dependent on the process, but the results are typically positive, with laser welding resulting in finer grain structures that improve hardness and strength.
The metallurgical impacts of the handheld laser welding process have shown particular promise in steel repair situations by eliminating cracking caused by arc welding repairs.
Reduced Porosity
Significant porosity – caused by gas trapped in the weld pool as it cools – can cause weld failure in otherwise solid welds. Laser welding reduces the chance of gas entrapment and porosity in three main ways:
- Smaller molten pools provide less opportunity for gas to form and become trapped by limiting gas generation and reducing the time they have to create large voids.
- Higher cooling rates also reduce void formation time and quickly stabilize the weld pool.
- Precision heat control minimizes the chances of weld pool turbulence and prevents defects caused by inconsistent heat input.
A note on porosity: laser welds are not immune to porosity, but porosity and spatter can be virtually eliminated in the majority of applications. If you are experiencing unacceptable levels of porosity, try adjusting your machine’s settings and other welding parameters (particularly gas pressure).
Beyond Theory: Real-World Strength Test Data
The principles and concepts above explain why laser welds are strong. Quantifiable data* provides proof and shows how strong they are in real-world conditions.
The following data comes from a series of transverse lap shear tests performed on ¼” thick 304L stainless steel with a 1” wide fillet lap weld. The tests measured the ultimate load – the maximum force the weld could withstand before failing – across different laser powers and welding wires.
As the data demonstrates, laser welding is more than capable of producing strong welded joints. Strength correlates directly with measurements of the throat and increased throat sizes can be consistently achieved with increased laser power and thicker welding wire.
*Our welding and testing is performed in collaboration with the Handheld Laser Institute and Applied Technical Services
Meeting (And Exceeding) Industry Standards
The performance of laser welds is also validated by their ability to meet the standards of various industry codes. Weld tensile tests performed in accordance with AWS D1.6 (Structural Welding Code for Stainless Steel) have produced ultimate loads of nearly 18,000 lbs. In these qualification tests, laser welds consistently meet or exceed the code’s standards for visual inspection, macro-etching, and bend testing.
Getting Started with LightWELD
Considering adding handheld laser welding to your toolkit? Our laser welding experts are here to help. Whether you just want to learn more about LightWELD or want to try it for yourself, getting started is easy.
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