Laser welding is a powerful technology that uses the heat generated by laser technology to fuse two materials (such as steel plates). Welding faces the challenge of weld distortion due to the natural reaction of metals to extreme heat. This is a deformation that compromises the integrity of the molten metal. Fortunately, there are ways to minimize distortion by implementing welding process parameters.
In this article, we will focus on what welding distortion is, what causes it, and how to minimize its effects. You'll also find effective tips to help you address the effects of welding distortion.
What is laser welding deformation?
Weld deformation or deformation involves changes in the shape and size of the metal structure. This is a natural effect of welding. When metals are laser welded, they are exposed to extremely high heat that causes them to melt. This causes the material to expand.
As the molten metal cools to room temperature, it solidifies and begins to shrink. This is residual stress. If the heating is localized and the rest of the metal surface is not heated, it will not expand or contract in the same way. This causes distortion.
Materials that are easily deformed after welding
To determine why a material is susceptible to welding distortion, you must understand the properties that affect distortion. Some materials are more susceptible to deformation due to both physical and mechanical properties.
1. Physical properties are measures of thermal expansion and thermal conductivity.
Thermal expansion is the movement of metal as it expands when heated and contracts when cooled. If there is a high coefficient, the material expands and contracts more easily - so it will twist more.
Thermal conductivity, on the other hand, measures the flow of heat along a material. High thermal conductivity dissipates heat faster. Since the presence of heat can make materials easily deform, low conductivity increases the chance of deformation during welding.
2. In terms of mechanical properties, two factors need to be considered, yield strength and elastic modulus.
Yield strength refers to how much pressure a material can withstand in response to external forces. Therefore, materials with higher yield strengths have higher residual stresses and thus deform more easily.
Modulus of elasticity refers to a material’s ability to expand and contract. A higher elastic modulus means the material has a greater ability to resist deformation.
Taking these properties into account, you can assume that if a material has a higher thermal expansion coefficient, lower thermal conductivity, higher yield strength, and lower elastic modulus
Materials that are easily deformed after welding
Comparing stainless steel to carbon steel, you can assume that the former is more susceptible to deformation because it has a higher yield strength and coefficient of thermal expansion as well as a lower thermal conductivity.
Between aluminum and copper, the former is prone to deformation because it has a higher yield strength thermal expansion coefficient, and lower thermal conductivity.
Types and causes of laser welding deformation
Several studies have been conducted to identify the real causes of deformation after the laser welding process. According to a study, 3 factors significantly affect laser welding yield: material, process, and geometry.
For example, laser welding deforms because the welding parameters are applied to the metal surface in different ways. Welding speed, current, angle, etc. will be concentrated on the parts being welded. When moving away from the welding area, the heat gradually decreases and thermal effects such as metal expansion are also reduced.
Therefore, it can be safely assumed that the expansion will vary depending on the intensity of heat received by the metal. In the example given, the welded portion expands the most because it receives the most heat from the laser source.
When the welding process ends, the metal will begin to cool and shrink. The metal will continue to shrink by the same amount as it expands. This is called residual stress.
If the stress is greater than the yield strength of the parent material, two types of stress may occur.
Compressive stress occurs in the area around the edge of the parent metal.
Tensile Stress This occurs when the contraction of a heated metal is resisted by the rest of the metal's surface (the unheated surface).
To further understand this, it is best to consider the different ways in which deformation can occur when welding is completed.
1. Longitudinal deformation
As the name suggests, this deformation occurs along the length of the material being welded. As it cools, the weld and the area around it shrink. As a result, the workpiece will become shorter. This causes the outer edges to look longer and the middle section to look arcuate.
Especially if the workpiece is not fixed correctly, the deformation will be maximum.
2. Lateral distortion
This type of distortion occurs when the edges of the metal are pulled toward each other. This deformation is caused by shrinkage exceeding the expansion that initially occurred during laser welding.
3. Angle distortion
Angle distortion occurs when the angle of the metal plate changes due to shrinkage after the welding task is completed. The edges of the sheet metal are pulled toward each other on one side, causing the material to appear curved.
If the weld joins metal at a vertical angle, the vertical metal will not appear straight, but rather curved.
4. Complex distortion
This type is a combination of the twists discussed earlier. It looks like it is buckling, bending, or warping. These are different types of bends and deformations that can compromise the integrity of the welded material. No matter how strong the metal is, if laser welding causes it to deform, the weld will fail.
10 ways to minimize weld distortion
While distortion is unavoidable, that doesn't mean there's nothing you can do to minimize it. Just as there are different types of deformation, there are also various ways to prevent stainless steel and other metals from deforming. This has nothing to do with the strength of the steel chosen. What matters more is what you do before, during, and after your welding task.
Here are 10 different welding ideas you can use.
1. Avoid excessive welding
Welding large areas increases the shrinkage that occurs. This is why you should plan your laser welding process especially when you need to work on huge surfaces. Adjusting surface dimensions minimizes welding distortion and residual stresses, thereby avoiding wasted metal and time.
2. Use intermittent welding
This is a technique that leaves space between welds. Instead of a continuous weld, you'll weld an inch and then leave space for the unwelded metal before making another weld. This can effectively reduce deformation after welding is completed.
3. Reduce the number of transfers
Another way to avoid distortion is to limit the number of passes in the welding process. Make sure one time is enough to avoid deformation. You can try performing one large weld pass instead of several small weld passes. According to TWI, a larger single weld produces less angular deformation than a weld formed by multiple small passes
4. Consider welding location
The location of the welding is also important. Ideally, you should place the weld close to the center or neutral axis of the material. This will minimize distortion when the laser weld begins to shrink because there will be less leverage as the shrinkage forces try to move out of alignment.
5. Try the back-step welding technique
Back-step welding is a technique where the welding direction is from left to right, but the weld bead segments are deposited from right to left. Doing this will expand the edges where the bead segments are placed to temporarily separate the sheet metal.
When the movement from left to right is completed, the continuation of the beads will cause the expansion to decrease as the process is completed. This is an effective way to minimize distortion.
6. Preset welding parts
This will involve some testing to ensure minimal distortion once the welding is complete. Determine the preset required for previous welding so you can estimate the shrinkage you will experience. This will allow you to make adjustments to minimize shrinkage and distortion.
7. Create a welding sequence
Don't just use a straight line to weld parts. Create a planned welding sequence that can counteract the shrinkage of another part of the material being assembled. Based on your knowledge of how metal shrinks, you can create a sequence that balances the reactions to prevent deformation.
8. Clamp the part to lock it in place
Another option is to use a jig when you are welding the parts together. This will secure them and prevent expansion or contraction from distorting them. Keep the parts in place until the process is complete. The lack of movement reduces distortion.
9. Consider thermal stress relief
This is a technology that controls the heating and cooling of parts joined by welding. That's when you increase the temperature and control the cooling management stress by wearing it while welding the product.
10. Shorten welding time
You can also shorten the welding time to reduce the risk of distortion. This would be challenging if you were to do this manually. The parts you weld first will cool before they are finished. However, if you have mechanized welding equipment, you can reduce processing time and keep distortion to a minimum.