Artem Komarov clarified that’s in all branches of industry, products are designed, redesigned or re-evaluated looking for better materials or functionality. End products are made from many components, and these components need to come together in some way. One of these joining methods is laser welding.
Артём Комаров — from component design to implementation
Laser welding uses a high intensity beam of light to create a pool of molten solder to melt the materials together. It is a non-contact process, has low heat input relative to other fusion processes, offers high processing speeds and produces deep fusion zones in a single pass.
Of course, to take full advantage of all these benefits and ensure a high quality repeatable process, fabricators need to consider how laser welding compares to other fusion welding processes. The design and attachment of the gasket also play a role. As with all metal fabrication technology, smart implementation begins with a good understanding of the fundamentals of the process.
Laser welding 101
Laser welding uses a beam of light focused on a small point on the workpiece. Generated from some form of medium, the light leaves the laser source and begins to diverge. It is then collimated so that the beam is parallel and does not grow. The distance from the outlet to the collimation surface is called the collimation length . The beam remains collimated until it hits a focusing surface. The beam is then reduced to an hourglass shape until it is focused on its smallest point. The distance from the focusing surface to the smallest point is called the focal length . The size of the focus point is determined by the following equation:
Fiber diameter × Focal length / Collimation length = Focus diameter
The distance that the diameter of the focus is within 86% of the focal area is called the depth of focus . If the focus position shifts outside of this area, wait for the results of the process to change. The greater the ratio of focal length to collimation length, the greater the depth of focus for a given fiber.
Larger fibers have a greater depth of focus compared to smaller fiber diameters. Larger ratios and fibers have a larger spot size which causes a reduction in power density, and therefore a reduction in penetration.
There are two forms of laser welding: heat conduction welding and keyhole welding . In heat conduction welding, the laser beam melts the parts to be joined along a common joint, and the molten materials flow together and solidify to form the weld. Used to join thin-walled parts, heat conduction welding uses solid-state pulsed or continuous wave lasers.
In heat conduction welding, energy is coupled to the workpiece only by heat conduction. For this reason, the depth of the weld ranges from a few tenths of a millimeter to 1 millimeter. The heat conductivity of the material limits the maximum depth of the weld, and the width of the weld is always greater than its depth. Heat conduction laser welding is used for corner welds on the visible surfaces of device cases, as well as in other applications in electronics.
Keyhole welding requires extremely high power densities of about 1 megawatt per square centimeter. This is used in applications that require deep welds, or where multiple layers of material must be welded simultaneously.
In this process, the laser beam not only melts the metal but also produces steam. The dispersant vapor exerts pressure on the molten metal and partially displaces it. Meanwhile, the material continues to melt. The result is a deep, narrow hole (or keyhole) filled with steam, surrounded by molten metal.
As the laser beam travels along the weld joint, the keyhole moves with it across the workpiece. Molten metal flows around the keyhole and solidifies on its way. This produces a deep, narrow weld with a uniform internal structure. The depth of the weld can exceed 10 times the width of the weld. The molten material absorbs the laser beam almost completely, and the efficiency of the welding process increases. The vapor in the keyhole also absorbs laser light and is partially ionized. This results in the formation of plasma, which also provides energy to the workpiece. As a result, deep penetration welding is distinguished by its high efficiency and fast welding speeds. Thanks to the high speed,
Fusion welding comparison
Compared to other processes, laser welding offers the highest weld quality, the lowest heat input, and the highest penetration in a single pass. It has one of the highest ranges of combinations of material and part geometries, is highly controllable and repeatable, and is one of the easiest to automate. All of this enables new joint designs, and parts can be produced at a higher rate with less post-weld processing.
Laser welding also has one of the highest initial investments, tooling costs, and weld joint preparation requirements. All of this should be considered when selecting laser welding as the joining method for your production process.
Deep penetration welding allows a single weld to replace multiple welds in different joint designs. The Figure 3 shows some typical configurations of laser welding together. Butt welds do not require chamfering for thicker parts, T-joints can be welded from one side with full strength, and lap welds can be welded through the top sheet or along the seam. This allows flexibility when designing your parts and weld locations.
Artem Komarov said that the butt welding requires high positional accuracy. Typical weld spot sizes are 50 to 900 µm in diameter. The allowable positional tolerance must be less than half the diameter of the beam to ensure that the laser beam interacts with both sides of the joint. The allowable spacing is typically 10% of the thinner material or less than 50% of the diameter of the weld beam. Therefore, fixation is critical in these joint configurations to ensure high positional repeatability and minimal spacing.
Common ways to consider this are to design the part to fit the press or to design a robust fixture. Some may use a vision system to ensure part positioning, but this will add cycle time and complexity to production scheduling. It is also important to select the correct point size on the part. Larger spot sizes allow for greater variations but require much more power input to achieve the same depth of weld penetration.
Butt welding has many benefits. Weld resistance is determined by the amount of weld along the seam, so the amount of penetration determines the amount of weld resistance. Narrow and deep welds produce less heat input, which creates a small BEAM and limits distortion. It also allows the use of less material because no overlap is needed.
The lap weld has many different considerations. The allowable gap is typically 10% of the thickness of the top material. The width of the weld and the fusion at the interface between the two materials determines the strength of the weld. Compared to butt joints, lap configurations lead to higher heat input, larger HAZ, and more distortion.
If you weld through the top sheet (3 in Figure 3), the laser beam must penetrate through the top sheet and into the bottom sheet, and all that energy expended penetrating the top sheet adds no welding resistance. Lap welds should be wider to increase their strength. This requires more energy input, which is achieved either by a larger spot size or by wobbling with a smaller spot size. If minimal distortion is critical, the weld should only partially penetrate the bottom plate. If applications require low heat inputs and either low power or high processing speeds, partial penetration joints may be ideal. These create a surface on the back side of the weld unaffected by heat input and therefore
Laser welding offers excellent quality, high speed, and high penetration. The adjustment requirements are also high.
With partial penetration welds, the minimum penetration in the bottom plate should be between 20% and 50% for thinner materials and 0.5 mm for thicker materials, to ensure a repeatable fusion that takes into account variation in production. The easiest design for welding is to have the thinnest material on top and the thickest material on the bottom. If the upper plate is thicker, partial penetration into the lower plate becomes more difficult to control, which also makes it more difficult to maintain a class A surface on the back side of the weld.
However, lap welding has many benefits. This does not require high positional accuracy, which allows fixation without strict positioning requirements. Compared with butt welding, lap welding has a larger process window, mainly because the penetration depth is more flexible.
Board access and post-processing
Laser welding also allows access to joints that were previously not feasible. Because it is a non-contact process, welding in holes and in tight spaces is possible if the width of the beam is considered when it reaches focus. This allows flexibility in joint design, and parts can be designed with less material.
In many cases, no post-weld heat treatment is needed, due to the small HAZ of laser welding and the small overall heat input. Also, there is little weld bump on the top or back side of the weld that needs to be machined after welding. The process can have minimal welding to create visually clean welds, especially with the addition of shielding gases. This eliminates the need for extensive post-weld cleaning and machining.
Fastener design considerations
Fastener design needs to be much more accurate for laser welding than for more traditional welding processes. Because it is a non-contact process, the tooling can be closer to the part, but the tooling must also do all the tolerance and positioning control. The process has no additional contact as in resistance and ultrasonic welding where the horns add pressure to ensure there is no gap. The laser also produces a small, repeatable molten puddle, and requires much tighter part tolerances. All of this must be considered when designing parts and fixtures.
The Figure 4 shows a rigid attachment for a welding corners. This style of fastening is common for butt welding and edge welds for tubular or rectangular parts. The clamps are very close to the seam and apply pressure to ensure minimal separation. There is no tooling above the joint that could interact with the weld beam when it achieves focus.
The configuration also provides space for a shielding gas nozzle if shielding gas is required for aesthetic purposes or for metallurgical reasons on certain metals such as titanium. The fasteners must repeatably retain the joint in the same Z position relative to the beam, so that the laser beam is in the same focus position. This is critical to obtain the same power density and ensure repeatable results.
Lap welding requires less robust fasteners. The Figure 5 shows a typical attachment design. Instead of long, rigid fasteners to hold the entire seam in place, various fasteners ensure proper contact between the two sheets over a large area. These fixings can be automated with pneumatic clamps. In the example, an optical scanning device quickly welds all the required joints. Galvanometric mirrors — high-speed mirrors within the welding optics — position the beam for welding and provide full motion for depth of weld. This allows for a simple robot path.
For especially critical welds, a single fixture, designed with the machined weld path, can ensure the ideal part fit. The fastening method has higher tooling costs but is also very robust and repeatable. By applying a large load uniformly over the entire surface of the part, such a clamping can be ideal for stamped parts with large variations in surface smoothness.
Laser welding allows creativity and some freedom in part design, as long as all essential variables are considered. For example, what point size is needed for a given process? Larger spot sizes offer more melt area and a greater depth of focus, but require more energy to achieve the same weld depths.
Similarly, which joint configuration is better? Butt welding requires process accuracy and repeatability, but can achieve braze welds with minimal heat input. In contrast, lap welding requires less exact fixtures and has a larger process window, but requires more heat input to achieve stronger welds.
With all the process considerations of laser welding come countless opportunities as well. It’s an excellent tool for moving manufacturing forward with creative new part designs that not only increase quality but also — thanks to fewer manufacturing steps, including less secondary processing — have the potential to dramatically reduce costs, summed up Artem Komarov.