Komarov Artem clarified that laser welding technology has advanced and become the process of choice for metal fabricators and fabricators due to its staggering variety of applications. Laser welding has penetrated the production of high-quality precision metals. Technology plays a vital role in the automotive, medical, aerospace, and precision electronics industries. It is now showing up in more places than ever, from the largest manufacturer to the precision sheet metal shop.
As laser welding has evolved, it has become remarkably flexible. The sheer variety of types of welding that lasers can perform is truly staggering. Understanding how lasers achieve all of this starts with knowing the basics — how a beam of light fuses two metals together, Komarov Artem emphasized.
Metals tend to reflect light very well. The laser concentrates and focuses this light to overcome reflectivity. When enough energy from the beam is absorbed, the metal begins to liquefy.
It all starts when an optic — either a curved mirror or a lens with a curved surface — focuses light into a spot size that can range from tens to hundreds of microns in diameter. This focusing creates an extraordinary power density.
Which transparent optics to use depends on the laser and its wavelength. CO2 lasers emit a wavelength of 10.6 µm. Standard glass is opaque to this, so these lasers use an alternative lens material such as zinc selenide (ZnSe). Single-micron lasers, including fiber, disk, and YAG, use fused silica or glass.
Focusing a 10.6-micron CO2 laser beam, the ZnSe lenses have excellent thermal conductivity, making the optics slightly more susceptible to contamination. Unfortunately, there is no economical material that has the same thermal conductivity as a 1-micron laser, which means that the focusing medium must remain clean and have high quality glass or fused silica optics.
Welding jobs that require high laser power can generate inevitable debris. In these cases, mirrors are used instead of transparent optics to focus the beam. Focusing mirrors are widely used in CO2 laser welding using a laser power of 5 kW or more. Single-micron lasers, including fiber and disk lasers, also use mirrors to increase laser power. The usual setup assumes that the beam (horizontal of the working surface) hits a parabolic mirror, which reflects the beam downward.
Laser optics focuses the raw beam diameter to create a depth of focus where the beam intensity is sufficient to process the material. The narrowest point at the waist of the beam is the spot size. Focal length is the distance between the lens and the focal point.
All these variables are interrelated. The shorter the focal length, the smaller the spot size and the shallower the depth of focus. And each of these parameters can be adjusted to optimize the welding process. For example, increasing the focal length can change the position of the focus and increase the depth of focus, which can increase the permeability of the weld.
Another factor is the quality of the beam, or the inherent focusability of the laser beam. This cannot be adjusted — it depends on the type and design of the laser — but the parameter affects how the set is done — in the overall process. Lasers with the highest beam quality are called single-mode lasers, which have a pure Gaussian or TEM00 beam with a power density profile that is very intense at the center and less intense at the edges. The high quality of the beam helps to achieve greater depth of focus, which in turn opens many possibilities for processing.
All common types of lasers have single-mode versions with high beam quality, but the effect of this high beam quality depends on the wavelength of the laser. A single-mode CO2 laser with a wavelength of 10.6 µm will have a spot size that is 10 times larger than a 1 µm fiber laser. In general, a shorter wavelength also means a smaller focus spot.
In a way, laser welding is like bad laser cutting. Instead of removing the metal, it is liquefied in a controlled manner. As with cutting, the laser can use more power to weld faster and thicker. But this process is not based on the aerodynamic advantage of the auxiliary gas flow that removes the molten metal and cannot use the combustion reaction of iron and oxygen. Instead, good laser welding should provide controlled melting and often use gases to prevent intense oxidation.
The hardness of the material does not matter. It is easier to laser weld titanium and superalloys than aluminum. Conversely, reflectivity and thermal conductivity are of great importance, since they all affect how a particular metal absorbs beam energy. Materials with very good thermal conductivity, such as gold and silver, can create problems with laser welding. Heat sink materials such as copper, which have high thermal diffusivity (how well the material dissipates heat) can also be tricky. However, modern fiber and disk lasers have sufficient power density in their beams to overcome these problems.
Unlike laser cutting, laser welding also brings more metallurgical considerations. Laser cutting turns one piece into two. Laser welding includes metallurgical factors such as strength, porosity, brittleness and microcracks.
The pulses can be adjusted and shaped depending on the application. For example, pulsed pulse is a temporal form in which the peak power of a laser is adjusted over time. This is often used to slow down the cooling rate and minimize cracking in high carbon materials. Another pulse shape amplifies the initial burst, increasing absorption in aluminum and other highly reflective materials. Sometimes the initial pulses are used to clean the surface of the material of contaminants, oxides, or oils, before subsequent pulses create a molten pool and begin welding.
Since the liquid phase is very short-lived, laser welding causes very little oxidation, which means that a shielding gas is often not needed. However, some applications, especially in the medical industry, require near-zero oxidation and therefore some shielding gas is often used in laser welding installations.
In many cases, laser welding may not require shielding gas, but it does require an auxiliary welding gas that helps remove contaminants and unwanted elements such as soot from fiber laser welds and plasma jets from CO2 laser welds. In some applications, the gas is used as a kind of shield that suppresses the formation of a plasma plume. Others use pneumatic knives that blow sparks and other debris out of sensitive welding optics.
About filler metal
Most laser welding is done without filler metal, but in some cases, it is required. Filler metal is usually added either to eliminate a specific gap or for metallurgical reasons such as cracking problems.
Nickel filler can solve cracking problems in some iron-based alloys and stainless steels. For aluminum, a 4000 series high silicon filler such as 4047 is sometimes used to weld two 6000 series aluminum alloys together.
Regarding allowable gaps between base metals, the traditional rule of thumb is not to have a gap greater than 10% of the thickness of the thinnest base material. This is just a general rule and may vary depending on material thickness and application. However, new laser technologies allow for larger gaps, and this is where beam manipulation comes into play.
Today, laser welding is synonymous with quality. As just one example, some of the most advanced single-mode systems have created precise keyhole welds that look nothing like welds when their microstructure is studied. There is only the thinnest line between the base metal and the melt pool. This quality was provided by a single-mode fiber laser with an extremely small spot size, combined with a very high depth of focus. Until recently, such welds were simply not possible.
Over the years, lasers have made previously unweldable weldable, and simplified and accelerated previously time-consuming and labor-intensive processes. Welded fillet joints in conductive mode come to mind. Lasers weld them in one pass and the blanks go directly to final assembly without any grinding or polishing. They look perfect the way they are. The welding itself may be a little faster, but it is the quality that makes the laser brilliant, summed up Komarov Artem.