Introduction to laser cutting and welding
Laser cutting is generally a fairly simple and straight-forward process: using a high-power laser beam focused into a tight spot and aimed at the work surface, the laser energy is absorbed in the workpiece and is then converted into heat, which in turn can cause vaporization or ablation of the metal, creating a hole with molten walls. Combining a scanner or a moving stage with this concept, one can produce a narrow cut in any desired path. Cooling of the surface and cleaning of the debris is also added to complete the system.
Surprisingly or not, the concept behind the process of combining two sheets of metal together is not so different than the one separating one sheet into multiple pieces. The process of laser welding is also a heat based process similar to laser cutting. There are generally two types of laser welding: heat conduction laser welding and deep penetration laser welding, and the defining factor between them being the power density.
Lower power densities enable only heat conduction laser welding, in which the beam melts the mating parts between two sheets of metal along a common joint. The molten materials flow and solidify to form the weld. This process is also known as “butt-joint welding” or if adding another bonder wire material it becomes “brazing”. In this process, the maximum weld depth is limited by the heat conductivity of the workpiece, creating a width/depth > 1 ratio of the weld.
When high power densities are obtained the heat cannot dissipate fast enough causing vaporization of the workpiece (as opposed to melting with lower power density). The vaporizing metal or plasma expands to create a keyhole or tunnel from the surface down into the depth of the weld. This process is referred to as deep penetration laser welding or keyhole welding. Here too, moving the beam across the workpiece by combining a scanner or a moving stage, one can create a deep and narrow weld in the desired path.
Advantages of shaping in welding & cutting
Weld strength, penetration, quality and seam height are determined by processing parameters, including the shape of the laser spot. By diverting some energy into a ring around the laser spot, for example, spattering can be reduced, increasing weld seam quality and strength. Another example are laser cutting processes, that often show improved edge quality and reduced kerf when a flat top beam shaper or M shaper is used.
In general, typical improvements achieved by spot shaping in processing include:
- In laser cutting:
- Cut width (kerf)
- Cut edges quality (squareness and striations)
- Cut underside appearance (dross)
- In laser welding:
- Seam angle
- Seam strength
- Heat affected zones (HAZ)
Types of beam shapers for laser welding and cutting
There are many types of fixed shape beam shapers used in welding & cutting processes. The main categories include:
- Refractive beam shaper – for example Holo/Or’s Broadband Diffusers (BD). This element generates a homogenic spot with a well-defined shape at high efficiency from any input beam.
- Diffractive Diffuser – a homogenizer element that can generate any well-defined freeform spot shape with perfect angular accuracy and no sensitivity to alignment or centralization tolerances. This element is suitable for use with low-coherence discrete wavelengths multi-mode laser beams.
- Diffractive beam shaper, also known as “top hat beam shaper” or “flat top beam shaper” – this beam shaping element generates a well-defined shape with a flat-top or otherwise controlled intensity profile (such as a C shaper) with very sharp transfer region. This element provides best beam shaping results when used with high-coherence single-mode laser beams, however it is a bit more tricky to use as it is more sensitive to tolerances and thus requires a more accurate assembly.
- Vortex phase plate – a diffractive optical element (DOE) that converts a Gaussian laser beam into a donut-shaped energy ring. The optical Vortex requires as input a collimated Single Mode (TEM00) Gaussian input beam, which it converts to a TEM01 axially symmetric mode
The need for Flexible shaping
The various types of Diffractive beam shapers above can provide outstanding shaping quality and angular stability for high power processes such as Laser Welding and Cutting. Due to being flat, thin windows with structures etched into a micro-thin layer, they have high LDT and are insensitive to temperature effects, providing stable shaping at all system operating conditions.
However, a standard DOE is a passive laser beam shaper, capable of creating a fixed spot shape. In welding and cutting machines, many tasks are often performed by the same system, each on a different workpiece with different thickness (and sometimes different materials). This means that a laser shape optimized for one process can be sub-optimal for another, limiting the usefulness of passive shapers.
To address this need, Holo/or has developed the Flexishaper concept, enabling control of the spot shape or intensity distribution within the shape . The Flexishaper is an optical element that contains more than one optical function on different areas of the same surface, called sub-apertures. It can be assembled with a beam switching device directing the beam to the different areas on a desired regime, mounted on a motorized or rotation stage or assembled as a part of a scanning system that directs the beam through different apertures.
Using Holo/Or’s Flexishaper, one can achieve the perfectly accurate beam shape with no angular tolerances, with either several varying functions on a single component or even with continuous adjustable functions like going from spot to ring and back in the same process, which have shown advantages in welding applications.
There are of course other types of variable beam shapers, including dynamic beam shapers such as structured light modulators, who have the benefit of enabling various shaping functionalities, however they cannot produce as accurate shaping as DOEs, and often cannot handle the high power levels (>1kW) in industrial welding processes. The uniqueness of Flexishapers is that they have all the benefits of passive DOE, while enabling greater processing optimization and flexibility.