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High-Power Laser Newsletter

Issue 1 | December 2018

Spatter Matters

Improve Fiber Laser Weld Quality and Yield with CleanWeld™

How CleanWeld™ Improves Your Fiber Laser Weld Quality and Yield

While fiber lasers have been used in welding for over a decade, efforts still continue to improve their utility and extend the results they deliver in terms of part quality, production throughput and process costs.

The CleanWeld initiative from Coherent is an integrated approach to fiber laser welding that addresses these issues and delivers up to 80% spatter reduction, as well as minimal cracking and porosity. And − in addition to improved process consistency − it allows some welding processes to be performed with 40% less laser power.

Understanding Key Laser Welding Parameters

To understand how our CleanWeld technology accomplishes these goals, it’s useful to review some basics of “keyhole” or deep penetration welding processes. 

Schematic of laser keyhole welding

As shown in this figure, a focused laser melts material, creating a “keyhole” of vaporized metal surrounded by molten material, which is held back by vapor pressure. This is a highly dynamic process, which can become chaotic, especially when multiple materials with different boiling points are present in the alloys. For example, lower melting point materials can produce bubbles in the melt pool which introduces spatter or reduces the porosity of the final weld seam. The quality of a weld is determined by the stability of the keyhole during the welding process. The goal is to achieve a stable energy balance through the entire depth of the keyhole to ensure uniform absorption and non-perturbed molten flow around the keyhole. This in turn provides consistent weld depth and, more importantly, minimized spatter and porosity in your welding process.

How to Optimize Keyhole Welding

In fact, there are several tools which can be employed to control the various process parameters, and therefore optimize a specific welding process, and this is the essence of our CleanWeld approach. Probably the most obvious of these are laser power and the focused laser spot size, as well as the spot’s spatial intensity distribution. In fact, it’s really only useful to consider these three factors together, since it is really the power density at the workpiece, rather than the overall power, which affects welding results. For example, even the highest power laser won’t weld at all if the focused spot size is too large. Focus position is also critical – the depth of smallest laser focus relative to the top material surface. 

  • Controlling the precise laser intensity distribution (laterally, and in depth) determines how laser energy is absorbed in the keyhole (and the keyhole’s subsequent shape and temporal stability) as vapor expansion characteristics are balanced with melt pool flow. This is because the laser intensity distribution directly determines the temperature gradient at the workpiece and in the keyhole.
  • Traditionally, fiber lasers have employed a single, round core fiber which essentially delivers either a single mode or multi-mode, circular spot. Today there are several approaches that deliver more complex intensity distributions. These methods range from square (or other shaped) cores to multi-core fibers (such as fibers which consist of two concentric cores). For the latter, Coherent’s Adjustable Ring Mode (ARM) lasers provide a ring beam with the highest brightness in the market.

  • Next, various process (focusing) optics can be used to further manipulate or change the focused spot size, shape and position. Beyond this, the effective power delivered at a given position on the workpiece surface, and the rate at which it is delivered, can be altered using power ramping, laser modulation and beam motion techniques, such as beam wobble.

What Else Can Matter?

In addition to focusing optics, laser welding process heads may also incorporate nozzles for delivering process gases, and for removing the expanding laser vapor. In particular, certain assist gases can be used to stabilize the heating of specific materials and calm melt pool dynamics. Gas flow can also be used to clear the plume and protect optics from debris.

Improved Welding Results

Typical applications that can benefit dramatically from CleanWeld range from the welding of powertrain components, zinc-coated steel, motor stator "hairpins", electric car battery lids, to the welding of aluminum hang-on parts.

By optimizing all relevant factors, we are able to supply you with a complete, cost-effective CleanWeld solution that delivers a specific weld quality and throughput speed, rather than just a laser with a given output power – since spatter matters.

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