Medium- and high-power optical fiber lasers that operate in the 1-μm region have proven their capabilities for cutting and welding a wide range of metals in industrial materials-processing applications. This report addresses the use of medium-power (400-W) fiber lasers in precision fusion welding, including performance and process capabilities, key requirements of the welding process, and their use in butt welding applications with two example case studies, welding of high-quality stainless steel sheet, and welding of coil tape for general manufacturing use.

Fiber lasers have been extensively deployed in advanced manufacturing applications, such as precision cutting, micromachining, rapid prototyping and welding; their performance advantages in terms of beam quality, depth of field, parametric dynamic range, wall-plug efficiency, process versatility, reliability, and cost, have been well recognized.

In welding applications, medium-power fiber lasers offer industrial process technologists and users new degrees of operational freedom and process control whilst concurrently delivering major benefits in terms of ongoing operational costs due to laser maintenance support, day-to-day process “tuning”, and consumables consumption.

These latter features can be of crucial importance to high-volume continuous manufacturing processes where 24×7 operations at high productivity and “right-first-time, every-time” are crucial to both cost and capacity.

Welding using fiber lasers
There are many choices of beam source for laser welding using a 1 μm source: Historically “welding lasers” would be “long-pulse” devices capable of delivering significant pulse energy (Joules) with kW-class peak-power in pulse durations of the order of milliseconds (ms). The maximum duty cycle for this class of pulsed source typically would be 10% (at full power), and the pulse repetition rate typically would be up to 1kHz; the limited pulse-repetition rate of this type of beam source could frequently be the process-limiting factor.

With this class of laser, the welding process is generally “conduction” based, whereby energy is delivered into the material in a series of pulses and is absorbed on the surface of the material. Sub-surface melting occurs and the energy thermally diffuses through the weld zone by conduction and convection in the melt. Process speed and penetration are determined by the laser parameters, in particular pulse frequency, pulse energy and average power. Consequently, conduction welds are semicircular in cross section with aspect ratios of 1:2 or less. Due to the (slow) thermal diffusion process for creating the melt, the heat-affected zone (HAZ) around the weld can be substantial.

High-brilliance CW lasers such as the fiber laser and the disc laser provide an alternative process for fusion welding: Provided that the focused intensity of the beam at the metal surface exceeds a threshold, the incident energy raises the metal temperature above its boiling point, produces a metal vapor channel (“keyhole”) that absorbs the incident laser power, and converts it into heat. Energy is absorbed as it progresses down through the channel, producing a deep and narrow weld for which the aspect ratio (depth/width) of keyhole laser welds can be as high as 10:1. Because the lasers can operate in continuous wave mode, once established, the keyhole can be sustained to produce uniform (narrow) welds with a much-reduced HAZ.

Importantly, CWM fiber lasers provide the process designer with the choice of welding mode between “pulsed mode” and “CW mode” welding. As noted above, CW provides sustained energy input and hence “speed”, whilst pulsing provides the option to reduce process speed in situations where “precision and control” are of primary importance. By appropriate arrangement of the beam delivery optics, the size and power density of the incident spot can be controlled according to processing needs.

The range of welding applications and materials addressed using medium-power fiber laser sources continues to expand from the fine-welding applications used in manufacturing of medical devices to more mainstream industrial applications. The smooth finish and visual quality of CW-welded seams can be important for both cosmetic effect and/or ease of sterilization (in medical applications). Where overall cost becomes critical, modifications of the beam spot size as discussed above to relax the tolerances for finish quality and fit-up tolerances of the welded parts become increasingly important.

Weld process considerations
In designing the weld process, appropriate selection of key parameters such as spot size, power level, weld speed and shield gas choice/flow conditions are made to provide a suitable weld profile (depth, width (or diameter for spot welds) with process margin to accommodate fit-up tolerances, material variations, and metal vapor plume effects. Figure 2 below shows the effect of using two different focused spot sizes (2:1 ratio) on weld penetration depth and speed in the welding of stainless steel. Increased spot size provides a larger fusion zone and increased process tolerance with a corresponding trade-off on process speed.

Note that for the two keyhole welding examples above, the penetration depth was comparable and the process speed is increased (~2x); this suggests that in the 15 μm case, the process-limiting factor is energy coupling into the material through the vapor plume created at the surface. Further increasing the spot size to 120 μm changes the fusion-zone profile to a “hybrid profile” comprising an upper conduction zone and a broader, less deep keyhole zone; increasing the volume of the weld in this way requires a corresponding reduction of process speed.

Butt-welding using fiber lasers
In many end-use applications, a butt-welding configuration is required (for functional / aesthetic reasons.) Although in process terms it requires the lowest heat input, permits the smallest HAZ, and the fastest speed, butt-welding places stringent requirements in terms of the dimensional accuracy, tolerances, and part-to-part match of the surfaces to be welded, the tooling and fixturing accuracy and repeatability of the welding equipment, and the in-process control of beam guiding system.

As a general guideline for butt welding, gaps in the joint between mating parts must be less than ~50% of the focused beam diameter to avoid the beam passing through the “aperture” in the butt joint without welding it at all. As noted above, for reliable welding the weld tooling must hold the parts in close contact and present the seam consistently to the laser beam; optical imaging systems are increasingly used to ensure that the laser beam aligns to and tracks the required weld path.

In situations where this level of precision cannot be assured because the seam location is not controlled accurately enough, a lap joint often can be used to provide a much larger tolerance on alignment. The only requirement on a lap weld is that piece parts are in contact and that the beam alignment and tracking impinges on this zone.

Case study 1: Butt-welding stainless steel sheet
304 grade stainless steel sheet used in the manufacture of domestic goods was welded using a 75 μm spot size. Trials were undertaken with controlled butt gaps and with both shear-cut and laser-cut materials. Figure 4a and 4b below shows the test arrangement and a through-the-head image of pre-welded samples respectively.

Sample welds are shown below for top and bottom welds in 0.5 mm sheet, made with the piece parts butted. The welds were produced using a 75 μm spot size.

The process speed was in excess of 2 m/min, and was deliberately reduced from the “bead-on-plate” maximum to increase the width of the fusion zone to ensure full fusion throughout the full depth of the weld.

Case Study 2: Butt-welding chromium steel coil strip
Plated steel coil strip is extensively used in the manufacture of a diverse range of products: Stainless steel strip is used in the manufacture of seam-welded tubes. Chromium-plated steel strip is used in elongated cylindrical items, such as protective sheaths on a wide range of cables, the steel sheath providing both increased mechanical strength and also rodent protection.

In the manufacturing of such products, processes can run continuously for several hours and may require in-process, on-the-run, tape-to-tape welding of coil strip batches. Typically the time available to introduce a new coiled roll and to weld it to the tail-end of the previous roll is less than two minutes; an excess-length tape-accumulator is used to provide “buffer” between coils and to enable the weld joint to be made in stationary manner.

As the weld joining the tape sections will itself be processed and incorporated into the finished item, the in-line tape joints must have high dimensional uniformity and mechanical integrity, and must generally be butt-welded. Tape materials with thicknesses in the range 50 μm to 250 μm are used according to application and specification requirements; thicker tapes provide greater protection but impose limitations in terms of flexibility and bend radius.

Figure 5 (above) shows images of an example weld in 15 mm wide, 0.2 mm thick tape joint. The weld was made at only 200 W power at a speed of 1.2 m/min; a low flow of argon gas was used to provide a controlled environment for the weld. Note that the incident beam was defocused (+3 mm) in order to increase the spot-size at the workpiece. The seam weld is made using two “dummy tapes” alongside the workpieces so that the seam weld can be over-run into the dummy tapes to avoid “notching” at the edge of the tape; the dummy tapes are readily detached from the workpiece tape after welding.

Single-mode, medium-power optical fiber lasers are well suited to precision welding of metals across a diverse range of applications.

Weld joint quality must satisfy the following criteria:
• Uniform flat seam
• No porosity
• Good edge quality (no “notches” at weld edge)

Crucial to the success of the welding process is the set-up tooling and the edge-to-edge match of the two tapes, both laterally and vertically (“bow”.) While the tape ends can be cut and prepared for welding using a precision shear, an optional alternative solution (free of the need for regular maintenance / shear-blade change-out) is to use the same laser to cut the tapes on the tool prior to welding them together. The laser-cut edge provides improved dimensional accuracy, free of distortion, and repeatable quality run-to-run, week-to-week.

Further information on alternative weld configurations and welding strategies contact SPI Lasers.