Fiber Laser Welding of 100 Micron Stainless Steel

Fiber Lasers are penetrating the precision Laser welding market. Precision welded parts in the medical or computer industry require very high quality but relatively low  penetration welds, typically less than 0.1 mm thick.

The average power required to weld this thickness of material for small medical components is relatively low, mostly less than 50 watts. This work shows an evaluation of welds produced on 0.1 mm thick, type 304, Stainless Steel – a widely used weld-able material. Metallographic examination of these welds shows them to be very high quality continuous welds with very smooth spatter-free top beads. Weld shape is shown to be controlled primarily by weld speed.

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Introduction

Since the start of the Laser industry high power CO2 Lasers have been used for welding sheet steel and a great deal of information is available on this subject. However, welding very thin steel sheets is a very challenging application as a much greater degree of control is required to avoid over penetrating and damaging the films. Until recently, this application was done exclusively by pulsed solid-state Lasers where a series of overlapping spots were used at a low pulse energy. The arrival of industrial Fiber Lasers at power levels up to 100 watts has introduced an alternative welding tool for this challenging application.

Fiber Lasers have several advantages. These can be summarised as:

  • Very stable output
  • Maintenance-free
  • Small footprint
  • Ease of integration
  • High level of process control

Results: Fiber Laser Welds

Figure 1: Welding speed: 2.5m/min, 30 Watts. Figure 2: Showing very even partial penetration weld at 30 Watts.

Figure 1: Welding speed: 2.5m/min, 30 Watts. Figure 2: Showing very even partial penetration weld at 30 Watts.

Keyhole welding

The term ‘keyhole’ refers to the plasma and vapour filled keyhole that may be formed when a high intensity Laser beam impinges on the surface of most metals.

This keyhole is then traversed through the joint area leaving a high aspect ratio weld of solidified material. Keyhole welding these very thin materials is usually considered very challenging.

Conduction limited welding

In this regime, a lower intensity Laser beam is used and high energy, low repetition rate pulses are used to produce a weld. Each pulse produces a single spot weld and these overlapping spot welds produce a quasi-continuous seam. This regime has been served by pulsed, low duty cycle solid-state Lasers in the < 100 watt average power regime.

In this regime, to achieve a weld with a consistent penetration, pulse-to-pulse stability of the Laser source is critical.

Hence for both these welding regimes, Laser stability is critical. The most important advantage of the industrial Fiber Laser for precision welding is therefore Laser stability.

Experimental details

A single focus lens direct optics configuration was used to generate bead-on-plate weld samples. Laser output power was changed to identify the power required to produce high quality welds through this thickness of material. Welding speed was also changed incrementally and these were therefore the primary variables studied. To confirm and to expand on visual observations, metallographic samples were prepared for examination. Transverse diametral cross-sections of several welds from each parameter setting were prepared using conventional metallurgical techniques.

Figure 1 shows a transverse cross-section and figure 2 shows a longitudinal cross section of bead-on-plate welds.

The weld shown appears to be of mixed mode with features from both conduction and keyhole welds. The low power weld shown in figure 2 shows the evenness of the weld penetration, this contrasts with the uneven saw-tooth nature of a weld seen when a typical low duty cycle pulsed laser is used for this type of weld.

These different weld shapes will be appropriate for different joint types and different weld applications. Welds made at these parameters exhibit a well controlled welding process with clean, spatter-free weld beads. Grain structure appears fine.

Benefits of SPI Fiber Lasers

  • High beam quality, high intensity and small spot size control using external optics
  • Exceptional output power stability over time and temperature due to closed loop power control
  • Average power control using continuously variable pulse width or modulation rate up to 100kHz
  • Maintenance free (no replaceable parts)
  • High efficiency energy source – up to 10x more efficient than Nd:YAG
  • Compact system 6U in total size (200W CW).

Conclusion

0.1 mm thick stainless steel has been successfully welded and excellent weld quality has been demonstrated using conventional metallurgical techniques. Weld speeds are very acceptable for industrial welding processes on small components. Visual observations of joint quality have been confirmed by metallographic examination of a range of weld joints. The smooth weld surfaces have been related to excellent porosity-free welds.

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