Minimizing Porosity in Fiber Laser Welding

Controlling defects, such as porosity in laser welds, is one of the key goals in creating a robust high power laser welding process. Although the presence of weld porosity is not necessarily catastrophic, it is generally undesirable.

We have demonstrated that there is less porosity formed during laser welding of stainless steel when using modulated laser output compared to that produced with CW (continuous wave) output.

Prima Power Laserdyne Applications Engineers have conducted numerous welding studies aimed at developing a deeper understanding of the laser beam-material interaction for high power fiber laser welding. Previously published reports have documented the influence of laser average power, power density, focus position, welding speed, type of shield gas, and method of delivering the shield gas on weld penetration and profile and weld microstructure for a number of commonly welded alloys and dissimilar metal combinations.

In this study, we compared porosity in butt welds and overlap welds of 304 stainless steel sheet produced using CW, sine wave (Figure 1a), and square wave (Figure 1b) output. Welds were sectioned and inspected using a metallurgical microscope to quantify the number, size, and location of pores.

(a)	(b)
Figure 1: Examples of (a) sine wave and (b) square wave modulated output used to control formation of porosity in stainless steel welds.

Results show that the number and size of pores is dramatically lower in welds produced with either of the modulated outputs. Welds produced with CW output generally have more observable pores.

With overlap welds produced with CW output, there was considerably more evidence of porosity in welds with partial penetration into the bottom sheet than for full penetration welds. This is presumably due to the path for escape of the gas within the pore provided by complete penetration.

For partial penetration overlap welds, pores were primarily present at the root of the weld. For these configurations, modulating the beam led to welds without porosity at the root.

The result is illustrated in Figure 2, which shows a comparison of the weld profile produced using CW (Figure 2a) and sine wave modulated output (Figure 2b). Results with square wave output were similar to those with sine wave modulation.

Why does modulation reduce porosity?

The wider weld bead observed in CW welding is normally associated with a plume of vaporized weld metal above the laser keyhole. This plume can scatter the laser energy, resulting in a wider top bead and a reduction in penetration and formation of pores.

Results from this work suggest that the pulsing action inhibits the formation of the plume allowing the laser to better interact with the stainless steel. This, in turn, improves process efficiency and reduces pores in the weld.

(a)	(b)
Figure 2: Lap weld of 1.5mm thick 304 stainless using Argon shield gas and 1.6 kW average power with (a) CW output and (b) 800Hz sine wave modulation.

Other applications

In addition to porosity control, modulated laser output is also beneficial when welding:

• Dissimilar materials. Modulation is used to control the formation of intermetallic compounds by controlling weld metal cooling and solidification rates.
• Crack sensitive aluminum based alloys. With aluminum alloys, a wide range of vaporization and solidification temperatures can lead to keyhole instability, porosity, blowholes, loss of mechanical properties, and various defects in the weld metallurgy such as hot cracking. Controlling the heating and cooling cycles using modulated output reduces these defects as well cracking.
• Welding components that benefit from reduced overall heat input. Controlling the total energy into the material through modulations leads to better control of temperatures within the weld joint.