In the present study, 3 mm thick sheets of high pressure die cast magnesium alloy AM50 were welded. The sheets had dimensions 100 × 170 mm and the surface of the sheets were prepared by wire brushing and degreasing with acetone prior to welding. For composition of AM50 according to ISO 16220(00) and composition measured with glow-discharge optical emission spectroscopy, see Table 1.

Welding was done with IPG 5 kW (with a 150 µm fiber, for twin-spot) or 10 kW (with a 200 µm fiber, for single-spot) fiber lasers. The fiber laser was equipped with one (for single-spot and twin-spot with beam splitter) or two optics (for twin-spot with primary and secondary optics). The primary optics was aligned perpendicular to the sheet to be welded, while the secondary optics had a 12 degree angle (see Figure 1). Laser welding parameters and optical setup were varied to study their influence on porosity. When using two optics, both optics had identical lens setups. The welding parameters varied were power, welding speed and focus position (see Table 2).

Both single-spot and twin-spot optics were used with different focus and collimator lenses. Twin-spot was produced in two ways, either with a beam splitter

in the primary optics resulting in two identical laser beams perpendicular to the surface, or by using two separate optics. In the case with two separate optics the primary optics are perpendicular to the sheet surface, while the secondary is placed in front of the process, but with a small angle. The laser power is equally divided between the two optics.

Both optic solutions had the focus position placed on the surface of the material as the standard setup.

Bead-on-plate welds were produced with 100 mm length across the sheet. Pure argon (gas type I1 according to ISO 14175:2008) was used as shielding gas both at the weld face and root, with a flow rate of 40 l/min and 5 l/min respectively. At the weld face a trailing gas shield was used with a “panpipe” design to

distribute the gas. The root gas was applied through an efflux channel with a 10 × 10 mm cross-section (Figure 1).

Transverse cross-sections were prepared to study the microstructure and the porosity of the resulting welds. After cutting, all sections were grinded with 4000 grit paper, followed by polishing with 6 µm diamond suspension slurry.

Micrographs were taken in LOM. The porosity was made clearly visible by using an extra external ring shaped light source (directed from the sides onto the sample) causing light to be reflected in the edges of the pores. This yielded a high contrast image suitable for image analysis. To count porosity, image analysis was done with “Image J” which is an open source Java-based image processing software [20].

In a previous study [16] it was found that a lower heat input for single-spot welding gave lower porosity content. The welding speed had the largest effect, i.e. a higher welding speed reduced porosity. However, for twin-spot welding in the present study, no clear relationship between heat input and pore area fraction could be seen (see Figure 4). This suggests that several parameters influence the porosity when using twin-spot, not only heat input.

Several mechanisms for porosity formation in the weld metal have been suggested in literature. One explanation [11] [12] [15] is that pores already existing in the base material grow larger when re-melted during welding. The undissolved gas expands to form bubbles in the liquid metal resulting in pores during solidification. The bubbles have little time to escape from the molten pool because of the rapid solidification in laser welding. This could be part of the explanation of the pore behavior in the present study, but it’s not the full explanation.

An alternative explanation, in line with the findings from Haboudou et al. [21] who performed twin-spot welding of cast aluminium, is that the keyhole is stabilized. Haboudou et al. stated that a twin-spot stabilizes the weld pool and

keyhole dynamics, which reduces the porosity amount to below 2%. If the keyhole is stable and somewhat larger when using twin-spot, the degassing of large pores should occur to a larger extent. This could be part of the explanation, but since several setups with twin-spot welding do have a large amount of porosity in the present study, at least one other factor do influence as well.

In Figure 5 a hypothesis for the observed porosity occurrence is suggested. Some of the existing inclusions (oxides, precipitates etc.) 1) in the material will remain undissolved and are distributed in the molten material during welding together with remainders of the broken up surface oxide 2). Most of the pores from 1) are dissolved in the melt due to higher solvability in liquid state. During cooling of the melt the inclusions can act as nucleation points 3) for pores. The higher the density of inclusions, the more nucleation points. While still in the liquid state, during subsequent heating, either as an effect of the energy input from the first beam or by the second beam (by beam splitter, or the second optics), the nucleated pores grow 4) due to that dissolved gas is diffusing to pores and increase volume and/or that small pores coalesce.

Depending on how the twin-spot procedure is designed, a degassing effect (5) is achieved with the second beam. If the energy input into the material (in this case controlled by defocusing: more defocusing ? less energy into the material) from the first beam is too high, the molten material will have little time to cool between the two beams and pores will not form until the final cooling of the melt and hence little degassing will take place. However, if the energy input from the first beam is well balanced, there will be time for nucleation and some growth before reheating by the second beam. The second beam then heats the melt and allows the pores to grow and also escape from the molten material. For illustration, see Figure 6.

The effect of defocusing on the porosity amount for the twin optics solution is explained in Figure 7. For (a) both beams are in focus. The energy input from the first beam is high, resulting in a small temperature reduction between the beams, hence little nucleation of pores occurs on cooling after passing of the first

beam. Consequently few pores will exist that can grow in size making degassing unlikely when the melt pool is heated by the second beam. Furthermore, nucleation and growth will occur on cooling after passing of the second beam, leaving a weld with high porosity content (e.g. Figure 2 W19/W25/W28). A similar scenario can be envisaged for (b) where the first beam is in focus, but the second beam is defocused. This results in a long cooling time which is unbeneficial from a porosity point of view resulting in nucleation and growth at the end of the sequence and little degassing (see Figure 2 W37). Looking at (c), the pores can nucleate on cooling after the first defocused beam has passed (as explained in Figure 6), since the energy input is relatively low. The second beam then provides heating required for growth of the pores and time for degassing (see Figure 2 W35).

Single-spot welding results in relatively high porosity content, roughly 9%. Using two-pass welding the porosity content is reduced to 3% [16] , which is considered low. However, two-pass welding is not production friendly in the aspect of process time. Twin-spot welding could be a good combination of a production friendly solution and high quality welding. Compared with single-spot optics, a twin-spot optic setup is more complex; however, the porosity content is low (4% - 6%). Furthermore, until today no published study has been found that correlates the amount of porosity in welded cast magnesium to strength of the weld, which would give a recommendation for a strict porosity content limit to aim for.

Summarizing, the optic setup is shown to be crucial when trying to minimizing the porosity while laser welding cast magnesium. Harooni et al. [19] used twin-spot optics for minimizing pores with the origin from the oxide layer at the faying interface between two sheets in lap joint configuration. The present study shows that porosity also can be reduced in butt joint configuration. Single-spot welding could give low porosity welds, but degassing is limited when using single-pass welding. More intensive degassing occurs when using twin-spot, which hence shows promising results. Future work should be to further optimize the twin-spot configuration, including power and welding speed, to have as low porosity as possible.