Effect of Laser Welding Parameters on Porosity of Welds in Cast Magnesium Alloy AM50

Pores in the weld metal lower the mechanical properties of the weld. It is therefore important to understand the pore formation mechanisms and find procedures that could reduce porosity. This study focused on laser welding of 3 mm thick magnesium alloy AM50, investigating how different parameters affect porosity formation. Low levels of porosity content were achieved by either increasing the welding speed or using a two-pass welding approach. It was found that higher welding speeds did not allow pores, which were pre-existing from the die-casting process, to have sufficient time to coalesce and expand. In the two-pass welding technique, pores were removed as a result of a degassing process which occurred through the second pass.


Introduction
Magnesium alloys are light-weight metals suitable for applications in several industries, such as automotive and aerospace. Compared with most materials they provide a possibility to reduce weight due to their high specific strength. However, their tensile strength is low (190-310MPa) compared with steels, which may limit their application; for example, to car interior parts such as seat frames, steering wheels or structural dashboard cross beams [1][2][3][4]. A common magnesium alloy is AM50 (4.4-5.5 wt% Al, 0.26-0.6 wt% Mn) which, compared with other magnesium alloys, is of relatively high strength, high hardness, high elongation and has excellent castability. Often magnesium alloys are cast into complex shapes using high pressure die-casting [5][6][7][8]. An alternative is to cast less complicated parts and join them by welding, commonly by tungsten inert gas (TIG) or metal inert gas (MIG) welding [2].
An alternative is laser welding, where high power densities are attained with small welding spots, allowing relatively high welding speed and low heat inputs to be achieved. Low heat input is an advantage for many metallic materials as a narrow fusion zone and HAZ will form, reducing negative effects on material properties [2]. Laser welding of magnesium alloys was reviewed by [9], who stated that crack-free laser welds with low porosity and good surface quality could be obtained when using appropriate welding parameters. Nevertheless, magnesium alloys may exhibit many processing problems and weld discontinuities, such as an unstable weld pool, spatter, drop-through, sagging, undercut, porosity, cracking, and oxide inclusions. Pores in the weld metal lower especially the tensile strength and may have a deleterious effect on fatigue performance if surface breaking. Therefore, it is important to understand the pore formation mechanisms and find procedures that could be used to reduce pore formation [2,10]. Porosity in welded magnesium alloys has been the subject of a number of previous investigations [2,[10][11][12][13][14][15]. In these studies, a range of different factors have been found to cause pore formation including: hydrogen, an unstable keyhole, pre-existing pores from the die-cast process, surface condition, gas entrapment, and alloying elements with a low vaporization temperature. In studies by [15] and [11]

ISSN: 2641-6921
porosity in laser welded AM60B (Mg-alloy with 5.5-6.5 wt.% Al and 0.24-0.6 wt.% Mn) was investigated. Pre-existing pores in the base metal coalesced and expanded in the weld metal during welding resulting in large diameter pores [16] presented three solutions to avoid porosity; specifically, removing the oxide layer with a separate plasma arc before welding, use of dual laser beam welding or using a two-pass laser welding procedure. The best results were obtained using a two-pass welding, with a pre-heating configuration for the first laser pass. However, a systematic study of how different parameters affect the amount of porosity in laser welded AM50 has not been previously performed. This study was therefore initiated to investigate how different parameters affect porosity formation in laser welded AM50, with the overall aim to ensure that high quality welds can be produced reliably and reproducibly. Investigations were undertaken on 3mm thickness AM50, since this is of common interest to many potential applications.

Material
Die-cast magnesium alloy AM50 sheets of dimensions 3(T) x100(L)x170(W) mm were welded. The composition according to ISO 16220(00) and the composition measured with glow-discharge optical emission spectroscopy are presented in Table 1.

Microstructure
EDS analysis showed that the matrix of the as-received AM50 sheet material contained a Mg-Al phase (corresponding to β-Mg17Al12 according to literature [18]), particles of Al-Mn (typically Al8Mn5 [18]) and as Mg-Al oxides Figure 2. Also, occasional cavities were found in the base material. These are most likely shrinkage pores from the high-pressure die-casting process.  The porosity analysis was typically performed using images taken of the cross-section's transverse to the welding direction.
Longitudinal section images Figure 3 were also analyzed to verify that the transverse cross-sectional images were representative of the full length of the weld. Table 3

Focus position
The focus position of the laser beam was varied in three steps; specifically, 3mm (-3) below the top surface of the work piece The number of pores per mm 2 was 80 for -3mm (W03), compared with 147 for 0mm (W01), and 58 for +3mm (W02).  , single-pass with 1.5m/min and 1100W (W04), two pass with 3m/min and 2200W using brushing between passes (W07) as well as two-pass with 3m/min and 2200W without cleaning (W08). The yellow contour shows the fusion zone. The area fraction pores clearly decreased when using two-pass welding. Results of porosity measurements are summarized in Table 4.

Porosity
There is no clear correlation between the number of pores and the area fraction pores in the cross-section. This can be understood from Figure 4 showing that the size distribution of the pores depends on, for example, the welding speed. It can be seen that Therefore, in this paper the area fraction porosity in the fusion zone cross-section was used.

Effect of welding parameters
The welds that had above 10 % area fraction pores had parameters settings giving the highest heat input or most defocused laser beam (W02, W06 and W10). On the other hand, welds with the lowest area fraction pores of around 3% had parameter settings with the highest welding speed or were two-pass welds (W07, W08 and W11). The effects of the heat input, welding speed and power are illustrated in Figure 9 & 10.

Pore formation
Three major reasons for porosity are detailed in existing literature: pre-existing pores from the die-casting process, pores formed by nucleation and growth of gas (most likely hydrogen) in the molten material, and porosity originating from process instabilities [9]. One explanation is that gases entrapped in preexisting shrinkage pores grow when the solid material becomes molten during welding [11,12,15]. These relatively large pores have difficulties to escape from the molten pool due to the rapid solidification in laser welding. This suggests that a higher heat input, giving a larger volume of molten material and allowing more entrapped gas to grow to larger pores, should result in a larger pore volume. It is also claimed that a higher welding speed reduces the available time to form and grow pores, resulting in fewer pores in the weld bead [15]. In line with this Zhao et al. [15] found that the porosity increased with increased heat input, i.e., increase in laser power and decrease in welding speed. This corresponds well with

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results from the present study. In a previous study it is described how the heat input is controlled while welding die-cast AM50 [14].
It needs to be high enough to achieve full penetration, but low enough to avoid poor weld surface quality and excessive porosity.
Porosity of 5% was achieved, which is in the range of the lowest porosity content of 3% in the present study.

Surface cleaning
Unexpectedly brushing (8.7%) or grit blasting (9.6%) resulted in more porosity than no cleaning (6.2%) and cleaning only with acetone (5.4%) Table 4. This suggests that cleaning has a negative impact on porosity. Harooni et al. [2] stated that the surface oxide on magnesium alloys was one of the causes of pore formation. When welding a lap joint where the oxide layer had been mechanically removed almost no porosity was shown, while welding with asreceived surfaces produced porosity at the faying surface of the two overlapping sheets. The explanation was that the oxide layer bonds moisture forming magnesium hydroxide. As a result of heating during welding water molecules were released and trapped in the molten metal thereby forming pores. Lower porosity content when plasma arc pre-heating was explained by degassing of the oxide layer. In the present study there were no faying interfaces for the bead-on-plate samples why porosity formation cannot readily be explained by moisture being released from surface oxides. This suggests cleaning of the surface would not have a major effect.
Another explanation is that the oxide layer was mixed in the molten metal creating nucleation points for porosity. However, the pores did not have time to grow, hence resulted in many small pores with low impact on the total pore volume. The absorption of the laser beam depends on the surface condition and thereby the cleaning procedure [2]. The higher porosity content in the cleaned samples could therefore possibly be due to a lower absorption of the laser beam and hence a resulting unstable welding process. An unstable keyhole could be one explanation for pore formation [9] however, this is more common when welding aluminium than magnesium due to the lower vapor pressure and higher surface tension causing the keyhole to collapse.

Two-pass welding
The lowest amount of porosity was achieved with two-pass welding with 2.9% (W07) and 3.5% (W08). The porosity decreased roughly 5% when applying the second pass. Obviously pores formed in the first pass, while the second pass had a degassing effect removing in particular the large pores. A higher heat input in a single-pass could be thought to have the same effect, however more material becomes molten and hence more porosity occur.
The two-pass welding only melts a small volume but gives longer time to de-gas. Two-pass welding is however not considered production friendly due to the lower productivity (double process time). Solutions like twin-spot laser welding would therefore be of interest for further investigations. Conclusion a) Magnesium alloy AM50 has been laser welded bead-on-plate to study pore formation. Effects of welding parameters including laser power, welding speed, focus position, single-pass and twopass welding and surface cleaning have been studied.
b) The lowest area fraction porosity of 3 % was achieved for 4 m/min with 2200 W and for two-pass welding at 3 m/min with 2200 W. Focus position was at the surface and oxide was not removed. c) Trends were that an increased welding speed or a decreased welding power resulted in less porosity. The focus position does not seem to influence the porosity. i. The surface oxide layer is mixed in the molten metal acting as nucleation points for porosity. This results in many small pores with low impact on the total pore volume.
ii. Removal of the oxide layer affects the absorption of the laser beam causing an unstable process resulting in porosity.