Laser beam welding of AZ31B-H24 magnesium alloy. Page: 4 of 11
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This study examines the conditions for laser beam welding of AZ31B-H24 magnesium alloys
in more detail. Welds were carried out with Nd:YAG and CO2 lasers. Absorptivity effects on
threshold irradiances and weldability of butt joints were examined. Weld hardness profiles were
determined and compared with the microstructure of the weld and heat affected zone. Chemical
analyses of the weld were carried out to determine loss of alloying elements and oxidation. The
results augment the magnesium laser welding work of Niemeyer et al .
Laser beam welding (bead-on-plate and butt) was performed on samples of 1.8 mm (nominal
thickness) magnesium alloy AZ31B-H24 which has a nominal composition of 3 wt% Al and 1 wt%
Zn. The H24 condition refers to strain hardening and partial annealing (recrystallization with no
grain growth). The alloy sheets was supplied with a protective chromate conversion coating.
Surfaces and edges were cleaned with acetone before welding. Chemical analysis for Al, Zn, and O
was carried out by Wah Chang Analytical (Albany, Oregon). The metallic elements were measured
by direct current plasma (DCP) spectroscopy, while the oxygen was quantified with a standard Leco
test. A comparison of the ASTM alloy specifications for AZ31B-H24  and the measured
composition is given in Table 1.
Table 1. Composition of Magnesium Alloy AZ31B (wt%)
Al Zn Mn Si Cu Ca Ni Fe Mg O (ppm)
Specification 2.5 - 0.7 - 0.2 0.05 0.05 0.04 0.005 0.005 balance none
3.5 1.3 min max max max max max
Measured 3.27 0.79 - - - - - - balance < 50
A 2.0 kW pulsed Nd:YAG laser (Electrox) with fiberoptic beam delivery was used. The fiber
size was 1 mm and 50 mm diameter lenses were used in the output optics. A 75 mm focal length lens
focused the upcollimated beam from the fiber to a spot of 600 Rm. Top gas shielding was provided
by a 40 1pm flow of argon in a trailing jet configuration delivered by a 0.8 cm diameter tube oriented
at 450 from the surface and 1 cm from the weld. Bottom shielding was accomplished with a 20 1pm
flow of argon within a rectangular slot below the butt joint seam. The optics were protected by an air
knife with a 50 lpm flow of nitrogen.
Welds were also obtained with a 6 kW CW CO2 laser (Rofin Sinar RS6000) using a near TEM20
(M2=4.1) beam and a 150 mm focal length off-axis parabola. The spot size of the focused beam was
400 Rm. Top gas shielding was provided by a flow of helium in a trailing or transverse jet
configuration delivered by a 0.5 cm diameter tube oriented at 45' from the surface and 1 cm from
the weld or by a special shroud that ensures oxygen free shielding of the weld area . Bottom
shielding was accomplished with a 25 1pm flow of helium within a rectangular slot below the butt
joint seam. The optics were protected by a crossflow device (Spawr Industries) with a 100 1pm flow of
The weld, heat affected zone, and the base material were examined by optical microscopy.
Transverse cross-sections of the autogenous welds were metallographically prepared and etched with
dilute Keller's reagent to determine penetration depth and weld microstructure. Vickers
microhardness measurements were made every 0.2 mm across the weld using a 50 g load and a dwell
time of 15 s.
RESULTS AND DISCUSSION
The thermophysical properties of magnesium, alloying elements and iron are listed in Table 2
[8-11]. The properties affect the beam irradiance required for welding and the overall weldability of
the alloy [11, 12]. The viscosity and surface tension of molten magnesium at the melting point are
lower than those of aluminum and substantially lower than those of steel. These low values contribute
to an unstable weldpool, production of substantial spatter and poor weld surface quality . At
weldpool temperatures, the relatively high vapor pressures of magnesium and zinc, as compared to
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Leong, K. H. Laser beam welding of AZ31B-H24 magnesium alloy., article, September 29, 1998; Illinois. (digital.library.unt.edu/ark:/67531/metadc623699/m1/4/: accessed October 23, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.