Laser beam welding of AZ31B-H24 magnesium alloy. Page: 5 of 11
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aluminum, indicate that only magnesium and zinc will have significant evaporative losses during
welding. Minimizing the irradiance incident upon the workpiece would reduce the spatter as well as
loss of zinc. Therefore, the approach was to determine the threshold irradiance at which consistent
coupling of the beam to the workpiece occurred so that the irradiance for achieving a prescribed
penetration can be minimized. The welding speed was adjusted to provide the necessary penetration.
Table 2. Thermophysical Properties of Al, Zn, Mg, and Fe.
Al Fe Mg Zn
Melting point Tm (K) 933 1808 923 693
Boiling point Tb (K) 2333 3003 1380 1203
Vapor pressure (Pa) @ Tm 10-6 2.3 360 23
1000K 0.000012 0 1360 12000
Viscosity (mPa s @ Tm) 1.3 6 1.25 3.5
Surface tension (N/m @ Tm) 0.91 1.87 0.56 0.78
Thermal conductivity of solid
@ Tm (W m-1K-1) 210 30 130 9
Absorptivity (%) @ Tm 1.06 jm 11 36(300K) 42(300K)
10.6 gm 3 5(300K) 3(300K) 10
Bead-on-plate (BOP) welds were used to develop the parameters for Nd:YAG laser welding of
magnesium. Initially, the energy per unit time (peak irradiance) was reduced to a level which just
produced consistent coupling (1.35 J/ms). Higher energy densities led to greater evaporation losses,
increased spatter, and uneven weld beads. The pulse length was then increased until consistent weld
bead appearance was obtained (5 ms). The maximum repetition rate (120 Hz) was constrained by the
maximum duty cycle (60%). Finally, the travel speed was adjusted to produce full penetration
(3 cm/s). For a travel speed of 3 cm/s and a beam diameter of 600 jm, the pulse overlap was 42%.
The average power at the workpiece was 0.8 kW (1.3 kW peak), which led to a mean peak irradiance
of 0.5 MW/cm2. The mean irradiance corresponds to the ratio of 86% of the power over the
cross-sectional area (containing 86% of the power) that was determined using a laser beam analyzer
Butt welds were made using parameters obtained from successful, full penetration bead-on-plate
welds. These conditions produced sound butt welds in magnesium sheet with shear cut edges (see
latter discussion and Y1 in Table 3 and Fig. 3a). The large beam diameter of the Nd:YAG laser made
the welding quality less sensitive to small gaps in the shear cut edges. The use of argon cover gas,
which is slightly denser than air, allowed good shielding of the workpiece evident by the shiny weld
A model based on conservation of energy for a moving heat source incident on a flat plate was
used to predict the threshold irradiance to initiate melting for the CW CO2 laser . The primary
source of uncertainty in the calculation is the surface absorptivity. For a flat polished surface, the
reference value of the absorptivity for 10.6 jm radiation at room temperature is 3% . The
threshold irradiance for melting with a travel speed of 12.7 cm/s and a beam diameter of 400 pm was
calculated to be 1.5 MW cm-2. The absorptivity of the AZ31B alloy was expected to be significantly
higher because of the chromate conversion coating, which would result in a lower threshold
BOP welds were also used to determine the average power and travel speed for CO2 laser
welding of magnesium. The BOP welds were made at a travel speed of 12.7 cm s-1. The power was
varied from 0.5 to 2 kW to produce mean irradiances ranging from 0.4 to 1.6 MW cm-2. Consistent
coupling occurred at an irradiance of 0.5 MW cm-2, which is one third the value predicted for melting
and approximately the minimum value required to sustain a keyhole . Since the predicted value
is a lower bound value for welding, the chromate coating must have increased the absorptivity from
3% (or a slightly higher value at the melting point) to >9% to allow consistent coupling at
0.5 MW/cm2. The experimental threshold irradiance for chromate coated magnesium is less than
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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/m1/5/: accessed April 22, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.