Fillet edge joints
28 Industrial Laser Solutions MAY/JUNE 2015 www.industrial-lasers.com
FIGURE 1. The effect of laser power, welding speed and sheet gap on porosity formation
in laser welding of AA6014 aluminium alloy [ 7].
PROCESS REDUCES POROSITY FORMATION
IN LASER-WELDED ALUMINUM ALLOYS
Aluminum (Al) alloys are typically used in the automotive, aero- space/aeronautical, and sport industries due to their light weight, ease of forming/machin- ing, and acceptable strength properties. Joining Al alloy panels and sheets is typically realized by mechanical riveting, arc
welding, brazing, friction-stir welding, laser welding, and hybrid
laser/arc welding. Laser welding can be very fast and generates low thermal distortions. However, laser welding of Al alloys
often results in significant amount of porosity, thus very low
joint strengths—typically 50–75% that of the parent material.
There are a number of reasons for porosity generation in
laser welding of Al alloys:
1. Surface contamination, including hydrogen in the surface
oxide layer. Hydrogen (H2) is typically
trapped in the surface oxide layer on
the Al alloys’ surfaces. During laser
welding, they can be released and
dissolved in the melt pool. Hydrogen
solubility in liquid Al is much higher
than that in the solid. In liquid, the H2
solubility [ 1] is:
(mL/100 g Al) at 273°K and 760 torr,
T is the temperature (K), and P is the
partial pressure of hydrogen ( 1 torr
=0.133 mbar) [ 2].
From the above, it can be seen
that the H2 solubility in Al liquid is
about 20 times of that in solid. Thus,
Al alloys can take in large amount
of H2 in the molten pool and release
them as porosity during solidification.
Other surface organic contaminants such as oil and grease can
also contribute to the hydrogen gas trapping in the weld zones.
Porosities generated by hydrogen trapping are normally small
(<0.5mm in diameter) and typically in a few 10s of micrometers.
Porosity in the melt pool moves slowly. The porosity moving
speed [ 2] can be found using:
µ = (2r2∙∆p∙g) 9µ
where r is the radius of the pore; ∆þ is the difference in
density between the liquid and the gas inside the pore; g
is gravitational acceleration; µ is the dynamic viscosity of
the liquid; and þ is the liquid density. For a 50µm-diame-
ter pore in the Al alloy, its moving speed is 4–6mm/s while
the liquid solidification speed is normally 20–50mm/s, thus
difficult to get out [ 3].
2. Keyhole porosity. This occurs typically in keyhole
(deep-penetration) welding, where the laser power density is
normally above 106W/cm2. For welding relatively thick (>3mm)
Al alloys, keyhole welding is necessary to enable the deep
penetration. Keyholes are formed as a result of high-pressure
vapor/plasma generation during laser welding that result in a
deep vapor hole due to the high recoil pressure as the vapor
leaves the melt pool. Such a keyhole allows the laser beam to
be absorbed in much deeper part of the material compared
with that at lower-power-density, conduction-based laser
welding. If the keyhole is not stable (when the material surface