Lasers advance the precision welding art Essay

Lasers advance the precision welding art



Welding with a laser has advanced the art of precisely joining and
sealing a range of ferrous and nonferrous metals. When compared to
conventional processes, laser welding enjoys many productive and
quality-improving advantages. No welding rods, fluxes, or protective
materials are needed, for example. Dissimilar materials can be joined
or sealed as well. And warping, internal stresses, and cracks are
minimized because the focused beam, with a spot size of 0.004 to 0.010
dia, generates heat only in the weld area.

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Flexibility is another plus. The beam can be directed with mirrors
so it impinges on surfaces difficult to reach conventionally. Also, a
laser can be made fully automatic by computerized numerical control (CNC). Running in this mode, a laser welder greatly improves precision,
repeatability, and productivity in processing complex shapes.



Two operating modes



The laser welding process can be performed two ways: continuous
wave (CW), or deep penetration by pulsing the beam.



In CW welding, which is mainly used to join sheets or plates, the
heat source is focused at the workpiece surface. The operation results
in fast welding speeds, but has the disadvantages of small weldbead
width and high heat input (and correspondingly large heat affected
zone).



These drawbacks are overcome with deep penetration welding, which
is more widely used. It’s accomplished by sinking a small keyhole
into the metal with the laser. Then, as the beam travels along the weld
path, the molten keyhole solidifies. Such welds result in deep
penetration with minimal heat input.


Keyhole welding lends itself to processing difficult materials
(such as aluminum) where high power densities are required to overcome
the metal’s reflectivity and thermal diffusivity. A technique used
to accelerate penetration, improve bead control and minimize the
heat-affected zone is to enhance the beam’s pulse, i.e., an
additional high-voltage power supply is used to increase pulse power.



Single, long pulses with enhanced edge spikes are ideal for welding
various metals. The leading edge spike quickly melts the metal, which
is necessary to overcome its reflectivity. The remaining pulse energy
now is absorbed readily by the molten puddle.



One benefit of welding this way is that the time required to bring
metal to a molten stage is reduced considerably, and this lowers the
heat input into the workpiece. The result is less distortion.



Deeper, more discrete weld penetrations in metals are obtained by
beam pulsing. For example, a beam operating in the enhanced pulse mode
(with a pulse length of 4 milliseconds and a repetition rate of 100
pulses/sec) can produce a weld rate of 30 ipm. Figure 1 illustrates the
difference in performance between CW and enhanced pulse modes for a
750-W CO2 laser welding AISI 304 stainless.



With a standard gas mix of nitrogen, CO2, and helium flowing
through the laser resonator, a CO2 laser designed for enhanced pulsing
can be operated up to 1000 pulses/sec. Pulse rates higher than 2500
pulses/sec are possible when a special, commercially available gas
mixture is used. This mixture is needed to shorten the enhanced
pulse’s decay time and eliminate pulse overlap.



Cloud cover



Wen a laser welds, a plume of metal vapor (plasma) forms above the
spot at which the focused beam reacts with the work surface. It’s
possible that this plasma can absorb most of the beam’s energy,
allowing little or none of it to react with the workpiece.



To counter this, either argon or a mixture of helium and argon is
used as a shield gas. These inert gases reduce the plasma halo and
surface oxidation, while aiding absorption of light energy at the joint.



Argon is a suitable shielding gas for lasers operating in the CW
mode; however, it ionizes too readily to be useful in beams of greater
power density such as those produced by pulsing. Helium, or a mixture
of He and Ar, is used as a shielding gas for pulsed welding beams.
Helium has a sufficiently high ionization temperature to resist forming
a large plasma cloud in the multikilowatt peak power levels produced by
enhanced pulsing.


A plasma formed by the beam of a pulsed laser is smaller than that
formed by a continuous wave with the same power input to the work
surface. This is because the plasma is allowed to dissipate (or relax)
between pulses.



On the edge



The workpiece must be held correctly to maintain intimate contact
when laser welding. Probability of a successful weld increases by
knowing when to spend effort for edge preparation.



Butt weld. The work material doesn’t require beveling at the
edges. Sheared edges are acceptable if they are square, straight, and
securely clamped together.



Lap weld. Square edges aren’t of great importance if
it’s either a full- or partial-penetration application, but air
gaps between the workpieces will severely limit both penetration and
welding speeds.



Flange joint. It’s important that the workpieces have
straight, square edges. Also, it is necessary that they are securely
clamped together and be in precise transverse alignment with one
another.



Record is impressive



Laser welding is a mature technology, and today’s industrial
units pack the kind of processing power needed to successfully handle a
variety of metals and applications. Design advancements have enabled
cost- and quality-conscious manufacturers to use industrial lasers on
high-alloy metals.



Coherent General, with an Everlase Fast Axial(TM) CO2 laser, has
successfully welded aerospace material. Butt welding of 0.060-thick
Inconel was done using an inert shielding gas at rates up to 300 ipm
with good fit-up and alignment. Inconel plate (0.125 thick) can be
welded at 40 ipm with full penetration.



Similar results have been observed with titanium, with the
exception that weld rates are somewhat slower with thinner materials.
Titanium (0.047 thick) was welded at 200 ipm, whereas 0.120-thick stock
was welded at 45 ipm, slightly faster than the Inconel.



Skip welding of materials held in a T configuration also is being
done. Here, the beam is directed at a 10- to 15-degree angle, with
respect to the cap of the T, and directly at the junction of the mating
parts. Again, inert gas is used. Superior welds of 0.060-thick Inconel
have been done at 80 ipm.



Further, lower power CO2 lasers have been applied to weld and seal
a variety of metals. A lap weld joining a 416 stainless cap to a 310
stainless body is shown in Figure 2. In another case, an electrical
lead-through was pulse welded in place without fracturing a ceramic
insulator that was close to the weld zone. A 575-W laser with a 2.5
focal-length lens was rotated around each pin to produce a hermetic weld
in 2 sec. Penetration was almost 100 percent on this 0.025-thick
titanium workpiece. Processing was at 30 pulses/sec. In a final
example, Figure 3 shows a 1.5-dia nickel-plated, cold-rolled steel
battery can with the top hermetically welded on a 0.040 lip.



There are many other success stories. This, however, doesn’t
mean that lasers are a panacea. What it does mean is that they lend
themselves to applications previously considered impossible or very
difficult to weld conventionally.



Photo: 1. Performance comparison of laser modes when welding AISI
304 stainless.



Photo: 2. This lap weld of a 416 stainless cap to 310 stainless
body met the requirements for a cosmetic as well as a deep penetrating
weld. The joint is made with a 575-W laser using a 5 focal-length lens
and a helium-gas shield. An enhanced pulse mode (100 pulses/sec) with 4
milli-second pulses is used to achieve penetration of 0.065 at 30 ipm.
The cross section shows no undercut at the edge of the weld bead, and
because a shielding gas is used, there’s no oxidation.



Photo: 3. Battery cans made of nickel-plated, cold-rolled steel
are hermetically welded by lasers. Weld time is 8 sec/can. A 375-W
laser (with a 2.5 focal-length lens) in a pulsed mode (80 pulses/sec)
was used to achieve 0.030 penetration.

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