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Ready J.F., et al. (eds.) LIA Handbook of Laser Materials Processing (LIA, 2001)(K)(600dpi)(T)(740s)(ISBN 0912035153)_EO_.djvu |
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Size 72.9Mb Date May 1, 2005 |
energy will increase weld volume by approximately 60-90% and
increase weld penetration by 30-70%...
Easily measurable attributes such
as weld penetration, strength, hermeticity, etc., work best...
The beam can be focused to a small area allowing for weld-
welding of very small components, which are often closely spaced...
Careful control of the weld joint material chemistry must
often be dictated to the part manufacturer...
The higher peak power of short pulse lasers allows the surface
to be broken down more easily and its reflectivity at the 10.6-
\im wavelength is thus reduced...
For example, a material which may be considered to have
good weldability for an appliance application may be unsuited
for an aerospace application where the weld cannot reliably en-
endure the rigorous aerospace environment, and failure could re-
result in loss of life...
Also, surface coatings
containing high aluminum content may promote smooth continu-
continuous welds in thin rimmed, capped, or semi-killed low-carbon steel...
Most alumi-
aluminum materials welded to date have been relatively thin, up to
one or two millimeters thick...
These types of components use a variety of materials, usually
selected for electrical performance, strength in thin sections, non-
nonmagnetic properties, or corrosion resistance...
Laser welds of brass, bronze,
and free machining grades (sulfur or lead added) of steels have
shown reduced quantities of these elements in the weld zone...
Although electron-beam welding produces a weld with similar
characteristics to laser beam welding, this process usually takes
place inside a vacuum system...
This relatively slow
process limits the depth that can be melted effectively and hence
limits the welding depth...
At low weld speed, the weld fusion zone is
large and wide, resulting in liquid metal drop-through; at high
enough weld speeds, weld penetration is incomplete, not pen-
penetrating through the metal thickness...
The resulting plasma
shields the workpiece from the laser and so precautions are nec-
necessary to inhibit the ionization process (see Section 11.2.8)...
The minimum energy density to melt metals ranges from
approximately 2.5 (Al) to 12.5 kJ/cm2 (W), and the energy den-
density to vaporize ranges approximately from 30 to 80 kJ/cm2...
The diagonal
lines in the figure represent constant melting efficiency, which
is defined as the ratio of the minimum energy required to melt
the specified volume of material divided by the laser energy...
Carbon dioxide systems utilize a circulating gas mixture of he-
helium, nitrogen, and carbon dioxide; the latter is the lasing me-
medium, while helium and nitrogen serve to improve conversion
efficiency and cooling...
For
reference, it is noted that the predicted minimum spot diameter
for a plane wave (K= 2.44) at 10.6 jam and an //number of unity,
is of the order of 0.025 mm...
Oscillation can significantly influence bead shape and width as
well as reduce defects, particularly in aluminum alloys...
Evaporation
of such materials, however, can lead to formation of a high-den-
high-density plasma above the workpiece surface, as discussed in Sec-
Section 11.2.8 and in Chapter 5, which can lead to a marked reduc-
reduction in welding performance...
A somewhat broader
weld can be achieved by locating the focal point slightly above
or below the material surface and welding at lower power and
speed...
In
some surface treating applications this may enhance beam-ma-
beam-material coupling and, in special circumstances, as noted in Sec-
Section 11.2.5, establishment of a controlled plasma may facilitate
attainment of desirable weld characteristics...
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