During a laser cladding
process dilution is expected to be minimized.
In cladding operations dilution is often defined
as the amount of intermixing of the clad and
substrate. Dilution is measured by visual analysis
or through a SEM elemental line scan. Visual
analysis allows the user to get a quick estimate
of the dilution of the clad, however this method
of measurement is not very accurate. Through
visual analysis dilution is defined as the distance
the clad layer extends below the substrate.
SEM analysis is a true, accurate measure of
the dilution, or intermixing of the clad and
substrate. Laser alloying is a process that
is often grouped with laser cladding operations.
Laser cladding and alloying are traditionally
distinguished by the relative amounts of the
consumable material added and substrate melted.
Generally the two categories are arbitrarily
separated by their relative amount of dilution,
laser alloying being classified as having greater
than 10% dilution, laser cladding having less
than 10% dilution4. In laser alloying it is
generally desired to mix portions of the coating
with the substrate to produce an alloyed layer,
thus a high dilution and high intermixing is
expected. It should also be noted that laser
alloying requires convection and laser cladding
does not. In many laser alloying processes the
cooling rate is often monitored to ensure intermixing
and the formation of unique metallurgical compounds.
Ultrafast quench rates of the order of 1011
Ks-1 are often required4 as well as a high solubility
of the clad material in the parent material.
Laser alloying experiments were not conducted
in this study, however throughout the experimentation
there was an expectation that at a low process
speed some alloying of the powder and substrate
may occur. This was not true for the HPDDL process
because laser alloying requires very high quench
rates and a keyhole as seen in Nd:YAG and CO2
lasers.
The denser microstructure and better
bonding of laser clads allows for enhanced corrosion
and wear resistance with a single pass. Laser
cladding is a viable alternative to plasma spraying
and TIG or MIG processes. The clad material deposit
does not intermix with the substrate in many applications;
therefore the dense, uniform microstructure of
the clad layer allows for enhanced single pass
corrosion or wear resistance in a HPDDL clad.
It is difficult to produce a clad with a TIG,
MIG or plasma spray system without having less
than 5% dilution, therefore as many as 15 overlapping
passes are required to obtain an undiluted clad
layer5. Conventional arc welding processes generally
impart a significant amount of heat into the part
resulting in a large heat affected zone and distortion.
Post-weld treatment can improve the properties
of the joint, but can also lead to distortion
of the component6. The surface finish of overlapping
passes produced with the HPDDL are relatively
flat, however a TIG cladding process often results
in distinct ridges and valleys, which lead to
cracking when bent7. In addition, the arc welding
processes often are also responsible for the losses
of alloying elements8. A direct comparison of
a laser clad layer with an arc-welded layer indicates
that the HPDDL clad has significant grain refinement,
which in some cases lead to an increased wear
resistance9. The HPDDL also surpasses flame spray
technology, since flame spray produces a more
porous coating with limited adhesion10.
