Aerospace & Defense Technology - August 2022 - 12

Aerospace Alloys
method simplifies the process from an
assembly standpoint, as faying surfaces
can be easily matched before placing a
charge into the furnace. Alignment
pins and tack welding may be used for
this process as well. Because no liquid
phase is present in diffusion bonding,
slight deformation occurs at the surface.
This results from applied pressure
used to flatten surface asperities and
break up residual surface oxides before
bonding.
In the liquid-phase diffusion bonding
process, an interlayer typically melts at
the faying surfaces. In this instance,
lower pressure than solid-state diffusion
bonding is used, and even less deformation
occurs at the surface. Depending
on the thermal cycle and composition
of the interlayer materials combination
and the interlayer selected, an interdiffusion
occurs between the interlayer
and base materials on either side of the
joint through the metallurgical processes
known as a eutectic or peritectic
phase change.
The thickness of the final intermetallic
reaction layer is a result of, first, the
liquid-phase diffusion and second, the
solid-state diffusion. Bond strength is a
function of the intermetallic compounds
formed, the thickness of the
intermetallic zone, and anomalies, such
as voids, at the interface.
Exploring Diffusion Bonding
There are several ways for an aeroCombining
the beneficial properties of different metals is the main reason to explore diffusion bonding.
bined, the channels can provide for cooling
or heat dissipation. The layers can be
bonded up to a stack height of 600 mm in
the MOV diffusion bonding press, retaining
the strength as the parent materials.
Another application related to conformal
cooling is for plastic injection molds
made in 2-layer designs of low alloyed tool
steel with stainless steel such as STAVAX.
Understanding Diffusion Bonding
Academia has researched dissimilar
joining for decades, and much of the
focus has been on high-performance
12
materials like titanium alloys. According
to Palamides, expanding beyond
commercially pure (CP) titanium, one
often finds the existence of an intermetallic
layer at the mating interface after
processing. " The influence of the solute
elements, such Cu, Ni, and Ag, are the
major drivers for the intermetallic layer
formation in titanium, " he says.
The diffusion bonding process can
occur as either a solid-state or a liquid-phase
bonding process. Similar
pure material bonding has historically
been as a solid-state operation. This
mobilityengineeringtech.com
space manufacturer to investigate how
diffusion bonding of dissimilar metals
could benefit their process. Much scientific
literature is readily available
for specific material combinations and
processing times and temperatures,
depending on the application. For
example, Kavian O. Cooke and Anas M.
Atieh conducted an exhaustive review
published in 2020 titled Current Trends
in Dissimilar Diffusion Bonding of Titanium
Alloys to Stainless Steels, Aluminum,
and Magnesium. Journal of Manufacturing
and Materials Processing.
While ample research exists on the subject,
design engineers can still find it challenging
to convert the information into
real-world manufacturing of a specific
part. When this is the case, it can be helpful
to partner with experts with an extenAerospace
& Defense Technology, August 2022

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Aerospace & Defense Technology - August 2022

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