Diffusion bonding

Definition
 Diffusion bonding is a solid-state welding
technique, wherein coalescence of the faying
surfaces is produced by the application of
pressure and temperature to carefully cleaned
and mated metal surfaces so that they actually
grow together by atomic diffusion.
 The process does not involve macroscopic
deformation or relative motion of the parts. The
process can join either like or dissimilar metals
with or without the use of another material
between.
2

Theory of diffusion Welding
3
Diffusion welding process involves two steps:
 Any surface to be diffusion welded is never
extremely smooth. It has a number of peak points
and valleys. Moreover, this surface may have,
(i) An oxidized layer
(ii) Oil, grease, dirt etc.,
(iii) Absorbed gas, moisture.

4
 The first stage is to achieve intimate metal to metal
contact between the two pieces to be diffusion
welded. This is done by the application of pressure
that deforms the substrate roughness and disrupts
and disperses the above mentioned surface layers
and contaminants.
 The pressure applied in diffusion welded ranges from
350 to 700 kg/cm2.
 The second stage involves diffusion and grain
growth to complete the weld and ultimately eliminate
the interface formed in the previous stage. The
second stage induces complete metallic bonding
across the area of contact.
 In order to increase diffusion rate, moderate heating
temperatures are used.

Schematic representation of diffusion
bonding using electrical resistance for
heating
5

Diffusion Bonding Process
a
b
c
d
e
a) Initial 'point' contact
b) Yielding and creep leading to
reduced voids
c) Final yielding and creep (some
voids left)
d) Continued vacancy diffusion,
leaving few small voids
e) Bonding is complete
6

Diffusion Bonded Methods
7
1. Gas pressure boding
2. Vacuum fusion bonding
3. Eutectic bonding
Gas pressure boding:
 Parts to be joined are placed together in intimate
contact and then heated to around 815 0C. During
heating, an inert gas pressure is built up over all
the surfaces of the parts to be welded.
 Non ferrous metals are joined with the help of gas
pressure bonding method.

8
Vacuum fusion bonding:
 Parts to be joined are pressed together
mechanically or hydraulically. A hydraulic pres used
for diffusion welding resembles that employed in
forging and is equipped to pressurize from three
directions.
 Heating is carried out the same way as in gas
pressure bonding.
 Process is carried out in vacuum chamber.
 Since, pressure higher than those in gas pressure
bonding can be applied in this process, vacuum
fusion boding is used for steel and its alloys.
 For diffusion bonding of steel, the temperature and
pressure required are approximately 1150 0C and
700 kg/cm2 respectively.

9
Eutectic fusion bonding:
 It is a low temperature diffusion welding process.
 A thin plate of some other material is kept between
the pieces to be joined.
 As the pieces are heated to a elevated temperature,
the filler material diffuses and forms an eutectic
compound with the parent metals.

Diffusion bonding parameters
10
Main diffusion bonding parameters are
1. Pressure
2. Temperature
3. Time
Others parameters are
4. Surface preparation
5. Metallurgical factors
6. Use of interlayer

11
Pressure:
 It assures consistency of bond formation.
 The initial deformation phase of bond formation is
directly affected by the intensity of pressure applied.
 For any given time temperature value, increased
pressure invariably results in better joints.
 However, increased pressures require costlier
equipment.

12
Temperature:
 It serves the important function of increasing the surface
energy.
 Temperature affects
(i) Plasticity,
(ii) Diffusivity
(iii) Oxide solubility
(iv) Allotropic transformation
(v) Recrystallization
 Temperature must be controlled to promote or avoid
these factors as desired.
 Generally, increasing temperature shortens diffusion
welding cycle and improves the economics of the
process.
 Diffusion welding temperature usually ranges from 0.55

13
Time:
 Time is a dependent process parameter.
 An increase in temperature shortens the time
required to complete the diffusion welding.
 Time required for diffusion welding varies from a few
minutes to several hours.

14
Surface preparation:
 Better prepared and cleaned surfaces lower the
minute the minimum diffusion welding temperature
or pressure.
 Surface to be diffusion bonded are
(i) Machined, ground or abraded so that they are
sufficiently smooth to ensure that the interfaces can
be passed to proper contact without excessive
deformation.
(ii) Cleaned of chemically combined films, oxides
etc.
(iii) Cleaned of gaseous, aqueous or organic surface
films.

15
Metallurgical factors:
1. Allotropic transformation:
Hardenable steels undergo allotropic
transformation and involve volume change during
diffusion welding. This may affect dimensional
stability of the welded component.
2. Recrystallization:
Many cold worked metals tend to recrystallize
during diffusion welding. This may be good for
certain materials but undesirable for others, e.g.,
refractory metals.
3. Surface oxides:
Beryllium, aluminium, chromium, etc., form
tenacious surface oxides. They and alloys
containing them are, therefore, more difficult to
weld than those which form less stable oxide films

16
Use of interlayers:
 It differing from base metals being joined is growing
in diffusion welding. An interlayer is a lower strength
intermediate or one containing a diffusive element.
An interlayer solves alloying compatibility problems
when joining dissimilar metals. Also, it being soft,
confines deformation to itself and thus minimizes
distortion of work pieces when pressed to contact.
 Interlayers may, however, give rise to decreased
strength or stability.
 The most common interlayer materials used at this
time are titanium, nickel and silver.

Materials diffusion bonded
17
 Many similar and dissimilar metals have been joined
by diffusion welding, but most applications of this
process have been with
Titanium alloys,
Zirconium Alloys and
Nickel base alloys.

Advantages of diffusion bonding
18
a) Welded having essentially the same physical,
chemical and mechanical properties as the base
metal can be produced.
b) Heat treating operations can be incorporated
during the bonding cycle.
c) Continuous, leak tight welds can be formed.
d) The process is well suited for welding dissimilar
metals and ceramics.
e) Numerous welds can be made simultaneously.
f) Weldability is largely independent of material
thickness.

Limitations of diffusion bonding
19
i. A major difficulty is the removal of oxide and the
contaminating layers present on practically all metals
exposed to natural or industrial environment.
ii. Opposing surfaces must be mated in size to within a
few angstroms of each other in order to achieve a
satisfactory metal bond.
iii. Diffusion welding requires a relatively long, time
consuming thermal cycle.
iv. With dissimilar materials, difficulties due to time /
temperature / pressure requirements are frequently
encountered.
v. Diffusion welding is not classified as a mass
production process.

Application of Diffusion bonding
20
1. Fabrication of reactor components in atomic
energy industries.
2. Fabrication of honeycomb, rocket engines,
helicopter rotor hub, turbine components, etc., in
aerospace missile and rocketry industries.
3. Two controversial aerospace vehicles have brought
diffusion bonding into the light e.g., B-1 bomber
and space shuttle.
4. Fabrication of composite materials.

Diffusion
 Many reactions and processes that are
important in the treatment of materials rely on
the transfer of mass either within a specific
solid (ordinarily on a microscopic level) or
from a liquid, a gas, or another solid phase.
This is necessarily accomplished by diffusion,
the phenomenon of material transport by
atomic motion.
 The phenomenon of diffusion may be
demonstrated with the use of a diffusion
couple, which is formed by joining bars of two
different metals together so that there is
intimate contact between the two faces.
21

22
(a) A copper–nickel diffusion couple
before a high-temperature heat
treatment.
(b) Schematic representations of
Cu (red circles) and Ni (blue
circles) atom locations within the
diffusion couple.
(c) Concentrations of copper and
nickel as a
function of position across the
couple.

23
(a)A copper–nickel diffusion
couple after a high-
temperature heat treatment,
showing the alloyed diffusion
zone.
(b) Schematic representations
of Cu (red circles) and Ni
(blue circles) atom locations
within the couple.
(c) Concentrations of copper
and nickel as a function of
position across the couple.

Inter diffusion
This result indicates that copper atoms have migrated or diffused into
the nickel, and that nickel has diffused into copper. This process,
whereby atoms of one metal diffuse into another, is termed inter
diffusion, or impurity diffusion.
Initially
After some time
24

Self-Diffusion
Inter diffusion may be discerned from a macroscopic
perspective by changes in concentration that occur over
time, as in the example for the Cu–Ni diffusion couple.
There is a net drift or transport of atoms from high to low
concentration regions. Diffusion also occurs for pure
metals, but all atoms exchanging positions are of the same
type; this is termed self-diffusion.specific atom movement
A
B
C
D
After some time
A
B
C
D
25

Diffusion Mechanisms
 From an atomic perspective, diffusion is just the
stepwise migration of atoms from lattice site to lattice
site. In fact, the atoms in solid materials are in constant
motion, rapidly changing positions. For an atom to make
such a move, two conditions must be met:
 (1) there must be an empty adjacent site,
 (2) the atom must have sufficient energy to break bonds
with its neighbour atoms and then cause some lattice
distortion during the displacement.
 This energy is vibrational in nature. At a specific
temperature some small fraction of the total number of
atoms is capable of diffusive motion, by virtue of the
magnitudes of their vibrational energies.
 This fraction increases with rising temperature. Several
different models for this atomic motion have been
26

Vacancy Diffusion
27
 One mechanism involves the interchange of an
atom from a normal lattice position to an adjacent
vacant lattice site or vacancy. This mechanism is
aptly termed vacancy diffusion.
 Of course, this process necessitates the presence of
vacancies, and the extent to which vacancy diffusion
can occur is a function of the number of these
defects that are present; significant concentrations
of vacancies may exist in metals at elevated
temperatures.
 Because diffusing atoms and vacancies exchange
positions, the diffusion of atoms in one direction
corresponds to the motion of vacancies in the
opposite direction. Both self-diffusion and inter
diffusion occur by this mechanism; for the latter, the

Interstitial diffusion
 The second type of diffusion involves atoms that
migrate from an interstitial position to a neighbouring
one that is empty. This mechanism is found for inter
diffusion of impurities such as hydrogen, carbon,
nitrogen, and oxygen, which have atoms that are
small enough to fit into the interstitial positions.
 Host or substitutional impurity atoms rarely form
interstitials and do not normally diffuse via this
mechanism. This phenomenon is appropriately
termed interstitial diffusion.
 In most metal alloys, interstitial diffusion occurs
much more rapidly than diffusion by the vacancy
mode, because the interstitial atoms are smaller and
thus more mobile.
 Furthermore, there are more empty interstitial29

Steady State Diffusion
32
 Diffusion is a time-dependent process that is, in a
macroscopic sense, the quantity of an element that
is transported within another is a function of time.
Often it is necessary to know how fast diffusion
occurs, or the rate of mass transfer. This rate is
frequently expressed as a diffusion flux (J), defined
as the mass M diffusing through and perpendicular
to a unit cross-sectional area of solid per unit of time.
In mathematical form, this may be represented as
At
M
J 

33
 In differential form, this expression becomes
 If the diffusion flux does not change with time, a
steady-state condition exists. One common example
of steady-state diffusion is the diffusion of atoms of a
gas through a plate of metal for which the
concentrations (or pressures) of the diffusing
species on both surfaces of the plate are held
constant. When concentration C is plotted versus
position within the solid x, the resulting curve is
termed the concentration profile. The slope at a
particular point on this curve is the concentration
gradient.
dt
dM
A
J
1

concentration gradient
dx
dC


For diffusion problems, it is sometimes convenient to express concentration
in terms of mass of diffusing species per unit volume of solid (kg/m3 or
g/cm3).
Steady State Diffusion Across A Thin
Plate
BA
BA
xx
CC
x
C
dx
dC






34

35
 The mathematics of steady-state diffusion in a single
(x) direction is relatively simple, in that the flux is
proportional to the concentration gradient through
the expression
 The constant of proportionality D is called the
diffusion coefficient, which is expressed in square
meters per second. The negative sign in this
expression indicates that the direction of diffusion is
down the concentration gradient, from a high to a
low concentration. Above equation is called Fick’s
first law.
 Sometimes the term driving force is used in the
context of what compels a reaction to occur. For
diffusion reactions, several such forces are possible;
dx
dC
DJ 

Non Steady State Diffusion
36
 Most practical diffusion situations are non steady
state ones. That is, the diffusion flux and the
concentration gradient at some particular point in a
solid vary with time, with a net accumulation or
depletion of the diffusing species resulting. Under
conditions of non steady state, use of Steady state
equation is no longer convenient; instead, the partial
differential equation known as Fick’s second law.
 If the diffusion coefficient is independent of
composition, above equation simplifies to


















x
c
D
xt
c
2
2
x
c
D
t
c











Calculate Activated Diffusion
 Room temperature (kBT = 0.026 eV)
 Typical activation energy Em (~ 1 eV/atom) (like Qv)
 Therefore, a large fluctuation in energy is needed for
a jump.
 Probability of a fluctuation or frequency of jump, Rj
0 exp m
j
B
E
R R
k T
   
 
R0 = attempt frequency proportional to
vibration frequency
37

1. Probability of finding a vacancy in an adjacent lattice
site:
2. Probability of thermal fluctuation
Calculate Activated Diffusion
The diffusion coefficient = Multiply
0 exp m
j
B
E
R R
k T
   
 
.exp v
B
Q
P Const
k T
   
 
.exp expm V
B B
E Q
D Const
k T k T
        
   
0 exp d
B
Q
D D
k T
   
 
38

Diffusion and Temperature
Diffusion coefficient increases with increasing T.
Activation energy - energy required to produce the movement of 1
mole of atoms by diffusion.
0 0exp expd dQ Q
D D D
RT kT
         
   
D0 - Temperature-independent (m2/s)
Qd - The activation energy (J/mol or eV/atom)
R - The gas constant (8.31 J/mol-K)
or
kB - Boltzmann constant ( 8.6210-5 eV/atom-K)
T - Absolute temperature (K)
39

Diffusion – Temperature Dependence
0
1
ln ln dQ
D D
R T
 
   
 
or 0
1
log log
2.3
dQ
D D
R T
 
   
 
The above equation has represented as straight
line equation
40

 The diffusing species, host material and temperature
influence the diffusion coefficient.
Diffusion of different species
41

Factors that Influence Diffusion
Diffusing Species:
 The magnitude of the diffusion coefficient D is indicative
of the rate at which atoms diffuse. Coefficients, both self-
and inter diffusion, for several metallic systems are listed
in Table 5.2.
 The diffusing species as well as the host material
influence the diffusion coefficient. For example, there is a
significant difference in magnitude between self-diffusion
and carbon inter diffusion in iron at 500 0C, the D value
being greater for the carbon inter diffusion (3 X 10-21 vs.
2.4 X 10-12 m2/s).
 This comparison also provides a contrast between rates
of diffusion via vacancy and interstitial modes as
discussed earlier. Self-diffusion occurs by a vacancy
mechanism, whereas carbon diffusion in iron is
42

43
Temperature:
 Temperature has a most profound influence on the
coefficients and diffusion rates. For example, for the
self-diffusion of Fe in -Fe, the diffusion coefficient
increases approximately six orders of magnitude (3
X 10-21 vs. 1.8 X 10-15 m2/s) in rising temperature
from 500 0C to 900 0C (Table 5.2). The temperature
dependence of the diffusion coefficients is
0 0exp expd dQ Q
D D D
RT kT
         
   

45
 The activation energy may be thought of as that
energy required to produce the diffusive motion of
one mole of atoms. A large activation energy results
in a relatively small diffusion coefficient. Table 5.2
also contains a listing of D0 and Qd values for
several diffusion systems. Taking natural logarithms
of above equation yields
Or
 Because D0, Qd and R are all constants, above
equation takes on the form of an equation of a
straight line:
 where y and x are analogous, respectively, to the
variables log D and 1/T.
0
1
ln ln dQ
D D
R T
 
   
 
0
1
log log
2.3
dQ
D D
R T
 
   
 
cmxy 

Diffusion bonding

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Diffusion bonding