13
Joining processes for powder
metallurgy parts
C. S e l c u k, Brunel Innovation Centre, UK
DOI: 10.1533/9780857098900.3.380
Abstract: Powder metallurgy (PM) processes are ideal for rapid production
of near net shape parts with complex geometries from a range of materials
that would often not be possible to combine otherwise. Certain alloy
combinations, chemical compositions in powder form are more favourable
for synthesis of materials. This allows PM to be extremely versatile,
maximising material utilisation. The net shape capability directly reduces
or eliminates secondary operations like machining. Despite this obvious
advantage of PM processes, joining materials synthesised from powders has
been associated with difficulties related to their inherent characteristics, like
porosity, contamination and inclusions, at levels which tend to influence the
properties of a welded joint. This chapter presents an overview of joining
PM components. It seeks to identify preferred joining processes and identify
apparent technology gaps, with an emphasis on offering solutions to welding
problems. It also highlights developing approaches.
Key words: joining, near net-shape processing, powder metallurgy, welding.
13.1
Introduction
In PM materials processing techniques, all or some constituents of a part
are employed in particulate form with certain characteristics of composition,
morphology and size, compacted into a high precision product (Figs 13.1
and 13.2).
The ability of PM to produce high quality, complex parts with close
tolerances and high productivity presents significant advantages, such as
energy efficiency, with potentially low capital costs. PM is widely used
for a range of applications, such as dental restorations, implants, bearings
and automotive transmission parts, from biomedical to automotive industry
sectors.
PM components are becoming increasingly attractive as substitutes for
wrought and cast materials in various applications. However, it is possible
to increase further the use of PM by exploiting the ability to manufacture
complex geometrical configuration by joining PM parts to one another or
to other cast/wrought products. The main issues restricting the welding of
PM parts have been porosity, impurities and the fact that some PM parts
have a high carbon content. Of these, the most important characteristic of a
380
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Design &
development
Product
Particulate
composition
Secondary
processing
Joining
Machining
Heat treatment
Green/
sintered part
Particulate
size
Particulate
shape
13.1 Important factors to consider in development of PM products.
Joining is an enabling step.
High quality
Construction
& engineering
tooling
bearings
Automotive
transmission
parts, gears
medical dental
restoration,
impants
Mining
Oil & gas
exploration
Power
Renewables
Generators
Magnets
Electronics
Aerospace
Defence
Household
Sports
PM
Energy efficiency
Part complexity
(Joining)
High
productivity
13.2 Ability of PM to create complex geometries for industry with the
help of joining.
PM part for welding has been porosity, created either deliberately to make a
porous part or incidentally due to insufficient densification. Powder particle
characteristics (such as particle shape, size and surface area) determine the
porosity or relative density of a powder compact, which in turn influences
several important physical properties of the preform, such as thermal
conductivity and hence hardenability, as well as thermal expansion. It has
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been noted that porosity can also act as a trap for impurities/inclusions that
could potentially have an impact on secondary operations such as welding
when entrapped impurities in the pores could be deleterious to the weldability
of the PM part, for example by encouraging solidification cracking. PM
parts have been reported to be susceptible to cracking in the heat affected
zone when welded, owing to the porosity of the preform and limited area
of interparticle bonding giving low ductility adjacent to the joint. Hence
these locations may be unable to resist the thermal stresses generated as
a result of contraction in a fusion weld. In addition, welding is commonly
associated with resultant distortion, whereas PM parts are known to provide
good dimensional and geometrical accuracy and thus, if machining operations
are to be avoided, distortion must be minimised via selection of appropriate
joining technology.
Therefore, widespread success in welding PM parts requires understanding
of the influence of porosity, chemical composition, impurity level and overall
cleanliness, upon weldment properties such as weld metal and heat affected
zone (HAZ) cracking, ductility and toughness, residual stresses and distortion.
These issues are addressed in this review.
13.2
Welding processes for powder metallurgy parts
13.2.1 Introduction
Joining processes applicable for PM parts can be categorised as solid state
and liquid state. The solid state processes such as diffusion bonding and
brazing have been predominantly used for lower density porous parts. In
comparison, parts with higher densities or minimal porosity are typically
treated as fully dense wrought materials, and these are typically welded using
fusion-based joining processes, including arc welding, that is gas tungsten
arc (GTA), gas metal arc (GMA), electron beam (EB) and laser welding.
Further joining techniques such as adhesive bonding and shrink fitting may
be used for some applications but are not considered here.
13.2.2 Arc welding
Arc welding of PM parts may give porosity with an associated detrimental
influence on the weld integrity.1 Gas metal arc (GMA) welding of powder
compacts, which are not fully dense, can result in porous welds and weld toe
cracking, the latter presumably resulting from low ductility of the original
PM part. The density of a PM part and its composition are expected to affect
the tendency for porosity during welding.2,3
Welding of ferrous PM parts can form a soft pearlitic microstructure as a
result of slow cooling, caused by porosity reducing the thermal conductivity,
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which can allow strains to be accommodated that would otherwise give rise
to cracking. An additional benefit of the porous structure is that hydrogen
can diffuse out of the metal into and through the interconnected pores, hence
reducing susceptibility to fabrication hydrogen cracking. Sintered steels are
thus considered to be more resistant than wrought steels.
It is also worth mentioning that, when subjecting ferrous PM parts to
steam treatment (heating the parts to a temperature in the region of 550°C
and exposing them to water vapour) which is a common secondary process
in production environment, a thin layer of Fe3O4 is formed both on the outer
surface and on the surfaces of the interconnected porosity. This treatment is
for improved corrosion resistance, increased surface hardness, compressive
strength and wear resistance. This or a similar post sintering heat treatment of
ferrous sintered parts will, however, prevent satisfactory welding. The problem
is due to a resulting oxide film, which is slightly porous, probably containing
moisture and has an insulating effect and hence is potentially a source of
weld metal porosity and a potential cause of cracking in weldments.
Arc welding (gas tungsten arc welding, GTAW, and gas metal arc welding,
GMAW) has also been attempted for non-ferrous metal matrix composite
(MMC) systems, for example, SiC particle-reinforced aluminium. Gross
porosity and delaminations in both weld metal and HAZ were reported to
be frequent. It was claimed that despite large volumes of particulates in
the Al alloy matrix, a wide range of MMCs can be fusion welded, with
weldability similar to Al.4 However, particulate characteristics (shape,
size and distribution) of reinforcement, their proportion within the matrix,
along with the homogeneity of the material and chemistry are critical for
welding. Any interfacial reactions between the reinforcement and matrix
that may result in the presence of secondary compounds or flux derivatives
can influence the weldability and its success. Filler metal compositions,
with respect to inclusions such as oxides and impurity contents (e.g. silicon
and phosphorous levels in notably ferrous parts), can change the output of
welding by influencing the microstructures attainable, especially in the heat
affected regions. This can manifest itself in formation of low melting point
eutectic phases at grain boundaries owing to segregation of impurities which
can lead to solidification cracks in the heat affected zone (HAZ). Porosity
distribution across the weld metal, HAZ and parent metal will affect the
strength of the joint. PM components have the potential to replace their
wrought counterparts in several applications owing to the advantages of net
shape capability, low cost of production and high added value in terms of
performance (e.g. strength, wear and fatigue). However, this can only be fully
achieved if they can be confidently joined to other components especially
with respect to design and structural considerations. Weldability of PM parts
is therefore crucial to establish when it comes to widening the horizons for
PM and future applications.
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Consumable
electrode
Flux
covering
Weld
metal
Core
wire
Evolved
gas shield
Slag
Arc
Weld
pool
Parent
metal
(a)
Gas
nozzle
Filler
rod
Non-consumable
tungsten electrode
Gas
shield
Arc
Weld
pool
Parent
metal
Weld
metal
(b)
13.3 Schematic representations of manual metal arc: (a) gas–metal
arc GMAW welding and (b) tungsten–inert gas gas–tungsten arcGTAW welding (courtesy of TWI Ltd).
13.2.3 Laser welding
There are advantages of laser welding for PM parts, as it is a highly automated
process, offering precision and control (Fig. 13.4). Welding speeds of several
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Key bole
(a)
(b)
13.4 Representation of key hole laser welding and a typical weld
(courtesy of TWI Ltd).
metres per minute are possible, with a low heat input, resulting in a small
HAZ and limited thermal distortion and residual stress.5
However, various defects, such as blowholes that are probably due to
entrapment of gases that cannot leave the melt during rapid solidification,
were observed in laser welding of sintered steel parts in general, together
with the occurrence of hydrogen cracking of medium carbon parts, cold,
and hot cracking which have been observed in laser welding of sintered
steel parts.6 It was recommended by one author that a filler wire could be
used to prevent all of these types of defect, although filler wire addition is
an extra complication in laser welding. Low C-steels (typically up to 0.3%
C) can be satisfactorily laser welded and it has been reported that a sintered
steel part can be acceptably laser welded to a wrought counterpart, as long
as both components have low carbon content.7 However, laser welding of
medium C‑steels (typically 0.3–0.6% C) appears to be difficult, owing to the
formation of a hard, brittle and hydrogen crack sensitive martensitic structure
in the joint, caused by rapid cooling. Pre-heating may help by inducing a
softer bainitic microstructure with some fine pearlite, which will increase
toughness and hence improve the defect tolerance of the joint and reduce
sensitivity to hydrogen cracking.
Gas carburised and oil quenched steels are considered to be difficult to
weld, because of significant blowhole formation, probably due to entrapped
oil and gas in the pores. Elsewhere, it was noted that smooth, discontinuityfree welds can be produced at slower travel speeds and lower beam powers.8
It was also reported that a beam weaving laser welding technique would
suppress porosity and that an increased width to depth ratio of the molten
metal was beneficial for the escape of bubbles in the weld zone.9
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Laser welding of sintered austenitic stainless steel (grade 316L) was
reported to be very easy, resulting in good joints.7 In trials with sintered
Al alloys, it proved difficult to obtain a sound joint because of porosity
formation, resulting in spongy welds.7 It is thought that oxidation was the
probable cause, possibly creating locally overheated spots when more laser
energy is absorbed by the oxides than the metal; another possibility is that
the oxide absorbs moisture and this is released during welding.
13.2.4 Electron beam (EB) welding
EB welding is normally carried out in high vacuum (e.g. 10–6 mbar).
Therefore it is a batch process and may be expensive and thus restricted to
high value parts only. It has a tendency to give high cooling rates and high
hardness in C-steels, similar to laser welding, but is likely to have a greater
tendency towards pore formation as the vacuum encourages trapped gas
to try to escape during welding. It is reported that porosity increased with
reduced travel speed. However, it has been demonstrated that weld metal
porosity content in sintered ferrous compacts with a range of porosities can
be controlled by beam parameters10 and a non-vacuum EB welding process
has been used for sintered parts.11
Any residual films, such as heat treatment quench oil trapped in the pores
of a PM part, were found to have a detrimental effect on EB welding.12
A fine grain size (< ASTM 8–9) in a sintered PM part was reported to be
essential for good EB weldability, presumably due to improved ductility, in
high temperature PM superalloys.13 This is related to increased ductility and
toughness of a fine grained material for absorbing strains upon solidification.
Cracking has been observed in EB welding of PM superalloys designed for
aerospace applications (engine components such as turbine discs) which
required further investigation.14 However, despite the problems described
above, EB welding has the ability to give low distortion, again similar to
laser welding, or at least uniform distortion effects, which is important
for preserving the dimensional stability of near net shape PM components
(Fig. 13.5).
13.2.5 Resistance projection welding
Projection welding is one of the most widely applied welding processes for
sintered PM parts. A significant aspect of projection welding is the limited
distortion associated with it, which is certainly advantageous in terms of
geometrical stability. However, one potential difficulty for the success of
projection welding is the cleanliness of the parts or presence of surface films
that can inhibit bonding. This can manifest itself in the likely presence of an
oxide layer on some parts, for example ones subjected to steam treatment,
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Vacuum
pumping
Workspace pressure
~10–2 – 10–4 mbar
(a)
10
mm
(b)
13.5 Example of (a) EB welding configuration with reduced pressure
and (b) reduced pressure EB welds in C-Mn steel (courtesy of TWI
Ltd).
typically on ferrous sintered parts. Such a layer could prevent satisfactory
welding through an insulation effect at the interface and as a potential source
of moisture and hence porosity.15,16 It is therefore recommended that any steam
treatment on PM parts, as described earlier, should be done after welding.
It has been possible to weld high carbon PM steels and case hardened parts
using resistance projection welding.17 Light alloys have also been projection
welded and the combined effect of pressure and temperature at the joint
can, in fact, help to densify the region by closing the pores. This can help
improve the strength of the weld region. The process enables PM parts to
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be joined to wrought materials and therefore allows a degree of flexibility
in creating complex geometries and dissimilar joints (Fig. 13.6).
13.2.6 Friction welding
Friction welding is a solid phase joining process, mainly used for wrought
products (e.g. light alloys of Al and Ti) in a range of geometries such as
extrusions (Fig. 13.7). Particular advantages are the absence of flux, filler
or need for a protective atmosphere, which makes the process extremely
attractive.15 The friction welding process is highly suitable for welding PM
parts as it enables pore closure, which can potentially lead to a pore-free weld
interface and a refined microstructure. This is particularly valid for Al-based
PM parts and MMCs.18 An additional advantage of friction welding for Al
alloy PM components, is that it is useful in breaking down any oxide layer
deposited on the particles by intense deformation within the weld region. This
in turn results in exfoliation of oxide and its rejection from the joint. As a
consequence, a clear bond line is achieved and often the bond homogeneity
and strength is improved. One potential disadvantage of friction welding
may be associated with the change in microstructure caused by reorientation
and deformation of sintered metal grains, which may create a potential
weak region in the joint, thereby reducing fatigue performance.16 Since its
invention in the early 1990s at TWI in the UK, friction stir welding (FSW),
which is a variant of friction welding, has come a long way and it is widely
applied in various industry sectors notably transport and aerospace. Al has
been a good material to demonstrate the applicability of process in its fully
industrialised form. The tooling and agility of the process have been critical
for complex parts and relatively difficult to weld materials. FSW presents a
good opportunity for PM materials, not only for joining but also for creating
MMCs in bulk or on the surface. The process can be utilised to combine
what would otherwise be difficult to co-exist alloy systems. This is partly
due to the fact that it is a solid state process like PM.
Projection
Sheet
(a)
(b)
(c)
13.6 Projection welding configurations: (a) embossed projection
section, (b) stud to plate, (c) annular projection, and an example
microstructure from a typical weld (courtesy of TWI Ltd).
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(a)
Load
Motion
(b)
Sufficient downward force to
maintain registered contact
Advancing
side of weld
Joint
Leading
edge of the
rotating tool
Shoulder
Probe
Trailing edge of
the rotating tool
Retreating side
of weld
(c)
13.7 Examples of friction welding: (a) rotary, (b) linear, (c) friction stir
configurations, and typical weld macrograph and microstructure in Ti
alloy forging, shown in (d) and (e) respectively (images courtesy of
TWI Ltd).
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10 mm
(d)
100 µm
(e)
13.7 Continued
13.3
Other joining processes for powder metallurgy
parts
13.3.1 Brazing
In brazing, the porosity of the PM part can, in fact, draw away the brazing
alloy from the joint region. This can leave insufficient material for bonding.
To overcome this problem, techniques have been developed where brazing
is integrated in the sintering process.19,20 Another potential issue which is
encountered commonly in brazing is the presence of secondary products,
for example due to flux reactions which would stop further infiltration by
blocking the pores. Factors that can affect the brazed joint in terms of strength
are surface condition of the particles (as is the case for other techniques)
and the part’s surface roughness;21 their influence can be significant. There
are new brazing techniques, such as laser brazing, and new brazing alloys
being developed for joining sintered components in large volume.22 Brazed
assemblies can typically consist of PM to PM or PM to wrought or cast
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structures, which can end up as a functionally graded material. Brazing, however
has been selected as a joining method for hot isostatically pressed (HIP)
parts.
13.3.2 Diffusion bonding
Diffusion bonding is typically utilised for ferrous parts which are fairly small
in size. Hence, part geometry can be a concern. To avoid this complication,
sintering and diffusion bonding can potentially be carried out in the same
furnace. A eutectic reaction is often employed to provide a transient liquid
phase for bonding at the interface.23 The presence of resulting reaction products
such as oxide compounds may reduce the bond strength.24 To induce high
bond strength diffusion can be activated via the addition of suitable elements,
such as Cu in, for example, ferrous PM parts.25 When compared with other
joining techniques, rather low strength joints can be expected from diffusion
bonding, which may be limited to certain geometries and alloy compositions.
Diffusion bonding has been a choice when handling more exotic compositions
where the chemistry of the part plays a significant role in the joining process
and its feasibility. Hence, applications have been focused on light materials
such as Ti alloys, MMCs and special products. More conventional C–Mn
steels have not been commonly diffusion bonded. Although an attractive
option, the success of the bond strength gained by this joining technique
critically depends on compositions and hence phase transformations and
may require protective atmospheres (inert such as argon, or reducing, such as
hydrogen) which bring additional considerations such as complexity and cost
of production. Small volume high performance and added value components
may therefore be more favourable for diffusion bonding and the technique
may well be the only option for joining such materials. Similar to brazing,
diffusion bonding has also found applications in HIP components.
13.3.3 Shrink fitting (press fitting)
In shrink fitting, dimensional changes between parts are employed to form
the joint, which will not be necessarily gas-or liquid-tight. This may be
a disadvantage in some applications.15 It has been demonstrated that, for
ferrous PM parts, dimensional change will be dependent on the powder
selection and the nature of the constituents.26 Shrink fitting is a cost effective
option for many applications, reducing machining, especially for parts that
could be manufactured by other methods such as metal injection moulding
(MIM) for particular geometrical reasons.19 Cylindrical and ring-shaped
compacts are most often joined by press fitting. Joint strength is dependent
on variations in densities between the parts, fitting pressure and degree of
taper angle, if any.27
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13.3.4 Adhesive bonding
The porosity of sintered parts is ideal for adhesive bonding, since surface
porosity can facilitate mechanical keying of the adhesive. Most PM parts can
simply be joined by gluing.12 However, the surface has to be cleaned and
free of contaminants. Potential advantages include uniform distribution of
stress and the ability to join thin to thick parts, making complex geometries
possible. The major disadvantage tends to be poor resistance to elevated
temperatures where the adhesive degrades; as a result the strength of the
joint can be significantly reduced. It is worth noting that the performance of
any adhesive strongly depends on the loading mode and therefore any testing
should be representative of the actual loading conditions in the application
of interest.19
13.3.5 Joining metal injection moulded (MIM) parts
At this juncture it is worth noting that the above considerations for different
processes are also valid for MIM parts even more so than PM parts owing to
the higher binder content in MIM which can affect the weldability, porosity
content and impurity levels in the parts. Therefore, careful handling of
the component chemistry, including residual levels as well as geometrical
considerations and strength/porosity of the parts to be welded, is paramount
in defining a window for joining and will enable choice of a suitable
process for MIM parts. To date both diffusion bonding and brazing have
been favourable processes for joining. It is worth highlighting that powder
injection moulding (PIM) of ceramics and metals can also be regarded as a
potential route for creating tool materials for friction stir welding of metals and
alloys.
13.3.6 Laser metal deposition
Laser metal deposition (LMD) is a unique technique, combining laser
and powder processing, which enhances material utilisation by enabling
manufacture of high precision near net shape components from powders.
Owing to the cost of specialised powder feedstock and laser equipments
with protective atmosphere in many cases, the technique is regarded as an
expensive but versatile option. Therefore, LMD has been of most interest
in high value added applications such as in aerospace and medicine which
can afford this developing process as a whole, in spite of the criticality of
performance and acceptance criteria in these industry sectors.
There are several benefits to using LMD for repair. Highly complex parts
can be repaired in an automated fashion and, because the heat input is low,
the heat affected zone is small. Therefore the strength of material is not
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affected. Distortion is also lower than for conventional welding techniques.
Anisotropy in the mechanical properties occurs, however, due to the layered
microstructure and residual stresses that are commonly present because of
steep thermal gradients. These are usually detrimental to the mechanical
properties of the parts produced. A current concern with LMD is that it is
very hard to predict the properties of LMD fabricated components because
there are so many process variables involved in the process. The parts have
very complex thermal histories, which depend on process variables. The
effects of process parameters on the microstructure are complex with a strong
dependence on the material system.
Despite the major advantages of LMD, there is, however, a continuous need
to develop a full understanding of the process–structure–property relationships,
with a particular emphasis on the effect of powder characteristics, such as
powder particle size, shape and distribution on process variables, and the
metallurgy and resulting mechanical properties, which are crucial for many
in service applications (Fig. 13.8, Table 13.1).28
13.4
Discussion
A broad range of powder metallurgy parts is available, in a wide range of
alloys, and there is no single best way to join them. However, there are a
number of welding characteristics of PM parts that are different from those
associated with wrought or cast equivalents, either as a consequence of the
PM production route or the typical applications of PM parts. For example, as
PM parts are used in a variety of high precision applications, it is desirable
to weld with a process that gives minimal distortion. This favours low
heat input processes such as laser and EB welding but any low heat input
process will also inevitably give rapid cooling and hence high hardness
in steel parts, particularly for higher C contents. It is not clear whether
sintered parts can be EB welded in a vacuum however, considering inherent
porosity where gases and impurities can be retained and entrapped in the
weld. Reduced pressure EB welding, which only requires a vacuum of the
order of 10–3 mbar, may be more suitable for welding sintered PM parts, to
overcome difficulties in achieving an adequate vacuum, but this is still under
development.
As with any welding, the main requirements for welding PM parts is that
the process should not introduce defects (Fig. 13.9). Powder metallurgy
parts contain porosity, either deliberately, and hence with a fairly high
volume fraction, or as a consequence of the inability to obtain complete
densification, in which case it is typically at a low level. Any porosity in
the PM part will tend to trap contaminants and gas, which can cause pores
in the weld metal and introduce species that increase sensitivity to both
hot and cold cracking mechanisms, for example sulphur and phosphorus
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Ti alloys
Laser DMD
Ferrous and non-ferrous
Friction
welding
Al PM, MMCs
Diffusion
bonding
Laser welding
Super alloys
Light materials, MMCs
EB welding
Brazing
Porous
Arc welding
Shrink
fitting
Resistance/
projection
welding
Ferrous
13.8 Layout showing relative disposition of joining processes with
respect to applications in terms of the nature of materials.
contamination will encourage solidification cracking, whilst moisture
and carbon contamination will encourage hydrogen cracking. In order
to minimise these problems, cleanliness of parts is vital. In this respect,
avoiding steam treatment will be beneficial and degreasing is important
prior to welding. Where contamination exists, use of a filler metal that
is more tolerant of contamination than the parent material, for example a
nickel alloy, may be beneficial, in which case arc welding processes are
preferred. One possible advantage of an interconnected porosity may be
that hydrogen can diffuse out via the open porous structure, in welding PM
steel parts, which may make them more resistant to hydrogen fabrication
cracking.
Where present at significant levels, porosity of the parent material may
lead to tearing of the material adjacent to the weld, simply due to the
development of plastic strain beyond the capacity of the PM part, perhaps
exacerbated by geometric effects at the joint. In such cases, use of low heat
input is preferred, to reduce the amount of material strained, and friction
welding may be advantageous, as the compression involved tends to close
pores. Indeed friction welding may be generally useful for PM parts owing
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Table 13.1 Qualitative ranking of joining processes for PM parts in terms of relative
distortion, geometrical constraint, porosity and cost as high (H), medium (M) or
low (L)
Joining
process
Distortion
Geometrical Porosity
constraint
Cost
Typical applications
Arc welding H/M
H/M
H/M
M/L
Ferrous and nonferrous materials
Laser
welding
M/L
M/L
H/M
H/M
Ferrous and nonferrous materials
EB welding
M/L
M/L
H/M
H/M
PM superalloys
Resistance
projection
welding
M/L
M/L
M/L
M/L
Ferrous materials
Friction
welding
M/L
M/L
M/L
H/M
Al based PM parts,
MMCs
Brazing
M/L
M/L
M/L
M/L
Ferrous: PM to PM
or PM to wrought
materials
Diffusion
bonding
M/L
H/M
M/L
M/L
Ferrous, light
materials, Ti,
MMCs
Shrink fitting M/L
M/L
M/L
M/L
Porous materials
(a)
(c)
(b)
(d)
13.9 Representation of typical welding flaws: (a) HAZ hydrogen
cracking and (b) weld metal hydrogen cracking, (c) lamellar tearing
and (d) solidification (hot) cracking (courtesy of TWI Ltd).
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to the compressive force involved and the fact that friction welding squeezes
the original surface layer, which may be contaminated, out of the joint.
It is apparent that powder particle characteristics, which influence
densification of a PM part and therefore its final porosity, have not received
much emphasis in relation to welding studies. In order to achieve better control
of porosity and minimise its detrimental effects in welding, consideration
should be given to the influence of powder particle characteristics such as
particle shape, size and surface area on the density and porosity of a powder
compact, as well as any interfacial reactions and subsequent formations such
as secondary phases for improved weldability and joint strength.
13.5
Conclusions
The following conclusions are drawn:
∑
Welding is widely used for a range of PM components for diverse
applications across several industry sectors but limitations exist owing
to the inherent porosity, contamination within the pores and the effect of
porosity on the ductility of the material and hence its ability to withstand
strain in a weld heat affected zone.
∑ For porous materials, use of low heat input is recommended to reduce
strains that develop in the heat affected zone and minimise the risk of
tearing adjacent to the weld. Where applicable, friction welding and
projection welding may be advantageous as they involve compression,
which tends to close pores in the joint area. Porosity in the weld metal is
likely to result when any fusion-based process is used and rapid cooling
rates may increase porosity owing to the limited opportunity for bubbles
to escape from the molten metal.
∑ Where contamination causes weld metal cracking, the use of a welding
process that allows introduction of a consumable filler material, such
as the various common arc welding processes, is beneficial. In extreme
cases, use of a tolerant nickel-based filler metal may be necessary to
avoid cracking. Any process that introduces contamination or oxide to
porous parts, such as steam treatment is likely to be detrimental to the
ability to make sound welded joints and should be avoided. Similarly,
cleaning the joint surfaces prior to welding is beneficial.
∑Laser and EB welding have also found application in welding PM
components when the inherent low distortion of these processes is an
advantage, that is for high precision parts, but these processes have rapid
thermal cycles, which encourage hardening and cracking of the weld
metal, with limited opportunity for filler addition, and are not always
applicable. EB welding in particular is likely to suffer from porosity and
the necessary high vacuum might not be achieved when gas contamination
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Joining processes for powder metallurgy parts
397
is extensive. Reduced pressure EB welding, which operates with higher
gas pressures, is attractive for sintered porous parts.
13.6
References
1. J. C. Thornley, Welding Design and Metal Fabrication, 1973, 12, 399–402.
2.K. Couchman, M. Kesterholt and R. White, ‘Seminar on secondary operations’,
Proceedings International Conference PM 1988, Orlando, 33–9.
3. M. A. Greenfield, R.F. Geisendorfer, D. K. Haggend and L.P. Clark, Welding
Research Supplement, 1977, May, 43–148.
4. J. H. Devletian, Welding Journal, 1987, June, 33–9.
5. A. Rocca and G. Capra, SPEI GCL-7th International Symposium on Gas Flow and
Chemical Lasers, Austria 1988, Vol. 1031, 635–45.
6. A. Joskin, J. Wildermuth and D. F. Stein, International Journal Powder Metallurgy
and Powder Technology, 1975, 11(2), 137–142.
7.E. Mosca, A. Marchetti and U. Lampugnani, Proceedings International Conference
PM Powder Metallurgy, Florence, 1982, 193–200.
8. S. Chiang and C. E. Albright, Journal of Laser Applications, 1988, 1(1), 18–24.
9. X. Zhang, W. Chen, G. Bao and C. Zhao, Science and Technology of Welding and
Joining, 2004, 9(4), 379–76.
10. G. M. Alexander-Morrison, A. G. Dobbins, R. K. Holbert and M. W. Doughty,
Journal Materials for Energy Systems, 1986, 8(2), 79.
11. J. A. Hamill, Jr, Welding Journal, 1993, February 37–45.
12. G. W. Halldin, S. N. Patel and G. A. Duchon, Progress in Powder Metallurgy, 39,
267–280.
13. J. H. Davidson and C. Aubin, Proceedings: High Temperature Alloys for Gas
Turbines, 1982, Liege, Belgium, 853–86.
14. P. Adam and H. Wilhelm, Proceedings: High Temperature Alloys for Gas Turbines,
4–6 October 1982, Liege, Belgium, 909–30.
15. W. V. Knopp, Automobile Engineering Meeting Toronto, Canada, Oct 21–25,
Society of Automotive Engineers, 1974, 740984.
16. J. E. Middle, Chartered Mechanical Engineer, 1980, 27(7), 55–60.
17.L. J. Johnson, G. J. Holstand, M. J. O’Hanlon, Fall Powder Metallurgy Conference,
19–20, Detroit, Michigan, MPIF/APMI, 1971, 193–203.
18. W. A. Baeslack III and K. S. Hagey, Welding Research Supplement, 1988, July
1395–495.
19. P. Beiss, Powder Metallurgy, 1989, 32(4), 277–84.
20. W. V. Knopp, Materials Engineering, 1975, 12–75, 34.
21.K. Okimoto and T. Satoh, International Journal Powder Metallurgy, 1987, 23(3),
163–69.
22. N. Janissek, DVS Berichte, Proceedings Brazing, High Temperature Brazing and
Diffusion Welding Conference, no. 243, Auchen, 19–21 June 2007, 1–5.
23. H. Duan, M. Kocak, K. -H. Bohm and V. Ventzke, 2004, Science and Technology
of Welding and Joining, 2004, 9(6), 513–17.
24. A. Akutso and M. Iijima, 1985, Modern Developments in Powder Metallurgy, 16,
195–208.
25. T. Tabata, Nasaki, H. Susuki and B.G Zhu, International Journal Powder Metallurgy,
1989, 25(1), 37–41.
© Woodhead Publishing Limited, 2013
398
26.
27.
28.
Advances in powder metallurgy
J. C. Thornley, Welding and Metal Fabrication, 1972, 390–5.
T. Tabata and S. Masaki, International Journal Powder Metallurgy Powder
Technology, 1979, 15(3), 239–44.
C. Selcuk, Powder Metallurgy, 2011, 54(2), 94–9.
© Woodhead Publishing Limited, 2013