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International Journal of Computer Integrated
Manufacturing
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/tcim20
Different build strategies and computer-aided
process planning for fabricating a functional
component through hybrid-friction stir additive
manufacturing
Ankan Das, Tanmoy Medhi, Sajan Kapil & Pankaj Biswas
To cite this article: Ankan Das, Tanmoy Medhi, Sajan Kapil & Pankaj Biswas (2023): Different
build strategies and computer-aided process planning for fabricating a functional component
through hybrid-friction stir additive manufacturing, International Journal of Computer
Integrated Manufacturing, DOI: 10.1080/0951192X.2023.2228258
To link to this article: https://doi.org/10.1080/0951192X.2023.2228258
Published online: 26 Jun 2023.
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Dierent build strategies and computer-aided process planning for fabricating
a functional component through hybrid-friction stir additive manufacturing
Ankan Das, Tanmoy Medhi, Sajan Kapil and Pankaj Biswas
Department of Mechanical Engineering, IIT Guwahati, Guwahati, Assam, India
ABSTRACT
Hybrid Additive Manufacturing (HAM) technologies leverage the strength of Additive
Manufacturing (AM) and Subtractive Manufacturing (SM) in fabricating complex geometries with
precision. Friction Stir Welding (FSW) has been acknowledged for joining sheets of similar or
dissimilar materials with improved mechanical/metallurgical properties. However, due to the
inherent pinhole defect and complicated toolpath planning, this process has not been explored
much for fabricating the objects in a layer-by-layer manner. Therefore, this work investigates
Computer Aided Process Planning (CAPP) for realizing fully functional metallic parts through
a HAM system. This HAM system uses a Sheet Lamination (SL) process, i.e. Friction Stir Additive
Manufacturing (FSAM), synergistically coupled with an SM process, i.e. machining. To realize
complicated geometries through the proposed HAM system, three dierent build strategies are
developed viz. Near-net block fabrication, Near-net shape fabrication via ‘form-then-bond
approach’ and ‘bond-then-form approach.’ Each build strategy consists of a detailed CAPP that
includes toolpath planning, tooling aspects, simulated outcome and so on. Further, these build
strategies are discussed for realizing the parts with Functionally Graded Materials (FGM) &
embedded structures. Finally, a monolithic object of aluminium has been fabricated as a case
study to demonstrate one of the aforementioned build strategies.
ARTICLE HISTORY
Received 1 August 2022
Accepted 14 June 2023
KEYWORDS
Friction stir additive
manufacturing; machining;
build strategies; computer-
aided process planning;
tooling aspects; hybrid
additive manufacturing
1. Introduction
Sheet Lamination (SL) is an Additive Manufacturing
(AM) technique by which a 3D object is realized from
sheets (see Figure 1) of various materials like polymer,
ceramic, metal, composite and so on. SL can be clas-
sied based on the raw material, cutting method,
build approach, the bonding method and joining
region, as represented in Figure 2. Its economic
aspects include fast print time, large structures and
adaptability to complex designs (Mueller and Kochan
1999; Ngo et al. 2018). However, an in/post-
processing via Subtractive Manufacturing (SM) is
essentially required for realizing functional parts, the
absence of which shall end up with a block fabrication
and a high stair-case eect (Yi et al. 2004). Therefore,
SL is often utilized as a hybrid system by synergisti-
cally coupling with an SM process.
The metallic sheets are typically joined by diusion
bonding (Bournias-Varotsis et al. 2019; Shimizu et al.
2014), soldering/brazing (Obikawa, Yoshino, and
Shinozuka 1999), heat activated resins (Bhatt et al.
2019) and Friction Stir Welding (FSW) (Yuqing et al.
2016), followed by an in/post milling operation to
produce a near-net-shape with improved surface n-
ish (Norfolk and Johnson 2015). This study is primarily
focused on FSW-based SL process, known as Friction
Stir Additive Manufacturing (FSAM) (see Figure 3)
(Palanivel et al. 2015). Unlike the conventional Metal
Additive Manufacturing (MAM) processes (such as
Powder Bed Fusion (PBF) and Direct Energy
Deposition (DED)), where the layers are joined by
fusion bonding (Frazier 2014), the FSAM utilizes solid-
state bonding. FSAM can overcome several limitations
associated with conventional MAM, such as low struc-
tural capacities, pores, warpage, inhomogeneous
microstructures, etc. (Palanivel and Mishra 2017).
Versatility in material combination is also possible in
FSAM and machining, and such exibility is not avail-
able in the case of casting and machining. Moreover,
several researchers demonstrated the capability of
FSAM to fabricate multi-layered blocks of high
mechanical and microstructural properties (He et al.
2020; Lim et al. 2016; Srivastava et al. 2019; Yuqing
et al. 2016), polymer-metal composites (Derazkola,
CONTACT Sajan Kapil sajan.kapil@iitg.ac.in Department of Mechanical Engineering, IIT Guwahati, FR 01, Extension building, Guwahati, Assam 781039,
India
INTERNATIONAL JOURNAL OF COMPUTER INTEGRATED MANUFACTURING
https://doi.org/10.1080/0951192X.2023.2228258
© 2023 Informa UK Limited, trading as Taylor & Francis Group
Figure 1. Typical process flow of a sheet lamination process.
Figure 2. Classification of Sheet Lamination (SL) process.
Figure 3. Schematic representation of Friction Stir Additive Manufacturing (FSAM).
2A. DAS ET AL.
Khodabakhshi, and Simchi 2020) and tailor-made
Functionally Graded Composite Material (FGCM)
(Sharma et al. 2017; Srivastava and Rathee 2020).
Like other SL processes, the FSAM also needs to be
a hybrid system to fabricate fully functional compo-
nents. Typically, this hybridization is achieved via
synergistic integration of FSAM with a multi-axis
CNC machine (Sealy et al. 2018). Such hybrid-FSAM
processes can economically fabricate large-scale
multi-material objects in a short period of time.
Edison Welding Institute (EWI) demonstrated
a hybrid-FSAM system to manufacture a large-scale
3D object of Aluminum 7075 (Cruz 2016). Similarly,
other researchers have utilized Friction Stir Spot
Welding (FSSW) to locally join the cut sheets to fabri-
cated dies and tools (Abdel-All, Frank, and Rivero
2017; Frank, Peters, and Karthikeyan 2010). A unique
strategy was also suggested to determine the number
of FSSW points and their locations irrespective of the
layer geometry. Some of the essential aspects of
a hybrid-FSAM system are summarized in a shbone
diagram shown in Figure 4.
For synergistically & eciently integrating these
aspects in a hybrid-FSAM system, a detailed
Computer-Aided Process Planning (CAPP) is
required. Various data-driven steps associated with
the manufacturing cycle, starting from the product
design to the nished product end of life, are
termed the digital thread or digitization of process
chain, or CAPP (Bonnard, Hascoët, and Mognol
2019). The literature has reported many interactive
and knowledge-based CAPP frameworks for
individual machining and AM processes. A study
showed that energy consumption during machining
could be added to multi-criteria process planning,
and a mathematical model was developed to vali-
date the logic (Newman et al. 2012). In this context,
a multivariable energy optimization has been per-
formed considering the radial depth of cut, cutting
speed and the feed as main parameters. Also, the
dedicated model takes into account the energy
absorbed by dierent machine components
(Albertelli, Keshari, and Matta 2016). Milling process
planning and scheduling optimization are done in
two stages, a process stage and a system stage,
augmented with intelligent mechanisms for enhan-
cing the adaptability and responsiveness to job
dynamics in machining shop oors (Wang et al.
2015). However, in the past few decades,
a strategic sector of AM digital thread was based
on old numerical solutions such as STL and G-code.
Kim et al. (2015) treated the digital thread for AM as
‘a set of the interconnected manufacturing process,
end to end: from scan or design to analysis and
simulation, through build planning and fabrication,
to end use of the part, all connected in a series of
feedback and feed-forward loops. Due to the smart
factory revolution (Yoon et al. 2016), the Industry 4.0
concept (Lee, Bagheri, and Kao 2015), including
cyber-physical systems, the Internet of things (IoT)
and cloud computing, enables the surge of numer-
ical data exchange in various manufacturing pro-
jects (Lu and Xun 2017). A data-intensive digital
chain can lead to redundancy and loss of
Figure 4. Parameters of a hybrid-FSAM system.
INTERNATIONAL JOURNAL OF COMPUTER INTEGRATED MANUFACTURING 3
information, increasing the duration and cost of
a manufacturing project (Bonnard and Hascoet
2017). To deal with such issues, the Standard for
the Exchange of Product model data compliant
Numerical Control (STEP-NC) has been introduced
in AM digital thread. It is a unique le with informa-
tion on design and manufacturing and enables the
manufacturing of high-value products directly with-
out numerical data conversion or post-processing
(Bonnard et al. 2018). Notably, the digital thread/
CAPP for combining additive and subtractive
approaches was also developed by a few research-
ers. A design for the manufacturing system was
created (Kerbrat, Mognol, and Hascoët 2011) to
achieve quantitative information during the product
design stage so that both materials adding and
subsequent machining get the benet. In the case
of manufacturing complex geometries with greater
precision, the hybrid process was engaged (Zhu
et al. 2014) and entitled a novel process planning
algorithm, addressing tool accessibility, production
time and dimensional accuracy (Newman et al.
2015). From part inspection as a built-in functional-
ity, quantifying geometrical and dimensional part
deviations have been added to the hybrid process
plan by dynamically sequencing laser deposition
and milling (Behandish, Nelaturi, and de Kleer
2018; Stavropoulos et al. 2020). Hybrid systems
were also utilized in a remanufacturing context
(Paris and Mandil 2017). A unique system has been
demonstrated by utilizing articial intelligence (AI)
to support the process planning for machining and
3D printing (Rojek et al. 2021) that will be capable of
gathering the necessary knowledge, analyzing data
and drawing conclusions to solve problems. From
the literature, it has been found that available pro-
cess planning introduced generic steps which were
mostly related to material deposition-based AM and
machining. Relatively inadequate literature has been
found for the AM digital thread/CAPP frameworks
for a hybrid-FSAM system. Handling sheets as a raw
material instead of powder and wire itself bestow
dierent challenges on the researchers.
Furthermore, an FSW process inherently has
a pinhole defect at the end of the scanning pass
while joining two sheets. Therefore, the elimination/
minimization/optimization of the pinhole defects in
a part fabricated by a hybrid-FSAM system has not
been explored by researchers. Moreover, very little
knowledge is available in the context of a metallic
sheet-based solid-state AM, possible build strate-
gies, sequence of operations, toolpath strategy and
tooling.
This article reveals a detailed CAPP framework for
a hybrid-FSAM system and demonstrates three dier-
ent build strategies suitable for realizing a wide range
of fully functional objects. Moreover, it has addressed
the common tooling aspects and shown the fabrica-
tion of a monolithic object as a case study to demon-
strate one of the aforesaid build strategies.
2. Methodology
A multi-stage process planning is required for utilizing
a hybrid-FSAM eciently for fabricating fully func-
tional objects. The required operations involved in
a hybrid FSAM are grouped into three categories:
pre, in-situ and post-processes, as shown in Figure 5.
Primarily, the pre-processes involved the CAPP, where
a CAD model of the desired part is analyzed to com-
prehend the sequence/strategies of further opera-
tions to be carried out. Figure 5 shows the detailed
CAPP of a hybrid FSAM, starting with a CAD le as
input and ending with a suitable NC le generation.
The in-situ processes comprise actual layer addition
(through FSW) and subtraction operations (through
milling) to produce a near-net shape of the desired
product. The post-processes mainly consist of opera-
tions to convert the object’s near–near shape to the
Figure 5. Process flow of a hybrid-FSAM system.
4A. DAS ET AL.
desired actual shape with the required accuracy and
tolerances. Sometimes heat treatment may also be
required to enhance the product’s mechanical and
microstructural properties as a post-process.
It can be noted that this work proposes three
methods for realizing functional parts by hybrid-
FSAM, viz. (i) Near-net block fabrication, (ii) Near-net
shape fabrication via ‘form-then-bond approach’ and
(iii) Near-net shape fabrication via ‘bond-then-form
approach’. Each of these methods has unique benets
and limitations and should be eciently utilized
based on the availability of the machine tools and
the requirement of the products. The CAPP corre-
sponding to each of these methods will be dierent;
hence, they are explained in detail in the subsequent
sections.
3. Near-net block fabrication
In this method, the rectangular plates (of similar or
dissimilar materials) are joined together (by FSW lap
joint) to realize the bounding block of the object. The
fabricated block is then processed to obtain the
desired shape through machining. The process ow
of this method is shown in Figure 6(b). Firstly,
a metallic sheet and prebuilt layers (/substrate) are
held together and clamped adequately. The clamping
force must be large enough to withstand the shear
force exerted by the FSW tool during the joining of
layers. The required area of the sheet is joined with
the prebuild layers (/substrate) by multi-pass Friction
Stir Lap Welding (FSLW). An intermediate face milling
operation then removes the ash generated during
the joining of layers. It generates a smooth surface
with an accurate build height (Z-height). The next
layer is then added to the prebuild layers following
the same steps until the desired build height is
achieved. Finally, a rough machining operation is car-
ried out on the fabricated block, followed by a nal
nishing operation to obtain the exact shape of the
object. Notably, this method nds application in fab-
ricating multi-material objects and objects with inter-
nal cooling channels.
3.1. CAPP for near-net block fabrication
The detailed CAPP for the near-net block fabrication
has been illustrated in Figure 6(a). The CAPP began
with a CAD model of the desired product and
transformed it into a.stl le format. The model is
then uniformly sliced into multiple layers by moving
a slicing plane at a constant distance from the bot-
tom surface to the top surface (Kapil et al. 2017).
Each plane generates an intersection curve during
slicing, and those closed curves are commonly called
closed contours. Those closed curves are enlarged by
adding some oset value in the outward direction to
adjust miscellaneous allowance during operation.
The minimum oset distance should be the FSW
tool’s Shoulder Radius (SR). As this approach utilizes
rectangular sheets, tapping at the middle of four
sides on the oset area along the periphery and
then inserting a screw will hold any new layer.
Otherwise, strap clamps, swing clamping and
hydraulic clamping are also suitable for holding the
stacked sheets.
The fabrication process will adopt the bottom-up
approach, which demands toolpath planning from
the bottom layer itself. After stock allocation for the
two bottom layers, the FSW toolpath has been
planned using the slice area lling strategy. Those
resultant oset curves are a limiting boundary for
the FSW toolpaths. Subsequently, intermediate face
milling toolpaths are planned so that the entire top
surface can be machined and set ready for the next
layer addition. The workplace coordinates need to
adjust prior to the next layer addition to compensate
for the z height. In a similar fashion, toolpaths are
designed for the rest of the layers, and NC programs
are generated in a suitable machine control unit.
Then, each program le is transferred to the machine
unit to execute the operations, as shown in
Figure 6(b).
After achieving the desired build height,
a roughing toolpath is planned on the stock to elim-
inate the allowances and take out the near-net shape
of the product. The nal nishing toolpaths are sched-
uled on the pre-machined store. The simulation win-
dow delivered the exact outcomes after machining, as
shown in Figure 6(a), which conrms the completion
of the process planning stage. NC programs are writ-
ten afterward, followed by transferring les to the
machine unit to execute real-time operations.
3.2. Applications
The block fabricating strategy aims at engineering
applications such as investment casting die,
INTERNATIONAL JOURNAL OF COMPUTER INTEGRATED MANUFACTURING 5
custom support blocks, medical uid dispensers
and so on. Moreover, this approach can easily
manufacture monolithic, multi-material compo-
nents of prismatic geometry and engineered
beams.
4. Near-net shape fabrication via ‘form-then-
bond approach’
In this approach, we rst need to cut a near-net
prole from the sheet metal; then, the formed
shapes will be adjoined by FSW. Intermediate
Figure 6. (a) CAPP for near-net block fabrication process. (b) Schematic illustration of process flow in machine unit.
6A. DAS ET AL.
milling is performed for feature nishing and to
remove ashes, and the nal nish milling is car-
ried out to eliminate the stair-casing eect that
appeared on the build due to joining of thick
layers. More explicitly, internal features can be
split into a few layers such that they can be pre-
machined as a near-net shape. For channeling, it
is necessary to accommodate into a single layer
so that machining does not create any potential
problems. A pictorial overview of this suggested
strategy has been briey represented in Figure 7.
The actual practice of this strategy leads to less
material wastage and no chance of machining
chips clogging into the interior features.
4.1. CAPP for near-net shape fabrication via
form-then-bond approach
The complete CAPP module of this approach has
been shown in Figure 7(a), and the comprehended
operations interlinked with the CAPP module are illu-
strated in Figure 7(b). This approach follows similar
steps until osetting the intersection curves, as dis-
cussed in Section 3. In some cases, it has been
observed that uniform slicing fails to extract informa-
tion at a few locations of internal features. Therefore,
the adaptive slicing strategy is adopted here. Two
resultant intersection curves are generated during
slicing for a single layer, one at the top and another
at the bottom. Both curves will be the same for
Figure 7. (a) CAPP steps for form-then-bond approach. (b) Process flow in machine unit.
INTERNATIONAL JOURNAL OF COMPUTER INTEGRATED MANUFACTURING 7
a prismatic section, but they vary in the case of a free-
form model. On such occasions, the maximum dia-
meter curves needed to be chosen as a shape-cutting
toolpath to compensate for the post-machining
allowances. The NC program les of those curves are
then transferred to the sheet-cutting unit to prepare
the near-net proles. T-slot stepped strap clamps are
most suitable to x those cut sheets as each layer
geometry may vary along the building height. Some
vacuum chucks or magnets (for ferromagnetic sheets)
are also helpful for holding the sheet in place.
The stock material is allocated for the two bottom-
most layers, as the bottom-up approach has been
adopted here to fabricate the component. The FSW
toolpath is planned using a slice area joining strategy
followed by intermediate face milling for z-height
accuracy. In the case of internal channel making,
a feature slot machining strategy is planned with an
active 2D pattern along the centreline of that entity.
Preferably, a ball end mill tool is selected for channel-
ing, having a semi-circular cross-section in the XY
plane. However, any circular section of the internal
channel can also be realized by ipping two layers
having the same semi-circular sections and joining
them together. The channel depth is achieved by
giving constant step-downs of the tool having an
equal diameter of the channel. This strategy will be
applicable for any channeling (cooling purpose or
electric wiring purpose) on an XY plane, and the
same can be performed on successive layers if any
channel is available in the CAD model. Another inter-
nal feature is a cylindrical hole that splits into three
consecutive layers. It is planned to machine the hole
during near-net layer preparation. Only a nal nish-
ing is planned along its internal periphery to retain
dimensional accuracy. It is recommended that any
internal feature be machined after the intermediate
face milling is done. This will prevent any height loss
of that feature. Most importantly, the FSW toolpaths
need to be planned so that a minimum number of
tool lifts occur during operation, and these pin exit
holes can be dumped at the outward oset boundary
by the toolpath end point manipulation strategy. In
this way, after joining the last layer, a near-net shape
of the product is achieved. Then, a nal nishing
strategy is planned to execute on that near-net
shape of the build, which gives it the desired shape.
Dierent simulated outcomes after each processing
stage are also shown in Figure 7(a). After inspecting
those outcomes, the NC programs are written on
a suitable machine control unit and transferred to
the machine library. As shown in Figure 7(b), each
operational step is executed.
4.2. Applications
This is a practical approach for constructing large
metallic parts with solid-state thermal bonding. This
approach may apply to realizing prismatic geometry,
internal features, channels, embedded structures or
intelligent components. This strategy’s targeted appli-
cations are heat exchanger extended ns, aero space
wings, fuselages, empennages, Bumper beams and
so on.
5. Near-net shape fabrication via ‘bond-then-
form approach
In this approach, an FSW tool is retrotted on a CNC
center which runs over the pre-planned tool path and
joins the sheets. Then, a milling cutter cuts the
required shape along with its outer periphery from
those thermo-mechanically bonded layers. Again,
a new sheet is deposited on the previous layer, and
the same process will continue until the additively
built object is completed, a process illustration
shown in Figure 8(a). This machining center can also
generate features in between the layer joining pro-
cess. However, machining chips need to be removed
from the internal features before adding a new layer
on top of it.
Large parts are easily fabricated through this
approach. The most advantageous side of this strat-
egy is the fabrication of multiple products in a single
setup and from a single sheet. Provided that all pro-
ducts must share equal layer thickness at a time, as
shown in Figure 8(b), three bottom three layers share
t
1
thickness, and the top four layers share t
2
thickness.
5.1. CAPP for near-net shape fabrication via
bond-then-form approach
The detailed CAPP for this strategy has been illu-
strated on a hemispherical dome-like structure,
shown in Figure 9. The uniform slicing of the CAD
model is unable to retain the exact boundary curve
for the top inner surface of the dome. This issue is
resolved by performing re-slicing between two
8A. DAS ET AL.
Figure 8. Illustrating process flow of bond-layer-and-then-form strategy (a) and equal layer height at a time for all products (b).
Figure 9. CAPP process flow steps for near-net-shape fabrication via bond-then-form approach.
INTERNATIONAL JOURNAL OF COMPUTER INTEGRATED MANUFACTURING 9
existing slices with a very minimum gap, as shown in
Figure 9. The FSW toolpath is generated with a slice
area joining strategy. In this approach, instead of
a limiting boundary which is used in the last two
CAPPs, a 2D pattern has been incorporated to indicate
the working area. This eliminates unnecessary tool
movement. A slot machining toolpath is planned
using a 2D curve prole to remove that extra sheet.
Intermediate milling has been planned under a 2D
curve area instead of opting for a direct face milling
strategy over the whole area. These steps are followed
for all consecutive layers. After each toolpath plan-
ning, the respective NC program is written and trans-
ferred to the machine library. The feature machining
halts due to the absence of any internal feature in the
CAD model. Meanwhile, the direct roughing strategy
has been implemented on a pre-casted model allo-
cated as a stock material to visualize the real-time
machining operations. Moreover, it delivers the
exact path planning for nal nishing, eliminating
unnecessary space cutter movement. That pre-
casted model is the result of all layer joining steps. It
indicates the near-net shape dome-like structure with
a stair casing eect.
A roughing strategy is planned on both sides of the
built, followed by a nal nishing strategy that
bestows the nal component’s desired shape. The
nal simulated outcomes of the desired product are
shown in Figure 9. The CAPP ends with NC code
generation for the nal machining operation.
However, the size dierence between the prebuilt
part and the new blank creates clamping issues.
A sizeable stepped vice jaw clamps temporarily hold
the blanks so that friction stir spot welding (FSSW) can
be carried out along the periphery at multiple spots.
Depending on the prebuild cross-sections, FSSW
numbers and locations may alter, and its algorithm
has been developed by Abdel et al. (2017).
5.2. Applications
This building strategy is aimed at many industrial
applications such as manufacturing large machine
components, shutter stocks, rocket engines, rotary
vane steering systems in marines, aerospace engine
compartments, yachts and so on. Furthermore, it can
be useful for batch production also.
The targeted applications addressed against each
build strategy can have their traditional routes of
manufacturing like casting, then machining and so
on. Here are some key advantages of using FSAM
and machining, highlighted in bullet points as
follows:
●Solidication defects (phase-transitional defects)
are associated with casting, whereas FSAM is
a solid-state process that works below the melt-
ing temperature of the substrate.
●Material combination compatibility in fabricating
functional components is possible in FSAM and
machining. Such exibility is not available in the
case of Casting and Machining.
●Products can be fabricated within a single hybrid
setup.
6. Multiple tooling aspects for proposed HAM
The FSAM-based HAM can fabricate monolithic com-
ponents and multi-material and FGM parts. However,
before starting the fabrication process, some essential
common aspects need to be addressed, such as the
minimum stock size estimation, number of layers’
estimation, selection of clamping mechanism, over-
lapping toolpath percentage, exit pinhole elimination
strategy, sheet cutting mechanism, types of joining
variation w.r.t. components and conditional toolpath
altercation during layer joining.
The single-track and multi-track joining is per-
formed depending on the component’s size and
application specicity, as shown in Figure 10. The
estimation of minimum stock size has been discussed
with the help of a single-track monolithic wall. The
minimum width (W) of the wall depends on the stirred
zone area and the clamping allowance left on the
bonded sheets. The stirred zone area directly depends
on the tool geometry, especially on shoulder dia-
meter. One of the important aspects of the eective
stirred zone is the shoulder-to-pin diameter ratio
(SPR). The SPR ratio is the ratio between the shoulder
diameter and the pin root diameter, irrespective of
pin geometry, as shown in Figure 11(a). It is critical
because the FSW tool shoulder is responsible for 80%
of frictional heat generation during the process
(Ahmed et al. 2021). So, the dierence between
shoulder and pin root diameter can be regarded as
the eective shoulder diameter which rubs against
the substrate. Notably, the SPR aects the weld qual-
ity, and its general range is mostly 2:1 to 5:1 (Mehta
10 A. DAS ET AL.
and Badheka 2016). However, this SPR range varies
with the sheet thickness. The minimum width of
a layer required to achieve a single-track monolithic
wall is given by
W¼SD þ2CA½ (1)
where W is the width of the wall, S
D
is the shoulder
diameter and C
A
is the clamping allowance dump on
both sides of the sheets. The approximate stock
length depends on the working approach of the
FSW. During welding, the FSW tool left some
unbonded areas at the beginning and end. So, the
minimum length (L) of a single layer can be given by,
L¼lþ2SD(2)
where l is the original design length, S
D
shoulder
diameter. Flash formation during FSW is a common
occurrence, and it depends on the plunge depth,
shoulder prole atness, excessive frictional heat gen-
eration and many other aspects (Mehta and Badheka
2016). The removal of ash materials with intermedi-
ate face milling results in the reduction of each layer’s
thickness. Gradually leads to the height reduction of
the fabricated part. The CAD model height (H) is
segmented into layers; then, the reduced height
needs to be accommodated by adding more layers
to the build. The total number of layers (N) required
can be calculated from
N¼Ht1
tα
þ1 (3)
where H is the model height, t
1
is the rst layer thick-
ness and t is the thickness for the rest of the layers.
Assume α is the layer thickness reduction during
intermediate face milling.
Stacking a new sheet over the prebuilt part is a very
typical process. Currently, many mechanisms are
available to the industries to execute such work.
A widespread tool in practice is the pick-and-place
mechanism, where a conveyor belt is used to get the
sheets to the machine unit, and the same sheet is
lifted by mechanical means and placed on the past
layer. However, the pick-and-place robots are very
sophisticated for this purpose, where the arm moves
the end eector to grip the sheet and place it in the
Figure 10. Single-track (a), and multi-track (b) and multi-layered monolithic component. Conventional clamping (c). Clamping with
Friction Stir Spot Welding (FSSW) (d).
Figure 11. (a) Stirred zone w.r.t. Shoulder-to-Pin Ratio (SPR). (b) Overlapping toolpaths for two layers.
INTERNATIONAL JOURNAL OF COMPUTER INTEGRATED MANUFACTURING 11
required location (Manyar et al. 2022). The controller
is used to operate the arm’s position and movements,
keep track of time and monitor the movements of the
manipulator. This controller comes in four categories:
mechanical, hydraulic, electrical and pneumatic.
In the case of large-size components, a multi-track
welding strategy (shown in Figure 11(b)) is utilized to
join the area. Here, the overlapping welding pass is
essential for fabricating a defect-free dense micro-
structural component. It has been identied that the
stirred zone overlapping in neighbouring welding
passes minimizes or eradicates crack-like unbounded
defects, usually called hooking, which is quite inher-
ent in the Friction Stir Lap Welding (FSLW) process
(Zou et al. 2017). The overlapping percentage (OP) of
the stirred zone is identied by the tool pin prole
and the step-over between the two neighbouring
welding tracks. OP for two adjacent welding tracks
can be given by,
OP ¼1L
D
100
%(4)
where L is the central axis distance between two
successive welding tracks, and D is the FSW tool pin
diameter.
Dierent areas joining toolpath patterns and exit
pinhole elimination strategies have been discussed in
this paragraph. In machining toolpath, tool lifts are
very common practice, but in the FSW process, tool
lift results in pin exit hole creation. However, in the
general welding process, an FSW tool follows a weld
seam line which may be a straight or curvilinear path,
and a pinhole appears at the end of the path. In a few
cases, pin holes are eliminated by post-machining, or
sometimes pinhole remains on the part. Nowadays,
researchers have developed some pinhole-lling
techniques such as metal chips inserted into the pin-
hole (Bhardwaj, Ganesh Narayanan, and Dixit 2019),
consumable tool pins (Bhardwaj, Narayanan, and Dixit
2022) and self-retracting tool pin (Ding and Oelgoetz
1999). However, FSAM deals with large cross-sectional
area joining in which dierent area scanning tool-
paths are utilized, unlike normal FSW toolpath. Basic
toolpath patterns work very well for any regular cross-
section, as shown in Figure 12(a-f). Any CAM platform
is capable of designing such basic path plans.
However, there are a few developments found in non-
retractable toolpath like Hilbert (Catchpole-Smith
et al. 2017), Trochoid (Rajput et al. 2022) (shown in
Figure 12(g, h)) and direction favouring toolpath
(Singh et al. 2022) planning for AM irrespective of
cross-sectional geometry. It implements a Travelling
Salesman Problem (TSP)-based algorithm to generate
a toolpath for ecient and accurate area lling with
fewer tool retractions and sharp turns. Dierent tool-
path strategies have been incorporated in FSW path
planning for a complex cross-section in FSAM, shown
in Figure 12(i). After several iterations of path plan-
ning, the FSW tool lifts were reduced at one time; as
a result, pin exit holes have been minimized/opti-
mized, as shown in Figure 12 (i: last pic). Although
the pin holes are minimized, one pin hole is still
occurring at the end of the toolpath. To overcome
this issue, a new subroutine of lead extension has
been added to the toolpath planning digital chain. It
extends the toolpath end at an absolute point that
can be user-dened. A dedicated outward oset
boundary has already been allocated with slice area
information in the CAPP module/digital thread to
accumulate the pinhole. A toolpath design algorithm
digital chain for FSW pinhole optimizing/minimizing
has been shown in Figure 12(j). This can be useful for
all the CAPPs as FSW path planning is common to any
build strategies discussed earlier, as shown in Figure
(5-9). The essential dierence in FSW path planning is
observed in internal feature integrated slices where
oset toolpath or restricted area toolpath strategy is
mandatory. The digital chain simulation window
enables a user to visualize the modied path plans
and move further. This results in the fabrication of
pinhole-free products with less material wastage. In
exceptional cases, it is possible to move the FSW tool
along the peripheral boundary of the geometry rst
and then join the rest of the space inside. The users
can decide what to set on priority depending on the
cross-sectional prole of the model.
The proposed HAM process can be useful for fab-
ricating functionally graded materials (FGMs) such as
multi-material alloys and sandwich structures, as
shown in Figure 13(a,b). These structures can be rea-
lized by FSAM very easily. Many researchers have
already investigated and established the successful
joining of dissimilar materials by FSW in the past few
years (Simar and Avettand-Fènoël 2017). However,
the radially varying alloy components, shown in
Figure 13(c,d), demand a dierent process planning
strategy and unique joint conguration. The intro-
spective process plan for those structures is shown
12 A. DAS ET AL.
Figure 12. Raster pattern (a); zig-zag pattern (b); contour parallel offsetting pattern-inward and outward (c, d); spiral pattern inward
outward (e, f); Hilbert curve pattern (g); trochoid (h); hybrid toolpath patterns (i); pin-hole-free area toolpath design algorithm digital
chain (j).
INTERNATIONAL JOURNAL OF COMPUTER INTEGRATED MANUFACTURING 13
sequentially in Figure 13. The possible slicing direc-
tions are shown in Figure 13 (d1, d2). The d1 slicing
strategy raises the welding problem for drastically
varying alloy thickness in similar/dissimilar materials.
This is because in the case of lap-joint conguration,
the upper layer thickness plays a vital role in layer
interface bonding (Behmand et al. 2016). As the layer
thickness increases, the heat input decreases at the
two-layer interface; as a result, improper weld pene-
tration in the bottom layer (Chitturi, Pedapati, and
Awang 2020). It reduces the material mixing at the
bottom layer’s stirred zone, which is attributed to less
joining strength (Zhang et al. 2020). Recently, the
maximum layer thickness joined in FSAM was 6.3
mm for a similar material (Lim et al. 2016). So, the d2
slicing strategy is more convenient for fabricating this
FGM part. The d2 slicing strategy asserts multiple joint
congurations like butt, lap and butt-lap. In this case,
the Form-then-Bond build strategy has been imple-
mented to realize the part. First, the bottommost layer
is assembled with a butt joint strategy. Then, from
the second layer itself, the formed layer shapes are
directly added to the prebuilt bottom layer by means
of the lap-butt strategy. The same strategy will con-
tinue until the build is complete. The rough milling
followed by the nal nish milling ascertain it is
a near-net shape. This type of FGM fabrication strat-
egy demands more introspection into the material
joining properties and their acceptability in real-
world applications. More importantly, for the shape
cutting from a blank sheet, several options are avail-
able for an industrial partner, such as plasma cutter,
laser cutter, or any mechanical means. Nowadays,
abrasive-water jet cutting is also used to cut thick
sheets of hard alloys (Hlavacova and Geryk 2017).
The embedded structures fabrication can be a unique
application for this proposed HAM. In Figure 14(a,b),
insulator-wrapped optical bers and electronics are
embedded in metallic structures, which portrays the
knowledge of a smart structure assembly. Notably, the
fabrication strategy for such components is genuinely
material-specic and demands relevant application-
oriented process planning.
These structures mostly deal with internal features
like holes, pockets, channels, or any arbitrary geometry.
So, an important aspect of such a fabrication strategy is
Figure 13. Build strategy of FGMs. Multi-material component (a); sandwich structure (b); radially varying alloy component (c, d); an
example of fabricating FGMs: Slicing strategy (d1, d2) and joining strategy (d2-a; d2-b).
Figure 14. Schematic of fibber-embedded part (a) and smart structure (b).
14 A. DAS ET AL.
the FSW toolpath restriction methodology. For example,
a hollow feature exists on the N
th
layer, as shown in
Figure 15(a); the toolpath on N
th
and (N + 1)
th
layer must
follow an oset feature contour parallel pattern, such
that the FSW tool does not enter the restricted zone.
Moreover, the minimum oset distance that needs to be
maintained from the feature boundary is equal to the
tool solder radius. This strategy prevents unnecessary
distortion of internal feature geometry and saves the
embedded part from any potential damage.
Sometimes, it has been observed that preventive
measures fail to serve the purpose due to excessive
plunge depth and dwelling time. This results in material
bulging along the boundary, as shown in Figure 15(b).
A nal feature nishing strategy will machine those
bulges.
7. Case study
A case study has been performed to demonstrate the
capability of the proposed build strategy in Section 2.
Aluminium alloy is the most commercially usable
material and shows good weldability in response to
FSW. That is why this case study has been conducted
on an aluminium built casing with two compartments
of dierent shapes, as shown in Figure 16. This type of
part can be used in multiple commercial applications
such as battery casing, gearbox housing and so on.
7.1. Computer-aided process planning and
experimental setup
This model is rectangular, no internal features are
present and relatively small in size; based on all
these arguments, a near-net block fabrication strategy
and a bottom-up approach have been adopted for
fabrication. Mandatory CAPP process ow steps are
shown in Figure 17. The required number of layers has
been calculated from Equation 3, where α is taken as
0.2 mm. The bottom three layers have followed the
raster pattern for the FSW toolpath plan, as shown in
Figure 17(c). In the top six layers, two compartments
are present, and the similar raster path plan for those
layers is shown in Figure 17(f), which is very
Figure 15. Showing fabrication strategy of embedded parts (a); bulges appear on boundary area (b).
Figure 16. Case study model: Casing.
INTERNATIONAL JOURNAL OF COMPUTER INTEGRATED MANUFACTURING 15
inconvenient due to unnecessary tool lifts along the
built periphery. On that account, a customized path
plan is designed, as shown in Figure 17(e), in which all
welding passes are planned to be performed along
the peripheral boundary of the building. A total of
four passes in the Y-direction and ve in the
X-direction. An intermediate face milling strategy is
applied to remove the ash after each layer joining
and smooth surface preparation for the next layer.
Moreover, at the end of welding, all exit pinholes are
dumped on that layer’s top and right sides. After
joining all the layers, rough milling toolpaths are
planned on that built block (shown in Figure 17(g))
to give it a near-net shape, followed by a nal nish-
ing (shown in Figure 17(h)), which ascertains the exact
shape of the product.
This case study was carried out on a vertical milling
machine modied into a HAM setup, shown in
Figure 18(a). A exible sheet clamping setup was
prepared to hold those sheets, as shown in
Figure 18(b). An FSW tool, a face milling cutter and
an end milling cutter were retrotted alternatively to
execute the process, as shown in Figure 18(c-e).
AA6061-T6 sheets (200 mm × 150 mm × 4 mm) are
used for product fabrication. This plate size includes
all kinds of clamping allowances, exit hole allowances
Figure 17. CAPP process flow for the case study.
Figure 18. Multi-axis CNC controlled FSAM-based HAM setup (a); sheet holding flexible clamping fixture (b); FSW tool (c); face milling
cutter (d); end milling cutter (e).
16 A. DAS ET AL.
and machining allowances. An FSW tool made of H13
tool steel was selected, having a shoulder diameter of
18 mm, a cylindrical taper pin prole with 7 mm and 5
mm as root and tip diameter, respectively, and a pin
length of 5 mm.
During fabrication, all welding passes were per-
formed in a pre-planned sequential manner.
Dierent welding and machining operations carried
out during this case study are shown in Figure 19. All
the process parameters used during the fabrication
process are listed in Table 1.
7.2. Results and discussion
This case study established the manufacturability of
a functional part by FSAM-based HAM. It gives
a better understanding of the selection of the build
strategies and tooling aspects during process execu-
tion. The case study reveals the importance of CAPP,
where an aluminum casing has been fabricated utiliz-
ing an aforesaid build strategy. It estimated the mini-
mum stock size and the exact number of layers (9)
required to achieve the model height. It also enables
the scope of selecting a better joining strategy for
Figure 19. Experimental steps. Layer joining in Y-direction (a, c). Intermediate face milled plate (b). Layer joining in X-direction (d).
Fabricated block (e). Block after machined out machining allowance (f). Face milling operation (g: Top face, j: Bottom face). Rough
machining with end mill cutter (h). Final finish milling (i).
Table 1. Process parameters.
Process name Parameters Selected values
Joining FSW tool rotational speed 600 rpm
Welding speed 60 mm/min
Tool plunge feed rate 0.7 mm/min
Tool tilt angle 1.5°
Welding toolpath step over 6 mm
Total number of sheets required 9
Total number of passes 78
Intermediate face milling Step down 0.2 mm
Spindle speed 400 rpm
Cutting feed rate 1500 mm/min
Plunging feed rate 200 mm/min
Spindle speed 600 rpm
Rough milling Step down 5 mm
Cutting feed rate 180 mm/min
Plunging feed rate 30 mm/min
Final finish milling Spindle speed 300 rpm
Step down 1 mm
Plunging feed rate 60 mm/min
Cutting feed rate 200 mm/min
INTERNATIONAL JOURNAL OF COMPUTER INTEGRATED MANUFACTURING 17
dierent cross-sections. As shown in Figure 17(f), 13
overlapping welding passes were estimated for one
layer, which turns into 78 passes for the other six
layers and multiple tool lifts, resulting in more pin-
holes appearing in the building part. However, in
another path planning, shown in Figure 17(e), only
54 passes were estimated with minimized tool lifting,
and pinholes were dumped in an identical location
each time. As a result, the fabrication process takes
comparatively less time and delivers pinhole-free cas-
ing. Above all, this study revealed that the simulated
results are in good match with its experimental
results, shown in Figure 20(a). It takes approximately
5 hr to complete the fabrication experimentally.
However, some scratch marks of machining are still
present on the build part, which urges the selection of
optimized machining parameters. It may be necessary
to increase the welding speed to realize large func-
tional parts. Moreover, this study ascertains the
acceptability of three dierent build strategies of
FSAM-based HAM and their dedicated CAPPs.
All the above strategies are capable of fabricating
a particular product. However, depending on the com-
plexity, tooling arrangement and material saving fac-
tors, the user has to choose a suitable build strategy. A
product with hollow features or internal channels will
create diculties in removing the machining chips if
Near-net Block Fabrication and Bond then Form strate-
gies are chosen. Moreover, the heat exchanger
extended ns-like structures are an appropriate exam-
ple of the ‘Form-then-Bond’ approach. This is because
in the conventional approach, they are prepared from
an ingot through machining where lots of material
wastage is typically unavoidable. However, in
Figure 20(b), an extended bimetallic n and cooling
plate structure have been portrayed that can quickly
fabricate through ‘Form-then-Bond’ approach.
Moreover, a demonstration has been made by fabricat-
ing an electronic embedded aluminum block through
the aforesaid ‘Form-then-Bond’ approach. Notably, the
functionality of the PVC-K-24 wire has been checked by
connecting a power source and a laptop cooling fan at
two ends of the wire, shown in Figure 20(c). Here, two
wires were inserted into a pre-machined grove, and
then the layers joined on one another. While joining
layers, two dierent build strategies were followed,
with and without osetting the FSW tool from the
grove periphery. Without an oset of the FSW tool,
the wire gets damaged due to the melting of the
insulator coating. In another case, the wire remained
in good condition as the tool oset was maintained by
10 mm from the grove periphery.
In a few words, the fundamental dierence between
the aforesaid build strategies (Sections 3 to 5) has been
highlighted. In the ‘Near-net Block Fabrication’ strat-
egy, each layer was placed as rectangular or square-
shaped sheets irrespective of the CAD geometry. This
ends up with a cubical block-shaped structure, and
then machining will give its nal shape. This strategy
is mainly dedicated to prismatic geometries with cubi-
cal shapes, whereas in the case of ‘Near-net Shape
Fabrication via Form-then-Bond approach’, each layer
was cut out from a raw sheet w.r.t. to the CAD model’s
boundary information. Those formed layers are joined
accordingly by FSW to achieve the near-net shape, and
a nal nishing operation realizes its original shape.
This strategy is solely dedicated to products having
prismatic or free-form surfaces, any at under-cut sec-
tions, or internal features or embedded parts. In the
third strategy, ‘Near-net Shape Fabrication via Bond-
then-Form approach’, two blank sheets were placed,
and then the FSW tool moved over the required sec-
tions, and after joining the layer, a milling cutter
cuts the remaining sheet along the periphery and
gives it near-net shape. Then again, a new blank
sheet was placed over the prebuild part, and the
exact steps were followed till the part was complete.
The essential advantage of this strategy is the ease of
fabricating multiple components in a single setup at
a time, which is impossible through the other two.
Although dierent strategies are useful for dierent
kinds of products, a general comparison is shown in
Table 2, based on the CAPP module discussed in each
section of the build strategy. From each step of the
digital chain/CAPP, the statistics part has been scruti-
nized for quantitative comparison, and qualitative deci-
sions were made based on the complexity of the CAD
model.
8. Conclusions and future work
The proposed HAM and its build strategies open up
a new avenue in MAM with a solid-state joining pro-
cess. It can handle thick layers, resulting in rapid tool-
ing and fast fabrication of functional parts. Moreover,
from this study, the following conclusions can be
made:
18 A. DAS ET AL.
Figure 20. (a) Simulated outcome from CAPP (top left corner) and experimentally fabricated aluminium casing. (b) An extended
bimetallic fin and cooling plate structure. (C) Aluminium-built embedded part.
Table 2. General comparison of three proposed build strategies.
Miscellaneous aspects Near-net block fabrication Form-then-bond approach Bond-then-form approach
Ease of CAPP ++ ˗+
Layer preparation + ++ -
Clamping + ˗++
Ease of layer joining + ˗++
Machining time ˗++ +
Internal features generation + ++ +
Total built time ˗++ +
Large component productivity ˗+ ++
Reduction of material wastage ˗++ +
Dimensional accuracy ++ ˗+
Complex geometry fabrication ˗++ +
Here, ’+’ represents convenient; ’++’ represents most convenient and ‘˗’ represents unlikely.
INTERNATIONAL JOURNAL OF COMPUTER INTEGRATED MANUFACTURING 19
●A complete introspective framework for FSAM-
based HAM has been outlined, and starting from
a CAD model to complete product fabrication
steps has been established successfully.
●Three dierent build strategies have been exhib-
ited with three dierent CAD models. Their CAPP
has been developed with detailed path planning
and simulated outcomes. Moreover, the target
applications of individual build strategies have
been proposed.
●An FSW toolpath planning algorithm has been
developed to optimize the toolpath, minimize
the tool lifts and eliminate the exit pinhole from
the actual scanning area, irrespective of the geo-
metrical complexity.
●Various tooling aspects have been analyzed, and
mathematical formulas have been derived to esti-
mate minimum stock size, number of layers and
so on. Moreover, this study yields deep insight
into the necessary toolpath strategy altercations
in the case of FGMs and embedded structures.
●An aforesaid build strategy has demonstrated
a case study on the aluminum casing. It established
the critical importance of CAPP for the proposed
HAM and revealed that the experimental and simu-
lated outcomes are highly compatible.
●An electronic embedded part was fabricated by
Near net shape fabrication via Form then Bond
approach and conrmed the functionality of the
embedded wire was intake.
This article represents a few essential fabricating
strategies, CAPPs, pinhole elimination algorithms
and tooling aspects. However, in view of the
industry 4.0 perspective, a few more research
scopes need to be addressed, like the automation
of the sheet stacking at an accurate position and
clamping process. Notably, while integrated with
a digital thread, the data server can control the
design analysis, adopt an appropriate slicing strat-
egy, build orientation selection and individual
steps of path planning. Moreover, the mechanical
and tribological testing results can be utilized as
the dening factor for the life cycle assessment of
the fabricated product. The digital thread can be
extensible, modularity provider and scalable, while
existing systems are compatible with technology
evolution. However, the numerical analysis of the
force and thrust exerted by the FSW tool on the
overhang part needs to be studied rigorously.
Acknowledgements
The authors acknowledge the contribution of the Central
Workshop members and Advance Welding and Fabrication
Lab research group members at the Indian Institute of
Technology Guwahati, India.
Disclosure statement
No potential conict of interest was reported by the author(s).
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