Abstract. In the construction industry, wire-based steel 3D printing (WBS-3DP) technology for structural parts
has yet to be explored.
As 3D printing becomes highly productive and cost-effective with the development of
qualification standards, it opens a wide range of opportunities for the steel-based additive construction segment
too. The major advantage of large-scale 3D printing techniques like wire-arc additive manufacturing (WAAM) is the
ability to develop components that are more compatible through topology optimization and with the mechanical
requirements of the structural standards. This research aims to discuss the application of WAAM in the fabrication
of space frame structures for T-K-Y joints with structural integrity. In this study, a space frame structure was
designed and developed with multi-branch tube connectors using WBS-3DP technology. These tube connectors
are used to accommodate any number of steel tubes at arbitrary angles, such that the resulting T-K-Y joints are
smooth and lightweight. The prototype space frame structure with a multi-branch tube connector is designed to
demonstrate how theWAAM process can produce more efficient structures by eliminating challenges involved
in tube joining and welding. Mainly there are three major challenges in the 3D printing of the tube connectors,
such as transforming T-K-Y joints into a topology-optimized tube connector design; developing a tool path for the
tube connector model, and printing the model by overcoming the issue of heat AQ1 management. This paper also
aims to bridge the gap between WBS-3DP and its application in the construction industry. It was concluded that
WBS-3DP has the potential to revolutionize the construction industry by producing innovative parts such as tube
connectors with ease of fabrication and improved digital construction techniques.
Keywords: WAAM · Tube connectors · T-K-Y joints · WBS-3DP
Introduction
Additive Manufacturing (AM), also known as 3D printing, is
a layer-based fabrication process that involves depositing thin
layers of material together based on a 3D computer model
to generate a physical model. Manufacturing industries such
as aerospace, automotive, power and energy have a strong
interest in realising metal 3D printing’s full potential, whereas
steel 3D printing in the construction industry appears to be
moving at
© The Author(s), under exclusive license to Springer Nature
Switzerland AG 2024
M. J. Tan et al. (Eds.): 3DcP 2023, STAM, pp. 1–12, 2024.
https://doi.org/10.1007/978-3-031-64269-2_31
• A. K. Perka et al.
a much slower pace for a number of reasons, such as the size
of the parts to be printed, high load capacity requirements and
service life of the structures etc., (Anis et al. 2019; Laghi et al.
2019).
Steel tubes are frequently used in the construction
industry for several applications such as trusses, frames, space frames etc., (Hassani et al. 2019). Some of the most
challenging structural steel welding and inspection problems
are encountered during the construction of round tubular
members, especially at the T, K, and Y end connections. The
conventional way of TKY connections fabrication requires
several steps such as, profile cutting of the tubes followed by
the edge preparation and joint fit-up assembly for welding as
shown in Fig. 1 (Kühne et al. 2019). Also, there are many specific
items that should be taken into consideration when planning
for structural tubular fabrication. Even more so than for other
types of structural fabrication at all phases such as welding
qualification, dimensional control, and non-destructive testing,
etc. The disadvantages of directly welding tubular joints include
stress concentration in the welding pass, high residual stress
in the members, design requirements in the fabrication and
assembly process, and great difficulties of fabrication when the
braces overlap each other. Also, in TKY joints, the importance
of complete joint penetration groove welds, eliminating “notch
effects” at the root and notably the cap of node welds, and
achieving the requisite weld profile have all received increased
emphasis. Itmay be necessary to grindwelds that are crucial
for fatigue endurance to a smooth curve. Brittle failure is less likely because of this approach. However, it also means that
fabrication and quality assurance/quality control requirements
are becoming more advanced and severe.
The typical welding procedure for TKY joints welding as per
the global standards is shown in Fig. 2 (Chen et al. 2019). The
tubulars will connect each other at an angle ranging from 30° to
90°. The interface between these profiles is divided into three
parts: crown, heel, and saddle. These profiles are only welded
using manual welding methods and require a different skill set
to do welding in each of these zones. These directional tubes
are capable of transferring forces in various directions, which
makes them ideal for interconnecting tabular joints in high-rise
building applications. Hence, the loadcarrying capacity of the
connection completely depends on the quality of the welding
along the profile (Shao 2007).
Alternatively, the TKY joints were proposed to be
manufactured using several methods using customized
designs to simplify the challenges involved with conventional
welding methods. Custom-designed tube connectors have
major benefits such as lower weight, and faster construction
by reducing the lead time and also the cost of the component
while ensuring structural performance. (Wang et al. 2020).
However, such tubular connection designs were observed
to have geometrical complexities, and curvature patterns
that were highly challenging to build using conventional
fabrication methods such as milling or casting or subtractive
manufacturing. The higher carbon content of cast steel also
possesses weldability issues and thickness defects compared
to conventional hot-rolled steel products. (Herion et al. 2010;
Wang et al. 2013). On the other hand, 3D printing will help in
resolving such challenges in manufacturing complex shapes
with better process control. This approach will also solve the
skilled personnel shortage that the steel construction industry
is now experiencing.
Based on a comparative analysis of the benefits & drawbacks
of directly welded tubes, this research proposes a new method
for fabricating tubular joints using wirebased steel 3D printing
(WBS-3DP). The major objective of this research is to study the
feasibility of joining steel tubes using 3D-printed connectors.
The innovative welding tubular junctionwill attract greater
interest and further investigation due to its simple fabrication
approach, precise dimensioning, and ideal mechanical qualities.
This research
• A. K. Perka et al.
will be beneficial in the field of digital construction. The basic
knowledge needed for structural optimisation with the 3D
printing method and the material characterisation has been
summarised with the various steps involved in the printing of
tube connectors.
Materials and Methods
• Prototype for Tubular Joints Design
A prototype spaceframe structure model was designed
as shown in Fig. 3, in such a way that the designed model
can accommodate all possible types of T-K and Y-type tube
connections. The design consists of 3 truss structures and 2
side frame structures using various diameters and thicknesses
of tubes. The lower part of the structure was designed using
tubes of a diameter of 114 mm and thickness of 5.4 mm, as
it takes a major part of the structural load; and upper part of the structure was designed using a diameter of 76 mm and
thickness of 4.5 mm as which takes the roof sheet load.
In this work, a topology optimisation tool from Altair Inspire
Print3D was used for the nodes/tube connectors in order to
reduce the material usage and print time. Figure 4 shows the
topology optimisation process on howthematerialwas reduced
from the entire components of tube connections based on
the structural load distribution, and further surfaces were
smoothened to avoid irregularities in the final design. Excess
overhangs and inclinations were also avoided while optimising
the final design.
• Tubular Joining Using Additive Manufacturing
The printing of the TKY connectors was conducted at
the Centre for AdvancedWelding & Joining (CAWJ), R&D,
Tata Steel, India. The facility has a robotic wire arc additive
manufacturing (WAAM), in which the metal 3D printing process
is carried out using integrated 3D printing equipment consisting
of Metal Inert Gas (MIG) welding which is connected to a 6-axis
robotic arm.
TheWBS-3DP technique, in combinationwith localization
and path-planning strategies, allows for local control of detail
geometry, allowing for the manufacture of tailored welded
connections that correct formaterial and construction
tolerances. Figure 5 shows the schematic representation of
the integratedWAAMsystem, including a power source, 6-axis
robot, fixed table and base plate arrangement in a cooling tank
& data acquisition set-up.
• Printing Process
The 3D CAD model is sliced into layers in order to print
the topology-optimised components. Robot tool paths were
generated using the Autodesk - PowerMill software. In this
study, a carbon steel wire of diameter 0.8 mm was used as
feedstock. The shielding gas was 82% Ar with an 18% Co2
mixture at a constant flow rate of 15 l/min. By using these
parameters, a bead height of 2 mm and a width of 6 mm were
planned to be maintained, with a constant bead over of 1.5 mm.
The temperature between the passes (interpass temperature)
was kept below 120 °C and controlled using temperature
sensors (Table 1).
Table 1. Properties of the filler wires
used
Wire type
Dia. (mm)
C
Si
Mn
Cr
Mo
Ni
Ys
(MPa)
UTS (MPa)
%El
Low carbon steel
0.8
0.07
0.83
1.48
---
---
---
>470
>565
>22
To validate the process parameters, an additive
manufacturing procedure specification (AMPS) was also
performed using identical process parameters. A steel block
of size 400 × 400 × 50 mm was deposited under similar
conditions used for the TKY joint printing. Further, mechanical
characterization was carried out on the printed samples.
The tension test specimens were prepared along the
horizontal plane condition, vertical plane condition, and angular
plane conditions in accordance with standard or sub-size
specimen dimensions as given in ASTM E8/E8M as shown
in Fig. 6.
• Metallurgical and Mechanical Characterization
The additively manufactured sampleswere characterized
to study themicrostructure that was formed as a result of
layer-by-layer printing. The mounted and polished samples
were etched using 2% nitric acid in ethanolic solution to reveal
the phases. Leica optical microscope was used to study the
microstructure, and grain size that were formed. Ziess Oxford
made SEM-Electron back scattered diffraction (EBSD) study
was done along the build direction and the print direction using
a step size of 1.5 μm to understand the crystallography of the
solidified grains.
Oxford-hkl software was used to analyse the inverse pole
f
igures and the texture. Instron universal tensile testing
machine was used to evaluate the tensile properties using a
cross-head velocity of 5 mm/min along different directions to
understand the anisotropy of the built component.
Results and Discussion
After topology optimisation, the dimensions and weight along
with layers and slices of each connection are shown in Table 2.
Table 2. Details of the T-K-Y connections after topology
optimisation.
Connection
Quantity
Part boundary dimensions (mm)
Weight (kg)
A
2
240 × 284 × 340
42
B
4
257 × 227 × 338
42
C
2
200 × 344 × 304
32
D
3
327 × 200 × 224
26
E
6
150 × 150 × 201
10
F
3
305 × 120 × 100
5.5
The details of each topology-optimised connection are shown
in Fig. 7. The final dimensions of all the tubular connectors
were found within the required size limits with a rough surface
f
inish. The deposition of nodes in this work was done in a
zig-zag pattern. Maintaining a constant layer height is one
of themost difficult aspects of following a zigzag tool path
pattern. To remedy this issue, a hybrid tool path approach was
presented, which combines the benefits of a zig-zag tool path
with the solution of the non-uniform layer height problem.
By overlapping the edges of the zig-zag pattern, an additional
boundary layer was created. The boundary layer was then
added with a 0.5 mm overlap.
During the printing process, the fundamental process
parameters are (i) the current and its voltage, (ii) the wire
diameter, (iii) the wire-feed rate, (iv) the welding speed and (v)
the vertical printed layer height. Therefore, it becomes crucial
to properly characterize WAAM metal parts related to the
specific process parameters, in terms of geometrical accuracy
and mechanical response.
The control of the printing parameters is possible directly
from the program by specifying the welding lists containing
information on wire feed speed, travel speed, gas pre-flow,
etc. Figure 8 shows the sample data of current, and voltage
captured during the printing process.
All six types of TKY connectors were printed using the same
WAAM procedure. The final produced parts are shown in Fig.
9. As per the proposed space frame design, it was required
to build 3 number of connections for F- connector, and after
following the above procedure repeatedly it was observed that
the weights of the three F-connectors were observed as 6.95
kg, 6.97kg and 6.93 kg. So, it clearly shows that the repeatability
of the WAAM procedure was almost 99%.
An attempt was made to validate the printing procedure
through test coupon printing, and the tensile test results in
0-degree, 45-degree and 90 degrees are shown in Fig. 10.
The tensile test results clearly show that there were no
significant differences found with respect to the direction
in which the tensile properties were observed. The tensile
properties were observed isotropic in nature, in line with the
wire properties with yield strength between 395-405MPa,
tensile strength range between 497-507MPa, and elongation
range between 32–40%.
The microstructure and EBSD results are shown in Fig. 11.
The microstructure of printed samples showed an equiaxed
ferrite matrix with 7–10% pearlite as the second phase. Each
printed layer showed two distinct grain sizes. The higher grain
size on top of the layer is due to the reheating of the deposited
layer and its grain growth. Largearea EBSD scans were done
along the print direction and the build direction. It can be
observed from the crystallography analysis that there is no
directional solidification along the build or the print direction.
The formation of solidification texture was avoided AQ2
using the print layer rotation strategy (900 rotation between
individual layers).
The developed 3D-printed connectors are fixed at the
designated multi-branch connection location using an interface
plate between the connector and the tube section. Figure 10
shows the final structure installed at the Centre for Advanced
Welding and Joining (CAWJ) research laboratory, R&D, Tata
Steel in India. As the locations of joints are accurate, it is
necessary to control the straightness of the tubular sections
so that the final structure can get accurate overall dimensions.
Also, in this approach, there is no need to groove weld these
complex profiles, a circular fillet weld can make the complete
connection between the printed connector and the tube using
an interface plate (Fig. 12).
Conclusions
This research proposes a method comprised of a digital
setup using a welding-based integrated WAAM system that
structures the detailing, processing, and production of tubular
connections. In this study, a space frame structure was designed
and developed with multi-branch tube connectors using
WBS-3DP technology. To prove the feasibility of the idea, six
types of TKY connectors were identified from the developed
spaceframe design, and 20 connectorswere successfully printed
using theWAAMmethod. The result shows that the printed
connectors can be installed in place of the TKY connection
with better tolerances. The test coupon results also confirmed
that the mechanical properties of the printed material have
achieved the desired tensile strength level, matching those of
conventional tubular steels.
Acknowledgments. The research described in this paper
was supported by Research and Development, Tata Steel,
Jamshedpur. The authors would like to express sincere
thanks to welding lab team for successfully conducting the
experiments during this work.
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