Metallography, Microstructure, and Analysis
https://doi.org/10.1007/s13632-020-00645-2
TECHNICAL ARTICLE
Mechanical and Microstructural Characteristics of Conventional
and Robotic Gas Metal Arc Welded Low Carbon Steel Joints:
A Comparative Study
S. Rajakumar1 · P. Vimal Kumar2 · S. Kavitha3 · V. Balasubramanian1
Received: 29 October 2019 / Revised: 24 April 2020 / Accepted: 5 May 2020
© ASM International 2020
Abstract
In the recent automobile era, SAE 1022, low carbon manganese steel is widely used in structural members of automobiles.
These steels possess good weldability and need proper heating treatment conditions when high thickness sections are being
used in the structural application. Mostly, gas metal arc welding (GMAW) is preferred in automobile industries to have which
produces better penetration with some minor defects like improper fusion of sidewall, etc. The increasing demand in the
field of the automobile led to the attention over automated GMAW is being preferred to reach better productivity with zero
defects. The samples were welded by the conventional GMAW method and automated through a robot. From the experimental
test results, welding current of 450A, welding speed of 350 mm/min and a land height of 4 mm yielded a maximum tensile
strength of 647 MPa and hardness of 275 HV in the coarse-grained heat-affected zone region with a robotic mode of GMAW.
The comparative study proved that automated process produced better mechanical properties and improved efficiency.
Keywords SAE 1022 · Robotic GMAW · Conventional GMAW · “Y” Groove · Single-pass · ER70S6 · Tensile strength ·
Elongation
Introduction A suitable heat treatment condition is required for the weld-
ing of a thicker section of low carbon steels. Fusion welding
Low carbon steels are generally preferred in automotive processes such as tungsten inert gas welding (TIG) and metal
sectors due to its better weldability, penetration and lower (or) gas metal arc welding (MIG/GMAW) are preferred
investment. These steels have good weldability compared to over the years because of better weld properties of which
others; they are generally weldable by all welding processes. GMAW is suited for joining higher thickness plates. This
process is widely preferred in the industries for its adaptable
* S. Rajakumar nature to weld both ferrous and non-ferrous materials. The
srkcemajor@yahoo.com major influencing welding parameters like welding current,
P. Vimal Kumar travel speed, metal flow rate govern the penetration of the
vimleshpg3201@gmail.com joint. High thickness joints require preparation of grooves
S. Kavitha to accumulate the material that is to be deposited over the
kaviraj_2003@rediffmail.com joint length. The effect of the welding parameter describes
V. Balasubramanian as there is an increase in welding current and welding speed,
visvabalu@yahoo.com the penetration and shape factor reach an optimum range,
1
whereas the range when exceeds leads to improper shape
Department of Manufacturing Engineering, Centre factors and penetration [1, 2]. These welding parameters
for Materials Joining and Research (CEMAJOR), Annamalai
University, Annamalai Nagar, Tamil Nadu 608 002, India describe the weld quality, integrity and productivity of the
2 process. So in recent times, industries automated GMAW
Quality Assurance Department, Automotive Axles Limited,
Mysore 570018, India process is preferred over conventional ones [3–7]. Zakaria
3 et al. [8] discussed the effect of the welding parameters on
Department of Electronics and Instrumentation Engineering,
NPMaSS MEMS Design Center, Annamalai University, welding industrial-grade low carbon steels and revealed
Annamalai Nagar, Tamil Nadu 608 002, India that the weld zone consisted of Widmanstätten ferrite and
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Metallography, Microstructure, and Analysis
grains of ferrite with some colonies of pearlite. The maxi-
mum hardness that was achieved was in the HAZ region due
to the presence of these structures in the softening region.
Olabi et al. [9] on his investigation on post-weld heat treat-
ment of AISI 1020 components concluded that maximum
mechanical properties were achieved after heat treatment at
650°C and also stated that cooling rate plays a major role in
improving the properties of materials. Lailesh Kumar et al.
[10] studied the effect of welding technique on mechanical
properties and microstructure of mild steel and concluded
the fusion-welded joints showed equivalent properties as
with solid-state welding process. Mir Mostafa et al. [11] dis-
cussed multi-run welding of HSLA steels by manual metal
arc welding and observed heterogeneous microstructures in
the final pass and finer grains in the penultimate pass with Fig. 1 Schematic representation of “Y” groove sample
mechanical properties variation in the plates. The author
also concluded with a point that automated welding can be a
better solution to achieve uniform joint properties and metal- of argon and carbon dioxide in the ration of 80–20% with a
lurgical reasons. Therefore, the main reason to implement flow rate range of 24–27 lpm. The voltage range was varied
robotic GMA welding in industries is to achieve an increased from 28 to 31 V. The root gap was maintained as 1.5 mm
production rate through the automation of the process which through the zero root gaps for preventing the root porosities.
reduces the manpower hazards and provides better moni- The specimens were extracted in the transverse direction to
toring of seam tracking concerning good penetration with the welding direction against the weld bead width and side-
better sidewall fusion and lower shape factor. As far as the wall fusion (SWF) measurement to study the effect of weld-
qualitative and quantitative productivity is concerned, the ment of the mechanical properties of R-GMAW and GMA
spray metal transfer mode is primarily used in the robotic welded carbon steel joints. The macro-etching has been done
gas metal arc (R-GMA) welding process [12–14]. İpek et al. as per the ASTM specification E 340, and weld geometries
[15] conducted an analysis of the design of the groove and its were measured as shown in Fig. 2. The specimens for uni-
effect of angle width, from which he concluded that grooves directional smooth tensile testing and notch tensile testing
with X and V grooves at an angle of 48° and 54° produced were prepared as per ASTM E 8 –13a and ASTM E 338-
maximum strength in compression and tension. 03 standards, respectively. Tensile testing of these sub-size
In this work, a comparative study on the conventional specimens had been carried out in a calibrated 5 TON load
GMA welding process with the robotic gas metal arc cell with the initial loading of 0.7 mm/min on the Universal
(R-GMA) welding process has been done with the imple- Testing Machine (UTM) with a 25 mm mechanical exten-
mentation of the “Y” groove to increase the penetration with someter for smooth tensile specimens.
a 1.5 mm root gap. The conducted study produced a maxi-
mum penetration for welding current of 490 A and a travel
speed of 400 mm/min with a land height of 5.5 mm, and the Results and Discussion
joint exhibited better ductility at a crack-free angle of 180°.
Mechanical Testing
Experimental Work The recorded values for smooth and notch tensile tested
specimens are tabulated in Table 2 and graphically repre-
The experiments were carried out with industrial-grade (SAE sented in Fig. 3. The samples tested resulted in maximum
1022) low carbon steel of dimension 300 × 100 × 14 mm strength of 490 and 647 MPa with the specimen welded with
with a root gap of 1.5 mm at Automotive Axles Pvt. Ltd. the current of 450 A, travel speed of 350 mm/min and land
Mild steel grade ER70S6 copper-coated wire of 2.0 mm height of 4 mm. The notch strength of 792 was attained for
was used as a consumable electrode and an electrode stick conventional welded samples at a current of 490 A, travel
distance of 20 mm was maintained throughout the process speed of 400 mm/min and land height of 5 mm, and the
and the welding coupons utilized before welding are shown notch strength of the robot welded sample was achieved in
in Fig. 1. The chemical composition of base material and the sample welded with a current of 610 A; travel speed of
filler is presented in Table 1. The experiment was carried 430 mm/min; and land height of 6.4 mm. The notch ten-
out with a shielding environment that consists of a mixture sile strength of R-GMAW is increased when the welding
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Metallography, Microstructure, and Analysis
Table 1 Chemical composition (wt.%) of base metal and filler wire
Chemical compositions, wt.% C Mn Si S, Max P, Max C, Max Cr V Ti Nb Ni Al N Mo, Max
Base metal (SAE 1022) 0.17 1.39 0.27 0.003 0.02 0.006 0.016 0.077 0.019 0.035 0.008 0.039 … …
Filler metal (ER70S-6) 0.11 1.49 0.857 0.0024 0.011 0.0776 0.0218 0.0039 … … 0.015 … 0.0026 0.15
Fig. 2 Macrostructure of transverse cross section of GMA welded specimen
Table 2 Tensile test results
Sl. no. Welding process Welding parameters Smooth tensile test NTS, MPa NSR
WC, Amps WS, mm/min LH, mm YS, MPa UTS, MPa E, %
1 Parent … … … 431 557 34 621 1.11
2 C-GMAW 450 350 4 445 590 12.6 746 1.26
R-GMAW 450 350 4 480 647 15.5 843 1.3
3 C-GMAW 470 370 4.8 446 576 12.3 784 1.36
R-GMAW 470 370 4.8 464 583 16 802 1.38
4 C-GMAW 490 400 5.5 479 575 13.5 792 1.37
R-GMAW 490 400 5.5 501 616 20 838 1.36
5 C-GMAW 510 430 6.4 490 585 14.7 788 1.34
R-GAW 510 430 6.4 468 611 21.5 913 1.49
6 C-GMAW 525 450 7 450 580 14.8 … …
R-GMAW 525 450 7 503 630 22.7 794 1.26
current increases from 470 to 525 A, 370 mm/min-450 mm/ The specimens failed in the parent material rather than the
min of welding current, where lower welding parameters weld area, showing the quality and integrity of the weld. The
achieved the maximum tensile properties, because of the strength of the weld joint was associated with the welding
lower heat input. The notch strength of the R-GMA welded parameters which influences the weld area properties. This
specimens was found more than the smooth tensile strength variation in the strength can be recorded for the stress in the
of the R-GMA welded specimens by 30–49%, and the notch thickness direction, as stressing provides a high degree of
strength of the conventional GMA welded specimens was elastic triaxiality and the existence of the transverse stresses
found more than that of the smooth tensile strength of con- raises the average value of longitudinal stresses at which the
ventional GMA welded specimen by 26–37%. The percent- yielding occurs. The metal deposition rate increased at the
age (%) of elongation was found 23 to 53% more in the lowest welding speed, and the solidification rate decreases
R-GMA welded joints than conventionally GMA welded which leads to widening the weld bead, i.e., the molten metal
joints. spread over the parent metal adjacent to the weld groove.
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Metallography, Microstructure, and Analysis
Fig. 3 Tensile test results of
R-GMAW and C-GMAW Joints
The land height of the “Y” groove creates the 2.5D heat
flow from the molten pool which increased the excess bead
width and sidewall fusion, and hence, the overall weldment
size also depends on the land height. The welding speed and
welding current have significant effects on the weld bead
geometries.
Guided Three‑Point Root Bend Test
The entire bend test specimen was subjected to 180° root
bend and found free from cracks or other flaws. This test
method covers a guided bend test for the determination of
the ductility of welds. Defects that are not detected through
X-rays appear on the surface of a specimen when subjected
to progressive localized over stressing. From the test results,
it has been observed that there is no significant difference
in the ductility of conventional GMA welded and robotic
GMA welded specimens as both the welding processes use
the same filler wire and welding parameters. The bend test
conducted over the samples welded by both modes exhibited
a similar range of ductility with minimal variations. Fig. 4 Microhardness graph of R-GMA and C-GMA welded samples
Microhardness Analysis dwell time of 8 to 10 s through the transverse direction to
the weld bead as shown in Fig. 4. From the microhardness
In this study, the microhardness was carried out on the results, it is observed that weld metal is showing more
specimen welded with a welding current of 450 A; travel hardness than that of base metal and lower than that of
speed of 350 mm/min of R-GMA welded; and conven- the coarse-grained heat-affected zone of the weldment.
tional GMAW process. The microhardness was studied The hardness in the heat-affected zone was also observed
using Reicharter’s microhardness tester. The microhard- higher than that of base metal. The weld metal hardness is
ness was analyzed on applying the 1 kg of the load for the uniform throughout the weld metal center for both R-GMA
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Metallography, Microstructure, and Analysis
welded and conventional GMA welded specimens. Macrostructural Analysis
The hardness in the coarse-grained heat-affected zone
(CGHAZ) is higher when compared to the weld metal; The welded specimens were cut along their transverse direc-
this is due to the formation of precipitates formed with tion to observe the macrostructure of the joints. Table 3
the microalloying elements present in base metal which shows the macrostructure for the conventional as well as
are not fully dissolved within the weld pool. the robot welded samples, and the shape coefficient for the
welds was calculated and is tabulated in Table 4.
Table 3 Macrostructure of cross section of C-GMA and R-GMA welded specimens
Sl. no. Welding process Welding parameters Macrostructure of weld joints Penetration
height, mm
WC, Amps WS, mm/min LH, mm
1 C-GMAW 450 350 4 12.2
R-GMAW 450 350 4 13.4
2 C-GMAW 470 370 4.8 11.2
R-GMAW 470 370 4.8 12.8
3 C-GMAW 490 400 5.5 14
R-GMAW 490 400 5.5 14
4 C-GMAW 510 430 6.4 11.3
R-GMAW 510 430 6.4 11.5
5 C-GMAW 525 450 7 10.5
R-GMAW 525 450 7 10.6
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Metallography, Microstructure, and Analysis
Table 4 Shape coefficient (WPSF)
Sl. no. Welding process Welding parameters Weld bead width Penetration Shape
factor (F),
WC, Amps WS, mm/min LH, mm W, mm P, mm W/P
1 C-GMAW 450 350 4 21.2 12.2 1.74
R-GMAW 450 350 4 21.6 13.4 1.54
2 C-GMAW 470 370 4.8 19 11.2 1.70
R-GMAW 470 370 4.8 19.4 12.8 1.52
3 C-GMAW 490 400 5.5 19.4 14 1.39
R-GMAW 490 400 5.5 19 14 1.36
4 C-GMAW 510 430 6.4 18.4 11.3 1.63
R-GMAW 510 430 6.4 17.5 11.5 1.52
5 C-GMAW 525 450 7 18 10.5 1.71
R-GMAW 525 450 7 17.4 10.6 1.62
Microstructural Analysis conventional GMA welded specimens than that of robotic
GMA welded specimens. The roughness of austenitic grain
Figure 5a, b illustrates the base metal microstructure. in HAZ heated in the zone of high temperature formats the
The base metal microstructure is having uniform α-ferrite coarser structures after its transformation at cooling, which
and lean pearlite corresponding to light and dark regions was noticed in the precipitation of the microalloying ele-
(α-Fe + Fe3C) along with the rolling bands having fine ment such as chromium, molybdenum, niobium, titanium
microalloy precipitates as observed in Fig. 5b. From the and aluminum attributes, particularly hardness and tensile
microstructure of conventional GMA welded and robotic strength this part of joint.
GMA welded specimens, it has been observed that the
interface of the weld joint was characterized by bands of
coarse grains in the same orientation. This coarse-grained Conclusions
heat-affected zone (CGHAZ) seems to have the grains that
tend to grow along a certain crystallographic direction. A set of experiments on robotic gas metal arc (R-GMA)
However, the center of weld metal is different from the welding and conventional gas metal arc (GMA) welding of
other zones, because it is characterized by pseudo-grains modified “Y” grooved SAE 1022 carbon steel plates in butt
and a microstructural inhomogeneity which is a result of joint configuration were performed. The main purpose of
the faster cooling rates. It appears that this zone contains a this comparative study weld joints was to evaluate the effect
major proportion of upper bainite ferrite with some colonies of R-GMAW on the mechanical properties and microstruc-
of pearlite. In the HAZ, the microstructures of this zone tural analysis of similar weld joints. The weld joints have
contain Widmanstätten ferrite and some colonies of pearlite. been made by implementing the 2.0 mm filler wire in single-
The phase transformations, in the coarse-grained region of pass butt welding of modified “Y” grooved 14.0-mm-thick
the HAZ, adjacent to the weld fusion zone contain grains carbon steel plate. From this study, it has been concluded
larger than those in the base metal. It has been found that that:
there are two-phase transformations that occur in the HAZ
during cooling. The first transformation corresponds to the 1. The comparative study of a unidirectional transverse
high-temperature transformation of δ-Fe to γ-Fe followed smooth tensile test for R-GMA welding and conven-
by γ-Fe to α-Fe transformation. The formation of the fine- tional GMA welding has been conducted, and it has
grained heat-affected zone (FGHAZ) could be because this been found that the failures occurred in the parent metal
region may be heat affected closely above the AC-3 and was proving the weld quality and integrity and the R-GMA
formed by acicular ferrite (AF, CAF) as shown in Fig. 5c, d welded specimens yielded higher strength (2 to 10%
for both the welding process. higher) than that of conventional GMA welded speci-
The microstructure of the HAZ region shown in Fig. 5e, f mens.
was formed by upper bainite (UB) and acicular ferrite (AF) 2. The variation in the strength of the smooth tensile test is
along the boundary of austenitic grains after austenite dis- because of the resistance to the slip distortion or elonga-
integration, which had locally Widmanstätten character. The tion due to the comparatively higher weld bead width
size of the Widmanstätten ferrite was observed coarse in found in robotic GMA welded specimens.
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Metallography, Microstructure, and Analysis
Fig. 5 Optical microstructures of conventional GMA and robotic GMA welded joints 3; a, b parent metal, c conventional GMA welded
FGHAZ, d robotic GMA welded FGHAZ, e conventional GMA welded CGHAZ, f robotic GMA welded CGHAZ
3. The ductility of welded specimens was studied by a Acknowledgements The authors extend thanks to Automotive Axles
three-point guided root bend test, and the specimens Limited—Mysore, India, for their extreme support toward the pre-
sented research work.
showed crack-free welds.
4. The microhardness of the coarse-grained heat-affected
zone (CGHAZ) was found to be the highest due to the
presence of Widmanstätten ferrite with microalloy- References
ing precipitates, whereas the microhardness of weld
1. S.Q. Moinuddin, A. Kapil, K. Kohama, A. Sharma, On process–
metal and fine-grained heat-affected zones was found
structure–property interconnection in anti-phase synchronized
to be intermediate but higher than that of parent metal. twin-wire GMAW of low carbon steel. Sci. Technol. Weld. Join.
Robotic GMA welded joints have quite finer Widman- 21(6), 452–459 (2016)
stätten ferrite in CGHAZ, a lower density of block alpha 2. K. Abbasi, S. Alam, M.I. Khan, An experimental study on the
effect of MIG welding parameters on weld bead shape character-
ferrite in the weld metal and finer uniformly distributed
istics. IRACST – Eng. Sci. Technol. 2(4), 599–602 (2012)
ferrite with grain boundary precipitates in FGHAZ. 3. K.R. Carpenter, B.J. Monaghan, J. Norrish, Analysis of fume
formation rate and fume particle composition for gas metal arc
13
Metallography, Microstructure, and Analysis
welding (GMAW) of plain carbon steel using different shielding crystallographic texture and mechanical properties of mild steel.
gas compositions. ISIJ Int. 49, 416–420 (2009) Mater. Sci. Eng., A 754, 400–410 (2019)
4. O. Kermorgant, A magnetic climbing robot to perform autono- 11. M.M. Hosseinouin et al., Investigation on multi-run metal made
mous welding in the shipbuilding industry. Robot. Comput. Integr. of HSLA steel- heterogeneous microstructure and mechanical
Manuf. 53, 178–186 (2018) properties. Mater. Test. 59(7–8), 661–672 (2017)
5. D. Skarin, J. Nilsson, D. Green, Robotic equipment for MIG weld- 12. I. Kim, J. Son, S. Lee, P.K.D.V. Yarlagadda, Optimal Design of
ing. MIG Weld Guide, (Woodhead Publishing, 2006) pp. 221–262 neural networks for control in robotic arc welding. Robot. Com-
6. E.A. Gyasi, P. Kah, H. Wu, M.A. Kesse, Modelling of an artifi- put. Int. Manuf. 20, 57–63 (2004)
cial intelligence system to predict structural integrity in robotic 13. E.A. Gyasi, P. Kah, H. Wu, M.A. Kesse, Modeling of an artifi-
GMAW of UHSS fillet welded joints. Int. J. Adv. Manuf. Technol. cial intelligence system to predict structural integrity in robotic
93, 1139–1155 (2017) GMAW of UHSS fillet welded joints. Int. J. Adv. Manuf. Tech.
7. Y. Xu, J. Zhong, M. Ding et al., The acquisition and processing of 93, 1139–1155 (2017)
real-time information for height tracking of robotic GTAW pro- 14. Y. Xu, J. Zhong, M. Ding et al., The acquisition and processing of
cess by arc sensor. Int. J. Adv. Manuf. Technol. 65, 1031–1043 real-time information for height tracking of robotic GTAW pro-
(2013) cess by arc sensor. Int. J. Adv. Manuf. Tech 65, 1031–1043 (2013)
8. Zakaria Boumerzoug, Chemseddine Derfouf, Thierry Baudin, 15. N. İpek, F. Elaldi (2012). Analysis of welding groove angle and
Effect of welding on microstructure and mechanical properties of geometry on strength of armor steel, Mater. Manuf. Process.
an industrial low carbon steel. Engineering 2(7), 502–506 (2010) 1437–1441
9. A.G. Olabi, M.S.J. Hashmi, The microstructure and mechani-
cal properties of low carbon steel welded components after the Publisher’s Note Springer Nature remains neutral with regard to
application of post weld heat treatments. J. Mater Proc. Technol. jurisdictional claims in published maps and institutional affiliations.
38(10), 88–97 (1996)
10. K.U. Lailesh Kumar, Sudipta Pramanik Yazar, Effect of fusion
and friction stir welding techniques on the microstructure,
13
