Vibration assisted welding processes and their influence on quality of welds

Abstract

Vibration assisted welding (VAW) has emerged as a successful replacement for heat treatments and post-weld vibration treatments of arc welds to reduce residual stresses and distortions and thus to improve its mechanical properties. This review paper tries to create a knowledge platform for such a next generation research by consolidating the findings, merits, demerits and shortfalls identified hitherto in the field of VAW. This paper presents a review on the various techniques and processes of applying vibration to the welding system and their effects on microstructure, mechanical properties and residual stress of welds, and the directions for future research are presented. Vibration of workpiece during welding, oscillation of weld pool, oscillation of molten droplet, oscillation of welding arc and vibration of welding electrode were identified from the literature as the possible ways of imparting vibration. The advancement in the direction of computational work is also observed.

Full text

Vibration assisted welding processes and their influence on quality of welds
M J Jose1,2, S Surya Kumar1, Abhay Sharma1,*
1Indian Institute of Technology Hyderabad, Yeddumailaram, Telangana, India - 502 205
2Govt. College of Engineering Kannur, Kerala, India - 670 563
Abstract
Vibration Assisted Welding (VAW) has emerged as a successful replacement for heat
treatments and post-weld vibration treatments of arc welds to reduce residual stresses and
distortions and thus to improve its mechanical properties. This review paper tries to create a
knowledge platform for such a next generation research by consolidating the findings, merits,
demerits and shortfalls identified hitherto in the field of VAW. This paper presents a review
on the various techniques and processes of applying vibration to the welding system and their
effects on microstructure, mechanical properties and residual stress of welds and the
directions for future research are presented. Vibration of work piece during welding,
oscillation of weld pool, oscillation of molten droplet, oscillation of welding arc, vibration of
welding electrode were identified from literature as the possible ways of imparting vibration.
The advancement in the direction of computational work is also observed.
1. Introduction
Different forms of vibrations such as mechanical vibrations and electromagnetic
vibrations imparted to the manufacturing system elements supply extra energy, the proper
handling of which leads to improved process performance. For the different types of
manufacturing processes, there are broadly two ways in which the vibration is applied,
* Corresponding Author: abhay@iith.ac.in

namely, tool vibration1-9and vibration of work piece.10-16Some of the specific instances for the
first category are vibration of dielectric fluid,1 vibration of tool electrode,2vibration of blank
holder in deep drawing process,3 vibrating wire drawing die,4 vibrating extrusion die5 and
vibrating die in upsetting.6For the second category, there are vibration of flowing melt during
extrusion,10 melt vibration17 and vibrating work pieces during welding.15,16External vibrations
in the foregoing processes lead to less cutting7,9 or drawing forces4 and better surface quality.8
In casting, the shrinkage defects, blow holes and porosity17,18can be controlled and the
microstructure,11 strength of composites12 and homogeneity13can be improved with the use
of vibrations.
In arc welding, vibration can be applied either during or after the process. When
applied after welding, it is generally termed as Vibration Stress Relief (VSR) which is a stress
relieving method in which controlled vibrations are being applied on welded pieces after
welding. Continuing the search for higher productivity, researchers are now putting their
effort to develop the process of arc welding during vibration i.e. Vibration Assisted Welding,
(VAW) which can cut most of the expenses related to post-weld vibrations or heat treatments.
Production lead-time can be considerably reduced due to the parallel processing of vibration
and welding. Moreover, VAW leads to improved microstructure14and better mechanical
properties.15 The different ways in which the vibration is being applied are listed below:
 Vibration of work piece during welding
 Weld pool oscillation
 Molten droplet oscillation
 Oscillation of welding arc
 Vibration of electrode

Different modes of vibration are associated with any one of the constituents of
welding system that includes work-piece, electrode, welding arc, weld pool and power source.
The first four constituents can be physically agitated whereas the power source can provide
vibration to the molten droplet by supplying pulsed current as shown in Fig.1. All the above
mentioned types of vibrations can be classified into two groups – (a) Vibration of work piece
in a welding system and (b) Vibration of various elements other than work piece in a welding
system. The first group consists of Vibration of work piece and Vibration of weld pool.
Vibration of work piece and Oscillation of weld pool are closely related to each other, because
the former leads to the later. However, direct agitation of weld-pool is also possible through
pulsating gas shielding described later.
Figure 1. Modes of vibration assisted welding (a) Vibration of work piece 15-16, 19-42, 44, 46-55
(b)Weld pool oscillation56-59, 61, 63-75 (c)Molten droplet oscillation using pulsed current76-79 (d)
Oscillation of welding arc80-92 (e) Vibration of electrode93-96
Number of investigations have been conducted with different welding processes
during which foregoing vibration methods were applied, as summarised in Table 1. Each

process of arc welding has its own set of parameters and the presence of these parameters
propose unique ways of applying vibration to various elements in the welding system. In other
words, the same type of vibration, when it is applied to different welding processes, needs to
be studied separately as the influential process parameters, their ranges and the process
outcomes are different. Thus in subsequent sections, vibration outcomes under each mode
due to different welding process are discussed.
Table1. Summary of different vibration methods and associated welding processes
No.
Method of vibration
Welding process and corresponding references
1
Vibration of work piece during
welding
GMAW 15,19,22,23,24,25,26,29,30, 34, 37, 54,55
FCAW16, 48
EGW 16
GTAW20,27, 28, 44, 46,49,52
Laser Cladding 21
SAW31,33,36,38, 39, 40, 41, 42
TIME 31, 42
ESW32
SMAW35,47, 50
Ultrasonic Welding51
FSW53
2
Weld pool oscillation
GTAW 56,59,61,63,64,65,66,67,68,69,70,72,73,75
Laser welding57
GMAW58,65,71,72
SMAW74
3
Molten droplet oscillation
GMAW76, 77, 78
SMAW79
4
Oscillation of welding arc
GTAW80, 81, 82, 83, 85, 86, 87, 89, 90, 91
SMAW 84, 88
GMAW92
5
Vibration of electrode
GMAW 93,95,96
GTAW94
With the evolution of micro structure of welds being an important factor in stress
reduction and distortion, VAW may prove more beneficial compared to VSR as post-weld
vibrations have no role in shaping the micro structure of welds. VAW actively influences the
process of solidification of weld metal which determines its mechanical properties. With the

improvement in mechanical properties and reduction in production lead time, residual stress
and distortion, associated with VAW, being unambiguously established, the need of the hour
is to extend its scope to hotter areas like possible savings in heat input and effects on element
transfer. Selection of suitable vibration parameters for VAW provides ample scope for
engineers to produce the welds of their choice and demand. Design of a competent VAW
system necessitates deep knowledge and expertise not only in welding engineering but also
in the areas of vibration engineering, metallurgy and design engineering. The interdisciplinary
nature of VAW demonstrates the seemingly available immense potential for its growth. The
beginning of such a leap needs a platform, consolidated with the summary of findings, merits,
demerits and shortfalls identified hitherto in the field of VAW, which is exactly the purpose of
this review paper. The techniques and processes of imparting externally controlled vibrations
to the different elements of the welding system along with their effects on microstructure,
mechanical properties and residual stresses related to welded joints are reviewed extensively.
Some of the analytical and mathematical models relevant to the context are also discussed.
The following sections present the state-of-the-art survey on the foregoing techniques
followed by a discussion on the future direction in the area of VAW.
2. Vibration of work piece during welding
There are different modes in which vibration is applied to work piece such as
longitudinal, transverse, vertical and random vibrations. In their oldest available paper,
Tewari et al.19 reported effect of longitudinal vibration on mechanical properties such as yield
strength, ultimate tensile strength and hardness. Schematic diagram for experimental set up
is shown in Fig.2. Work pieces were clamped on a table vibrating longitudinally with the help
of an electro-dynamic vibrator and welded using Manual Metal Arc Welding (MMAW). Rutile-

coated mild steel electrodes were used and the welding current, voltage, energy input, arc
length, and travel speed of electrode were kept constant. The vibrating conditions i.e.
frequency and amplitude were in the range of 0-400 Hz and 0-40 µm respectively. In general,
low frequency and low amplitude vibrations are given to work pieces during welding by
considering the mass of the system to be vibrated. But, the use of ultrasonic vibrations of
frequency of 20 kHz and amplitude of 10-15µm in vibration assisted Gas Tungsten Arc Welding
(GTAW) was also reported.20The maximum output power of vibration was 1.4 kW. The filler
metal ER70S-G was melted into water-cooled copper mould which was vibrated ultrasonically
with the help of a horn.In addition to the MMAW and GTAW, other welding and allied
processes like Gas Metal Arc Welding (GMAW), Submerged Arc Welding (SAW), Flux Cored
Arc Welding (FCAW) and Electro Gas Welding (EGW),16 and Nd: YAG pulsed laser ceramic-
metal composite cladding21were also reported as working with vibrating workpieces. The
vibration of workpiece leads to various qualitative changes in the welds. The majority of those
are given as follows.
2.1 Effect on mechanical properties
Tewari et al.19 reported that significant enhancement of mechanical properties was
observed within the range of 80-400 Hz for frequency and 5-30 µm for amplitude.In another
investigation, breaking strength, UTS and yield strength of specimen made under longitudinal
vibratory conditions registered an improvement of 39%, 26% and 2% respectively.22However,
the effect of specimen thickness on tensile properties was insignificant. The reason for
improvement in mechanical properties is grain refinement due to vibration. The grain
refinement also leads to reduction in elongation by 5.5%. Under the identical conditions of
vibration, the hardness improved both at the centre and at the extreme ends of weldments

and this increase was by and large uniform in contrast to what was observed in stationary
welding.23Vibration during welding produces finer grains and uniform distribution of grains
that results in improvement in hardness. The energy absorbed by the welds during Izod test
is found to be higher for those prepared under vibratory conditions compared to those made
normally.24 Ultimate tensile strength, yield strength and breaking strength of weld made
under conditions of transverse oscillations were higher and the improvement was apparent
within the ranges of 80-400 Hz for frequency and 5-30 µm for amplitude. The ductility was
diminished and finer grains were formed.15
Figure 2.Schematic diagram for the experimental set up19
Longitudinal vibration of the base metal during arc welding of galvanised carbon steel
sheets was reported to be helpful in reducing the number of blow holes formed by vaporized
zinc gas bubbles.25 In the vibratory set up, the base metal was fixed to a jig which oscillated
to and fro through a connecting rod due to rotation of an eccentric cam. The movement of
welding torch was independent of vibration. Back and forth flow of hot metal within the weld
pool due to vibrations resulted in less number of blow holes. Further investigation suggested
that the longitudinal vibration of weld pool could also be realised by pulsed current wave
form groups such as of high peak/short time and low peak/long time. It was observed that
there existed an optimum frequency of 30 Hz for the above two cases at which the number

of blow holes was the minimum. The removal of super saturation blow holes developed due
to nitrogen was also assisted by the vibration.
Vibration during the process helped to reduce the shrinkage cavity defect for smaller
depth of penetration in high power Nd: YAG pulsed laser ceramic-metal composite cladding
on aluminium alloy A6061.21 Vibration waves of different forms like rectangular, sinusoidal
and burst were generated at frequencies 50, 100, 200, 500, 1000, 1500 and 2000 Hz.
Acceleration sensing device monitored the frequency and amplitude of vibration helping to
control the amplitude of specimen vibration more accurately. The sine wave modulation
turned out to be more significant in improving the shrinkage cavity.
When the specimens were vibrated in rigid body motion mode during welding, the
process was ineffective in reducing both the longitudinal and transverse residual
stresses.26Single pass bead weld was performed on flat bar using MIG welding set up at
frequencies of 50 Hz and 500 Hz. During semi-automated GTAW of Nickel alloy 690 to form
single V-butt joint,27,28 the work pieces were vibrated by a rotating cam through a vibrating
table and the experiments were conducted at three chosen frequencies of 0Hz, 48Hz: sub-
resonant vibration and 58Hz: resonant vibration. The hardness of weldments made with
vibration was found to be less than that without vibration. Vibration affected the process of
solidification breaking up the dendrites without allowing them to grow larger and thus
produced finer grains. Diminished residual stress, lower tensile strength and elongation and
less hardness were also reported. Finer microstructures and lower residual stresses were
observed with the vibration assisted semi-automated multi-pass GTAW of Inconel alloy 690.14
Sub-resonant frequencies and resonant frequency were chosen for the experiment in
addition to a welding condition of without vibration. Sub-resonant vibration resulted in more
randomized grain orientation and reduction in residual stresses was higher compared to

those developed with resonant vibration and without vibration. Tensile strength, ductility and
hardness of weld metal were found diminished because of the externally applied mechanical
vibration during welding.
When vibration was applied to cold rolled mild steel specimens during MIG welding,
it resulted in grain refinement, reduction in residual stress and increase in hardness.29The
frequency of vibration was maintained at 25 Hz during vibratory welding. Heat input, welding
speed and time of vibration were also kept constant and amplitude was varied. During post-
weld vibratory treatment, the applied stress was kept constant and time of vibration was kept
varying. Residual stress was diminished in both the cases. Post-weld vibratory treatment was
reported as not having any significant impact on the microstructure of welded zone. The high
amplitude vibrations produced a joint of higher hardness and more refined grains.
Flexural vibration as applied to specimens made of 0.18 wt-%C steel during the single
pass MIG bead-on-plate welding led to fall in both transverse and longitudinal residual
stresses.30 The flexural vibrations at a non-resonant frequency of 25 Hz were applied to the
specimen. Effect of amplitude of vibration on the development of residual stresses was
studied keeping frequency constant and the residual stress was found as a function of
amplitude and thus that of applied stress. Level of amplitude of vibration was kept constant
to study the effect of time of vibration and it was proved to be insignificant. Application of a
frequency as high as 341.5Hz did not result in a reduction in residual stress, as expected due
to increase in energy input. Optimum amplitude of vibration was observed at which the
decline in both transverse and longitudinal residual stresses was the maximum.
The effect of vibration of specimen during welding on hardness and toughness of weld
metal was studied in the case of two welding processes namely, SAW and TIME31. TIME
(Transferred Ionized Molten Energy) is a high-performance process variant of MIG/MAG

welding. TIME process is the most economical one for very long weld seams with large cross-
section and high metal deposition rate. This process has greater ‘stick-out’, faster wire speed
and modified shielding gases. Niomol490K was the base material used to make K butt joint
and the frequency of vibration was a sub-resonant one. The hardness distributions for the
two processes were different owing to different welding parameters and filler materials.
Average value of hardness for vibration assisted SAW was less than that without vibration.
Both the impact toughness and fracture toughness in multi-pass welding got enhanced with
vibration and the later was increased up to 80%. There was not any substantial change in
microstructure for multi-pass weld made with vibration. It was found that in comparison with
TIME, the change in micro structure was more evident in SAW. Fig. 3(a) and Fig. 3 (b) show
the difference between structures of normal and vibrated weldments in SAW. The former is
more dendritic and oriented than the later.
(a)
(b)
Figure3. Micro structure of weld by SAW without vibration (a) and with vibration (b).31
VAW has been proved to be beneficial for Electro Slag Welding (ESW) also.32The weld
was made under two different types of vibratory conditions namely, 0.3g and 0.6g
acceleration. Residual stresses were diminished largely with the maximum stress being less
than half of the yield strength. VAW helped to produce finer grains and grain size in the case
of 0.6g acceleration was smaller than that of other. Again, continuing the vibration even after

welding resulted in further relieving of residual stress. When the specimens made of
Niomol490K were welded using SAW method under the condition of specimen vibrating, the
toughness was found improved.33 In addition to single pass welding, the effect of multi-pass
welding was also simulated and toughness increased in all the cases. Even though the
difference between toughness values of vibrated and normal welds was small in the case of
single pass welding, it was apparent in multi-pass welding. It was concluded that such
microstructures favourable to high toughness might have been formed during the process of
vibration that further heating could not affect them much.33The vibration during welding also
promoted ductile fracture of welds.
Figure 4. Residual stress distribution (a) Longitudinal direction (b) On the bead.34
Reduction in residual stress was also observed when random vibration was applied to
the work pieces during welding using automatic acid carbon gas shielded welding machine.34
Random vibration comprised of wide range of frequencies and white noise and filtered white
noise were chosen as representatives for the process. Thin plates of rolled steel were butt

welded along X-shaped groove and tensile residual stresses were found to be diminished by
22% and 14% on first and second welded sides respectively. Fig.4 shows the distribution of
residual stress both along the longitudinal direction and over the bead.
The ultrasonic vibrations applied to the work pieces during the bead-on-plate SMAW
of AL-6XN super-austenitic stainless steel could completely eliminate the formation of
unmixed zone.35 Unmixed zone is a boundary layer adjacent to the fusion boundary and
consists of base metal that melts and solidifies during welding without undergoing mechanical
mixing with filler metal.35Ultrasonic vibrations at frequency of 20 kHzwere applied on
specimen in a direction perpendicular to it using ultrasonic generator and ultrasonic radiator.
The proposed process was claimed to be capable of increasing the corrosion resistance of the
weldment. Charpy toughness and microstructure of weld metal in vibration assisted SAW
were further studied and it was found that vibration during multi-pass welding helped to
improve the weld metal toughness while vibration during single-pass welding did not have
any significant effect on toughness.36 The single pass welds were made on high strength low
alloy steel with vibration and without vibration and there was difference in microstructure of
the two.
The effect of imparting ultrasonic vibrations of different frequencies to the two thin
plates made of rolled steel during butt welding by acid carbon gas sealed arc welding machine
was studied by some researchers.37 In the first part of their experiment, ultrasonic vibration
at frequency of 60 kHz was applied on one plate and tensile residual stress at the centre of
bead got reduced. In the second part, ultrasonic vibrations at frequencies of 60 kHz and 27.5
kHz were applied to the two thin plates leading to higher rate of reduction in residual stress.
Further, with the help of a two dimensional spring-mass model, the reduction in tensile
residual stress in and around the bead was demonstrated. The concept behind the proposed

model was that the low yield strength of material at the weld zone immediately after welding
facilitated relieving of residual stresses through plastic deformation by absorbing energy from
ultrasonic vibrations.
Introducing VAW into the multi-pass single wire SAW of full welded valves, both
residual stresses and transverse contraction distortion were lessened largely and finer grains
were formed for weld metal.38 A105 forged steel being the base material, the work pieces
were vibrated during welding. The vibratory set up comprised of a rotating eccentric wheel
which produced oscillations, an acceleration sensor that measured vibratory accelerations, a
controller to control the accelerations and a test platform supported by rubber pads. The
maximum residual stress was brought down with VAW to a point lower than yield strength
making the distribution of residual stress more uniform and the welded joint safe.
Experimental study of multi-pass girth-butt welded pipes, made through the process
of single wire SAW and assisted by VAW, showed that the process had profound effect on
residual stresses and distortion.39The forged steel A105 was used as base material. The work
pieces were vibrated during welding with the help of the rotation of an eccentric wheel and
a platform supported by rubber pads. The VAW process succeeded in reducing the residual
hoop stresses at the outer surface and its maximum value. Residual axial stresses at the outer
surface were only slightly affected. With VAW, the residual stresses were found to be less
than the yield stress. While the process reduced the radial distortion by 50%, it could not do
much in the case of axial distortion. The VAW was applied to investigate its effect on the
properties of valves welded through SAW and there was no significant change in yield
strength and tensile strength.40,41 The work piece material was forged steel, A105. Two
cylinders of equal outer diameter and different inner diameter were butt-welded in multi-
pass SAW with a vibrating platform on which the work pieces were clamped. The bending

property was found to be increased with vibration. Grain refinement, enhanced removal of
gas and hence reduced porosity, significant reduction in welding deformation and residual
stress and uniform welded joint were cited as other advantages of applying VAW. The Charpy
impact toughness was increased by 21% for weld metal and decreased by 0.9% for HAZ. The
energy due to vibration helped to break up the growing dendrites thus producing finer grains.
Effect of vibrating the specimen during Flux Cored Arc Welding (FCAW) and Electro
Gas Welding (EGW) of AH-32 steel and DH-32 steel respectively on residual stress and
microstructure was studied.16 FCAW was performed as multi-pass welding for which weld
metal formed with vibration was found as having higher toughness and its microstructure was
finer and isotropic without any columnar structure. The stirring effect created by vibration on
weld pool made the rate of cooling larger preventing the formation of large dendrite
structures. In single pass EGW, appreciable difference was there between microstructures of
vibrated and normal welds. At the same time, multi-pass welding with vibration produced
hardly any change in microstructure.
VAW was found to be adding to Charpy toughness of weld metal in an experimental
study42 in which four single welded plates of base material Niomol490 K were made with SAW
process under four different conditions, namely, as welded, heat treated to 500 degree
Celsius after welding for 20 minutes, vibratory weld conditioned and combination of above
two with vibration during heating and the weldments were subjected to impact testing. The
vibration enhanced the impact energy of single welded plates, tempered single-pass welds
exhibited less toughness and multi-pass welds acquired higher toughness.
VAW was proposed as a process capable of replacing post-weld heat treatment in
reducing the welding residual stresses in fully welded body ball valve.43 Work pieces made of
forged steel A105 were vibrated during welding and residual stresses were measured using

blind hole drilling method. Vibration during welding could reduce the residual hoop stress at
the outer surface and residual axial stress hardly changed at the outer surface. It was
observed from experiments that the residual stresses were lower than yield stress thereby
ensuring the safety of welded valve. The Crack Tip Opening Displacement (CTOD) values for
HAZ and weld metal with vibration during welding were found more than that without
vibration. These CTOD values were larger than the critical value by which it was confirmed
that there was no need of further post-weld heat treatment.
During GTA welding of aluminium alloy AA7075,it was found that the process led to
reduction in hot cracking and finer grain size in comparison with those weldments produced
without vibration.44 Vibrations of frequency within the range of 100-2050 Hz were generated
using a piezoelectric transducer and applied to the work pieces during welding. Houldcroft
test was conducted to assess the resistance to hot cracking and the hardness of the weld
metal was found increasing with the frequency of vibration.
The mechanical properties of the weld such as ductility and tensile strength along with
homogeneity were found to be enhanced with the help of VAW.45 The process of manual arc
welding was performed on hot rolled mild steel plates with the assistance of harmonic
vibration at frequencies of 3, 5, 10 and 100 Hz during the welding and the subsequent bending
test and tensile test disclosed that both the ductility and tensile strength were increased.
While the pieces made through VAW developed no cracks, those made without vibration did
it. It was claimed that the vibration energy provided helped to have better homogeneity in
welded zone.
High-frequency electromagnetic vibrations imparted to the work pieces during GTA
welding of two dissimilar materials 25Cr-35Ni heat resistant steel (HP steel) and Alloy 800
superalloy could successfully eliminate the unmixed zone.46 Copper coil, located under the

work table, produced the alternating magnetic field for the electromagnetic vibration. The
experiment was conducted at four different potentials, namely, 0 (without vibration), 3, 12
and 24 V. The vibration resulted in stirring of weld pool causing complete mixing of filler
material, Alloy 82, with base materials and eliminated unmixed zone. The level of mixing was
higher with higher value of potentials. Vibration affected the solidification behaviour of
molten pool. Longitudinal vibration of 8mm MS work pieces during welding led to
improvement in yield strength, UTS and breaking strength but caused reduction in percentage
of elongation.47The range of frequency was 0-400 Hz and 0-40 µm was the range for
amplitude. The tensile properties increased with increase in frequency but they decreased
generally with increase in amplitude.
The fatigue strength of weldment made of SM 490A steel of weldable grade was
improved only slightly with mechanical vibrations of work pieces during FCA welding.48The
experimental set up had a mechanical vibration apparatus consisting of rotary vibrator,
programmable logic controller, sensor and analyser. The experiment was conducted at the
resonant frequency of 60 Hz. The vibration resulted in grain refinement and a different micro
structure and mechanical properties of specimen such as tensile strength, impact strength
and hardness were improved.
Figure 5. The schematic diagram for the ultrasonic vibration assisted GTAW process.49

Micro structure improvement through grain refinement was reported in ultrasonic
vibration assisted SMAW of 304 stainless steel plates of 12 mm thickness.50The experimental
set up included ultrasonic generator, waveguide and welding equipment. The direction of
ultrasonic vibration was normal to that of the plane of specimen. The vibration changed the
microstructure of weld metal from columnar dendritic to fine equiaxed dendritic
microstructure.
Aluminium was joined to galvanized steel with aluminium at the top and steel at the
bottom through ultrasonic vibration assisted GTAW.49 Ultrasonic probe of 28 mm diameter
rested vertically over the aluminium sheet at a distance of 30 mm from weld pool and vibrated
axially at a frequency of 20 kHz. The weight of probe and vibration holder acted on work piece
downwards. The Fig. 5 shows the schematic diagram for the process. Grain refinement of
weld metal and increase in micro hardness for both HAZ and weld metal were reported as the
result of this process. An increase of 27% in tensile strength could be achieved with the
ultrasonic vibration assisted GTAW process. The acoustic cavitation happening in weld pool
due to the ultrasonic vibrations was stated to be the reason for greater nucleation and
dendrite fragmentation which results in grain refinement.
Heat generation and temperature distribution in ultrasonic welding with Cu and Al
were studied by Chen and Zhang51 using a 3-D finite element model. The model composed of
workpieces, sonotrode and backing anvil. Uniformly distributed force was applied normally
on the Cu and Al plates through sonotrode and the sonotrode vibrated laterally over the
plates. Frequency of ultrasonic vibration, applied force and amplitude of sonotrode vibration
were considered as the welding process parameters. It was observed that the heat generated
due to plastic deformation was nearly a quarter of the total heat generated which was the
sum of heat generated due to plastic deformation and interface friction. Because of the huge

amount of frictional heat generation, the maximum temperature was found to be located at
the contact surface of specimens. Grain refinement in fusion zone was reported by Xu et
al.52based on their experimental study on ultrasonic vibration assisted TIG welding for making
Mg/Ti joints. The ultrasonic vibration system was of 1.6 kW maximum output power, 30 µm
output amplitude and 20 kHz output frequency. The direction of vibration was normal to the
workpiece. The average size of α-Mg grains was reduced to 1/4th. In comparison with joints
made without ultrasonic vibration, the joint strength was increased by about 18.1%.
The softening effect of ultrasonic vibration helped to achieve good weld formation
and mechanical properties in Friction Stir Welding (FSW)53. The base metal used was of 3 mm
thick commercial 2024Al-T4rolled plates and butt welding were performed with and without
localized ultrasonic vibration. Schematic of the process is given in Fig. 6. The vibration tool
head which was connected to an ultrasonic amplitude transformer transmitted vibration into
material around the too pin at a distance of 20 mm ahead the tool. Ultrasonic vibration system
worked with a frequency of 20 kHz, output power of 300 W and amplitude of 40 µm. It was
reported that the butt welding speed could be increased with such vibrations. Microhardness
of the weld nugget zone and tensile strength of joints were improved.
Figure 6. Schematic of ultrasonic vibration assisted FSW.53
A comparative study between ultrasound and Ultrasonic Impact Treatments (UIT) was
conducted in multi-pass gas metal arc welding with 16 mm thick steel plates54. The ultrasonic

transducer was applied to the weld toe with and without mechanical impacts as post-weld
treatments. The vibration amplitude of transducer was 28 µm and the operating frequency of
transducer was 27 kHz. Residual stresses were measured along two orthogonal directions. It
was found that the UIT was effective both in releasing and re-distributing residual stresses.
Application of only ultrasonic transducer at the weld toe was more effective than UIT in re-
distributing the residual stresses uniformly. The combination of ultrasound with mechanical
impacts was reported to be an effective method to release residual stresses. Another post-
weld treatment to reduce residual stresses and improve the surface mechanical properties
was reported by Ye et al.I55 and this process is called Electropulsing assisted Ultrasonic Impact
Treatment (EUIT). The base material was 12 mm thick medium carbon steel plates and metal
active gas welding was performed at a speed of 3.8 mm/s. UIT was applied at the weld surface
and simultaneously, electropulsing was applied at both ends of welded components. EUIT
appeared to be better in eliminating welding residual stresses than conventional UIT. EUIT
helped to improve microhardness and modify surface microstructure. Electroplasticity due to
electropulse current was said to be reducing the deformation resistance and improving the
ultrasonic strike efficiency.
3.Weld pool oscillation
Electromagnetic stirring of molten pool was found to be favouring the grain
refinement in GTA welds of thin sheet made of aluminium alloys.56 The electromagnetic
stirring apparatus had a magnetic coil with a ferrous core in centre. The coil was fixed under
the specimen and in line with the axis of welding torch. The stirring effect was produced in
molten pool as the result of interaction of alternating magnetic field with arc current.
Existence of optimum frequency for alternating magnetic field was observed for grain

refinement and this frequency was proportional to welding speed. Formation of porosities
was reduced due to stirring and an optimum condition of frequency and intensity of magnetic
field existed for this. The stirring helped to eliminate formation of feathery crystal and
homogenize the composition of weld metal. Stirring of molten metal during CO2 laser welding
with dynamic polarised laser beam (DP) resulted in smooth top bead surface.57The work piece
was made of steel St37 with Nickel foils placed between steel sheets as contrast material. The
contrast material helped for appropriate visualization of molten pool stirring. Stirring at
250Hz led to the lowest top bead roughness. The characteristics of weld like penetration can
be controlled with the knowledge of natural frequency of vibration of weld pool as the
frequency is directly linked to the properties of weld pool and its size and shape. The size and
shape of weld pool are decided by the heat input, thickness of base metal and type of base
metal. The oscillating force and shape of weld pool determine the modes of oscillation. The
natural frequency of weld pool decreases with increase in size and it can be detected with the
help of resonance phenomenon due to the oscillation imparted to base plate.58The natural
oscillation was successfully amplified by using modulated pulse.
Two theoretical models, one a Lumped Parameter Model and another a Distributed
Parameter Model, were proposed for natural frequency of oscillations of partially penetrated
weld pools in the case of stationary GTA welding.59 The models help to predict weld pool
geometry from the knowledge of natural frequency of oscillations which can be measured by
monitoring arc voltage and current. The natural frequency of weld pool from the Lumped
Parameter Model is:


 
 

 (1)

Where,  resonant cyclic frequency of weld pool oscillations
D = each portion of weld pool acts as an inductance element whose length is D
 acceleration due to gravity
= surface tension of liquid metal
 = width of weld pool
 = ratio of diameter of inner tank to total weld pool width 
 
= liquid density
As per the Distributed Parameter Model, natural frequency of the infinite depth weld pool
is:



 (2)
Natural frequency of the finite depth weld pool is:



 
  (3)
In the case of zero surface tension the models from the two different approaches match
completely. It was learned from all these models that frequency of weld pool increases with
increase in surface tension, decrease in density and decrease in weld pool size. While the
effect of gravity dominates in natural frequency for larger pools, it is the surface tension that
governs the natural frequency for smaller pools.
Modelling of weld pool oscillation during fully penetrated welding of thin plate was
described by some researchers.60The theoretical analysis showed that the mode of oscillation
of elliptical weld pool varies as per its shape defined by the ratio of major axis to minor axis.
While the oscillation mode kept constant, the natural frequency increased with a decrease in
size of weld pool. The natural frequency of primary mode in Hz is modelled as

f 

 kh
kh (4)
h = plate thickness (m)
= surface tension of molten metal (N/m)
= density (kg/m3)
=gravitational acceleration (m/s2)
3.1 Weld pool oscillation due to pulsation of current
Natural frequency of weld pool oscillation can also be found with the help of pulsation
of current. Excitation of weld pool using current pulses synchronized with its natural
frequency leads to resonance and thus the natural frequency may be determined. A Phase-
Locked Loop (PLL) system was used for implementing this synchronous weld pool pulsing
technique.61The oscillation frequencies of weld pool were detected using an optical sensor
and a model that relates the frequency to geometries of weld pool was developed. The pool
width or area was to be measured. The expected resonant frequency from models was
compared with the actual resonant frequency to find the weld penetration. Controlling of
penetration was made possible with the knowledge of relation between penetration and
natural frequency of weld pool. The weld pool dynamic model was developed as


 (5)
where,  = frequency for the first mode of oscillation in Hz
=surface tension (N/m)
=pool mass (kg)

The weld pool was found to possess the dynamic response of a fluid droplet rather than that
of a membrane.
Pulsed GMAW has many advantages in terms of weld quality. The process uses a
higher peak current to produce one molten droplet per pulse and maintain the arc stability
with the help of lower background current.62 The pulsation increases depth of penetration
due to arc oscillation, refines grains in welded zone and causes variation in arc pressure and
thus level of aspiration of air in weld pool which leads to change in porosity of weldment.62
Desired quality of weld can be obtained by effectively controlling the pulse parameters.
It has been experimentally verified that the frequency spectrum of arc voltage could
be used for detecting weld pool oscillations during the partially penetrated GTA welding.63 A
narrow pulse with sampling beginning on the negative edge of the pulse was selected as the
optimum waveform for weld pool excitation signal from several types of current waveforms
like constant current, white noise, square wave and pulse waveforms. Arc voltage is
proportional to arc length and hence the frequency of oscillations of weld pool could be
determined by monitoring the arc voltage. The experiments were conducted with stationary
welds on steel plate using helium as shielding gas. Measurements were taken for arc length
of up to 13 mm. The relation between weld pool size and its natural frequency of oscillation
was observed as inverse and this relationship is shown in the Fig. 7.

Figure 7. Resonant frequency versus weld pool diameter for various lengths.63
During pulsed GTA welding, the oscillations of larger amplitudes could be imparted to
the weld pool using extra current pulses superimposed on the welding pulse and the
frequency of oscillations was used to sense and control the weld penetration.64 At the
beginning of pulse time and base time, extra pulses were applied. Continuous measurement
of arc voltage along with use of FFT provides the frequency distribution. Depending on the
welding conditions, the three situations of optimal penetration, under penetration and over
penetration could be occurred. One high frequency peak and one low frequency peak in the
frequency distribution leads to optimal penetration. One high frequency peak leads to under
penetration and one low frequency peak leads to over penetration. Thus the sensing of
penetration was made possible by monitoring the peaks in frequency distribution. The in-
process control of weld penetration may be done by controlling the heat input.
Figure 8. Relation between switching frequency and frequency of molten pool vibration.65

Figure9. Relation between switching frequency and amplitude of vibration.65
If two different unit pulse conditions are selected for cyclic switch over, the switching
frequency was proved to be having substantial effect on frequency of molten pool
vibration.65The use of two different unit pulse conditions helped to realize low frequency
pulsed MIG welding which was otherwise not possible due to instability in arc behaviour.
The base material used was Al-Mg alloy A5052 of 3 mm thickness. The stirring of weld pool
due to vibration led to greater grain refinement. Uniform grain refinement was achieved
when the switching frequency reached 30 Hz at which amplitude was the highest. The larger
the difference between pulsed current values of two unit pulse conditions, the more the grain
refinement. Improved grain refinement made the weld less susceptible to solidification crack.
The Fig. 8 shows relation between switching frequency and frequency of molten pool
vibration. The Fig. 9 shows the relation between switching frequency and amplitude of
vibration.
Dynamics of molten weld pool was analysed for stationary, partial penetration GTA
welding of 6.4 mm thick cold rolled MS clamped to a copper cooling block.66A linear constant
current source provided the welding power. A short pulse above normal welding current was
given to the arc to oscillate the pool. The pulse produced oscillations in weld pool and thus
caused change in arc length which led to a proportional change in arc voltage and from the
measurement of arc voltage using oscilloscope, the natural frequency of oscillations was
determined. The oscillation frequency of arc voltage was the same as that of weld pool. High
speed films of arc and weld pool taken during the process of pulse welding could be used for
ascertaining the pool oscillations and its frequency. It was found that the amplitude of voltage
oscillation increased with current.

The natural frequencies of oscillation of stationary weld pools during the GTA welding
of both Fe 360 mild steel and stainless steel AISI 304 were determined by applying current
pulses of approximately rectangular shape.67 The oscillation of arc voltage was measured
using oscilloscope and thus the oscillation frequencies of weld pools of different sizes were
found. Since the natural frequency of oscillation strongly depends on the penetration depth
of weld, the same can be used for in-process control of penetration. There was an abrupt
transition in the behaviour of oscillation frequency for a shift from partial penetration to full
penetration. When the bottom diameter of weld pool was relatively small, weld pool acted
similar to that of partial penetration and when it was large, it followed a different mode of
oscillation. For partial penetration, the natural frequency of oscillation varied between 100
and 400 Hz and decreased with D in a manner of D-3/2 where D was the diameter of weld pool.
For full penetration, it varied from 25 to 40 Hz and decrease as D-1.
Twin-arc TIG welding is another method to excite the weld pool by injecting ultrasonic
vibrations68 in which welding may be performed in combination with ultrasonic arc and
ordinary DC arc. Two separate power sources were responsible for producing these two arcs
and the ultrasonic arc injected ultrasonic vibrations into the common weld pool. DC arc
remained as the main source of heat supply as the heat from ultrasonic arc was smaller. The
ultrasonic arc is produced by combining direct current with DC pulse current of ultrasonic
frequency. The experiments on steel 304 resulted in thinning the microstructure of weld
besides the improvement in tensile strength and welding efficiency. The presence of
ultrasonic vibrations in the molten pool was ascertained using an ultrasonic receiver probe
which, when employed near the pool with the help of a couplant, produced piezoelectric
signals.

Weld pool oscillations due to AC pulses during the full penetration GTA welding of
AZ31 Magnesium alloy resulted in grain refinement and led to improvement in fusion zone
hardness, tensile strength and ductility.69 In addition to variation in temperature gradient and
improved solidification rate, the oscillation of weld pool was also cited as the reason for the
significant grain refinement. When the pulsed current was applied near to the natural
frequency of oscillation of weld pool, it started to resonate. With resonance, the amplitude
was higher and the grain refinement occurred also was higher. Oscillation of weld pool was
understood to have favoured the fragmentation of dendrites. Columnar grains were mostly
prevented from forming equiaxed grains. Both the amplitude and frequency were important
parameters for grain refinement. The values of pulse frequency applied were 2,4,6,8 and 10
Hz at a travel speed of 4.16 mm/s and arc voltage of 11.5 V.
Analysis of weld pool oscillations during GTA welding of mild steel Fe 360 with
travelling arc was conducted by some researchers.70 Application of short current pulses made
the weld pool oscillating and from the oscillation of arc voltage, the amplitude and frequency
of weld pool oscillations were determined. The torch was kept fixed and test plate was
moved. The weld pool was elongated and there was a shift for welding arc from the geometric
centre of pool to front edge of pool. The natural frequency of oscillations could be determined
based on the phenomenon of resonance. Both the partially penetrated and fully penetrated
pools had different modes of oscillations. Pulse frequency for partial penetration welding
varied from 5 to 25 Hz and that for full penetration was 12.5 Hz with the frequency at
transition from partial to full penetration being 12.5 Hz. An abrupt change in frequency was
observed with the transition from partially to fully penetrated welds and this could be used
for the in-process control of weld penetration. The Fig.10 shows this abrupt change in
frequency. The higher the size of pool, the lower the natural frequency of oscillation was.

Figure 10. Oscillation frequency and weld pool geometry as function of base
current.70
Another technique for employing pulse current is Double Pulse-GMAW (DP-GMAW)
in which one low frequency pulse current is superimposed on a high frequency current pulse
and the oscillation of weld pool depends on thermal pulse frequency (low frequency pulse).71
Appropriate low frequency pulse value should be selected for having adequate amplitude of
weld pool oscillation. The high frequency pulse controls the droplet transfer and thus
influences the weld penetration.
3.2 Weld pool oscillation through waveguide
Ultrasonic vibrations as applied to the weld pool with the help of a wave guide could
change the structure of weld and these changes were dependent on the wave phase.72The
base material was 2017A aluminium alloy and the effect of vibration was evident both in TIG
and MIG welding. The experimental set up consisted of welding torch, linear manipulator,
waveguide, ultrasonic transducer and concentrator. The 2017A aluminium alloy waveguide
was in the form of a cylinder of diameter of 0.045 m and length of 0.254 m. The vibration
frequency was 20 kHz. The length of waveguide was equal to the wavelength of 1λ with

respect to the 20 kHz frequency. At the maximum displacement of vibration wave, face width
and weld penetration depth increase in TIG welding and face width decreases in MIG welding.
3.3 Weld pool oscillation using Pulse Shielding Gas (PSG) oscillating method
The PSG oscillating method is an alternative to Pulse Current (PC) method to excite
oscillations in the weld pool of TIG arc welding of thin steel plate.73 Apart from PC in which
the current pulses are applied to create oscillation in weld pool, in PSG oscillating method,
the shielding gas does the duty. An arc sensor detects the oscillations of weld pool and its
natural frequency of oscillation is determined. Since there is a relation between pool
geometry and its natural frequency, the penetration of weld can be determined. The PSG
method was stated to be easier and better compared to PC method in terms of amplitude of
oscillation and robustness in measurement of frequency. A system for controlling the weld
penetration by controlling the welding current with respect to oscillation frequency was
constructed. The Fig.11 shows the welding torch for PSG method.
Figure 11. Welding torch for PSG method.73
3.4 Weld pool stirring using a mechanical stirrer
Mechanical vibrations were successfully introduced into the molten pool with the help
of a stirrer during SMAW of 10 mm thick austenitic stainless steels to form butt joints.74 The

stirrer which was a Thoriated Tungsten rod having melting point of 34100C trailed behind the
welding arc along the length of weld producing high frequency and low amplitude vibrations
in weld pool through stirring. The resulted weld metal had more micro-indentation hardness
and finer microstructure. The welded joint had higher yield strength and tensile strength and
maintained the same ductility.
3.5 Weld pool oscillation through vibrating filler material
Ultrasonic vibrations could be transmitted to the weld pool through the
vibrating filler material.75 In one such experiment, the excitations generated by an ultrasonic
transducer were amplified through a horn and passed on to a guide tube through which the
filler metal was fed to the weld pool. The filler metal vibrated perpendicular to the weld
direction at a frequency of 19 kHz. The work piece material was high purity ferritic stainless
steel and full penetration bead-on-plate GTA welds were made. Formation of equiaxed grains
was enhanced in the central region of weld metal by ultrasonic vibration. The more such
grains formed, the more was the welding speed. Tensile strength and tensile fracture
elongation of weld metal were higher for welds with vibration.
4 Molten droplet oscillation
The mode of metal transfer has a lot of influence on the quality of welds. Pulsed
current GMAW is a way to achieve controlled metal transfer. An analytical model was
developed for explaining the droplet oscillation and detachment process in pulsed current
GMAW.76,77In pulsed GMAW, the pulse cycle comprises of two periods, droplet growth period
and droplet detachment period. During the pulse-on time, droplet grows and when the
current falls to base value, the detachment period starts. The sudden fall in the current

reduces the electromagnetic force acting on droplet making it oscillating along the vertical
direction. The peak value of current is switched on at a suitable time so as to make use of the
downward momentum of droplet in its detachment. The timing of excitation is so important
that both the downward movement of droplet and its sufficient downward momentum are
ensured. Hence, the droplet oscillation actively helps in achieving controlled metal transfer.
It was learned from the model that the level of excitation and its timing have greater effect
on droplet detachment and thus the metal transfer. The much sought after mode of one
droplet per pulse may be obtained, by adjusting these two parameters, at a peak current
lower than the transition current. Thus the modified pulsed current GMAW ensures a far
better controlled metal transfer.77
In general, the arc voltage is directly proportional to arc length in GMAW. But in pulsed
GMAW, because of the droplet oscillation, this relationship may not be correct always as
reported by some researchers.78In an experimental study, bead on plate welds were made on
MS plates, using a constant current power source. Frequencies of droplet oscillations were
measured both by using a high speed video camera and through analysis of arc voltage signals.
The findings were compared with that predicted by an analytical model developed based on
spring-mass system. The three methods gave matching values of frequency of oscillation for
smaller sized droplets. For larger droplets, the value from analysis of arc voltage signal
differed from other two and was higher than them. It was concluded that the oscillation of
droplet affects the behaviour of arc thereby deviating from the expected linear relationship
for them.
A numerical model was developed for explaining the droplet oscillation and
detachment phenomena in pulsed GMAW.79 The model explained that the natural frequency
of molten droplet decreases with increase in droplet size. The droplet detachment is largely

assisted by periodical forces adjusted to create resonance in oscillation of droplet. The
numerical model was validated by experiments using water drops.
5 Oscillation of welding arc
The oscillation of welding arc was proved to be another method of generating
agitations in weld pool. A two dimensional mathematical model was developed to study the
effect of electromagnetically oscillated arc on the microstructure and temperature
distribution of bead on plate welds on thin Tantalum sheets made through GTAW.80 The
associated experiment was conducted in a glove box in which the pressure was maintained
slightly above its atmospheric value. The arc was transversely oscillated up to a frequency of
25 Hz with maximum amplitude of 6.5 mm. In tune with the arc oscillations, oscillations in
temperature were observed in the weld zone. Due to the nucleation mechanism because of
the large temperature oscillations, the grain structure was refined considerably in the fusion
zone. When the frequency of oscillation of arc crossed 20 Hz, amplitude of temperature
oscillation decreased. Amplitude of temperature oscillations was found to be increasing with
increase in amplitude of oscillations of welding arc up to a certain value after which it showed
a behaviour of independence.
Magnetic oscillation of the arc was achieved using a four-pole magnetic arc
oscillator.81 DCSP GTA welding was carried out on 5052 aluminium alloy sheets of 1.6 mm
thickness with pure Argon as the shielding gas. Constant arc voltage was maintained during
welding with the help of an automatic arc voltage controller. Circular, transverse and
longitudinal types of arc oscillations were produced using the four-pole magnetic arc
oscillator for which the frequency range was 0.9 – 35 Hz. Digital oscilloscope measured the
frequency of oscillation and a split anode measured the amplitude. Both at low frequency

range of around 1 Hz and high frequency range of around 20 Hz, reduction in solidification
cracking was observed and the alteration of grain orientation in the former case and grain
refinement in the latter case were attributed to be the reasons for it. At the intermediate
frequencies where neither alteration of grain orientation nor grain refinement has happened,
the solidification cracking was severe. Grain refinement and reduction in solidification
cracking occurred at circular, transverse and longitudinal oscillations at high frequencies.
Alteration of grain orientation and reduction in solidification cracking happened for all the
modes of oscillation at low frequencies except longitudinal one.
In a Narrow Gap GTA welding of AISI 304L Stainless Steel plates, the electromagnetic
arc oscillation did not have any influence on lateral fusion of joint but it reduced the axial
fusion.82 But the oscillation increased the concavity and improved bead shape. With the
reduction in gap width, the tendency to form undercutting was increased. Special power
supply was used for the electromagnet.
Transverse oscillations of arc produced with the use of an alternating external
magnetic field in GTA welding of α-β Titanium alloy could make considerable amount of grain
refinement in the fusion zone.83The process was of autogenous, full penetration and bead on
plate. The electromagnetic arc oscillation equipment consisted of a water-cooled
electromagnetic probe mounted on to welding torch and a control unit. The amplitude and
frequency of the square wave current signal were independently adjusted during the
experiment. Thus the alternating magnetic field also was adjusted accordingly. Welding was
performed within the frequency range of 1-20 Hz and at 0.6 mm amplitude. Optimum values
for amplitude and frequency were found at which the grain refinement was the maximum.
The specimens made through electromagnetic oscillation of arc were more ductile than the
ordinarily welded specimens even after the post-weld heat treatment.

By exerting high frequency electric signal to welding circuit, the welding arc in CO2 gas
shielded arc welding of mild steel plates, was modulated to emit ultrasonic energy into molten
pool.84 The experimental set up is shown in Fig.12.
Figure 12. The experimental set up for emitting ultrasonic energy through welding arc.84
To monitor ultrasonic emission and record acoustic signal, a transducer, oscilloscope
and PC data-collection card were employed within the system. The frequency of arc-
ultrasonic substantially helped in grain refinement of welds. The phenomenon of resonance
was observed and the non-resonant frequencies could not do much towards grain
refinement.
AC pulsing, magnetic arc oscillation and the combination of these two were proved to
be beneficial to improve grain refinement in autogenous GTA welds of 2219-T87 plates.85
Peak current and base current were 180A and 60A respectively in pulsed bead on plate
welding with frequency ranging from 2-10 Hz. Frequency within the range of 2-10 Hz and peak
current of 120A were used in magnetic arc oscillation. For the PC+AO, respective parameters
remained the same with peak current being 180A. Linear and elliptical modes of arc
oscillations were employed during AO and PC+AO methods. All the above techniques led to
fine equiaxed grains in welds. Frequencies of oscillating arc and pulse current were found to
be key players in achieving grain refinement. The three processes were effective in increasing
tensile properties and ductility of the welds.

The structure of weld made through a process of vibration welding along with arc
oscillations can possess orientation in preferred directions.86A mass-eccentric vibrator helped
to apply mechanical vibration during autogenous GTA welding at resonant frequency of 47.3
Hz and amplitude of 0.203 µm. The parameters at welding were 180A, 13V and 20mm/min.
The speed of oscillation was 3.3 mm/s for 35 mm width. The vibration arc oscillation welding
was superior to vibration-less arc oscillation welding in the aspects of micro structure.
Secondary dendrite arms were observed rarely during vibration welding. But those dendrites
with an orientation parallel to vibration grew easily and strongly in vibration welding. This
established that the vibration welding had more preferred orientation than one without
vibration.
Significant grain refinement was reported for the weld made of Inconel 690 when it
was welded in constant current GTAW with a filler material of Inconel 52M under the
conditions of circular magnetic arc deflection.87 The oscillation device consisted of a
magnetising coil with two pole pairs, which surrounded the tungsten electrode when fitted
to the torch. Magnetic poles near the welding arc generate orthogonal magnetic fields which
deflected the arc in circular or oval patterns. Phase and magnitude of these magnetic
deflections were so adjusted that the arc was rotated in a circular path with specified
frequency and amplitude. The circular arc motion stirred molten metal in pool through
corresponding rotation of electromagnetic force field induced by arc current. A micro
structure of small, equiaxed grains was observed at a stirring frequency of 7 Hz.
The shape of indirect arc could be modified using applied magnetic field in twin-wire
indirect arc gas shielded welding88.Only little current passes through the base material and
the channel for current is along the ‘U’ path in twin-wire indirect arc welding. The Fig. 13
shows the scheme of indirect arc.

Figure13. The scheme for indirect arc.88
Resistance to hot cracking was found to be improved in Transverse Mechanically Arc
Oscillated (TMAO) autogenous AA2014T6welds.89A mechanical oscillator generated
transverse arc oscillation by the movement of a single axis slide. The frequency of oscillations
was limited to 10 Hz. The process with amplitude of 0.9 mm and frequency of 0.5 Hz resulted
in significant reduction of tendency for hot cracking. Increase in hot cracking resistance was
due to the grain refinement happened because of the mechanical agitation and increase in
cooling rate due to arc oscillation. Liquation crack in Partially Melted Zone (PMZ) was
completely eliminated and this was because of reduced extent of grain boundary liquation
and subsequent reduction in the width of PMZ. Change in the morphology of bead due to arc
oscillation led to uniform residual tensile strains which also helped to reduce the tendency
for hot cracking.
TMAO caused grain refinement in Al-Mg-Si alloy TIG welds also.90The same mechanical
oscillator as described above was used here. The experiments were conducted at fixed
amplitude of 1.4 mm and at three different frequencies of 0.28, 0.92 and 1.5 Hz. Frequency
was the key parameter in getting refined grain for welds. The higher effective welding speed
in TMAO improved the cooling rate which led to less net linear heat input producing finer

grains. The mechanical properties of weld like yield strength, UTS and percentage elongation
were improved due to the grain refinement caused by arc oscillation.
TMAO produced finer grains in welds made of mild steel in GTAW.91 Welding arc was
oscillated at a frequency of 0.7 Hz and amplitude of 1.5 mm with the help of a mechanical
oscillator. Arc gap of 3 mm was maintained with DCEN power source. While columnar grains
were appeared in welds made without TMAO process, smaller equiaxed grains were observed
in those made with process of TMAO. Higher strength and ductility were reported for welds
made with oscillated arc due to higher grain refinement. The process of TMAO caused less
hardness for weld metal and higher hardness in HAZ. The TMAO increased the cooling rate in
weldment and the same affected the properties of welded joint.
A swinging arc system for Narrow Gap (NG) GMAW was designed92 by which high
quality welds were obtained at low cost. In the newly designed torch, a motor of hollow axis
turned a micro-bent conductive rod for swinging the arc as shown in Fig. 14.
Figure 14. The swing arc system for NG-GMAW process.92

The process may be used for better penetration onto the sidewalls of the groove. Convenient
weaving modes may be selected by adjusting the turning angle and direction of motor. Swing
frequency, swing amplitude and at-sidewall staying time are the parameters that may be
adjusted for each mode of weaving. It was found that with the increase in frequency of swing
and staying time, the depth of penetration of grove sidewalls and surface curvature of weld
increase.
6. Vibration of welding electrode
Supplying the vibration energy to electrode is another method to achieve agitation in
the welding. The vibration may be given both in TIG and GMAW types of welding and the
resulting impact may be different for them. Electrode vibration has succeeded in improving
the metal transfer rate in GMAW. As pointed out by some earlier researchers93 in GMAW, the
vibrating electrode cannot simply give sufficient mechanical energy to directly melt additional
metal. As per them, the energy previously used to overheat the drops is available to melt
additional metal with vibrating electrode. Intelligent control of metal transfer and thus the
weld quality is possible with duly adjusting the parameters of electrode vibration.
The ultrasonically vibrated welding electrode could produce more arc pressure in TIG
welding94. The electrode vibrated axially and the vibrations were passed on to weld pool
through arc plasma. The ultrasonic vibrations created using ultrasonic generator was
transmitted to welding torch through a mechanical coupling and bead-on-plate welding was
carried out on stainless steel pieces. The study revealed that there was an improvement in
arc pressure distribution in a way favourable to the process of welding and the radial
distribution of arc pressure was close to Gaussian distribution. The process could increase

fusion zone area, weld depth-to-width ratio and weld penetration depth in comparison with
conventional TIG welding.
The Fig. 15 shows the scheme of ultrasonic vibrated welding torch GMAW
system.Ultrasonic assisted GMAW method was developed and applied to bead-on-plate
welding of 8 mm thick mild steel pieces95.
Figure 14. The scheme of ultrasonic GMAW system.95
The welding set up included a specially designed hybrid welding torch which vibrated
axially, welding power supply and ultrasonic power supply. The welding torch as a whole
consisted of ultrasonic transducer to develop mechanical vibrations, horn to amplify the
vibrations and a provision to feed electrode wire through a centrally made hole. Here, the
mechanism was such that the work piece remained stationary and the torch got vibrated. The
electrode wire supplied through the central hole did not get vibrated. The ultrasonic wave
radiated from the end of horn and reflected from the surface of work piece. An acoustic
radiation field was created together by the incident and reflected waves. The acoustic
radiation wave improved the metal transfer by affecting the nature of droplet detachment.96
In comparison with the conventional process, the arc length was compressed. The mode of

metal transfer changed to ‘short-circuiting transfer’ from ‘globular transfer’ and weld surface
was significantly different.
7 Future directions
As mentioned earlier, the multi-disciplinary nature of the process of VAW makes its
scope wider. Some of the possible directions of work in the field of VAW are given below:
(1) Vibration is a form of energy and the vibrations imparted during welding may be used to
substitute a part of the total heat input required for welding. The study in this direction
necessitates deep knowledge and expertise in metallurgy and material science. One needs
to dig into the effects of micro level vibrations rather than macro level vibrations.
(2) Vibration of molten pool is all expected to cause greater level of diffusion between weld
metal and base metal. Element transfer during VAW is a matter of interest as it can be
tailored to suit the purpose by controlling the vibration parameters. Developing welds of
desired chemical composition will contribute heavily to the quality of welded structures.
The work will necessitate expertise in chemical engineering.
(3) The extent of computational work reported in VAW is very less and for a fast development
of VAW, computation is very much essential.
(4) Hybrid VAW systems may be developed by combining the vibrations of two or more
elements of the system. For example, VAW combined with pulse welding. The effects of
such hybrid systems on the quality of welds may be studied.
(5) Such hybrid systems can be tried, in particular, to eliminate residual stresses after welding
which has a prominent role in the quality of welds. The cascading effect of two or more

VAW processes may be explored to reach at a point where the residual stress is the
absolute minimum.
8 Conclusions
External vibrations have greatly helped to improve the quality of welds in arc welding
through grain refinement, improved mechanical properties such as tensile strength,
toughness and hardness, less residual stress and more homogeneous welded joints. It has
been concluded that the properly designed and fabricated vibration assisted arc welding
systems can enhance productivity. Computational approach finds extreme importance in
establishing such systems because the realization of a particular process performance
parameter at a desired level would otherwise become nearly impossible in view of the highly
complex nature of interdependence among process parameters. The multi-disciplinary nature
of VAW makes its scope for growth wider and development of hybrid systems may do
wonders, especially in our effort to minimize residual stresses and distortions. In VAW, the
external vibration may be given to the different elements of the system such as work piece,
welding torch, arc plasma, welding electrode, filler material and weld pool for producing
better quality welds. The common outcome of vibration during welding, irrespective of which
element of the welding system is vibrated, is the grain refinement of the weld metal. Vibration
of work piece has emerged as a competent process capable to replace post-weld vibration
and heat treatments in terms of its advantages of less total energy input, decrease in
production lead time and reduction in residual stress and distortions. The vibration of weld
metal during its solidification makes the process of VAW unique and the associated change in
micro structure of welds is believed to be the cause of altered mechanical properties like
tensile strength, impact strength, hardness and fatigue strength. There are many more areas

of VAW that can be worked on in future, namely, element transfer during VAW, micro level
vibrations, computational work, and hybridization of VAW with other process variants.
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