Girish Padhy | Shandong University, China (original) (raw)

Papers by Girish Padhy

A preliminary investigation was carried out into the application of angularly exerted ultrasonic... more A preliminary investigation was carried out into the application of angularly exerted
ultrasonic vibrations in friction stir welding for the joining of AA 6061-T4 alloy to AZ31B at
different tool rotation speeds. The variations in the welding process and weld properties due
to the applied acoustic field were investigated. The process temperature was increased, the
material flow path was widened and mechanical interlocking features at weld interfaces
were improved in the presence of ultrasonics. Morphology and distribution of intermetallic …

Influence of stable ultrasonic power on the process effectiveness and weld quality are investiga... more Influence of stable ultrasonic power on the process effectiveness and weld quality
are investigated in the friction stir welding of 3 mm thick AA 2024-T3 with and without
ultrasonic vibrations. It is found that the ultrasonically enhanced process experiences a
lower downward flow stress and higher material flow. The resulted weld exhibits higher
refinement and distribution of precipitates, better mechanical properties and higher amount
of ductile fracture. Variations in high angle grain boundary show the progress of …

Friction stir welding (FSW) has achieved remarkable success in the joining and processing of alu... more Friction stir welding (FSW) has achieved remarkable success in the joining and
processing of aluminium alloys and other softer structural alloys. Conventional FSW,
however, has not been entirely successful in the joining, processing and manufacturing of
different desired materials essential to meet the sophisticated green globe requirements.
Through the efforts of improving the process and transferring the existing friction stir
knowledge base to other advanced applications, several friction stir based daughter …

Intermetallic compound layers of friction stir welded Al-Mg joints without and with ultrasonic v... more Intermetallic compound layers of friction stir welded Al-Mg joints without and with
ultrasonic vibrations were investigated. Microstructure and chemical analysis showed that
the conventional Al/Mg weld interface constitutes an intermetallic bi-layer of Al 3 Mg 2+ Al 12
Mg 17. Application of ultrasonic vibrations in welding process diminished the bi-layer into
mono-layer of reduced overall thickness. Tool offset influenced the chemical composition of
intermetallic compound layer in the joints with ultrasonic vibrations. Complete removal of …

Arc shape, droplet transfer, weld pool behavior and temperature fields were studied to investiga... more Arc shape, droplet transfer, weld pool behavior and temperature fields were studied
to investigate the influence of low current auxiliary TIG arc on weld formation and
microstructure in TIG-MIG hybrid welding and compared with conventional MIG welding.
Results indicated that the MIG arc is stabilized by a leading TIG arc while it is destabilized by
a trailing TIG arc. Regardless of the TIG arc leading or trailing, it decentralized the MIG arc
force and reduced the impingement of droplet. The backward fluid flow in the hybrid …

This review paper aims to introduce systematic in-depth analysis of diversified aspects of ultra... more This review paper aims to introduce systematic in-depth analysis of diversified
aspects of ultrasonic application in metal joining and processing including its limitations,
future prospects and assessments. Allied welding and metal processing technologies
employing ultrasonic vibrations either as the primary source to accomplish the intended
operation or as an assistant source to improve the operation efficiency and product quality
are classified and discussed. The detailed state-of the art, experimentation and …

Specific locations in horizontal cross-sections of AA6061-T6 weld nuggets, prepared with and wit... more Specific locations in horizontal cross-sections of AA6061-T6 weld nuggets,
prepared with and without precursor ultrasonic vibrations in friction stir welding, were
orientation imaged using electron back scattered diffraction and compared. With ultrasonic
vibrations, grains exhibited higher orientation propensity along (101) and (111) and higher
grain refinement at middle of weld nugget. Misorientation analysis was conducted using a
novel approach where grains with misorientation> 45,< 5 and between 15 and 45 were …

The main process variables were measured, characterized and compared in friction stir welding wi... more The main process variables were measured, characterized and compared in friction
stir welding with/without ultrasonic vibration. The exerted ultrasonic vibration causes
reduction in traverse force, tool torque and axial force of welding. Based on the
measurement of thermal cycles in friction stir welding process, the thermal effect of ultrasonic
vibration is negligible, and the mechanical coupling between ultrasonic vibration and tool
action plays a more important role in affecting both the welding process effectiveness and …

While dry machining is an emerging trend for maintaining sound surface integrity of materials, i... more While dry machining is an emerging trend for maintaining sound surface integrity of
materials, it poses cost-effective productivity as a major concern in industries. In the
machining of difficult-to-cut materials such as aerospace turbine engine materials,
application of coolants is indispensable. Flood cooling is the most conventional cooling
method employed in the machining process although alternative cooling technologies like
minimum quantity lubrication (MQL), cryogenic cooling, solid lubrication, and high …

Molten metal flow on weld pool surface in gas metal arc welding process is investigated using a ... more Molten metal flow on weld pool surface in gas metal arc welding process is investigated
using a vision-based sensing system and an interpolation algorithm. Bead formation is
investigated by analysing flow patterns and its driving forces of weld pool under different
welding speed, welding current and shielding gas. Results show that if longitudinal to
transverse velocity ratio exceeds 2.0 in the front of weld pool, outward molten metal mainly
driven by arc force cannot reach the widest section of the weld pool. Meanwhile, the

Recrystallization fractions in the stirred zone of Al 6061-T6 friction stir welds, prepared with... more Recrystallization fractions in the stirred zone of Al 6061-T6 friction stir welds,
prepared with and without ultrasonic vibrations, were evaluated using recrystallization
fraction maps. Based on the maps, it was suggested that the microstructure evolution can be
described as different dislocation manipulation processes. It was observed that
superposition of static load of FSW on residual ultrasonic softening induces subgrain
formation. Subgrain formation was substantial at the center of the stirred zone where the

During ultrasonic vibration enhanced friction stir welding (UVeFSW) process, the ultrasonic ener... more During ultrasonic vibration enhanced friction stir welding (UVeFSW) process, the
ultrasonic energy is transmitted directly into a localized area of the workpiece near and
ahead of the rotating tool by a specially-designed sonotrode. Understanding the effect of
ultrasonic vibration on the thermal and material flow behaviors in friction stir welding (FSW)
is of great significance for optimizing the UVeFSW process. In this study, a numerical model
of UVeFSW is developed using computational fluid dynamics method coupled with

Amodified material constitutive equation characterizing the acoustic softening effect on aluminum... more Amodified material constitutive equation characterizing the acoustic softening effect on aluminumalloys is proposed
and validated for modeling the ultrasonic vibration enhanced friction stir welding process. The dependence
of the acoustic softening effect on temperature and strain rate is quantitatively analyzed. The numerical
investigation based on the constitutive equation shows that it can describe the effect of ultrasonic vibration on
friction stir welding of aluminum alloy plates reasonably.

tTwo sets of vision-based sensing systems were established to investigate the behavior of droplet... more tTwo sets of vision-based sensing systems were established to investigate the behavior of droplet transfer,arc shape and molten metal flow by varying shielding gas mixtures. Orthogonal experiments and analysisof variance indicated that the CO2content in shielding gas was the major welding parameter comparedwith welding current and arc voltage, which affected the size of undercutting defects. Results demon-strated that the suppression of undercutting defects was mainly caused by the reduction in backwardflow velocity of molten metal in weld pool due to the lowering in droplet impact and arc force when CO2content increased from 10% to 100%. Non-dimensional fitting method was used to establish the relation-ships between welding parameters and undercutting defects. It showed that appropriate pulse weldingcurrent could suppress undercutting and spatter with high CO2content in shielding gas.

Representative strain states in Al 6061-T6 friction stir weld nuggets prepared with and without u... more Representative strain states in Al 6061-T6 friction stir weld nuggets prepared with and without ultrasonic vibration were evaluated using strain component data from electron back scattered diffraction analysis. Strain components of various weld nugget locations in final grain structures and approximate strain components of the similar locations at a pseudo-intermediate grain size were accessed by varying average grain size of locations during data processing. Net strain components at the pseudo-intermediate states were higher than those at the final grain structure. Variations in net strain components at each location were analyzed to elucidate grain structure evolution with and without the ultrasonic effect. Results revealed that strain component distributions varied from location to location. Net strain component at center location increased with depth while it decreased towards weld nugget extremities. The net strain components are slightly higher in the presence of ultrasonic vibrations. Relative effect of ultrasonic energy on grain structure evolution is interpreted based on specific strain component (SSC). In the presence of ultrasonic vibrations, SSC is high which causes higher strain dependent grain refinement and recrystallization is most affected by strain. When SSC is low, additional factors control recrystallization. Position of ultrasonic factor reflects the ultrasonic effect.

This paper briefly accounts the current progress and trends in the secondary energy assisted FSW ... more This paper briefly accounts the current progress and trends in the secondary energy assisted FSW processes

Apparent hydrogen diffusivity in structural steel is determined at better accuracy.Hydrogen diffu... more Apparent hydrogen diffusivity in structural steel is determined at better accuracy.Hydrogen diffusivity increases with temperature and decreases with alloy content.Alloying can increase hydrogen cracking susceptibility in steel.Higher cracking susceptibility of alloy steel is due to higher remaining hydrogen.Hydrogen evolution data can be used to avoid HAC in steel welds.In the present study, apparent hydrogen diffusivities in 2.25Cr–1Mo and mild steel in the temperature range of 25–400 °C were determined from the hydrogen evolution profiles from specimens, electrochemically charged with hydrogen to saturation. These profiles were generated by measuring the volume of hydrogen diffused out from a specimen using a hot extraction-gas chromatography technique. The diffusivity values agree well with the literature data on apparent diffusivity of hydrogen in steel. Results from this study along with apparent diffusivity of hydrogen in 9Cr–1MoVNbN steel obtained from an earlier study are compared to reveal the effects of alloying on diffusivity of hydrogen in steel. The results indicated that the apparent hydrogen diffusivity in steel decreases with alloy content of steel. It is also shown that the activation energy for apparent diffusion of hydrogen in steel increases with increase in alloy content. Based on these, it is also shown that for the same amount of hydrogen charged into different steels; the hydrogen diffused out for a given time will be less for alloyed steel than for the unalloyed steels. Practical implications of these results in the diffusible hydrogen measurements in steel welding consumables and choosing the post-heating during welding of steels are discussed.