A major challenge in welding ferritic stainless steel is the pronounced grain growth that occurs in both the weld zone and the heat-affected zone (HAZ), which adversely affects the mechanical properties of the weld joint. To address this issue, a low heat-input welding approach using a repetitive nanosecond (ns) pulsed laser was employed. This experimental study investigates dissimilar spot welding between 0.8 mm thick ferritic and austenitic stainless steel sheets using a 200 W average-power nanosecond pulsed fiber laser. The experiments were designed to examine the influence of laser processing parameters on microstructural evolution and mechanical performance. Mechanical properties were evaluated through tensile shear testing and microhardness measurements, while microstructural characteristics were analyzed using optical microscopy and scanning electron microscopy (SEM). The results demonstrated that ns pulsed laser welding effectively suppressed grain coarsening in the HAZ of ferritic stainless steel and provided superior tensile strength compared to conventional welding methods.

Zircaloy-4 (Zry-4) fuel tubes encapsulating sintered high-density uranium dioxide pellets are used in Pressurized Heavy Water Reactors (PHWR) due to their favourable mechanical and nuclear properties. End cap welding of these tubes is a critical fabrication step, conventionally performed using upset butt resistance welding. Despite its high throughput, resistance welding poses challenges in non-destructive evaluation (NDT) due to the small and irregular nature of weld defects, and also produces an external upset that requires post-weld machining. In this study, a systematic investigation of pulsed Nd:YAG laser welding of Zry-4 end caps to Zry-4 fuel tubes was conducted to explore its suitability as an alternative for welding plutonium-bearing fuels inside a glove box. The existing joint design was modified to accommodate laser welding, resulting in a negligible (<1%) reduction in fuel column length. Weld penetration was found to be adequate, and all welds were radiographically acceptable. Chemical analysis confirmed weld chemistry to remain within specified limits. Microstructural characteristics of the welds were examined using optical microscopy, and electron microscopy (SEM-EBSD, and TEM). Weld mechanical integrity was evaluated through tensile pull tests, closed-end burst tests, and microhardness measurements. Corrosion resistance of the laser welds was found to be adequate. Furthermore, weld residual stresses did not result in any unfavourable hydride morphology or distribution. In summary, the study presents a comprehensive comparison of resistance versus laser welding for Zry-4 end cap joining, and demonstrates that laser welding offers a viable, automatable alternative with improved NDT compatibility and controlled microstructural and mechanical performance.

In this work, we report the drilling of microhole arrays on a pure aluminum foil of 50 μm thickness using a 200 W average power nanosecond pulsed fiber laser. Holes were made by trepanning with six different pulses of varying duration from 29 to 2020 ns. The surface morphology of the wall of the holes was studied using a scanning electron microscope and was found to be different for holes made with different optical pulses. Laser induced periodic surface structures were found to be generated when the hole was drilled by optical pulses of 29 and 53 ns durations. These laser induced structures were found to be responsible for altering the wettability of the hole walls, thereby making the hole walls suitable for use as filters to separate oil and water from oil-water mixture. The filtration ability of these microhole arrays in separating oil-water mixture was assessed, and the separation efficiency of the filters made with 29 and 53 ns optical pulses having a hole diameter of ∼55 μm was found to be ∼99%. The laser processed array of holes showed switchable wettability. The purity of the filtered water was tested by Fourier transformed infrared spectroscopy.

In this work, we report on handheld laser welding of AISI 304 L stainless steel sheets and pipes using various joint configurations. Sheet welds were performed using butt, corner, “T”, lap, and edge joints with a 2 kW continuous-wave handheld laser welding system. AISI 304 filler wire with a diameter of 1 mm was employed during welding. The process was carried out manually by manoeuvring the welding torch along the joint, utilizing an oscillating beam at a constant speed. The effects of oscillation amplitude, oscillation frequency, and laser power on weld pool shape and penetration were investigated. Weld quality was evaluated through X-ray radiography, optical microscopy, tensile pull testing, and micro-hardness measurements. Porosity was observed in welds produced under argon shielding gas. In contrast, using high-purity nitrogen gas resulted in porosity-free welds. Tensile tests showed that welds made with nitrogen shielding sustained a peak load of 23 kN, 26 kN and 12 kN for butt, “T” and lap joint welds respectively before fracture, significantly outperforming those welded under argon. Microstructural analysis revealed a duplex structure, with ferrite as the minor phase distributed in various morphologies within the austenitic matrix. All standard joint configurations were successfully welded using the handheld laser welding system.

Dissimilar welding between aluminium and copper thin sheets has gained industrial interest over the last decade due to its applications in many industries, mainly in the field of e-mobility. The present paper is aimed at assessing the possibility of using a nano-second pulsed fiber laser of average power of 200 W for seam welding between 0.6 mm thick pure aluminium sheet and 0.3 mm thick pure copper sheet in lap joint configuration and to study the effect of the IMC layer on the functional properties of the joint. Laser welding was carried out in oscillation mode with heat inputs varying from 0.6 to 3.8 J/mm. The thickness of the inter metallic compound (IMC) layer was found to vary from similar to 1 to 5 mu m. Minimum thickness of the IMC layer with Al2Cu stoichiometry was observed when the weld heat input was similar to 2.3 J/mm. The welds made with this energy input were found to be mechanically strong and sustained a load in excess of 450 N before failure during tensile shear testing. The electrical contact quality of the joint was also found to be satisfactory with the value of the quality factor measured to be 1.4.

In this communication we report a modification in the conventional design of wire wrapping process for fast reactor fuel pins and related results. The fuel pins for sodium cooled fast reactors are wire wrapped helically along the length of the pin. In the conventional design of wire wrapping the end of the wire is mechanically crimped prior to Tungsten Inert Gas (TIG) spot welding with the end plug. In the modified design, we have incorporated resistance spot welding (RSW) of the wire with the plug in place of the hydraulic crimping operation prior to TIG spot welding. The quality of the welds were evaluated by metallography and tensile testing. Digital Image Correlation (DIC) studies were carried out for strain mapping during tensile testing. The modified design resulted in enduring similar to 23% more load than the conventional design before fracture. The failure location during tensile pull test of spacer wire was observed at the resistance weld location. DIC studies during tensile testing showed that maximum strain was experienced by the resistance spot weld. Incorporation of modified design will ease the bottom end plug welding and wire wrapping process in addition to substantial cost reduction in fabrication of end plugs.

In this work, we report full penetration welding of 1.6 mm thick AISI 304L stainless steel sheets in a butt joint configuration using a pulsed nanosecond fiber laser of an average power of 200 W. The welding was carried out by a focused laser beam oscillating in a circular path. The effects of beam oscillation parameters, e.g., amplitude, frequency, and weld speed, on weld morphology and microstructure were studied. Electron back scattered diffraction was used to characterize the weld microstructure and to map the distribution of austenite and ferrite phases in the weld. The solidification mode of the weld was found to change from the equilibrium FA (Ferrite-Austenite) to AF (Austenite-Ferrite) to A (Austenite) on an increase in the cooling rate with a concomitant drop in the fraction of δ-ferrite. The welds were found to be without any cracks with the sporadic presence of porosities. The welds were found to be mechanically strong.

The heat generated during the welding of stainless steel results in the oxidation of the surface that, apart from aesthetics, adversely affects its corrosion property too. This communication reports the successful removal of heat tint generated by the gas tungsten arc welding process off a stainless steel surface in addition to an improvement in its pitting corrosion resistance by exposure to the emission of a nanosecond pulsed fiber laser. The laser cleaning experiments, conducted with six different laser pulses having different temporal shapes and duration that varied from 20 to 1020 ns, revealed that removal of the weld heat tint was possible in all conditions provided the effective laser fluence exceeded a definite threshold value. This threshold value, in turn, was found to increase with an increase in pulse duration. The laser cleaned specimens were subsequently analyzed for surface morphology, roughness, and pitting corrosion resistance. The pitting corrosion resistance of the laser-treated specimens was compared with the as-welded specimens and welded specimens with heat tint removed using conventional methods like wire brush cleaning and chemical pickling. The superior quality of tint removal, improved pitting corrosion resistance, and the ease of noncontact operation indicates the significant potential of the fiber laser-based weld tint removal method to substitute the conventional mechanical or chemical cleaning methods currently in use in the manufacturing industry.

In the present work, we have compared the results of microstructural and mechanical property characterization of AISI 430 ferritic stainless steel spot welds made remotely using a repetitive nanosecond pulsed fiber laser and a CW fiber laser. Optical/electron microscopy, microhardness test, and tensile shear tests were performed for evaluation of the welds. Welds made by the pulsed nanosecond laser were found to be superior due to the presence of the very narrow heat-affected zone along with finer grains in the fusion zone. The welds were also found to withstand more load before fracture and were more ductile. Use of short duration laser pulses with lower heat input were found to be responsible for the refinement of grains in the fusion zone and improvement of mechanical properties of nanosecond laser weld specimens.

Joining of materials, with a strong bearing on the manufacturing industry, has remained an active area of research for decades. The demand for the fabrication of a variety of miniature components has placed more emphasis, in particular, on the welding of thin materials. Welding of thin materials is a challenging task, and more so if they are foils of dissimilar materials, as even a tiny weld flaw invariably can lead to a rapid blemishing of the job. Aluminum with its good thermal and electrical properties, low specific weight, and low cost is often considered a preferred material in many applications. In this communication, the authors present the result of a feasibility study of laser-assisted welding of stainless steel (AISI 304) and pure aluminum foils. A repetitive single-mode nanosecond fiber laser was used to carry out the weld in the lap joint configuration. Welding between the foils was done in the weld brazing mode. Electron microscopy, microhardness measurements, and tensile testing were carried on the weld to evaluate its microstructural and mechanical properties. In the course of welding, stainless steel remained in solid state, while aluminum underwent localized melting over a narrow zone at the interface. The use of very short duration repetitive laser pulses with lower heat input restricted the bulk diffusion of elements across the interface and thereby the generation of the intermetallic compound/second phase with minimum Heat Affected Zone and almost no distortion. This study establishes a nanosecond laser-assisted welding technique as an option for microwelding between stainless steel and aluminum foil.

This communication reports the result of a comparative study of laser marking with conventional mechanical stamping of AISI 304 stainless steel sheet by subjecting the marked specimens to stress corrosion cracking susceptibility test. A single mode fiber laser was made use of for this work to mark the steel specimens while a hydro-pneumatic press was used to effect marking in the conventional way. Mechanically stamped specimens exhibited better resistance to stress corrosion cracking compared to specimens marked with laser. The cracking susceptibility of laser marked samples was found to be critically dependent on the fluence of laser used to cause marking. Extent of cracking was found to increase with fluence. Reduced stress corrosion cracking susceptibility of mechanically stamped specimens was attributed to the presence of compressive residual stresses in and around the mark. This work establishes that upon scaling down its fluence appropriately, a laser can be a workable tool for marking stainless steel components meant for application in corrosive media.

This communication reports a comparative study on the seam welding of CP grade-2 Ti foils with thicknesses of 10 µm, 50 µm and 100 µm using a repetitive nanosecond pulsed fibre laser and a lamp-pumped pulsed Nd-YAG laser. The welding was accomplished in a lap joint configuration. A fine acicular microstructure was observed in the welds made by the nano second fibre laser, whilst a coarse basket weave microstructure was observed in the welds made by the Nd-YAG laser. The tensile properties of the welds carried out by the nano-second fibre laser was found to be superior with respect to the welds carried out by the pulsed Nd-YAG laser. Welds made by the nano-second fibre laser could withstand ~17% more load, whereas elongation before fracture was almost double to that of the welds made by the Nd-YAG laser. This variation is comprehensible on the basis of the finer weld microstructure evolved in nanosecond laser welding due to the very low heat input and high cooling rate associated with this process. This study established with certitude that nanosecond duration pulsed fibre lasers are a better choice for welding thin titanium foils in comparison to long pulsed Nd-YAG lasers.

We report here on the generation of simulated defects on sintered uranium dioxide fuel pellets by taking advantage of the underwater laser ablation procedure and their subsequent characterization. This work, we believe, can play a role towards the validation of the performance of fuel pellet inspection machines. A repetitive fiber laser, capable of delivering pulses of nanosecond duration, in conjunction with a galvo-scanner served as the machining tool in the experiment. The study of the dependence of mass ablation rate on laser fluence, water column height, repetition rate, and beam scanning speed formed the bulk of this work. A water column of height similar to 3 mm above the pellet surface in combination with a laser fluence of lying within 6-7 J/cm(2) yielded the maximum ablation rate. The generated defects were analyzed using optical and electron microscopy. Clean defects, e.g. longitudinal cracks, circumferential cracks, pits, and end caps of different sizes and depths were created on the pellet surface by varying the laser parameters, beam travel trajectory, and beam scanning parameters. The mechanical pressure arising out of shock wave generated as a result of the confinement of the laser-produced plasma and the collapse of cavitation bubbles together with the microfluid jet formed due to the implosion of the bubbles pushed the molten ablated products from the interaction zone thereby facilitating the generation of cleaned machined surfaces. An insight into the physical processes' operative in the underwater laser machining process has been offered, albeit qualitatively. (C) 2020 Elsevier B.V. All rights reserved.

This is the first study undertaken towards development of mixed-matrix membranes (MMMs) with enhanced radiation resistant attributes by reinforcement of nanostructured Gd2Zr2O7 (GZO) within polysulfone (Psf) host-matrix. The study describes synthesis and characterization of GZO in disordered, defect-fluorite structure, having average crystallite size of 31(±3) nm. Membranes prepared with different loading of GZO (up to 2 w/w% of Psf) are exposed to γ-radiation up to a dose of 1000 kGy in aqueous environment. The effect of radiation on the structural, mechanical, and thermo-oxidative properties of MMMs has been compared with that of radiation-sensitive Psf membrane. The ultrafiltration performance of the (un)irradiated Psf and MMMs reveal that an optimum reinforcement of GZO at 1 (w/wPsf)% offers ~10 times radiation resistant MMM, compared to that of Psf membrane. The MMM with 1 (w/wPsf)% GZO was rolled into 2512 spiral configuration and a highly-compact device was developed for treatment of radioactive effluent. Based on the flux decline behaviour over 18 months duration, the fouling behaviour of the MMM module was modelled. The life-span of the MMM was predicted based on the absorbed radiation dose, GZO leaching study, and fouling behaviour. It is proposed that GZO mitigates a fraction of the impinged γ-energy in swapping of the Gd3+ and Zr4+ sites, which protects the polymeric host-matrix in-situ from radiation induced degradation. The abundance of Gd3+ and Zr4+ associated with polar O2− over the impregnated GZO further stabilizes MMMs through scavenging of the oxidizing and reducing radiolysed products of water. Thus, we report a smart approach to develop novel MMM with greater life-expectancy against high-energy radiation and further fabricate a membrane-based device for management of liquid radioactive waste.

Joining of materials is an essential process step in manufacturing industry. The need to weld dissimilar thin materials is growing lately with applications in MEMS devices involving different materials. Welding of thin dissimilar materials is a challenging task in comparison to their bulk counterparts as they are quite susceptible to damage. This paper is concerned with the feasibility of welding extremely thin stainless- steel foil (AISI 304) with pure copper foil of thickness of tens of microns in lap joint configuration using a single mode nano-second pulsed fiber laser. Welds were made in “weld brazing” mode as well as in “full fusion” mode with linear heat inputs of 0.1 J/mm and 0.145 J/mm respectively. Optical microscopy and electronic microscopy were used to evaluate the weld microstructure, X-ray radiography was employed to evaluate the integrity of the weld, while tensile testing and micro-hardness measurements of the welds were carried out to ascertain the mechanical properties the weld. Welds could be made without any defects, e.g. lack of fusion, cracks and porosities. Welds made in “weld brazing” mode was found to be mechanically superior to the welds performed in “full fusion” mode. The decided advantage of the usage of nanosecond single mode laser for this application is also elucidated.

This work explores laser assisted removal of uranium-di-oxide and thorium-di-oxide particulates from zircaloy substrate. An electro-optically Q switched Nd-YAG laser operating on three wavelengths of 1064 nm, 532 nm, 355 nm was used as the cleaning tool to compare the efficiency of the decontamination process. Laser assisted surface cleaning that offers the dual advantage of remote operation and minimizing the generation of secondary waste has therefore emerged as the most attractive technique with regard to decontamination of radioactive surface. In case of loose contamination wherein the contaminant particulates are often attached to the substrate surface by short range van-der-wall's force, the absorption of energy from the incident radiation field by both particulates and substrate can contribute towards generation of cleaning force leading to expulsion of the particulates. It was found that laser parameters like its wavelength, fluence, number of exposures and particulate shape/size play an important role in the cleaning process. A single laser exposure with a fluence of -0.3-0.5 J/cm(2) was found to be enough to obtain decontamination efficiency of -70-80% while removing the contaminants from zircaloy substrate. Subsequent exposure of laser pulses to the contaminants further increases the efficiency of cleaning. Decontamination efficiency was found to be always higher for uranium-di-oxide particulates than virgin thorium-di-oxide particulates under identical cleaning conditions. However, decontamination efficiency of milled thorium-di-oxide particulates was found to be marginally higher than uranium-di-oxide particulates. (C) 2018 Elsevier Ltd. All rights reserved.

Metallic waste contaminated with radioactive materials accounts for a large proportion of the nuclear wastes generated in nuclear facilities. Storage of these highly contaminated wastes is known to attract high cost. Reduction of activity of these wastes can significantly bring down the cost of their storage. Further, for recycling of the precious metal, it's mandatory that it's surface contamination is first brought down to the accepted level. We report here a practical method of significant reduction of volumes of contaminated wastes to be stored by way of employing coherent radiation emanating from a laser to ablate fixed radioactive contamination alone from the surface of metallic wastes. A single mode pulsed fiber laser was used to remove fixed contamination from stainless steel substrate by ablation. Optimisation of the removal process was effected with respect to the laser power, repetition rate, laser beam scanning speed and number of passes. Ablation depths ranging from few microns to few hundreds of microns could be readily achieved as per the requirement simply by suitably varying the above processing parameters. Samples simulated with U-232 contamination as well as samples from the pipe line of a Boiling Water Reactor (BWR) were found to be decontaminated containing only marginal residual activity using this technique. (C) 2017 Elsevier B.V. All rights reserved.

Efficient removal of Uranium-di-oxide (UO2) particulates from stainless steel surface was effected by Nd-YAG laser induced plasma shock waves in air as well as in water environment. The propagation velocity of the generated shock wave was measured by employing the photo-acoustic probe deflection method. Monitoring of the alpha activity of the sample with a ZnS (Ag) scintillation detector before and after the laser exposure allowed the estimation of decontamination efficiency defined as the percentage removal of the initial activity. Experiments were carried out to study the effect of laser pulse energy, number of laser exposures, orientation of the sample, the separation between the substrate surface and the onset point of the shock wave on the de-contamination efficiency. The most optimised cleaning was found to occur when the laser beam impinged normally on the sample that was immersed in water and placed at a distance of similar to 0.7 mm from the laser focal spot. Analysis of the cleaned surface by optical microscopes established that laser induced shock cleaning in no way altered the surface property. The shock force generated in both air and water has been estimated theoretically and has been found to exceed the Van der Waal's binding force for spherical contaminant particulate. (C) 2017 Elsevier Ltd. All rights reserved.