Aniruddha Kumar

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 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.

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.

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.

This communication reports a comparative study on the seam welding of CP grade- 2 Ti foils with thickness of 10 µm, 50 µm and 100 µm using a repetitive nano-second pulsed fiber 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 fiber laser, while 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 fiber laser was found to be superior in respect of the welds carried out by the pulsed Nd-YAG laser. Welds made by the nano-second fiber 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 nano-second 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 fiber lasers are a better choice for welding thin titanium foils in comparison to long pulsed Nd-YAG lasers.