Fiber laser – Compact source for micro-welding

Metal Inert Gas welding (MIG) is a widely used welding method for mild steel (low carbon steel) in industry due to its high weld quality, good penetration and comparatively low investment. It is an arc welding process wherein coalescence is produced by heating the job with an electrical arc struck between a filler rod and the job. A Shielding gas (argon, carbon dioxide) is used to avoid atmospheric contamination of the molten weld pool. A filler material is added for joining the work piece by MIG welding. Many researchers investigated on different welding process parameters on mechanical & microstructural characteristics. Some researchers used the welding speed, welding current welding voltage, number of welding passes & work peace thickness for stainless steel, naval grade steel & aluminum. But the shielding gas flow rate & gas mixture also important factor which affects mechanical properties of mild steel. For this study, process parameters such as welding current, gas flow rate and gas combination used in the range 170-210A, 13-17 Lit/min & 0-100 % of CO2 respectively. The material used E-250 grade mild steel with 12 mm thickness. The effect of the welding parameter is investigated by designing the experiments using central composite matrix. Using the data generated by the experimentation, an empirical relation is obtained for tensile strength. The optimum value of tensile strength 356 N/mm 2 is observed at 190A welding current, 15 Lit/min gas flow rate & 50% CO2+50% Argon gas combination. Index Terms —Co2 Welding, v-butt Welds, welding current, gas flow rate, Gas combination, Tensile strength.

The effect of Ar and He shielding gas on fibre laser weld shape and microstructure in AA 2024-T3

This paper investigates the effect of material type, material thickness, laser wavelength, and laser power on the efficiency of the cutting process for industrial state-of-the-art cutting machines. The cutting efficiency is defined in its most basic terms: as the area of cut edge created per Joule of laser energy. This fundamental measure is useful in producing a direct comparison between the efficiency of fiber and CO2 lasers when cutting any material. It is well known that the efficiency of the laser cutting process generally reduces as the material thickness increases, because conductive losses from the cut zone are higher at the lower speeds associated with thicker section material. However, there is an efficiency dip at the thinnest sections. This paper explains this dip in terms of a change in laser–material interaction at high cutting speeds. Fiber lasers have a higher cutting efficiency at thin sections than their CO2 counterparts, but the efficiency of fiber laser cutting falls faster than that of CO2 lasers as the material thickness increases. This is the result of a number of factors including changes in cut zone absorptivity and kerf width. This paper presents phenomenological explanations for the relative cutting efficiencies of fiber lasers and CO2 lasers and the mechanisms affecting these efficiencies for stainless steels (cut with nitrogen) and mild steel (cut with oxygen or nitrogen) over a range of thicknesses. The paper involves a discussion of both theoretical and practical engineering issues.

Recent progress in fiber laser technology has provided lasers operating at mid-infrared wavelengths, at power levels and temporal regimes previously inaccessible to bulk crystal lasers. This paper reports on the development of new continuous wave fiber lasers; a Tm-silica fiber laser operating at 1.98 micrometers , an Er,Pr:ZBLAN fiber laser at 2.78 micrometers and a Yb:Er-silica fiber laser at 1.5 micrometers , and pulsed fiber laser sources; gain-switched and Q-switched Tm- silica systems, specifically targeting applications in medicine. The first studies on the interaction of high- power, continuous wave mid-infrared fiber laser light and soft biological tissues are presented, demonstrating the ability of 2 and 3 micrometers fiber lasers to remove soft tissue cleanly, and with minimum collateral damage.

One function of shielding gases used in welding processes, such as hydrogen (H2), oxygen (O2), carbon dioxide (CO2), nitrogen (N2), helium (He), argon (Ar) and their mixtures, is protection of the weld pool against harmful contamination that could generate defects. In addition to this primary function, shielding gases significantly affect the shape of the weld, weld geometry, seam appearance, metallurgical and mechanical properties, welding speed, metal transfer, arc stability or beam and fume emissions. The shielding gas is thus a key factor in determining weld joint properties and welding process efficiency. As welding processes have become enhanced and welding research has advanced, different combinations of shielding gas mixtures have become available under a wide variety of trademarks, each claiming to offer the best efficiency. The shielding gas flow rate in GMAW welding is usually set according to empirical experiment. The flow generally remains unchanged throughout the entire welding process and is set at maximum values of the welding parameters so that there is sufficient gas cover. This setting means, however, that unnecessarily large quantities of shielding gas may be consumed in other phases of the welding process. In view of constantly increasing prices and shortfalls in helium supply, there is a need to optimize the use of shielding gas. Consequently, an ability to closely monitor the shielding gas blend and reduce waste can provide valuable cost savings. This paper examines the effects of shielding gas mixtures and their components, presents a cross-comparison of shielding effects in fusion welding and suggests guidelines for adaptive controllability of shielding gas in advanced adaptive fusion welding. The study reviews scientific case studies and experiments from the point of view of the effect of the shielding gas on the process efficiency and process outcome. The study considers shielding gases for welding of both ferrous metals (i.e. carbon steels, stainless steels, high-strength steels) and non-ferrous metals (i.e. aluminium and its alloys, nickel and its alloys and copper and its alloys). Appropriate choice of shielding gas and use of an optimum flow rate results in better quality in terms of increased productivity, reduced gas consumption and improved weld geometry properties, microstructure and mechanical properties. Although some blends can be used effectively in many different processes, other blends appear process-dependent; they produce far poorer results when utilized in non-appropriate processes. Particle image velocimetry (PIV) and Schlieren techniques can be used for visual sensing of gas flow during fusing welding. Moreover, an adaptive alternative gas supply can improve welding performance and weld quality and reduce harmful fume emission.

In this work we report on a novel optical design for beam shaping and focalization of high-power diode laser bars. The goals of our study are: the increase the optical throughput of the beam shaping device with respect to standard solutions and either to enhance the irradiance on a target or to inject the laser beam into a smaller fibre than with respect to beam shaping system based on plane surfaces. The high power diode laser bars pose serious difficulties in their optical handling due to their strong difference between the two transverse axes, which induce a strong astigmatic and asymmetric output radiation. As is well known, the beam quality is very different in the two axes called slow axis and fast axis, and in particular the slow axis is composed by the superposition of several multimodal sources. The beam quality in this axis is very low (its etendue may exceed 2000 mm mrad). On the other hand, the fast axis has a very high beam quality, near diffraction limited, although with very high divergence (30°-50°). The common solution for the application of the laser radiation is a fast axis aspheric micro lens in front of the emitters, in order to achieve its collimation. Typical values of the fast axis collimated beam are 0.7mm and less than 6mrad. However, the so obtained collimated beam is poorly focusable with a standard lens, and a few methods were proposed to overcome the problem. The more relevant solutions include: the stepped mirror technique, the plane parallel mirrors pair, micro prisms array and confocal micro lens array. Each of these techniques is based on the equalization of the beam parameter product by the subdivision of the beam in the slow axis and its reshaping. For all these techniques the efficiency spans from 50% to 70%. The best focalization results allow the coupling in a fibre of 400μm diameter, with NA-0.22. The aim of this work is the design and the realization of a new device, that is considered as target the following aspects: 1) the maximum optical efficiency in the beam shaping process, 2) the optimal equalization of the beam parameter product for the two axes, 3) the use of few optical elements and 4) a very compact size. These goals are addressed by a scheme that splits the collimated beam from the laser diode into different portions while the length of the optical paths of each sub element is kept constant, and by the subsequent use of short focal length aspheric lenses for the focalization of the transformed beam. Each sub-beam is deflected by a couple of plane parallel mirrors, whose normal is directed to equalize the BPP without any mutual shadowing. An optimal solution can be easily envisaged for a laser source of common size of 0.7 x 10 mm. The condition on equal optical path length has the noticeable property of placing the virtual position of the individual portions into which the original beam is split at the same distance with respect to target. Thanks to this, their subsequent focusing is unaffected by the axial displacement of the common solution by the stepped mirrors. In fact, to correct this effect, this latter technique requires the use of a prism pair, involving complexity, size enlargement and higher costs. In this work both an extensive ray tracing and optical analysis is presented as well as the experimental characterization of an experimental model. Moreover, we also report on the technique for the realization of th tilted-face plane mirrors of which is composed our beam shaping device. The scheme of beam shaping here reported can be extended to higher power beam by means of the technique of the beam combination by polarization coupling or that of the optical beam compression. Examples of theses developments are discussed in the paper, and experimental results presented. The most direct applications of the class of optical devices here reported are the high power diode laser direct application in material processing or manufacturing, the coupling into multimode optical fiber of the diode laser radiation as well as the fiber laser end pumping.

Spatter formation is a major issue in deep penetration welding with solid state lasers at high welding speeds from 8 up to 20 m/min. One approach to describe spatter formation is based on the assumption of an unstable keyhole. This leads to a temporary constriction of the keyhole due to the melt pool whereby the keyhole pressure increases. Finally, the keyhole collapses and spattering occurs. Therefore, the stabilization of the capillary is a possible way to limit spatter formation. For this purpose, several potential solutions have been tested in the past. However, investigations regarding a precisely local adjustable shielding gas flow on the keyhole stability under the condition of high welding speeds (≥ 8 m/min) is not given in the state of the art yet. To investigate these interactions, a shielding gas supply was developed, which can be adjusted in four axes with a reproducibility of 0.02 mm. Furthermore, the assembly was provided with a coaxial alignment laser for determining the interaction region of the gas. Under the processing of stainless steel (1.4301), different flow rates of argon, helium and nitrogen were tested. Additionally, Schlieren videography was used to visualize the gas flow. The spatial orientation has been varied in angles from 20° up to 48°. The experiments were recorded by means of HV-camera and subsequently analyzed by image processing (number, velocity and trajectory of spatters). Thereby, it was possible to reduce spattering by up to 91 % at welding speeds of up to 16 m/min.

With an ongoing demand to improve the efficiency of the gas metal arc welding process, steps are being taken to reduce the shielding gas consumption. However, sufficient shielding gas coverage of the weld region is essential for the generation of high quality welds, and drafts can be detrimental to its efficiency. In industry, the general practise to ensure coverage is to increase the shielding gas flow rate, however, too high a flow rate can induce undesirable turbulence in the shielding gas column, whilst adding unnecessary cost to the process. A simplified computational fluid dynamics model has been generated, and validated through extensive experimental trials, to accurately model the shielding gas flow when subjected to the adverse effects of cross drafts. Several nozzle geometry changes have been investigated with the aim of improving the shielding gas column’s resistance to drafts, eliminating the requirement to increase the shielding gas flow rate.

We report a continuous-wave Tm-doped fiber (TDF) laser with a single fiber Bragg grating (FBG). The Fresnel effect presenting in the passive fiber end-face and high-reflectivity FBG established the resonator. The peak wavelength of the laser was 1942.25 nm with a spectral linewidth of 194 pm. The maximum output power was 37.5 W, corresponding to a slope efficiency of 63.7% with a beam quality factor of M 2 ∼ 1.51. To the best of our knowledge, this is the highest slope efficiency of a TDF laser pumped by 793 nm laser diode.

Quasi continuous wave (QCW) fiber lasers operating in the near IR at 1070 nm have unique properties such high pulse energy with high peak power and very good beam quality. Consequently extreme high power densities are possible allowing for adequate coupling and high quality machining of materials that are typically transparent at these wavelengths. QCW lasers can operate with variable pulse length in pulsed mode at high repetition rate as well as in CW mode at high average power which translates into high throughput machining. Single mode and multimode fiber lasers can be used to match processing needs.In this paper we will present results covering laser processing with QCW fiber lasers of materials that are typically difficult to machine at near IR wavelengths, namely ‘white ceramics’ and sapphire.Results are shown for alumina, aluminum nitride and zirconia toughened alumina (ZTA). These are ceramics used in industries such as LED packaging as well as RF and microwave packaging. High speed drilling up to 3000 holes/sec is shown with hole diameters ranging from 100 to 15 microns, in material thicknesses ranging from 635 to 100 microns. Examples of high speed cutting and scribing are also presented.Results are shown for high quality cutting of sapphire up to 3 mm thick. Sapphire has received increased attention in the consumer electronics industry due to its increased use in applications ranging from mobile phone and tablet displays, to their lens cover, in addition to more common applications such as windows for the semiconductor and defense industry.Quasi continuous wave (QCW) fiber lasers operating in the near IR at 1070 nm have unique properties such high pulse energy with high peak power and very good beam quality. Consequently extreme high power densities are possible allowing for adequate coupling and high quality machining of materials that are typically transparent at these wavelengths. QCW lasers can operate with variable pulse length in pulsed mode at high repetition rate as well as in CW mode at high average power which translates into high throughput machining. Single mode and multimode fiber lasers can be used to match processing needs.In this paper we will present results covering laser processing with QCW fiber lasers of materials that are typically difficult to machine at near IR wavelengths, namely ‘white ceramics’ and sapphire.Results are shown for alumina, aluminum nitride and zirconia toughened alumina (ZTA). These are ceramics used in industries such as LED packaging as well as RF and microwave packaging. High speed drilling up to...

Mid-infrared ultrafast pulses are useful to drive nonlinear effects towards supercontinuum generation. These mid-infrared supercontinuum light sources are useful in label-free spectral imaging on biological tissues for disease identification. Other potential applications include spectroscopy and spectral imaging for food safety and quality inspection. A high photon flux is also desirable especially in single-shot applications enabling high signal-to-noise ratios or when probing moving targets such as living cells. Fiber lasers are particularly attractive as laser sources as they offer compactness and excellent beam quality. Furthermore, their efficient heat dissipation along long fiber lengths allows high pulse repetition rates and consequently high output powers. However, there are challenges to scale the output powers in fiber lasers due to the deleterious effects of nonlinearities and modulation instability. High-power ultrafast mid-infrared lasers are also useful for material processing of semiconductors and clear polymers, and for interactions with water-rich biological tissues and biomaterials. These applications will benefit from the flexible delivery of the ultrafast pulses with low losses whilst maintaining good beam quality. However, there are challenges in delivering ultrafast mid-infrared pulses, due to silica absorption, peak power damage thresholds in glass and dispersion which distorts the pulse temporal profile. These applications will also benefit from a shorter pulse width which eliminates thermal-related effects. In this thesis, some strategies useful for power scaling in thulium-based fiber lasers namely spectral gain shaping, resonant pumping and chirped pulse amplification will be discussed. The spectral bandwidth and shape of the propagating laser light may suppress nonlinear effects. Thus, a design method to shape the spectral gain of thulium-based sources based on cascaded segments of fibers, variable pumping schemes and other parameter optimization was proposed. Experimentally, a 3-dB bandwidth of 178 nm centered at 1944.75 nm through the cascaded two-segment TDF system was achieved. The use of resonant pumping at 1940 nm in a pulsed thulium fiber amplifier system was investigated. An output power of 40 W was attained with 53 W launched pump power from the 1940 nm CW fiber laser, corresponding to an efficiency of 87%. The gain fiber 5 did not require active cooling due to the low quantum defect, thus resonant pumping method eases the cooling requirements and may potentially reduce the onset of transverse modulation instability (TMI). A compact megawatt picosecond thulium-based fiber laser system based on the chirped pulse amplification (CPA) technique was constructed. The CPA technique allowed amplification at high pulse energies, and we achieved an output pulse energy of 46.3 μJ after compression from a multipass CVBG-based compressor at 2 μm wavelengths. The megawatt picosecond pulses were then used for processing of hydrogel, a temperature-sensitive biomaterial commonly used in cell culture studies. Microchannels, pores and surface foaming were created by direct writing without the need for additives or additional processing. A relatively new hollow-core fiber, the antiresonant hollow-core fiber (AR-HCF), was studied through finite element method analysis and experimental work as a potential candidate for the delivery and compression of high-power ultrafast mid-infrared pulses. The use of a cone type AR-HCF for flexible single-mode delivery of up to 39.1 W average power of 2 μm nanosecond pulses was demonstrated, with transmission efficiencies of up to 77%. The bending performance of the AR-HCF was evaluated and the fiber maintains a Gaussian-like beam profile up to 15 cm bending diameter. The transmission of picosecond and femtosecond 2 μm pulses using the AR-HCF was studied and verified its usefulness for ultrashort pulse delivery. Finally, the use of a gas-filled AR-HCF for nonlinear pulse compression was studied through numerical modelling and experimental demonstration. The pulse propagation in a gas-filled AR-HCF was modelled based on the numerical implementation of the nonlinear Schrödinger equation. Then, the gas cell to contain each fiber end was designed and fabricated, allowing separate pressure control at both fiber ends. The shortest pulse duration obtained was 55.4 fs using a 48 cm-long AR-HCF.

In this paper, the plasma arc welding (PAW) is studied and stainless steel temperature field is gained in this process. The thermal effect of plasma arc that specially depends on the plasma and shielding gases type and temperature field of it in work piece, which is the main key of analysis and optimization. Simulation of this process begins precisely and the separated fields for work piece, plasma arc, shielding gas and field for environment as well considered. These field equations are derived and process optimization is done by SIMPELC method and by ANSYS software coding. This simulation for gaining the temperature field of stainless steel, the effect of parameter variation on temperature field and process optimization for different cases of plasma and shielding gases (Argon, Helium and mixture of them), are done. Finally, the results of experimental tests are used to compare. The results that are derived from thermal fields show the affected process parameters which can be used for PAW process optimization. The numerical results show the timedependant distributions of arc pressure and current density, and heat transfer at the work piece surface are different from presumed Gaussian distributions in previous models.

The main aim of this paper is to study laser welding of AISI stainless steel (SS) 410 and mild steel (MS) at different values of welding power, welding speed and frequency. The welding of dissimilar metals is most useful in structural, automobiles, furniture and decorative applications. But there is a difficulty in welding of mild steel and stainless steel due to the loss of carbon from mild steel and precipitation of chromium in stainless steel. Laser welding is best suitable process which is used to control the problems occurred during welding on base metals joints. A 2 kW continuous wave fibre laser welding machine was used to weld 2 mm thick stainless steel (SS) and mild steel (MS) plates. The photos of the welded joints were assessed and then tested to investigate the mechanical properties of the welded joints.

We present a study of cooling enhancement in optical refrigerators by the implementation of advanced non-resonant optical cavities. Cavity designs have been studied to maximize pump light-trapping to improve absorption and thereby increase the efficiency of optical refrigeration. The approaches of non-resonant optical cavities by Herriott-cell and totalinternal- reflection were studied. Ray-tracing simulations and experiments were performed to analyze and optimize the different light-trapping configurations. Light trapping was studied for laser sources with high quality beams and for beams with large divergences, roughly corresponding to the output from fiber lasers and from diode lasers, respectively. We present a trade-off analysis between performance, reliability, and manufacturability.

In this work, a novel continuous wave fiber laser, pumped at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\lambda }_p = \ 1550\ nm$</tex-math></inline-formula> and emitting at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\lambda }_s = \ 4\ \mu m$</tex-math></inline-formula> , has been designed and optimized. It is based on a step-index, double-cladding, praseodymium-doped fluoroindate glass fiber, available on market, having dopant concentration <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${N}_{Pr} = \ 8000\ ppm$</tex-math></inline-formula> . For a realistic design, measured spectroscopical parameters have been taken into account, writing a five-level rate equation model. The design is carried out by employing a homemade code solver. The best predicted slope efficiency of about <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\eta \ = \ 33{\bm{\ \% }}$</tex-math></inline-formula> and pump power threshold <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${P}_{th} = {\bm{\ }}0.007\ W$</tex-math></inline-formula> have been obtained for a fiber length <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${L}_{fiber} = \ 0.4\ m$</tex-math></inline-formula> and output mirror reflectivity <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${R}_{out} = \ 30{\bm{\ \% }}$</tex-math></inline-formula> . These values are very interesting with reference to the state of the art and promise the fabrication of high beam quality optical sources in the middle infrared range, by employing conventional erbium-doped fiber pumping lasers, with a potentially easy all-in-fiber integration.