Hybrid laser-arc welding of laser- and plasma-cut 20-mm-thick structural steels

It is already known that the laser beam welding (LBW) or hybrid laser-arc welding (HLAW) processes are sensitive to manufacturing tolerances such as gaps and misalignment of the edges, especially at welding of thick-walled steels due to its narrow beam diameter. Therefore, the joining parts preferably have to be milled. The study deals with the influence of the edge quality, the gap and the misalignment of edges on the weld seam quality of hybrid laser-arc welded 20-mm-thick structural steel plates which were prepared by laser and plasma cutting. Single-pass welds were conducted in butt joint configuration. An AC magnet was used as a contactless backing. It was positioned under the workpiece during the welding process to prevent sagging. The profile of the edges and the gap between the workpieces were measured before welding by a profile scanner or a digital camera, respectively. With a laser beam power of just 13.7 kW, the single-pass welds could be performed. A gap bridgeability up to 1 mm at laser-cut and 2 mm at plasma-cut samples could be reached respectively. Furthermore, a misalignment of the edges up to 2 mm could be welded in a single pass. The new findings may eliminate the need for cost and time-consuming preparation of the edges.

Hybrid Laser-Arc Welding of Thick-Walled, Closed, Circumferential Pipe Welds

The application of hybrid laser-arc welding (HLAW) for joining closed circumferential welds is a challenge due to the high risk of forming a defective overlap area with a shrinkage void or solidification cracks in the material thickness. A series of HLAW experiments were performed to understand the development of a faulty overlap area when closing the circumferential weld. Welding trials on flat specimens and pipe segments were supported by numerical analyses in which the thermomechanical behavior of the welds in the overlap area was investigated. Different process control strategies were tested, including variations in defocusing levels and the overlap length. The newly developed HLAW head, including laser optics with a motor-driven collimation system, made it possible to defocus the laser beam during welding without disturbing the stability of the welding process. High-level defocusing of the laser beam of more than 40 mm relative to the specimen surface with a resulting beam diameter of > 2.9 mm, and in combination with a short overlap length of 15 mm, was promising with respect to the formation of a desired cup-shaped weld profile that is resistant to solidification cracks.

Influence of laser power in nanosecond laser texturing for a hydrophobic state on SS316L

The use of lasers in surface engineering has recently made significant progress. The hydrophobic surface is commonly studied because of the application in various fields, including vehicles, aerospace, biomedicine, etc. Since these laser methods require many combination parameters, such as laser power (P), frequency (ƒ), scan speed (ʋ) and laser beam diameter (D), the effect of the parameters must therefore be investigated to produce the hydrophobic condition. This research tries to relate the laser power with the morphological properties and contact angle of the SS316L surfaces. Samples are subjected to laser texturing with different laser power settings. The surface is then characterised by surface roughness, and the contact angle is measured according to a specific time interval. The laser power output and energy density function on the surface and contact angle were investigated in these contexts experimentally. Surface roughness was defined and validated to show that the laser parameters' effect is effective and controllable. This study shows that the laser output intensity significantly contributes to regulating surface roughness and the substrate's wetting activity. The 18W and 24W laser outputs produce a spiked surface with various peaks that cause the surface to become hydrophobic over time because of the air-trap that happens in the valley.

Numerical Simulation of Enhancement of Superficial Tumor Laser Hyperthermia with Silicon Nanoparticles

Biodegradable and low-toxic silicon nanoparticles (SiNPs) have potential in different biomedical applications. Previous experimental studies revealed the efficiency of some types of SiNPs in tumor hyperthermia. To analyse the feasibility of employing SiNPs produced by the laser ablation of silicon nanowire arrays in water and ethanol as agents for laser tumor hyperthermia, we numerically simulated effects of heating a millimeter-size nodal basal-cell carcinoma with embedded nanoparticles by continuous-wave laser radiation at 633 nm. Based on scanning electron microscopy data for the synthesized SiNPs size distributions, we used Mie theory to calculate their optical properties and carried out Monte Carlo simulations of light absorption inside the tumor, with and without the embedded nanoparticles, followed by an evaluation of local temperature increase based on the bioheat transfer equation. Given the same mass concentration, SiNPs obtained by the laser ablation of silicon nanowires in ethanol (eSiNPs) are characterized by smaller absorption and scattering coefficients compared to those synthesized in water (wSiNPs). In contrast, wSiNPs embedded in the tumor provide a lower overall temperature increase than eSiNPs due to the effect of shielding the laser irradiation by the highly absorbing wSiNPs-containing region at the top of the tumor. Effective tumor hyperthermia (temperature increase above 42 °C) can be performed with eSiNPs at nanoparticle mass concentrations of 3 mg/mL and higher, provided that the neighboring healthy tissues remain underheated at the applied irradiation power. The use of a laser beam with the diameter fitting the size of the tumor allows to obtain a higher temperature contrast between the tumor and surrounding normal tissues compared to the case when the beam diameter exceeds the tumor size at the comparable power.

DYNAMICS OF THE ELECTRON BEAM GENERATED BY THE MAGNETRON GUN WITH DIFFERENT CONFIGURATIONS OF THE MAGNETIC FIELD IN THE TRANSPORTATION CHANNEL

The dynamics of the dimensions of the electron beam generated by the magnetron gun in the particle transport channel and the efficiency of focusing the tubular electron beam in the gradient magnetic field are investigated. The experiments were carried out with magnetron guns with secondary-emission cathodes (cathode diameters 36 and 16 mm, anodes diameters 78 and 36 mm) at cathode voltage of 20...80 kV. Magnetic fields were created both by the solenoid and jointly by the solenoid and the permanent magnet. The dependence of the radial distribution of the beam on metal targets on the amplitude and gradient of the magnetic field along the axis of the system is inves-tigated. The possibility of controlling the beam diameter by varying the magnetic field is shown. The imprints of collimated beams were obtained experimentally on targets located at selected distances. The obtained experimental data agree with the results of numerical simulation. It is shown that with an increase in the amplitude of the gradient magnetic field, the effect of radial focusing of the beam is more pronounced.

High power selective laser melting of crack sensitive nickel-base alloy CM247LC, including dimensional analysis and modelling

Compared to reference parameters in the low power and scan velocity range, which lead to dense and crack-free CM247LC LPBF samples due to in-situ crack healing, high power, high scan velocities and increased laser beam diameters are investigated, to decrease the production time further. By keeping the maximum laser intensity from the reference and the laser power to scan velocity ratio constant, the intensity approach provides an initial estimation for the laser spot size regarding the measured Archimedean density and crack density in the high power and high scan velocity range. The investigated cracks are identified as re-melting cracks. Solidification or hot cracks are not observed, since the crack healing effect for those kinds of cracks still occurs. Furthermore, a melt pool depth range is discovered, where not only solidification cracks can be avoided, but also re-melting cracks, which are resulting from higher laser power inputs. This theory can be proven by further laser spot size optimization, where the melt pool depth comes closer to the mentioned range. The Archimedean density and crack density results, in case of the 600 W power parameter with 2400 mm/s scan velocity and a beam diameter of 164 µm, are close to the one obtained from the reference with 200 W, a scan velocity of 800 mm/s and a laser spot of 90 µm. With the intensity approach and laser beam diameter optimization, the production time can be reduced by 300%. Based on dimensional analysis, a model, which combines the samples density with the crack density through the melt pool depth, is presented. Six main and two additional process and laser parameters are taken into relation. The result from the model and the measured values from experiments are in good agreement. Additionally, the influence of the doubled layer thickness and an increased hatch distance by 50% with varying scan velocities on the Archimedean density and crack density is analysed.

Wear and Corrosion Behavior of nano carbide dispersed AISI304 Stainless Steel by laser surface processing

In this study, dispersion of tungsten carbide on AISI 304 stainless steel substrate has been carried out by laser melting of the sand blasted substrate using 5 kW continuous wave (CW) Nd-YAG laser (with the beam diameter of 3 mm) with an output power ranging from 1.5 - 2 kW and scan speed varying from 12 to 16 mm/s and simultaneous feeding of premixed WC+Co in the ratio of 1:4 (with a flow rate of 10 mg/s). The microstructure of the composite zone is dendritic or cellular in morphology and consists of nano-tungsten carbide (both WC and W2C) and M23C6 precipitates. There is an enhancement in hardness from 220 VHN of the as-received substrate to 290 - 400 VHN. The wear resistance is improved significantly with a maximum enhancement observed in the sample processed with an applied power of 2 kW and a scan speed of 12 mm/s. The corrosion rate in a 3.56 wt.% NaCl solution is significantly reduced due to laser processing. However, there is a deterioration of pitting corrosion resistance (in terms of shifting of Epit towards active direction) for all the samples, except for the sample processed with an applied power of 2 kW and a scan speed of 12 mm/s.

Light Scattering in Glass Ceramic X-ray Imaging Plates

<p>Glass ceramic materials have been suggested as a possible high resolution replacement for current commercial storage phosphor imaging plates. The low spatial frequency of the current plates is caused by strong scattering of the laser light incident on the plate during the read-out process. Glass ceramic materials show very small scattering due to their transparent nature, which should lead to a higher resolution. However, a competing argument is the small amount of scattering that does occur travels a much greater distance in the plate, limiting the resolution. The aim of this thesis was to simulate the scattering of light in imaging plates and use this to optimise the trade-off between resolution, sensitivity and transparency which is implicit in plate design. Additionally, experiments were performed to determine the resolution of glass ceramic and commercial imaging plates. Simulations show that high resolution can be achieved in both the strong and weak scattering limits, corresponding to opaque and transparent materials. Increasing the absorption of the laser light increases the resolution, as does decreasing the laser beam diameter and power. An increase in the resolution almost always comes at a cost of a decrease in the sensitivity. The resolutions of an Agfa MD30 and glass ceramic imaging plate were found to be 4:5 line pairs/mm and 6:5 - 8:0 line pairs/mm respectively for an MTF equal to 0:2.</p>

Light Scattering in Glass Ceramic X-ray Imaging Plates

<p>Glass ceramic materials have been suggested as a possible high resolution replacement for current commercial storage phosphor imaging plates. The low spatial frequency of the current plates is caused by strong scattering of the laser light incident on the plate during the read-out process. Glass ceramic materials show very small scattering due to their transparent nature, which should lead to a higher resolution. However, a competing argument is the small amount of scattering that does occur travels a much greater distance in the plate, limiting the resolution. The aim of this thesis was to simulate the scattering of light in imaging plates and use this to optimise the trade-off between resolution, sensitivity and transparency which is implicit in plate design. Additionally, experiments were performed to determine the resolution of glass ceramic and commercial imaging plates. Simulations show that high resolution can be achieved in both the strong and weak scattering limits, corresponding to opaque and transparent materials. Increasing the absorption of the laser light increases the resolution, as does decreasing the laser beam diameter and power. An increase in the resolution almost always comes at a cost of a decrease in the sensitivity. The resolutions of an Agfa MD30 and glass ceramic imaging plate were found to be 4:5 line pairs/mm and 6:5 - 8:0 line pairs/mm respectively for an MTF equal to 0:2.</p>

Absorption of laser radiation in a laser-produced plasma of Xe – hydrodynamic effects and nonequilibrium ionization

Experiments on measuring absorption of an IR laser radiation in the laser-produced plasma of Xe are described. The absorbed fraction of up to 65% has been obtained when the gas-jet target was illuminated by a wide, defocused beam, whereas it barely reached 8.5% in the case of a sharply focused beam. The phenomenon is explained on the basis of a hypothesis of the plasma hydrodynamic expansion according to which the plasma leaves the illuminated area the faster, the smaller its size. Based on the experimental results, an attempt to estimate plasma parameters (N, T, <Z>) is undertaken, with the mean ion charge, <Z>, being calculated using ionization cross-sections for ions from +7Xe to +14Xe which were obtained by means of a quantum-mechanical numeric simulation especially for the present work. A similarity of the EUV output and the laser energy absorption as functions of the laser beam diameter needs an additional study in a future.