Varying Laser Power Research Articles

Effect of laser processing parameters and carbon black on morphological and mechanical properties of welded polypropylene

In the present work, laser process window and welding suitability of the neat isotactic polypropylene (iPP) and its carbon black (CB: 0–3 wt.%) composites was defined for laser transmission welding (LTW) with the help of thermal characterisation and thermal simulation. This novel approach of welding feasibility moves from trial and error to robust analytical methods. The non-contact spacer method is also a novel feature of this study, followed by X-ray diffraction of the laser-treated iPP composites. This study emphasises the importance of crystallinity in defining weld integrity and quality. The welding experiments were conducted with line energies of 0.06–0.12 J/mm using the Response Surface Methodology approach by varying laser power and welding speed. The composites with 0.5 wt.% and 1.0 wt.% of CB were welded successfully with neat iPP, but composites with CB (>1 wt.%) were difficult to weld due to degradation. Weld lap shear strength and apparent weld width measured during mechanical testing showed better weld quality for 1 wt.%CB welded at line energy 0.12 J/m. Mathematical models have been developed based on the experimental results of central composite design after backward elimination. The present study demonstrates that an increase in CB to an optimised value fulfils the requirements of high welding speed with improved crystallinity and weld strength. This study is of keen interest to industrialists and researchers.

Control of Ni-Ti phase structure, solid-state transformation temperatures and enthalpies via control of L-PBF process parameters

In this work, nitinol samples were produced via Laser Powder Bed Fusion (L-PBF) in the horizontal and vertical orientations with systematic variations in laser power, scan speed and hatch spacing parameters. Increased density was positively correlated with increased laser power, scan speed and hatch spacing for the horizontally built samples but not for the vertically built samples. A smaller difference in the average temperature within a printed layer, associated with the vertically built samples, was linked with reduced porosity and reduced porosity variability between samples. Control of the L-PBF parameters was found to allow control of the resulting part chemical composition which also directly affected phase transformation temperatures, and related phase structures. The laser process parameters were found to have a significant effect (p < 0.01) on the martensite start/finish temperature, austenite start/finish temperatures, and the total temperature span. The volumetric energy density was also found to have a direct correlation with both the cooling (r = 0.52) and heating (r = 0.53) enthalpies, which was found to be due to increased nickel evaporation. Such control of phase change properties afforded from L-PBF is important for many of the end applications for nitinol components including within the energy and precision actuation sectors.

Experimental investigations of channel profile and surface roughness on PMMA substrate for microfluidic devices with mathematical modelling

Poly-methyl-methacrylate (PMMA) microchannel based devices are gaining popularity, particularly in medical applications. Experiments were carried out on a PMMA based microchannel profile of thickness 10 mm through a 100-Watt CO2 laser machine by varying laser power, scanning speed and number of passes of the laser machine. The full factorial design of the experiment was implemented to investigate the microchannel profile, depth and surface roughness of the microchannel. The analytical model was developed to predict the microchannel profile and depth for the prescribed number of laser passes. The developed analytical model has been compared with the experimental investigations and also with the previously developed models available in the literature. Results of the analytical model predicted the fabricated profile with appreciable accuracy. Moreover, the close agreement of the results with the available literature models validates the acceptability of the proposed analytical model. The surface roughness predictive modeling using linear regression, polynomial regression of order 2,3 and 4, exponential Box-Lucas method and sigmoid Boltzmann models have been compared. The statistical analysis of all the models was performed, with the analysis of variance, mean squared error, Root Mean Square Error, mean absolute percentage error and coefficient of determination. Based on the comparative assessment, the best fitted and accurate model obtained was polynomial regression of order 3. Thus, the improved surface roughness generated with the laser machining would be a time and cost technique for the fabrication of microfluidic devices.

Effect of a constant laser energy density on the evolution of microstructure and mechanical properties of NiTi shape memory alloy fabricated by laser powder bed fusion

As the key parameter in the laser powder bed fusion (L-PBF), the laser energy density can directly determine the comprehensive quality of NiTi shape memory alloys (SMAs). However, the specific role of laser parameters under the energy density value is rarely discussed. In this work, the effect of specific laser power and scanning speed variation on the properties of NiTi alloy under the same energy density (62.5 J/mm3) was studied. Results show that the phase transformation temperatures decrease as the Ni content was increasing with the simultaneous increases in laser power and scanning speed. Under the influence of Ni content, the SMA samples with low phase transformation temperatures were fully austenite at room temperature. In the cyclic compression test at Af + 15 °C, the sample built under the lowest laser power and scanning speed showed the best superelasticity effect. Higher scanning speed resulted in a stronger <100>//BD texture, and the increase of laser power produced a deeper molten pool, which remelted the solidified part at the bottom and further enhanced the <100>//BD texture. The TEM characterization shows that more Ni4Ti3 precipitates with larger sizes are obtained in the sample with the lowest laser power and scanning speed. The finite element analysis shows the lowest peak temperature in the molten pool under the lowest laser power and scanning speed, which can promote the precipitation of Ni4Ti3 particles. The promoted formation of Ni4Ti3 precipitates enhanced the austenite stability and further enhanced the superelasticity of NiTi alloy fabricated at the lowest laser power. The present work shows that the specific laser parameter under the constant energy density value can further improve the comprehensive performance of L-PBF NiTi SMAs.

Effect of preheating and cooling of the powder bed by laser pulse shaping on the microstructure of the TiC based cermets

For the fabrication of brittle materials like cermets through Selective Laser Melting (SLM), the pulse shaping technique has proven to be an effective and optimal way to produce cermet with no cracks. The purpose of this work is to investigate the effect of laser peak in pulse shaping as a preheating and cooling approach for the melt pool of TiC − Fe based cermet using the pulse shaping technique. Several samples were fabricated with variations in laser power and exposure time refereed as pre-pulse and post-pulse before and after the laser peak power (LPP). Microstructural and phase analyses were performed on the fabricated SLM sample using SEM and XRD to study the effect of pulse shaping with variation in exposure time and therefore energy density. Mechanical properties like microhardness were calculated and hence the effect of hardness and energy density were studied. Results indicated that preheating and cooling through laser pulse during pulse shaping has a beneficial effect on the fabrication of the crack-free brittle cermet.

Printability, mechanical and degradation properties of Mg-(x)Zn elemental powder mixes processed by laser powder bed fusion

• Printability of binary Mg-Zn alloys with 0, 0.5, 1, 2, 5 and 8 wt% were investigated by laser powder bed fusion. • Density parameter maps for varying laser power and scan speed were recorded for each alloy composition. • Mg0.5 Zn and Mg1Zn possessed the highest density results. • UTS of Mg1Zn was 126 MPa at a strain at failure of 5.7% and Mg0.5 Zn averaged 113 MPa at 5.4%, but showed better reproducibility. • Mg0.5 Zn and Mg1Zn possessed similar degradation behavior but were inferior to a LPBFed WE43 reference.

Improving the quality of AISI H13 tool steel produced by selective laser melting

AISI H13 tool steel is intended for making various tools and die-casting molds, which generally are of complex shapes. The application of selective laser melting (SLM) can significantly speed up their production, but it is accompanied by the appearance of various defects, such as high surface roughness, cracking, and reduced density. In particular, there is a fairly limited amount of data on the influence of SLM process parameters on the surface roughness of tool steels. The aim of this work is to study the possibility of improving the quality of AISI H13 steel by varying laser power and scanning speed during SLM. Optical profilometry and microscopy are used as methods of investigation, with statistical data analysis being carried out. It has been discovered that both surface roughness and porosity can be effectively influenced by varying the laser power and scanning speed during SLM. A direct relationship between surface roughness and porosity has been established, and this makes it possible to control effectively the quality of AISI H13 tool steel produced by selective laser melting. Steel of the highest quality with the lowest surface roughness and porosity has been obtained at a laser power of 250 W and a scanning speed of 350 mm/s. It has also been found that varying laser power and scanning speed does not prevent crack formation, and other technological solutions should be sought to eliminate cracking.

Influence of laser-Sn powder coupling on microstructure and mechanical properties of fusion welded AZ31B-alloy to DP590-steel

Microstructure and mechanical properties were studied in fusion welded AZ31B-alloy to DP590-steel at different weld conditions such as with varying laser power, and Sn powder added to lap joint with prefabricated width-settled concave groove. Temperature field and flow field for laser welding including chemical potential of each element are also calculated. The results suggested that Al atom tends to diffuse toward Fe side which contributes to form a FeAl compound reaction layer at interface. Meanwhile, Al accelerates the diffusion of Sn and Fe as a catalyst. The shear force of joint climbs at first, then declines, and the maximum value reaches 1711 N when laser power is 1 kW, which is improved by 38% compared with that of 0.9 kW. The improved joint properties are dominated by metallurgical regulation of Sn power, which benefits from the accelerated flow of molten pool and increased temperature leading to the phase composition transformation from α-Mg + Mg2Sn + FeAl to α-Mg + Mg2Sn + FeSn + Fe3Sn + FeSn2 + FeAl in direct laser irradiation region. However, under the condition of excessive laser power, overreaction occurs at side boundary interface, which leads to the formation of thick IMC and the production of hot crack defect, thus the joint performance decreases.

On the role of volumetric energy density in the microstructure and mechanical properties of laser powder bed fusion Ti-6Al-4V alloy

The volumetric energy density (EV) is considered as a comprehensive index to evaluate the complex parameters in laser powder bed fusion (L-PBF) of metallic parts. This work aims to investigate the influence of varying laser power and scanning velocity coupling under a specific EV value on the microstructure, texture, and tensile properties of L-PBF Ti-6Al-4 V (Ti64) alloy to illustrate the limitation of EV. Three sets of laser parameters under a specific EV of 58.8 J/mm3 were employed to fabricate dense parts. Analyses by electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) revealed that the same EV in the L-PBF process can result in distinct thickness of α′ lath, crystallographic texture and tensile behavior. The α′-lath was refined from 0.89 µm to 0.66 µm while laser power increased from 180 W to 240 W. Meanwhile, the crystallographic orientation of prior-β grains was oriented < 111 >β direction to parallel building direction (BD) from < 001 >β//BD, which determined the orientations of α′ lath. Tensile test results exhibit that the yield strength, ultimate tensile strength and elongation to fracture all increase with the linearly increases in laser power and speed. An excellent balance between yield strength of ~1042.2 ± 37.1 MPa and total elongation of ~ 10.8 ± 1.1% was obtained after the variant optimization. The finite element analysis shows that even under a constant EV value, varying laser power and scanning velocity coupling will carry out distinct thermal histories, leading to the grain refinement and higher dislocation density. This work shows the limited significance of EV in the guidance of the L-PBF process with complex physics. More efforts should be made on microstructural control via the combination of specific laser parameters.

Laser patterning assisted devitrification and domain engineering of amorphous and nanocrystalline alloys

We have investigated laser-patterning and devitrification as a novel method to realize emergent properties in modified regions at the ribbon surface consisting of periodic localized laser spots thermally treated under rapid heating and cooling conditions. Fe-based amorphous ribbons were annealed for systematically varied laser power. Extremely high heating and cooling rates on the order of 108-1010K/s have been estimated by finite element analysis simulations. Observations suggest surface melting followed by rapid solidification fast enough to quench and form an amorphous structure upon cooling. Diffusion of heat occurs from the laser irradiated surface region so that a finite volume of material surrounding this region rises in temperature above the crystallization temperature relevant for conventional isothermal annealing experiments. The underlying mechanism of laser annealing and consequences of heat transients involved are discussed in terms of impacts upon micro/nanostructure, residual stresses, and magnetic domain structure surrounding the laser irradiated region. The study illustrates the potential to exploit spatially optimized phase transformations in a scalable manufacturing process of amorphous and nanocrystalline alloys to locally access otherwise inaccessible extreme heating and cooling rates.

Study of the Microstructure and Corrosion Properties of a Ni-Based Alloy Coating Deposited onto the Surface of Ductile Cast Iron Using High-Speed Laser Cladding

To improve the surface corrosion resistance of ductile iron, Ni-based alloy coatings were prepared using a high-speed laser cladding technology with different levels of laser power. The microstructure, phases, and corrosion properties of the coatings were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), and an electrochemical workstation. Variations in laser power did not change the main phases of the coatings, which were composed of γ-Ni, Ni3B, Ni2Si, and Cr23C6. With an increase in power, the degree of segregation in the coating decreased, sufficient melting between elements was achieved, and the chemical composition became more uniform. Enhancement of the laser power resulted in more energy being injected into the cladding, which allowed adequate growth of tissue, and dendrites continued to grow in size as the power increased. The self-corrosion potentials of the coatings at laser power levels of 1.6, 2.0, and 2.4 kW were −625.7, −526.5, and −335.7 mV, respectively. The corrosion potential of the 2.4 kW coating was the highest, and the corroded surface of the cladding layer included mainly sizeable continuous structures with a light degree of corrosion and the highest corrosion resistance.

Effect of S-curve laser power for power distribution control on laser oscillating welding of 5A06 aluminum alloy

Laser oscillating welding of 5A06 aluminum alloy is widely used in industry. In order to improve the performance of Al-based welded joints and reduce welding defects, this paper details the effects of employing an S-curve laser power which is distributed along the laser oscillating path (sine wave) to improve the microstructure and mechanical properties of the joints. By applying the S-curve and an equal power condition to the 5A06 aluminum alloy, bottom butt sinusoidal path laser oscillating welding was performed. The welding appearance, microstructure and mechanical properties of the welded joints under two laser oscillating welding processes were investigated. Our results showed that laser oscillating welding using the S-curve power allows to obtain good joints when compared to the joints produced with constant power. When the linearly varying laser power of “high-low-high” applied periodically was employed, the grains in the joint structure showed the cyclic “growth-inhibition-growth” phenomenon. This growth mechanism not only refined the grain structure, but also narrowed the columnar region, and improved the mechanical properties of the joint. Compared with the laser oscillating process with constant power, the tensile strength of the joints using the S-curve power was increased by 35.3 %. Moreover, the tensile strength of the welded test coupons with S-curve power was 337 MPa, which is about 99.1% of the base metal (340 MPa). The welded joints using the S-curve power showed larger and deeper dimples, highlighting higher ductility and good plasticity. This paper provides a new approach for the research of laser oscillating welding and the control of laser power to improve the mechanical properties of welded joints.

Microstructural Development in Inconel 718 Nickel-Based Superalloy Additively Manufactured by Laser Powder Bed Fusion

Excellent weldability and high temperature stability make Inconel 718 (IN718) one of the most popular alloys to be produced by additive manufacturing. In this study, we investigated the effects of laser powder bed fusion (LPBF) parameters on the microstructure and relative density of IN718. The samples were fabricated with independently varied laser power (125–350 W), laser scan speed (200–2200 mm/s), and laser scan rotation (0°–90°). Archimedes’ method, optical microscopy, and scanning electron microscopy were employed to assess the influence of LPBF parameters on the relative density and microstructure. Optimal processing windows were identified for a wide range of processing parameters, and relative density greater than 99.5% was achieved using volumetric energy density between 50 and 100 J/mm3. Microstructural features including melt pool geometry, lack of fusion defect, keyhole porosity, and sub-grain cellular microstructure were examined and quantified to correlate to LPBF parameters. A simple empirical model was postulated to relate relative sample density and LPBF volumetric energy density. Melt pool dimensions were quantitatively measured and compared to estimations based on Rosenthal solution, which yielded a good agreement with the width, but underestimated the depth, particularly at high energy input, due to lack of consideration for keyhole mode. In addition, the sub-grain cellular-dendritic microstructure in the as-built samples was observed to decrease with increasing laser scan speed. Quantification of the sub-micron cellular-dendritic microstructure yielded estimated cooling rate in the order of 105–107 K/s.

In situ absorption synchrotron measurements, predictive modeling, microstructural analysis, and scanning probe measurements of laser melted Ti-6Al-4V single tracks for additive manufacturing applications

In this work, the fundamental processing-structure-property (PSP) relationships that govern laser-based additive manufacturing were investigated with the Ti-6Al-4V alloy. X-ray synchrotron imaging carried out in conjunction with in-situ integrating sphere radiometry enabled real-time energy absorption measurements for a range of melting conditions that varied laser power and velocity. A thermal conduction model that incorporated the in-situ absorption data and final melt pool geometry was used to predict the thermal histories and diffusion distances along the heat-affected zone (HAZ) in the Ti-6Al-4V alloy to provide insight into the solid-state phase transformations that occurred in the unmelted regions adjacent to the melt pool. Resulting microstructural features were quantified using scanning electron microscopy techniques to elucidate changes in solidification behavior. Significant changes to α/β-Ti phase fractions were measured in the unmelted HAZ, across all test cases. Nanoindentation and scanning probe microscopy revealed differences in the hardness, modulus, and Volta potential across the resolidified melt pool, HAZ, and wrought base material. These measurements and simulations can be used to predict how processing changes lead to differences in the as-built performance of titanium parts that are used in aerospace and biomedical applications. This work demonstrates the utility of coupling in-situ absorption data with a conduction-only high speed model, which leads reasonable agreement with the synchrotron imaging measurements and microstructural transformations observed herein.

Advanced mechanism of multiphysics fields tip enhancement induced with varied laser power to fabricate pattern-transformable subdiffraction limit nanostructures.

The nanofabrication platform was carried out using an atomic force microscope (AFM) system and a continuous wave (cw) laser to investigate the influence of laser power on the underlying mechanism of nanostructures fabricated by multiphysics fields tip enhancement (MFTE) induced by a cw laser irradiating the AFM probe tip. The nanostructure fabrication of nanopits and grooves and nanodots and lines on a polymethyl methacrylate thin film was conducted in an ambient environment by changing the incident laser power. The dependence of the MFTE on laser power was numerically analyzed, too. The lateral dimensions of nanopits and grooves and nanodots and lines characterized in situ were 154 nm, 96 nm, 188 nm, and 25 nm, respectively, breaking the optical diffraction limit. It turned out that the nanostructures converted from craters (pits and grooves) to protrusions (dots and lines) when altered with the laser power. Different laser powers can trigger the MFTE to change, thus, inducing varied coupling energy, which is the essential mechanism for nanostructure conversion. We also established a model to analyze the nanostructures transition and to predict the dimensions of nanostructures. The simulation results demonstrate that the MFTE has an essential effect on the formation of nanostructures, which are in good agreement with the experimental results.

Influence of Ring-Shaped Beam Profiles on Process Stability and Productivity in Laser-Based Powder Bed Fusion of AISI 316L

A major factor slowing down the establishment of additive manufacturing processes as production processes is insufficient reproducibility and productivity. Therefore, this work investigates the influence of ring-shaped beam profiles on process stability and productivity in laser-based powder bed fusion of AISI 316L. For this purpose, the weld track geometries of single tracks and multi-track segments with varying laser power, scan speed, hatch distance, and beam profile (Gaussian profile and three different ring-shaped profiles) are analyzed. To evaluate the process robustness, process windows are identified by classifying the generated single tracks into different process categories. The influence of the beam profiles on productivity is studied by analyzing the molten cross-sectional areas and volumes per time. When using ring-shaped beam profiles, the process windows are significantly larger (up to a laser power of 1050 W and a scanning speed of 1700 mm/s) than those of Gaussian beams (laser power up to 450 W and scanning speed up to 1100 mm/s), which suggests a higher process robustness and stability. With ring-shaped beam profiles, larger volumes can be stably melted per track and time. The weld tracks created with ring-shaped profiles are significantly wider than those generated with Gaussian profiles (up to factor 2 within the process window), allowing enlargement of the hatch distances. Due to the higher scanning speeds and the enlarged hatch distances for ring-shaped beam profiles, the process can be accelerated by a factor of approximately 2 in the parameter range investigated.

Additive manufacturing of cobalt-based alloy on tool steel by directed energy deposition

Cladding hard-surfacing alloys on tool steel is an effective approach to enhance the surface properties of tool steel. In this study, a Co-based alloy was deposited on tool steel by Directed Energy Deposition (DED) following a three-factor three-level design of experiment matrix with varied laser power, scan speed, and powder flow rate. The microstructure of the deposits was characterized using scanning electron microscopy (SEM). The residual stress on the surface of the samples was measured by the X-ray diffraction (XRD) sin2ψ technique. The parameters that produced promising deposits were used to fabricate samples for tensile test, four-point bending test, Charpy impact test, and hardness measurement. The result reveals that the processing parameters have a significant role in the residual stress of the coatings. Residual stress reduces with the increase of laser energy density. Cracks were found at samples with energy density below a threshold. Tensile testing of the coating/substrate combined structure reveals fracture at the coatings with an ultimate tensile strength of 633.9 ± 54.7 MPa. The bi-material interface survived the tensile test, indicating a strong interfacial bond. The four-point bending test of coating/substrate laminates shows an ultimate flexure strength of 860.6 ± 36.9 MPa. Cracks initiated from the coatings ignored the interface and penetrated the substrate, suggesting a solid bi-material bond. Charpy impact test shows the absorbed energy of coating/substrate laminates is more than doubled that of the substrate.

Investigations of wavelength division multiplexing-orthogonal frequency division multiplexing (WDM-OFDM) system with 50 Gb/s optical access

Abstract In the present work, orthogonal frequency division multiplexing (OFDM) is combined with wavelength division multiplexing-passive optical networks (WDMPONs) to obtain the combined benefits of both technologies for overall system improvement. performance of five OFDM signals using WDM is analyzed. An optical system is designed using the OFDM signal, and its performance is analyzed using various matrices like bit error rate (BER) and Q-factor under the effect of varying laser power. Mapping of OFDM signals is done using 16-QAM modulation. The system shows an enhancement in the performance by increasing the laser power but up to some limit, and efficient results are obtained in the 70–130 mW laser power range.

Multiple electron beam generation from InGaN photocathode

In this study, we generated 25 multielectron beam (MEB) using an InGaN photocathode with a negative electron affinity state irradiating with 25 multilaser beam. The uniformity of the MEB and the total electron beam current were evaluated. A laser beam was split into 25 laser beams using a spatial light modulator. The coefficient of variation (CV) of laser power was 20%. The CV of quantum efficiency was 1.1%. The CV of electron beam current was 12%, and the total current was about 1.2 μA. These results will enhance the development of the MEB-defect inspection using the InGaN photocathode.

Characterization of Ni-Cr-Mo alloy phase structures under DLD process

Direct laser deposition of Ni-Cr-Mo alloy has been carried out to study the phase structure transformation under various operating parameters. To assess the phase composition SEM, TEM and XRD analysis were performed. It clearly seen that the precipitation of straightening phases (γ' and γ") depends on the temperature gradients caused by different laser power. The nucleation of γ' phase starts at 750 W while γ" phase at 1000 W. The changes in temperature gradients lead to different diffusion condition across all the stage of laser power variation and formation of δ-phase, primary MC carbides and Laves phase. δ-phase of all samples did not change the shape and characterized only by plate-like shape.