Pulsed Wave Laser Research Articles

Laser powder bed fusion of the Ni superalloy Inconel 939 using pulsed wave emission

The aim of this work is to determine the laser powder bed fusion (LPBF) processability window for the fairly weldable Inconel 939 (IN939) alloy using pulsed wave (PW) laser emission. With that goal, a design of experiments strategy was put in place with the aim of finding out the laser power, scan speed and hatch distance parameters that would render the lowest defect density. Also, a combination of characterization techniques, including optical microscopy, electron backscattered diffraction, and transmission electron microscopy was utilized to examine the microstructure of as-built and heat treated samples at different length scales. Finally, mechanical tests were carried out at temperatures ranging from room temperature to 950 °C and quasi-static strain rates in order to compare the mechanical performance of the additively manufactured specimens with that of cast samples. This work demonstrates that it is possible to process IN939 by PW LPBF with densities exceeding 99.5% and that a relatively large flexibility in microstructural design is allowed within the optimum processability window. In particular, both polycrystalline samples with weak textures and irregularly shaped grains, as well as columnar structures with relatively strong <001> fibers can be manufactured. It is shown that, using the standard post-processing heat treatment, the mechanical behavior of PW LPBF Inconel 939 specimens is superior to that of cast specimens at T ≤ 700 °C due to a strong Hall-Petch effect, but that pronounced softening takes place at T > 700 °C due to an enhanced role of diffusion-based processes. This study provides guidelines for the manufacturing of PW LPBF IN939 specimens with superior mechanical behavior at a wide range of temperatures.

Investigation of Intermetallics Formation and Joint Performance of Laser Welded Ni to Al

In this paper, laser welding Ni to Al using pulsed wave (PW) and continuous wave (CW) lasers was investigated. Weld quality and strength were evaluated in terms of cross-section examination, intermetallic compounds formation, microhardness, shear test and 90-degree peel test. The results show that deep penetration welding Ni to Al causes high melting pool temperature and severe material mixing, which could result in dominant AlNi3 and AlNi intermetallics (IMCs) in the weld. These IMCs could significantly increase the hardness of the welding zone, but could also lead to the formation of defects, as well as reducing the ability to withstand the shear force and peel force applied to the weld. In comparison, using process optimization to maintain a shallow penetration or form a weld-braze joint, low melting pool temperature and minimum material mixing can be achieved. Hence, low-hardness Al3Ni IMCs are prevalent in the weld. This helps generate a defect-free dissimilar weld joint to withstand higher shear force and peel force. The findings show promising applications, such as the battery management system of electric vehicles, in which joining a Ni adaptor to an Al bus bar is required.

Single-frequency linearly-polarized gain-switched DFB pulsed fiber laser

We report a single-frequency linearly-polarized gain-switched distributed feedback (DFB) pulsed fiber laser. A 976 nm laser diode (LD) which can generate both pulsed and continuous wave (CW) laser is used as the pump source to initiate gain-switched progress. Pulsed fiber laser with single-frequency and linearly-polarization is obtained by applying pressure on a 50 mm long phase shifted fiber Bragg grating (PS-FBG). Through parameter optimization, the laser generates single-frequency linearly-polarized laser pulses with a center wavelength of 1030.78 nm, a narrowest linewidth of 36.1 MHz, a maximum peak power of 3.78 W, and a polarization extinction ratio of better than 19 dB. And the repetition rate of output pulses can be flexibly changed from 1 kHz to 400 kHz with a pulse width of 20–200 ns.

Effects of groove clearance size on gap bridging capacity in PWLBW of Ti6Al4V alloy sheet assembled in butt joint configuration: Numerical simulation and experimental assessment

In laser tailor-welding of metal sheets, it is difficult to keep the assembly clearance fixed due to plate specification deviation, tooling error, and thermal deformation. In this paper, the influence of clearance size on gap bridging capacity and in turn the seam appearance quality was investigated for thin Ti6Al4V alloy plates, configured by an improved pulse wave laser beam welding (PWLBW) scheme. The welding experiment and the numerical simulation were performed at 1.2 mm-thick workpieces assembled in a butt joint with a reserved clearance. With the clearance size enlarged from 0.2 mm to 0.4 mm, the gap bridging type changes from bilateral to unilateral, which results in the failure of the keyhole to rebound during the pulse interval, and causes an obvious reduction in metal bridge area at the cross-section. As the clearance increases further, a through-cavity geometry is generated between the weld spots and finally preserved within the solidified weld bead, indicating a poor overlapping effect. The dynamics of the weld pool, including thermal cycle, Marangoni convection, and molten metal flow rate are exquisitely sensitive to the change in clearance size. The clearance size during assembly and welding processes should be controlled below 0.4 mm to attain seam integrity, continuity, and sound tensile property.

Numerical study of thermal fluid dynamics and solidification characteristics during continuous wave and pulsed wave laser welding

Pulsed wave (PW) mode laser welding is often preferred under conditions where specific heat input and tailorable cooling rate are required. Previous works on numerical modelling of laser welding mainly focus on continuous wave (CW) mode, while physical understanding of PW mode from numerical investigation is scarce. In this work, a 3D multi-physics thermal fluid model is developed considering both CW and PW modes laser welding. The main physical factors involving heat transfer, thermal fluid flow, Fresnel absorption, recoil pressure induced by metal evaporation, Marangoni effect driven by surface tension, free surface tracing, and laser multiple reflections inside the keyhole are included in this model. The model is tested and validated against the corresponding experimental results for laser welds of stainless steel 316 L. The simulated weld fusion cross sections and surface molten pool agree well with the experimentally observed results for both CW and PW modes. The results show that PW mode exhibits a higher depth of penetration and a smaller molten pool at the same heat input with CW mode. In CW mode laser welding, the shape of keyhole is unstable, while its depth keeps relatively stable. There are two main flow patterns in the molten pool of CW mode welding: an outward flow from keyhole outlet to the rear weld pool and a clockwise flow from the keyhole tip to the rear weld pool. While for PW mode, keyhole and molten pool show periodic behaviors with a period of the laser energy output used. The periodic fluid flow pattern is mainly affected by the gradually changed surface tension and suddenly changed recoil pressure. Furthermore, it is found that a finer microstructure with a smaller secondary dendrite arm spacing is obtained in PW mode due to a higher cooling rate from both the calculated and experimentally measured results.

Improvement of Part Accuracy by Combination of Pulsed Wave (PW) and Continuous Wave (CW) Laser Powder Bed Fusion

In this paper, a processing strategy is investigated to increase the geometric accuracy of parts fabricated by laser powder bed fusion (LPBF). Pulsed wave emission (pw) is used for contour exposure in combination with a continuous wave (cw) emission for bulk volume exposure. With the goal of discrete solidification of adjacent melt pools, process parameters of laser power PL, scanning speed vs, and relative pulse overlap ∆xo are developed for contour exposure. Samples with variable contour angles are fabricated to investigate the effect of the pw contour exposure on excessive melting in critical areas of the part prone to overheating. Geometric accuracy and surface roughness are evaluated using SEM images of surface topography and optical surface roughness measurements, respectively. It is observed that excessive melting can be suppressed by using pw contour exposure and usage of modified process parameters. Due to the discretized energy input in pw emission mode, smaller melt pools with lower melt pool fluctuation and powder erosion are produced. The maximum applicable scanning speed is limited by the solidification time of the melt pool and is significantly lower compared to conventional cw contour exposure parameters. Therefore, a combination of cw volume exposure with high process productivity and pw contour exposure for high geometric accuracy is beneficial to limit productivity losses and increase accuracy in part building.

Physical processes during ultracold plasma expansion

Using the method of molecular dynamics, the expansion of a two-component, pulsed laser-produced ultracold plasma is directly calculated for various values of the number and density of particles and their electron temperatures. A new method is presented for generating and diagnosing a steady-state ultracold plasma formed under continuous wave laser irradiation. The performed calculations show the difference in the properties of an ultracold plasma obtained by pulsed and continuous wave laser irradiation.

The Effect of Laser Ablation Pulse Width and Feed Speed on Necrosis and Surface Damage of Cortical Bone

Bone cutting is of importance in orthopaedic surgery but is also challenging due to its nature of brittleness—where severe mechanical and thermal damages can be introduced easily in conventional machining. Laser machining is a new technology that can allow for complex cut geometries whilst minimising surface defects i.e., smearing, which occur in mechanical methods. However, comparative studies on the influence of lasers with different pulse characteristics on necrotic damage and surface integrity have not been reported yet. This paper for the first time investigates the effects of laser type on the necrotic damage and surface integrity in fresh bovine cortical bone after ex-situ laser machining. Three lasers of different pulse widths, i.e., picosecond, nanosecond and continuous wave lasers have been investigated with different feed speeds tested to study the machining efficiency. The cutting temperature, and geometrical outputs have been measured to investigate the thermal influence on the cooling behaviour of the bone samples while high-speed imaging was used to compare the material removal mechanisms between a pulsed and continuous wave laser. Furthermore, an in-depth histological analysis of the subsurface has revealed that the nanosecond laser caused the largest necrotic depth, owing to the high pulse frequency limiting the dissipation of heat. It has also been observed that surface cracks positioned perpendicular to the trench direction were produced after machining by the picosecond laser, indicative of the photomechanical effect induced by plasma explosions. Therefore, the choice of laser type (i.e., in terms of its pulse width and frequency) needs to be critically considered for appropriate application during laser osteotomy with minimum damage and improved healing.

LiDAR Based Self-Driving Car

LiDAR, typically used as an acronym for “’light detection and ranging’”, is essentially a sonar that uses pulsed laser waves to map the distance to surrounding objects. It is used by a large number of autonomous vehicles to navigate environments in real time. Its advantages include impressively accurate depth perception, which allows LiDAR to know the distance to an object to within a few centimetres, up to 60 metres away. It’s also highly suitable for 3D mapping, which means returning vehicles can then navigate the environment predictably —a significant benefit for most self-driving technologies. One of the key strengths of LiDAR is the number of areas that show potential for improvement. These include solid-state sensors, which could reduce its cost tenfold, sensor range increases of up to 200m, and 4-dimensional LiDAR, which senses the velocity of an object as well as its position in 3-D space. However, despite these exciting advances, LiDAR is still hindered by a key factor; its significant cost. LiDAR is not the only self-driving detection technology, with cameras as the major rival, championed by Tesla as the best way forward. Elon Musk has described LiDAR as “a fool’s errand” and “unnecessary”. The argument runs that humans drive based only on ambient visible light, so robots should equally be able to. A camera is significantly smaller and cheaper than LiDAR (although more of them are needed), and has the advantage of seeing in better resolution and in colour, meaning it can read traffic lights and signs. However, cameras have a wide host of characteristics that make them tricky to use in common driving conditions. Whereas LiDAR uses near infra-red light, cameras use visible light, and are thus more susceptible to issues when faced with rain, fog, or even some textures. In addition, LiDARs do not depend on ambient light, generating their own light pulses, whereas cameras are more sensitive to sudden light changes, direct sunlight and even raindrops

Achieving high quality nitinol parts with minimised input thermal energy by optimised pulse wave laser powder bed fusion process

Nitinol alloys have great applications especially in the biomedical industry due to their unique super-elastic and shape memory properties which depends on the balance between the element composition Ni–Ti. For this reason, controlling the applied volumetric thermal energy (VED) during the manufacturing process is very essential in order to identify the final part's property. It is known that the boiling temperature of nickel is far less than that of titanium thus is easier to vaporise and alter the chemical composition of the alloy. In this study, pulse operating mode was employed in a Laser-Powder Bed Fusion (L-PBF) process to produce additively manufactured (AM) parts with high density and high mechanical properties with minimised input thermal energy. The aim was always to achieve full melting situation and to maintain the original chemical composition of the virgin powder. It has been shown that fully dens parts with significant compression strength, comparative modulus of elasticity, and stable chemical composition can be produced by modulating the laser pulse by 50% duty cycle. This means reducing the applied VED by 50% and working in low levels of ∼6.5 J/mm3, avoiding excessive heating and vaporization of metals and enhancing the sustainability of the process. All the tested samples exhibit a catastrophic fracture accompanied by loud noise and spark during compression. A new approach was used to calculate the VED in a more accurate way by calculating the pulse energy and the exact number of pulses per unit area.

A Review on Macroscopic and Microstructural Features of Metallic Coating Created by Pulsed Laser Material Deposition.

Owing to the unparalleled advantages in repairing of high value-add component with big size, fabricating of functionally graded material, and cladding to enhance the surface properties of parts, the laser material deposition (LMD) is widely used. Compared to the continuous wave (CW) laser, the controllability of the laser energy would be improved and the temperature history would be different under the condition of pulse wave (PW) laser through changing the pulse parameters, such as duty cycle and pulse frequency. In this paper, the research status of temperature field simulation, surface quality, microstructural features, including microstructures, microhardness, residual stress, and cracking, as well as corrosion behavior of metallic coating created by pulsed laser material deposition have been reviewed. Furthermore, the existing knowledge and technology gaps are identified while the future research directions are also discussed.

New phenomenological model for comparison of lasers with different temporal outputs

Laser welding is distinguished by low heat input, low distortion, high travel speeds and accuracy. Traditional high-power pulsed wave (PW) lasers are being replaced by high-frequency low-pulse energy fibre lasers. However, as these lasers operate at very high frequencies, near continuous wave (CW) operation, it is not clear the benefit of such frequencies in comparison to CW lasers for micro-welding. In this project, two lasers, one in high-frequency PW and another in CW are operated at the same conditions, including average power, average peak power, spot size and travel speed, and the differences in material response are investigated. It has been shown that frequency is one of the important parameters that affect the heat loss between individual pulses, referred to as inter-pulse losses. At low frequency, the PW laser provided lower melting efficiency and higher penetration efficiency than CW. On the other hand, at high frequency, the PW resulted in lower melting and penetration efficiency than CW. In addition, a new definition of interaction time has been proposed to capture conduction losses by travel speed and heat inter-pulse losses due to periodic lack of laser power. This allows a like-for-like comparison of CW and PW lasers and can be used to predict penetration depth with processing parameters.

Nonlinear localization of high energy long wave laser pulses in fully correlated 3D turbulence.

We study the interplay between three-dimensional (3D) fully correlated optical turbulence and nonlinearity in time and 3D space resolved long-wavelength infrared pulsed beam propagation. Here the average self-trapped beam waist exceeds the inner scale in contrast to near-infrared filaments, and we find that their nonlinear self-channeling remains robust even in the presence of strong turbulence. More surprisingly, our simulation results invite a conjecture that in regimes where diffraction and nonlinearity are roughly balanced, turbulence can result in a tighter localization of the nonlinear beam core.

Residual stress control of 316 L stainless steel using pulsed-wave laser additive manufacturing

In this paper, 316 L stainless steel were fabricated by direct laser deposition (DED) using both continuous-wave (CW) and pulsed-wave (PW) laser modes. Effects of laser modes on residual stress, geometric dislocation density (GND) and solid phase transformation with a given heat energy input were investigated. Results showed that PW mode tends to generate less concentrated, lower residual stress than that CW mode does, which the decline range was reached about 42% at the joint of the deposition and the substrate and approximately 75% on the deposition respectively. This is determined by the process characteristics of PW mode. Firstly, the relatively small temperature gradient in PW mode directly causes small thermal stress. Secondly, the characteristic of periodic temperature history and an improved cooling rate of molten pool in PW mode leads to more thermal stress relaxation. The released thermal stress can be used as a driving force for the formation of dislocations at grain boundaries. In addition, solid phase transformation does not occur with PW mode, which has little contribution to the reduction of residual stress.

Improving surface quality and superficial microstructure of LDED Inconel 718 superalloy processed by hybrid laser polishing

In the present research, a hybrid laser polishing technology combining pulsed laser and continuous wave laser was applied to polish the surface of laser directed energy deposition (LDED) Inconel 718 superalloy components. The surface morphology, microstructure evolution and microhardness of the as-fabricated, the single pulsed laser polishing (SPLP) and the hybrid laser polishing (HLP) processed samples were investigated. The results revealed that the as-fabricated sample has a rough surface with sintered powders. In the matrix, the NbC carbide and Cr2Nb based Laves phase array parallel to the build direction and the small γʺ-Ni3Nb particles precipitate in matrix uniformly. The surface roughness of the as-fabricated sample is 15.75 μm which is decreased to 6.14 μm and 0.23 μm by SPLP and HLP processing, respectively. The SPLP processing refines the grains and secondary phase significantly in the remelted layer which is reconstructured with the cellular structure and plenty of substructures. The HLP processing also refines the grain and secondary phase but the secondary phases still exhibit array distribution. In addition, the tangled dislocations pile up along the interface of secondary phases. Compared with the as-fabricated sample, the SPLP processing decreases the surface microhardness but the HLP processing increases the surface microhardness, and the Young's elasticity modulus of surface layer is improved by SPLP and HLP processing to 282 ± 5.21 GPa and 304 ± 5.57 GPa, respectively.

Low-cost camera-based pulsed and modulated continuous wave laser detection for laser-based optical communications

A novel, low-cost, camera-based method of detecting a single nanosecond (ns) laser pulse and kHz modulated continuous wave and pulsed lasers has been developed. The detector uses a simple optical modification to a standard rolling shutter color camera combined with image processing techniques to distinguish lasers from other illumination sources and extract a laser’s wavelength and pulse repetition frequency. In addition, the detector is also capable of detecting a single ns laser pulse at any given time. Such a detector has applications in free-space optical communications (FSO comms) as a low-cost broadband method of extracting information from multiple sources and as a detector of laser range finders. A low-cost prototype (≈£600) has been developed using entirely off-the-shelf components and assessed in laboratory and field trial conditions, with the ability to measure laser wavelengths to ±5 nm and pulse repetition frequencies to within ±5 % at a distance of 660 m. In the laboratory, the prototype was also able to detect each of the 1000 pulses generated by a 10 Hz 10 ns 532-nm pulsed laser, as well as 100 pulses sent at random intervals, highlighting its capability to detect a ns pulse at any given time. This novel technology offers a low-cost method of detecting lasers and extracting their pulse repetition frequencies, with a wide field of view and high spatial resolution. Ultimately, this technology has applications in FSO comms for between vehicles or platforms of interest, with a capability of communicating with multiple targets simultaneously.

Comparison of continuous and pulsed wave lasers in keyhole welding of stainless-steel to aluminium

A continuous wave (CW) and a nanosecond pulsed wave (PW) lasers were used to join 1-mm thick sheets of SS304L (SS) austenitic stainless-steel to AA5251 (Al) aluminium alloy in an overlap joint configuration. The weld shape (penetration depth and width), intermetallic compounds concentration, weld quality (cracking and porosity) and mechanical strength were correlated with the process energy and compared between each laser temporal mode. Successful CW joints were produced with the SS sheet on top of the Al, but the opposite configuration revealed to be impossible for the range of parameters tested. The PW joints were successful with the Al sheet on top of SS, but all the joints cracked at the interface when the opposite configuration was used. The mechanical tests showed that even though it is possible to achieve higher tensile shear load in CW welds due to the larger bonding area, the tensile shear strength revealed to be almost 5 × higher for PW welds at similar applied energy.

Additive manufacturing of NdFeB magnets by synchronized three-beam laser powder bed fusion

Neodymium iron boron (NdFeB) magnet that is manufactured using laser powder bed fusion (LPBF) machines has attracted attention since the complex magnetic parts can be directly manufactured. This study used LPBF systems with single and three-beam lasers to fabricate NdFeB components, and the effects of different process parameters (pulsed or continuous wave laser, powder layer thickness, laser power, and scanning velocity) on the magnetic characteristics were investigated. The results from the synchronized three-beam method using the two-zone scanning strategy improved the maximum magnetic energy product by 33.42 kJ/m3, and the processing time was decreased by 38% compared to the single-beam method.

Effect of energy on Ti plate fabrication by vacuum selective laser melting for uniformity of grain size

Grain size homogenization by controlling heat input using modulated pulses in additive manufacturing was verified. A Ti plate was fabricated with a modulated pulsed laser by vacuum selective laser melting. 20-layer fabrication was performed with modulated pulsed laser and continuous wave laser. As a result of cross-sectional observation, it was found that the variation of the grain size was smaller when the modulated pulsed laser was used. These experimental results suggest that it is possible to control the grain size by using a modulated pulsed laser.

Weld pool dynamics and joining mechanism in pulse wave laser beam welding of Ti-6Al-4V titanium alloy sheets assembled in butt joint with an air gap

A validated CFD model coupling the multiple phases and multiple physics was developed for the dynamic simulation of pulse wave laser welding (PWLBW) process of 1.2 mm-thick Ti-6Al-4V alloy sheets assembled in butt joint with a reserved air gap. A laser spot diameter of 700 μm was used to compensate for the energy loss with the clearance applied at 0.2 mm and therefore improve the gap bridging ability. The distributions and evolutions of temperature, velocity, phase interface and weld pool dimensions were characterized and discussed to reveal the joining mechanism and the heat and flow behaviors during welding. The results show that the liquid bridges firstly occur in the middle height of sheets as the welding begins, and rapidly extend to the whole thickness with the increasing heat input. The reduction on laser power causes the rebounding of keyhole surface and the backfilling of molten metal. The keyhole closes at root surface and rebounds completely in 2 microseconds, leading to the merging of liquid bridges, bottom-up moving of high temperature area, and temporarily enlargement on weld pool size at root surface. During the pulse interval, the weld pool volume shrinks significantly and the molten metal slows down from Marangoni vortexes to thermal buoyance flows. The proposed process parameters allow an air gap accounting for 16.67% of specimen thickness and result in a well-formed and defect-free weld bead.