Laser Line Scan Research Articles

Effect of the scanning path on the nanosecond pulse laser welded Al/Cu lapped joint

In this research, Al/Cu lapped joints with a thickness of 0.2 mm were joined via nanosecond pulsed laser welding. The influence of laser scanning paths on the weld formation, microstructure and bonding strength was investigated. Sound weld formations were produced when the outer spiral laser scanning path was used. Porosities and cracks were easily formed in joints produced by concentric circle and straight laser scanning paths due to the interval duration between the neighboring welding processes. The interfacial morphology was a wave structure when the laser scanning paths were an outer spiral and concentric circle, and continuous interfacial intermetallic compound (IMC) was formed at the interface. θ–CuAl2 was generated in the vicinity of the Cu substrate, while α-Al + θ–CuAl2 was generated in the outer region and the weld seam. When a straight line laser scanning path was adopted, the wave structure disappeared. Discontinuous IMC and cracks were also produced at the interface. The tensile-shear load results indicated that the outer spiral laser scanning line was the most beneficial for the bonding strength of the joint and could reach up to 198 N. A satisfactory weld formation, the pinning effect due to the wave structure at the interface and continuous interfacial IMC all contributed to its highest bonding strength.

Error compensation for laser line scanners

This paper presents an algorithm to compensate the systematic error of lasers line scanned data based on the measurement error model of the measurement system. The iterative process of the algorithm is used to determine geometrical features accurately, solely making use of an optical measurement device. The systematic error on the task-specific measurement is reduced by the algorithm. The validation of the algorithm consists of the evaluation of multiple compensated measurements on calibrated ring gauges with different diameters and similar surface properties as the acquisition artefact of the measurement error model. Furthermore, a validation is performed on a gauge block. Experimental research shows the reduction of the systematic error as a function of the scan strategy due to the compensation by the algorithm.

Synthetic image data augmentation for fibre layup inspection processes: Techniques to enhance the data set

In the aerospace industry, the Automated Fiber Placement process is an established method for producing composite parts. Nowadays the required visual inspection, subsequent to this process, typically takes up to 50% of the total manufacturing time and the inspection quality strongly depends on the inspector. A Deep Learning based classification of manufacturing defects is a possibility to improve the process efficiency and accuracy. However, these techniques require several hundreds or thousands of training data samples. Acquiring this huge amount of data is difficult and time consuming in a real world manufacturing process. Thus, an approach for augmenting a smaller number of defect images for the training of a neural network classifier is presented. Five traditional methods and eight deep learning approaches are theoretically assessed according to the literature. The selected conditional Deep Convolutional Generative Adversarial Network and Geometrical Transformation techniques are investigated in detail, with regard to the diversity and realism of the synthetic images. Between 22 and 166 laser line scan sensor images per defect class from six common fiber placement inspection cases are utilised for tests. The GAN-Train GAN-Test method was applied for the validation. The studies demonstrated that a conditional Deep Convolutional Generative Adversarial Network combined with a previous Geometrical Transformation is well suited to generate a large realistic data set from less than 50 actual input images. The presented network architecture and the associated training weights can serve as a basis for applying the demonstrated approach to other fibre layup inspection images.

Application of Reverse Engineering for Automotive Plastic Components – Case Study

AbstractThe advancement in control and measuring technologies, the widespread acceptance and the growing popularity of the reverse engineering concept made the possibility of reproducing objects with unknown specifications an easier task. Although industrial practice offers many reverse engineering methods, this technology is not yet used at its fully potential. This article presents a case study involving the application of reverse engineering technology on a complex geometry plastic part from the automotive industry. The aim of the paper is to present a step‐by‐step reverse engineering method, starting from the creation of the point cloud obtained through scanning of the parts surfaces (using a 7.5 axis portable measuring arm with a laser line scanner) to the reproduction of the physical part.

Development and Modeling of a Robotic Restoration System Based on Laser Directed Energy Deposition

Laser-based directed energy deposition (DED) technique has the potential for precise restoration of localized damage in high value components. This paper describes the indigenous development and sub-systems integration of a robotic restoration system based on laser DED. This system comprises of four modules: a robot for imparting desired outcome; 3 kW fiber laser as the energy source; powder feeder; and a quartz block head connected deposition head with co-axial powder delivery nozzle. In addition, a damage detection technique using a laser line scanner has been developed. Experimental characterization of the developed system is also presented. Laser DED involves a complex interplay of several physical phenomena, such as powder particle injection, melt pools formation, deposition spreading, and coupled metallo-thermomechanical behavior which induces residual stresses in the deposition and the substrate. In order to understand the complete process, each of these vital mechanisms needs to be studied. Computational fluid dynamics models have been developed to predict the flow and injection of powder particles via coaxial annular nozzle, melt pool spreading and deposition geometry. The major challenge in restoration via DED is the compromised service life of the restored part due to unfavorable residual stresses originating from differential thermal expansion/contraction, volume dilation and transformation plasticity. A fully coupled metallo-thermomechanical model has been presented in this paper. The residual stresses obtained from the model have been validated using residual stress measurements from X-ray diffraction and neutron diffraction techniques. Finally, process maps have been developed based on these physics-based models to obtain optimal process parameters to ensure favorable residual stresses, dilution and geometry in DED.

Effect of laser spot overlap ratio on surface characteristics and adhesive bonding strength of an Al alloy processed by nanosecond pulsed laser

• Nanosecond laser ablation treatment enhances adhesive bonding strength for Al alloy. • Laser-induced surface physiochemical modifications increase with overlap ratio. • Excessive overlap ratio of laser spots reduces the strength of laser-treated joints. • Micropores in adhesive layer caused by insufficient wetting reduce joint strength. • Corrosion resistance of laser-treated substrate drops with increasing overlap ratio. The laser ablation treatment has been validated to be an effective surface treatment method in improving adhesive bonding performance for metal or composite materials. This study was further conducted to investigate the effect of laser spot overlap ratio on surface characteristics and resultant adhesive bonding strength for AA6022-T4 Al alloy. The laser spot overlap ratio was changed by varying the laser processing speed and line spacing between laser scanning lines. Surface characteristics analyses show that the laser ablation treatment at higher overlap ratio produces rougher surface morphology and thicker aluminum oxide layer on substrate surface, which are considered to be more favorable for adhesive bonding strength. Lap-shear strength results show that the laser ablation treatment at overlap ratio of -49 % (corresponding with separated laser spots) improves the joint strength by 6.8 % and 17.1 % before and after water soak exposure compared to as-received condition. Meanwhile, the desired cohesive fracture mode is also achieved which indicates that the joint strength reaches to the saturated value depending on mechanical performance of adhesive. This explains that the continuous increase of laser spot overlap ratio from -49 % to 63 % exhibits negligible improvement on joint strength. Nevertheless, although inducing a higher degree of surface physicochemical modifications on Al substrates, the laser ablation treatment with an overlap ratio beyond 63 % decreases the joint strength. Detailed studies of SEM micrographs verified that the decreased joint strength is attributed to insufficient wetting of adhesive on the rough surfaces of laser-treated substrates resulting in entrapment of air between substrate and adhesive layer, which forms micropores defects in the adhesive layer and consequently weakens the adhesive-bonding joints. In addition, the improved corrosion resistance of adhesive-bonded Al joints by laser ablation treatment is attributed to the barrier effect of laser-produced rough surface morphology to water diffusion along bonding interface.

Reducing Lifecycle Costs due to Profile Scanning of the Powder Bed in Metal Printing

First time right is one major goal in powder based 3D metal printing. Reaching this goal is driven by reducing life cycle costs for quality measures, to minimize scrap rate and to increase productivity under optimal resource efficiency. Therefore, monitoring the state of the powder bed for each printed layer is state of the art in selective laser melting. In the most modern approaches the quality monitoring is done by computer vision systems working with an interference on trained neural networks with images taken after exposure and after recoating. There are two drawbacks of this monitoring method: First, the sensor signals - the image of the powder bed - give no direct height information. Second, the application of this method needs to be trained and labeled with reference images for several cases. The novel approach presented in this paper uses a laser line scanner attached to the recoating machine. With this new concept, a direct threshold measure can be applied during the recoating process to detect deviations in height level without prior knowledge. The evaluation can be done online during recoating and feedback to the controller to monitor each individual layer. Hence, in case of deviations the location in the printing plane is an inherent measurement and will be used to decide which severity of error is reported. The signal is used to control the process, either by starting the recoating process again or stopping the printing process. With this approach, the sources of error for each layer can be evaluated with deep information to evaluate the cause of the error. This allows a reduction of failure in the future, which saves material costs, reduces running time of the machine life cycle phase in serial production and results in less rework for manufactured parts. Also a shorter throughput time per print job results, which means that the employee can spent more time to other print jobs and making efficient use of the employee’s work force. In summary, this novel approach will not only reduce material costs but also operating costs and thus optimize the entire life cycle cost structure. The paper presents a first feasibility and application of the described approach for test workpieces in comparison to conventional monitoring systems on an EOS M290 machine.

Defect detection in steel bars up to 600 °C using laser line thermography

Crack detection in steel bars at high surface temperatures is a critical problem in any manufacturing industry. Surface breaking cracks are the major problems during the billet casting. Many NDT techniques are proven its capability in crack detection at room temperature. Here, we are demonstrating the possibility of exposure of cracks using laser line thermography at higher surface temperatures (up to 600 °C). A continuous-wave (CW) laser is used to excite the sample kept at higher surface temperatures. The temperature distribution over the sample due to the laser line scanning is captured using a temperature calibrated infrared (IR) thermal camera. The response of the sample temperature in crack detection is investigated using a validated FE model. The impact of the oxide layer in crack detection is investigated by using two types of samples; one without any oxide layer and the second is with the oxide layer. The influence of laser power in the detection of defects at high temperatures is studied. 3D numerical models were developed for the cases; when the sample is with oxide layer and without any oxide layer for a better understanding of physics. The surface temperature rise due to laser heating is higher for the scaled sample compared to the no-scale sample. The presence of the oxide layer above the parent metal will reduce the reflectivity of the surface. Lower reflectivity will lead to increased absorption of incident energy so that the surface temperature rise will be higher than the surface with no scale. Thermal contrast linearly depends on laser power, which means higher laser power will increase the defect detectability even at a higher surface temperature.

Laser line scanning of a shape of moving objects with various degree of transparency

Laser scanning methods for surface geometry reconstruction have become a common tool in many areas of application due to relative simplicity, high accuracy, and versatility. Nevertheless, scanning of glossy or translucent surfaces still poses a problem for these techniques. The present work aims at assessing the applicability of the laser line scanning method for evaluation of the height profile of a waste stream on a conveyor belt, taking into account that the waste stream may contain a large number of semi-transparent objects. In particular, preliminary results of height profile measurements with regard to the laser emission wavelength for polyethylene terephthalate (PET) bottles with various degrees of transparency, common in the municipal solid wastes, are reported in the paper.

On incorporating scanning strategy effects into the modified inherent strain modeling framework for laser powder bed fusion

Laser powder bed fusion ( L -PBF) has been the most popular metal additive manufacturing (AM) process thus far. However, residual deformation of the metal builds has been a significant issue. Laser scanning strategies adopted in the laser-assisted fabricating process have proved to have important influence on the residual stress and deformation. As the main contribution of this paper, the effects of different laser scanning strategies are incorporated into the modified inherent strain modeling (MISM) framework for the first time to enable accurate simulation for residual deformation of the L -PBF metal parts. Anisotropy in mechanical property of the fabricated components caused by the laser scanning strategies is also fully considered. For the rotational laser scanning strategies, only a small-scale representative volume element (RVE) is modeled by employing both the inherent strains and asymptotic homogenization. By employing the homogenized inherent strains, residual deformation can be predicted for the L -PBF manufactured components using different rotational scanning strategies accurately. Regarding the unidirectional parallel line scanning strategy, the directional inherent strain vector and orthotropic mechanical properties are used in the MISM-based layer-wise simulation. Good accuracy of the proposed framework is fully validated through comparing the simulated residual deformations with the experimental results for Inconel 718 parts produced by different laser scanning strategies. Thus, it is demonstrated that the effects of both parallel line and rotational laser scanning strategies have been successfully integrated into the MISM framework for predicting residual deformations of the L -PBF builds.

Laser line scanner aptitude for the measurement of Selective Laser Melting parts

When looking for any metrological verification of parts manufactured by metal laser printing with optical equipment, it is necessary to ensure the traceability of the measurements that can be obtained. The difficulty of this process lies in the fact that these measurements are obtained on point clouds captured from surfaces with high form errors and poor surface finishes, even when this type of surface usually undergoes processes to improve the surface finish, such as sandblasting. This research focuses precisely on the analysis of the metrological suitability of a laser line scanner (laser triangulation sensor) on parts manufactured by Selective Laser Melting (SLM).The study starts from the design of a test part specifically oriented to the printing process with SLM metal powder bed. This test part was printed in 17-4PH stainless steel and then sandblasted. The test part was measured in a Coordinate Measuring Machine (CMM), obtaining reference GD&T values. The measurement was carried out under pre-sandblasting ("as built") and post-sandblasting conditions, thus providing interesting information about the erosion rate of this post process. A state-of-the-art laser sensor was employed for the metrological comparison, mounted on the same available CMM that was used for contact measurements.In this research three analyses were carried out: the quality of 3D metal printed parts with respect to CAD model, the effect of the sandblasting post-process, and the accuracy of the measurements obtained with the laser line sensor. In addition, this work conducts an in-depth study about the influence of point cloud treatment and filtering procedures, by comparing the filtering methods applied by different reverse engineering software packages. The study leads to the conclusion that filters based on the standard deviation of the point cloud are the best candidates in order to obtain laser measurements closer to the contact measurements.

Joint Ecological and Archeological Shipwreck Surveys Using Submersible Video and Laser‐Line Scanning

Archeological and ecological studies of shipwrecks are often conducted separately. We provide a proof of concept for a cross-disciplinary approach. Here, we extract ecological metrics from archeological surveys on two World War II shipwrecks that rest at ~200 m depth off North Carolina, USA. We used two advanced technologies (video and laser-line scanning), both from human-occupied submersibles to assess fish communities from imagery and models intended for archeological investigations. Combining video and laser technologies allowed us to identify fish and pinpoint their locations. The resulting fish data revealed that both shipwrecks hosted high densities of groupers, primarily concentrated around tall shipwreck features. These photographs illustrate the article “Extracting ecological metrics from archaeological surveys of shipwrecks using submersible video and laser-line scanning” by Katrina H. Johnson, Avery B. Paxton, J. Christopher Taylor, Joseph Hoyt, John McCord, and William Hoffman Ecosphere. https://doi.org/10.1002/ecs2.3210.

Dual-beam stealth laser dicing based on electrically tunable lens

In this paper, we present the development of a high-speed dual-beam stealth laser dicing (D-SLD) method for processing semiconductor wafers based on electrically tunable lenses (ETLs). An SLD system utilizes a laser beam to dice a wafer by inducing interior defects without modifying the surface characteristics of a wafer, thereby presenting significant advantages over conventional dicing methods, e.g., blade dicing or laser ablation. Currently, the throughput of SLD is limited by the serial scanning process; in addition, wafer misalignment and the warpage effects together deteriorate the dicing quality and yield and demand high-cost multi-axis precision stages. To address the issue, we parallelize the dicing process by splitting the laser focus into two foci, where the laser intensity at different depths are automatically adjusted to achieve optimal dicing condition. By combining the height sensor and ETLs, each laser focus, i.e., laser scan lines, can be rapidly controlled axially to compensate the wafer curvature and misalignment errors in real time, leading to substantially improved precision (kerf width < 2 μm) and speed, demonstrated by our experimental results. The new dual-beam laser dicing system presents an effective solution for high-speed wafer dicing as well as other important engineering applications, e.g., fabrication of micro-devices and optical components.

Influence of laser fluences and scan speeds on the morphologies and wetting properties of titanium alloy

The influence of laser fluences and laser scanning speeds on the morphologies and wetting properties of titanium alloy surfaces was investigated by fs laser line scanning. The fs laser textured surfaces showed some micro-cracks, and laser-induced periodic surface structures (LIPSS) with some nanoparticles decoration. The water wetting response of the samples were measured by static contact angle measurements. The obtained laser-textured titanium alloy surfaces were hydrophilic, and the water contact angle (CA) decreased with increase of the laser fluence used and also the surface roughness. The laser-textured surfaces with laser fluence of 0.79 J/cm2 and scan speed of 50 mm/min were found to retain its hydrophilic charateristic for a long time storage in ambient atmosphere. However, some other laser-textured surfaces were changed from hydrophilic to hydrophobic range. The micro-Raman spectra were also used to analyze the surface phases of titanium alloy after laser treatments. The results imply the potential applications of these surfaces in a variety of fields.

Super resolution laser line scanning thermography

In this paper we propose super resolution measurement and post-processing strategies that can be applied in thermography using laser line scanning. The implementation of these techniques facilitates the separation of two closely spaced defects and avoids the expected deterioration of spatial resolution due to heat diffusion.The experimental studies were performed using a high-power laser as heat source in combination with pulsed thermography measurements (step scanning) or with continuous heating measurements (continuous scanning). Our work shows that laser line step scanning as well as continuous scanning both can be used within our developed super resolution (SR) techniques. Our SR techniques make use of a compressed sensing based algorithm in post-processing, the so-called iterative joint sparsity (IJOSP) approach. The IJOSP method benefits from both - the sparse nature of defects in space as well as from the similarity of each measurement. In addition, we show further methods to improve the reconstruction quality e.g. by simple manipulations in thermal image processing such as by considering the effect of the scanning motion or by using different optimization algorithms within the IJOSP approach. These super resolution image processing methods are discussed so that the advantages and disadvantages of each method can be extracted.Our contribution thus provides new approaches for the implementation of super resolution techniques in laser line scanning thermography and informs about which experimental and post-processing parameters should be chosen to better separate two closely spaced defects.

Microscope vision system based on micro laser line scanning for characterizing microscale topography.

A microscope vision system to characterize a microscale surface via micro laser line projection is presented. The characterization is performed by means of surface descriptors, which include the root mean square, kurtosis, skewness, homogeneity, entropy, contrast, and correlation. These descriptors are computed from surface irregularities, which are retrieved by means of the micro laser line projection. The characterization is carried out by an optical microscope system on which a CCD camera and a 36 µm laser line are attached. Thus, the microscope vision system projects the micro laser line on the surface, and the CCD camera captures the line reflection, which provides the surface contour. The contour dimension is computed via Bezier networks by means of the micro laser line coordinates. Thus, the surface descriptors are computed by means of the surface contour to accomplish the characterization. The proposed characterization improves the accuracy of the optical microscope imaging systems, which characterize the microscale surface by means of the gray-level intensity. Thus, the capability of the characterization via micro laser line projection is established by means of the descriptors' accuracy. This contribution is corroborated by characterizing metal and paper surfaces.

Multifunctional laser speckle imaging.

We have developed a multi-functional laser speckle imaging system, which can be operated in both the surface illumination laser speckle contrast imaging (SI-LSCI) mode and the line scan laser speckle contrast imaging (LS-LSCI) mode. The system has been applied to imaging the chicken embryos to visualize both the blood flow and morphological details of the vasculature. The experimental results demonstrated that LS-LSCI is capable of detecting and quantifying blood flow in blood vessels smaller and deeper than those detectable by conventional SI-LSCI. Furthermore, the line scan mode is also capable of producing depth-resolved absorption-based morphological images of tissue, augmenting flow-based functional images.

Virtually structured detection enables super-resolution ophthalmoscopy of rod and cone photoreceptors in human retina.

High resolution imaging is desirable for advanced study and clinical management of retinal diseases. However, spatial resolution of retinal imaging has been limited due to available numerical aperture and optical aberration of the ocular optics. This study is to develop and validate virtually structured detection (VSD) to surpass diffraction limit for resolution improvement in in vivo retinal imaging of awake human. A rapid line scanning laser ophthalmoscope (SLO) was constructed for in vivo retinal imaging. A high speed (25,000 kHz) camera was used for recording the two-dimensional (2D) light reflectance profile, corresponding to each focused line illumination. VSD was implemented to the 2D light reflectance profiles for super-resolution reconstruction. Because each 2D light reflectance profile was recorded within 40 μs, the intra-frame blur due to eye movements can be ignored. Digital registration was implemented to further compensate for inter-frame eye movements, before the VSD processing. Based on digital processing, the modulation transfer function (MTF) of the imaging system was derived for objective identification of the cut-off frequency of ocular optics, which is essential for robust VSD processing to ensure reliable super-resolution imaging. Dynamic motility analysis of the super-resolution images was implemented to further enhance the imaging contrast of retinal rod and cone photoreceptors. The VSD based super-resolution SLO significantly improved image quality compared with equivalent wide-field imaging. In vivo observation of individual retinal photoreceptors has been demonstrated unambiguously. Dynamic motility analysis of the super-resolution images enhanced the contrast of retinal rod and cone photoreceptors, and revealed sub-cellular structures in cone photoreceptors. In conjunction with rapid line-scan imaging and digital registration to minimize the effect of eye movements, VSD enabled resolution improvement to observe individual retinal photoreceptors without the involvement of adaptive optics (AO). An objective method has been developed to identify MTF to enable quantitative estimation of the cut-off frequency required for robust VSD processing.

Polarization sensitive microstructures fabricated on lithium niobate surfaces by using femtosecond laser pulses.

Polarization sensitive microstructures with different morphologies were induced by irradiating dual lithium niobate crystals with femtosecond laser pulses. An upper lithium niobate crystal served as a mask plate to tailor light field, which led to the formation of crater and arc-shaped structures on the surface of a lower lithium niobate crystal. In single-shot irradiation, the orientation and morphology of resultant microstructures can be tailored by controlling the focusing position, because focus splitting took place when a focused laser light propagated through dual lithium niobate crystals. In scanning, the width and morphology of laser scan lines can be governed using various combinations of focusing position and scanning direction. Furthermore, large-area micro/nanostructures with different topography features were successfully fabricated on the crystal surface and their absorption spectra indicated that the absorptance in the visible wavelength range was strongly dependent on fabricated micro/nanostructures. This new type of structured lithium niobate surfaces can be potentially applied in optical and photonic devices.

Effect of Laser Pulse Overlap and Scanning Line Overlap on Femtosecond Laser-Structured Ti6Al4V Surfaces.

Surface structuring is a key factor for the tailoring of proper cell attachment and the improvement of the bone-implant interface anchorage. Femtosecond laser machining is especially suited to the structuring of implants due to the possibility of creating surfaces with a wide variety of nano- and microstructures. To achieve a desired surface topography, different laser structuring parameters can be adjusted. The scanning strategy, or rather the laser pulse overlap and scanning line overlap, affect the surface topography in an essential way, which is demonstrated in this study. Ti6Al4V samples were structured using a 300 fs laser source with a wavelength of 1030 nm. Laser pulse overlap and scanning line overlap were varied between 40% and 90% over a wide range of fluences (F from 0.49 to 12.28 J/cm²), respectively. Four different main types of surface structures were obtained depending on the applied laser parameters: femtosecond laser-induced periodic surface structures (FLIPSS), micrometric ripples (MR), micro-craters, and pillared microstructures. It could also be demonstrated that the exceedance of the strong ablation threshold of Ti6Al4V strongly depends on the scanning strategy. The formation of microstructures can be achieved at lower levels of laser pulse overlap compared to the corresponding value of scanning line overlap due to higher heat accumulation in the irradiated area during laser machining.