Laser remote-fusion cutting with solid-state lasers

During laser deep penetration welding process, defects cannot be completely avoided yet. Due to the dynamic process, especially process pores and spatters occur. Keyhole dynamics are assumed to be responsible for the initiation of both pores and spatters. However, it is not completely clear yet how spatters and pores are formed. A semianalytical model of a keyhole is used to simulate dynamic keyhole properties of laser beam welding of aluminum. These are related to characteristics of spatters and pores occurring during laser deep penetration welding recorded using high-speed-imaging technique and x-ray analysis, respectively. Correlations of keyhole shapes and pore formation show that the volume underneath a keyhole collapse can be sufficient to capture the gas that is necessary for the formation of a pore and a keyhole expansion is not necessarily needed to produce the pores found in the weld seam. Calculated keyhole wall fluctuations are found to be not sufficient to detach spatters from the keyhole wall without additional forces from the melt pool.

A computational analysis is achieved during laser deep penetration welding. Multiple reflection and absorption are implemented simultaneously with the ray tracing technique in the keyhole. With all the governing equations including continuity, momentum and energy equation, the VOF method is adopted to trace the free surface of the molten pool. Temperature field is achieved by numerical simulation. The laser keyhole welding experiments on 304 stainless steel sheet showed that the computational results agree well with experimental results.

Adopting coaxial and side blown protective gas to control plasma, many experiments of CO2 laser deep penetration welding on the stainless steel are finished. Considering the protective gas effect of melten characteristics on laser welding,the experiments choose the gas species and composition to promote the collision and energy exchange of gas and plasma; The laminar flow state is holded by controlling the gas runoff, the angle and direction, increaseing the gas stiffness to prevent plasma punctured, and the plasma energy exchange range is close on the surface of the workpiece so that the energy of laser beam is less absorbed by plasma than the workpiece.The experiments indicate that shielding gas suppression are adverse factors on plasma to use alone Ar, He and N2. Adopting He-Ar-N2 and the ratio of 7:4:l as a protective gas when gas flow rate is about 30L/min, the deep and wide ratio is large,the deep penetration welding seam surfaces obtained are smooth,the defects aren't obvious;the protective effect is poor if the gas flow is too large or too small.

Keyhole stability during laser welding—Part II: process pores and spatters

In aluminum welding, the grain structure of produced seams is an essential factor with respect to the seam properties. In the casting technology the effect of mechanical vibrations on the grain growth during the solidification of liquid metals is known as a refinement method. In this paper, the transferability of this approach from comparatively long-time processes in the field of casting to the short-time process of laser deep penetration welding is investigated. Therefore, specimens were sinusoidal vibrated with frequencies up to 4 kHz during bead-on-plate full-penetration welding experiments. The resulting grain size was determined by applying the circular intercept procedure on the center of a cross-section micrograph of each weld. The results show that grain refinement is in general achievable by mechanical vibrations in the audible frequency range during laser full penetration keyhole welding of the aluminum alloy EN AW-5083.

A mathematical model that is capable of simulating the fluid flow and heat transfer during fully penetration laser welding is developed to study: the shape of the keyhole and its stability; velocity vectors and distribution in the weld pool. Firstly, a new combination heat source model consisting of a rotary Gaussian volumetric heat source and a double ellipsoidal volumetric heat source during laser keyhole welding is developed. It represents substantial characteristic of heat transfer of the keyhole. Secondly, A mathematical model for the simulation of heat transfer and fluid flow in weld pool phenomena during deep penetration laser beam welding based on a numerical solution of the conservation equations of energy, momentum and mass is presented. and verification and validation tests were carried out.A mathematical model that is capable of simulating the fluid flow and heat transfer during fully penetration laser welding is developed to study: the shape of the keyhole and its stability; velocity vectors and distribution in the weld pool. Firstly, a new combination heat source model consisting of a rotary Gaussian volumetric heat source and a double ellipsoidal volumetric heat source during laser keyhole welding is developed. It represents substantial characteristic of heat transfer of the keyhole. Secondly, A mathematical model for the simulation of heat transfer and fluid flow in weld pool phenomena during deep penetration laser beam welding based on a numerical solution of the conservation equations of energy, momentum and mass is presented. and verification and validation tests were carried out.

Machine learning-based in-process monitoring for laser deep penetration welding: A survey

Laser deep penetration welding is characterized by the formation of a vapor capillary (keyhole) and a high overall absorption (coupling rate) of the laser radiation in comparison to heat conduction welding. Due to multiple reflexions within the highly dynamic keyhole, the absorption mechanisms are very complex. The questions, how a laser beam interacts with the existing keyhole, and, if shadowing effects occur, are a matter of concern for several researchers. In this study, the hypothesis that the beam of a laser source can be transmitted to a significant extent through an existing keyhole and acts predominantly at the keyhole bottom was investigated. Experiments were carried out on mild steel and technical pure aluminum, using a specially designed optical system, which allows to coaxially combine two laser beams and to adjust their settings like the energy and the focal plane individually. A combination of a disk laser with a focal diameter of 390 µm and a single-mode fiber laser with a focal diameter of 30 µm was used. Weld depths and seam shapes were analyzed in cross and longitudinal sections. Based on the results, it is shown that the much smaller focal diameter of the single-mode laser beam acted at the bottom of the keyhole and caused a kind of “keyhole-in-keyhole formation” in the form of a local maximum in weld depth whose expression depended on the focal plane of the single-mode laser beam.

It is not generally appreciated that there is a gradation of the focal spot size from the anode to cathode end of an x-ray field that can dramatically affect radiographic detail from one end of a radiograph to the other. The authors name this gradation the "focal effect." Gradations in radiation intensity and focal spot size were measured from anode end to cathode end of a 14 x 17-inch field and were visually demonstrated with lymphangiograms and line-pair patterns. The degradation in spatial resolution along the anode-cathode axis was found to be as much as 75%, depending on the orientation of the patient with respect to the x-ray tube. Radiographic detail is, therefore, significantly improved (even when the large focal spot is used) by exploiting the focal effect and placing the body part requiring the best radiographic detail at the anode end of the table.

Acceleration of X-ray computed tomography scanning with high-quality reconstructed volume by deblurring transmission images using convolutional neural networks

A monolithic thick-wall polycapillary X-ray lens (MTW-PXRL) was designed to eliminate image splitting in a multipoint X-ray microscope, a limitation of traditional thin-walled PXRLs. The MTW-PXRL, composed of 1951 thick-walled monocapillaries, simplifies the experimental setup by eliminating the need for masks and optical alignment. A ray-tracing simulation program was developed to evaluate the performance of the MTW-PXRL in a multipoint X-ray microscope. The results show that the MTW-PXRL can generate a sparse secondary X-ray source array without image splitting and signal loss, enabling acquisition of 3D sample information in a single exposure. The effects of various MTW-PXRL parameters on multipoint projections were systematically examined by the simulation program. The projection resolution is now determined by the output inner diameter of the monocapillary, rather than the X-ray source size. Thus, this approach allows the use of a high-power laboratory X-ray source with a large focal spot, facilitating the development of rapid 3D X-ray microscopy.

In mammography, it is most important to consistently produce high-contrast, high-resolution images at the lowest radiation dose possible. During the past 20 years, there have been many significant technologic improvements in mammographic x-ray equipment and in screen-film recording systems (Haus 1990, 1991, 1994). Twenty years ago, many x-ray units were used that were not dedicated for mammography. These x-ray units had tungsten target tubes which were originally designed for general-purpose medical imaging procedures, such as chest radiography. Some of these units had compression devices that were homemade; therefore, breast compression was less than optimal by today’s standards. Many of these units had very large focal spots and/or short focal spot-to-breast surface distances which could result in significant geometric blur (unsharpness). Direct exposure (industrial type) x-ray films were being used, which often required long exposure times (causing blur due to motion) and which resulted in high radiation exposure.KeywordsFocal SpotModulation Transfer FunctionLatent ImageLine PairAutomatic Exposure ControlThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Ray-tracing is the commonly used technique to calculate the absorption of light in laser deep-penetration welding or drilling. Since new lasers with high brilliance enable small capillaries with high aspect ratios, diffraction might become important. To examine the applicability of the ray-tracing method, we studied the total absorptance and the absorbed intensity of polarized beams in several capillary geometries. The ray-tracing results are compared with more sophisticated simulations based on physical optics. The comparison shows that the simple ray-tracing is applicable to calculate the total absorptance in triangular grooves and in conical capillaries but not in rectangular grooves. To calculate the distribution of the absorbed intensity ray-tracing fails due to the neglected interference, diffraction, and the effects of beam propagation in the capillaries with sub-wavelength diameter. If diffraction is avoided e.g. with beams smaller than the entrance pupil of the capillary or with very shallow capillaries, the distribution of the absorbed intensity calculated by ray-tracing corresponds to the local average of the interference pattern found by physical optics.

Laser welding of die-cast AZ91D magnesium alloy

Detection of Forces and Displacements along the Axial Direction in an Optical Trap