Quality assessment methods and techniques for laser welding have been developed both in- and post-process. This paper summarizes and presents relevant studies being classified according to the technology implemented (vision, camera, acoustic emissions, ultrasonic testing (UT), eddy current technique (ECT)) for the quality inspection. Furthermore, the current review aims to map the existing modeling approaches used to correlating measured weld characteristics and defects with the process parameters. Research gaps and implications of the quality assessment in laser welding are also described, and a future outlook of the research in the particular field is provided.

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Quality assessment in laser welding: a critical review

An efficient all-fiber optic source is presented; it adopts absorbing films, deposed directly over the fiber tip, as targets. It is demonstrated that the use of absorbing films made of pure graphite, or graphite powder mixed with epoxy resin, has produced a conversion efficiency increase of two orders of magnitude with respect to metallic materials. It is observed that the conversion efficiency increases monotonically as thickness is reduced down to the material optical penetration depth. Moreover, the conversion efficiency rises with the concentration of graphite powder. Principal advantages of this kind of source are the ease of production and miniaturization, the excellent electromagnetic compatibility, wide ultrasonic bandwidth and, consequently, high spatial resolution. The ultrasonic bandwidth can be controlled by varying the laser pulse duration. The possibility of generating ultrasonic signals with high frequency and flat spectral distribution makes the proposed device suitable for biological tissue spectral characterization.

https://cern.ch/dmeniche/documenti_david0/BiagiUFFC01.pdf

Efficient laser-ultrasound generation by using heavily absorbing films as targets

This paper proposes the speed control of a separately excited dc motor varying armature voltage. The novelty of this paper lies in the application of nonlinear autoregressive-moving average L2 controller for the speed control of SEDM. This paper also discusses speed control of a SEDM using chopper circuit. The performance of the proposed system has been compared with the traditional one using conventional controllers. The entire system has been modeled using MATLAB 7.0 toolbox. It has been found that both PI and hysteresis current controllers could be eliminated by the use of NARMA-L2 controller.

Speed Control of Separately Excited DC Motor

https://upcommons.upc.edu/bitstream/handle/2117/8540/arias_thermoelastic_2003.pdf;sequence=1

Thermoelastic Generation of Ultrasound by Line-Focused Laser Irradiation

This paper deals with the elastodynamic finite integration technique for axisymmetric wave propagation in a homogeneous and heterogeneous cylindrical medium (CEFIT). This special variant of a finite difference time domain (FDTD) scheme offers a suitable method to calculate real three-dimensional problems in a two-dimensional staggered grid. In order to test the accuracy of the numerical CEFIT code, problems for which analytical solutions are available are presented. These solutions involve wave propagation in an elastic plate, the scattering of a plane longitudinal wave by a spherical obstacle, and ultrasound generation by a thermoelastic laser source. For the latter problem experimental results are included. The CEFIT code also allows the treatment of more complicated problems. Further possible applications are the investigation of elastic waves generated in an acoustic microscope, the simulation of impact-echo measurements in multi-layer systems, axisymmetric wave propagation in arbitrary bodies of revo...

The elastodynamic finite integration technique for waves in cylindrical geometries

A real-time ultrasound-based system for controlling robotic weld quality by monitoring the weld pool is presented. The weld penetration depth is one of the most important geometric parameters that define weld quality, hence, remains a key control quantity. The sensing system is based on using a laser phased array technique to generate focused and steered ultrasound, and an electromagnetic acoustic transducer (EMAT) as a receiver. When a pulsed laser beam is incident on the surface of a condensed matter, either the thermoelastic expansion or ablation induces mechanical vibrations that propagate as ultrasound within the specimen. Both the ultrasound generation by the laser phased array and the reception by the EMAT are noncontact, which eliminates the need for a couplant medium. They are capable of operating at high temperatures involved in the welding process. The ultrasound generated by the laser phased array propagates through the weld pool and is picked up by the EMAT receiver. A signal-processing algorithm based on a cross-correlation technique has been developed to estimate the time-of-flight (TOF) of the ultrasound. The relationship between the TOF and the penetration depth of the weld has been established experimentally and analytically. The analytical relationship between the TOF and the penetration depth, which is obtained by the ray-tracing algorithm and geometric analysis, agrees well with the experimental measurements.

Charles Ume

Papers

Real-Time Weld Penetration Depth Monitoring With Laser Ultrasonic Sensing System

Non-contact optical fibre phased array generation of ultrasound for non-destructive evaluation of materials and processes

Laser generated ultrasound: a thermoelastic analysis of the source

Temporal modulation of a laser source for the generation of ultrasonic waves

Longitudinal wave generation in laser ultrasonics