Laser ultrasonic testing for near-surface defects inspection of 316L stainless steel fabricated by laser powder bed fusion

The laser powder bed fusion(L-PBF)method of additive manufacturing(AM)is increasingly used in various industrial manufacturing fields due to its high material utilization and design freedom of parts.However,the parts produced by L-PBF usually contain such defects as crack and porosity because of the technological characteristics of L-PBF,which affect the quality of the product.Laser ultrasonic testing(LUT)is a potential technology for on-line testing of the L-PBF process.It is a non-contact and non-destructive approach based on signals from abundant waveforms with a wide frequency-band.In this study,a method of LUT for on-line inspection of L-PBF process was proposed,and a system of LUT was established approaching the actual environment of on-line detection to evaluate the method applicability for defects detection of L-PBF parts.The detection results of near-surface defects in L-PBF 316L stainless steel parts show that the crack-type defects with a sub-millimeter level within 0.5 mm depth can be identified,and accordingly,the positions and dimensions information can be acquired.The results were verified by X-ray computed tomography,which indicates that the present method exhibits great potential for on-line inspection of AM processes.

Laser ultrasonic technique for surface defects detection of 6061 aluminum alloy

In order to estimate and detect the surface defect depth of metals, the transmission method of laser ultrasonic surface waves is used in this work. The laser ultrasonic detection platform taking use of thermoelastic mechanism as acoustic signal excitation method and interference receiver as acoustic signal receiver method was built, by which B-scan images of detected specimens with surface defects were collected to establish the relationship between the transmission coefficient and depth of the surface defect. Experimental results show that the amplitude of transmitted acoustic signal is related to the depth of surface defect. At last, a fitted curve of transmission coefficient using measured experimental data is obtained to estimate depth of surface defect on the 6061 aluminum alloy. Furthermore, a surface defect depth of 0.3 mm is estimated by the fitting curve with an estimated error of 16%. Therefore, a experimental method using the transmission method by laser ultrasonic is presented in this paper.

Numerical simulation of laser ultrasonic detection of the surface microdefects on laser powder bed fusion additive manufactured 316L stainless steel

A numerical model is presented in this article to investigate the interactions between laser generated ultrasonic and the microdefects(0.01 to 0.1 mm),which are on the surface of the laser powder bed fusion additive manufactured 316L stainless steel.Firstly,the influence of the transient sound field and detection positions on Rayleigh wave signals are investigated.The interactions between the varied microdefects and the laser ultrasonic are studied.It is shown that arrival time of reflected Rayleigh(RR)waves wave is only related to the location of defects.The depth can be checked from the feature point Q,the displacement amplitude and time delay of converted transverse(RS)wave,while the width information can be evaluated from the RS wave time delay.With the aid of fitting curves,it is found to be linearly related.This simulation study provides a theoretical basis for quantitative detection of surface microdefects of additive manufactured 316L stainless steel components.

Study of mode transformation and energy attenuation of wedge waves with different apex angles by laser ultrasonic techniques

This research focuses on measuring the mode transformation and energy attenuation of wedge waves for different apex angle wedges by using laser ultrasound techniques.Pulsed laser excitation and optical deflection beam methods for detection were used to measure the wedge waves.In experiment,various wedge waves at different locations were recorded by scanning the excitation laser along the wedge tip,and different orders of wedge waves were observed.By fixing the distance between the excitation and detection positions and scanning the samples along the direction normal to the wedge tip,the modes transformation process was obtained.It was found that the energy of the acoustic waves for 20°,25° and 30° wedges decreased exponentially to a steady energy of Rayleigh wave within the main wavelength range of the acoustic modes,while the energy of acoustic waves of 45° and 60° wedge remained stable as the propagation line was scanned away from the wedge tip.

Laser ultrasonic generation and optical detection and processing

History, present situation and importancy of the laser-generated ultrasonic technique are presented. Basic principles and some experimental results of laser ultrasonic generation and optical detection and processing are discussed. Several problems about applying this technique to NDT are also discussed in this paper.

Experimental Set-up of Laser Ultrasonic Techniques and Applications

A laser ultrasonics generation and non contact detection system has been developed. The weak generated ultrasonic signal of nanosecond duration and nanometer amplitude can be detected with a confocal Fabry Perot interferometer (CFPI). It can be applied to determination of elastic constants of material and flaw detection. The experimental results show that laser ultrasonic technique is practical and effective.

Generation of narrowband Lamb waves based on the Michelson interference technique

An optical method of generating narrowband Lamb waves is presented. It is carried out with a laser line array in a thermoelastic regime implemented by the Michelson interference technique, where the formed array element spacing can be flexibly and conveniently changed to achieve selective mode excitation. In order to simulate the displacement response generated by this array, its intensity distribution function is presented to build a theoretical analysis model and to derive the integral representation of the displacement response. The experimental device and measuring system are built to generate and detect the Lamb waves on a steel plate. Numerical calculation results of narrowband Lamb wave displacement signals based on the theoretical model show good agreement with experimental results.

Ultrasonic displacement field generated by laser pulse line source

Applying the Fourier transform to the wave equations of elastic medium at its surface a laser pulse line source is acted, the integral representations of solutions are obtained. Displacement waveforms are calculated numerically by using double FFT. The calculated results include the displacements of the elastic half space of Aluminum medium, and epicenter and off-epicenter of an Aluminum plate. The two exciting sources of thermoelastic and ablating generation are considered respectively. The experiment was made on the Aluminum medium with a Nd:YAG laser and the normal displacement signals are detected by a laser interferometer. The numerical results are quite in agreement with experiments.

The application of laser reflective tomography in near-field measurements of ultrasonic transducers

With Laser Reflective Tomography（LRT）,the near fields of ultrasonic transducers were measured and analyzed.The principle of LRT measurement of ultrasonic field distribution was introduced and an experimental system was set up.Acoustic pressure of a multiple element piston transducer was measured by using of a laser vibrometer.Its distribution in amplitude and phase was obtained.The acoustic pressure in the same region was measured with a needle hydrophone to validate the LRT method.Furthermore,through reconstruction of acoustic fields,it indicated that LRT method is suitable for predicting the distribution on transducers＇surface and conditions of active elements.

Design and characterization of a novel Cu_(2.3)Al_(1.3)Ni_(1.7)SnCr_(0.3) multi-principal element alloy coating on magnesium alloy by laser cladding

The evaporation and dilution of substrate seriously limit the performance of laser cladding coatings on magnesium alloys.In order to overcome the above shortcomings,a multi-step ultrasonic assisted laser remelting technology was proposed to improve the performance of the coating.In this work,a novel Cu_(2.3)Al_(1.3)Ni_(1.7)SnCr_(0.3) multi-principal element alloy coating(MPEAC)was prepared on the surface of mag-nesium alloy.Characterization techniques such as transmission electron microscopy(TEM),electron back scatter diffraction(EBSD)and scanning electron microscopy(SEM)were employed to characterize the microstructure and phase composition of the coatings.And the phase structure and morphology at the interface between the coating and the substrate were also studied via focus ion beam(FIB)and TEM method.In addition,the corrosion and wear resistance ability of the coatings were monitored by potentiodynamic polarization(PDP),and electrochemical impedance spectroscopy(EIS),hardness and friction tests.The results show that Cu_(2.3)Al_(1.3)Ni_(1.7)SnCr_(0.3) MPEAC with ultrasonic assisted is composed of FCC phase and eutectic phases(Cu_(10)Sn_(3) and Cu_(2)Ni_(3)Sn_(3)).Due to the forced convection generated by ultrasonic waves,some Cu and Ni phases are precipitated around Cu_(2)Ni_(3)Sn_(3) phases,which is beneficial to enhance the corrosion resistance.Because of the grain refinement effect caused by ultrasonic,the wear resistance of the coating is also improved.Furthermore,ultrasonic vibration can effectively weaken and eliminate the texture density of the Cu_(2.3)Al_(1.3)Ni_(1.7)SnCr_(0.3) MPEAC fabricated by laser cladding.

Visualization of ultrasonic traveling wavefront by CW modulated laser diode

A new method is presented to display ultrasonic traveling wavefront by colltinuously modulated laser diode based on the temporal correlation theory. It is proved by experimeot.

MEASUREMENT AND ANALYSIS OF LASER GENERATED RAYLEIGH AND LAMB WAVES CONSIDERING ITS PULSE DURATION

In this article, laser generated Rayleigh and Lamb waves are studied by taking into account its pulse duration. The physical model and theoretical solution are presented to predict the corresponding waveforms for aluminum samples under the ablation generation regime. The waveforms of the excited Rayleigh and Lamb waves by laser with selected pulse duration were measured by laser interferometer and analyzed theoretically, and the agreement between measurement and analysis is demonstrated for the validation of the theoretical model and solution. The broadening of the Rayleigh wave and the disappearing of high order Lamb wave modes can be found with the increase of the pulse duration by the laser ultrasonic technique.

Effect of optical penetration on laser generatedthermoelastic ultrasound in solids

The effect of optical penetration on laser thermoelastically-generated ultrasound in non-metallic solids is studied theoretically on the basis of the thermoelastie equations. In one-dimensional model, on the assumption of some finite laser penetration, the general expression of wave displacement is obtaincd by solving the thermal diffusion and thermoelastic displacement equations, from which the influencc of optical absorption on the displacement amplitude is analyzed. In the case of two-dimensional axial symmetry, on neglecting the thermal diffusion, we solve the thermoelastic wave equation by means of the integral transform method and obtain analytic expressions of the wave displacement components in the far-field.Directivily patterns of both longitudinal and shear waves are presented and discussed.