The paper is devoted to researching various post-processing methods that affect surface quality, physical properties, and mechanical properties of laser additively manufactured steel parts. The samples made of two types of anticorrosion steels—20kH13 (DIN 1.4021, X20Cr13, AISI 420) and 12kH18N9T (DIN 1.4541, X10CrNiTi18-10, AISI 321) steels—of martensitic and austenitic class were subjected to cavitation abrasive finishing and vibration tumbling. The roughness parameter Ra was reduced by 4.2 times for the 20kH13 (X20Cr13) sample by cavitation-abrasive finishing when the roughness parameter Ra for 12kH18N9T (X10CrNiTi18-10) sample was reduced by 2.8 times by vibratory tumbling. The factors of cavitation-abrasive finishing were quantitatively evaluated and mathematically supported. The samples after low tempering at 240 °C in air, at 680 °C in oil, and annealing at 760 °C in air were compared with cast samples after quenching at 1030 °C and tempering at 240 °C in air, 680 °C in oil. It was shown that the strength characteristics increased by ~15% for 20kH13 (X20Cr13) steel and ~20% for 12kH18N9T (X10CrNiTi18-10) steel than for traditionally heat-treated cast samples. The wear resistance of 20kH13 (X20Cr13) steel during abrasive wear correlated with measured hardness and decreased with an increase in tempering temperatures.

https://www.mdpi.com/2075-4701/10/11/1540/pdf

Effect of Cavitation Erosion Wear, Vibration Tumbling, and Heat Treatment on Additively Manufactured Surface Quality and Properties

The results of studies of acoustic anisotropy and anisotropy in terms of Young's and shear moduli, Poisson's ratio, and transverse strain of low-carbon 2% Mn-doped steel manufactured by laser powder bed fusion technology are presented in this article. The results on non-uniformity of anisotropy over the surface area of disc-shaped samples are also presented. The electromagnetic-acoustic method is used to input and receive longitudinal and transverse waves of various polarizations. The dependences of the tH-wave velocity and Poisson's ratio on the modes of heat treatment of the samples are obtained. The angular indicatrices of the change in the velocity of the transverse tH-wave depending on the direction of polarization and the indicatrix of Poisson's ratio are presented, as well as the calculated values of the weighted average Poisson's ratio over the plane of the samples. 

Acoustic properties of low-carbon 2% Mn-doped steel manufactured by laser powder bed fusion technology

In the automatic manufacturing of robotic welding, real-time monitoring of weld quality is a difficult problem. Meanwhile, due to volatilization of zinc vapor in galvanized steel and complexity of welding process, the existence of welding defects greatly affects industrial production process. And real-time detection of welding defects is a key step in development of intelligent welding. To realize real-time monitoring of weld surface defects, an active visual monitoring method for weld surface defects is proposed. Firstly, after applying Gabor filter to remove interference signals such as arc and noise, obtain weld centerline image; then employ the slope analysis method to extract peak valley coefficient of weld defects, extract five feature points of weld surface quality by Douglas-Puke algorithm, and analyse geometric and spatial distribution features of different types of defects in weld laser stripe images. Finally, using eight feature vectors extracted from weld features, design a weld defect recognition random forest model based on decision tree. After optimizing the decision tree depth and number of model evaluators, compared with the traditional decision tree ID3 and CART algorithm model, this model has better performance than traditional machine learning algorithms on five weld surface defect data sets. The experimental results show that accuracy of weld defect identification in the training set is 99.26%, accuracy of weld defect recognition in the test set is 96%, and processing time of a single image is only 5.3 ms, which overcomes difficulty of real-time weld defect detection in intelligent welding and ensures real-time and accuracy.

An active visual monitoring method for GMAW weld surface defects based on random forest model

Additive manufacturing has evolved over the past decades into a technology that provides freedom of design through the ability to produce complex-shaped solid structures, reducing the operational time and material volumes in manufacturing significantly. However, the surface of parts manufactured by the additive method remains now extremely rough. The current trend of expanding the industrial application of additive manufacturing is researching surface roughness and finishing. Moreover, the limited choice of materials suitable for additive manufacturing does not satisfy the diverse design requirements, necessitating additional coatings deposition. Requirements for surface treatment and coating deposition technology depend on the intended use of the parts, their material, and technology. In most cases, they cannot be determined based on existing knowledge and experience. It determines the scientific relevance of the analytical research and development of scientific and technological principles of finishing parts obtained by laser additive manufacturing and functional coating deposition. There is a scientific novelty of analytical research that proposes gas-discharge plasma processing for finishing laser additive manufactured parts and technological principles development including three processing stages—explosive ablation, polishing with a concentrated beam of fast neutral argon atoms, and coating deposition—for the first time.

https://www.mdpi.com/2227-7080/9/2/27/pdf

Surface Quality of Metal Parts Produced by Laser Powder Bed Fusion: Ion Polishing in Gas-Discharge Plasma Proposal

Results of the tomographic inspection of a prototype sample additive manufactured by selective laser alloying have been evaluated. Specific features of revealing “incomplete fusion” flaws and the influence of various parameters of inspection on its results are considered.

Some Specific Features of Studying Additive-Manufactured Parts by X-Ray Computed Tomography

—The paper presents the results of studying the detectability of surface volumetric defects by ultrasonic method of nondestructive testing. The possibility of using Rayleigh surface waves excited by an electromagnetic-acoustic transducer (EMAT) for these purposes is considered. Blind vertical drillings of various diameters and depths were used as artificial defects to simulate defects in low carbon steel samples. Based on the results of measurements, the dependence of the amplitude of the received signals on drilling parameters has been constructed. When statistically processing the results, the signal-to-noise ratio during the excitation of the Rayleigh wave with the help of the EMAT on defect-free areas of samples was taken into account. Optimal models of the distribution of signal amplitudes were determined to construct the curves of probability of detection (PoD) of a defect. Under the conditions of the experiments, based on the PoD curves constructed, conclusions have been drawn about the minimum dimensions of surface volumetric defects identified with 90% probability with allowance for the confidence interval of 90% and about the possibility of adjusting the parameters of ultrasonic testing by signals reflected from vertical drillings.

Investigating the Detectability of Surface Volumetric Defects in Ultrasonic Testing with the Use of Rayleigh Waves Generated by an Electromagnetic-Acoustic Transducer

The possibilities and limitations of applying head and surface waves to ultrasonic inspection of articles manufactured by selective laser alloying are considered. The optimum angles have been determined for the generation of head and surface waves in a product made of the domestic EP648 powder. The peculiarities of revealing crack-like flaws by the ultrasonic echo method are demonstrated.

On the Possibility of Using Ultrasonic Surface and Head Waves in Nondestructive Quality Checks of Additive Manufactured Products

Issues of quality control of welded joints in hot-tool butt-welded polyethylene pipelines are considered. It is indicated that the main scope of control currently falls on inspections based on the parameters of welding and on selective mechanical testing of welded joints. Physical nondestructive post-welding testing methods are used little, which is generally not typical for the practice of testing welded joints. The reason for this is the general problem of ultrasonic methods, viz., the low detectability of contracted defects, partially transmitting ultrasound. To ensure the detectability of such defects, we evaluate the possibility of increasing the sensitivity of ultrasonic testing by using the ultrasonic time-of-flight-diffraction (TOFD) technique. It is proposed to use the tip of vertical drillings to simulate defects weakly scattering ultrasound. It is shown that when tuning is performed based on such targets, the inspection sensitivity can be increased by at least 6 dB, i.e., two times or more. In this case, the drilling tip can simulate the upper and lower edges of a planar defect vertically oriented in the section of the weld. The results of experimental studies aimed at revealing multidirectional drills with a diameter of 1 mm in welded seams of polyethylene pipes, including pipes with an additional removable outer layer, are presented. The expediency of gathering statistical data on the identification of real defects in welded butt joints of polyethylene pipes using the considered testing method is shown.

Studying Applicability of TOFD Technique to Inspection of Welded Joints in Polyethylene Pipes

The problems of detecting operational plane surface defects by the ultrasonic method of nondestructive testing with the use of Rayleigh surface waves generated by an electromagnetic-acoustic transducer are considered. The paper presents an experimental study of the detection of plane defects simulated by an artificial reflector of the “groove” type with different opening, depth, and angle of slope. The dependences of the signal amplitude on the listed parameters are constructed and their nature is evaluated. The optimal amplitude models for constructing the curves of the probability of detection of defects (PoD) have been determined. A conclusion is made about the minimum dimensions of a plane defect of the operational type detected by the considered method with a probability of 90% taking into account the confidence interval of 95%.

N. V. Krysko

Papers

Investigating the Detectability of Surface Volumetric Defects in Ultrasonic Testing with the Use of Rayleigh Waves Generated by an Electromagnetic-Acoustic Transducer

On the Possibility of Using Ultrasonic Surface and Head Waves in Nondestructive Quality Checks of Additive Manufactured Products

Studying Applicability of TOFD Technique to Inspection of Welded Joints in Polyethylene Pipes

Studying Detectability of Plane Surface Defects by Ultrasonic Method Using Rayleigh Waves

Characteristic of cracks detection during ultrasonic testing of weld joints of vertical stock tanks’ wall