Defect and Diffusion Forum Vol. 448

Abstract: In this study, natural convection of a hybrid nanofluid inside a cubic cavity under the influence of a constant external magnetic field is numerically investigated by using control volume method. The cavity is partially heated from the left wall with uniform temperature and cooled from the opposite wall while the other sides are kept adiabatic. Analysis is focused on the impact of some parameters, including Hartmann number (0≤Ha≤100), Rayleigh number (10³≤Ra≤10⁶), nanoparticle volume fraction (0≤Φ_s≤0.06) and heater band width (1/3≤ ɛ ≤1). The Analysis of the results related to the dynamic and thermal structures, as well as the average Nusselt number, revealed that the effect of the external magnetic field exerts a negative influence on heat transfer within the cavity. However, more favorable findings were observed when the volume fraction of nanoparticles was increased, as well as when the width of the heater band was increased.

Abstract: To describe the flow of a gas in a micro cavity, two different approaches can be adopted; namely the macroscopic approach or the microscopic approach. The objective of the present work is to use the Boltzmann lattice method (LBM) as a numerical alternative of kinetic methods to describe the behavior of rarefied gases. It is an intermediate mesoscopic method between the two approaches that allows to estimate the solution of the Boltzmann transport equation with a reduced computational time. In this report, the flow of a rarefied gas in a micro cavity with presence of an obstacle in its middle has been studied. Simulation results are presented for different degrees of rarefaction (). The choice of boundary conditions plays an important role in the stability and accuracy of the numerical schemes. For this reason, several types of boundary conditions have been used to correctly determine the slip velocity and the temperature jump at the solid walls, which allow a good description of gaseous micro-flows.

Abstract: Lattice boltzmann method (LBM) has emerged as a powerful numerical technique for simulating fluid flows due to its inherent simplicity and efficiency. In this paper we studied a convective heat transfer occurring naturally within a cubic enclosure differentially heated filled with air (Pr=0.71) to compare the obtained results with those obtained from the literature. For this a fortran code program utilized for simulating natural convective phenomena in two and three dimensions (2d and 3d LBM) considerate a single relaxation time LBM (SRT-LBM). The results are presented in terms of isotherms, velocity and average nusselt number. The verification of the lattice boltzmann method code shows a good agreement with the literature.

Abstract: In the present study, we have performed a numerical investigation of the effect of aspect ratio (AR), solid-to-fluid volume fraction (χ) of the heated rectangular block and Richardson number on mixed convection heat transfer in a lid-driven square cavity having centered rectangular heated block inside. The vertical walls of the cavity are exposed to the cold temperature while the horizontal walls are kept at adiabatic, with top wall moving to the right with a constant velocity. The cavity is filled with air (Pr = 0.71) as working fluid. A wide range of Ri (0.01 ≤ Ri ≤ 100) by varying Reynolds number at fixed Rayleigh number Ra = 10⁴, aspect ratio (0.5 ≤ AR ≤ 2) and the solid-fluid volume fraction of the block (20% ≤ χ ≤ 50%) are considered. The obtained results indicate that the total average Nusselt number depends strongly on the Richardson number, the aspect ratio and the solid-to-fluid volume fraction, which reaches its maximum for higher values of χ and for AR = 2 (horizontal rectangular block), at low values of Ri. Additionally, it is observed that more effective cooling of the cavity is generally achieved in the scenario where the aspect ratio is 1 (square heated block) and the solid-fluid volume fraction is 20%. In addition, for Ri = 1, when changing the volume fraction of solid-fluid from 20% to χ = 35% / (χ = 50%), an increase around 37.94% /(90.42%) of Nu is achieved at AR = 1. Nomenclature

Abstract: This article presents a comprehensive numerical study of natural convection in a water-filled triangular cavity using the multi-relaxation time lattice Boltzmann method (MRT-LBM). The main objective of this study is to thoroughly analyze the influence of the heated chip's position along the left wall and the Rayleigh number on crucial aspects such as isotherms, streamlines, velocity, and temperature profiles, as well as the Nusselt number. In this setup, the hypotenuse wall is kept completely cold, while the other parts of the left wall and the bottom wall are adiabatic. Simulations are conducted for three different positions of the heated chip, with Rayleigh numbers, Ra, set at 10³ and 10⁵. The results of these investigations reveal that the heating position plays a crucial role in optimizing control, providing significant implications for various applications. Validation results demonstrate satisfactory agreement with existing literature, reinforcing the robustness and reliability of our numerical approach based on MRT.

Abstract: During the study of laser cladding processes for manufacturing of structural elements from high-alloy corrosion-resistant steel on a thin-walled base, the issue of reduction of the powder material corrosion durability, applied by such technologies, during their use in corrosive environments, was considered. The aim of this study is to determine the effect of laser radiation intensity, used to form a deposited layer on a thin-walled base made from AISI 316L high-alloy corrosion-resistant steel, on its corrosion resistance. Samples, utilizing a laser cladding method, developed for creation of structural elements on pre-made thin-walled parts, were tested for pitting and intergranular corrosion (IGC) resistance using standard methods. IGC resistance was assessed by optical metallography. According to the results of corrosion tests, it was determined that samples of the layers of high-alloy corrosion-resistant steel AISI 316L, applied utilizing laser cladding technology on a thin-walled base, made from high-alloy corrosion-resistant steel, can be considered resistant to pitting and intergranular corrosion, while maintaining the range of values of power density at 30...50.0 kW/cm². These results align with the results of various studies by other authors who have been testing similar cases in other industries. The results of this study were used for further development of laser surfacing technologies for thin-walled parts used in various extreme conditions and further deepening of knowledge about modern laser cladding processes and expansion of the scope of this technology.

Abstract: Stress-corrosion cracking (SCC) of buried gas pipelines has remained a global problem for over 50 years. As pipelines age, new SCC cases emerge, including in welded joints. Improving ductility in these joints is one way to enhance SCC resistance. A welded joint of X70 steel was made using single-arc submerged arc welding (OK10.74, Sv-08GNMA wire). SCC behavior was studied under cathodic polarization in NS4 solution using ANSYS modeling, slow strain rate tests, and scanning electron microscopy (SEM). Results showed that SCC in joints without a stress concentrator occurs along the base metal at a polarization potential of -1.050 V (vs. Ag/AgCl), indicating high weld quality and resistance to brittle fracture. Finite element modeling revealed stress concentration in the base metal during rupture. SEM analysis confirmed increased brittle fracture zones in these joints. With a V-shaped stress concentrator, SCC propagated along the weld, avoiding the heat-affected zone and base metal, which highlights the joint’s ductility. Numerical modeling showed maximum deformation beneath the notch, where fracture initiates. These findings demonstrate that ductile welded joints can effectively resist SCC, and stress concentrators significantly influence crack propagation paths. The study emphasizes the importance of weld quality and joint design in maintaining pipeline integrity under corrosive conditions.

Abstract: Actuality. The accumulation of damage due to fatigue, plastic deformation, and wear significantly reduces the service life of railway rolled metal products. The development of a fatigue crack to its critical length (main cracks) leads to failure at stress levels much lower than the material's strength limit. In industrial-grade steels, there may be chemical micro-inhomogeneity of the main element—carbon. Objective of the study: To determine the effect of chemical micro-inhomogeneity (carbon content variation of 0.02%) on fatigue failure characteristics (crack growth rate, threshold stress intensity factor, fatigue life, and critical defect size) of railway wheel steels of grades ER7 and ER8 according to EN 13262. Results. Segments of the fatigue crack growth rate (FCGR) diagram were constructed to characterize the development of fatigue cracks. The crack growth rate on the second linear section of the diagram and the critical value of the stress intensity factor at which failure occurs were determined. It was found that on the linear portion, which describes the crack growth process, the indicator values vary slightly (up to 10%), indicating that the crack growth rate differs minimally between these steels. Fatigue life—the number of loading cycles until failure—was also determined, and the critical size of the fatigue crack was calculated. A carbon content fluctuation within 0.02% by mass leads to a reduction in fatigue life by approximately 10% for ER7 steel and about 20% for ER8 steel, and a reduction in the critical crack size by around 8% for ER7 and 18% for ER8. Conclusion. Chemical micro-inhomogeneity with carbon content variation in the range of 0.02% in ER7 and ER8 railway wheel steels leads to a decrease in fatigue life (as determined from specimens with cracks) and in the critical size of the fatigue crack (up to 20%). However, it has only a minor effect (about 10%) on the stable fatigue crack growth rate.

Abstract: Steel 1.7220 is widely used in mechanical engineering for heavily loaded components operating at temperatures up to 450 °C. Femtosecond laser processing enables the formation of controlled microstructures without overheating or deformation, reducing the contact area and serving as reservoirs for lubricants. This study aims to evaluate the effect of laser microstructuring (Mesh and LIPSS types) on the tribological properties of steel, particularly under dry lubrication with powder. Micro- and nanostructures were analyzed using optical and scanning electron microscopy. Tribological tests were conducted using the ball-on-plate reciprocating method, with a load of 200 g, a frequency of 2 Hz, and a stroke amplitude of 1 cm for 30 minutes. For LIPSS structures, the influence of the friction direction relative to the orientation of the surface structures was investigated. The change in surface morphology after laser texturing was investigated. The effects of lubrication and surface structuring on tribological properties were analyzed, and the role of periodic structures in enhancing tribofilm stability was demonstrated.