فصلنامه علوم و مهندسی سطح ایران
پیاپی 50 (زمستان 1400)

Laser shock peening process is a complex phenomenon of interactions among various parameters in a nanosecond time frame with the goal of generating residual stress in the surface. In this research, the distribution of residual stress and surface deformations after the laser shock peening process has been investigated via finite element method (FEM) and statistical analysis. To simulate the process, ABAQUS explicit dynamic finite element method coupled with Johnson-Cook model to analyze the nonlinear behavior of Ti-6Al-4V alloy and the results were compared with experimental data from other articles. The design of experiment was used to investigate various process sates with the aid of simulation and analyzing the effect of process parameters on the distribution of residual stress and surface deformation. The aforementioned process parameters are laser spot size, laser spot overlap, laser power density, number of laser passes and laser pulse width. To achieve the correlation between the parameters and the process outcomes namely, the distribution of residual stress and surface deformation, linear regression was incorporated. Additionally, genetic algorithm was used to optimize the process and achieve more accurate results. Defining the compound variable for effective laser parameters and implementing the genetic algorithm resulted in linear regression with 93% accuracy for predicting the surface deformation and 94% accuracy for the prediction of the distribution of residual stress. A good correlation between the simulated model and other articles regarding shock peening in specific conditions was observed. The simulation results indicated that the value of compressive residual stress and surface deformation increase when power, pulse width, overlapping and the number of repetitions are increased. On the other hand, an increment in laser spot size has an opposite effect on the aforementioned parameters.

Direct Laser Deposition (DLD) is among the important industrial processes for fabricating and repairing parts. In this method, injection of metallic powder subject to laser beam causes the power to melt and deposit, thus allowing the part to grow in volume in three dimensions. INCONEL 718 is a high-strength, corrosion-resistant superalloy widely employed in the military and aerospace industries. This study assesses the scanning pattern on solidification texture of Inconel 718 after additive fabrication. After performing additive fabrication, the specimens were prepared using three scanning patterns, namely circular, single direction raster, and bi-direction. The results indicated that directions <100> with maximum heat transfer are formed mostly along the fabrication and transverse directions. Using concentric circles pattern, a larger number of dendrites assumed a similar orientation, making circular scanning the most suitable choice for oriented structures. The components of the main texture in the specimens were cubic, rotated cubic, and Gauss. The bi-direction pattern produced the weakest texture intensities in the specimens, and circular scanning achieved the most suitable texture intensity, with SDRS in a close second. The hardness results showed no significant changes with changes in fabrication pattern. M oreover, the hardness and its variations decrease after annealing.

The results of various research activities show that coating is one of the effective ways to increase the corrosion resistance and wear of metallic substrates. Synthesizing composite coatings using nanoparticles can also further protect the substrate. In this study, using electroplating process and PTFE particles (with concentrations of 10, 20 or 30 g / l), Ni-P-PTFE coatings were prepared and their corrosion and abrasion properties were investigated and compared with Ni-P coating. Using scanning electron microscopy (SEM ) and X-ray diffraction (EDS) methods, the surface morphology and elemental composition of the coatings were studied and finally, using open circuit potential (OCP) techniques, electrochemical impedance spectroscopy (EIS) and polarization techniques, the corrosion resistance of the resulting coatings were evaluated in NaCl solution. M icrohardness and pin on disk tests were also utilized to investigate the effect of PTFE concentration on the tribological properties of the coatings. The results of SEM and EDS studies confirmed the formation of nanocomposites. Electrochemical studies also showed that Ni-P-PTFE coatings, at a concentration of 20 g/L PTFE, had the highest electrochemical corrosion resistance. M icrohardness decreased with increasing PTFE particles in the coating and reached its lowest value in 20 g/l PTFE . Using the wear test, the lowest coefficient of friction obtained in composite coatings obtained at a concentration of 20 g/l, which shows the applicability of PTFE particles as a solid lubricant in Ni-P coatings.

The purpose of this study is to investigate the microstructure and surface properties of titanium-aluminum nitride (TiAlN) nanostructured coatings produced at different deposition temperatures. The TiAlN nanostructured coatings were deposited on nitrided hot-work tool steel (H11) using pulsed direct current plasma assisted chemical vapor deposition method (DC- PACVD). The coatings were produced under the same conditions such as duty cycle, frequency and H2/Ar/N2 gas ratio at 33%, 10 kHz and 400/150/50 sccm, respectively. Also the specimens were deposited at temperatures of 470, 485, 500 and 515 °C by PACVD method. TiN coating interlayer was used to improve the adhesion strength of TiAlN coatings to the substrate. FESEM , SEM and XRD devices were used to characterize the coatings and Vickers micro-hardness and scratch test were used to evaluate the mechanical properties and adhesion strength of the coatings. The results indicated that the maximum hardness of TiAlN coating produced at 485 °C. It is attributed to various factors such as lattice parameter, crystallite size and amount of chlorine in the chemical composition of the coating. Also, due to the formation of a nanoscale- thin layer of Fe4N at the interface between the coating and the substrate at the temperatures above 500 °C, the adhesion strength of the TiAlN coating reduces. By quantitative evaluation of the adhesion of the coatings, it was found that the coating formed at the temperature of 485 °C withstands the maximum critical load (23 N) of the scratch.

In this article, the goal is to build a super-hydrophobic diffusion layer to preserve the facade of the building. In this research, first, the surface of fumed silica nanoparticles was modified by polydimethylsiloxane polymer, and then a suspension of modified nanoparticles was made based on water/ethanol and the stabilizing agent sodium dodecyl sulfate. The super- hydrophobic diffusion layer was applied on the brick surface of the buildings using brushing and spraying process. In order t o evaluate the properties of the super-hydrophobic diffusion layer on the brick surface, Rilem test, breathability, efflorescence, contact angle and water drop sliding angle were used. The results showed that the zeta potential for the suspension containing hydrophilic fumed silica nanoparticles and the suspension containing hydrophobic fumed silica nanoparticles is -25.4 mV and -55.9 mV, respectively. Based on the Fourier transform infrared spectrometry test, since no significant shift of the spectra was observed, therefore, the mechanism of connection of modifier molecules with the surface of nanoparticles was reported as physical. The minimum and maximum amount of polydimethylsiloxane polymer for hydrophobization the surface of fumed silica nanoparticles was 5 and 15% by weight. The contact angle of the water drop on the surface of the brick, which is coated with a hydrophobic diffusion layer, is more than 150◦ and the sliding angle is 4◦, which shows the super-hydrophobic property of this layer. This super-hydrophobic property was achieved according to the Cassie-Baxter mechanism due to the creation of a surface with micro/nano scale roughness and very low surface energy. According to Rilem test, after 20 minutes, water did not penetrate into the hydrophobic brick. Also, according to the efflorescence test, after 15 days, no salt deposits were observed on the surface of the brick. In addition, the super-hydrophobic diffusion layer created a favorable breathability on the brick surface of the building facade.