جستجوی مقالات مرتبط با کلیدواژه « frequency » در نشریات گروه « مواد و متالورژی »

In this research, Ti-6Al-4V alloy sheet with a thickness of one millimeter with butt joint design was welded by tungsten-gas arc welding process using pulse current (PCGTAW) and using AMS 4954G filler metal. In this study, the effect of pulse system frequency on microstructure and mechanical properties was investigated by optical microscopy, Vickers hardness and tensile strength tests. In the non-frequency welding sample, due to the lack of pulse current and lower cooling rate of the molten pool, the formation of large amounts of soft phases of the Weidmann-Statten layer in the weld metal region is possible. Finally, in this method, the lowest average hardness of 341 Vickers was obtained. The experimental results showed that using pulsed current and increasing the pulse frequency up to 450 Hz increased the cooling rate of the molten pool, followed by increasing the amount of martensitic phase α 'in the form of a basket in the weld metal region and finally increasing the average microhardness in this region. In other words, using the maximum frequency led to a significant increase in hardness up to 367 Vickers in the weld zone. Finally, using the tensile strength test, it was shown that in all the samples, failure occurred from the base metal area, which was a very good phenomenon due to the proper welding quality of the samples.

Plasma electrolytic oxidation is a novel and promising method for coating the light alloys especially magnesium alloys. Several parameters affect the coating quality and its properties, where in this study; the effects of frequency on properties of coatings produced by PEO on AZ31 Mg alloy were investigated. The coating process was performed in a silicate-based electrolyte containing KF and K2TiF6 using a soft-sparking waveforms with 30 % cathodic pulse width at 1, 2, and 3 kHz. The surface morphology of the coatings was a net-like (scaffold) containing a micro-pores network, micro-cracks, and granules of oxide compounds, in which, no noticeable change on coating morphology occurred with increasing the frequency. Also, the coatings were composed of MgO, MgF2, Mg2SiO4, SiO2 (amorphous), and TiO2 (crystalline and amorphous) phases contributed by the elements from both substrate and electrolyte in the coating process. Based on the cross-sectional images, the thickness of coatings displayed no significant increase with increasing in frequency, but the porosity percentage was increased. Also, the very small pores were presented in the inner layer with increasing frequency. Corrosion behavior of the coatings was evaluated using electrochemical impedance spectroscopy (EIS) in 3.5 wt. % NaCl until 7 days of immersion. The results of the EIS showed that with increasing frequency and porosity, the larger surface of the coating is in contact with the solution, and the presence of porosity in the more compact layer of the coating allowed the access of solution to the substrate, which reduces the barrier performance of the coating.

In this research, pure zinc, Zn-SiC and Zn-TiO2 nanocomposite coatings were fabricated through pulse electroplating process from a sulphate bath. While pulse electroplating, the effect of several parameters like maximum current density (ip), frequency and duty cycle were investigated. In order to characterize the obtained coatings, surface morphology of the coatings was investigated by scanning electron microscopy (SEM & FESEM), composition of the coatings by elemental energy dispersive spectroscopy (EDS) analysis, microstructure by means of X-ray (XRD) method, corrosion measurement by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS), surface topography by the use of atomic force microscopy (AFM) and also microhardness of the coatings were investigated. The highest amount of incorporated SiC nanoparticles was 8.73 Vol. % and for TiO2 nanoparticles was 7.12 Vol. %. Also, minimum corrosion current density for pure zinc coating was 7.1, for Zn-SiC nanocomposite coating was 4.49 and for Zn-TiO2 nanocomposite coating was 4.53 μA/cm2. It was observed that utilization of simultaneous of composite coating system and also pulse electroplating process leads to significant increase in corrosion resistance. XRD analysis also showed that preferred orientation for pure zinc coating is through (002) plane. By incorporation of SiC and TiO2 nanoparticles into the zinc matrix, the intensity for (002) plane decreases and (101) plane intensity increases. Average roughness for three samples of Zn, Zn-SiC and Zn-TiO2 coatings was 528.11, 627.32 and 815.41 nm respectively. The microhardness of the coatings was also improved after SiC and TiO2 incorporation.