Volume 112, Issue 4

In the present work different approaches to improve the mechanical properties of a resistance spot welded 1200 MPa transformation induced plasticity-aided bainitic ferrite steel are evaluated. An extension of the welding time results in coarsening of the microstructure of the outer fusion zone and the maximum force derived by the cross tension strength test did not improve significantly. A temper pulse after a long cooling time leads to pronounced softening of the fusion zone, as determined by hardness mapping, which resulted in enhanced weld properties. A recrystallization pulse modifies the shape of the prior austenite grains at the outer fusion zone, which was visualized with electron backscatter diffraction. This also resulted in a significant improvement of maximum force. For all approaches the failure mode improved, which can be attributed to an increased fraction of high angle grain boundaries at the edge of the fusion zone.

We address here the continuing challenge and scientific gap in obtaining high impact toughness in medium-Mn steels. While addressing the challenge, the objective of the study described here is to obtain a fundamental understanding via critical experimental analysis of the reasons underlying high impact toughness that was successfully obtained in Fe-0.2C-6Mn-3Al medium-Mn TRIP steel. Electron microscopy and X-ray diffraction studies clearly underscored the absence of the TRIP effect in Fe-0.2C-6Mn-3Al medium manganese steel during impact and the volume fraction of austenite played a determining role in governing impact toughness. The highest impact toughness of 213.6 J · cm – 2 was obtained when the steel was subjected to an intercritical hardening temperature of 700 °C and low tempering temperature of 200 °C. The presence of martensite in the microstructure reduced the impact toughness on quenching from 750 – 850 °C.

The microstructural evolution, solidification behavior, phase relationships, microhardness and room temperature magnetic properties of the hypoeutectic Fe 60 Al 40- n Hf n ( n = 1, 3, and 5 at.%) compositions are investigated in detail. The micro-structure of the studied alloys is composed of Fe–Al based dendrites and eutectic mixture. The components of the eutectic are B2 Fe–Al based and HfFe 6 Al 6 τ 1 phases. With increasing Hf fraction, the amounts of eutectic and τ 1 phases increase continuously which results in significant strengthening. Based on the differential scanning calorimetry results, the eutectic reaction temperature is measured as 1251 °C. Fe 60 Al 40- n Hf n alloys show weak room temperature ferromagnetism and their magnetization values also increase with increasing Hf content.

The effects of ultrasonic melt treatment (UST) on the microstructure and tensile properties of Al-25% Si alloys with and without phosphorus addition were investigated. The average size of the primary Si firstly decreased and then increased with the rise of the ultrasonic intensity. The minimum value is about 25.4 and 32.8 μm when the ultrasonic intensity is 900 W with phosphorus addition and 1 200 W without phosphorus addition. The average number of primary Si per unit area firstly increased and then decreased in both alloys with increasing of ultrasonic intensity. Both tensile strength and elongation were improved mainly due to the refinement of microstructure by UST. Finally, the mechanism of UST on the primary Si is discussed.

The effect of pulsed magneto oscillation (PMO) on the microstructure and tensile properties of Al-25% Si alloys with and without phosphorus addition was investigated. When PMO was applied to the melt during solidification, the results show that the PMO treatment is an effective method for refining the primary Si of Al-25% Si alloy with and without phosphorus addition. The morphology of primary Si was considerably refined via coarse dendritic and polygon shape to fine and regular block under the PMO treatment, and the average size of primary Si was refined to 30.8 μm and 45.3 μm under the optimal processing conditions for the alloy with and without phosphorus addition, respectively. The tensile strength and elongation of the alloys first increased and then decreased with rise of current. Analysis shows that application of PMO decreases Gibbs free energy and critical nucleus radius of the Al-25% Si alloy during solidification. The nucleation rate first increased and then decreased with increasing PMO intensity. The variation tendency of average number of primary Si per unit area is consistent with the theory analysis.

In this study, powder metallurgy was applied in a furnace atmosphere to form titanium boride layers on a commercially pure Ti surface. Experiments were carried out using the solid-state boriding method at 900 °C and 1000°C for 12 h and 24 h. Samples were produced by pressing the commercially pure Ti powders under 870 MPa. The sintering process required by the powder metallurgy method was carried out simultaneously with the boriding process. Thus, the sintering and boriding were performed in one stage. The formation of the boride layer was investigated by field emission scanning electron microscopy, optical-light microscopy, X-ray diffraction, and elemental dispersion spectrometry analyses. In addition, microhardness measurements were performed to examine the effect of the boriding process on hardness. The Vickers microhardness of the boronized surface reached 1773 HV, which was much higher than the 150 HV hardness of the commercially pure Ti substrate. The X-ray diffraction analysis showed that the boriding process had enabled the formation of TiB and TiB 2 on the powder metallurgy Ti substrate surface. Consequently, the production of Ti via powder metallurgy is a potentially cost-effective alternative to the conventional method, and the boriding process supplies TiB and TiB 2 that provide super-high hardness and excellent wear and corrosion resistance.

Ni/WC/graphene oxide (GO) composite cladding fusion coatings were fabricated through the vacuum cladding technique on a medium carbon structure steel (45# steel) substrate whose carbon content was 0.45 ± 0.03%. The microstructural characteristics, phase composition, and electrochemical polarization characteristics of the composite cladding fusion coatings were analyzed with scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and the electrochemical workstation CHI660E. Results show that the microstructure was compact and was micro-crack free, and without inclusions or other defects. It was comprised of four micro-zones, namely, the composite, transition, diffusion fusion, and diffusion-affected zones with thicknesses of approximately 4 mm, 1 mm, 20 μm, and 250 μm, respectively. The main phases of the composite coating were γ-Ni solid solution, WC, Cr 7 C 3 , Ni 2.9 Cr 0.7 Fe 0.36 , Cr 23 C 6 , Ni3Fe, Ni 3 Si, Ni 3 B, W 2 C, and C. The self-corrosion potential of the composite coatings had increased by 0.3269 V compared with that of the substrate, and the corrosion current density of the composite coatings had decreased by nearly two orders of magnitude. The Ni-based solid solution region with relatively high C and Cr contents was difficult to dissolve.

In the current study, zinc–aluminum (Galfan) and zinc (galvanized) coatings were coated on steel sheets using the hot-dip technique. The effect of process parameters on the microstructure, corrosion behavior, hardness, and formability of the coatings was assessed. The results demonstrated that both the corrosion area and corrosion rate of the galvanized coatings were reduced by increasing the immersion time. In contrast, the process temperature did not affect the corrosion behavior. Furthermore, by prolonging the dipping time, the hardness of the coatings decreased. The Galfan coatings showed higher formability and the superiority of mechanical and corrosion properties of Galfan coatings over conventional galvanized coatings was indicated.

Polyacrylonitrile yarn hard waste and multiwall carbon nano-tubes nanofibrous mat was prepared by the electrospinning technique. The nanofibrous composite mats were characterized using thermo-gravimetric analysis, Fourier transform infrared spectroscopy, FT-Raman, scanning electron microscopy, and X-ray diffraction. Adsorption studies were conducted with various physical and chemical parameters such as contact time, solution pH and initial dye concentration. The maximum methylene blue dye removal efficiency of nanofibrous composite was found to be 73.4%at optimized pH 10. The pseudo-second order kinetics and Freundlich isotherm are suitable for methylene blue dye adsorption of nanofibrous composite.