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In the present study, the effect of various cooling environments on macrostructural, microstructural, and hot cracking sensitivity of Nd: YAG pulsed laser welded Inconel-718 superalloy was investigated. Laser welding was performed under air, water, brine and oil cooling environments with heights of 1–4 mm of the environment from surface of the samples. By measuring the macrostructural aspects in the presence of cooling environments, the depth of penetration of welds under water, brine and oil cooling conditions increased, respectively. By measuring the values of dendrite arm spacing in the weld microstructure, it was found that the brine environment with the highest ability of cooling intensity produces the finest and minimal dendrite arm spacing. Subsequently, the water and oil environments, due to the lower cooling intensities, cause relatively coarser microstructures in comparison to brine environment. It was determined that the samples welded under water and brine showed the lower quantities of maximum solidification crack length and maximum liquation crack length leading to improved weldability in comparison to the reference specimen. Also, the height of 3 mm of water and brine environments from the surface shows no solidification and liquation cracks. By using the oil environment with the height of 4 mm, the presence of cracks decreased as compared to the reference specimen. The hardness of welded zone under brine shows the maximum value due to the highest cooling rate causing the finest microstructure and stronger solid solution. The results obtained in the current research can be utilized to design a new and practical cooling procedure during welding as a post weld spraying or quenching in order to control the cooling rates of the welds for improving the properties, weldability and microstructure of austenitic nickel-based superalloys. [ABSTRACT FROM AUTHOR]

A 3D finite-element thermal model has been developed to estimate weld geometry and analyze thermal gradients and cooling rates in pulsed laser welding of Inconel 718 superalloy. Since Inconel 718 superalloy is sensitive to hot cracking, the extent of hot crack sensitive areas including heat affected zone, mushy zone and partially melted zone is very important in the investigation of weldability for this alloy. Because the welding process is very fast and the welding width is very thin, the numerical modeling approach can be very helpful to overcome the difficulties and limitations of experimental procedure. In this study, a finite element model is presented to simulate the process of pulsed laser welding of Inconel 718 and to predict the temperature distribution and shape of the weld pool and finally to estimate the extent of the hot cracking sensitive regions. Heat source was modeled by the combination of conical Gaussian and ellipsoid Goldak. Then, the results of numerical model were compared to the experimental results at preheat temperatures of 308, 393, 473 Kelvin. The proposed numerical model exhibits good agreements with the experimental results including the internal and external weld pool geometries and the extent of the hot cracking susceptible regions. Furthermore, detailed pulsed temperature distribution and thermal cycle containing solidification latent heat are obtained. Accordingly, this pulsed temperature history is correlated to the internal and external geometry of pulsed solidified weld pool. [ABSTRACT FROM AUTHOR]

Micro-nano scale surface modification of Ti-6Al-4V was investigated through the fascinated modern fiber engraving laser method. The process was performed at a high laser speed of 2000 mm/s, under different laser frequencies (20–160 kHz) and groove distances (0.5–50 μm). Topographic evaluations such as Atomic Force Microscopy (AFM) and Field Emission Scanning Electron Microscopy (FE-SEM) were used to identify the quality and regularity of patterns. The proliferation of human osteoblast-like osteosarcoma cells (MG63) was analyzed by MTT assay for up to 72 h. Also, the plate counting method was used to quantify the viability potential of the modified surface against Escherichia coli bacteria. The cellular viability of the sample modified at the laser frequency of 20 kHz and grooving distance of 50 μm increased up to 35 and 10% compared to the non-treated and control samples, respectively. In the case of the surface modification at lower grooving distances range between 0.5–50 μm, the maximum laser frequency (160 kHz) applied leads to lower pulse's energies and less bacterial adhesion. Otherwise, at groove distances more than 50 μm, the minimum laser frequency (20 kHz) applied reduces the laser pulse overlaps, increases the cell adhesion and antibacterial properties. Surface modification by the fiber engraving laser process significantly enhances the cell adhesion on the surface. As a result of such roughness and cell adhesion enhancement, the surface toxicity feature diminished, and its antibacterial properties improved. [ABSTRACT FROM AUTHOR]

Recently, Zn-based alloys have been considered a promising biodegradable material due to their excellent physiological degradable behavior and acceptable biocompatibility. However, poor mechanical performance limits its application, especially for cardiovascular stents. In this work, novel biodegradable Zn-xCu-0.8Mn-0.4Ag (x = 1, 2, 5 %Wt) alloys were prepared. The effects of Cu addition on microstructure, mechanical properties, and cytotoxicity of alloys were investigated. To modify the microstructure and improve the properties of the alloys, the secondary process of hot extrusion was applied. The results of XRD analysis showed that the above alloys have four phases of α-Zn, CuZn 5 , MnZn 13 , and Zn 3 Ag. After hot extrusion, the secondary phase particles were broken and spread throughout the structure. According to the hardness test, due to the presence of CuZn 5 hard secondary phase particles, the hardness increased gradually. The tensile test showed that adding copper and performing a hot extrusion process can significantly improve mechanical properties. The as-extruded alloys exhibited suitable mechanical performances mainly due to the combined effects of solid solution strengthening, and second phase precipitation hardening. In particular, yield strength, ultimate tensile strength, and elongation for Zn-xCu-0.8Mn-0.4Ag (x = 5 wt) alloy reached the value of 255.47, 359.82 MPa, and 35.84 %, respectively, which meets the requirements of cardiovascular stent well. Cytotoxicity test also showed that homogenized and as-extruded alloys have acceptable in vitro cytocompatibility. According to the obtained results, Zn-xCu-0.8Mn-0.4Ag extruded alloys can be considered as potential candidates for cardiovascular stents. [Display omitted] • Newly-developed Zn-xCu-0.8Mn-0.4Ag (x = 1, 2, 5 %Wt) alloys are studied to be nominated for biodegradable vascular stents. • Cu1%, Cu2% and Cu5% alloys have four phases α-Zn, CuZn 5 , MnZn 13 and Zn 3 Ag in the casting condition. • Cu5% alloy showed the yield strength, ultimate tensile strength, and elongation of 255.4 MPa, 359.8 MPa, and 35.8 %, respectively. • In vitro evaluation showed that all as-homogenized and as-extruded alloys have acceptable toxicity to human endothelial cells. • High mechanical properties and cytocompatibility of new alloys make them candidate for cardiovascular stents application. [ABSTRACT FROM AUTHOR]

• Dissimilar laser welding of Kovar to AISI 304L has been investigated. • Solidification crack and lack of complete mixing in the weld metal are seen. • The heat input must have an optimum value equaling 19.1 J/mm. • At heat input less than 15 J/mm, non-mixing is clearly observable. Kovar, due to its close thermal expansion coefficient to glass, can be used as a mediating alloy in joint of glass to metals such as AISI 304L. In the present research, the effect of parameters of pulsed laser on dissimilar welding of Kovar to AISI 304L has been investigated. For observation and investigation of the results, metallography, Energy dispersive X-ray spectroscopy (EDS) and microhardness tests have been used. Results obtained from microstructural examinations, indicated the existence of solidification crack and lack of complete mixing in the weld metal. According to the results, the heat input must have an optimum value equaling approximately 19.1 J/mm. At higher levels such as 32.6 J/mm, the TCL (total crack length) value reached 760 μm. Increasing the heat input lead to increasing thermal stresses and formation of crack in the weld metal. On the other hand, at heat input levels less than the value of 15 J/mm, cracks are eliminated and non-mixing is clearly observable because sufficient time is not provided for mixing and, then, solidification for the weld metal. The weld metal hardness is also higher than the base metals due to the fine dendrite structure and the presence of hardening elements. [ABSTRACT FROM AUTHOR]

In this study, weldability and mechanical properties of dissimilar joint between Nb-1Zr and 321 stainless steel produced by electron beam welding were investigated. The results indicated that the optimum parameters to make this dissimilar joint possible are the electron beam offset of 50–50 and the current of 13 mA. Because of high melting temperature of Nb relative to 321 stainless steel and also high thermal conductivity of Nb, the molten weld pool was drawn to 321 stainless steel side containing 85.1%Vol 321 stainless steel and 14.9%Vol Nb. Non uniform regions were observed in the weld zone which were due to melt turbulence and its rapid solidification. The Fe 7 Nb 6 and Fe 2 Nb intermetallic compounds formed in various regions of the weld zone directly govern the mechanical properties and weldability of the weld. Tensile strength of the weld was 170 MPa which was equal to 60.5% of that in autogenous weld of Nb-1Zr alloy. Tensile strength of the autogenous weld of Nb-1Zr alloy was 281 MPa. According to the Rosenthal's equations and measurements of the weld section, the recrystallization temperature of the Nb-1Zr was identified 1810 °C in the EBW condition. [ABSTRACT FROM AUTHOR]

A 700 W pulsed Nd:YAG laser welding was employed to weld 5456 aluminum alloy plates with thickness of 5 mm. The weld penetration and tendency of liquation cracking in the heat affected zone and solidification cracking in the weld metal are investigated. It is found that hot cracks can be avoided by pulsed laser in preheated status. Increasing the laser average power in fixed pulse frequency and pulse duration resulted in a significant reduction in hot cracking tendency in pulsed laser welding. Also, the hot cracking susceptibility is reduced with increasing pulse frequency up to 50 Hz in fixed average power and pulse duration. The hot cracking tendency can be decreased by pulse duration decrement to 6 ms in fixed average power and pulse frequency. The factors contributing to the prevention or reduction of hot cracks in laser welding of aluminum alloy 5456 are the reduction in cooling rate causing change in solidification mode from columnar to equiaxed grains, and backfilling of liquated grain boundaries. With a 0.4 mm beam diameter Nd:YAG pulsed laser at 300 W average power, 6 ms pulse duration, 20 Hz pulse frequency, 5 mm/s travel speed and 150 °C preheating temperature, hot crack-free weld cross sections were achieved. [ABSTRACT FROM AUTHOR]

The effect of electron beam welding current changes on the microstructure and mechanical properties of the Nb-based alloy has been investigated. The electron beam welding was applied with 4 different values for electric current 20, 24, 30 and 35 mA on 3 mm thick plates. The aspects including different welding regions, geometry and depth of welding penetration, as well as the effect of heat input on the mechanical properties are investigated. The mechanical properties including tensile and microhardness values of the weld was measured. The results show that in a sample with a 30 mA welding current, the optimum conditions for the depth of penetration, microstructure, mechanical properties and the geometry of the weld are obtained. The welds showed a cellular structure, and intercellular dendrites in the central region of the weld have been caused due to microsegregations created between the cells. In heat-affected zone (HAZ), severe recrystallization and grain growth has occurred. Because of the high thermal conductivity of niobium, the HAZ size is relatively large. Based on the 3D Rosenthal's equation, the recrystallization temperature of alloy was calculated as 713 °C. It is observed that as G × R increases, the grain size in the central line of the weld decreases. The hardness profile shows that the hardness of the weld zone and the HAZ is significantly less than that of the base metal due to elimination of work hardening effect. The tensile strength of the weld for a sample with a current of 30 mA was 281 MPa, which is 53% of the tensile strength of the base metal and the weld was broken in the HAZ. [ABSTRACT FROM AUTHOR]

Dissimilar butt welding of Nb-1Zr to the 321 stainless steel was studied using electron beam welding. The aim of this study was investigation of the offset and microdilution effects on the weldability, microstructure and mechanical properties of this dissimilar joint. The offset of 50-50 and welding parameters of 13 mA and 1343 V showed the best weldability and penetration for the joint. The microdilution of Nb in the weld changes by going far from the base metals. The microdilution of Nb is the minimum at root of the weld (∼0.2) leading to formation of less brittle phases. As a result, no cracks are observed at the root. The microdilution of Nb at the top of weld just near to Nb-based alloy at a 200 μm narrow zone under surface increases (∼0.8) which leads to formation of more brittle phases and subsequent microcracks. Applying 200 μm thickness machining on the surface of weld can give a completely free of crack welds that show strength more than the Nb-alloy base. It is concluded that a sound weld can be achieved by 50-50 offset and Nb microdilution less than 0.6. No interlayer and/or transition piece are required, and appropriate mechanical properties can be obtained. [ABSTRACT FROM AUTHOR]

Developments in electronic industries for telecommunications and demands for decreasing electromagnetic radiation pollution result in developing researches on microwave absorption materials. The target of the present study is to design materials with high absorption properties for electromagnetic waves in the 12–18 GHz range. Thus, Fe 3 O 4 magnetic nanoparticles were syntheses through chemical co-precipitation reinforced by ultrasonic. Then, BaTiO 3 nanocrystalline powder was synthesized by the hydrothermal sol-gel method under atmospheric oxygen. Next, nano-particles of barium titanate were deposited on the multi-walled carbon nanotubes (BaTiO 3 @CNT). It was concluded that a magnetic-dielectric nanocomposite has superior microwave absorption properties in comparison to individual magnetic or dielectric absorbers. Also, in order to obtain an optimum absorption in a wide frequency band, dielectric-CNT nanocomposites represents higher properties than magnetic-CNT composites. It is concluded that composites with more magnetic percentage showed better absorption in low frequency band (12 GHz), whereas composites with more dielectric percentage exhibited superior absorption for high frequency band (18 GHz). 80–93% absorption was obtained in the frequency range of 16.7–18 GHz by composite 40M.20F.40C (40% paraffin, 20% magnetite, 40% multi-walled carbon nanotubes). Also, composite 40M.20B.40B@C (40% paraffin, 20% barium titanate, 40% barium titanate deposited on multi-walled carbon nanotubes) showed the absorption of 80–90%. [ABSTRACT FROM AUTHOR]

The present study has evaluated the surface rejuvenation of aged Inconel 617 superalloy by both GTAW and pulsed Nd:YAG laser techniques. The gas tungsten arc welding (GTAW) by heat input per unit length [Q/V (J/mm) ] of 280, 291.67, 309.74 and 225.48 (J/mm), and the pulse Nd:YAG laser process by the 15.71, 19.43 and 22.32 (J/mm), were employed. The Rosenthal equation was used for calculation of mushy zone (MZ) and partially-melted zone (PMZ). Size of MZ and PMZ in GTAW are more than 31 and 6 times than that of formed in pulsed Nd:YAG laser. According to the characterizations, solidification and liquation cracks were observed in these areas produced by GTAW whereas no cracks were identified in laser treated samples. Also, line scan EDS analyses demonstrated the interdendritic chromium and molybdenum segregation, which facilitated formation of hot cracks. With reduction in heat input per unit length, the hardness increased and the size of solidified metal microstructure reduced in pulse Nd:YAG laser. These comparative results showed that pulse Nd:YAG laser can easily be utilized as a new rejuvenation technique for aged Alloy 617 in comparison to the conventional processes due to extremely narrow MZ and HAZ and better surface soundness and mechanical properties. [ABSTRACT FROM AUTHOR]

Abstract: In the present research, the modern fiber laser beam welding of newly-designed precipitation-strengthened nickel-base superalloys using various welding parameters in constant heat input has been investigated. Five nickel-base superalloys with various Ti and Nb contents were designed and produced by Vacuum Induction Melting furnace. The fiber laser beam welding operations were performed in constant heat input (100Jmm−2) and different welding powers (400 and 1000W) and velocities (40 and 100mms−1) using 6-axis anthropomorphic robot. The macro- and micro-structural features, weld defects, chemical composition and mechanical property of 3.2mm weldments were assessed utilizing optical and scanning electron microscopes equipped with EDS analysis and microhardness tester. The results showed that welding with higher powers can create higher penetration-to-width ratios. The porosity formation was increased when the welding powers and velocities were increased. None of the welds displayed hot solidification and liquation cracks in 400 and 1000W welding powers, but liquation phenomenon was observed in all the heat-affected zones. With increasing the Nb content of the superalloys the liquation length was increased. The changing of the welding power and velocity did not alter the hardness property of the welds. The hardness of welds decreased when the Ti content declined in the composition of superalloys. Finally, the 400 and 1000W fiber laser powers with velocity of 40 and 100mms−1 have been offered for hot crack-free welding of the thin sheet of newly-designed precipitation-strengthened nickel-base superalloys. [Copyright &y& Elsevier]

Abstract: Dissimilar welding of γ′- and γ″-strengthened nickel-base superalloys has been investigated to identify the relationship between the microstructure of the welds and the resultant mechanical and weldability characteristics. γ′-Strengthened nickel-base Alloy 500 and γ″-strengthened nickel-base Alloy 718 were used for dissimilar welding. Gas tungsten arc welding operations were utilized for performing the autogenous dissimilar welding. Alloy 500 and Alloy 718 base metals showed various types of phases, carbides, intermetallics and eutectics in their microstructure. The results for Alloy 500 weld metal showed severe segregation of titanium to the interdendritic regions. The Alloy 718 weld metal compositional analysis confirmed the substantial role of Nb in the formation of low-melting eutectic-type morphologies which can reduce the weldability. The microstructure of dissimilar weld metal with dilution level of 65% wt.% displayed semi-developed dendritic structure. The less segregation and less formation of low-melting eutectic structures caused to less susceptibility of the dissimilar weld metal to the solidification cracking. This result was confirmed by analytic modeling achievements. Dissolution of γ″-Ni3Nb precipitations took place in the Alloy 718 heat-affected zone leading to sharp decline of the microhardness in this region. Remelted and resolidified regions were observed in the partially-melted zone of Alloy 500 and Alloy 718. Nevertheless, no solidification and liquation cracking happened in the dissimilar welds. Finally, this was concluded that dissimilar welding of γ′- and γ″-strengthened nickel-base superalloys can successfully be performed. [Copyright &y& Elsevier]

• 17-4 PH SS welded by pulsed Nd:YAG laser was free of crack. • Aging at 550 °C for 30 min increased hardness of weld metal and HAZ. • Yield and ultimate strengths, were 995 MPa, 1185 MPa for as-welded samples. • Microstructure of weld metal in aged condition was δ-ferrite in martensite. Pulsed Nd:YAG laser welding of 17-4 martensitic precipitation hardening (PH) stainless steel (SS) is investigated. In order to achieve optimum laser welding parameters, bead on plate (BOP) welding was conducted on 17-4 PH SS. No crack was detected in the weldments, and microstructure of weld metal was δ-ferrite in the martensite matrix. Resistance to cracking of weldment was correlated to the presence of δ-ferrite in the weld metal, and using pulsed Nd:YAG laser as a low heat input welding method. Four HAZs were distinguished and microstructure evaluation in each zone is discussed. In the second part of this research, pulsed Nd:YAG laser welding of similar 17-4 PH SS in butt joint configuration was performed. Microstructure characterization and mechanical properties of weldment in as-welded and aged in 550 °C for 30 min conditions were investigated and compared. Aging caused precipitation of copper-rich clusters and increasing hardness of weld metal and homogenizing the hardness in HAZ. Tensile test results demonstrated that the failure occurred in the base metal and showed features of ductile fracture. [ABSTRACT FROM AUTHOR]

For planar perovskite solar cells (PSCs) to have satisfactory performance and acceptable stability, an ETL (electron transport layer) is needed that provides a high level of efficiency. It should be noted that when considering the implementation of n-type metal oxide materials, the ZnO (zinc oxide) can be a great choice for an ETL because it provides a relatively high level of mobility. The current study proposes a new design involving a reduced graphene oxide (rGO) sheet/Ag-doped ZnO bilayer to be utilized as ETL in planar PSCs. Extensive experimental and computational analyses including nano-micro structural, optical and electrical properties are carried out in order to compare the performance and governing mechanisms of rGO/Ag-doped bilayer-based planar PSC to ordinary un-doped ZnO. The results from the performed experiments show that using the rGO sheet as the buffer layer and Ag-doped ZnO significantly increase the power conversion efficiency (PCE) by more than 30% compared to un-doped ZnO ETL in planar PSCs. These improvements in the performance of PSCs can be explained based on fine tuning the energy levels of ZnO/perovskite, since this increases the charge-carrier-extraction from the perovskite layer. Moreover, the results of the study confirm the using the rGO/Ag-doped ZnO bilayer ETL as a new methodology for the creation of stable planar PSCs with low charge recombination rate and high performance. [ABSTRACT FROM AUTHOR]

• In laser welding of aluminum alloys, the weld penetration is dependent on Mg content of the alloy. • The presence of Mg vapor in the plasma plume over the weld pool affects the laser absorption, and it is through this phenomenon that the penetration is affected. • Increasing the laser pulse energy, Mg evaporation is increased, and that, in turn, increased the effective laser absorption coefficient. It is observed that in laser welding of aluminum alloys, magnesium can evaporate, and the weld penetration is dependent on Mg content of the alloy and Mg loss from the weld pool. In this research, it is proposed that the presence of Mg not in the base metal alloy, but rather the presence of Mg vapor in the plasma plume over the weld pool affects the laser absorption, and it is through this phenomenon that the weld profile and penetration is affected. Numerical simulation was performed to determine the relationship between the weld profile to estimate the effective laser absorption coefficient of four Al alloys and in parallel EPMA technique was used to determine the Mg losses of the weld metals. The combined analysis of the results showed that increasing the laser pulse energy (decreasing laser pulse frequency), Mg evaporation is increased, and that, in turn, increased the effective laser absorption coefficient. However, more laser power absorption does not necessarily mean more weld penetration. Laser absorption results in weld penetration, once the threshold Mg evaporation rate of 200 × 10 - 6 g/cm2 is passed. [ABSTRACT FROM AUTHOR]