Zone Of Alloy Research Articles

Nucleation mechanisms of equiaxed grains in the fusion zone of aluminum-lithium alloys by laser welding

The formation of different microstructures in aluminum-lithium alloys welding joints by laser welding has not been fully understood at the moment. In this study, the nucleation mechanisms of equiaxed grains in the fusion zone of aluminum-lithium alloys during laser welding are investigated. Two different kinds of equiaxed grains, i.e., non-dendritic equiaxed grains and dendritic equiaxed grains, are discovered. An improved method based on an overlap welding procedure is proposed to clarify the nucleation mechanisms. Two nucleation mechanisms, i.e., grain detachment and heterogeneous nucleation, are simultaneously responsible for the formation of equiaxed grains. The grain detachment, caused by the flow in the molten pool, leads to the formation of non-dendritic equiaxed grains and a part of coarse dendritic equiaxed grains. The direct evidence that the convection of laser welding molten pool causes grains to be detached from the base metal is found for the first time. Most of the dendritic equiaxed grains grow on heterogeneous nucleation particles. D0 22 -Al 3 Ti and D0 23 -Al 3 Zr are the main heterogeneous nucleation phases during laser welding.

Synthesis of ex-situ Al5083 reinforced with mechanically-alloyed CNTs and Fe2O3 nanoparticles using friction stir processing

Friction stir processing (FSP) was employed to synthesis ex-situ Al5083 nanocomposite reinforced with different mechanically-alloyed aluminium nanocomposites containing carbon nanotubes (CNTs) and ferric oxide (Fe 2 O 3 ). The mechanically-alloyed nanocomposite reinforcements were initially prepared using ball milling and introduced into the stir zone (SZ) of rolled Al5083 alloy during subsequent FSP. The FSPed nanocomposites were successfully processed with uniform dispersion of CNTs and Fe 2 O 3 nanoparticles resulting in the achievement of unique properties. Addition of nanocomposite reinforcements significantly reduces the grain size of FSPed nanocomposites. The least grain size belongs to hybrid FSP5 (Al/CNTs/Fe 2 O 3 ) nanocomposite with grain size of 6.9 ± 0.4 μm which is 7.4 and 2.6 times smaller than base metal (BM) and un-reinforced FSPed (FSP1). Microstructure of FSP5 nanocomposite contain largest fraction of high angle grain boundaries (HAGBs) of 87 %. Among all FSPed nanocomposites, hybrid FSP5 nanocomposite has the best mechanical properties in the terms of microhardness, Yield strength, ultimate tensile strength and wear resistance; with 1.6, 2.7, 3.5, 3.1 times, respectively, superior to BM. The significant improvement is attributed to the desirable microstructural changes including well dispersion of nanoparticles and extensive grain refinement. Hybrid FSP5 has an excellent magnetic and electrical resistivity when compared with the BM, however, its magnetic properties is slightly slower than FSP4 specimen due to de-magnetization effect of CNTs.

Influence of Beam Power on Young’s Modulus and Friction Coefficient of Ti–Ta Alloys Formed by Electron-Beam Surface Alloying

In this study, we present the results of Young’s modulus and coefficient of friction (COF) of Ti–Ta surface alloys formed by electron-beam surface alloying by a scanning electron beam. Ta films were deposited on the top of Ti substrates, and the specimens were then electron-beam surface alloyed, where the beam power was varied from 750 to 1750 W. The structure of the samples was characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD). Young’s modulus was studied by a nanoindentation test. The coefficient of friction was studied by a micromechanical wear experiment. It was found that at 750 W, the Ta film remained undissolved on the top of the Ti, and no alloyed zone was observed. By an increase in the beam power to 1250 and 1750 W, a distinguished alloyed zone is formed, where it is much thicker in the case of 1750 W. The structure of the obtained surface alloys is in the form of double-phase α’and β. In both surface alloys formed by a beam power of 1250 and 1750 W, respectively, Young’s modulus decreases about two times due to different reasons: in the case of alloying by 1250 W, the observed drop is attributed to the larger amount of the β phase, while at 1750 W is it due to the weaker binding forces between the atoms. The results obtained for the COF show that the formation of the Ti–Ta surface alloy on the top of Ti substrate leads to a decrease in the coefficient of friction, where the effect is more pronounced in the case of the formation of Ti–Ta surface alloys by a beam power of 1250 W.

Corrosion resistance of the dissimilar alloy AL6XN-Inconel 718 in 0.5 M NaCl

In the present work, the corrosion resistance of the dissimilar alloy AL6XN-Inconel 718 bound by the Gas Tungsten Arc Welding (GTAW) method was studied. Different alloy zones were analyzed: base materials (BMs), heat affected zones (HAZs) and welds. The effect exerted by the GTAW method on the corrosion resistance displayed by such dissimilar alloy in 0.5 M NaCl was established by means of corrosion studies employing potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) tests. The electrochemical results revealed that the alloy HAZs are more susceptible to corrosion due to microstructure changes caused mainly by elements such as Ni, Cr and Mo during the GTAW process.

Fretting wear behavior of nano ZrO2 doped plasma electrolytic oxidation composite coatings on TC21 titanium alloy

Basic and ZrO2 doped composite plasma electrolytic oxidation coatings were prepared on the surface of TC21 titanium alloy. Subsequently, fretting wear tests of these two kinds of coated samples and the substrate alloy against GCr15 steel balls were conducted at 50 μm and 150 μm displacement amplitudes. The morphology, composition, valence states and topography of the fretting wear tracks were characterized and the wear mechanisms were studied. The results indicated that the porosity of the composite coatings was lower than that of the basic coatings, and the microhardness, adhesion strength and toughness were higher. When the fretting was in the partial slip regime (50 μm displacement amplitude), the fretting damage of the three materials was slight. Meanwhile, the cracks and delamination in the microslip zone of the TC21 alloy were not observed in the coated samples. When the fretting was in the slip regime (150 μm displacement amplitude), although the TC21 alloy and basic coatings experienced local wear, there was no significant material loss of the composite coatings, which demonstrated that the composite coatings could improve the fretting wear resistance.

Strain-tunable triple point Fermions in diamagnetic rare-earth half-Heusler alloys

Topologically non-trivial electronic structure is a feature of many rare-earth half-Heusler alloys, which host atoms with high spin-orbit coupling bringing in the non-triviality. In this article, using the first-principles simulations, rare-earth half-Heusler YPdBi, ScPdBi, LaPdBi, LuPdBi, YPtBi and LuPtBi alloys are studied under strain to reveal multiple band inversions associated with topological phase transitions. From our simulations we find that, as a result of first band-inversion, the Brillouin zone of the diamagnetic half-Heusler alloys hosts eight triple points whereas, the second band inversion causes the emergence of sixteen more triple points. These band-inversions are observed to be independent of the spin-orbit coupling and are the reason behind increasing occupation of bismuth 7s orbitals as volume of the unit cell increases. The surface electronic transport in different triple point semi-metallic phases is found to evolve under strain, as the number of Fermi arcs change due to multiple band inversions. Once the second band inversion occurs, further application of tensile strain does not increase the number of triple points and Fermi arcs. However, increasing tensile strain (or decreasing compressive strain) pushes the triple point crossing to higher momenta, making them more effective as source of highly mobile electrons. These observations make a pathway to tune the bulk as well as surface transport through these semi-metals by application of tensile or compressive strain depending on the unstrained relative band-inversion strength of the material.

The impacts of grain boundary on the scattering of intermetallics in friction-stir-welded AA6061-T6

The type of intermetallic precipitates and their distribution during the friction stir welding (FSW) have significant effects on the microstructure and mechanical performance. Therefore, the present study aimed to investigate the type and volume fraction of grain boundaries in different zones of the FSW-ed AA6061-T6 alloys and their impacts on the scattering of intermetallic precipitates. For this purpose, EBSD and FESEM/EDS analysis along with the Rietveld refinement were performed. The results revealed the tendency of the AA6061-T6 to form intermetallic precipitates, i.e., Mg2Si, Mg5Si6, and Al0.70Fe3Si0.30, where both grain boundaries and grains interior were favored locations for the development of the intermetallics. This achievement could be a milestone in the development of affordable and sustainable welding technology for applications ranging from aircraft fittings to magneto parts.

Superficial residual stress, microstructure, and efficiency in similar joints of AA2024-T3 and AA7475-T761 aluminum alloys formed by friction stir welding

Friction stir welding (FSW) represents a conceptually simple technique that consists of joining either similar or dissimilar solid-state materials through higher plastic deformation rates. FSW is an important technique in the aeronautical and aerospace industries, and its development is vital because of the significant difficulty in joining higher resistance AA 2000 and AA 7000 aluminum alloys with conventional techniques, like fusion welding, due to porosity and mechanical property losses. Thin sheets with a 1.6-mm nominal thickness of AA2024, heat treated to condition T3, and thin sheets with a 1.6-mm nominal thickness of AA7475, heat treated to condition T761, were used to investigate the influence of welding parameters under superficial residual stress and the efficiency of joints by FSW of AA2024-T3 and AA7475-T761 aluminum alloys. A central composite design (CCD) was used as a statistical model in this study (23 factorial points, six stellar points, two central points, and two replicas). Micrographic analysis showed that in the nugget zone of the AA7475-T761 alloy, there was hardness recovery. The fractography images showed that failures occurred mainly due to the joint line remnant defect, evidenced by the presence of cracks. The superficial residual stresses show a maximum value of 81 MPa at the advancing side in run 27 (hot welding) of AA2024-T3, whereas in AA7475-T761, a value of 57 MPa was found in the same run. Finally, tensile strength represents an efficiency of ~92% of the AA2024-T3 base metal value, while for AA7475-T761, this value was ~85%. From a component design perspective, the parameter window of this study is identified as interesting for its evaluation in the possible application in component manufacturing, due to the low values of superficial residual stresses found compared to those in previous work.

Microstructure and wear behavior of IC10 directionally solidified superalloy repaired by directed energy deposition

• The microstructure and wear behavior of different zones of IC10 alloy repaired by directed energy deposition are studied. • The size of primary γ′ phase in HAZ is smaller than that in the substrate, and the size and fraction of γ′ phase are decreased along building direction. • The dominant wear mechanism of the repaired alloy is abrasive wear, and the wear resistance is closely related to the fraction of γ′ phase. Directed energy deposition has been used to repair superalloy components in aero engines and gas turbines. However, the microstructure and properties are generally inhomogeneous in components because of the different processing histories. Here, the microstructures and wear behavior of different zones (substrate, HAZ, and deposit) are investigated for the IC10 directionally solidified superalloy repaired by the directed energy deposition process. It is found that the microstructure of the deposited layers is strongly textured with a <001>-fiber texture in the building direction, and the texture intensity is continuously increased along the building direction. Two kinds of γ′ phase (primary and secondary γ′ phase) can be found in the heat-affected zone (HAZ), and the average size of primary γ′ phase is smaller than that in the substrate due to liquation. In the deposit layers, the size of γ′ phase is much smaller than those in the substrate and the primary γ′ phase of HAZ; both size and the fraction of the γ′ phase decreases with the increase of building height. The wear rate of the substrate is the smallest, indicating the best wear resistance; while the wear rate of HAZ is the largest, indicating the worst wear resistance in the repaired sample. The wear rates in the deposit layers increase from the bottom to the top zones, showing a decreasing wear resistance. Abrasive wear is found to be the dominant wear mechanism of the repaired alloy, and the resistance to which is closely related to the fraction of γ′ phase in the microstructure. The understanding of the influence of microstructure on wear resistance allows for a more informed application of inhomogeneous superalloy components repaired by directed energy deposition in industry.

Revealing the structural heterogeneity of metallic glass: Mechanical spectroscopy and nanoindentation experiments

Microstructural heterogeneity is an important issue in understanding the mechanical and physical properties of metallic glasses (MGs). Theoretically, physical aging below the glass transition temperature relax the MGs systems to a lower potential energy state on the potential landscape (PEL) while the cold rolling rejuvenates MGs to a higher potential energy state. In the current work, we systemically turned the structural heterogeneity through dynamic mechanical relaxation and rejuvenation. Correlations among the structural heterogeneity, internal friction, and shear transformation zones (STZs) of the model alloy was connected. Based on the cooperative shear model and statistical analysis, the scale variation of the STZs was revealed. This study demonstrated that the spatial structural heterogeneity can be enhanced by severe plastic deformation which can reduce the energy barrier for STZs activation. And we proposed the barriers decrease and more nucleation sites of STZs from physical aging to rejuvenation, which may be potential mechanism for improving the plasticity in MGs.

Microstructural analysis and mechanical behavior of TC4 titanium alloy and 304 stainless steel by friction stir lap welding

The optimization of welding parameters of the welded joints of TC4 titanium alloys/304 stainless steel (TC4/304 SS) by friction stir lap welding (FSLW) based on orthogonal test was researched. The results show that under low heat input (low rotating speed and high traversing speed), there was a damaging defect in stir zone (SZ) and cracks started from the defect under the tension. Under high heat input (high rotating speed and low traversing speed), the intermetallic compounds were barely formed, and the cracks formed from 304 SS blocks existed in thermal mechanically affected zone of TC4 titanium alloys (TMAZTC4), eventually obtaining the highest fracturing load. However, under middle heat input (middle rotating speed and low traversing speed), the intermetallic compounds and the maximum hardness were found at the interface, and cracks started from these compounds under the tension, getting a middle fracture load. In conclusion, the joints with good properties can be obtained with high rotating speed and low traversing speed.

Grain polydispersity and coherent crystal reorientations are features to foster stress hotspots in polycrystalline alloys under load.

The mechanical properties of metallic alloys are controlled through the design of their polycrystalline structure via heat treatments. For single-phase microstructures, they aim to achieve a particular average grain diameter to leverage stress hardening or softening. The stochastic nature of the recrystallization process generates a grain size distribution, and the randomness of the crystallographic orientation determines the anisotropy of a mechanical response. We developed a multiscale computational formalism to capture the collective mechanical response of polycrystalline microstructures at unprecedented length scales. We found that for an averaged grain size, the mechanical response is highly dependent on the grain size distribution. The simulations reveal the topological conditions that promote coherent grain texturization and grain growth inhibition during stress relaxation. We identify the microstructural features that are responsible for the appearance of stress hotspots. Our results provide the elusive evidence of how stress hotspots are ideal precursors for plastic and creep failure.

Microstructure evolution of Ag-Cu/BaCo0.7Fe0.2Nb0.1O3-δ joints in air and argon atmospheres

ABSTRACT Microstructure evolution of the joints formed by Ag-6.6 mol% Cu brazing BaCo0.7Fe0.2Nb0.1O3-δ (BCFN) ceramic after long-term heat treatment in different atmospheres was revealed. After heat treatment in air, the layered structure in the original interfacial reaction layer disappears, accompanied by the aggregation and growth of the interfacial products. The thickness of the reaction layer increased first and then stabilized. Under the action of the concentration gradient, the residual copper oxide diffused along the pores of BCFN matrix and continued to react with the matrix until it exhausted. The types of interfacial products remained unchanged, which were Ba2Cu2O5 and Co-Cu-O (solid solution). However, serious decomposition of interfacial products (Ba2Cu2O5) occurred due to the low oxygen partial pressure after heat treatment in argon, resulting in the formation of holes and cracks. Part of cobalt precipitated from the BCFN matrix and formed a cobalt-rich phase at the junction of Ag-Cu alloy and interfacial reaction zone. Results showed that the joint microstructure is unstable at high temperature. Controlling the amount of the interfacial products formed during brazing is key to improving the stability of the joint, which provides a theoretical support for further optimizing the sealing quality of membrane components.

How microstructure affects localized corrosion resistance of stir zone of the AA2198-T8 alloy after friction stir welding

In this study, the microstructure and corrosion resistance of the stir zone (SZ) of the AA2198-T8 Al-Cu-Li alloy welded by friction stir welding (FSW) were investigated by microscopy, immersion tests and electrochemical techniques such as measurements of open circuit potential variation with time, and scanning vibrating electrode technique (SVET) measurements. A low chloride-containing solution (0.005 mol L−1 NaCl) was employed in the corrosion studies and severe localized corrosion (SLC) was observed in the SZ related to intergranular attack. The results were compared to those of the non-affected areas by FSW, also known as base metal (BM). In the BM, SLC was found and the type of attack related to it was intragranular. In both zones, BM and SZ, SLC was due to precipitates of high electrochemical activity, specifically T1 (Al2CuLi) phase in the BM, whereas TB (Al7Cu4Li) / T2 (Al6CuLi3) in the SZ. Scanning vibrating electrode technique (SVET) analysis was very useful in the study of SLC in the AA2198-T8 alloy showing the development of high anodic current densities at the mouth of the SLC sites.

Mathematical modelling and optimization of laser beam parameters on microhardness and surface scratch resistance of Al-TiZn alloy

ABSTRACT There is high demand of Aluminium material for various engineering applications but the main problem being low wear and corrosion resistance. Laser beam application for surface modification of engineering materials gets high attention in research and industries due to superior surface performance for wear and corrosion resistance. In the present research article, an attempt has been made to improve the properties of Aluminium by laser surface alloying technology. Laser surface alloying of commercially pure aluminium with titanium and zinc metal powders at 50% combination has been conducted using CO2 laser power from 1600 to 2000 W. The experiments have been planned through design of experiment using Response Surface Methodology (RSM). The mathematical modelling and optimisation of process parameters such as specific laser power scan speed and heat input for optimum value of Micro hardness has been carried out using RSM with three factors and five levels. The statistical significance of the data has been carried out using Analysis of Variance (ANOVA).The alloy made by Al-TiZn shows improved hardness by 47% due to good laser surface alloying which has created dendrite microstructure without any metallurgical defects. The optimised value of laser energy density was found to be 37 J/mm2. It has been investigated that the optimum values of laser speed and heat input are 84 m/s and 61.50 watt respectively for micro hardness value of 56HV. Hardness decreases from top alloy zone towards substrate material since high energy creates more depth where the concentration of alloying elements is less. The friction coefficient of Al-TiZn alloy was found to be less by 23% than unalloyed aluminium substrate. The depth of laser alloyed has been increased with laser power and heat input from laser beam. Higher electron microscopy analysis shows that a sound alloying has been created without surface and subsurface defects. The present investigation is expected to help automobile and aerospace industries where the wear and corrosion resistance of the material are of major concern.

The effect of bonding time on the microstructure and mechanical properties of dissimilar transient liquid phase bonding between UNS N08825 alloy and UNS S32750 super duplex stainless steel using the BNi-2 interlayer

• The dissimilar transient liquid phase (TLP) bonding of the UNS N08825 alloy to UNS S32750super duplex stainless steel was successfully accomplished using the BNi-2 interlayer. • The complete isothermal solidification occurred at 1050 °C and the bonding time of 300 s. • Shear strength of the TLP joints improved by increasing the holding time. In this study, the microstructure and mechanical properties of transient liquid phase, dissimilar joints between UNS N08825 alloy and UNS S32750super duplex stainless steel was investigated. The joining process was carried out at 1050 °C for 0, 60, 120, and 300 s using an BNi-2 interlayer. The transient liquid phase bonding area was divided into three zones: (i) the isothermal solidification zone which contained γ-Ni solid solution, (ii) the a-thermal solidification zone which consisted of Ni- and Cr-rich borides and Ni 3 Si precipitates, (iii) and the diffusion affected zone in which Cr-Ni-Fe-Mo-Cu-rich borides constituents existed. The hard and brittle eutectic compounds in the bonding zone posed deleterious effects on the mechanical properties of the joints. By increasing the holding time, the amount of these detrimental phases in the bonding zone decreased before finally a eutectic-free joint was achieved at the bonding time of 300 s, which was notably low as compared to the previous studies. The joint held for 0 s had the widest a-thermal solidification zone and the highest hardness was measured at the center of it (546 Hv). The average hardness of the diffusion affected zone on the UNS S3750 super duplex stainless steel side was approximately 450 Hv, which was markedly higher than that of the diffusion affected zone on the UNS N08825 side (302 Hv). The joint held for 0 s showed the lowest shear strength (239 MPa), while the maximum shear strength was obtained for the joint with a holding time of 300 s, approximately 487 MPa. Moreover, it was demonstrated that eutectic compounds were the principal cause of the brittle fracture in the joints containing the a-thermal solidification zone, whereas the joint with a complete isothermal solidification experienced a brittle fracture that occurred at the diffusion-affected zone of UNS N08825 alloy.

Formation mechanism of interfacial microstructures and mechanical properties of Ti2AlNb/Ni-based superalloy joints brazed with NiCrFeSiB filler metal

Dissimilar brazing of Ti2AlNb-based alloy and Ni-based wrought superalloy (GH536) was studied using NiCrFeSiB filler metal. The Ti2AlNb/GH536 joints were analyzed by scanning electron microscope (SEM) equipped with an electron probe micro-analyzer (EPMA), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The formation mechanism of interfacial microstructure and mechanical properties of Ti2AlNb/GH536 joints were studied. The results indicated that Ti2Ni(Al,Nb), AlNi2Ti and TiB2 reaction layers were formed in the joint adjacent to Ti2AlNb base metal. These layers resulted in high micro-hardness and the weak link of the joint. γ solid solution was formed through isothermal solidification and β1-Ni3Si phase precipitated in the γ solid solution during cooling process. Ni3B, β1-Ni3Si and CrB phases appeared in the centre of the joint. Blocky and needle-like borides formed within the diffusion affected zone of GH536 base alloy. The maximum tensile strength of Ti2AlNb/GH536 joints reached 425 ​MPa ​at room temperature and the strength value of 373 ​MPa was maintained at 923 ​K.

Corrosion and corrosion-fatigue synergism on the base metal and nugget zone of the 2524-T3 Al alloy joined by FSW process

The corrosion processes and its influence on the FCG curves of the 2524-T3 alloy joined by FSW was studied in the nugget and BM. To assess the corrosion process, SVET, pH micro-potentiometry and EIS were used. The localised corrosion results showed similar BM and nugget zone electrochemical behaviour, while the TMAZ/HAZ region was electrochemically the weakest zone, susceptible to anodic dissolution. The FSW process and saline environment was found to modify the BM and nugget FCG resistance, being more detrimental for the BM due to the primary IM particles that are not present in the nugget, making it chemically more homogeneous.

Analysis of test results under static load conditions of 316L steel

Metal powder 3D printing technology is gaining popularity due to the possibility of producing structural elements of complex geometry, which production with the methods used so far is difficult or impossible to obtain. An example of a material used in the parts production by the additive method is 316L steel, which is used in the production of bone support screws, surgical tools and needles, or in other industries for the production of exhaust manifolds, parts of furnaces or heat exchangers. The study investigated the mechanical properties, hardness and microstructure of 316L steel produced in the selective laser melting process (SLM). Based on the tests, the following mechanical properties of 316L steel were obtained: Su = 566.7MPa, Sp0.2 = 484MPa, E = 113820MPa, A = 79.5%, Z = 72.3%. The hardness test results show a significant increase in hardness as the tensile test approaches the sample fracture. The structure of 316L steel in the grip part is characterized by the formation of visible semi-elliptical zones of the material alloy, the pools with crystallized grains with a cell-column structure oriented in the direction of the thermal gradient. This type of microstructure is characteristic of technology in which, after solidification, the cooling process takes place at high speed.

Some studies of nanoparticle properties for dissimilar materials on the surface features created by EBW and LBW

Study of various microstructural properties and mechanical properties of the weld zones of different alloys, which were welded using the EBW and LBW. This study reveals all the changes occurring to the microstructural and mechanical properties due to variation in the input values such as at which speed the welding is done, the angle at which the component is welded. EBW is the most sophisticated process for welding both similar and dissimilar metals and alloys. As the process is a high energy density process, the heat produced or the heat input is comparatively lower than the convention welding process, thereby producing smaller width of the fusion zone (FZ) and heat-affected zone (HAZ). Due to the HAZ, the microstructure and microhardness of the Base Metal (BM) are getting affected. And the optical microscopy studies also revealed how the microstructure changes affect the microhardness and the tensile strength. Some of the test results were also incorporated to add strength to the details provided.