Higher Microhardness Values Research Articles

Optimization of Process Parameters and Comparative Study on Bi-Metallic Joints Developed by High Temperature Nicrobrazing, Laser and GTAW Joining Processes

High Temperature Nicrobrazing of AISI 316 and 316 L stainless steels has been studied by comparing with pulsed laser and gas tungsten arc welding processes. Fabrication of instrumented irradiation capsule for evaluating the irradiation performance of fuel and structural materials in a nuclear reactor requires development of thin wall joints capable of withstanding high temperature and/or internal pressure. Thin wall joints for high temperature (∼550 °C) applications can be made by Laser Beam Welding (LBW), Gas Tungsten Arc Welding (GTAW) and High Temperature Brazing (HTB) methods. Using these methods, joints have been prepared and tests such as helium leak test (HLT) and microhardness test have been carried out. Microstructures were also evaluated. A comparative study has been performed on these joining methods to identify the suitable methods for high temperature applications. No observable leak was found during HLT. Microstructures of joints shows variation in solidification morphology due to difference in heat sources. It has been observed that the joint prepared by high temperature brazing process is having higher microhardness value than the joints prepared by LBW and GTAW processes. Further, it has been observed that LBW joints have better mechanical properties than GTAW joints and HBW joints.

The Formation of Composite Ti-Al-N Coatings Using Filtered Vacuum Arc Deposition with Separate Cathodes

Ti-Al-N coatings were deposited on high-speed steel substrates by filtered vacuum arc deposition (FVAD) during evaporation of aluminum and titanium cathodes. Distribution of elements, phase composition, and mechanical properties of Ti-Al-N coatings were investigated using Auger electron spectroscopy (AES), X-ray diffraction (XRD), transmission electron microscopy (TEM) and nanoindentation, respectively. Additionally, tribological tests and scratch tests of the coatings were performed. The stoichiometry of the coating changes from Ti0.6Al0.4N to Ti0.48Al0.52N with increasing aluminum arc current from 70 A to 90 A, respectively. XRD and TEM showed only face-centered cubic Ti-Al-N phase with preferred orientation of the crystallites in (220) direction with respect to the sample normal and without precipitates of AlN or intermetallics inside the coatings. Incorporation of Al into the TiN lattice caused shifting of the (220) reflex to a higher 2θ angle with increasing Al content. Low content and size of microdroplets were obtained using coaxial plasma filters, which provides good mechanical and tribological properties of the coatings. The highest value of microhardness (36 GPa) and the best wear-resistance were achieved for the coating with higher Al content, thus for Ti0.48Al0.52N. These coatings exhibit good adhesive properties up to 30 N load in the scratch tests.

The Effects of Laser Surface Hardening on Microstructural Characteristics and Wear Resistance of AISI H11 Hot Work Tool Steel

AbstractThe present study deals with the effects of laser surface treatment on microstructure evolution and wear resistance of AISI H11 hot work tool steel in quenched and tempered condition. The most upper laser-affected zone is characterized by re-melted microstructure consisting of dendrite cells with fresh non-tempered martensite, retained austenite and inter-dendritic carbidic network. The subsolidus microstructure just beneath the re-melted zone represents the most laser surface hardened zone consisting of fresh non-tempered martensite with fine and coarse carbides as a result of overheating the original QT substrate microstructure. The highest microhardness values in the range from 775 to 857 HV were measured for the LSH microstructure and the most softened microstructure exhibited the minimum hardness of 530 HV. The laser treated samples showed the improvement of their surface wear resistance by 35%.

Deposition of the Ti-Al coatings on different metallic substrates by mechanical alloying and subsequent laser treatment

Ti-Al intermetallic coatings thickness of about 60–150 μm was fabricated on Al, Ti, and hypo-eutectoid steel substrates by mechanical alloying (MA) using a vibratory ball mill. The coatings was then laser treated to obtain homogenous and equilibrium structure. Phase composition and the cross-sectional microstructure of the coatings were investigated using X-ray diffraction and the Scanning electron microscopy respectively while chemical analysis of the coatings was carried out using Bruker energy-dispersive X-ray Spectrometer (EDS). Micro-hardness (HV) of the obtained coatings was observed to be increased more than twice after the deposition and subsequent laser treatment in comparison with microhardness of the initial substrates materials.. Surface roughness of the obtained coatings was also reduced significantly after laser treatment i.e. for Al substrate it reduced from 52 μm to 26 μm. The parameters of mechanical alloying and laser treatment used in this work provided a successful way to deposit nano-structured intermetallic coatings with homogenous and smooth surface finish and high value of micro-hardness.

Microhardness distribution and finite element method analysis of Al 5452 alloy processed by unconstrained high pressure torsion

High pressure torsion (HPT) is one of the successful and efficient methods of severe plastic deformation (SPD). In this research, disk-shaped specimens of aluminum alloy were exposed to high pressure and torsion, which are the key factors of HPT. Simultaneously applying high pressure and torsion causes shear strain and thus enhancement of mechanical properties such as microhardness. In order to understanding the behavior of local deformation on disks after HPT, the process has been simulated by using finite element analysis method in ABAQUS/Explicit software. Results of simulation showed that by increasing applied pressure and number of turns, more effective strain would be applied to the disks. By comparing results of experiment and simulation, it was concluded that there is a region in the middle of the disk that has higher microhardness value in comparison to other regions, which is caused by more strain that was applied to it. Also, dimensions and equivalent plastic strain (PEEQ) obtained in experiments and simulations are compared. It was observed that expected dimensions and PEEQ of simulations are in good agreement with experimental results.

Ageing response of sub-transus heat treated Ti–6.8Mo–4.5Fe–1.5Al alloy

In this study, phase transformations occurring during ageing of Timetal LCB (Ti–6.8Mo–4.5Fe–1.5Al in wt.%) and their influence on mechanical properties were investigated. The initial condition was prepared by solution treating above β-transus directly followed by heat treatment in α/β temperature regime. This treatment resulted in precipitation of α phase on β/β grain boundaries. The subsequent ageing response of this material was studied during isothermal ageing at 400 °C, 450 °C and 500 °C. The phase transformations were studied by X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Mechanical properties were investigated using microhardness measurements. During ageing at 400 °C, the ω phase was detected in all studied conditions. Due to its presence in the material, very fine and homogeneously distributed α particles were formed, leading to a high microhardness. At 450 °C, the ω phase disappeared already after a few hours of ageing. The highest microhardness values at this temperature were reached in samples aged for a few hours after the dissolution of ω particles. No ω was detected in samples aged at 500 °C. The α phase morphology after ageing at 500 °C consisted of large plate-shaped precipitates which coarsened rapidly with increasing ageing time. This coarse microstructure corresponded to a low microhardness.

Buckypaper of polyvinyl chloride/p‐phenylenediamine modified graphite and PVC/graphite via resin infiltration technique

Resin‐infiltration technique has been employed as a versatile method to improve nanofiller dispersion in matrix. In this article, fabrication of polyvinyl chloride (PVC)/p‐phenylenediamine (PPD)/functional graphite (F‐G) and PVC/F‐G composite paper have been reported through resin‐infiltration techniques. Synthesis and characterization of PVC/PPD/F‐G and PVC/F‐G composite papers for enhanced structural, morphological, thermal, hardness, wettability, and conductivity properties have been studied. Moreover, the effect of increasing amount of F‐G and PPD/F‐G nanofiller on composite paper properties was scrutinized and compared. Transmission electron microscopy and scanning electron microscopy depicted better graphite nanofiller dispersion in PVC matrix and network formation in PVC/PPD/F‐G composite paper. Maximum degradation temperature (Tmax) of PVC/PPD/F‐G 0.08 composite paper (517°C) was found higher than PVC/F‐G 0.08 composite paper (484°C). Dynamic contact angle analysis shows that the PVC/PPD/F‐G and PVC/F‐G composite papers have better surface wettability than neat PVC. PVC/PDD/F‐G 0.1 and PVC/F‐G 0.1 composite papers possess high microhardness values up to 39.67 and 32.76 Hv, respectively. Electrical conductivity of 8.97 × 10−3 and 5.89 × 10−3 S m−1 was observed for 0.08 wt% PPD/F‐G and F‐G loaded samples. POLYM. COMPOS., 39:4176–4187, 2018. © 2017 Society of Plastics Engineers

Effects of the Casting Temperature in the Leakage of Zamak 5

Recent studies evidenced that the Zamak 5 corrosion is associated to solidification defects arising from the injection process that occur mainly in areas where the solidification rate is faster. Given this factor, the aim of this research was to evaluate the influence of Zamak 5 casting temperature. For this study, Zamak 5 was casted at different temperatures (430 °C, 450 °C, 500 °C, 550 °C) with respective solidification curves. Zamak 5 was analyzed for chemical composition and microstructure. Moreover, complementary tests of microhardness and electrochemical analyses of open circuit potential and polarization were performed. From these analyses it was observed that the highest casting temperature of Zamak 5 originates larger grains and higher microhardness value, due to the lower solidification rate. In none of the leaking temperatures porosity defects were observed on the samples, which is coherent with previous researches, being originated from the injection process.

Fabrication of nickel–aluminum alloys with tungsten and molybdenum borides by the method of self-propagating high-temperature synthesis

Intermetallic alloys in the Ni–Al system strengthened through inclusions of molybdenum and tungsten borides by means of sequential exothermic reactions in a two-layer charge are fabricated. NiAl and Ni2Al3 intermetallic phases and molybdenum (Mo2B5) and tungsten (W2B5) borides are identified in alloys by the methods of X-ray diffraction and elemental analysis. It is established that Mo2B5 and W2B5 inclusions are characterized by high values of microhardness (Mo2B5—24.4 GPa and W2B5—29.2 GPa).

Nanostructured compacts and composites based on diamond-like boron nitride

Three ways of thermobaric synthesis of nanocompacts and nanocomposites based on diamondlike boron nitride under the pressure of 8 GPa are considered. The structure of specimens is probed via X-ray diffraction and electron microscopy. It is shown that using the titanium and boron nitrides powders accelerates sintering, resulting in high microhardness values and elastic moduli of the obtained superhard composites.

Evaluation of Co-Ni/SiC Nanocomposite Coating Obtained by Electrodeposition on the Corrosion Resistance of API 5L X80 Steel

Co-Ni/SiC nanocomposite coatings were obtained by codeposition of SiC nanoparticles in a bath with nickel and cobalt, using API 5L X80 as the substrate. The influence of cathodic current density on deposit properties was investigated. Measurements of electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PP) were performed in 3.5 wt% NaCl solution to investigate a corrosion performance. The morphology and chemical analysis of the surfaces were analyzed by scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) and related with micro-hardness and X-ray diffraction (XRD). The results indicate which composite coatings have acicular morphology, with higher micro-hardness values than those found for the carbon steel substrate. The Co-Ni/SiC coating increased the corrosion potential and the resistance to polarization, which resulted in lower corrosion rates than those obtained without uncoated steel.

Structural, mechanical and thermal characterization of an Al-Co-Fe-Cr alloy for wear and thermal barrier coating applications

A structural, mechanical and thermal characterization of an Al-Co-Fe-Cr coating alloy was performed. An atomized powder with atomic composition of Al71Co13Fe8Cr8 was thermally sprayed by high velocity oxygen fuel (HVOF) on a steel substrate. X-ray diffraction, scanning and transmission electron microscopy and differential scanning calorimetry were used to characterize the atomized powder and the sprayed coating. Vickers microhardness and pin-on- plate wear test were carried out for mechanical and tribological characterization and an infrared camera was used to evaluate the insulation capacity of the coating material. The results show that both the atomized powder and the coating material were composed predominantly by a quaternary extension of the hexagonal Al5Co2 phase and by the monoclinic Al13Co4. Both phases are quasicrystalline approximants of a decagonal Al-Co quasicrystal. The coating samples presented high values of micro-hardness, close to 500HV and substantially low friction coefficient values, around 0.05. The coatings were good thermal insulators, decreasing by 30% the surface temperature of a sample exposed to a hot plate in comparison with the carbon steel substrate.

Nanoscale structural defects in electrodeposited Ni/Al2O3 composite coatings

This article attempts to explain how Al2O3 nanoparticles and saccharin added to and dispersed in an electrolytic bath affect the microstructure of electrodeposited composite coatings. The micro- and nanoscale observations of the structure of the pure Ni and Ni/Al2O3 coatings revealed the presence of numerous growth type faults in the Ni matrix, including dislocations and high density (111) nanotwins along the [011] zone axis. The addition of alumina nanoparticles and saccharin to the electrolyte caused an increase in the density of the nanotwins and refinement of the Ni grains, which improved the mechanical properties of the coatings. The highest values of microhardness were reported for the Ni/Al2O3 coating obtained from the bath containing the organic compound. The presence of alumina nanoparticles in the electrolyte changed the texture component of the Ni deposit. The addition of both the ceramic and saccharin did not affect the preferred crystallographic orientation of the Ni grains but resulted in a lower level of residual stresses in the coatings.

Laser Metal Deposition of the Intermetallic TiAl Alloy

Laser metal deposition of the commercial intermetallic Ti-47Al-2Cr-2Nb alloy was investigated. A large number of experiments were conducted under controlled atmosphere by changing the processing parameters to manufacture a series of beads, thin walls, and massive blocks. Optimal process parameters were successfully found to prevent cracking which is generally observed in this brittle material due to built-up residual stresses during fast cooling. These non-equilibrium cooling conditions tend to generate ultra-fine and metastable structures exhibiting high microhardness values, thus requiring post-heat treatments. The latter were successfully used to restore homogeneous lamellar or duplex microstructures and to relieve residual stresses. Subsequent tensile tests enabled us to validate the soundness and homogeneity of the Intermetallic TiAl alloy. Finally, a higher mechanical performance was achieved for the LMD material with respect to cast+HIP and EBM counterparts.

Influence of pulse operational parameters on pure nickel electrodeposits: Part II. Microhardness and corrosion resistance

Nanocrystalline nickel electrodeposits were fabricated from an additive-free Watts bath by pulse current electrodeposition technique. The coatings were obtained at different current densities, duty cycles and pulse frequencies and were characterised using Field Emission Scanning Electron Microscope and X-ray diffraction. The corrosion properties of the coatings in 3.5 wt-% NaCl solution were investigated using potentiodynamic polarisation and electrochemical impedance spectroscopy. All the nickel samples exhibited active–passive potentiodynamic polarisation behaviour. A strong relationship was observed between the crystallographic orientation of the nickel electrodeposits and their corrosion resistance. Commonly the corrosion current density increases with I(200)/I(111) ratio (I(hkl) is the peak intensity corresponding to (hkl) crystal plane). The passive layer is more stable on [211] oriented grains than on [100] oriented grains. The Nyquist impedance curves were composed of two depressed semicircles corresponding to charge transfer process and diffusion process through a porous passive layer. [211] orientation and small grain size favour high microhardness values.

Phase formation criteria assessment on the microstructure of a new refractory high entropy alloy

This work focuses on the evaluation of various parametric models for the formation of solid solution phases in high entropy alloys. The actual observation of MoWHfZrTi microstructure, along with the theoretical suggestions for single phase selection is discussed. The micro-segregated obtained structure is explained in terms of introducing a possible mechanism based on the estimation of thermodynamic parameters and further analysis of the experimental evidence upon solidification. High values of both micro and macro hardness characterize the synthesized system, indicating a solid solution strengthening mechanism.

Fabrication of new metallic glassy Ti40.6Cu15.4Ni8.5Al5.5W30 alloy powders by mechanical alloying and subsequent SPS consolidation

Mechanical alloying approach, using a low-energy ball milling technique was employed for synthesizing metallic glassy Ti40.6Cu15.4Ni8.5Al5.5W30 alloy powders. The powders of the product obtained after 200h of milling possessed high thermal stability, characterized by a wide supercooled liquid region of 90K and crystallization temperature began at 827K. The powders were successfully consolidated into full dense bulk metallic glass at temperature laid above its glass transition temperature, using spark plasma sintering technique. The as-consolidated bulk metallic glassy alloy possessed extraordinary mechanical properties, such as high values of microhardness (∼11.61GPa). Moreover, the measured friction coefficient of this new bulk metallic glassy alloy was 0.32.

Investigación sobre fusión de contacto en materiales compuestos laminados Cu/Al

Se presentan los resultados de composición química, microdureza y conductividad eléctrica obtenidos en materiales compuestos laminados Cu/Al, tratados térmicamente a temperaturas superiores a las del eutéctico Cu - Al. El bimetal Cu/Al fue obtenido por soldadura por explosión. La composición química del material tras los tratamientos térmicos se analizó por medio de análisis EDS (Espectroscopia de Energías Dispersiva de Rayos X). La conductividad eléctrica se midió mediante ensayos con corrientes Eddy. Las zonas endurecidas en el material soldado por explosión fueron identificadas. El valor experimental obtenido de la conductividad eléctrica está en concordancia con el calculado por la regla de las mezclas. Los tratamientos térmicos dan lugar a la formación de múltiples intercaras de alta dureza, compuestos de intermetálicos. La conductividad eléctrica de las intercaras identificadas es significativamente menor que las correspondientes al Cu y al Al.

Strengthening mechanism of friction stir processed and post heat treated NiAl bronze alloy: Effect of rotation rates

In this work, NiAl bronze (NAB) alloys were subjected to friction stir processing (FSP) at a constant traverse speed of 100mm/min and rotation rates of 600rpm, 800rpm, 1000rpm and 1200rpm, respectively. Thereafter, heat treatment was performed at 675°C for 2h. The effects of rotation rates on strengthening mechanisms of friction stir processed and post heat treated NAB alloy were studied. The results showed that friction stir processed NAB alloy microhardness was increased as the rotation rate increased. During friction stir processing, martensite nanotwins could be formed due to high strain rates and peculiar martensitic structures of NAB alloys. A rotation rate increase, increased peak temperatures and strain rates in friction stir processed NAB alloys, leading to a significant amount of martensite nanotwins formation. As rotation rates increased from 600rpm to 1200rpm, in addition to grain refinement effects, the strengthening mechanism of friction stir processed NAB alloys gradually changed from secondary phase strengthening to solid solution, dislocations and nanotwin strengthening. During post heat treatment, discontinuous static recrystallization occurred and β′ phase decomposed into α and κ phases. Highest microhardness values were obtained at the rotation rate of 800rpm and the uniformly distributed second phases formed during friction stir processing contributed mainly to higher microhardness.

Friction Stir Welds of Al Alloy-Cu: An Investigation on Effect of Plunge Depth

Abstract In the present study, butt joints of aluminum (Al) 8011-H18 and pure copper (Cu) were produced by friction stir welding (FSW) and the effect of plunge depth on surface morphology, microstructure and mechanical properties were investigated. The welds were produced by varying the plunge depth in a range from 0.1 mm to 0.25 mm. The defect-free joints were obtained when the Cu plate was fixed at the advancing side. It was found that less plunging depth gives better tensile properties compare to higher plunging depth because at higher plunging depth local thinning occurs at the welded region. Good tensile properties were achieved at plunge depth of 0.2 mm and the tensile strength was found to be higher than the strength of the Al (weaker of the two base metals). Microstructure study revealed that the metal close to copper side in the Nugget Zone (NZ) possessed lamellar alternating structure. However, mixed structure of Cu and Al existed in the aluminum side of NZ. Higher microhardness values were witnessed at the joint interfaces resulting from plastic deformation and the presence of intermetallics.