Friction Stir Welding of Dissimilar Joints Between Commercially Pure Titanium Alloy and 7075 Aluminium Alloy

In this study, a joint between commercially pure titanium alloy and 7075 aluminium alloy was butt welded by using friction stir welding at a rotational speed of 1120 rpm and a traverse speed of 50 mm*min-1. The evaluation of hardness and microstructure was performed by using scanning electron microscopy. The phases in the weld area were identified by apply-ing the X-ray diffraction technique and the Energy Dispersive X-ray Spectroscopy (EDS) analysis was used for the evaluation of intermetallic compounds of the weld area. The weld zone is cone-shaped and consists of aluminium and titanium particles that play an important role in increasing hardness and tensile strength. The weld area has three zones, namely the titanium base metal zone, the aluminium base metal zone, and the titanium-aluminium inter-metallic compound mixed zone. It was also observed that the joint area on the aluminium side includes the stirred area, the thermo-mechanically affected zone, and the heat-affected zone, while the titanium joint area contains the stirred zone and the heat-affected zone. The hard-ness value of the weld area was around 360 HV, which means that in this area, compared to the base metal of titanium and aluminium, hardness has increased by 6% and 20%, respectively. This can be attributed to severe plastic deformation and formation of intermetallic compounds of titanium and aluminium in this area

Microstructural characterization, biocorrosion evaluation and mechanical properties of nanostructured ZnO and Si/ZnO coated Mg/HA/TiO2/MgO nanocomposites

Nano-ZnO monolayer and nano-Si/ZnO double-layer coatings were deposited on a Mg/HA/TiO2/MgO nanocomposite using radio frequency magnetron sputtering. The composition and surface morphology of the specimens were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, transmission electron microscopy, atomic force microscopy and field-emission scanning electron microscopy equipped with energy dispersive X-ray spectroscopy. Potentiodynamic polarization and immersion tests were used to investigate the corrosion behavior of samples. The Si/ZnO nanocomposite coating, which had an average thickness of 1.7µm, exhibited a uniform and dense film, consisting of a ZnO outer layer (~1.1µm thick) and a Si inner layer (~0.6µm thick). However, some pores and cracks were observed in the ZnO monolayer coating (~1.5µm thick). In addition, the Si and ZnO nanoparticles had a spherical morphology with an average particle size of 28-40nm. Higher polarization resistance values were obtained for the nano-Si/ZnO coated sample (~2.55kOcm2) compared with that of the ZnO coated (~1.34kOcm2) and uncoated (~0.14kOcm2) samples in a simulated body fluid (SBF). The results from the hydrogen evolution studies indicated that the nano-Si/ZnO-coated sample had a lower degradation rate (1.07ml/cm2/day) than the ZnO-only coated sample (2.25ml/cm2/day) and the uncoated sample (4.42ml/cm2/day). After 168h of immersion in a SBF solution, a larger amount of hydroxyapatite precipitated on the Si/ZnO coating than the ZnO coating, which resulted in an improvement in the bioactivity. The compressive strength and elongation of uncoated Mg/HA/TiO2/MgO decreased from 253MPa and 9.8% to 104MPa and 5.2% after 28days of immersion in SBF solution, whereas the Si/ZnO coated sample indicated a compressive strength of 148MPa and elongation of 7.2%. Therefore, the double-layer Si/ZnO composite coating prepared by magnetron sputtering on the Mg/HA/TiO2/MgO nanocomposite is more suitable for biomedical applications

Synthesis and biodegradation evaluation of nano-Si and nano-Si/TiO2 coatings on biodegradable Mg-Ca alloy in simulated body fluid

In the present study, nano-Si and nano-Si/TiO2 composite coatings have been successfully synthesized on the surface of Mg-1 wt%Ca alloy by the physical vapor deposition (PVD) method. The surface morphology and compositions of the coated specimens were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), transmission election microscopy (TEM), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The results show the formation of thin and compact coating layers which homogeneously cover the surface of Mg alloy. Some micro-pores and micro-cracks were observed in the Si and Si/TiO2 films. It was also found that the Si and TiO2 nanoparticle had a spherical morphology with an average particle size of 30-40 nm and 70-80 nm, respectively. Electrochemical studies revealed that nano-Si/TiO2 coating offers a significant reduction in the corrosion rate (0.57 mm/year) compared to the Si coated (0.91 mm/year) and the uncoated alloys (6.21 nun/year) in simulated body fluid (SBF). Hydrogen evolution studies showed a lower degradation rate of nano-Si/TiO2 (1.57 mIkm(2)/day) than that of nano-Si coated alloy (2.22 ml/cm(2)/day). Immersion test showed that the nano-Si/TiO2 coating presented a greater nucleation site of hydroxyapatite (HA) than the uncoated sample. Thus nano-Si/TiO2 composite coating prepared by PVD on the Mg-Ca alloy is more appropriate for biomedical applications

The Effect of Holding Time on Dissimilar Transient Liquid-Phase-Bonded Properties of Super-Ferritic Stainless Steel 446 to Martensitic Stainless Steel 410 Using a Nickel-Based Interlayer

The dissimilar joining of martensitic and ferritic stainless steels have been developed that needs corrosion resistance and enhanced mechanical properties. In this study, the transient liquid-phase bonding of martensitic stainless steel 410 and super-ferritic stainless steel 446 was conducted with a nickel-based amorphous interlayer (BNi-2) at constant temperature (1050 °C) and increasing times of 1, 15, 30, 45, and 60 min. For characterization of the TLP-bonded samples, optical microscopy and scanning emission microscopy equipped with energy-dispersive X-ray spectroscopy were used. To investigate the mechanical properties of TLP-bonded samples, the shear strength test method was used. Finally, the X-ray diffraction method was used for microstructural investigation and phase identification. The microstructural study showed that the microstructure of base metals changed: the martensitic structure transited to tempered martensite, including ferrite + cementite colonies, and the delta phase in super-ferritic stainless steel dissolved in the matrix. During the transient liquid-phase bonding, the aggregation of boron due to its diffusion to base metals resulted in the precipitation of a secondary phase, including iron–chromium-rich borides with blocky and needle-like morphologies at the interface of the molten interlayer and base metals. On the other hand, the segregation of boron in the bonding zone resulted from a low solubility limit, and the distribution coefficient has induced some destructive and brittle phases, such as nickel-rich (Ni3B) and chromium-rich boride (CrB/Cr2B). By increasing the time, significant amounts of boron have been diffused to a base metal, and diffusion-induced isothermal solidification has happened, such that the isothermal solidification of the assembly has been completed under the 1050 °C/60 min condition. The distribution of the hardness profile is relatively uniform at the bonding zone after completing isothermal solidification, except the diffusion-affected zone, which has a higher hardness. The shear strength test showed that increasing the holding time was effective in achieving the strength near the base metals such that the maximum shear strength of about 472 MPa was achieved

Effect of Vanadium and Rare Earth on the Structure, Phase Transformation Kinetics and Mechanical Properties of Carbide-Free Bainitic Steel Containing Silicon

Carbide-free bainitic (CFB) steels with a matrix of bainitic ferrite and thin layers of retained austenite, to reduce the manufacturing costs, usually do not contain alloying elements. However, a few reports were presented regarding the effect of alloying elements on the properties of these steels. Thus, this study evaluates the effects of vanadium and rare earth (Ce-La) microalloying elements on the structure, phase transformation kinetics, and mechanical properties of carbide-free bainite steel containing silicon fabricated by the casting and austempering procedure. Optical and scanning electron microscopy (OM and SEM), electron backscatter diffraction (EBSD), and X-ray diffraction (XRD) were used to study the microstructure and phase structure. The transformation kinetics were examined by a dilatometry test. Hardness, tensile, and impact tests evaluated the mechanical properties. Due to adding alloying elements, the fracture toughness and change in matrix phases relation was studied by the crack tip opening displacement (CTOD) test and SEM fractography. The microstructure of the silicon added sample was completely carbide-free bainite. The test results showed vanadium helped CFB formation, even in continuous cooling. The primary austenite grain (PAG) size grew by vanadium addition. The EBSD phase map illustrates an increment in the percentage of retained austenite by vanadium. In contrast, the addition of 0.03 wt% rare earth reduced the primary austenite grain size and reduced the retained austenite content. The results of the dilatometry test confirmed that vanadium and rare earth addition both reduced the critical cooling rate of the bainite transformation. Vanadium leads to an earlier cessation of bainite transformation, while rare earth elements postpone this transformation. Mechanical tests showed that the tensile strength of carbide-free bainite steels was strongly influenced by the morphology and volume fraction of austenite. Retained austenite, when transformed to martensite during the transformation-induced plasticity (TRIP) phenomenon, leads to increased tensile strength and fracture toughness, or retained austenite with a film-like shape prevents the growth of cracks by blinding the crack tip. The result of the CTOD test exhibited that retained austenite plays the leading role in increasing crack resistance when TRIP occurs

Investigation of microstructure and mechanical properties of copper shell produced by shear spinning in different rotation directions

In this study, the effect of alteration in the direction of forming during the shear spinning of C11000 copper metal on mechanical properties, microstructure, texture, and anisotropy was investigated. Shear spinning causes the grains stretching along the axial direction besides increasing the grain length in the circumferential direction. Strain-path change in the shear spinning specimens has somewhat resulted in finer grains, more grain refinement, and a higher percentage of high-angle boundaries. More change of strain direction in the shear spinning specimens resulted in approximately 9% to 11% reduction in strength, from 1% to 9% decrease in hardness, and increased elongation from 7% to 37% more than in the specimen without path change. Shear spinning specimens in different paths had different orientations and texture intensities. In the specimen without strain-path change, most of the texture is related to {123}〈412〉 orientation and copper texture with {112}〈111〉 orientation. In the shear spinning specimens in other paths, textures with {001}〈100〉, {011}〈011〉, and {211}〈011〉 orientations and brass texture with {110}〈112〉 orientation were strengthened. Due to the change in texture and mechanical properties, the strain-path change in the shear spinning process reduced the anisotropy in the C11000 copper metal

Effect of Vanadium and Rare Earth on the Structure, Phase Transformation Kinetics and Mechanical Properties of Carbide-Free Bainitic Steel Containing Silicon

Carbide-free bainitic (CFB) steels with a matrix of bainitic ferrite and thin layers of retained austenite, to reduce the manufacturing costs, usually do not contain alloying elements. However, a few reports were presented regarding the effect of alloying elements on the properties of these steels. Thus, this study evaluates the effects of vanadium and rare earth (Ce-La) microalloying elements on the structure, phase transformation kinetics, and mechanical properties of carbide-free bainite steel containing silicon fabricated by the casting and austempering procedure. Optical and scanning electron microscopy (OM and SEM), electron backscatter diffraction (EBSD), and X-ray diffraction (XRD) were used to study the microstructure and phase structure. The transformation kinetics were examined by a dilatometry test. Hardness, tensile, and impact tests evaluated the mechanical properties. Due to adding alloying elements, the fracture toughness and change in matrix phases relation was studied by the crack tip opening displacement (CTOD) test and SEM fractography. The microstructure of the silicon added sample was completely carbide-free bainite. The test results showed vanadium helped CFB formation, even in continuous cooling. The primary austenite grain (PAG) size grew by vanadium addition. The EBSD phase map illustrates an increment in the percentage of retained austenite by vanadium. In contrast, the addition of 0.03 wt% rare earth reduced the primary austenite grain size and reduced the retained austenite content. The results of the dilatometry test confirmed that vanadium and rare earth addition both reduced the critical cooling rate of the bainite transformation. Vanadium leads to an earlier cessation of bainite transformation, while rare earth elements postpone this transformation. Mechanical tests showed that the tensile strength of carbide-free bainite steels was strongly influenced by the morphology and volume fraction of austenite. Retained austenite, when transformed to martensite during the transformation-induced plasticity (TRIP) phenomenon, leads to increased tensile strength and fracture toughness, or retained austenite with a film-like shape prevents the growth of cracks by blinding the crack tip. The result of the CTOD test exhibited that retained austenite plays the leading role in increasing crack resistance when TRIP occurs

Deposition of nanostructured fluorine-doped hydroxyapatite-polycaprolactone duplex coating to enhance the mechanical properties and corrosion resistance of Mg alloy for biomedical applications

The present study addressed the synthesis of a bi-layered nanostructured fluorine-doped hydroxyapatite (nFHA)/polycaprolactone (PCL) coating on Mg-2Zn-3Ce alloy via a combination of electrodeposition (ED) and dip-coating methods. The nFHA/PCL composite coating is composed of a thick (70-80 μm) and porous layer of PCL that uniformly covered the thin nFHA film (8-10 μm) with nanoneedle-like microstructure and crystallite size of around 70-90 nm. Electrochemical measurements showed that the nFHA/PCL composite coating presented a high corrosion resistance (Rp = 2.9 × 103 kΩ cm2) and provided sufficient protection for a Mg substrate against galvanic corrosion. The mechanical integrity of the nFHA/PCL composite coatings immersed in SBF for 10 days showed higher compressive strength (34% higher) compared with the uncoated samples, indicating that composite coatings can delay the loss of compressive strength of the Mg alloy. The nFHA/PCL coating indicted better bonding strength (6.9 MPa) compared to PCL coating (2.2 MPa). Immersion tests showed that nFHA/PCL composite-coated alloy experienced much milder corrosion attack and more nucleation sites for apatite compared with the PCL coated and uncoated samples. The bi-layered nFHA/PCL coating can be a good alternative method for the control of corrosion degradation of biodegradable Mg alloy for implant applications

Microstructural, mechanical properties and corrosion behavior of plasma sprayed NiCrAlY/nano-YSZ duplex coating on Mg-1.2Ca-3Zn alloy

In this study, microstructural evolution, mechanical properties and corrosion behavior of plasma sprayed NiCrAlY/nano-yttria stabilized zirconia (nano-YSZ) dual-layered coating on Mg-1.2Ca-3Zn alloy were investigated. NiCrAlY underlayer is composed of large amount of porosities and micro-cracks with thickness around 80-90 µm. However, nano-YSZ overlayer shows bimodal microstructure consisting of columnar grains and some partially molten parts of the nanostructured powders with thickness around 270-300 µm. The microhardness of dual-layered NiCrAlY/nano-YSZ coating is significantly higher than that of single-layered NiCrAlY. Despite that, the bonding strength of dual-layered coating is slightly higher than single-layered plasma sprayed coating. Results also showed that both single-layer NiCrAlY and dual-layer NiCrAlY/nano-YSZ coatings decreased the corrosion current density of Mg alloy from 217.1 µA/cm2 to 114.5 and 82.4 µA/cm2, respectively

Effect of bonding temperature and bonding time on microstructure of dissimilar transient liquid phase bonding of GTD111/BNi-2/IN718 system

The effect of bonding temperature and bonding time on the microstructure of transient liquid phase (TLP) bonding named GTD111 and IN718 superalloys, using a commercial Ni–B–Cr filler alloy (BNi-2) interlayer were evaluated. The sandwich assembly was kept in a vacuum furnace at temperatures of 1050, 1100, and 1150 °C for 1, 15, 30, 45, 60, and 80 min until the TLP process occurred. Microstructural characterization was carried out via optical microscopy, scanning electron microscopy (SEM) equipped with field emission energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). Microstructural assessments displayed those in little bonding times, the joint microstructure includes continuous eutectic intermetallic phases and longer times cause eutectic free microstructure. The bonding temperature affects the isothermal solidification rate, while, at low bonding temperatures microstructure of the joint centerline is controlled by diffusion of melting point depressant (MPD) elements. Despite, at high bonding temperature effect of base metal alloying elements on the joint microstructure development was more marked. The results showed that athermally solidified zone (ASZ) size reduces with increasing bonding temperature and time due to diffusion of boron into the base metal