Improvement of hardness and wear resistance of (TiC,TiB)/Ti-6Al-4V surface-alloyed materials fabricated by high-energy electron-beam irradiation

The objective of this study is to investigate microstructure, hardness, and wear properties of three kinds of (TiC,TiB)/Ti-6Al-4V surface-alloyed materials fabricated by high-energy electron-beam irradiation. The mixtures of Ti+C, TiC+TiB₂, and Ti+B₄C powders and CaF₂ flux were deposited on a Ti-6A1-4V substrate, and then high-energy electron beam was irradiated on these mixtures. The surface-alloyed layers of 0.9 to 1.6 mm in thickness were homogeneously formed, and contained a large amount (30 to 44 vol. pct) of hard precipitates such as TiC and TiB in the martensitic matrix. This microstructural modification improved the hardness and wear resistance of the surface-alloyed layer 2 times and 6 to 9 times, respectively, greater than that of the substrate. Particularly, the surface-alloyed material fabricated with Ti+B₄C powders had a larger volume fraction of TiB and TiC homogeneously distributed in the martensitic matrix, and thus showed the best hardness and wear resistance. These findings suggested that the surface-alloying using high-energy electron-beam irradiation was economical and useful for the development of titanium-base surface-alloyed materials with improved hardness and wear properties.     

Improvement of the hardness and wear resistance of (TiC, TiN)/Ti-6Al-4V surface-alloyed materials fabricated by high-energy electron-beam irradiation

This study is concerned with the microstructural analysis and improvement of the hardness and wear resistance of Ti-6Al-4V surface-alloyed materials fabricated by a high-energy electron beam. The mixtures of TiC, TiN, or TiC + TiN powders and CaF₂ flux were deposited on a Ti-6Al-4V substrate, and then the electron beam was irradiated on these mixtures. In the specimens processed with a flux addition, the surface-alloyed layers of 1 mm in thickness were homogeneously formed without defects and contained a large amount (over 30 vol pct) of precipitates such as TiC, TiN, (Ti _x Al_1−x )N, and Ti(C _x N_1−x ) in the martensitic or N-rich acicular α-Ti matrix. This microstructural modification, including the formation of hard precipitates and hardened matrices in the surface-alloyed layers, improved the hardness and wear resistance. Particularly in the surface-alloyed material fabricated by the deposition of TiN powders, the wear resistance was greatly enhanced to a level 10 times higher than that of the Ti alloy substrate. These findings suggested that surface alloying using high-energy electron-beam irradiation was economical and useful for the development of titanium-based surface-alloyed materials with improved hardness and wear resistance.     

Correlation of microstructure with the hardness and wear resistance of (TiC,SiC)/Ti-6Al-4V surface composites fabricated by high-energy electron-beam irradiation

The correlation of microstructure with the hardness and wear resistance of (TiC,SiC)/Ti-6Al-4V surface composites fabricated by high-energy electron-beam irradiation was investigated in this study. The mixtures of TiC, SiC, or TiC + SiC powders and CaF₂ flux were placed on a Ti-6Al-4V substrate, and then an electron beam was irradiated on these mixtures using an electron-beam accelerator. The surface composite layers of 1.2 to 2.1 mm in thickness were formed without defects and contained a large amount (up to 66 vol pct) of precipitates such as TiC and Ti₅Si₃ in the martensitic matrix. This microstructural modification, including the formation of hard precipitates and a hardened matrix in the surface composite layer, improved the hardness and wear resistance. Particularly in the surface composite fabricated with TiC + SiC powders, the wear resistance was greatly enhanced to a level 25 times higher than that of the Ti alloy substrate, because 66 vol pct of TiC and Ti₅Si₃ was precipitated homogeneously in the hardened martensitic matrix. These findings suggested that high-energy electron-beam irradiation was useful for the development of Ti-based surface composites with improved hardness and wear properties.     

Microstructure of TiB2/carbon steel surface-alloyed materials fabricated by high-energy electron beam irradiation

The processing and the microstructural analysis of TiB₂/carbon steel surface-alloyed materials using the irradiation of a high-energy electron beam were investigated in this study. The mixtures of TiB₂ powders and flux were deposited on a plain carbon steel substrate, and then electron beam was irradiated on these mixtures using an electron beam accelerator. The microstructure of the irradiated surface layer was composed of a melted region, an interfacial region, a coarse-grained heat-affected zone (HAZ), and a fine-grained HAZ. A few residual micropores were found in the melted region of the specimen processed without flux because of irregular thermal transfer, but their number was decreased in the specimens processed with a considerable amount of flux. As a result of irradiation, the Ti content was homogeneously maintained throughout the melted region, whose hardness was greatly improved. This was associated with the microstructural modification including the segregation of Ti and B along solidification cell boundaries and the formation of fine Ti(C, N) particles. The proper flux mix ratio was 15 to 30 pct to obtain excellent surface alloying and a homogeneous microstructure.     

Microstructural analysis of vanadium carbide/steel surface-alloyed materials fabricated by high-energy electron-beam irradiation

This study is concerned with the microstructural analysis of vanadium carbide (VC)/steel surface-alloyed materials fabricated by high-energy electron-beam irradiation. The mixtures of VC powders and MgO-CaO flux were deposited on a plain carbon steel substrate, and then electron beam was irradiated on these mixtures using an electron-beam accelerator. Microstructures of the irradiated surface regions were examined by optical microscopy, scanning electron microscopy, and transmission electron microscopy. Residual pores were found in the specimen processed without flux, but hardly found in the specimens processed with a considerable addition of flux. As a result of irradiation, vanadium content was homogeneously maintained throughout the melted region, and fine vanadium carbides were formed in the melted region. These microstructural modification including the formation of vanadium carbides greatly improved hardness, especially high-temperature hardness up to 500 °C.     

Correlation of microstructure with hardness and wear resistance of VC/steel surface-alloyed materials fabricated by high-energy electron-beam irradiation

Correlation of microstructure with hardness and wear resistance of VC/carbon steel surface-alloyed materials fabricated by high-energy electron-beam irradiation was investigated. The mixtures of VC powders and flux (50 pct MgO-50 pct CaO or CaF₂) were deposited on a plain carbon steel substrate, and subsequently irradiated using a high-energy electron beam. The surface-alloyed layers of 1.2 to 3 mm in thickness were homogeneously formed without defects, and contained a large amount (about 10 vol pct) of VC precipitates in the bainitic or martensitic matrix. This microstructural modification including the formation of hard precipitates and hardened matrix in the surface-alloyed layers improved hardness and wear resistance. Particularly in the surface-alloyed material fabricated with the lower input energy density, the wear resistance was greatly enhanced over the steel substrate because of the increased size and volume fraction of VC particles, although the thickness of the surface-alloyed layer decreased. Microstructural modifications including melting, solidification, precipitation, and phase transformation of the surface-alloyed layer were also predicted from a thermal transfer modeling and a Fe-V-C ternary phase diagram. The predicted results were found consistent with those data from actual electron-beam irradiation and microstructural analysis.     

Correlation of microstructure with wear and fracture properties of two-layered VC/Ti-6Al-4V surface composites fabricated by high-energy electron-beam irradiation

Correlation of microstructure with the hardness, wear resistance, and fracture toughness of two-layered VC/Ti-6Al-4V surface composites fabricated by high-energy electron-beam irradiation was investigated in this study. A mixture of VC powders and CaF₂ flux was deposited on a Ti-6Al-4V substrate, and then an electron beam was irradiated on these powder mixtures to fabricate an one-layered surface composite. A two-layered surface composite was fabricated by irradiating an electron-beam again onto the powder mixture deposited on the one-layered surface composite. The composite layers of 1.2 to 1.5 mm in thickness were homogeneously formed without defects and contained a large amount (25 to 40 vol pct) of carbides in the martensitic or β-Ti matrix. This microstructural modification, including the formation of hard carbides and hardened matrix, improved the hardness and wear resistance. Particularly in the two-layered surface composite containing more carbides, the wear resistance was greatly enhanced to a level 7 times higher than that of the Ti-6Al-4V substrate. In-situ observation of the fracture process showed that microcracks were initiated at carbides and propagated along these microcracked carbides and that shear bands were formed in the matrix between these microcracks. In the two-layered surface composite, numerous microcracks were initiated at many carbides and then rapidly propagated along them, thereby lowering the fracture toughness.     

Correlation of microstructure with hardness and wear resistance in (CrB, MoB)/steel surface composites fabricated by high-energy electron beam irradiation

Correlation of microstructure with hardness and wear resistance of (CrB,MoB)/carbon steel surface composites fabricated by high-energy electron beam irradiation was investigated in this study. Three kinds of powder mixtures, i.e., 50CrB-50MgF₂(flux), 50MoB-50MgF₂, and 25CrB-25MoB-50MgF₂ (wt pct), were placed on a plain carbon steel substrate, which was then irradiated with the electron beam. In the specimens fabricated with flux powders, the surface composite layer of 0.8 to 1.3 mm in thickness was successfully formed without defects, and contained a large amount (up to 48 vol pct) of Cr_1.65Fe_0.35B_0.9 or Mo₂FeB₂ in the martensitic matrix. The hardness and wear resistance of the surface composite layer were directly influenced by the hard borides, and thus were about 3 to 7 times greater than those of the steel substrate. Particularly, in the surface composite fabricated with CrB and MoB powders, the hardness of eutectic solidification cells and martensitic matrix was very high, and borides formed a network structure along cells, thereby leading to the best hardness and wear resistance. These findings suggested that the high-energy electron beam irradiation was useful for the development of surface composites with improved hardness and wear resistance.     

Correlation of microstructure and abrasive and sliding wear resistance of (TiC,SiC)/Ti-6Al-4V surface composites fabricated by high-energy electron-beam irradiation

This study is concerned with the correlation of microstructure and abrasive and sliding wear resistance of (TiC,SiC)/Ti-6Al-4V surface composites fabricated by high-energy electron-beam irradiation. The mixtures of TiC, SiC, Ti + SiC, or TiC+SiC powders and CaF₂ flux were deposited on a Ti-6Al-4V substrate, and then an electron beam was irradiated on these mixtures. The surface composite layers of 1.2 to 2.1 mm in thickness were homogeneously formed without defects and contained a large amount (30 to 66 vol pct) of hard precipitates such as TiC and Ti₅Si₃ in the martensitic matrix. This microstructural modification, including the formation of hard precipitates in the surface composite layer, improved the hardness and abrasive wear resistance. Particularly in the surface composite fabricated with TiC + SiC powders, the abrasive wear resistance was greatly enhanced to a level 25 times higher than that of the Ti alloy substrate because of the precipitation of 66 vol pct of TiC and Ti₅Si₃ in the hardened martensitic matrix. During the sliding wear process, hard and coarse TiC and Ti₅Si₃ precipitates fell off from the matrix, and their wear debris worked as abrasive particles, thereby reducing the sliding wear resistance. On the other hand, needle-shaped Ti₅Si₃ particles, which did not play a significant role in enhancing abrasive wear resistance, lowered the friction coefficient and, accordingly, decelerated the sliding wear, because they played more of the role of solid lubricants than as abrasive particles after they fell off from the matrix. These findings indicated that high-energy electron-beam irradiation was useful for the development of Ti-based surface composites with improved abrasive and sliding wear resistance, although the abrasive and sliding-wear data should be interpreted by different wear mechanisms.     

Microhardness of laser ablated alumina coating on Ti-6Al-4V

Alumina coated on Titanium alloys find wide tribological applications due to the improvement in hardness of substrate. This paper presents the effect of deposition of alumina by pulsed laser ablation on Vickers hardness of Ti-6Al-4V substrate. Nd: YAG laser of wavelength 1064nm is used with sintered alumina disc as target for ablation. The variation of Vickers microhardness with load in Ti-6Al-4V shows indentation size effect. Proportional Specimen Resistance (P.S.R) model is applied to separate load -independent hardness from the load -dependent hardness. Composite hardness of Alumina coated Ti-6Al-4V is measured for different laser processing conditions. The film hardness has been separated from the composite hardness of the film-substrate system by the use of an approach based on the law of area of mixtures model taking into an account of ISE, due to proportional specimen resistance of the material to indentation. Film hardness of different films produced by varying the target — substrate distance is presented. As the film thickness increases its hardness decreases as compared to bulk hardness. These studies will be useful in the selection of appropriate coating thickness and substrate hardness to achieve a required composite hardness in the design and production of wear-resistant parts of engineering devices.     

两种3D打印用雾化Ti-6Al-4V粉末的对比研究

本研究分别利用水冷铜坩埚真空感应熔炼气雾化(VIGA-CC)和等离子旋转电极(PREP)两种技术制备出球形Ti-6Al-4V合金粉末,作者利用SEM、同步辐射CT扫描-三维重建和氩气含量测试等分析手段对不同粒径的Ti-6Al-4V合金粉末的孔洞缺陷和氩气含量、硬度值进行了表征。实验结果表明, VIGA-CC粉末粒度分布宽,细粉收得率较多,粉末粒度分布在40~180 μm之间, PREP粉末的粒度分布较窄,主要集中在110~180 μm之间;金属粉末内部的孔隙率、气体含量和孔尺寸随着粉末粒度的增大而增大,且同一粒径范围内VIGA-CC粉末的气孔概率多于PREP粉末;随着粉末粒径减小,粉末截面组织逐渐细化,其硬度值逐渐升高,整体上VIGA-CC粉末硬度值高于PREP粉末。     

Microstructure and Texture Evolution During Sub-Transus Thermomechanical Processing of Ti-6Al-4V-0.1B Alloy: Part I. Hot Rolling in (α + β) Phase Field

In the current study, the evolution of microstructure and texture has been studied for Ti-6Al-4V-0.1B alloy during sub-transus thermomechanical processing. This part of the work deals with the deformation response of the alloy by rolling in the (α + β) phase field. The (α + β) annealing behavior of the rolled specimen is communicated in part II. Rolled microstructures of the alloys exhibit either kinked or straight α colonies depending on their orientations with respect to the principal rolling directions. The Ti-6Al-4V-0.1B alloy shows an improved rolling response compared with the alloy Ti-6Al-4V because of smaller α lamellae size, coherency of α/β interfaces, and multiple slip due to orientation factors. Accelerated dynamic globularization for this alloy is similarly caused by the intralamellar transverse boundary formation via multiple slip and strain accumulation at TiB particles. The (0002)_α pole figures of rolled Ti-6Al-4V alloy shows “TD splitting” at lower rolling temperatures because of strong initial texture. Substantial β phase mitigates the effect of starting texture at higher temperature so that “RD splitting” characterizes the basal pole figure. Weak starting texture and easy slip transfer for Ti-6Al-4V-0.1B alloy produce simultaneous TD and RD splittings in basal pole figures at all rolling temperatures.     

Fabrication of carbide-particle-reinforced titanium aluminide-matrix composites by laser-engineered net shaping

TiAl-based titanium aluminide alloys and their composites reinforced with ceramic particles are considered to be important candidate materials for high-temperature structural applications. Laser-engineered net shaping (LENS) is a layered manufacturing process, which involves laser processing fine powders into three-dimensional components directly from a computer-aided design (CAD) model. In this work, the LENS process was employed to fabricate carbide-particle-reinforced titanium aluminide-matrix composites using TiC and gas-atomized Ti-48Al-2Cr-2Nb powders as the feedstock materials. The composites deposited by the LENS process were susceptible to solid-state cracking due to high thermal stresses. The microstructures of the laser-deposited monolithic and composite titanium aluminide materials were characterized using light optical microscopy (LOM), scanning electron microscopy (SEM), X-ray energy-dispersive spectroscopy (EDS) analysis, electron-probe microanalysis (EPMA), and X-ray diffraction (XRD) techniques. Effects of the LENS processing parameters on the cracking susceptibility and microstructure were studied. Crack-free deposits were fabricated by preheating the substrate to 450 °C to 500 °C during LENS processing. The fabricated composite deposits exhibit a hardness of more than twice the value of the Ti-6Al-4V alloy.     

Correlation of Microstructure,Hardness, and Fracture Toughness of Fe-Based Surface Composites Fabricated by High-Energy Electron Beam Irradiation with Fe-Based Metamorphic Alloy Powders and VC Powders

Iron-based surface composites were fabricated with Fe-based metamorphic alloy powders and VC powders by high-energy electron beam irradiation, and the correlation of their microstructure with hardness and fracture toughness was investigated. Mixtures of metamorphic powders and VC powders were deposited on a plain carbon steel substrate, and then the electron beam was irradiated on these powders without flux, to fabricate surface composites. The composite layers 1.3 to 1.8 mm in thickness contained a large amount (up to 47 vol pct) of hard Cr₂B and V₈C₇ particles formed in eutectic colony regions and inside colonies, respectively. The hardness of the surface composites was approximately 2 to 4 times greater than that of the substrate because of Cr₂B and V₈C₇ particles. According to the microfracture observation of the composite fabricated with mixing 30 wt pct VC powders, microcracks initiated at coarse V₈C₇ particles ins inside colonies as well as at Cr₂B particles in colony regions, and were connected with other microcracks in a zigzag shape. Thus, it showed a higher fracture toughness and hardness twice as high as the composite fabricated without mixing VC powders.     

The Effect of Scan Length on the Structure and Mechanical Properties of Electron Beam-Melted Ti-6Al-4V

Electron beam melting (EBM) is a powder bed fusion-based additive manufacturing process in which selective areas of a layer of powder are melted with an electron beam and a part is built layer by layer. EBM scanning strategies within the Arcam AB^® A2X EBM system rely upon governing relationships between the scan length of the beam path, the beam current, and speed. As a result, a large parameter process window exists for Ti-6Al-4V. Many studies have reviewed various properties of EBM materials without accounting for this effect. The work performed in this study demonstrates the relationship between scan length and the resulting density, microstructure, and mechanical properties of EBM-produced Ti-6Al-4V using the scanning strategies set by the EBM control software. This emphasizes the criticality of process knowledge and careful experimental design, and provides an alternate explanation for reported orientation-influenced strength differences.     

Mechanochemical processing of nanocrystalline Ti-6Al-4V alloy

Synthesis of nanocrystalline Ti-6Al-4V was explored using mechanochemical processing. The reaction mixture was comprised of CaH₂, Mg powder, anhydrous AlCl₃, anhydrous VCl₃, and TiCl₄. The milled powder (reaction product) primarily consisted of nanocrystalline alloy hydride having a composition (Ti-6Al-4V)H_1.942, along with MgCl₂ and CaCl₂ as by-products. Aqueous solutions of nitric acid, sulfuric acid, and 1 pct sodium sulfite were found to be very effective in leaching of the chlorides from the milled powder. The (Ti-6Al-4V)H_1.942 on dehydrogenation at 375°C resulted in nanocrystalline Ti-6Al-4V alloy powder.     

Correlation of microstructure with hardness and wear resistance in (TiC,SiC)/stainless steel surface composites fabricated by high-energy electron-beam irradiation

Stainless-steel-based surface composites reinforced with TiC and SiC carbides were fabricated by high-energy electron beam irradiation. Four types of powder/flux mixtures, i.e., TiC, (Ti + C), SiC, and (Ti + SiC) powders with 40 wt. pct of CaF₂ flux, were deposited evenly on an AISI 304 stainless steel substrate, which was then irradiated with an electron beam. TiC agglomerates and pores were found in the surface composite layer fabricated with TiC powders because of insufficient melting of TiC powders. In the composite layer fabricated with Ti and C powders having lower melting points than TiC powders, a number of primary TiC carbides were precipitated while very few TiC agglomerates or pores were formed. This indicated that more effective TiC precipitation was obtained from the melting of Ti and C powders than of TiC powders. A large amount of precipitates such as TiC and Cr₇C₃ improved the hardness, high-temperature hardness, and wear resistance of the surface composite layer two to three times greater than that of the stainless steel substrate. In particular, the surface composite fabricated with SiC powders had the highest volume fraction of Cr₇C₃ distributed along solidification cell boundaries, and thus showed the best hardness, high-temperature hardness, and wear resistance.     

Brazing of Ti-6Al-4V and niobium using three silver-base braze alloys

Evaluations of the (infrared)-brazed Ti-6Al-4V and niobium joints using three silver-base braze alloys have been extensively studied. According to the dynamic wetting angle measurement results, the niobium substrate cannot be effectively wetted by all three braze alloys. Because the dissolution of Ti-6Al-4V substrate causes transport of Ti into the molten braze, the molten braze dissolved with Ti can effectively wet the niobium substrate during brazing. For infrared-brazed Ti-6Al-4V/Ag/Nb joint, it is mainly comprised of the Ag-rich matrix. The TiAg reaction layer is observed at the interface between the braze and Ti-6Al-4V substrate. In contrast, Ti-rich, Ag-rich, and interfacial TiAg phases are found in the furnace-brazed specimen. The dominated Ti-rich phase in the joint is caused by enhanced dissolution between the molten braze and Ti-6Al-4V substrate. The infrared-brazed Ti-6Al-4V/72Ag-28Cu/Nb joint is mainly comprised of the Ag-rich matrix and Ag-Cu eutectic. With increasing the brazing temperature or time, the amount of Ag-Cu eutectic is decreased, and the interfacial Cu-Ti reaction layer(s) is increased. The infrared brazed joint has the highest average shear strength of 224.1 MPa. The averaged shear strength of the brazed joint is decreased with increasing brazing temperature or time, and its fracture location changes from the braze alloy into the interfacial reaction layer(s) due to excessive growth of the Cu-Ti intermetallics. The infrared-brazed Ti-6Al-4V/95Ag-5Al/Nb joint is composed of Ag-rich matrix and TiAl interfacial reaction layer. With increasing the brazing time, the amount of Ag-rich phase is greatly decreased, and the interfacial reaction layer becomes Ti₃Al due to enhanced dissolution of Ti-6Al-4V substrate into the molten braze. The average shear strength of the infrared-brazed joint is 172.8 MPa. Additionally, the existence of an interfacial Ti₃Al reaction layer significantly deteriorates the shear strength of the furnace-brazed specimen.     

Correlation of microstructure with hardness and wear resistance in (TiC,SiC)/stainless steel surface composites fabricated by high-energy electron-beam irradiation

Stainless-steel-based surface composites reinforced with TiC and SiC carbides were fabricated by high-energy electron beam irradiation. Four types of powder/flux mixtures, i.e., TiC, (Ti+C), SiC, and (Ti+SiC) powders with 40 wt. pct of CaF₂ flux, were deposited evenly on an AISI 304 stainless steel substrate, which was then irradiated with an electron beam. TiC agglomerates and pores were found in the surface composite layer fabricated with TiC powders because of insufficient melting of TiC powders. In the composite layer fabricated with Ti and C powders having lower melting points than TiC powders, a number of primary TiC carbides were precipitated while very few TiC agglomerates or pores were formed. This indicated that more effective TiC precipitation was obtained from the melting of Ti and C powders than of TiC powders. A large amount of precipitates such as TiC and Cr₇C₃ improved the hardness, high-temperature hardness, and wear resistance of the surface composite layer two to three times greater than that of the stainless steel substrate. In particular, the surface composite fabricated with SiC powders had the highest volume fraction of Cr₇C₃ distributed along solidification cell boundaries, and thus showed the best hardness, high-temperature hardness, and wear resistance.     

Room-temperature tensile and high-cycle-fatigue strength of fine TiB particulate-reinforced Ti-22Al-27Nb composites

To further improve the mechanical properties of a Ti-22Al-27Nb (mol pct) alloy, based on the ordered orthorhombic Ti₂AlNb (O phase), a TiB particulate-reinforced Ti-22Al-27Nb matrix composite was prepared using the gas-atomized powder metallurgy method. Because of the rapid solidification during the gas atomization process, the TiB particulates dispersed in the composite were extremely fine, with an average diameter of less than 1 μm and lengths ranging up to 5 μm. This composite (PM composite) showed higher tensile and high-cycle-fatigue properties at room temperature than both an unreinforced Ti-22Al-27Nb matrix alloy and a Ti-22Al-27Nb/TiB composite produced using a conventional ingot metallurgy method (IM composite) with relatively coarse (average diameter 5 μm and average length 40 μm) TiB particulates. These coarse TiB particulates in the IM composite were thought to provide only classical composite strengthening effects. On the other hand, the fine TiB particulates in the PM composite showed additional effects, such as blocking the movement of dislocations.