Electron Beam Surface Melting Research Articles

Transition from micro-rod to nano-lamella eutectics and its hardening effect in niobium/silicide in-situ composites

To improve the mechanical properties of niobium/silicide in-situ composites via rapid solidification, the evolution of eutectic geometry and the corresponding hardening effect in a prototype Nb–18Si (at.%) composite upon electron beam surface melting (EBSM), i.e., a rapid remelting and solidifying sequence, were studied. Results show that rod-like Nb solid solution (Nbss)/Nb3Si eutectics prevail in the arc-melted state, yet evolve into lamellar arrangements after EBSM. Atomic scale scanning transmission electron microscopy (STEM) and near-atomic scale atom probe tomography (APT) were employed to characterize the three-dimensional stacking of nano-laminated Nbss/Nb3Si eutectics and their compositions. Compared with the rod-like eutectics, the lamellar eutectics via EBSM demonstrate a prominent eutectic refinement (39.5 nm in spacing) and an increased volume fraction of Nbss (~41%). Nano-indentation testing reveals that with the microstructural transition from micro-rod to nano-lamella eutectics, a significant increment in hardness up to 13.9 GPa is achieved.

Tuning microstructure and enhancing corrosion property of Ti-6Al-3Nb-2Zr-1Mo alloy through electron beam surface melting

Herein, we employ electron beam surface melting (EBSM) technology to process Ti-6Al-3Nb-2Zr-1Mo alloys and compare their microstructure and corrosion performance with wrought counterparts. The mechanism of microstructural evolution has been illustrated. The EBSM sample exhibits a better corrosion resistance than the wrought one, which can be ascribed to the formation of a relatively more stable passive film due to a higher fraction of phase boundaries induced by the fine α′-phase, and the alleviated heterogeneous distribution of compositions between α (α′)- and β-phases stemming from rapid cooling during EBSM process, further leading to the absence of strong galvanic-corrosion between different phases.

Morphological Heredity of Intermetallic Nb5Si3 Dendrites in Hypereutectic Nb-Si Based Alloys via Non-Equilibrium Solidification

For hypereutectic Nb-Si based alloys, primary Nb5Si3 phases typically grow in a faceted mode during equilibrium or near-equilibrium solidification, which damages the ductility and toughness. To address this issue, here we artificially manipulate the growth morphology of Nb5Si3 using electron beam surface melting (EBSM) and subsequent annealing treatments. Results show that such a non-equilibrium solidification pathway enables the transition from faceted growth to non-faceted dendritic growth of Nb5Si3, along with evident microstructure refinement, generation of metastable β-Nb5Si3 phases and elimination of chemical segregation. The transformation from β-Nb5Si3 to α-Nb5Si3 and Nb solid solution (Nbss) particles is triggered by the annealing treatment at 1450 °C for 5 h. Also, we find the annealing-mediated formation of inherited Nb5Si3 dendrites that maintain the dendritic morphology of the original as-solidified β-Nb5Si3 dendrites. This work thus provides a feasible routine to obtain thermally stable and refined α-Nb5Si3 dendrites in hypereutectic Nb-Si based alloys.

Interplay Between Eutectic and Dendritic Growths Dominated by Si Content for Nb-Si-Ti Alloys Via Rapid Solidification

Abstract Rapid solidification techniques such as electron beam additive manufacturing are considered as promising pathways for manufacturing Nb-Si-based alloys for ultra-high-temperature applications. Here, we investigate the microstructure diversity of a series of Nb-Si-Ti alloys via electron beam surface melting (EBSM) to reveal their rapid solidification behaviors. Results show that the microstructural transition from coupled to divorced Nbss/Nb3Si eutectics can be triggered by increasing Si content. The formation of fully lamellar eutectics, evidenced by scanning transmission electron microscopy and atom probe tomography (APT), is achieved in the EBSM-processed Nb18Si20Ti alloy (at%), in contrast to the hypereutectic microstructures in arc-melted counterparts. The dendritic microstructures containing divorced eutectics are generated with a higher content of Si during rapid solidification. The transition from faceted to non-faceted growth of intermetallic Nb3Si occurs with the formation of primary Nb3Si dendrites. The interplay between eutectic and dendritic growths of silicides is discussed to provide insights for future alloy design and manufacture.

Electron beam surface melting of MoSiBTiC alloys: Effect of preheating on cracking behavior and microstructure evolution

In order to investigate the feasibility of the electron beam melting (EBM) process for fabricating brittle Mo-Si-B based alloys, the effect of preheating on cracking behavior and microstructure evolution was systematically investigated using two different MoSiBTiC alloys. Single track and multi tracks were scanned on the cast MoSiBTiC alloy substrates with and without preheating. In the single-track scanning, the crack density significantly decreased with increasing preheating temperature; however, only tiny cracks remained even at high preheating temperature of ~1250 °C. In contrast, the application of multi-track scanning with preheating, which more closely resembles the actual EBM building process, enabled complete cracking suppression regardless of the same beam parameter. The surface and cross-sectional microstructure observation revealed that the difference in cracking behavior between single-track and multi-track scanning is due to the decrease in cooling rate by in-situ heating effects through successive track scanning in addition to initial preheating. Moreover, it was confirmed that the cracking could be prevented even in the case of a very brittle intermetallic phase (Mo5SiB2)-matrix MoSiBTiC alloy. This groundbreaking result expands the possibilities of using EBM for the manufacturing of promising ultrahigh-temperature and high-strength materials, which have been unable to be processed by conventional routes.

Surface microstructure modification of hypereutectic Nb-Si based alloys to improve oxidation resistance without damaging fracture toughness

To achieve a good balance between oxidation resistance and fracture toughness of hypereutectic Nb-Si based alloys, we tune the alloy surface microstructure by an approach of rapid surface remelting. The remelted layer with a refined and modified hypereutectic microstructure is created on a bulk cast Nb-20Si-24Ti-2Cr-2Al (at.%) alloy via electron beam surface melting (EBSM). The EBSM-remelted layer markedly improves the oxidation resistance of the experimental alloy, showing a decreased parabolic rate constant and a reduced weight gain upon oxidation at 1250 °C. Three-point bending tests show that fracture toughness of the EBSM-treated alloy is comparable to that of the cast alloy without EBSM treatment. This work thus demonstrates the capability of surface microstructure modification enabled by rapid remelting for improving the oxidation resistance of Nb-Si based alloys while maintaining the fracture toughness.

Tuning microstructures and improving oxidation resistance of Nb-Si based alloys via electron beam surface melting

The bottleneck for the application of Nb-Si based alloys as high-temperature alloys is their poor oxidation resistance. Here we present an approach to tune the microstructures and enhance the oxidation resistance via electron beam surface melting (EBSM) treatment. Significant microstructural refinement, higher volume fraction of silicides and controlled phase regime are achieved by EBSM. The oxidation tests at 1250 °C demonstrate a substantially improved oxidation resistance of the EBSM-processed alloy, with a thinner, less wrinkled oxide scale. A continuous silica layer within the oxide scale is produced on the remelted surface. The oxidation response to tunable microstructures is discussed.

Surface Remelting‐Mediated Improvement in Oxidation Resistance of Cr2Nb‐Containing Nb‐Si‐Based Alloys at High Temperatures

The poor oxidation resistance of Nb‐Si‐based alloys generally limits their high‐temperature application. A surface remelting strategy for the formation of protective oxide scales and the improvement of oxidation resistance is reported. Electron beam surface melting (EBSM) is performed on arc‐melted Nb‐14Si‐18Ti‐12Cr‐7Al‐0.2Y (at.%) alloys. The microstructure is significantly refined, without altering the phase regime that consists of an Nb solid solution, βNb5Si3 and Cr2Nb. After oxidation at 1200 °C, the EBSM‐remelted layer forms a flatter and more compact oxide scale without significant cracking or spalling than that on the arc‐melted alloy. A continuous SiO2 oxide layer is formed within the oxide scale on the EBSM‐remelted layer. A reduction of ≈47% in the oxide scale thickness, indicative of improved oxidation resistance, is achieved by the EBSM treatment. The present work provides a promising engineering approach toward the design and fabrication of Nb‐Si‐based alloys with balanced properties.

Effect of Electron Beam Surface Melting on the Structure and Wear Characteristics of Cobalt-Based Hardfacing and Its Mo-Alloyed Version

Electron beam surface melting has been applied as a posttreatment to plasma-transferred, arc-deposited Stellite 6 hardfacing and its molybdenum-alloyed version. Molybdenum addition increased the surface-related properties—that is, hardness and wear resistance—via enriching the microstructure with carbides. Application of electron beam surface melting caused a further increment in hardness and wear resistance of the molybdenum-alloyed hardfacing by encouraging hypereutectic solidification to form a network of carbides surrounding the cobalt matrix. However, electron beam surface melting of the unalloyed hardfacing, which favored individual precipitation of fine carbides in the microstructure, decreased the wear resistance in contrast to the hardness.

Effect of electron beam surface melting on the microstructure and wear behavior of Stellite 12 hardfacing

PurposeThe purpose of this study is to investigate the effect of electron beam surface melting (EBSM) on the properties of Plasma Transfer Arc (PTA) deposited Stellite 12 hardfacing.Design/methodology/approachFor this purpose, structural characterization and dry sliding wear tests have been conducted on the hardfacings at room temperature. The wear tracks formed on the surfaces of the hardfacings were examined by a stylus-type profilometer and a scanning electron microscope.FindingsRefinement of the matrix and the carbides following EBSM process led to about 15 per cent increment in hardness as compared to PTA state. Despite an increase in the surface hardness, EBSM’ed hardfacing exhibited about 50 per cent lower sliding wear resistance than PTA hardfacing against alumina ball. According to the worn surface examinations, reduction in the wear resistance of Stellite 12 after EBSM process has been associated with the extensive refinement of the carbides which made them easier to be removed from the matrix during the sliding contact.Originality/valueThe authors of current study have applied EBSM to PTA deposited Stellite 12 hardfacing alloy to investigate if the surface structure and properties could be improved. More specifically the dry sliding wear performance of PTA and EBSM’ed hardfacings have been focused in the scope of this study. To the best of the authors’ knowledge, this approach, i.e. use of EBSM as a post deposition treatment of Stellite 12 hardfacings, has not been reported in open literature.

Surface modification of structural material for nuclear applications by electron beam melting: enhancement of microstructural and corrosion properties of Inconel 617

Inconel 617 is widely used in nuclear industry, due to its excellent elevated temperature mechanical and chemical properties. In this study, austenitic super alloy Inconel 617 has been subjected to electron beam surface melting using an indigenously developed electron beam welding unit (80 kV, 12 kW) for microstructural homogenization and understanding its effect on corrosion resistance property. Electron beam surface melting has been carried out at a gun voltage of 60 kV, current of 30 mA, and scan speed of 1000 mm/min inside a vacuum chamber with a vacuum level of 1.6 × 10−5 mb. Surface melting induced by EBM led to development of refined microstructural features consisting of γ dendrites and precipitates of Ni3 (Al, Ti) in the interdendritic regions, which further improved microhardness of the Electron Beam (EB) treated surface. The effect of electron beam melting on the kinetics of aqueous corrosion has been determined in acidic 3.56 wt% NaCl media. The improvement in corrosion resistance can be attributed to the microstructural refinement leading towards the redistribution and homogenization of alloying elements and surface purification.

Competitive growth of nano-lamellae Nb/Nb3Si eutectics with enhanced hardness and toughness

Here we propose to employ electron beam surface melting (EBSM) to refine microstructural length scale and obtain suited microstructure morphology of Nb-Si based alloys. As characterized by scanning transmission electron microscopy (STEM) and atom probe tomography (APT), nano-lamellar eutectics are created in Nb-20Si (at.%) alloys upon EBSM. Theoretical analysis based on the competitive growth models demonstrates the preferred formation of Nb solid solution (Nbss)/Nb3Si eutectics under selected processing parameters. The large cooling rate during EBSM, higher than 8 × 104 K/s, promotes the formation of the nano-scale eutectics. Compared to the initial coarse eutectics by arc melting, simultaneous enhancement of hardness and fracture toughness is enabled owing to the controlled microstructural length scale and morphology.

Wear behaviour of Electron beam surface melted Inconel 718

This study concerns understanding the wear behavior of electron beam surface melted Inconel 718. Electron beam surface melting has been carried out using an indigenously developed electron beam welding machine with a capacity of 80kV, 12 kW. Surface melting has been carried under a varied combination of beam current and scan speed to optimize the process parameter of electron beam melting corresponding to the development of defect free processed zone. Detailed investigation of microstructure, phase, hardness, and reciprocating friction wear have been carried out. Finally, the fretting wear behavior of electron beam melted Inconel 718 has been evaluated against tungsten carbide ball under fretting wear mode and compare with the fretting wear behavior of as received Inconel 718. Finally, the mechanism and kinetics of wear have been investigated threw a detail microstructure analysis of the worn surface.

Inclusions in metallic materials as origin for microcraters formation at a pulsed electron-beam surface melting

In the present work, the effect of difference between thermal properties of the base material (matrix) and inclusion on the temperature behavior of the target in the regions of inclusion locations has been considered. The calculations were based on the solution of two-dimensional nonlinear nonstationary heat equation. It has been shown that the most strong effect on the temperature field takes the difference between heat conductivities of the matrix and inclusion. The simulations of temperature fields have been carried out on the real systems matrix-inclusion, namely, stainless steel 316 L/manganese sulfide (SS 316L/MnS), TiNi/Ti2Ni, TiNi/TiC. The results obtained allows suggestion on the origin of microcraters appearing at the irradiated surface as a result of thermocapillary convection in overheated sites of location of inclusions.

Investigation on microstructure, mechanical and tribological properties of in-situ (TiB + TiC)/Ti composite during the electron beam surface melting

The (TiB + TiC)/Ti composite was successfully in-situ synthesized by the electron beam surface melting. The supplied energy input has significant effect on the microstructure evolution and properties of the titanium matrix composites (TMCs). Some aggregations of TiB whiskers around un/semi reacted B4C particles appeared in the TMCs with low energy input, resulting in the deterioration of mechanical performances, especially the wear resistance. The B4C powders fully interact with Ti matrix when the energy input increases. However, TiB whiskers, the in-situ reaction product, grow and coarsen when the energy input continuously increases, which deteriorates the mechanical and tribological properties of the TMCs. The energy input is thereby optimized to be 3.2 J/mm in order to obtain the fine in-situ reinforcements without the un/semi melted B4C particles, exhibiting the highest hardness, elastic modulus and wear resistance. This study unravels the microstructure controlling methods during the electron beam surface melting process and the optimization mechanism of the mechanical and tribological properties, which is of significance for the application of the in-situ ceramics reinforced titanium matrix composites. Overall, the results indicate that the input energy should be controlled to avoid the presence of un/semi reacted B4C and coarse TiB whiskers for the electron beam surface melted TMCs with superior properties.

Heat treatment induced phase transition and microstructural evolution in electron beam surface melted Nb-Si based alloys

The hardness, phase and microstructural development of Nb-18Si-24Ti-2Cr-2Al (at.%) alloys processed by electron beam surface melting (EBSM) and subsequent heat treatments were investigated. The EBSM experiments were performed using an electron beam based 3D printing system. Results showed that Nbss and Nb3Si phases were obtained via EBSM with a significantly refined microstructure. The eutectoid reaction of Nb3Si→Nbss+αNb5Si3 was triggered by heat treatments (HT) at 1200°C or 1450°C for 5h. The growth and the coarsening of αNb5Si3 grains were promoted with a higher HT temperature. The hardness of the EBSM alloy was remarkably reduced by HT.

Wear resistance improvement of a commercially pure titanium by high current pulsed electron beam treatment

A commercially pure titanium was selected as target material and treated by high current pulsed electron beam. The phase and structure changes occurring in the modified surface layers were observed with optical microscopy, scanning electron microscope, and transmission electron microscope. The increase in the wear resistance was observed for the pure titanium samples after pulsed electron beam surface melting. The mechanism for wear resistance modification was discussed. The results indicated that the presence of ultrafine martensite and defects in the treated surface layer were key factors for the improvement of the wear properties of pure titanium treated by high current pulsed beam treatment.

Studies on Electron Beam Surface Melting of AISI 316 Stainless Steel and AISI 347 Stainless Steel

In the present study, AISI 316 stainless steel and AISI 347 stainless steel are subjected to electron beam surface melting using an indigenously developed electron beam welding unit (80 kV & 12 kW). Electron beam processing was carried out at a gun voltage of 60 kV, current of 30 mA, scan speed of 1000 mm/min in a vacuum chamber with a vacuum level of 1.6 x 10-5 mb. Followed by surface melting, a detailed characterization of the surface was carried out and the effect of electron beam melting on the corrosion resistance property was evaluated. The effect of electron beam melting on the kinetics of aqueous corrosion has been determined in acidic 3.5 wt. % NaCl media.

Corrosion behaviour of Al86.0Co7.6Ce6.4 glass forming alloy with different microstructures

It has been extensively reported that Al-TM-RE amorphous alloy has excellent mechanical properties and corrosion resistance. In this paper, the corrosion behaviour of an Al86.0Co7.6Ce6.4 glass forming alloy with different microstructures is investigated through electrochemical experiments and microscopy. Results show the effect of microstructure. Laser and electron beam surface melting processes produce rapidly solidified microstructures with different extents of passivation compared to the as-cast alloy. An amorphous surface layer produced by these surface treatments had superior corrosion resistance compared with the crystalline alloy. As-cast and laser treated Al86.0Co7.6Ce6.4 suffered localised corrosion in the Al/Al11Ce3 eutectic region whereas the amorphous material exhibited uniform corrosion. Compared with the electrochemical behaviour of AA2024 and Alclad 2024, the fully amorphous layer prepared by combined laser-electron beam treatment exhibited advantages such as the more negative corrosion potential, the higher pitting potential and the uniform corrosion mechanism, which indicates that this material is a potential anode candidate in the protection of AA2024.

Corrosion behaviour of a rapidly solidified Al87.4Co7.9Ce4.7 layer prepared by large area electron beam irradiation

In this study, a bulk crystalline glass forming alloy with the composition of Al87.4Co7.9Ce4.7 (at.%) was rapidly solidified by large area electron beam (LAEB) surface melting of an as-cast starting material. Glancing angle XRD results show that, providing appropriate processing parameters are selected, a predominantly amorphous layer ∼5μm thick can be produced. However, under non-optimum processing conditions treated layers are compositionally inhomogeneous and comprise, in the main, crystalline regions. LAEB treated layers also exhibit a degree of surface cracking as well as micron-sized localised craters and ejected particles formed during electron beam irradiation. Potentiodynamic polarisation tests show that treated layers exhibit enhanced corrosion resistance compared to the as-cast bulk alloy; with the predominantly amorphous layer having the lowest current density. Examination of corroded samples following potentiodynamic tests reveals that cracks limit the effectiveness of the treated layer as a barrier to corrosion.