Combination Of High Specific Strength Research Articles

Influence of scanning strategies on L-PBF processing of titanium aluminides

Titanium Aluminides (TiAls) are highly appealing alloys in aerospace and automobile industries due to their combination of high specific strength and low density. However, their poor ductility is challenging during subtractive processing. Additive Manufacturing (AM) technology such as Laser Powder Bed Fusion (L-PBF) offers new horizons for processing different materials with complex shapes and high quality. Nevertheless, the TiAl parts fabricated by L-PBF process were typically characterized by low density and internal defects due to rapid cooling after laser melting. Recently, the sequential thermal scanning (STS) has been introduced as a cost-effective L-PBF strategy to produce high quality TiAl parts with low defects. Despite the success of the presented strategy, the literature still has a shortage of knowledge about the application of other STS strategies. Therefore, this work proposes novel STS strategies, besides the presented in the literature, and investigates the influence of using different geometrical scanning strategies. The results showed that the double post-heating STS strategies could produce parts with higher density, better surface quality, and less internal defects. The microstructure investigation demonstrated that the cracks formed inside the TiAl samples are solidification and thermal induced cracks. These cracks could be reduced significantly when applying the pre-heating STS strategy at higher laser melting energy or the post-heating STS strategy at lower melting energy. The XRD spectra and EBSD analysis demonstrated that the samples fabricated by STS strategies are dominated by the α2-Ti3Al phase due to the Al vaporization that reached up to 6.71 %. In addition, the equiaxed grain growth and alloy's isotropy increased with the application of the STS strategies. On the other side, the investigation of the parts fabricated by STS strategy using different geometrical scanning showed that the chessboard (Ch) strategy could produce dense TiAl sample with 99.3 %, while the sequential bidirectional tracking (X-Y) could enhance the surface quality. In addition, the 45° layer rotation ensured homogenous texture on the side surfaces. A three-steps heat treatment (HT) was performed to enhance the microstructure and relieve the residual stresses. A duplex microstructure with enhanced ductility could be achieved. In addition, compressive residual stresses were induced, which had a beneficial effect on the crack reduction. Finally, the microhardness has been measured and a high value of 562 HV was recorded for the STS samples. Besides, the microhardness homogeneity significantly increased when applying the post-heating STS strategies.

Recent Advances in the Design of Novel β‐Titanium Alloys Using Integrated Theory, Computer Simulation, and Advanced Characterization

Metastable β‐titanium (β‐Ti) alloys, because of their combination of high specific strength, superior corrosion resistance, and excellent biocompatibility, have found increasingly widespread application in aerospace, automobile, biomedical, and chemical industries. Many different pathways, available for phase transformation and deformation between a high‐temperature β and a low‐temperature α phase, provide ample opportunities to engineer the desired microstructure through thermal mechanical processing for optimal properties targeting specific applications. Based on an integrated approach (theory, crystallography, multiscale modeling, and advanced characterization), recent studies have obtained fundamental insights in both designing strategies by exploring a variety of nonconventional solid‐state phase transformation and deformation pathways, including 1) pseudospinodal decomposition; 2) precursory ω‐phase‐assisted α precipitation with a wide range of tunable size scales and number densities; and 3) abnormal high‐index deformation twinning modes. Herein, the revolutionary new concepts for alloy development and deployment and the underlying physical mechanisms are reviewed in detail. Perspectives on the future research directions in the development of metastable β‐Ti alloys are also offered. The mechanisms and alloy design concepts as well as the advantage of using state‐of‐the‐art computational and characterization tools in a correlative manner for alloy design are applicable to a wide range of metallic materials beyond Ti alloys.

Электроискровое нанесение порошка карбида вольфрама на титановый сплав Ti6Al4V

Titanium alloys are attracting the attention of researchers and engineers because of their unique combination of high specific strength, corrosion resistance and biocompatibility, but they are characterized by high wear. Therefore, the study of new methods for making protective coatings on titanium alloys is relevant. A mixture of titanium granules with tungsten carbide powder was used to prepare coatings on the titanium alloy Ti6Al4V by the method of electrospark granules deposition. Three mixtures of granules with a tungsten carbide content of 2.1, 4.1 and 6.0 vol.% were prepared. According to the data of X-ray analysis, it was found that the following phases were observed in the composition of the coatings: WC, W2C, W, αTi and β-(W, Ti)C1−x. Carbides W2C, (W, Ti)C1−x and metallic tungsten were formed as a result of the decarburization of WC upon its interaction with molten titanium under the conditions of an electric discharge. Large inclusions of tungsten carbide surrounded by a metallic Ti-W-C binder were observed in the microstructure of the coatings. According to the data of energy dispersive analysis, the concentrations of tungsten and carbon decreased when scanning from the surface layers of the coating to the substrate. The average values of the microhardness of the coatings increased from 7.9 to 9.2 GPa with an increase in the concentration of WC powder in the mixture of granules. The average values of the friction coefficients of the coatings were in the range of 0.33 – 0.48, which is 35 ± 3 % lower than that of the Ti6Al4V alloy. Tests for wear in the dry friction mode at loads of 25 and 70 N showed that the wear rate of the coatings ranged from 0.38 ×10−5 to 1.68 ×10−5 mm3 / Nm. The best wear resistance under both loads was demonstrated by the coatings deposited with the addition of 4.1 vol.% WC, which make it possible to increase the wear resistance of the Ti6Al4V alloy up to 18 times. Thus, the prospect of electrospark deposition of cermet coatings with enhanced mechanical properties on a titanium alloy using tungsten carbide powder mixed with titanium granules is shown.

Silicon Nitride Whisker-Reinforced Aluminum Matrix Composites: Twinning and Precipitation Behavior

Aluminum composites reinforced with ceramic whiskers exhibited a unique combination of high specific strength and superior specific modulus. A 20 vol.% Si3N4w/Al-11.5Si-1.0Mg-0.5Cu-0.5Ni (wt.%) composite was fabricated via squeeze casting in the present study. It was observed that the addition of silicon nitride (Si3N4) whiskers in the Al-Si cast alloy promoted extensive twinning in the eutectic silicon particles due to a coupled role of thermal stresses between the matrix and silicon and residual stresses present in the composite. Double aging peaks were present in the age-hardening curves. The precipitation mechanism involved the formation of Mg2Si and Al2CuMg phases. The presence of Si3N4 whiskers in the composite retarded the nucleation process of Mg2Si precipitate while enhancing its growth rate.

Lightweight, flaw-tolerant, and ultrastrong nanoarchitected carbon

It has been a long-standing challenge in modern material design to create low-density, lightweight materials that are simultaneously robust against defects and can withstand extreme thermomechanical environments, as these properties are often mutually exclusive: The lower the density, the weaker and more fragile the material. Here, we develop a process to create nanoarchitected carbon that can attain specific strength (strength-to-density ratio) up to one to three orders of magnitude above that of existing micro- and nanoarchitected materials. We use two-photon lithography followed by pyrolysis in a vacuum at 900 °C to fabricate pyrolytic carbon in two topologies, octet- and iso-truss, with unit-cell dimensions of ∼2 μm, beam diameters between 261 nm and 679 nm, and densities of 0.24 to 1.0 g/cm3 Experiments and simulations demonstrate that for densities higher than 0.95 g/cm3 the nanolattices become insensitive to fabrication-induced defects, allowing them to attain nearly theoretical strength of the constituent material. The combination of high specific strength, low density, and extensive deformability before failure lends such nanoarchitected carbon to being a particularly promising candidate for applications under harsh thermomechanical environments.

Cold Spraying of Armstrong Process Titanium Powder for Additive Manufacturing

Titanium parts are ideally suited for aerospace applications due to their unique combination of high specific strength and excellent corrosion resistance. However, titanium as bulk material is expensive and challenging/costly to machine. Production of complex titanium parts through additive manufacturing looks promising, but there are still many barriers to overcome before reaching mainstream commercialization. The cold gas dynamic spraying process offers the potential for additive manufacturing of large titanium parts due to its reduced reactive environment, its simplicity to operate, and the high deposition rates it offers. A few challenges are to be addressed before the additive manufacturing potential of titanium by cold gas dynamic spraying can be reached. In particular, it is known that titanium is easy to deposit by cold gas dynamic spraying, but the deposits produced are usually porous when nitrogen is used as the carrier gas. In this work, a method to manufacture low-porosity titanium components at high deposition efficiencies is revealed. The components are produced by combining low-pressure cold spray using nitrogen as the carrier gas with low-cost titanium powder produced using the Armstrong process. The microstructure and mechanical properties of additive manufactured titanium components are investigated.

Analysis of abrasive wear behavior of PTFE composite using Taguchi’s technique

AbstractPolymeric composites are widely used for structural, aerospace, and automobile sectors due to their good combination of high specific strength and specific modulus. These two main characteristics make these materials attractive, compared to conventional materials like metal or alloy ones. Some of their typical benefits include easy processing, corrosion resistance, low friction, and damping of noise and vibrations. Wear behavior of Polytetrafluoroethylenes (PTFE) and its composites including glass-filled composites and carbon-filled composites are investigated using a pin-on-disc configuration. A plan of experiments in terms of Taguchi technique is carried out to acquire data in controlled way. An orthogonal array (L9) and the analysis of variance are employed to investigate the influence of process parameters on the wear of these composites. Volume loss increased with abrasive size, load, and distance. Furthermore, specific wear rate decreased with increasing grit size, load, sliding distance, wh...

Experimental Investigation on Electrochemical Grinding of Inconel 718

Electrochemical grinding (ECG) is a variation of electrochemical machining (ECM), in which material is removed from the workpiece by simultaneous electrochemical reaction and mechanically abrasive action. It offers a number of advantages over conventional grinding, such as low induced stress, large depths of cut, and increased wheel life. It has been reported that ECG was employed in machining stainless steel 304, metal-ceramic hard alloy of WC-Co groups, and Tungsten carbide. Inconel 718 is used extensively in aerospace because of their excellent combination of high specific strength (strength-to-weight ratio) which is maintained their fracture resistant characteristics, and their exceptional resistance to corrosion at elevated temperature. The machinability of Inconel 718 is generally considered to be poor owing to several inherent properties of the materials. Poor thermal conductivity, chemically reactivity and low elastic modulus are the common problems.In this paper, ECG is employed to process Inconel 718. To prolong the wheel life, the brazed diamond wheel is introduced to be a tool instead of the electrodeposited diamond wheel. Experiments illustrated that the tool durability has been improved from 15hours to 50hours when a brazed diamond wheel was employed to replace an electrodeposited diamond wheel. In addition, a proper high applied voltage and electrolyte temperature was verified to be conductive to improve the maximum electrode feed rate and material removal rate. A electrode feed rate of 6.6mm min-1 was obtained at 30V and 36°C, respectively.

Effect of two-step aging on the mechanical properties of AA2219 DC cast alloy

Abstract With its combination of high specific strength, good machinability and excellent weldability, AA2219 direct chill (DC) cast alloy has become a new category of materials for manufacturing large molds for the plastics and automotive industries. The effect of two-step aging on the microstructural evolution and mechanical properties of AA2219 DC cast alloy was investigated. The precipitate microstructure was characterized under different heat treatment conditions using differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The poor mechanical properties of the air-quenched alloy were attributed to the presence of quench-induced coarse θ′ and θ precipitates, which had very limited contribution to the precipitation hardening during the aging treatment. The two-step aging treatment of the air-quenched AA2219 alloy involved the precipitation of GP zones in the first step followed by their transformation into fine θ″ strengthening precipitates in the second step, which considerably improved the mechanical properties. After undergoing 120 °C/36 h+190 °C/8 h two-step aging, the hardness, YS and UTS of the air-quenched alloy were increased by 27%, 46% and 15%, respectively, compared with 190 °C/8 h one-step aging.

Investigation on Chip Formation during Machining Using Finite Element Modeling

Titanium alloys are desirable materials for aerospace industry because of their excellent combination of high specific strength, lightweight, fracture resistant characteristics, and general corrosion resistance. Therefore, the chip morphology is very important in the study of machinability of metals as well as the study of cutting tool wear. The chips are generally classified into four groups: continuous chips, chips with built-up-edges (BUE), discontinuous chips and serrated chips. . The chip morphology and segmentation play a predominant role in determining machinability and tool wear during the machining process. The mechanics of segmented chip formation during orthogonal cutting of titanium alloy Ti–6Al–4V are studied in detail with the aid of high-speed imaging of the chip formation zone. The finite element model of chip formation of Ti–6Al–4V is suggested as a discontinuous type chip at lower cutting speeds developing into a continuous, but segmented, chip at higher cutting speeds. The prediction by using finite-element modeling method and simulation process in machining while create chips formation can contribute in reducing the cost of manufacturing in terms of prolongs the cutting tool life and machining time saving.

Isochronal sintering of the blended elemental Ti–35Nb alloy

Titanium alloy parts are ideally suited for advanced aerospace systems and surgical implants because of their unique combination of high specific strength at both room temperature and moderately elevated temperature, in addition to excellent corrosion resistance. In this work, results of the Ti–35Nb alloy sintering are presented. This alloy, due to its lower modulus of elasticity and high biocompatibility, is a promising candidate for surgical and aerospace applications. Samples were produced by mixing of initial metallic hydride powders followed by uniaxial and cold isostatic pressing with subsequent densification by isochronal sintering between 700 and 1500 °C, in vacuum. Sintering behavior was studied by means of microscopy and density. Sintered samples were characterized for phase composition, microstructure and microhardness by X-ray diffraction, scanning electron microscopy and Vickers indentation, respectively. Samples sintered at high temperatures display a fine plate-like α structure and intergranular β. A few remaining pores are still found, and density above 97% for specimens sintered at 1500 °C is reached.

Production of titanium alloys for advanced aerospace systems by powder metallurgy

Titanium alloys parts are ideally suited for advanced aerospace systems because of their unique combination of high specific strength at both room temperature and moderately elevated temperature, in addition to excellent corrosion resistance. Despite these features, use of titanium alloys in engines and airframes is limited by cost. The alloys processing by powder metallurgy eases the obtainment of parts with complex geometry. In this work, results of the Ti-6Al-4V and Ti-13Nb-13Zr alloys production are presented. Samples were produced by mixing of initial metallic powders followed by uniaxial and cold isostatic pressing with subsequent densification by sintering between 900 up to 1500 °C, in vacuum. Sintered samples were characterized for phase composition, microstructure and microhardness by X-ray diffraction, scanning electron microscopy and Vickers indentation, respectively. It was shown that the samples were sintered to high densities and presented homogeneous microstructure from the elements dissolution with low interstitial pick-up.

Electric field modification during superplastic deformation of 15 vol% SiCp/LY12 Al composite

Silicon carbide particulate-reinforced aluminium (SiCp–Al) composites possess a unique combination of high specific strength, high elastic modulus, good wear resistance and good thermal stability, which makes them attractive as metal matrix composites (MMCs). Some work focusing on the superplasticity of SiC particulate reinforced composites has been conducted in order to improve the formability. The study of superplastic deformation in the presence of an external electric field is a relatively new development, but to the authors' best knowledge, has not yet been reported for MMCs. In this paper, 15 vol% SiCp/LY12 Al composite was fabricated by spray atomization and co-deposition and pre-treated by isothermal hot compression as well as isothermal hot forward extrusion (extrusion ratio: 10.0). The superplastic deformation behaviour of the 15 vol% SiCp/LY12 Al composite under an external electric field has been investigated. The electroplastic effect of 15 vol% SiCp/LY12 Al composite and the influence of deformation process parameters on the superplasticity have also been studied. The experimental results show that: (i) the optimum intensity of the electric field improves the superplasticity of 15 vol% SiCp/LY12 Al composite, i.e. the strain-rate sensitivity index ( m) is increased by 13% and the elongation ( δ) is 420%, being increased by 20%; (ii) the optimum intensity of the electric field decreases the flow stress by 16% during the superplastic deformation of 15 vol% SiCp/LY12 Al composite; (iii) the optimum intensity of the electric field makes the grain size finer during the superplastic deformation of 15 vol% SiCp/LY12 Al composite, which provides the microstructural conditions for the improvement of superplasticity, but the electric field has only a slight influence on the microstructure before superplastic deformation.

Connecting high-performance carbon-fiber-reinforced polymer cables of suspension and cable-stayed bridges through the use of gradient materials

Carbon-fiber-reinforced polymer (CFRP) cables offer a very attractive combination of high specific strength and modulus (ratio of strength or modulus to density), outstanding fatigue performance, good corrosion resistance, and low axial thermal expansion. The high specific strength permits the design of structures with highly increased spans. The high specific modulus translates into a high relative equivalent modulus. This factor is very important in view of the deflection constraints imposed on large bridges. A relative high modulus coupled with a low mass density offer CFRP cables already an advantage for spans above 1000 m. Since 1980 EMPA has been developing CFRP cables for cable-stayed and suspension bridges that are produced as assemblies of parallel CFRP wires. The key problem facing the application of CFRP cables, and thus their widespread use in the future, is how to connect them. A new reliable anchoring scheme developed with computer-aided materials design and produced with advanced gradient materials based on ceramics and polymers is described. Early 1996 such CFRP cables with a load-carrying capacity of 12 MN (1200 metric tons) have been applied for the first time on a cable-stayed road bridge with a 124-m span. Each cable is built up from 241 CFRP wires having a diameter of 5 mm.