Tungsten Heavy Alloys Research Articles

Experimental investigation of tool wear characteristics and analytical prediction of tool life using a modified tool wear rate model while machining 90 tungsten heavy alloys

Tungsten heavy alloys are widely used in the manufacturing of weights for aircraft, missiles, boats and race cars; penetrators; radiation shielding; and radioisotope containers. Manufacturing these components needs machining as a secondary operation. Since tungsten heavy alloys are difficult to machine, the in-depth analysis of tool wear growth and mechanism during machining of these alloys becomes essential. Hence, this work focuses on the experimental study of flank wear growth and its effect on other machining outputs for two different tool geometries (−5° and 2° rake angles) during turning of 90 tungsten heavy alloys. The predominant wear mechanism was identified as adhesion based on scanning electron microscopic analysis. Finally, three commonly used analytical tool wear rate models and one newly proposed model (modified Zhao model) were utilized for the prediction of flank wear growth and tool life. It was observed that the modified Zhao model could predict tool flank wear fairly well within error percentage of 4%–7% and thus could be used as a benchmark while machining difficult-to-cut alloys.

Testing of the Granulation Process for the Preparation of a Mixture with the Chemical Composition of a Heavy Tungsten Alloy

The subject of this paper are the results of testing the process of preparation of a granulate made of a mixture of powders with the chemical composition of a heavy tungsten alloy. Different methods of granulation are discussed which reveal how widely they vary in physical and chemical phenomena. It is a testament to how vast the research field of granulation of matter is. The importance and significance of granulation methods were proven with the distinguishing features of the granulated form of the stock materials and products compared to their powdered (dusty) form. Granulation is applied to produce convenient forms of products or intermediates which are acceptable to end users, and to facilitate or even enable the application of products and intermediates in appropriate technologies.

Plasma and X-ray radiation-induced damage mechanisms in a tungsten heavy alloy

Tungsten-based materials are among the main candidate plasma-facing materials for future fusion reactors where extreme conditions of thermomechanical stresses, radiation, and energetic particles must be withstood. Here a hybrid X-pinch platform combined with a pulsed power machine was used to study damage mechanisms in 90W–4Cu–6Ni tungsten heavy alloy as a result of exposure to high energy density plasma, X-ray radiation, and the resulting thermal shock. It was found that the plasma-facing surface of the material undergoes severe melting and ablation with the extent of ablation being higher in the Cu–Ni phase than the W particles. Rapid melting and solidification transform the W particles into fine dendrite morphologies embedded in the Cu–Ni solid solution. Cracking was also observed in the subsurface layers at the interfaces between the W particles and the Cu-rich phase of the Cu–Ni matrix. X-ray radiation, on the other hand, cause slight melting on the surface without significant ablation and no cracking.

MECHANICAL PROPERTIES OF FORGED TUNGSTEN HEAVY ALLOYS

Tungsten heavy alloys are composite materials containing spherical tungsten particles embedded in binder matrix. Their excellent mechanical properties can be further improved by rotary forging. This paper aims to gain deeper understanding of the forging process by investigating the local elastic modulus, hardness, and residual stress of individual phases in W6Ni3Co pseudo-alloy. The resulting global properties of the composite material such as stress-strain behavior, fracture toughness and fatigue crack growth rate behavior are also studied. The results show that sintered and quenched material consists of highly textured matrix containing nearly perfect single crystal spheres of pure W. The rotary forging leads to significant lattice deformations destroying the texture and significantly increasing the hardness of both WNiCo matrix and W particles and making residual stresses in W particles anisotropic with increased compression along the longitudinal axis of the forged part.

Microstructure, Texture, Tensile Flow and Work Hardening Behavior of Tungsten Heavy Alloys in Swaged and Swaged + Aged Conditions

Present work describes the development of microstructure, texture, tensile flow and work hardening behavior of tungsten heavy alloys in swaged and swaged + aged conditions. The microstructural attributes of both the alloys such as the average W-grain size, shape, volume fractions of matrix phase and contiguity are nearly same in swaged and swaged + aged conditions. The textures of W-grains and matrix are different in swaged and swaged + aged alloys. The overall ODF intensities of W-grains and matrix phase are moderate and weak, respectively. Both the alloys show higher strength and slightly lower elongation after aging of the swaged samples. The aging has significantly reduced the impact toughness values of both the alloys in swaged condition. The true stress–true strain plots of both the alloys in swaged and swaged + aged conditions exhibit continuous increase in stress from σYS to σUTS (except yield point phenomenon in swaged + aged samples). Both the alloys in swaged + aged condition reveal the presence of yield point phenomenon. Bulk hardness values of both the alloys increase from swage to swaged + aged conditions. The true stress–true plastic strain curves on log–log scale of both the alloys in swaged and swaged + aged conditions display single slope only and follow Swift constitutive equation due to presence of pre-strain in the materials. The overall nature of the instantaneous curves is same in swaged and swaged + aged conditions. These curves exhibit three distinctive regimes of strain hardening that are stage I, stage II and stage III. The linear part of stage III has been explained using KME model in both alloys.

Substitution of Ni with NiB prevents shape distortion of liquid phase sintered 90W–6Ni–2Fe–2Co heavy alloys

The shape distortion of liquid phase sintered tungsten heavy alloys (WHAs) depends on different factors. Here, the effect of partial and full substitution of Ni with its monoboride (NiB) on distortion characteristics of 90W–6Ni–2Fe–2Co (wt.%) heavy alloy is investigated systematically without changing wt. fraction of W. The phase and microstructural evolution during sintering and effect of microstructural parameters on the distortion is studied. In order to understand the phase and microstructure evolution of the matrix phase of sintered samples, free-standing samples were prepared using the composition of the matrix phase under similar sintering conditions. XRD patterns of sintered sample show characteristic W-peaks along with austenitic matrix phase, whereas, in the case of free-standing samples majority of reflections corresponding to matrix phase (fcc, lattice parameter = 0.360 nm) eliciting a peak shift to slightly higher 2θ values as opposed to those of sintered samples. The FESEM micrographs of free-standing samples reveal microstructure resembling a hypoeutectic system with decreasing inter-lamellar and proeutectic spacing with the extent of NiB substitution. In the sintered 90W–6Ni–2Fe–2Co system, shape distortion resulted in elephant-foot formation (distortion parameter: 22.9). Owing to the partial and full substitution of Ni with NiB, the shape distortion is restricted by formation of uniformly contiguous microstructure, inherent microstructural changes of the matrix phase, increase in dihedral angle, and grain connectivity in partially (90W–3Ni–3NiB–2Fe–2Co) and fully (90W–6NiB–2Fe–2Co) NiB substituted samples, respectively. As a result, the distortion parameter reduces from 22.9 to 9.3 and to 3.7 for partially and fully substituted NiB samples.

Effects of Sintering Conditions on Structures and Properties of Sintered Tungsten Heavy Alloy.

Probably the most advantageous fabrication technology of tungsten heavy alloys enabling the achievement of required performance combines methods of powder metallurgy and processing by intensive plastic deformation. Since the selected processing conditions applied for each individual processing step affect the final structures and properties of the alloys, their optimization is of the utmost importance. This study deals with thorough investigations of the effects of sintering temperature, sintering time, and subsequent quenching in water on the structures and mechanical properties of a 93W6Ni1Co tungsten heavy alloy. The results showed that sintering at temperatures of or above 1525 °C leads to formation of structures featuring W agglomerates surrounded by the NiCo matrix. The sintering time has non-negligible effects on the microhardness of the sintered samples as it affects the diffusion and structure softening phenomena. Implementation of quenching to the processing technology results in excellent plasticity of the green sintered and quenched pieces of almost 20%, while maintaining the strength of more than 1000 MPa.

Microstructure and strengthening mechanisms of 90W–7Ni–3Fe alloys prepared using laser melting deposition

Abstract The microstructure and mechanical properties of a 90W–7Ni–3Fe heavy alloy manufactured using laser melting deposition (LMD) have been investigated with a goal of understanding the LMD process and the relevant strengthening mechanisms. A reference sample prepared using liquid phase sintering (LPS) with similar composition has been used for comparison to isolate effects of the processing rate. The results show that the nearly fully-dense LMD sample consists of two periodically alternating sublayers containing different volume fractions of W particles. Compared to the reference LPS sample, the LMD sample has a higher W content in the binder matrix, a more refined microstructure, a higher dislocation density and a lower contiguity between W particles. Furthermore, the LMD sample has a yield strength of 822 ± 30 MPa, which is about 200 MPa higher than that of the reference LPS sample. This strength difference is quantitatively analyzed based on the microstructural observations. It is shown that the increased fraction of W-matrix interfaces, a higher interface bonding strength and more pronounced constraint effects, resulting from the higher processing temperature and fast cooling of LMD, provide extra strengthening. The present study provides an important basis for optimizing the LMD process for manufacture of high W content tungsten heavy alloys with improved mechanical properties.

Changes in electrochemical properties of a heavy tungsten alloy during its processing under the influence of DC current in ammonia-alkali solutions

The method of linear voltammetry in the potentiodynamic mode was used to study the change in the anode behavior of a heavy tungsten VNZh alloy (in wt.%: 78.8 W; 15.2 Ni; 6.0 Fe), as well as the phase composition and morphology of its surface during its electrochemical dissolution under the influence of DC current. It was determined, that the phase composition of VNZh alloy surface is a solid solution of iron-nickel after tungsten dissolution from the surface of the alloy. It has been established, that in ammonia-alkali electrolytes the process of tungsten passing into solution does not stop after dissolution of the metal from the alloy surface, but continues both due to diffusion of the metal from the depth of the sample and the pits formation in the processed alloy.

AC electrochemical oxidation of nickel and VNZh alloy in alkaline-ammonium solutions

It was studied the electrochemical oxidation of nickel in an electrolyte of the composition NaOH 2M + NH4OH 4M by using linear and cyclic voltammetry. A significant increase in the process rate has been revealed due to the cathodic polarization of the electrode under study. It was shown the possibility of heavy tungsten alloy processing, such as VNZh-type alloy (in wt%: 78.8 W, 15.2 Ni, 6.0 Fe), under the influence of alternating electric current of industrial frequency in alkaline-ammonium solutions.

System Development for Diffusion Bonding of Multiple Unit Tubes to Produce Long Tubular Tungsten Heavy Alloys

A diffusion bonding system to fabricate long tubular parts by joining of two- or more-unit tubes made of tungsten heavy alloys (THAs) is proposed and characterized in this study. The difficulty of powder processing of THA originates from the weak strength of the green compact and the high weight of the THA powders. The long tubular green compact is difficult to handle due to its weak structural integrity. Furthermore, gravity-induced slumping during liquid phase sintering induces dimensional distortion and degrades the mechanical performances. As a clue for solving these problems, the unit tubes are fabricated. However, the mass of green compacts for unit tubes is not sufficiently great as to cause problematic slumping; tubular unit tubes can be obtained without significant difficulty. Fabricated unit tubes are stacked in a furnace chamber and diffusion-bonded to produce a long tubular part having bond strength substantially equal to that of a monolithic tube. The proposed diffusion bonding system was well characterized and successfully applied to the industrial production line. The feasibility was also confirmed by investigating the bond quality, which can be assessed by metallographic microstructure and mechanical property.

Microstructure and mechanical properties of spark plasma-sintered La2O3 dispersion-strengthened W–Ni–Fe alloy

An investigation of lanthanum oxide (La2O3) addition to tungsten heavy alloy (WHA) with a ternary composition of W–7Ni–3Fe was reported in this study. The mixed powders were sintered using spark plasma sintering (SPS) technique. La2O3 was added in increments of 0.25 wt%, 0.50 wt%, 0.75 wt% and 1.00 wt% to WHA, respectively. The sintered samples were characterized for microstructural evolution and mechanical properties. The influences of La2O3 addition on density, grain size, hardness, ultimate tensile strength (UTS) and ductility on W–7Ni–3Fe system were discussed in this study. The highest relative sintered density of 87.95% was obtained for 0.25 wt% La2O3 addition to W–7Ni–3Fe. The lowest grain size of 7.89 μm was observed for 1.00 wt% La2O3 addition. Similarly, the highest hardness and UTS of HV 533 and 1110 MPa, respectively, were also obtained for the same composition. Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) of the samples revealed homogenous distribution of La2O3 in the alloy matrix. Fractography of the sintered alloy samples revealed W–W intergranular fracture.

Densification, microstructural evolutions of 90W-7Ni-3Fe tungsten heavy alloys during laser melting deposition process

Thin walled 90W-7Ni-3Fe tungsten heavy alloys (WHAs) have been prepared by a laser melting deposition (LMD) additive manufacturing technique using different input laser volume energy densities. Detailed investigations on densification and microstructural evolutions of the LMD process have been carried out. The result shows that the sample density increases with increasing input energy density by elimination of “lack of fusion” defects. However, some gas delivered with powders gets trapped in the molten pool due to the fast cooling rate and complex shapes of W particles, resulting in a prevalence of small round pore defects even under high input energy densities. Near full density can be obtained when the energy density reaches 74 J/mm3. The LMD WHAs have two phase microstructures consisting of polygonal or dendritic W particles embedded in a nickel‑iron matrix, which has large sized columnar grains due to epitaxial growth. The polygonal particles are partially dissolved/melted W powder particles. The dendritic particles are newly formed grains due to the reprecipitation of supersaturated W in the nickel‑iron matrix during solidification. The proportion of dendritic particles increases with the increasing input energy density. A microstructure evolution process adapted from traditional liquid phase sintering process containing three stages of rearrangement, solution-reprecipitation, and solid state is proposed for the LMD process.

Ultrafine Grain Tungsten Heavy Alloys with Excellent Performance Prepared by Spark Plasma Sintering

Ultrafine grain tungsten heavy alloys (WHAs) were successfully produced from the nanocrystalline powders using spark plasma sintering. The present study mainly discussed the effects of sintering temperature on the density, microstructure and mechanical properties of the alloys. The relative density of 98.12% was obtained at 1 050 °C, and the tungsten grain size is about 871 nm. At 1 000 °C-1 200 °C, the mechanical properties of the alloys tend to first rise and then goes down. After SPS, the alloy exhibits improved hardness (84.3 HRA at 1 050 °C) and bending strength (987.16 MPa at 1 100 °C), due to the ultrafine-grained microstructure. The fracture mode after bending tests is mainly characterized as intergranular or intragranular fracture of W grains, interfacial debonding of W grains-binding phase and ductile tearing of binding phase. The EDS analysis reveals a certain proportion of solid solution between W and Ni-Fe binding phase. The good mechanical properties of the alloys can be attributed to grain refinement and solid solution strengthening.

Study on the Improving Penetration Performance of Tungsten Heavy Alloy Penetrator by Heat Treatment

Study on the Improving Penetration Performance of Tungsten Heavy Alloy Penetrator by Heat Treatment

Microstructures and properties of 90W-4Ni-6Mn alloy prepared by vacuum sintering

Tungsten-nickel-manganese alloy is an excellent potential replacement of depleted uranium alloy for kinetic energy penetrator (KEP) due to its high self-sharpening effect. In this work, the 90W-4Ni-6Mn alloy was prepared by vacuum sintering, and the effects of the sintering temperature and oxygen content on its microstructure and properties were discussed. Sintering temperature had a great influence on the properties of tungsten heavy alloy. The optimal properties of 90W-4Ni-6Mn with the relative density of 99.43%, the W grain size of 3.80 μm, and the compressive strength of 2790 MPa were obtained at the sintering temperature of 1125 °C for 60 min under a vacuum of 10−2∼10−3 Pa. With the increase of oxygen content, the densification of powders became more difficult, and the microstructure homogeneity and mechanical properties of sintered samples decreased accordingly. The results provide a new effective way to prepare W–Ni–Mn alloy with high performance.

Instantaneous work hardening behaviour of two-phase tungsten heavy alloys: a phenomenological approach

ABSTRACT The present work describes a phenomenological approach to explain the instantaneous behaviour of tungsten heavy alloys (WHAs) in heat-treated and swaged conditions. The strengths and elongation values of heat-treated materials are lower and higher than those of the swaged samples respectively. The heat-treated materials exhibit two slopes in true stress–true plastic strain curves and follow the Ludwigson constitutive equation. On the other hand, swaged materials display a single slope and adhere to typical Swift constitutive equation. The latter reflect the presence of pre-strain in the materials due to swaging deformation. The fracture surfaces in heat-treated materials consist of W-W decohesion along with matrix rupture and W-cleavage, while swaged samples consist of mainly W-cleavage. Both the materials display three typical stages (I, II and III) of work hardening. The second derivatives of true stress–true plastic strain curves of these alloys exhibit a perfect parabola although the nature of true stress–true strain as well as true stress–true plastic strain curves is quite different in heat-treated and swaged materials. This has been observed for the first time in WHAs consisting of matrix and W-grains. The shape of the parabola is simple and easy to fit. The fitting parameters of parabolas have been successfully employed to explain the flow behaviour of a large number of tungsten heavy alloys having two-phase microstructure in different processing conditions.

On the flow and work hardening behavior of tungsten heavy alloy 92W-5.5Ni-2.5Fe

Present work describes flow and work hardening behavior of Tungsten Heavy Alloy 92W-5.5Ni-2.5Fe (wt%) in heat treated and swaged conditions. The morphology of W-grains in heat treated and swaged materials is equiaxed and elongated in longitudinal direction with same volume fraction, respectively. The swaged material depicts different contiguity values along transverse and longitudinal directions. The strength and elongation values in heat treated condition are smaller and larger than those of the swaged material. The difference in σYS and σUTS values is very small in swaged material. The true stress-true plastic strain curve of the alloy on log-log scale exhibits two slopes in heat treated condition and follows Ludwigson constitutive equation. This can be attributed to the differences in micro hardness values of W-grains and matrix phase. In contrast, the true stress-true plastic strain curve of the swaged alloy on log-log scale displays only single slope and follows Swift equation due to the presence of pre-strain in the material. Both the materials exhibit three typical stages (I, II and III) of work hardening and linear part of stage III follows KME model. The fracture surface of heat treated material consists of W-cleavage, W-W decohesion and matrix rupture in considerable proportions. In contrast, the swaged material displays the presence of mainly W-cleavage. The fraction of W-matrix failure is significantly small and more or less same in both the materials.

On the remarkable fracture toughness of 90 to 97W-NiFe alloys revealing powerful new ductile phase toughening mechanisms

Tungsten is generally too brittle to serve a robust structural function. Here, we explore the fracture toughness of 90–97 wt%W Ni-Fe liquid phase sintered tungsten heavy alloys (WHAs). The room temperature (RT) maximum load fracture toughness (KJm ≈ 69–107 MPa√m) of the WHA, containing only 3–10 wt% of a Ni–Fe ductile phase (DP), is ≈ 9–13 times higher than KIc typical of monolithic W (≈ 8 MPa√m). All the WHAs show extensive stable crack growth, and increasing blunting line toughness averaging ≈ 170 MPa√m, prior to significant crack extension. In contrast to classical ductile phase toughening, that is primarily due to macrocrack bridging, the WHA toughness increase mainly involves new mechanisms associated with arrest, blunting and bridging of numerous dilatational shielding process zone microcracks in the macrocrack process zone. Tests down to –196 °C, to partially emulate irradiation hardening, show decreasing toughness and a transition to elastic fracture at a temperature of –150 °C for 90W to –25 °C for 97W. However, even at –196 °C, the leanest DP 97W WHA KIc is ≈ 3 times higher than that of monolithic W at RT. Possible effects of the small specimen size used in this study are briefly summarized.

“Self-sharpening” tungsten high-entropy alloy

“Self-sharpening”, a material maintaining its acute head shape during penetration, is highly desirable in a wide range of engineering applications. However, it remains a great challenge to make it occur in conventional single-principal-element alloys. Here, we develop a new chemical-disordered multi-phase tungsten high-entropy alloy that exhibits outstanding self-sharpening capability, in sharp contrast to conventional tungsten alloys only showing mushrooming. This alloy consists of a BCC dendrite phase and a rhombohedral μ phase embedded in the continuous FCC matrix, and such a unique microstructure leads to a 10–20% better penetration performance than conventional tungsten heavy alloys. We show that emergence of the self-sharpening is triggered by the ultrastrong μ phase stimulated dynamic recrystallization softening mediated shear banding. This study sheds light on the origin of self-sharpening and might open new opportunities for developing high-performance penetrator materials.