Tungsten Particle Size Research Articles

Effects of sintering conditions on mechanical properties of mechanically alloyed tungsten heavy alloys

The effects of sintering conditions on the microstructural evolution and mechanical properties of mechanically alloyed tungsten heavy alloys were investigated. W, Ni and Fe powders were mechanically alloyed in a tumbler ball mill at a milling speed of 75 rpm, ball-to-powder ratio of 20∶1 and ball filling ratio of 15%. The mechanically alloyed powders were compacted and solid-state sintered at a temperature of 1300°C for 1 hour in a hydrogen atmosphere. The solid-state sintered tungsten heavy alloy was subsequently liquid-phase sintered at 1470°C with varying sintering times from 4 min to 90 min. The solid-state sintered tungsten heavy alloy showed fine tungsten particles of 3 μm in diameter and high relative density above 99%. The volume fraction of the W-Ni-Fe matrix phase was measured, as 11% and tungsten/tungsten contiguity was 0.74 in solid-state sintered tungsten heavy alloys. Mechanically alloyed and two-step sintered tungsten heavy alloys showed tungsten particles of 6–15 μm and a volume fraction of the W-Ni-Fe matrix phase of 16% and tungsten/tungsten contiguity of 0.40. The solid-state sintered tungsten heavy alloy exhibited a yield strength of about 1100 MPa due to its finer tungsten particles, while it showed low elongation and impact energy due to its large tungsten/tungsten contiguity. The yield strength of two-step sintered tungsten heavy alloys increased with the decreasing of tungsten particle size and volume fraction of the W-Ni-Fe matrix.

High-velocity microprojectile mediated DNA delivery into Phaseolus vulgaris callus cells

We report the method for the establishment of rapidly growing callus cultures of Phaseolus vulgaris and the conditions required for efficient transformation using high velocity microprojectiles and high level of transient gene expression. Using hypocotyl explant and vertical culture on B5 medium with lmg/1 kinetin and 2 mg/1 2,4-D, we can recommend to get a rapidly growing callus from bean which is a good starting material to introduce foreign DNA into bean cells. The GeneBooster particle delivery system was used for the bombardment of bean callus and the Hgm resistance gene (Hgmr) was used as a selectable marker gene. 25mg/I hygromycin (Hgm) concentration was sufficient to kill the control callus. We used the standard physical factors, the appropriate pressure of N2 gas for the bombardment of the callus tissue, the shooting distance and the size of tungsten particles used as microprojectiles. Selective and nonselective tests were made by transferring the healthy green and white calluses, subcultured for 4 months on selective and nonselective medium. Several Hgm resistant calli had been obtained. Selective pressure was maintained over a period of 10 months.

Effects of Sintering Conditions on Mechanical Properties of Mechanically Alloyed Tungsten Heavy Alloys

The effects of sintering conditions on the microstructural evolution and mechanical properties of mechanically alloyed tungsten heavy alloys were investigated. W, Ni and Fe powders were mechanically alloyed in a tumbler ball mill at a milling speed of 75 rpm, ball-to-powder ratio of 20∶1 and ball filling ratio of 15%. The mechanically alloyed powders were compacted and solid-state sintered at a temperature of 1300°C for 1 hour in a hydrogen atmosphere. The solid-state sintered tungsten heavy alloy was subsequently liquid-phase sintered at 1470°C with varying sintering times from 4 min to 90 min. The solid-state sintered tungsten heavy alloy showed fine tungsten particles of 3 μm in diameter and high relative density above 99%. The volume fraction of the W-Ni-Fe matrix phase was measured, as 11% and tungsten/tungsten contiguity was 0.74 in solid-state sintered tungsten heavy alloys. Mechanically alloyed and two-step sintered tungsten heavy alloys showed tungsten particles of 6–15 μm and a volume fraction of the W-Ni-Fe matrix phase of 16% and tungsten/tungsten contiguity of 0.40. The solid-state sintered tungsten heavy alloy exhibited a yield strength of about 1100 MPa due to its finer tungsten particles, while it showed low elongation and impact energy due to its large tungsten/tungsten contiguity. The yield strength of two-step sintered tungsten heavy alloys increased with the decreasing of tungsten particle size and volume fraction of the W-Ni-Fe matrix.

Preparation of tungsten powder by the combustion of CaWC4/Mg

Tungsten powder was prepared by a Self-Propagating High-Temperature Synthesis(SHS) Process of a CaWO4-Mg mixture. Mg vapor was discovered to significantly affect such combustion parameters as the combustion temperature(Tc), the combustion velocity(U), the relative mass change(δm) and the relative elongation(δh) of the sample. These effects could be reduced by decreasing the internal Ar pressure, sample density and reducing agent content. The oxygen content in tungsten was decreased by using 5% excess Mg; the carbon content was diminished when the combustion process occurred under PAr=0.1 MPa. The MgO and CaO in the product was leached with an HC1 solution. Such parameters as combustion temperature(Tc), U, Am, and milling time(τ) were found to affect the tungsten particle size and morphology.

Effect of size and shape of tungsten particles on dynamic torsional properties in tungsten heavy alloys

The effect of the size and shape of tungsten particles on dynamic torsional properties in tungsten heavy alloys was investigated. Dynamic torsional tests were conducted on seven tungsten alloy specimens, four of which were fabricated by repeated sintering, using a torsional Kolsky bar, and then the test results were compared via microstructure, mechanical properties, adiabatic shear banding, and deformation and fracture mode. The size of tungsten particles and their hardness were increased as sintering temperature and time were increased, thereby deteriorating fracture toughness. The dynamic torsional test results indicated that in the specimens whose tungsten particles were coarse and irregularly shaped, cleavage fracture occurred predominantly with little shear deformation, whereas shear deformation was concentrated into the center of the gage section in the conventionally fabricated specimens. The deformation and fracture behavior of the specimens having coarse tungsten particles correlated well with the observation of the in situ fracture test results, i.e., cleavage crack initiation and propagation. These findings suggested that there would be an appropriate tungsten particle size because the cleavage fracture mode would be beneficial for the “self-sharpening” of the tungsten heavy alloys.

Effect of size and shape of tungsten particles on penetration performance in tungsten heavy alloys

The effects of the size and shape of tungsten particles on dynamic torsional properties in tungsten heavy alloys were investigated. Dynamic torsional tests were conducted on seven tungsten alloy specimens, four of which were fabricated by repeated sintering, using a torsional Kolsky bar, and then the test results were comparedvia microstructure, mechanical properties, adiabatic shear banding, and deformation and fracture mode. The size of tungsten particles and their hardness were increased with the increasing of the sintering temperature and time, thereby deteriorating the fracture toughness. The dynamic torsional test results indicated that a cleavage fracture occurred predominantly with little shear deformation in the specimens whose tungsten particles were coarse and irregularly shaped whereas shear deformation was concentrated into the center of the gage section in the conventionally fabricated specimens. The deformation and fracture behavior of the specimens having coarse tungsten particles correlated well with the observation of thein situ fracture test results,i.e., cleavage crack initiation and propagation. These findings suggested that there would be an appropriate tungsten particle size condition in the penetration performance since the cleavage fracture mode would be beneficial for the self-sharpening of tungsten heavy alloys.

Effect of tungsten particle size on properties of infiltrated tungsten-copper compacts: K.S.Hwang. (Taiwan National University, Taiwan.)

Pure titanium and titanium alloys are potential materials for the fabrication of cast dental appliances. One important factor in producing sound castings is the capacity of the metal to fill the mold. This study used a wedge-shaped mold to compare the mold filling of titanium with that of conventional dental casting alloys. The metals used were CP Ti, Ti–6Al–7Nb, Ti–6Al–4V, Ti with 1 and 4 wt% Cu and ADA Type III gold alloy and an Ni–Cr alloy. The castings were cut into four pieces parallel to the triangular surface. Mold filling was evaluated as the distance between the tip of the cast wedge and theoretical tip of the triangle. The mold filling of the gold alloy was superior compared to all the metals tested, while the mold filling of the Ni–Cr alloy was the worst. There were no statistical differences at the 30° marginal angle for all the cast titanium metals. At the sharper 15° angle, CP Ti and Ti–6Al–7Nb was superior to both the Ti–Cu alloys. Although the mold filling of titanium was inferior compared to the gold alloy, the data justify the use of titanium for the production of dental appliances.

The effect of tungsten particle size on the processing and properties of infiltrated W-Cu compacts

Three tungsten powders with average particle sizes of 8.7, 23.2, and 65.2 µm were used to make W-15Cu compacts. The compacting pressure and sintering temperature were adjusted for each powder to attain the desired skeleton density. Sintered skeletons were then infiltrated with oxygen-free copper at 1200 °C in hydrogen and in vacuum. Results showed that as the tungsten particle size decreased, higher compacting pressures and sintering temperatures were required for the same desired skeleton density. The processing parameters and the tungsten particle size caused variations in the amount of closed pores and the W-W contiguity, which in turn resulted in different infiltrated densities and resistivities. Direct infiltration on green compacts was also examined, and higher infiltration densities and lower electrical resistivities were obtained compared to those obtained by infiltrating sintered compacts. These results are discussed based on infiltrated density, differences in microstructure, and the W-W contiguity.

Effects of tungsten particle shape and size on dynamic deformation and fracture behavior of tungsten heavy alloys

Dynamic torsional tests were conducted using a torsional Kolsky bar for five alloys, one of which was fabricated by double-cycled sintering process, and then the test data were compared via microstructures, mechanical properties, adiabatic shear banding, and fracture mode. The double-sintered tungsten alloy specimen whose tungsten particles were very coarse and irregularly shaped showed cleavage fracture in the central area of the gage section with little shear deformation, whereas shear deformation was concentrated in the central area of the gage section in the other alloys. The deformation and fracture behavior of the double-sintered alloy correlated well with the observation of the impacted penetrator specimen and thein situ fracture test results,i.e., microcrack initiation at coarse tungsten particles and cleavage crack propagation through tungsten particles. These findings suggested that the cleavage fracture mode would be beneficial for the serf-sharpening effect and, thus, the improvement of the penetration performance of the double-sintered tungsten heavy alloy would be expected.

Investigation of Slow‐Propagation Tungsten Delay Mixtures

AbstractThe burning characteristics and reliability of slow‐propagation tungsten delay mixtures (characteristic time 38–42 s/in) were studied experimentally by using various mixture compositions and tungsten particle sizes. Thermal analysis showed that potassium perchlorate is vital to initiate the reaction. Gravimetric analysis indicated that during the reaction overall mass loss of the delay mixtures did not exceed 5% compared to their initial mass. A mixture containing a tungsten powder of 6–8 μm was found to have the lowest effective activation energy, facilitating good ignition and burning zone propagation. The results of this study were successfully used to design a tiny delay which exhibited reliable ignition, burning and propagation characteristics under extreme conditions.

Effect of Tungsten Particle Size and Copper Content on Working Behaviour of W–Cu Alloy Electrodes During Electrodischarge Machining

The effect of the average particle size (APS) of tungsten powders and copper content on the working behaviour of W–Cu alloy electrodischarge machining electrodes fabricated by pressing, sintering, and infiltration was investigated. Five batches of tungsten powder with an APS of 1·0–3·0 μm were used. The green density of tungsten compacts increased with increasing compacting pressure and APS of tungsten powder, but the volume change which occurred during sintering and infiltration decreased with increasing compacting pressure and APS. When machining a die steel, a low erosion ratio and high erosion rate resulted. Only slight erosion of the electrode occurred and the experimental data showed no obvious difference between specimens from the five batches of tungsten powder. In heavy erosion tests machining WC–Co annular workpieces, the W–Cu alloy made from the tungsten powder with an APS of 2·0 μm exhibited the lowest erosion ratio and highest erosion rate. Increasing the tungsten content of the W–Cu alloy reduced the erosion ratio markedly, but the erosion rate of the die steel workpieces slightly decreased. PM/0660

Tungsten particle size optimization in a large inclined rotating tube furnace : H. Palmour (North Carolina State University, Raleigh, North Carolina, USA)

Tungsten particle size optimization in a large inclined rotating tube furnace : H. Palmour (North Carolina State University, Raleigh, North Carolina, USA)

Effects of tungsten particle size and powder treating techniques on the sintered properties of tungsten-copper alloys: B. Yang and R.M. German (Pennsylvania State University, Pennsylvania, USA)

Effects of tungsten particle size and powder treating techniques on the sintered properties of tungsten-copper alloys: B. Yang and R.M. German (Pennsylvania State University, Pennsylvania, USA)

Experimental investigation of particle size distribution influence on diffusion controlled coarsening

The influence of initial particle size distribution on coarsening during liquid phase sintering has been experimentally investigated using W-14Ni-6Fe alloy as a model system. It was found that initially wider size distribution particles coarsened more rapidly than those of an initially narrow distribution. The well known linear relationship between the cube of the average particle radius, r3, and time was observed for most of the coarsening process, although the early stage coarsening rate constant changed with time, as expected with concomitant early changes in the tungsten particle size distribution. The instantaneous transient rate constant was shown to be related to the geometric standard deviation, lnσ, of the instantaneous size distributions, with higher rate constants corresponding to larger lnσ values. The form of the particle size distributions changed rapidly during early coarsening and reached a quasi-stable state, different from the theoretical asymptotic distribution, after some time. A linear relationship was found between the experimentally observed instantaneous rate constant and that computed from an earlier model incorporating the effect of particle size distribution. The above results compare favorably with those from prior theoretical modeling and computer simulation studies of the effect of particle size distribution on coarsening, based on the DeHoff communicating neighbor model.

Tensile fracture and shear localization under high loading rate in tungsten alloys

The influence of loading rate and microstructure on the tensile and compressive failure properties of three microstructurally dissimilar tungsten alloys has been investigated. Dynamic tensile fracture properties were characterized through fracture toughness tests performed at a stress intensity loading rate of 106 MPa $\sqrt{{\rm m}}$ s-1, and by tensile testing at a strain rate of 103 s-1. Shear banding phenomena were investigated by means of compression tests performed at strain rates of 5 ×103 s-1. Under rapid loading conditions, nickel-cobalt-tungsten alloys were found to be tougher than nickel-iron-tungsten alloys ; the tungsten/tungsten interface was identified as the governing microstructural factor. Quantitative micromodeling using simple fracture models was found to provide a mean of correlating toughness with microstructures. Compression-induced shear localization was found to be facilitated within systems characterized by either elongated tungsten particles or an adiabatic shear-prone matrix. The shear band width was observed to be proportional to tungsten particle size.

Microstructure effects on dynamic welding of Ag/W contacts

A computer-controlled device was used to simulate contact bounce on closing to evaluate the dynamic weld strength of some silver-tungsten contact materials. The effects of composition, microstructure modifications (such as different tungsten particle sizes), and the thickness of the silver-rich surface layer on the tendency to weld were evaluated. Probability analysis showed that silver has the highest tendency to weld. The addition of tungsten lowers the weld strength, with a minimum in the range of 35-62 vol.% of tungsten. The weld probability is lower than that predicted by a volume-mixing-rule calculation. The tungsten particle size was found to be a very important parameter in determining weld tendency. The optimum tungsten particle size for lowest weld tendency was dependent on the volume percent of tungsten. In general, an increase in tungsten particle size increased welding tendency. Silver enrichment of the contact surface by electrochemical etching also increased weld tendency. The welding tendencies were found to be related to the metallurgical properties of the contacts and the welding process.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Stereological investigation of tungsten-copper pseudoalloys. II. Effect of the production method and of the particle-size distribution of the initial tungsten powder on changes of the microstructure of materials

The authors use transmission and scanning electron microscopy to carry out a stereological analysis of the microstructure of a copper-tungsten pseudoalloy and its dependence on tungsten particle size distribution and on production conditions in sintering and hot molding. Comparative data are presented for different process parameters. The correlations they derive make it possible to control the properties of the alloy by targeted control of their structure in the production process.

Surface composition and barium evaporation rate of “pedigreed” impregnated tungsten dispenser cathodes during accelerated life testing

A study has been made of the surface composition and barium evaporation rate of “pedigreed” impregnated tungsten dispenser cathodes. The effect of air exposure on coated cathodes was examined and was found to have no significant effect on barium evaporation rate although in some cases longer reactivation times were required. No changes in surface topography were apparent following air exposure and reactivation. Life testing was done at 100°C above the typical operating temperature for the cathode, where the typical operating temperature was taken to be 950°C for coated cathodes and 1050°C for uncoated cathodes. The cathodes were examined at different stages of life testing, up to 1200 h. Significant decreases in barium evaporation rates were found after as few as 500 h of life testing. After 1000 h the evaporation rate had decreased more than an order of magnitude. Changes in surface composition were also found. The effects of tungsten particle size, used in manufacture of the billet, on barium evaporation rate were also studied but no correlation was found.

Effect of tungsten particle size on sintered properties of heavy alloys

In the present investigation, the sintering of CuNi (10 wt.%) bonded heavy alloys has been studied using two different particle sizes of tungsten, submicron range (0.74 μm) and micron range (3.03 μm). The sintering temperatures were 1400, 1450 and 1500 °C, and the sintering time was 1 h. Results show that an increase in the sintering temperature and/or the nickel content of the binder, enhances densification. The properties of the heavy alloys reveal an improvement when the fine tungsten powder is used. The results have been discussed on the basis of a solution-reprecipitation mechanism for liquid phase sintering, which correlates well with the spheroid coarsening of the tungsten particles.

The properties of tungsten processed by chemically activated sintering

Two tungsten powders have been treated with small concentrations of sintering activators to provide for enhanced low temperature sintering. The experimental study focused on the determination of the processing effects on properties such as sintered density, grain size, hardness, and strength. Variables in the plan included tungsten particle size, type of activator, amount of activator, compaction pressure, and sintering temperature. The sintered density is found to have a dominant effect on strength and hardness. The various processing variables are analyzed in terms of their effects on density. At high sintered densities, grain growth acts to degrade the strength. Additionally, the nature of the sintering activator influences the fracture strength. In this study optimal strength occurred with a 0.7 μm tungsten powder treated with 0.29 wt pct Ni, sintered at 1200 °C for one hour. The resulting density was 18.21 g/cm3, with aR A hardness of 69 and a transverse rupture strength of 460 MPa.