Ultrashort-time liquid phase sintering of high-performance fine-grain tungsten heavy alloys by laser additive manufacturing

Liquid phase sintering(LPS)is a proven technique for preparing large-size tungsten heavy alloys(WHAs).However,for densification,this processing requires that the matrix of WHAs keeps melting for a long time,which simultaneously causes W grain coarsening that degenerates the performance.This work develops a novel ultrashort-time LPS method to form bulk high-performance fine-grain WHAs based on the principle of laser additive manufacturing(LAM).During LAM,the high-entropy alloy matrix(Al_(0.5)Cr_(0.9)FeNi_(2.5)V_(0.2))and W powders were fed simultaneously but only the matrix was melted by laser and most W particles remained solid,and the melted matrix rapidly solidified with laser moving away,producing an ultrashort-time LPS processing in the melt pool,i.e.,laser ultrashort-time liquid phase sintering(LULPS).The extreme short dwell time in liquid(-1/10,000 of conventional LPS)can effectively suppress W grain growth,obtaining a small size of 1/3 of the size in LPS WHAs.Meanwhile,strong convection in the melt pool of LULPS enables a nearly full densification in such a short sintering time.Compared with LPS WHAs,the LULPS fine-grain WHAs present a 42%higher yield strength,as well as an enhanced susceptibility to adiabatic shear banding(ASB)that is important for strong armor-piercing capability,indicating that LULPS can be a promising pathway for forming high-performance WHAs that surpass those prepared by conventional LPS.

LOW TEMPERATURE THERMAL DEBINDING BEHAVIOR OF WAX-BASED MULTI-COMPONENT BINDER FOR TUNGSTEN HEAVY ALLOY

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

Ultrafine grain tungsten heavy alloys (WHAs) were successfully produced from the nano-crystalline 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 ℃,and the tungsten grain size is about 871 nm.At 1 000 ℃-1 200 ℃,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 ℃) and bending strength (987.16 MPa at 1 100 ℃),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.

COMPUTER NUMERICAL SIMULATION OF MECHANICAL PROPERTIES OF TUNGSTEN HEAVY ALLOYS

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Fabrication and Characterization of Tungsten Heavy Alloys Using Chemical Reduction and Mechanical Alloying Methods

A novel reduction technique has been developed to synthesize nano-sized tungsten heavy alloys powders and compared with the same powders processed by mechanical alloying technique. In the first method, nano-sized tungsten heavy alloys powders have been obtained by reduction of precursors obtained by spray drying of several appropriate aqueous solutions, which were made from salts containing tungsten, cobalt, and nickel. By adjusting the stoichiometry of the component of the solutions, it is possible to obtain the desired chemical composition of the tungsten heavy alloys powders. In the second method, highly pure elemental powders of tungsten heavy alloys have been mechanically alloyed in a tumbler ball mill for different milling time. The investigated tungsten heavy alloy powders with the composition (95%W-3.5%Ni-1.5%Fe), (93%W-4.5%Ni-1.0%Fe-1.5%Co), and (90%W-6%Ni-4%Cu) have been prepared using both methods. The prepared powders have been compacted at 70 bar (200 MPa) and sintered in vacuum furnace at 1400℃. Vacuum sintering was carried out to achieve full densification of the produced tungsten heavy alloys. The investigated materials were going to be evaluated the physical and mechanical properties of the sintered parts such as density;electrical conductivity, hardness, and transverse rupture strength. The results reveal that, the grain size of alloys fabricated by chemical reduction technique (53.1 - 63.8 nm) is smaller than that fabricated by mechanical alloying technique (56.4 - 71.4 nm).

Application of the Backpropagation Neural Network Method in Designing Tungsten Heavy Alloy

The model describing the dependence of the mechanical properties on the chemical composition and as deformation techniques of tungsten heavy alloy is established by the method of improved the backpropagation neural network. The mechanical properties＇ parameters of tungsten alloy and deformation techniques for tungsten alloy are used as the inputs. The chemical composition and deformation amount of tungsten alloy are used as the outputs. Then they are used for training the neural network. At the same time, the optimal number of the hidden neurons is obtained through the experiential equations, and the varied step learning method is adopted to ensure the stability of the training process. According to the requirements for mechanical properties, the chemical composition and the deformation condition for tungsten heavy alloy can be designed by this artificial neural network system.

Adiabatic shear banding of hot-extruded tungsten heavy alloy under cryogenic temperature

The effect of cryogenic temperature on adiabatic shear banding (ASBing) of tungsten heavy alloy (WHA) processed by hot-hydrostatic ex-trusion was investigated.Results show that,when the initial temperature is decreased,the dynamic flow stress,the critical failure time,and the dynamic failure energy of specimens show an increasing tendency,while the susceptibility to ASB of WHA shows a decreasing tendency,which is characterized by decreased shear strain and increased width of shear bands.Microstructure analysis shows that the number of mi-crocracks within ASB exhibits an increasing tendency with decreased initial temperature,and the dynamic recrystallization (DRX) process within ASB is evidently suppressed at the lower temperature.As a result of the lower temperature,the motion and rearrangement of disloca-tion are effectively suppressed,which is mainly responsible for the incomplete DRX process within ASB and decreases susceptibility to ASB.

Microstructure and properties of liquid-phase sintered tungsten heavy alloys by using ultra-fine tungsten powders

The microstructure and properties of liquid-phase sintered 93W-4.9Ni-2.1Fe tungsten heavy alloys using ultra-fine tungsten powders (medium particle size of 700 nm) and original tungsten powders (medium particle size of 3 μm) were investigated respectively. Commercial tungsten powders (original tungsten powders) were mechanically milled in a high-energy attritor mill for 35 h. Ultra-fine tungsten powders and commercial Ni, Fe powders were consolidated into green compacts by using CIP method and liquid-phase sintering at 1 465 ℃ for 30 min in the dissociated ammonia atmosphere. Liquid-phase sintered tungsten heavy alloys using ultra-fine tungsten powders exhibit full densification (above 99% in relative density) and higher strength and elongation compared with conventional liquid-phase sintered alloys using original tungsten powders due to lower sintering temperature at 1 465 ℃ and short sintering time. The mechanical properties of sintered tungsten heavy alloy are found to be mainly dependent on the particles size of raw tungsten powders and liquid-phase sintering temperature.

Influences of sub-micrometer Ta and Co dopants on microstructure and properties of tungsten heavy alloys

Tungsten heavy alloys are aggregates of particles of tungsten bonded with Ni/Fe or Ni/Cu via liquid-phase sintering. The sub-micrometer Ta Co powder was added to this aggregate to strengthen the bonding phase. It is found that the main fr acture pattern of the alloys is cleavage of tungsten grains and ductile rupture of bond phase,leading to improved tensile strength and elongation. Dopant Ta ca n act as grain size inhibitor in tungsten heavy alloys.

Grain refinement of W-Ni-Fe heavy alloys by tantalum element adding

90W-7Ni-3Fe and (90-x)W-xTa-7Ni-3Fe (x=1,3,5,7,10) specimens were attained by liquid phase sintering. A model describing the process of liquid forming and spreading was proposed to point out the differences between alloys doped with tantalum and traditional tungsten heavy alloys. Tantalum priority of entering matrix and a relative high solubility in liquid matrix depress tungsten solubility in liquid matrix, which decreases kinetic rate constant K and consequently results in the reduction of W grain size. The grain refinement is influenced by Ta content and becomes more obvious when Ta content is over 5%. The sample with less than 3%Ta has dominant W and matrix phases. While besides W and matrix phases, intermetallic phases emerge in 85W-5Tai-7Ni-3Fe sample. Ta is superfluous and forms a new tantalum phase when more than 7% Ta is added into alloys.

91W-Ni-Fe-Co合金的常温、低温力学性能及微观组织

通过粉末冶金方法制备91W-5.5Ni-3.3Fe-0.2Co和91W-5.5Ni-2.5Fe-1Co合金。采用硬度测试、室温低温拉伸试验、扫描电镜(SEM)和能谱(EDS)等方法,研究合金的微观组织演变和室温、低温力学行为。研究结果表明:在室温条件下,Co质量分数为1%的合金的力学性能更优异,抗拉强度和伸长率分别为1 400 MPa和11.7%,拉伸断裂行为以钨解理断裂和黏结相撕裂为主。微观组织上,相比于低Co合金,高Co合金的晶粒更加细小,钨-钨连续度降低,黏结相体积分数提高,黏结相中钨溶解度增大,合金性能得到优化;随着温度降低,2种合金的强度提高,伸长率下降;在-60℃时,高Co合金的伸长率相比于低Co合金的伸长率提高了20%,同时,合金的断裂模式仍以韧性相撕裂为主,而低Co合金在低于-20℃时,断裂由钨解理断裂主导。在一定范围内,Co质量分数增加能抑制杂质元素在晶界处偏聚,提高钨晶粒塑性变形程度,降低低温脆性。

旋转锻造加工高比重W-Ni-Fe合金的退火工艺研究

使用粉末冶金工艺制备Ni/Fe为7∶3的W-Ni-Fe高比重钨合金,在使用旋转锻造方法进行形变强化后进行热处理。结果表明,锻造态W-Ni-Fe合金经过一定温度的锻后退火,强度有所提高,而塑性下降。样品的强度在一定的退火温度和保温时间范围内能得到提升,更高温度和更高时间的保温会使样品强度下降。不同钨含量的W-Ni-Fe合金,强度提升最大的退火温度和时间条件不同。SEM图像显示,退火前样品钨颗粒因旋转锻造变形呈现长条状织构,退火后钨颗粒的形状无明显变化。这些现象说明,锻造变形的合金样品经过锻后退火发生应变时效现象,使强度得到提升。研究明确了不同钨含量的W-Ni-Fe合金最佳的锻后退火工艺参数。

Co含量及烧结温度对97W-Ni-Fe合金组织和性能的影响

钨基高比重合金因具有高密度、高强度、良好的塑性与韧性、优异的耐腐蚀性等综合性能,被广泛应用于航空航天、兵器、机械等各领域。本研究采用金属粉末注射成形技术制备97W-Ni-Fe合金,研究了不同Co含量和烧结温度对97W-Ni-Fe合金微观组织、密度、抗拉强度、延伸率和硬度的影响。结果表明,添加适量Co能够改善97W-Ni-Fe合金中钨颗粒与粘结相的润湿性,起到活化烧结的作用,降低烧结温度,提高合金相对密度;Co对合金的相组成影响不大,添加Co和未添加Co的两种合金相组成基本一致,均由W相和(Fe,Ni,Co)固溶体组成,没有金属间化合物出现。当Co含量为0.2%(质量分数)时,在氢气气氛下,烧结温度为1495℃、保温1 h,制备的97W-Ni-Fe合金具有最优异的性能,其相对密度为99.1%、抗拉强度为938 MPa、延伸率为11.3%和洛氏硬度为29.5。

Morphology and microstructure characterization of 95W-3.5Ni-1.5Fe powder prepared by mechanical alloying

The mechanism of mechanical solid-state reactions for formation of tungsten heavy alloy powder was discussed. A highenergy ball mill operating at room temperature was used for preparing tungsten heavy alloy powders, starting from elemental tungsten （W）, nickel （Ni）, and iron （Fe） powders. X-ray diffraction （XRD）, particle size analyzer, scanning electron microscopy （SEM）, and transmission electron microscopy （TEM） were used to follow the progress of the mechanical solid-state reaction of W, Ni, and Fe powders. These morphological studies revealed three stages in the milling process. In the first stage, the particle deformation changes the irregular structure of the as-received powder particles to flattened morphology, and the average particle size increases. In the second stage, the powder is sufficiently deformed and the tendency to fracture predominates over welding, and the particle size decreases. With continuous milling, the system reaches steady state, and relatively small and uniform particle size distribution is obtained after 20 h of milling.

Effect of microstructural parameters on the properties of W-Ni-Fe alloys

The aim of this research was to examine the effect of microstructural parameters on the tensile properties of different compositions of tungsten heavy alloys. The microstructural parameters （grain size, connectivity, contiguity, and solid volume fraction） were measured and were found to have a significant effect on the tensile properties of tungsten-based heavy alloys. The microstructural parameters of W-Ni-Fe alloys are sufficiently different to present a range of mechanical properties. It is concluded that the mechanical properties of tungsten heavy alloys largely depend on the microstructural parameters and their ductility is particularly harmed when grains are contiguous.

Experimental study on WFeNiMo high-entropy alloy projectile penetrating semi-infinite steel target

The appearance of high-entropy alloys (HEAs) makes it possible for a material to possess both high strength and high ductility. It is with great potential to apply HEAs under extreme conditions such as in the penetration process. In this paper, experiments of WFeNiMo HEA and tungsten heavy alloy (WHA) projectiles penetrating medium-carbon steel were conducted by using the ballistic gun and two-stage light-gas gun that can accelerate projectiles to impact velocities ranging from 1162 m/s to 2130 m/s. Depth of penetration (DOP) at elevated impact velocities of HEA and WHA projectiles were obtained firstly. Combined with the macroscopic and microscopic analysis of the residual projectiles, the transition of the penetration mode of the WFeNiMo HEA projectile was identified systemically. The experimental results indicated that the penetration mode of the HEA projectile changes from self-sharpening to mushrooming with the increase of impact velocity, while for the WHA projectile, the penetration mode is always mushrooming. The microstructure of the residual HEA projectiles showed that the phases tangle with each other and the morphology of the microstructure of the phases differs in the two penetration modes. Besides, the evolution of shear bands and fractures varies in the two modes. The evolution of the microstructure of HEAs causes the sharp-pointed nose to disappear and the HEA projectile ultimately becomes blunt as the impact velocity increases.

Injection molding of nano structured 90W-7Ni-3Fe alloy powder prepared by high energy ball milling

Blended elemental 90W 7Ni 3Fe (mass fraction, %) powder was mechanically alloyed in a planetary ball mill. Nano crystalline grains were obtained after 10 h milling. The nano structured powder was processed to full density by metal injection molding approach. Compacts from the optimal powder binder mixture were studied for molding and sintering behaviors. Milling significantly increases the maximum powder loading and homogeneity of the feedstock, and enhances the sintering densification process. When solid state sintered at 1 350～1 450 ℃, the alloy shows very fine grains (～3 μm), high tensile strength (>1 130 MPa) and almost no distortion. [

Synthesis and characteristics of W-Ni-Fe nano-composite powders prepared by mechanical alloying

The mixture of 90W 7Ni 3Fe(mass fraction, %) powders was milled in a planetary ball mill. Its structure changed during milling, the surface characteristics and thermal stability of the milled powders were studied with X ray diffraction(XRD), Brunaure Emmett Teller (BET) nitrogen adsorption technique and differential thermal analysis(DTA). The results show that high energy ball milling leads to the formation of composite powders with amorphous binder phase and supersaturated W(Ni, Fe) nano crystalline grains in which great lattice distortion exists. The crystallization temperature of the amorphous binder phase during heating decreases with milling time. The specific surface area and the pore size of the powder mixtures decreases with milling time due to agglomeration and welding between particles.

Aging Behaviors of W-Ni-Fe Ternary Alloys with High Ni-to-Fe Ratios

The hardness variation of two kinds of alloys with 36 wt pct W content and 7/3, 9/1 Ni-to-Fe ratios during strain aging at 800℃ was studied. The microstructures of the aged alloys were analyzed by X-ray diffraction and TEM. The results show that the strain aging hardness of W-Ni-Fe ternary alloy with 7/3 Ni-to-Fe ratio decreases monotonically with the increase of aging time. Under the same conditions, the hardness of 9/1 Ni-to-Fe ratio alloy decreases in the initial aging stage, but then increases as aging process goes on. X ray diffraction and TEM analysis show that there is not any precipitation depositing from the alloy with 7/3 Ni-to-Fe ratio during aging. The monotonic decrease in hardness of this alloy during aging process results from the recovery, recrystallization and solid solubility declining. In the alloy of 9/1 Ni-to-Fe ratio, the fine β phase precipitates dispersively during aging which hardens the alloy. The two different kinds of mechanisms (the softening one and the hardening one) decide the hardness variation of the alloy with 9/1 Ni-to-Fe ratio mentioned above.

Mechanical properties of tungsten heavy alloy and damage behaviors after hypervelocity impact

93W-4.5Ni-l.5Fe-lCo （W） was prepared by powder metallurgic method, and then dynamic mechanical properties of this material were tested at high temperature by means of high-temperature split Hopkinson pressure bar （SHPB）. The results show that the material possesses high- dynamic mechanical properties, significant temperature effects, and strain hardening behaviors. Used two-stage light gas gun, the penetration test of 93W projectile was finished. After the completion of the test, through the microstructure observation of the residual 93W projectiles with the aid of scanning electron microscopy （SEM） and transmission electron microscopy （TEM）, it can be found that there are obvious signs of hot melt existing on the surface of the projectile, a lot of adiabatic shear bands inside the projectile, and microcracks exist at the end of adiabatic shear bands. The test results show that adiabatic shear is the main form to cause the projectile failure and it is the emergence of the adiabatic shearing phenomenon that makes 93W display good self-sharpening property in the process of hypervelocity penetration. At the same time, the results of TEM observation show that there are high- density dislocations at the interface between W and Ni-Fe- Co-based alloy inside the 93W.