Density of WHA

Density of WHA (tungsten heavy alloy) is the single most important property that makes WHA the material of choice for a given application.  The selection of a specific composition for a given density application may ultimately be made on the basis of concurrent mechanical property requirements or sintering considerations, unless a specific value of density is critical.  WHAs can approach the density of pure tungsten and DU without the high cost of the former or the licensing and special handling requirements of the latter.

As is true with most mechanical designs, the optimum design is the one that adequately addresses all critical parameters and offers the best compromise of the remaining options.  As the density of WHA is increased, the available ductility decreases.  Alloy density varies according to tungsten content, as can be seen in the following table of Tungsten Products standard alloys.

TP

Alloy*

Composition

(Wt. %)

MIL-T-21014D

Classification

Typical Density

(g/cc)       (lbs/in2)

Magnetic

Permeability (m)

SD170

90W-(3Ni/Fe)

Class 1

17.14         0.619

>6.0

Dens21

90W-(7Ni/Fe)

Class 1

17.20         0.622

1.02-1.05

SD175

92.5W-(3Ni/Fe)

Class 2

17.62         0.637

4.5-5.0

Dens23

92.5W-(7Ni/Fe)

Class 2

17.66         0.638

1.02-1.05

SD180

95W-(3Ni/Fe)

Class 3

18.13         0.655

4.0-4.5

Dens25

95W-(7Ni/Fe)

Class 3

18.16         0.656

1.01-1.02

SD185

97W-(3Ni/Fe)

Class 4

18.57         0.671

2.5-3.0



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Tungsten Alloy EFP Warhead & Shaped Charge Liners

A typical tungsten alloy EFP (explosively formed penetrators) warhead mainly consists of shaped charge liners, high explosive, shells and initiation system. The shaped charge liners and explosive component are usually fixed by pressure screw.

When the tungsten alloy EFP exploding, the shaped charge liners squeeze, roll over and stretch into penetrators whose shape is expected and speed up to 2000m/s under the effect of Chapman-Jouguet pressure. It can damage efficiently to armor target in the range of charge diameter whose bursting height is more than 1000 times. The maximum depth of penetration can reach more than one time charge diameter. So it is possible to attack the weakest top of armor vehicle from far (long) distance.


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Introduction to WHA

Tungsten heavy alloy (WHA) are ideally suited to a wide range of density applications, offering a density approaching that of pure tungsten but without the very costly processing and inherent size and shape limitations of the former.  WHAs are produced by a powder metallurgy (P/M) technique known as liquid phase sintering (LPS), in which completely dense, fully alloyed parts are formed from pressed metal powders at a temperature less than half the melting point of pure tungsten.   While sintered steel and copper alloy parts commonly contain significant residual porosity that may require polymeric infiltrants to seal, sintered WHAs have a nonporous surface.

WHA parts are manufactured from very fine, high purity metal powders – typically tungsten, nickel, and iron.  The blended metal powder is compacted under high pressure (up to 30 ksi) to form a specific shape that is very close to the geometry of the final part.  By utilizing this near net shape forming approach, economy is realized by the elimination of excess material and the time and energy necessary to remove unwanted stock from mill shapes.  Pressed parts are then subjected to high temperature sintering in hydrogen.  As the parts are slowly heated, the hydrogen reduces metal oxides present and provides a clean, active surface on each of the very small metal particles.  As temperature increases further, chemical diffusion takes place between particles.  Neck growth occurs between particles, and surface energy drives pore elimination and part densification.  The pressed part shrinks uniformly, with about 20% linear shrinkage (equating to approximately 50% volumetric shrinkage) being typical.  Once the temperature is sufficiently high to form the liquid phase, any remaining densification occurs very quickly as the alloy assumes a "spheroidized" microstructure by a mechanism know as Ostwald Ripening.  The sintered structure of a common commercial WHA is two-phase, consisting of a linked network of tungsten spheroids contained in the ductile matrix phase.


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Why Use WHAs?

WHAs (tungsten heavy alloys) provide a unique combination of density, mechanical strength, machinability, corrosion resistance, and economy. Consequently, WHAs are widely used for counterweights, inertial masses, radiation shielding, sporting goods, and ordnance products.  These versatile materials provide distinct advantages when compared to alternate high density materials, as seen in the table below.

Material

Density

(g/cc)

Tensile

Strength

Stiffness

Machin-

ability

Toxicity

Radio-

activity

Cost

WHA

17.0-19.0

moderate

high

excellent

low

none

moderate

Lead

11.4 max.

very low

very low

very low

high

none

low

Uranium

18.7-18.9

moderate

medium

special

high

present

high

As can be seen from these data, WHA overcomes the toxicity, deformability, and inferior density of lead and its alloys.  Likewise, it can provide equivalent density to depleted uranium (DU) but without the special machining considerations (necessary due to its pyrophoricity) and licensing requirements for a radioactive substance.  WHA is truly the material of choice for high density applications.  These unique alloys provide the designer with many new freedoms.

There is one special category of density applications in which WHAs should not be used.  For applications in which the service temperature will exceed ~300°C, slight surface oxidation will occur in air.  It is important to note that at service temperatures exceeding ~500°C, WHA strength will fall off rapidly even in a protective atmosphere.  For these special cases, pure tungsten may provide a better option.  If reactive atmospheres are present in combination with elevated temperature, the best choice for very dense materials will be the platinum group metals – but at extremely high cost.


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Introduction to Tungsten

The name "tungsten" is derived from the Swedish term meaning "heavy stone".  Tungsten has been assigned the chemical symbol W after its German name wolfram.  While sometimes regarded as a scarce or exotic metal, its abundance in nature is actually about the same as that of copper.  The largest known tungsten reserves are in mainland China, though plentiful reserves also exist in North America.

Tungsten has the highest melting point (3410°C or 6170°F) of all metals.  The extremely high melting point of pure tungsten makes all the common manufacturing techniques used for metals such as iron impractical.  Specialized methods make possible the processing of pure tungsten into rod, sheet, and wire for a wide variety of high temperature applications including incandescent lamp wire, TIG welding electrodes, and high temperature heat shielding.

Another important industrial property of tungsten is its high density of 19.3 g/cc (0.70 lbs/in3).  In addition to high gravimetric density, its high radiographic density makes it an ideal material for shielding or collimating energetic x- and g-radiation.  For such applications, tungsten is commonly alloyed in order to circumvent the extremely high processing temperatures that would otherwise be required to melt and cast the pure metal. 


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