Tungsten Heavy Alloys

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Introduction to Tungsten Properties

The name "tungsten" derives from the Swedish term meaning "heavy stone." Tungsten has been assigned the chemical symbol W after its German name wolfram. Among all metals, tungsten possesses the highest melting point at 3410°C (6170°F), making conventional manufacturing techniques used for common metals impractical for pure tungsten processing.

Specialized methods have been developed to process pure tungsten into rod, sheet, and wire for various high-temperature applications, including incandescent lamp filaments, TIG welding electrodes, and high-temperature heat shielding components.

Another remarkable property of tungsten is its exceptionally high density of 19.3 g/cc (0.70 lbs/in³). This high gravimetric density, combined with its high radiographic density, makes tungsten an ideal material for shielding or collimating energetic x-ray and gamma radiation. For such applications, tungsten is commonly alloyed to avoid the extremely high processing temperatures that would otherwise be required to melt and cast the pure metal.

Manufacturing Process of Tungsten Heavy Alloys

Tungsten heavy alloys (WHAs) are produced through a powder metallurgy (P/M) technique known as liquid phase sintering (LPS). This process creates completely dense, fully alloyed parts from pressed metal powders at temperatures less than half the melting point of pure tungsten. Unlike sintered steel and copper alloy parts that often contain significant residual porosity requiring 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 manufacturing process follows these key steps:

• The blended metal powder is compacted under high pressure (up to 30 ksi) to form a shape close to the final part geometry.
• This near-net-shape forming approach eliminates excess material and reduces the time and energy needed to remove unwanted stock.
• Pressed parts undergo high-temperature sintering in hydrogen, which reduces metal oxides and provides a clean, active surface on each small metal particle.
• As temperature increases, chemical diffusion occurs between particles, with neck growth between particles driving pore elimination and part densification.
• The pressed section shrinks uniformly, with approximately 20% linear shrinkage (about 50% volumetric shrinkage) being typical.
• Once the temperature is high enough to form the liquid phase, remaining densification occurs rapidly as the alloy assumes a "spheroidized" microstructure through a mechanism known as Ostwald Ripening.

Figure 1: WHA "spheroidized" typical microstructure of tungsten heavy alloys with tungsten spheroids in a metal matrix

The sintered structure of common commercial WHAs is two-phase, consisting of a linked network of tungsten spheroids contained in a ductile matrix phase. The spheroidized microstructure shown in Figure 1 is typical for most commercial WHA products. The rounded phase (approximately 30-60 μm in diameter) is essentially pure tungsten, surrounded by a metallic nickel-iron binder phase containing some dissolved tungsten. This structure provides maximum mechanical properties for a given alloy composition.

Properties and Advantages of Tungsten Heavy Alloys

WHAs provide a unique combination of density, mechanical strength, machinability, corrosion resistance, and economic value. They offer excellent radiation resistance, thermal and electrical conductivity, and corrosion resistance while remaining machinable. These versatile materials provide distinct advantages when compared to alternative high-density materials, as shown in Table 1.

Table 1. Compared properties of high-density materials - Table comparing WHA, Lead, and Uranium across properties including density, strength, machinability, toxicity, radioactivity, and cost

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	max 11.4	very low	very low	very low	high	none	low
Uranium	18.7-18.9	moderate	medium	special	high	present	high

As evident from this data, WHAs overcome the toxicity, deformability, and inferior density of lead and its alloys. They can provide equivalent density to depleted uranium (DU) without the special machining considerations (necessary due to DU's pyrophoricity) and licensing requirements for a radioactive substance. WHA is therefore truly the material of choice for high-density applications, providing designers with extensive design flexibility.

Applications and Grade Selection

Tungsten heavy alloys are widely used for counterweights, inertial masses, radiation shielding, sporting goods, and ordnance products. Tables 2 and 3 show the field of application and properties of various tungsten heavy alloy grades.

Table 2. Tungsten Heavy Alloy Grade Application Guide

Grade	Application
HA170	HA 170 is the most ductile and readily machinable grade in the tungsten heavy alloy family. Common application areas include counterbalancing and inertial damping weights for the aviation and aerospace industries, crankshafts and chassis weights for auto racing, bucking bars for rivet setting, and radiation shielding.
HA175	HA175 is commonly used to produce chatter-resistant boring bars, grinding quills, and tool shanks as well as radiation shielding components.
HA180	HA180 is often applied where size is a factor in the placement of balance or ballast weights. Other applications include radiation shields and collimators of x-ray or gamma ray beams.
HA185	The densest of the the Ni-Fe binder alloys, HA185 is the preferred grade for radiation shielding in the medical imaging industry.
HA170C	Employing copper as a substitute for iron in the binder phase, HA170C is nonmagnetic and ideal for radiation shielding where the shield is in close proximity to a magnetic field.
HA180C	HA180C is a denser version of HA170C that offers somewhat greater shielding efficiency in situations where large shields in a nonmagnetic alloy are needed.

Table 3. Tungsten Heavy Alloy Grade Properties

FCC Grade	HA170	HA175	HA180	HA185	HA170C	HA180C
Matrix (wt.%)	90.0%W	92.5%W	95.0%W	97.0%W	90.0%W	95.0%W
Binder	10.0%Ni-Fe	7.5%Ni-Fe	5.0%Ni-Fe	3.0%Ni-Fe	10.0%Ni-Fe	5.0%Ni-Fe
MIL-T-21014RevD	Class 1	Class 2	Class 3	Class 4	Class 1	Class 3
SAE-AMS-T-21014	Class 1	Class 2	Class 3	Class 4	Class 1	Class 3
ASTM B777-07	Class 1	Class 2	Class 3	Class 4	Class 1	Class 3
AMS 7725C	7725C	-	-	-	-	-
Nominal Density (g/cm3)	17.0	17.5	18.0	18.5	17.0	18.0
Nominal Density (lb/in3)	0.614	0.632	0.650	0.668	0.614	0.650
Typical Hardness (Rc)	26	26	28	30	26	28
Ultimate Tensile Strength-Min (psi)	110,000	110,000	105,000	100,000	94,000	94,000
0.2% Offset Yield Strength-Min (psi)	75,000	75,000	75,000	75,000	75,000	75,000
Magnetic Characteristics	Slightly magnetic	Slightly magnetic	Slightly magnetic	Slightly magnetic	Nonmagnetic	Nonmagnetic

Each grade offers specific advantages for particular applications:

• HA170 provides maximum ductility and machinability, making it ideal for aviation and aerospace counterweights
• HA185 offers the highest density among nickel-iron binder alloys, preferred for medical imaging radiation shielding
• Copper-containing grades (HA170C, HA180C) provide non-magnetic alternatives for specialized shielding applications

Conclusion

Tungsten heavy alloys represent an optimal solution for high-density material requirements across numerous industries. Their unique combination of physical and mechanical properties, coupled with relatively straightforward manufacturing processes, positions WHAs as superior alternatives to both lead and depleted uranium for many technical applications. The variety of available grades allows engineers to select the precise formulation needed for specific performance requirements, from radiation shielding to precision counterweights and beyond.