Molybdenum and tungsten are extraordinarily resistant to heat. Both have very high melting points relative to common metals. Low thermal expansion coefficients provide exceptional material stability and facilitate close-tolerance machining. Both metals are highly wear- and corrosion-resistant. There’s significant similarity when comparing molybdenum vs. tungsten, but a more detailed review of their differences and uses should provide a better understanding of these heavy (refractory) metals.
Tungsten: A Heavy Metal That Can Take the Heat
Tungsten is very dense and is ideal in manufacturing applications where weight is required in small or constrained spaces such as counterbalances in crankshafts. Even though tungsten is exceptionally hard, it is extremely ductile. With a low coefficient of thermal expansion and an exceptional ability to handle heat, it can be drawn into fine wires or ribbons used as filaments in lights and electronics. Tungsten is also ideal for the manufacturing of high-temperature components for furnaces. Tungsten electrodes are an essential component of the Tungsten Inert Gas (TIG) welding process.
Machining Tungsten Heavy Alloys
In most production applications, machine shops and fabricators typically use tungsten heavy alloys (WHA) because of their lower cost and superior machinability compared to pure tungsten. These alloys contain between 89% and 97% pure tungsten with the balance being nickel and copper or nickel and iron, depending on the alloy grade. The alloying elements improve machinability. Tungsten’s low thermal expansion allows very close machining tolerances for these alloys. WHAs are used in the manufacturing of aircraft counterbalances, ballast, racing weights, vibration reduction, radiation shielding, and military ordnance. One of the most-frequently-used WHAs is Densalloy.
Tungsten Densalloy is 92.5% tungsten, 5.25% nickel, and 2.25% iron. This alloy is easily milled, turned, and drilled using carbide cutting and shaping tools. Coolant should be used for roughing and Densalloy can tolerate up to 1/8” depth cuts. For final finishing—up to .030”—coolant isn’t required. Densalloy can be cold worked by swaging.
One disadvantage of Densalloy is its poor weldability. And even though the alloy provides good corrosion resistance, if the end-use environment is extremely humid, consider finishing the component with nickel or cadmium plating, or acrylic or epoxy paints.
Molybdenum: Lower Melting Point and Density
Molybdenum has a lower melting point and density than tungsten. Its high thermal conductivity, low coefficients of thermal expansion, and strength make it ideal for use in semiconductor base plates, industrial motors, aerospace, and defense applications. Molybdenum is also used in steel alloys to improve their strength and electrical conductivity, as well as their resistance to corrosion and wear.
Molybdenum is highly machinable, comparable to stainless steel when using certain techniques. It can be machined with conventional equipment, but cutting tools need to be sharp, as dull tools will result in edge break. Because molybdenum is abrasive, expect cutting tools to wear out more quickly compared to machining iron or steel. Similar to tungsten alloys, rough turning of molybdenum is acceptable with depths of cut up to 1/8″. Finishing work should be done with a depth cut of 0.005” to 0.015”.
When welding molybdenum, begin with a chemical cleaning to remove impurities. To achieve the best welding results, perform in a vacuum as opposed to an open atmosphere—where you risk the material absorbing nitrogen and oxygen. Absorption can lead to oxidation and brittleness.
Molybdenum vs Tungsten: Cost-Efficient Sources
With such close similarities in their properties, whether or not to use tungsten alloys or molybdenum for a component is determined by design engineers, not machinists. Regardless of the choice of materials, either of these heavy metals can be machined without the need for special equipment or techniques.
Perhaps the greatest challenge for many machine shops is finding a reliable supplier who offers these metals at competitive prices. As an alternative to purchasing tungsten alloys or molybdenum sourced directly from the manufacturer, consider suppliers who specialize in verified remnants. In comparison to new materials, molybdenum and tungsten alloy remnants can be purchased at a lower price point. The best suppliers will work with you to closely match the remnant size to the dimensions of the parts you’ll be machining. With precision measurements, you’ll end up with less waste.
Whether it’s a long-term production run or a small order for a special project, purchasing verified tungsten or molybdenum remnants can often be the most cost-efficient approach to sourcing these heavy metals.