The Fabricator - December 2015

By Dale Harper and Gary Tydings

Most metals can be melted and “atomized” by liquid (normally water) or gas to form etal powders. Atomization is the most common method for producing metal and pre-alloyed powders, or powders consisting of two or more elements alloyed in the powder manufacturing process. Powder materials include iron and steel, stainless and nickel, tool steel, aluminum and copper, and their many alloys. Iron and steel account for 80 percent by weight of all metal powders produced annually.

Atomization accounts for nearly 70 percent by weight of all metal powders produced in North America. It is the main process for powder production because of its high production rates and economies of scale. Atomization also is the only way to produce pre-alloyed powders.

Producing metal powders involves melting raw
materials in a crucible (induction melting furnace or
electric arc being most common) and then pouring
liquid metal into a vessel through one or several ori-
fices (see Figures 1 and 2). When the melt stream en-
ters the vessel, the liquid metal is atomized by either
water or gas, forming a spray of solid-state particles
that can be irregular, pear, or spherical in shape.

Atomizers are sized by the melt capacity of the furnace, which may range from production batches of 250 kilograms to 100 tons (see Figure 3). Powder production generally is optimized for its intended particle size distribution. Particle sizes range from submicron to 600 microns, depending on the finished product specification. After the powders are processed, they are classified, inspected, packaged, and delivered.

Markets for Powdered Metal

Powdered metals are used in many areas of manufacturing, including business machines, the electronics sector, and telecommunications. Generally, though, powdered metal technology falls into three distinct areas.

The first is powder metallurgy. To make P/M parts, hot-forged and hot-isostatic-pressure (HIP) technologies press and sinter the powdered metal to net or near-net shapes. This process mainly uses iron-based alloys.

The second is additive manufacturing. Working from a 3-D model, electron beam and laser sintering technology build up A/M parts layer by layer. Additive manufacturing is used for small, complex, or customized precision parts. Alloys include maraging steels, stainless steels, nickel-based materials, cobalt-chrome, titanium, and aluminum.

The third technology area for powdered metals involves thermal spray and weld overlay coatings, surface treatment and enhancement methods processed through specialized equipment. The metal powders used here mainly include nickel-, cobalt-, and copper-based alloys.

P/M is by far the largest market for metal powders. It represents about 80 percent of the total metal powder tonnage consumed in North America. Conversely, the A/M market is a developing one.

For those in welding and metal fabrication, though, the third area for metal powders—thermal spray and weld overlay—is probably the most familiar. It’s a smaller market, but it’s also one of the most diverse.

Thermal Spray Applications

Thermal spray is a surfacing technology in which metal powder is used to add to or modify the surface of a component to enhance its design and application functionality. It is recognized by the American Welding Society (AWS) as a group of processes in which finely divided metallic or nonmetallic powders are melted and accelerated onto the surface of a part to form a coating. The molten (or semimol-ten) particles of sprayed material plastically deform upon impact with the prepared and roughened surface, forming a mechanical bond.

According to a 2014 report from TechNavio Insights, the thermal spray market is shared by North

America ( 33. 2 percent), followed by Europe and the
Middle East ( 31 percent), and the rest of the world
( 35. 8 percent). The report breaks down the market
into coating service providers ( 75 percent), materi-
als ( 20 percent), and equipment-makers ( 5 percent).
Aerospace precision parts coating is the largest
application for thermal spray. Coatings protect the
aircraft engine turbine blades and combustion lin-
ers from extreme temperatures and pressures.
Meanwhile, coating demand for biomedical and
medical instruments is expected to grow significantly
over the next seven years. Thermal spray coatings
improve wear resistance and boost biocompatibility
of medical implant prosthetics and dental implants.

In the industrial markets, significant growth has come from companies using thermal spray coatings as an alternative to hard-chrome plating, which has regulatory restrictions and environmental implications.