Powder metals press new frontiers

Although manufacturers have sintered powder metal (PM) for decades, many engineers still consider the process a poor cousin to other methods of producing metal parts, such as cutting and casting. However, powder producers are investing richly, anticipating a wealth of new applications.

For instance, Hoeganaes Corp., Riverton, NJ, is expanding its high-alloy facility so it will produce more stainless-steel powder than currently supplied to North America by all manufacturers combined, according to Howard Kopech, the firm's business unit manager. Why the bright future? Kopech attributes much of it to first-tier auto suppliers who are beginning to favor PM stainless-steel parts for exhaust systems.

"Governmental clean-air standards are causing exhaust-manifold makers to re-engineer their systems so exhaust temperature goes up," Kopech explains. "At 900F, carbon-steel is OK, but at 1,200 to 1,600F, you need stainless steel. Plus, Detroit is talking about ten-year, 100,000-mile warranties that cover everything, including the exhaust system. That says 'stainless,' too."

Although the U.S. is the world leader when it comes to auto emissions standards, legislative mandates for cleaner air can be expected in most industrialized nations. Positioning for increased powder stainless-steel demand, then, becomes a matter of timing.

The European Powder Metallurgy Association (EPMA), Shrewsbury, U.K., regards educating industry about PM as one of its duties. In conjunction with the Powder Technology Program of the European Union, the EPMA defines the appropriate uses for the PM its members produce (see chart).

A 1993 EPMA survey shows that nearly 70% of all European PM parts were earmarked for the automotive industry. Nevertheless, the organization notes recent efforts among powder producers and parts makers to develop a greater range of PM engineering components. Many of these components consist of highly alloyed metals (superalloys) and other high-strength materials, such as titanium.

Bryce Clark, director of the Cobalt Development Institute, Wickford, U.K., adds that increased interest in hard-metal and superalloy powders creates a greater demand for cobalt as a binder and alloying material. "The hard-material industry depends on production of fine, pure cobalt powders," Clark says.

Cobalt--whose name is derived from the goblins (German: kobold) that supposedly tainted silver with a blue cast--has a number of desirable properties that favor its use for PM parts. Among these: a high melting point (2,719.4F) and high-temperature strength. Clark also notes that cobalt aids sintering, as it forms an eutectic state at normal sintering temperatures. This liquid phase pulls a sintered part together by surface tension, eliminating gaps.

Specialty Products, Edmonton, Alberta, Canada, produces a fine cobalt powder intended for producing sintered tungsten-carbide parts and metal-bonded diamond tool components, such as drills. For these applications, the firm uses a hydrometallurgical autoclave process to produce loosely agglomerated, spheroidal crystallites with narrow particles and high surface area. The powders provide uniform distribution in blends and are suited for sintering at lower temperatures.

Steeling for the future. So-called high-speed steels are widely regarded as the most promising growth area for powder metallurgy. Donald White, executive director of the Metal Powder Industries Federation, Princeton, NJ, told engineers at the recent '96 World Congress on Powder Metallurgy & Particulate Materials that shipments of iron powder are growing at about 5% per year, compared with 2% growth for shipments of all powders. The automotive, aerospace, electronics, and manufacturing tool industries are largely responsible for this demand.

As a result, international producers of metal powder are preparing to meet this growing demand. From Great Britain to China, manufacturers are designing more ambitious, high-performance parts that take advantage of powder metallurgy. PM producers, then, are responding with greater quantities of high-alloy powders.

In one such endeavor, the Powder Metallurgy Division of Mannesmann Demag AG, Monchengladbach, Germany, recently completed a new melting and atomizing unit to produce powder that is alloyed prior to atomization. This facility turns out the company's MPS brands of prealloyed steel powders, which have varying amounts of nickel, molybdenum, and other alloying elements. Mannesmann acquires its metals in the form of scrap ingots, tested for purity prior to melt. The facility can process up to 5,000 tons of liquid steel annually.

"Prealloyed steel powders have more homogeneous microstructures than diffusion-alloyed grades," explains Klaus Vossen, Mannesmann's sales manager for iron and steel powders. "This makes them well suited to applications where the highest strength and densities are required--synchronizer hubs and sleeves, clutch hubs, and gears."

Powder metallurgy enables producers to create exactly crafted powders with properties targeted for specific applications. "In some cases, it achieves results that cannot be attained using conventional metallurgy," Vossen says.

For example, phosphorous adds qualities of strength and rust resistance when alloyed with iron to make steel. However, such steel is brittle due to separation of the elements when the melt solidifies. Mannesmann has developed a diffusion-alloyed iron-phosphorous powder that, when sintered, results in parts without brittleness because there is no melt. Furthermore, because the powder is alloyed by diffusion, customers can specify phosphorous content prior to packaging, or request that other materials such as copper or graphite be added.

Diffusion-alloying of pure elements allows very fine control of powder properties. Mannesmann's Ultrapac line of diffusion-alloyed steel powders contain amounts of copper, nickel, molybdenum, and graphite mixed with the iron tailored for specific applications.

Powder makers who serve the sintering industry (rather than, say, the chemical or battery industries, which are also large consumers of powder) formulate their product to take various aspects of the sintering process into account. For example, Powdrex, Ltd., Tonbridge, U.K., markets water-atomized, high-speed steel powders based on a patented process to reduce oxygen content. The softened particles have irregular shapes to aid in compression, which provides sufficient pre-sintering ("green") strength for pressed parts to be handled.

Kenneth Mingard, technical manager at Powdrex's Munday Works, says the company's M-series steel powders contain molybdenum, tungsten, chromium, vanadium, and, in some grades, cobalt for more wear resistance and high-temperature performance. Applications depend on how the powders are sintered. "M-series powders that are vacuum-sintered can be used for cutting tools and other abrasive wear applications," Mingard notes. "As a gas-sintered material, it is suited for applications requiring high wear resistance and reasonable structural strength."

Powdrex will custom formulate its high-speed steel powders. Vacuum-sintering of M-series powders to full densities generally requires temperatures of 2,264F. If a customer's furnace cannot achieve this, Powdrex will add higher carbon and silicon contents to compensate for the lower temperatures.

Powder metallurgy processes can be used to produce the large steel components. For instance, Powdermet Sweden AB, Surahammar, Sweden, uses hot isostatic pressing (HIP) to produce such parts for steam turbines, boilers, and offshore oil and gas platforms. In HIP, gas-atomized, stainless-steel powders are compacted in a flexible, metallic mold. The process employs inert gas at pressures up to 20,300 psi and temperatures up to 2,280F. Powdermet has produced 10-ton gas-turbine rotors using HIP.

Aiming high. Mechanical alloying is also a major application for PM. Inco Alloys, Ltd., Hereford, U.K., uses a mechanical process to produce alloys for diesel and gas turbine aircraft engine components from powders. The powders, mixed into grinding ball mills, are cold-welded together. This is followed by a fracturing stage where the composite particles are broken down into finer particles with uniform composition.

Inco high-strength alloy products can be extruded or hot pressed. Incoloy, an iron-chromium-aluminum alloy, resists creep, oxidation, and sulfidation. Inconel nickel-chromium alloys have been used for nozzle guide vane assemblies in jet engines. And Inconel is being considered for a thermal protection system that will equip NASA's X-33 reusable launch vehicle prototype.

Progress continues in the quality of commercially available powders, die material and design, and sintering technology, according to the EPMA. Analysis techniques employing FEA and CFD codes under development should help designers work more efficiently with powder metallurgy.

Design tips for sintered components

There are a number of basic rules that can help the designer when faced with sintered components. These are a few of the more important ones.

Radii are preferred: Sharp corners on tooling can lead to tool failure. This is best avoided by designing in radii wherever possible. However, chamfers are preferable to radii on component edges.

Re-entrant features have to be re-designed: Components with re-entrant angles cannot be ejected from the die during the pressing cycle and will therefore require machining after processing. Parallel sides are easily produced and in some instances draft angles are desirable.

Thin components with large surface areas are difficult to produce: Large density variations are the problem. Also, a thin part is fragile and tends to crack during production. Projections should be as thick as possible and all sharp edges designed out by radiusing.