Powder metallurgy advanced materials and processes were featured at the MPIFIAPMI PM2TEC conference in New Orleans, Louisiana, held 13 -17 May 2001. Several are discussed here, and some of the winners of the 2001 International PIM Design Competition are shown.

Joseph M. Capus* Beaconsfield, Quebec, Ca11ada

Advanced materials inu·oduced at the con­ference included a wide range of ferrous and nonferrous alloys. Titanium P /M was the focus of one special-interest pro­

gram, and the significant accomplishments in rhe­nium PI Mover the past three years were described in another session. A new linear slot gas atomizer has been developed in Germany for the bulk pro­duction of fine powders for metal injection molding, and a new sprayforming plant has been opened in Denmark.

Titanium P/M resurgent The P /M titanium program was introduced by

Pro£. Sam Froes, University of Idaho, who reviewed recent progress and pointed to some signs that this segment of titanium technology could be poised for expansion. Titanium has several properties that make it one of the most important advanced ma­terials, but it has a seliotts disadvantage in cost com­pared with competitive materials. Powder metal­lurgy has been seen as one of the ways to lower the cost of fabricating titanium components.

In his discussion, Dr. Froes included technolo­gies such as laser-forming, metal-injection molding, spray forming, near net shape forming, and rapid solidification, as well as other research-based processes. On the raw materials side, several sup­pliers now provide titanium powders prepared by plasma or gas atomizing, hydride-dehydride processes, and the traditional sponge fines. Prices range fromabout$10/lbto$200/lb, depending on process and purity. *Life member of ASM International

2 7

Fig. 1 -- Simplified drawing of the Widejlow linear melt film atomization nozzle system. l: high pressure section; 2: low pressure "spraying tower" sectio11; 3: linear de Laval gas nozzle; 4: linear melt nozzle; 5: melt; 6: melt film; 7: disintegmted spray plume of droplets.

Many powder sizes are available, some now in tonnage quantities. As mentioned later by Prof. Rand Ge:rman, much of the early work on titanium MIM was hampered by the lack of titanium powder of suitable quality. However, Dr. Froes notes that non-aerospace applications do not require the strin­gent limits in oxygen levels demanded for aero­space components.

Sprayforming of molten titanium has been demonstrated, but is nowhere near ready for com­mercial production. Cold spraying ( <500°C) with high-velocity gas has attracted recent interest for both shatpes and coatings: titanium cold-sprayed onto steel avoids the formation of brittle inter­metallic phases and provides good bond strength.

Titanium near-net shape applications include press-and-sinter, cold isostatic pressing, hot iso­static pressing, and metal-injection molding. Starting materials for these processes include blended elemental powders, usually based on sponge JEines plus master alloy powder (such as 60 I 40: AJ /V, to make Ti-6Al-4V). These materials have bee:n successfully fabricated into stinger mis­sile parts, nuts and bolts, and sporting goods such as golf club heads and softball bats.

Much higher mechanical properties, equivalent to cast or wrought titanium, can be developed if blended elemental components are fabricated from salt-free powders such as hydride/ dehydride ma­terial. In contrast, the prealloyed powder approach generally involves spherical powder produced by melting: gas atomization or the plasma rotating

ADVANCED MATERIALS & PROCESSES/OCTOBER 2001 43

Manual transmission synchronizer ring A complex synd1ronizer ring made bt; Sinterstahl GmbH, Fussen, Germn11y, for

Ford-Werke AG, Cologne, Ger-many won the grand prize in the overseas category. Tlre part is wm·m-compacted to a density of more than 7.3 gfcmJ in tire teeth, and more than 7.1 g/cmJ in tile ring body.

A friction 1 in ing of br01 zze and brass powder with silica and graphite is sintered onto a 0.4 mm (0.016 in.) sheet, which is az­pacitor discharge-welded onto the cone of the ring.

The part is made from a diffu­sion alloyed, sinter-hardened steel, and ]UIS a minimum tensile strength of 850 MPa (123 ksi). The ring is actually a friction clutch that synchronizes the ro­tational speed of gears in the mmwal tra11smission before shifting, assuring smooth en-gagemeHt and avoiding gear clash.

Warm compaction provides density levels and mechanical properties compa­rable to double-pressing. Green strength increased to almost 28 MPa ( 4 ksi). T11e pmuder metallurgJJ ring offers a 38% cost savings compared witf1 convention­alLy produced parts.

For more information: Michael Krehl, Sin terstahl GmbH, Hiebelerstrasse 4, D-87629 Fuessen, Germany; tel: 49-8362-506-111; fax: 49-8362-506-188.

-Circle144

electrode process. Such powders are usually canned and HIPped to produce aerospace components.

According to Dr. Froes, such components can be large and complex, such as parts for the U.S. Navy F-18 Hornet strike fighter. Difficult-to-produce al­loys, such as the intermetallic TiAl, can be formed advantageously by the near-net shape approach. Dr. Froes went on to say that despite the large ex­pendihrre on R & D over a number of years, rela­tively few commercial successes have been achieved so far, with the overall worldwide market for PI M titanium estimated at about 20,000 lb I yr. However, future prospects have been improved with the CLIT­

rent availability of high-quality, low-cost pow ders. He concluded by su ggesting that the big growth opportunity was in small complex-shaped parts formed by metal injection molding.

Titanium MIM: economic barriers Continuing in the titanium special program, Prof.

Randall German, Pennsylvania State University, declared that for titanium metal-injection molding "the days of research are over, and now it is a ques­tion of economics." Titanium is a good target m a­terial for MlM because it is attractive for high per­formance materials, and it permits near 100% material usage (typically 95% ). The process also fa­cilitates the addition of alloying elements to make cermets such as Ti-TiC.

Titanium injection molding w as first d emon­strated in 1988, and became commercially suc­cessful in the mid-1990s. Powders of titanium, Ti-6Al-4V, TiAl intermetallic, Ti-Mo-Al, NiTi, and

other titanium-based m aterials have all been suc­cessfully injection molded .

Contamination_ chiefly with oxygen, is th e m ajor s tum­bling block to economic pro­duction. The key barrier here is powder supply . Dr. German says that while over a d ozen producers supply powder, relatively few of the powders offered are really suited to injection molding. The finer powd ers required for MlM tend to have higher oxygen contents, and nanoscale titanium powders tend to be ve1y impure.

As far as processing is con­cerned, Acrawax (an EBS wax) is on e of the cheapest and best solutions as a binder. When debinding after molding, oxygen mon­itoring is manda tmy, and

argon should be the furnace atmosphere instead of nitrogen. H owever, this m akes the process more expensive than for other materials. With suitable powder and processing, high performance prop­erties are possible, with strengths of 500 to 630 MPa (70 to 90 ksi) and elongations of 3 to 20%. Applica­tion successes noted by Dr. German include auto­motive (shifter component), medical devices and den tal implants, golf putter heads (200 to 300 g), and luxury watch cases.

Larry La Voie of Titanium Products Inc., chimed in with many of the above comments, in reviewing his own company's experiences in d eveloping ti­tanium injection molding over the past three years. H e noted in particular the difficulty of sourcing fine titanium powders suitable for MJM, adding tha t these could cost from <$50 to $100 /lb. Oxygen con­tamination during processing is a problem, and re­quires debinding in argon or vacuum, because oxygen picked up during debinding or sintering could make the parts useless. Sintering should be done in vacuum, with only non-reactive shelves made of materials such as zirconia, allowed to come into contact with the titanium parts.

Ultrafine powders in bulk quantities Fine metal and alloy powders for MlM and other

specialized applications are frequently produced as a by-product of gas atomization . The preferred fine spherical particles m ust be sep arated from coarser fractions, resulting in high cost. Traditional gas atomization plants h ave relatively low pro­du ctivity if fine particles are required, because of the relationship between the specific transfer of en­ergy from the gas to the liquid metal, and the re­sulting particle sizes.

Wideflow Metal Powder Production and Engi­neering GmbH, of Ohrdruf, Germ any, has solved this problem. The company has patented a concept in which the molten metal or alloy falls through a linear melt nozzle into a linear de Laval gas nozzle,

44 ADVANCED MATERIALS & PROCESSES/OCTOBER 2001

where it is atomized by the accelerating gas flow (Fig. 1). Aspects of this development were presented in two papers by Gun­ther Schulz (e-mail: schulz@wideflow.de)

lhe melt 1ihn is sta­bilized by the gases flowing in parallel, and at the right pres­sure the metal is at­omized very effi­ciently. The process provides high throughput and re­sults in fine spherical powders with mean particle sizes between 10 and 50 microns at very reasonable costs, due to high produc-tivity and low specific gas consumption.

Dr. Schulz reviewed the growing number of ap­plications for this type of powder, beginning with metal-injection molding, for which Wideflow has made several fine alloy powder compositions. Thermal spray is another sector in which the trend is toward finer metal powders with a narrow par­ticle size, such as in HVOF and cold spray. Other sectors likely to benefit from the availability of fine powders in bulk quantities include rapidly solidi­fied materials, catalysts, metallic glasses, metallic pigments, rocket fuels (ten-micron aluminum powder for the fu·st stage rocket boosters), electro­chemical p ower sources, micro-solders for elec­tron.ics, metal-matrix composites, and binders for hardmetals and diamond tools.

In a second paper, a theoretical discussion and analysis of the gas atomization process indicated that increasing the gas back-pressure could increase the break-up of liquid metal droplets by secondary atomization, thus decreasing the size of the resulting powder. Experiments with molten tin were de­signed to explore this aspect with the Wideflow linear nozzle. Results showed that increasing the atomizing tower back-pressure from 1 bar to 8 bar (0.1 to 0.8 MPa) had little effect, suggesting that the high efficiency of the primary disintegration left little room for improvement.

P/Mrhenium lThenium is a heavy refractmy metal possessing

some exceptional properties. It has a melting point of 3453K (3180°C, 5756°F), the highest elastic mod­ulus of the refractory metals, and high yield strength. It is ductile at both room temperature and elevated temperatures, and does not form a carbide when exposed to methane or graphite, evert at very high temperatures. According to Todd Leonhardt of Rhenium Alloys Inc., such unique properties have made rhenium the material of choice for extreme service conditions, such as tactical-missile and latmch-vehicle throats, pintles, satellite thrusters,

Stainless steel industrial pump ball guide A complex ball guide made lr.J FMS

Corp., Mhmeapolis, Minn.,for Graco Inc., also of Minneapolis, wo11 the grand plize i11 the stainless steel category. Made from MPIF material SS-316N1-25, the complex part has a mi11imum yield strength of172MPa (25 ksi) and a typ­ical tensile strength of282 NfPa (41 ksi).

The part functions in a one-way ball valve in a high pressure stainless steel Graco pump for sprayi11g paints and sol­vmts. Its complex ID co,ifiguration of three ribs that are not connected in the center allows fluid to flow more efficiently through the ball valve than a standard desigrt. The valve was subjected to severe testing. Graco ran20 pumps through two million Cljcles at a maximum pres­sure of31 MPa (4500 psi). PIM offered significant cost savings over other man­ufacturillg techniques, such as invest-ment casting, while resisting corrosion a11d wear.

For more information: ]oh11 F. Sweet, FMS C01p., Powder Metal Division, 8635 Harriet Avenue Sou th, Minneapolis, MN 55420; tel: 952/888-7976; fox: 9521888-7978; e-mm1: info®finscOJporation.com; Web site: wzmv.fmscorporation. com. Circle 145

divert systems, large gurt-barrelliners, and nuclear -applications. Until recently, it was not possible to produce rhenium components large enough for many of these applications, and the cost of tradi-tional methods was prohibitive.

Mr. Leonhardt went on to discuss the progress in development of PI M net shape fabrication of rhenium and rhenium alloy components that have enhanced future prospects for these materiaJs. Some of the work was ftmded under a NASA Phase II SBIR program.

Rhenium metal has traditionally been produced from hydrogen-reduced powder in fully densified mill shapes such as rod, bar, and plate. Compo­nents such as engine thrusters have been electro­discharge machined from solid bar, which results in a large amount of scrap. However, Rhenium Al­loys has developed a procedure for the successful manufacture of rocket thrusters from low-density rhenium flake powder by a combination of cold and hot isostatic pressing.

In this process, hollow cylindrical components are first wet-bag cold isostatically pressed in a con­tour can and mold with a two-part pressing man­drel, to form the near net-shape green part. The green par t is pre-sintered, then sintered at 80% of the melting point, and finally hot isostatically pressed. The parts are finished by electro-disd1arge machining and diamond grinding to ensure di­mensional accuracy. By means of this process, the company has successfully manufactured the 440N and 490N high-performance liquid-fueled Apogee engine thrusters for low-earth orbit and / or geo­stationaty orbit for satellite positioning systems.

In another program, partly funded by a Ballistic Missile Defense Organization Phase I SBIR pm­gram, Rhenium Alloys has developed low-oxygen

ADVANCED N\ATERIALS & PROCESSES/OCTOBER 200 1 45

Mortise deadbolt A stailtless steel alloy 316 mortise dead­

bolt for commerciallocksets in prisons, nu­clear facilities, and other high-security/re­stricted areas was made by ASCO Sintering Company, Commerce, California. It won an award of distinction in the stain­less steel category.

The part is made from MPIF material SS-316Nl-25. It has n minimum densitt; of6.65 g/cm3, a minimum yield strength of170 MPn (25 ksi,) nnd a typical tensile strength of276 MPn (40 ksi). TI1e part passed several brenk tests and a 96 hour snit spray test; the PIM dendl7olt withstood 200 hours. Powder metallttl'glJ was signif­icantly less expensive than competing processes such as machiningfrom bar m1d investment casting. Secondary operations i11clude deburring and si11ter bonding a stainless steel pin into the deadbolt.

• For more infonnation: Mauret; Wimer,

Asco Sinterirtg Co., 2750 Gmfteld Avenue, Commerce, CA 90040; tel: 3231723-5121; fax: 323/888-9968; e-mail: ascosin.tering @aol.com. Circle 146

- spherical rhenium powders made by a gas-assisted rotating electrode process. A plasma torch melts a rhenium rod rota ting at 15,000 rpm in an atmos­phere of argon, and the rod throws off droplets that solidify into spherical pru:tides. Spherical rhenium powder made in this way is mostly sized between 100 and 300 microns, with the size decreasin g as the rotating rod diameter is increased. The resulting powders have apparent densities above 12 g / cm3 and flow at about 50 grams per five seconds.

Much finer spherical powders (<40 microns) h ave been produced by a plasma atomization process. Spherical Re, Re-Mo, and W-Re powders have also been produced by plasma-atomizing.

In experiments at NASA Glenn Research Center, Cleveland, Ohio, the plasma-atomized powders enabled vacuum-plasma spraying of rhenium to produce coatings up to 5 mm thick, and direct HIP­ping of plasma-atomized spherical rhenium powder was demonstrated to achieve a density 98% of theoretical.

Progress made in rhenium powder metallurgy during the past three years will allow increased ap­plications in satellite propulsion thrusters. Fur­thermore, the development of spherical rhenium powder opens up the possibility of applying the advanced P IM techniques already developed for other materials.

Sprayformed tool steels to fill a gap Spray forming of nonferrous metals and alloys

h as been commercialized for several years now. The attraction of the sprayforming route for cold work tool steels are similar to those of conventional PI M. In addition to the fine and uniform micro­structUl'e, fewer steps are required in the produc­tion of a sprayformed tool steel billet compared with ingot metallurgy, drastically reducing the risk of oxidation and contamination.

Oaus Spiegelhauer of Dan Spray A / S, Taastrup, Den­mark, reported on the im­plementation of a full-scale commercial tool steel spray­forming p lant at his com­pany, in a joint presentation with Odd Sandberg ofUd­deholm Tooling AB, H ag­fors, Sweden.

Dr. Spiegelhauer noted the wide gap, both technologi­cally and commercially, be­tween conventionally pro­duced tool steels and P I M tool steels. Conventional tool steels cover a wide range of compositions and dimen­sions, while P IM steels tend to concentrate on special products in high-speed steels and cold-work steels. H e says that sprayforming offers the possibility of closing the gap between the two.

The Dan Spray plant sprayforms large tool steel billets with diameter of 500 mm and lengths up to 2500 mm, with a weight of metric tons. It is capable of producing 2000 metric tons per year on a one-shift operation. Billets produced at the plant are in the process of being evaluated, and th e sprayformed material is being tested in variou s applications.

PI M and sprayfonned high-alloy Cr-Mo-V -type steels with up to 10% vanadium show microstruc­tures with fine and homogeneous distributions of small, hard, wear-resistant vanadium carbide par­tides. Abrasive wear is superior to conventional 02 steel, and better impact values suggest higher re­sistance to chipping and cracking. •

For more information, or for copies of the papers: Peter Johnson, Metal Powder lndustries Federation, 105 Col­lege Road East, Princteon, NJ 08540-6692; tel: 609 I 452-7700; fax: 609 / 987-8523; Web site: www.mpif.org.

How useful did you find the infonnntion presented in this m·ticle? Very useful, Circle 271

0£ general interest, Circle 272 Not useful, Circle 273

46 ADVANCED MATERIALS & PROCESSES/OCTOBER 2001