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Powder metal parts shine
By Joseph Ogando, Senior Editor -- Design News, September 9, 2002
Orlando, FL — Judging from the 2002 International P/M Design Competition, held by the Metal Powder Industries Federation, powder metallurgy each year tackles an increasingly diverse range of applications. This year's winners included parts used in robotics, food processing equipment, industrial machinery, and consumer electronics. And even in that traditional realm of powder metal, automotive gearing, the winning entry upped the ante on part size and complexity. Here's a closer look at the winning parts:
Sprocket beefs up. A six-level steel drive sprocket for the Hummer H2's transfer case won the top award in the ferrous category. Designed by BorgWarner TorqTransfer Systems (Muncie, IN) and made by the Automotive Gear Div. of Stackpole Ltd. (Mississauga, ON), the 6.3-lb sprocket distributes 1,555 ft-lb of torque to the rear wheels when the Hummer goes into four-wheel drive mode. This selectively densified part has an overall density of 7.0 g/cm3, but the outer teeth have been densified to 7.75 g/cm3 to impart some additional strength and wear resistance. The part has both a tensile strength and minimum yield strength of 120,000 psi.
The sprocket stands out for a couple of reasons. For one, "it's the largest selectively densified part in the world," says Stackpole Vice President Ron Duke, who adds that selective densification usually targets parts under two pounds. For another, the part maximizes the net-shape capabilities of powder metal. "It requires no machining after the pressing," Duke notes.
Judges flip over phone hinges. A hinge assembly for the V60C flip phone from Motorola (Schaumburg, IL) prevailed in the metal injection molded (MIM) category. The Parmatech Corp. (Petaluma, CA) molds the assembly's four pieces from two different stainless steel powders. Two knuckles and a cam make use of a 7-4PH stainless steel. All three of these parts have a density of 7.6 g/cm3, a minimum tensile strength of 115,000 psi, a minimum yield strength of 94,000 psi, and a 4% elongation. The hinge barrel, meanwhile, is produced in a 316L powder to a density of 7.6 g/cm3 . It has a minimum tensile strength of 65,000 psi, a minimum yield strength of 20,000 psi, and an elongation of 40%.
Plenty of MIM parts have complex shapes and thin walls. But these four parts, some of which have wall sections as thin as 0.030 inches, pair the usual geometry challenges with some additional headaches associated with a consumer electronics product—including hefty production volumes, tough-to-meet dimensional constraints, and cosmetic requirements. "The demands on this project were the most stringent we've faced," says Rajiv Tandon, Parmatech's director of technology. Production volumes of several hundred thousand parts per month, for example, required Parmatech to work with complex multi-cavity tools, which can multiply the chances of part-to-part variations. "It would be hard enough to make these parts in a single-cavity tool," Tandon notes. What's more, Motorola's dimensional specifications for features such as the length, inside diameter, and internal slots also forced Parmatech to develop a highly automated coining process to keep up with the production goals. Finally these parts had to look good. Achieving a reliable Class A surface took about six months, during which time Parmatech engineers had to find a vendor with a consistent buffing process, qualify that process to Motorola's standards, and finally help the vendor scale up for this big job.
Brass robot balance. A 5.8-lb brass counterweight for a robotic arm won the grand prize in the nonferrous category. Made by Metal Powder Products Co. (Anaheim, CA), the part goes into a tape library retrieval system from Storage Technology Corp. (Louisville, CO). Chock full of complex features—including asymmetrical deep pockets, raised bosses, cam ramps, and counterbores—the counterweight had to be made as a two-piece bolted and bonded assembly. Metal Powder Products also coins the parts for precision. Formed to an average density of 7.7 gram/cm3, the counterweight parts have a tensile strength of 28,000 psi, a yield strength of 16,000 psi, and an elongation of 14%.
With its complex shape and asymmetrical features, the counterweight at first glance defies the limits of what pressed powder metal can do, according to Don Bach, Metal Products' western regional manager. "Many engineers would look at it and say, 'You can't make that in powdered metal'," he says. The company worked around all of what Bach calls "extreme geometry" by making the part as an assembly. Fulfilling its counterbalancing function also created challenges. As Bach explains, "The assembly has to fit within a small space yet still be heavy enough." For that reason, MPP makes the counterweight in CZ3014, a relatively heavy brass alloy.
Titanium outwears tool steel. Dynamet Technology Inc. (Burlington, MA) took home the prize in the advanced particulate material category for shot sleeve liners used in aluminum diecasting. Produced for Prex Corp. (Greenville, OH), these shot sleeve liners consist of a ductile titanium alloy matrix reinforced with titanium carbide particles. With a tensile strength of 145,000 psi and a hardness of 40HRC, this metal matrix composite (MMC) takes the place of nitrided H13 tool steel used in previous liners. Dynamet forms the liners in several sizes, up to 42 lbs and 4.5-inch diameter, using cold and hot isostatic pressing.
According to Dynamet President Stanley Abkowitz, tooling for a harsh manufacturing environment firmly establishes the ability of MMC technology to add wear resistance to titanium's list of desirable attributes. "The MMC liners last five to ten times as long as the H13 liners they replace," he reports.
Rotary pump keeps the food flowing. In what could literally be a cheesy application, powder metallurgy has extended its reach into the food processing industry. Sintered Specialties Inc. (Janesville, WI) took top honors in the stainless steel category for a set of thirteen 316 L rotor hubs used in a food-processing pump from APV Americas (Lake Mills, WI). The parts together weigh 6.4 lbs and measure about 4.5 inches tall. They're made to a density of 7.7 g/cm3 and exhibit an ultimate tensile strength of 85,000 psi, a yield strength of 85,000 psi, and an elongation of 45%—properties comparable to annealed wrought metal and higher than cast.
The hubs have a couple of notable characteristics. Size is one. "It's the largest part by weight that we make," says Brian Hetzel, market development manager for SSI. Achieving a high enough density to meet the requirements of the food processing industry is another. "P/M parts generally don't qualify for food applications due to their high porosity and poor surface finish," Hetzel says, explaining that porous surface can be tough to keep clear of bacteria. To keep density consistently high throughout this tall part, SSI sinterbonds the 13 individual components into one rotor hub—as opposed to manufacturing a monolithic hub whose density would have likely varied throughout its height.
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