Powder Metal Parts Combine Benefits
When people think about how metal parts are made, they usually envision heavy machines cutting chips, or molten metal flowing into three- dimensional dies. But there is a process in between--making parts from metal powders--that combines the benefits of both.
Alloys of zinc, magnesium, and aluminum are relatively easy to die-cast. They melt in the range of 700 to 1200F. But some of the strongest metals and alloys cannot be used in die-casting. Their melting temperatures are too high. Converting those stronger metals into a powder helps solve that problem. The powder can be shaped in a die.
Powder metal parts can be made of some of the strongest elemental metals, as well as superalloys. And because they are die-formed to net shape, or near net, there is no money wasted on machined-away scrap metal. This cost saving feature is one reason for the increasing use of powder metal for making a variety of parts.
Also, the process is extremely versatile because powder composition is versatile. Simply, all powders are not the same. In fact, the powder particles of different elemental materials or alloys are actually different shapes and sizes. The shape of a powder particle, which depends on the way it was made, influences the density, surface area, permeability, and flow characteristic of the powder composition. By controlling these characteristics, the properties of the end-product are controlled. By adding a little bit of this, and a little bit of that, the composition of metal powders can be tailored to produce the characteristics needed for almost any application.
The largest market for powder metal parts has traditionally been the automotive industry. And, it is an increasing market as automobile manufacturers more frequently opt for high quality alloys in many parts, such as stainless steel exhaust systems.
But other powder metal applications are increasing as well. The aerospace, home appliance, lawn and garden, computer, and power tool industries are a few of them. Sinter Metals, Inc., maybe the largest pressed powder part maker, does 40% of its business outside the automotive industry.
One of the key reasons for this growth has been the advancements in the technologies of making parts from metal powder. Previously, cost saving was the primary consideration. But as techniques have been developed to increase the density of metal powder parts, the performance of these parts has led to new uses.
According to the Metal Powder Industries Federation, the powder metal parts and products industry in North America has sales estimated to be over $2 billion. The industry consists of 150 companies that make conventional powder metal parts and products from iron and copper-base-powders. The industry has about 50 companies that make specialty powder metal products such as superalloys, tool steels, porous products, friction materials, strip for electronic applications, high strength permanent magnets, magnetic powder cores, tungsten carbide cutting tools, and wear parts.
The Metal Powder Industries Federation is a "not-for-profit" trade association in Princeton, NJ that promotes the technologies of the metal powder producing and consuming industries. An excellent source of information about the powder metal industry, the organization can be reached at (609) 452-7700.
Metal powder can be converted to a solid part in different ways. But there are several steps that are common to most of the techniques.
Iverpac Corporation of Huntington Beach, California is an expert in using the traditional method of making parts from metal powders. President Tony Ray Iversen described how it is done.
In the first step, mixing, various metal powders are combined to produce the desired material properties. Many of the alloy powders are available premixed--the powder already has the correct material characteristics. Other ingredients, such as die lubricants, are usually added to the mix. In metal injection molding, described later in the article, a binder is added to hold the powder metal together.
The next step, compaction, is to compress the mixed powder in a die at pressure that usually ranges from 10 to 60 tons per square inch. Starting with about 2.5 times the final part volume, the powder mixture is pressed by punches moving in from both the top and bottom of the die.
After compaction, the part--now referred to as a "green" part--is ejected from the die. In most powder metallurgy techniques, the green part has the shape and size of the finished part. Therefore, very little secondary machining is required.
The green part is strong enough to undergo the very important step of sintering--a process in which the metal powder particles fuse together without melting. In sintering, the green parts are conveyed through a controlled atmosphere furnace, usually at temperatures a bit over 2000F, but below the melting point of the materials. The process metallurgically bonds the metal particles, without melting and without oxidation of the parts.
If the part has been pressed to near net shape, rather than net shape, Iverpac will perform secondary operations such as machining, grinding, or drilling.
Some of the more common raw materials used in metal powder part making are iron, copper, tin, and nickel. Alloys such as bronze, brass, and stainless and carbon steels are also commonly premixed into powders ready for use. Using metal powder is one of the most effective ways to make parts from difficult materials such as tungsten, molybdenum, or tungsten carbide.
The best way to determine if a part's material requirements can be met with metal powders is to consult with one of the qualified powder metal part producers that advertise in JobShop.com or Design-2-Part Magazine, or exhibit in the Design-2-Part Shows.
It's also a good idea to consult during part design. In conventional powder metal part making, the powder mixture does not flow easily, as would a fluid. As a result, die geometry must be conform to certain criteria to ensure die fill. This determines the type of parts that are most effectively produced with conventional powder metallurgy.
For example, sharp edges and thin walls are not a good idea. Rounded edges and corners permit a more adequate die fill. Also, thin parts are frequently too fragile, and some part designs cannot be ejected from the dies. Round, square, or D-shaped holes in the pressing direction can be made with rods in the die. Other details such as names or logos can be pressed into the workpiece. Special features, such as threads or side holes, can be machined-in after sintering.
An assembly that cannot be made from one powder metal part can be made from two or more parts, including parts of dissimilar metals. Parts of like metal powder can be joined during sintering. Parts of dissimilar metals can be joined in traditional methods such as brazing, welding or press fitting.
The conventional metal powder part making technique is especially effective for parts such as bearing races, gears, and cams. The connecting rods in automobile engines are one application that has achieved significant success with powder metal.
Parts can be produced from powder metal at rates from a few hundred to thousands per hour. They are usually relatively small, under five pounds, but larger parts, (30 to 40 pounds) can be produced.
Improved Part Performance
The Wakefield Corporation of Wakefield, Massachusetts provides an excellent example of an improved component after being redesigned for powder metallurgy.
Superwinch's X-series has become a common sight on recreation and utility vehicles over the last 25 years. This winch uses a ring gear and housing die cast in either 380 aluminum or ZA-27 alloy, depending on the winch's load capacity.
This spring it will be replaced by the S-series. The key element in the new design's success lies in the gearing design.
Working closely with the technical staff of The Wakefield Corporation, Superwinch replaced the die cast ring gears with powder metal gears. By designing a package that incorporates steel ring gears in an aluminum housing, Superwinch achieved significant performance improvements without a corresponding cost increase.
The nickel-steel ring gears are double the strength of the aluminum predecessor with a five-to-ten-fold increase in life expectancy. Besides greater strength and wear characteristics, the powder metal gears run quieter, both in noise level and quality.
The new powder metal gears have an apparent hardness of Rb 78 and minimum tensile strength of 65,000 psi. The stationary gear has a 5.30" outer diameter, while the rotating ring gear outer diameter is 5.25".
Both ring gear diameters are produced with a tolerance of 0.004". Other critical design requirements include a maximum pitch diameter runout of 0.004", a maximum concentricity on all diameters relative to the outer diameter of 0.005", and a maximum radius on all gear teeth of 0.005".
Parts made from conventional powder metal processing are often porous. That's not a deficiency--it's a characteristic that is not only controlled, but used advantageously. For example, powder metal parts are often impregnated with oil so they will be self-lubricating--especially useful to make bearings. When parts that have been impregnated with oil are heated through friction, the oil expands and moves to the surface of the part. When the part cools, the oil soaks back into the part through capillary action.
The amount of porosity is a function of the material being processed and the technique being used. It is also the opposite of the part's density. If a part is 90% dense, it's also 10% porous. Newer techniques being practiced by many powder metal shops now can produce parts of about 99% density when high performance, rather than controlled porosity is required.
If powder metal parts will be used to contain fluids, they can be impregnated with resins under vacuum and pressure to seal all porosity. Or, they can be infiltrated with another metal with a lower melting point, like copper or a copper alloy. A slug of the infiltration metal is applied to the green part during sintering. The slug melts and filters into the pores of the green part.
Permeability, the ability to allow fluids or gasses to pass through, is another controllable characteristic of powder metal parts that is often sought in design, especially for producing filters.
New Compaction Techniques
New techniques are being used by some powder metal part producers to increase the density, hardness, and strength of the part being sintered.
One such technique is known as hot forging, or powder forging. The process "re-strikes", or forges, the green part to its final density after sintering. This technique is often used to make automotive connecting rods.
Isostatic pressing, either hot or cold, increases density but is also used to make parts that have more complex shapes. During isostatic pressing, the powder is contained in flexible molds of the part shape. Cores in the powder can be used to form internal shapes. Multi-directional pressure is exerted onto the mold, either with a liquid in cold isostatic pressing systems, or with a heated gas such as argon in hot isostatic pressing. Equipment is available to process parts up to about 50" in diameter and 12' long.
Hot isostatic pressing is often used with superalloys for the gas turbine engine industry. It is also very effective with tool steels for high-speed cutters. The process may see increasing use for making parts from composites because composites are difficult to machine without breaking fibers and damaging the part.
Metal Injection Molding
One powder metal part making technique that deserves to be examined closely is metal injection molding (MIM). This process combines the thermoplastic injection molding and conventional powder metallurgy processes. It offers the same level of design freedom for highly configured metal components as is available for plastic parts in the plastic injection molding process. The technology produces very high-density parts with mechanical properties comparable to wrought materials.
Metal injection molding applies to a wide range of applications. Its ability to reduce component cost is centered on its ability to produce small, complicated, three-dimensional shapes.
According to Kinetics, a Dynacast Company of Wilsonville, Oregon, the MIM process starts by mixing fine metal powders with a polymer binder to create a feedstock suitable for injection molding. The feedstock is injected into standard plastic injection molds that have been designed about 20 percent larger than the desired final product. The oversized mold is required due to the presence of the binder, which is subsequently vaporized from the molded part in a furnace.
The part is sintered at temperatures above 2200F. This releases the tremendous surface energy stored in the fine-mesh metal powder and fuses the metal particles together, shrinking the part to the final shape and size in a precisely controlled manner. The as-sintered part retains all of its molded features.
Many of the same design principles used for designing plastic parts apply to designing MIM parts. They will exhibit features characteristic to the molding process such as parting lines, gate, and ejector pin marks.
MIM saves money for highly complex parts, defined as parts with at least four machined features. If a component is produced by stamping or die-casting, and it meets design and performance requirements, it is probably being produced by the most economical approach for that component. MIM does not compete with screw machined components unless they require two or more secondary operations.
If investment castings are used in the as-cast condition or conventional powder metallurgy parts used without secondary operations, then those processes should be retained. However, if the investment casting or powder metallurgy parts require secondary machining operations, then MIM may offer cost savings. Compared to investment casting, MIM is able to provide a better surface finish and finer feature details.
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