Powder Metallurgy: A State-of-the-Art Process
When people think about how metal parts are made, they usually envision heavy machines cutting chips or molten metal flowing into three-dimensional dies. 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 1200°F. 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 then be shaped in a die.
Powder Metallurgy (PM) parts can be made from the strongest elemental metals, as well as super-alloys, 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 PM for making a variety of parts.
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 PM 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 PM applications are increasing as well. The aerospace, home appliance, lawn and garden, computer, and power tool industries are a few of them.
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 PM parts, the performance of these parts has led to new uses.
According to the Metal Powder Industries Federation (MPIF), the powder metal parts and products industry in North America has sales estimated to be over $3 billion. The industry consists of 150 companies that make conventional PM 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.
MPIF 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 PM industry, the organization can be reached at (609) 452-7700.
Conventional Powder Metallurgy (PM) Process – Press-and-Sinter
Metal powder can be converted to a solid part in different ways. But there are three steps that are common to most of the techniques, mixing, compaction and sintering.
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, a binder is added to hold the metal powder 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 PM 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, the part will undergo secondary operations such as machining, grinding, or drilling.
Some of the more common raw materials used in PM parts 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 our Engineering Department to obtain an estimate and communicate with those seeking cost estimations.
It’s also a good idea to consult during part design. In conventional PM part making, the powder mixture does not flow easily, as would a fluid. As a result, die geometry must conform to certain criteria to ensure die fill. This determines the type of parts that are most effectively produced with conventional PM. For example, parts with sharp edges and thin walls are not a good candidate. 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 core rods in the punches. Other details such as names or logos can be pressed into the workpiece, or laser-etched. 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 PM part making process 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 PM. Parts can be produced from metal powder 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.
Parts made from conventional powder metallurgy (PM) 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 PM parts that is often sought in design, especially for producing filters.
Alternative Compaction Techniques
New techniques are being used by some powder metallurgy (PM) 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 (PF). The process “re-strikes”, or forges, the preform 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. Multi-directional pressure is exerted onto the mold, either with a liquid in cold isostatic pressing (CIP)systems, or with a heated gas such as argon in hot isostatic pressing (HIP). The powder is contained in flexible molds (CIP) or ridged “cans” (HIP) that form the part shape. Cores in the powder can be used to form internal shapes. Equipment is available to process parts up to about 50′′ in diameter and 12′ long.
HIP 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 metallurgy (PM) part making technique that deserves to be examined closely is metal injection molding (MIM). This process combines the thermoplastic injection molding and conventional PM 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.
MIM 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.
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.