The Fundamental Principles of Powder Metallurgy
No method of producing parts is perfect; no solution covers all problems. Powder Metallurgy (P/M) has solved literally thousands of design problems. Some solutions are simply a matter of a comparable part to other processes at less cost or with less wasted material. P/M parts are used in many products such as automobiles, home appliances, power tools and hardware, riding lawn mowers and farm equipment, business machines and sporting goods.
Powder metallurgy is a highly developed method of manufacturing reliable ferrous and nonferrous parts. Made by mixing elemental or alloy powders and compacting the mixture in a die, the resultant shapes are then sintered or heated in a controlled-atmosphere furnace to metallurgically bond the particles. Basically a 'chipless' metalworking process, P/M typically uses more than 97% of the starting raw material in the finished part. Because of this, P/M is an energy and materials conserving process.
The P/M process is cost effective in producing simple or complex parts at or very close to final dimensions at production rates that can range from a few hundred to several thousand parts per hour. As a result, only minor, if any, machining is required. P/M parts also can be sized for closer dimensional control that essentially eliminates secondary fabrication steps and/or coined for both higher density and strength.
In addition, ferrous and nonferrous P/M parts can be oil impregnated to function as self-lubricating bearings; resin impregnated to seal interconnecting porosity; infiltrated with a lower melting point metal for greater strength and shock resistance; and heat treated and plated when required.
Most P/M parts weight less than 5 pounds (2.27 Kg), although parts weighing as much as 35 pounds (15.89 Kg) can be fabricated in conventional P/M equipment. Many of the early P/M parts, such as bushings and bearings, were very simple shapes, as contrasted with the complex contours and multiple levels that are often produced economically today. In many cases, functions that normally would require intricate multiple parts and assembly steps are consolidated in a single P/M part that minimizes production procedures and reduces cost.
Since the P/M process is not shape sensitive and normally does not require draft, parts like cams, gears, sprockets and levers are very economically produced. In many cases, designs require that parts such as a cam and gear, or a spur gear and a pinion gear, be joined together by a secondary assembly method. These additional assembly steps very often can be eliminated with multiple level designs that combine the separate shapes into a one-piece P/M part. In other instances, two P/M parts may be assembled after pressing, then bonded into a one-piece part during sintering.
The basic versatility of P/M is applied in numerous industries, including automotive, business machines, aerospace, electrical and electronic equipment, small and major appliances, agricultural and garden equipment, hand and power tools. Thousands of different cost-saving reliable P/M designs now serve these industries in a wide range of engineering applications that fall into two main groups. In one group are parts which are very difficult to make by any other production method. For example, parts of difficult-to-fabricate materials such as tungsten and molybdenum, or tungsten carbide, cannot be made efficiently by any other process. Also, porous bearings, filters and many types of hard and soft magnetic parts are exclusively products of the P/M process.
Another, and larger, application group consists of components where P/M is an effective alternative to machined parts, castings and forgings. P/M technology now is expanding into precision hot forging from sintered blanks and preforms, thus extending its capabilities to providing P/M parts with increased strength.
- Eliminates or minimizes machining.
- Eliminates or minimizes scrap losses.
- Maintains close dimensional tolerances.
- Achieves a wide variety of alloy systems.
- Produces good surface finishes.
- Provides materials which may be heat-treated for increased strength or enhanced wear resistance.
- Provides controlled porosity for self-lubrication or filtration.
- Facilitates manufacture of complex or unique shapes which would be impractical or impossible with other metalworking processes.
- Suited to moderate-to-high volume component production requirements.
- Offers long-term performance reliability in critical applications.
Typical parts can be produced at rates of several hundred to thousands per hour with the P/M process. Although normally associated with high volume production, P/M also is feasible for economic part production in lower volumes. For maximum efficiency, however, lower volume parts should be comparatively simple, permitting low tooling and maintenance costs. Also, extending tooling use to produce more than one part, such as in varying thicknesses or a common part with different size holes, will enhance the low volume economic rationale for P/M. Another consideration is that the part design should take advantage of the ability of the P/M process to minimize secondary processing operations in comparison to these requirements if the part were to be produced by competitive methods. When these factors are implemented, P/M parts often offer cost and performance advantages in runs as low as 1,000 pieces.
The three basic steps for producing conventional density parts by the powder metallurgy process are mixing, compacting and sintering.
Precisely engineered materials, metal powders are available in numerous types and grades designed for the P/M process to meet a wide range of performance requirements. Mechanical properties of P/M parts compare favorably with metal parts made by other metalworking methods.
Most metal powders are produced by atomization, electrolysis, or chemical or oxide reduction. In powder form, the materials include iron, tin, nickel, copper, aluminum and refractory and reactive metals. These metals can be mixed to produce alloy compositions during sintering where the powder particles are metallurgically bonded.
Molten prealloyed compositions of low-alloy steels, bronze, brass, nickel silver and stainless steel, in which each powder particle is itself an alloy, also can be atomized. In addition, metal and non-metal powders can be combined to provide composite materials with specialized properties.
The engineering properties of the P/M part are significantly determined by the metal powder processing and fabrication techniques. Among the controlling elements are particle size and shape, and size distribution or sieve analysis; apparent or bulk density of the powder; rate of powder flow into the die cavity and powder compressibility in the die.
Iron powder, the most widely used P/M material for structural parts, is sometimes used alone but most frequently with small additions, singly or in combination, to improve mechanical properties of the pressed and sintered part. Among the powders added are carbon, copper, nickel or molybdenum.
Copper and aluminum-base powders are the most widely used nonferrous P/M materials, although most nonferrous metals and alloys can be converted to powder and fabricated into engineering components. Copper base materials include the brasses, bronzes and nickel-silver.
The P/M process, similar to other part fabrication methods, has its own set of design guidelines for producing soundly engineered, economical products. The particular aspects and requirements of P/M production should be kept in mind. Design advantages then can be gained that are unique to the P/M process, and the limitations of the process will have been taken into account.
Designing for powder metallurgy requires close cooperation between the part user, or buyer, and the producer, especially in the initial design stages. The P/M part should be designed in the context of the whole assembly, with the parts manufacturer kept informed throughout the process. An improved, lower-cost P/M part often can be achieved through small changes in an assembly. Not infrequently, early designer-manufacturer interaction results in an expansion of the P/M concept that nets overall design production simplification and cost reductions.
The P/M part design and the complexity of tooling to produce it are closely related. Consultation between the part and tool designers often can yield minor design changes in the part and more simple, economical tooling.
The feasibility of making a part by powder metallurgy depends on being able to economically press the metal powder in the die to obtain the desired shape, dimensions, detail and density. Two major factors in the compacting operation influence or control part design, the flow behavior of metal powders and the pressing action. Metal powders do not flow hydraulically because of friction between the particles and the dies and therefore, the design should assure that adequate powder will be placed in the die cavity to allow adequate compaction to reasonable density. Because metal powders have limited lateral flow, there are also some limitations on the contours that can be produced.
The pressing action is applied only from the top and bottom, largely governing the shape, dimensional details and length of part that can be made with the desired density. Another consideration of the pressing action is that the part shape must allow ejection from the mold.
Part Size--Although there is no known theoretical limit, maximum practical size is governed chiefly by powder characteristics, part density and available press capacity. The majority of conventionally pressed P/M parts range in projected area from about 0.006 to 25 sq. in. and between 1/32 and 6 in. in height (direction of pressing). The practical height limit is closer to 3 in. For a given metal powder, the lower the density required, the larger the part that can be produced on a particular press.
Parts that are relatively long in the direction of pressing are more difficult to produce with adequate density through the total length of the part, particularly in the center section. Another factor that limits part length is the apparent density of metal powders. In general, the compression ratio is at least 2 to 1 meaning that the die depth must be at least twice the height of the pressed part. Sometimes the thickness limitation is not a function of the press but of part and tooling design, such as where difficulty in filling thin wall sections may occur or core rod length may exceed a practical limit.
Shapes--The shapes most suitable for the P/M process, although a variety of shapes, sections and profiles can be produced, should have uniform dimensions in the direction of pressing. Included are simple cylindrical, square and rectangular shapes and those with the contour in a plane at right angles to the direction of pressing. Examples are parts with radial projections and contours, like cams and gears, and with no changes in thickness, which are relatively simple to press.
Since the tooling is subject to very high stresses, shapes that require fragile tools should be carefully considered. Under normal circumstances, the contour of parts must allow ejection of the green compact from the die with an upward motion of the bottom punch.
Perfect spheres cannot be made by the P/M process, since powders do not flow hydraulically, but must be compressed. Spherical P/M parts are designed with straight or flat areas around the equator. Parts that must fit into ball sockets are repressed after sintering to produce a more spherical shape. Hemispheres, such as those used in automotive ball 'joints, can be readily compacted. Spherical depressions up to a hemisphere are also possible.
Multi-Level Shapes--Simple variations or levels in part thickness can often be provided with face forms on upper and/or lower punches. These would include such details as countersinks, steps not exceeding 15% of the overall thickness, numbering or lettering and other similar features. Some minor variations in density from level to level will be present when this tooling method is used. It does employ more simple tooling techniques, resulting in more economical tooling and part cost.
More complex, multiple-motion tooling is required to maintain uniform density throughout parts with pronounced variations in steps or levels. Sophisticated pressing equipment with the ability to move tooling components independently is used to provide a proper vertical transfer of the metal powder before the compacting sequence begins. This allows parts with 5 or more levels to be produced with a minimum of density variation between the various levels. Both mechanical and hydraulic presses are used with tonnage available to make parts weighing from 1/2 ounce or less to 26 pounds or more.
Flatness--Flatness obtainable on a part is a function of a number of factors. Total measured flatness obviously depends on surface area. Also, it is affected by part thickness; usually thin parts tend to distort more than thick parts during sintering or heat treatment. Because of possible density variation in complex shapes and cross sections, flatness is more difficult to maintain than in parts of simple shape and cross section. Flatness can be improved in the case of soft metal parts by repressing, and in the case of hard materials by grinding. In some cases, small bosses or pads placed in critical areas minimize the total surface area that must be held 'true'.
Powder metallurgy components with wrought properties are now available. A popular material is the type 4600 steel with varying carbon levels and conventional heat-treatment when necessary. Various systems within the P/M industry produce millions of parts annually, primarily for automotive original equipment.
The basic process common to most higher density systems is to manufacture a 'green' compact called a 'preform,' heat and then restrike/forge the 'preform' to the required final density.
Isostatic pressing generally is used to produce P/M parts to near-net sizes and shapes of varied complexity. Unlike conventional press compaction or molding, isostatic pressing is performed in a pressurized fluid such as oil, water or gas. A flexible membrane or hermetic container surrounds the powder mass and provides a pressure differential between its contents and the pressurizing medium.
Among the benefits of isostatic pressing are:
- Complex shape capability.
- Minimized expensive powder input.
- Minimized material removal in making complex shapes.
- Applicability to difficult-to-compact materials.
- Highly effective compaction, usually without need for binders.
- Ability to make a near-net shape.
- Uniform density and properties.
The importance of discussing the part application with an MPIF-member P/M parts manufacturer cannot be over stressed. It is the best way to take advantage of the latest techniques in a rapidly growing industry. P/M is a high volume industry that can provide meaningful cost reductions combined with high quality.
When requesting a quotation, accurate part information must be provided. Refer to the Metal Powder Industries Federation Standard 35 for P/M materials, properties and specifications. Typical information needed includes the following:
- Order quantities, annual usage and future estimates, to take advantage of high volume cost reductions.
- Detailed drawings of the part, including assembly drawings and samples of existing parts or prototypes. Identify materials that have preformed satisfactorily for the application.
- Can part design be modified without affecting function? If so, where?
- Will the P/M part replace one currently in production, or is this a new application? Is the application commercial, military, aerospace, medical, etc?
- Actual service conditions: heat, moisture, impact, corrosive, etc.
- Necessary physical, mechanical, corrosion or special properties (tensile, elongation, hardness, flatness, conductivity, impact, fatigue, etc.).
- The finish required (plating, oxide coating, surface finish).
- If any machining operations are required, will they be performed by the P/M supplier?
- For bearings: load, shaft material and finish, speed and diameter should be noted.
- For gears, splines, sprockets, etc., specific data are required: a) number of teeth, b) theoretical pitch diameter, c) pressure angle, d) measurement over wires, e) tooth thickness, f) backlash, g) helix angle, h) AGMA quality class. New P/M applications can be evaluated with minimum dollar expenditure by testing parts machined from recommended P/M sample material. There is no low cost temporary tooling for die pressed parts. Materials for machining prototypes can be supplied by P/M fabricators at reasonable cost. Tooling does not have to be committed until after prototype samples have been tested.
Customers frequently request that the P/M material have properties that are identical to the material specified. Most properties usually can be met, but identical properties are not always possible and often may not be necessary and can increase cost.
The powder metallurgy process can allow some favorable cost variation if specific part requirements are not rigidly defined. Widely differing costs may result from differences in quality levels, changes in tolerance or design, or failure to specify minimum tensile properties. There may be more than one MPIF standard material that can satisfy the customer's functional requirements.
Due to high volume production capability, repeatability and attractive cost benefits, an acceptable quality level (AQL) should be agreed upon with the P/M parts manufacturer. To achieve an acceptable quality level for critical dimensions or functional tests (torque, bending, etc.), it is recommended that identical sets of inspection fixtures or gages, etc. be provided by the customer (one to the P/M supplier and one retained by the customer). This will ensure that both supplier and customer evaluate the parts in the same manner.
The acceptable quality level required can materially affect the cost of the P/M part.
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