Metal powders are available in many formulations whose properties approach those of the wrought sort of metal.
The processing involves preparing a perform by compacting the powder in a very die under air mass. Sintering at a warm temperature to fuse the powder into an even mass is that the next step.
Re-pressing is usually done to boost the properties or dimensional accuracy of the part.
One disadvantage of PM parts is that they're sometimes brittle and will not be employed in applications where high-impact loading is anticipated.
Another important application is in sintered bearings, which are made to a comparatively density with consequent high porosity.
The bearing is impregnated with a lubricant which will be sufficient for the lifetime of the part.
Manufacturers of metal powders have many
proprietary formulations and grades.
Several processes are applied to realize particular properties and to satisfy production goals. Brief descriptions are given here.
■■ Press and Sinter
■■ Constituent powders within the proper percentage for the required material are blended uniformly.
■■ Blended metal powders are fed into a die.
■■ A press compacts the powders into the required shape, producing a green part having relatively low strength; the part is ejected from the die.
■■ A heating process, called sintering, metallurgically bonds the grains of powder into a solid, strong form.
■■ Optional manufacturing steps is also performed pro re nata, such as:
■ Repressing to realize refined properties and
greater dimensional accuracy
■■ Machining of features that would not be made within the die
■■ Heat-treating to realize higher mechanical strength
■■ Plating or other surface treatments
POWDER FORGING
■■ The steps described for the press and sinter process are completed as before. However, the geometry isn't yet in final form or size.
■■ Because the part is withdrawn from the sintering furnace,
it is coated with a lubricant, transferred to a forging press in an exceedingly close-form die where it's hot worked, causing plastic flow of the fabric to its final shape.
■■ The forging process refines the strength of the part, nearly equaling that of wrought varieties of similar metals, while tending to orient the interior structure in favorable directions in line with the anticipated loads.
ISOSTATIC PRESSING
■■ The blended metal powders are placed into a hermetic chamber and restrained by a versatile membrane.
■■ A air mass is exerted on the membrane compressing the powder uniformly and producing a high density.
■■ Compressing at temperature is termed cold isostatic pressing.
■■ Compressing at elevated temperatures is termed hot isostatic pressing, which accomplishes the forming to the required shape and simultaneously sintering the powders, thus eliminating one step within the process.
■■ The flexibility to create larger, more complex parts and also the use of a wider range of powder types are advantages of using the isostatic pressing processes.
■■ The density of the finished part is often more uniform as compared with the press and sinter process and also the resulting density and mechanical properties are very near those obtained in wrought styles of metals of comparable composition.
METAL INJECTION MOLDING (MIM)
■■ Blended metal powders are enhanced with binders and additives like waxes and thermoplastic polymers.
■■ The mixture is fed into an injection molding machine, like plastics injection molding systems to form the green part.
■■ The green part is extracted from the mold and sintered like other powder metal processes.
■■ More complex shapes are often made with MIM than with other processes and dimensional accuracy is improved.
■■ Mechanical properties of the finished metal part are nearly adequate to those of wrought sorts of similar composition.
Production of powder metal products often requires expensive tooling, presses, and special equipment, lending its use for top volume or high-value products.
GUIDELINES FOR SPECIFYING POWDER METAL PARTS
While it's not practical to define all possible conditions, the subsequent discussion presents the framework during which powder metal products fall.
Consultation with an experienced supplier is usually recommended.
1. Part size is usually limited to those with a projected area of compressed powder of fifty in2 (32 000 mm2) supported press capacity.
2. Part length should be but 6 in (300 mm) but greater than 0.06 in (1.5 mm).
3. The length/diameter ratio should be but 5.
4. The length/wall thickness ratio should be but 8.
5. The part shape must be specified it is formed by linear pressing by a ram.
6. No undercut, threaded, or re-entrant forms may be made.
7. Density variations happen from normal press and sinter processing for relatively long parts.
8. Relatively high production volumes are normally required to amortize tooling and equipment costs.
9. Consultation with the supplier is suggested on powder composition, processing method, and performance testing of the finished product.
10. Some PM parts may retain some porosity that the product might not be pressure-tight without subsequent impregnation of sealing materials.
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