Powder Forging Powder Preparation

The most extensive application of powder forging is automotive drive train parts using high strength steels. Water-atomized low alloy steel 4620 is commonly used as well as iron-copper-carbon blended elemental powders (Ref 2). Powder characteristics, such as size and shape, are unimportant in the forging deformation process itself, but they may have important effects on the compaction and sintering processes prior to forging. For example, whether the powders are spherical or irregularly shaped, fine or coarse in size, they will lose their identity through the extensive deformation and accompanying densification during forging, and will have little effect on the deformation process. For the preform compaction process prior to forging, however, irregularly shaped powders are preferred because of their ability to interlock during compaction and produce compacts with sufficient green strength for handling. Coarse powders might also be preferred over fine powders because of ease of flow during compaction die filling, and because they present less surface area for contamination during sintering or heating for forging.

The chemical composition of the powders involved in powder forging is important, primarily in terms of oxide content, because the volume fraction of contaminants strongly influences the final properties of the powder forged component. If the oxide appears on the surface of the particles, it can be readily reduced by proper sintering practice or broken up and dispersed by shear deformation during forging.

Preforms for forging are fabricated primarily by conventional die compaction, although dry bag isostatic compaction has also been used for some applications. The overall density of the preform has no significant effect on the density of the forged parts, but variations in density from region to region of the preform are often designed to control metal flow and avoid defects during the forging process. Another consideration is the influence of preform density on the sintering process. Preforms of low density contain extensive interconnected porosity, allowing the sintering furnace atmosphere to reduce larger amounts of surface oxides on the powder particles. On the other hand, lower density preforms are more prone to internal oxidation and carburization during exposure to air while the preform is transferred from the furnace to the forging press. For this reason, protective coatings are generally applied to the preform before heating.

Control of the preform weight is critical because hot forging is carried out in a trap die without flash, as shown in Fig. 1(a). Excessive preform weight may lead to tool breakage or stalling of the press. Conversely, underweight preforms will not achieve full densification by forging and may not fill extreme corners of the die.

Sintering is the final critical step in preparation of preforms for forging. Sintering not only bonds the powder particles by diffusion, it also provides a protective or reducing atmosphere to prevent contamination and reduce the levels of oxide contamination that may be present in the particles. The oxygen level of the sintered preform, which depends on the sintering temperature, sintering atmosphere, dew point, and furnace type, determines the oxygen level of a forged part. The oxygen level of sintered preforms decreases with higher sintering temperature and lower dew point. Mechanical properties in general, and dynamic properties (impact and fatigue strength) in particular, are strongly influenced by the oxygen level of the forged part. Low oxygen levels lead to higher dynamic properties because the metallurgical bonds developed between particles during forging are stronger if the particle boundaries are free of oxides; see Fig. 3 (Ref 3).

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