Fig 3 Variation of reduction of area and impact strength as a function of oxygen content Source Ref 3 Forging Process

A major key to successful powder forging is proper preform design because it has a significant effect on the metal flow and distribution of stresses in the material during forging. These factors, in turn, affect the densification and probability of fracture. In general, sufficient metal flow must take place to achieve full density and good bonding across collapsed pores. However, increasing the amount of metal flow also increases the possibility of fracture. Thus, preform shape design involves a tradeoff between the lower limit of deformation to achieve the required properties, and the upper limit that would cause fracture.

As shown in Fig. 1(a) and 2(a), the deformation during forging involves considerable lateral flow and shearing of the powder particles. This shearing action causes any oxide films on the powder particles to be broken up, exposing clean metal, and enabling a strong metallurgical bond across collapsed pore interfaces. As a result, the dynamic properties of the material are enhanced. Figure 4, for example, shows that the impact energy of as-forged alloy steel powder preforms increases steadily with increasing deformation up to --■60% reduction. Similarly, Fig. 5 shows that the fatigue limit for powder forged 4620 steel increases with increased deformation (Ref 4).

Fig. 4 Increase in impact resistance with increasing forging deformation of sintered nickel steel (4600 series) powder

Fig. 5 Axial fatigue of P/M forged 4620 steel for various levels of forging deformation. Fatigue limit increases as deformation level (height strain) increases. Source: Ref 4

Large deformation of sintered powder material can easily lead to fracture because tensile stresses usually develop, and pores in the material provide many sites for concentration of these stresses, leading to fracture. To overcome this limitation a fracture criterion has been developed for predicting fracture during powder forging. Based on the occurrence of fracture during upset compression testing, a locus of surface strains at fracture can be generated, as shown in Fig. 6 (Ref 5). For a given material, strain combinations in the material above this line will lead to fracture, while strain combinations below the line are safe. Such fracture lines can be generated for any material.

Fig. 6 Tensile strain vs. compressive strain at fracture of 4620 steel powder cylinders and sintered under various conditions. Dashed line represents homogeneous deformation (zero tensile stress).

To illustrate the important role of preform design on defect-free powder forging, consider the part shown by shading within the tooling configuration in Fig. 7. This part is axisymmetric, consisting of an outer rim (upper section) formed by the upper punch, and a hub (lower section) formed by the lower punch. A flange is formed between both punches, and an outer die and core rod complete the tool set. A preform containing the rim and hub features is not considered because metal flow would be limited, leading to incomplete densification. Rather, the preform for this part would simply be a cylinder with a hole at the center. Forging the part from a cylindrical preform will involve various combinations of back extrusion of metal into the rim, forward extrusion into the hub, and lateral (radial) flow in the flange. However, as seen in Fig. 8, for a given volume of material, the inside and outside diameters of a cylindrical preform can have a variety of combinations. Using the fracture criterion in Fig. 6 and knowledge of the stresses in the various modes of deformation, the proper preform can be determined.

Fig. 7 Prototype part for illustration of preform design

Fig. 8 Preform options for forging the part shown in Fig. 7. See text for discussion of (a) through (d).

In Fig. 8(a), the preform leads to successful forging because radial expansion of the outer surface is prevented, forcing inward flow to form the hub. This inward flow is in compression and does not lead to fractures. Meanwhile, the rim forms by backward extrusion without radial expansion and is also safe from fracture. The preform in Fig. 8(b) allows considerable radial expansion and flow of material around the upper corner radius into the rim, which easily leads to fracture on the inside of the rim. In Fig. 8(c), the preform is constrained from lateral flow on both the inner and outer diameters; the resulting forward extrusion into the hub section causes large tension on the surface of the hub, leading to fracture. The preform in Fig. 8(d), like Fig. 8(b), leads to fracture on the inside surface of the rim as material flows around the upper corner of the punch into the rim.

It is clear that, even though a cylindrical preform is the obvious choice for this part, the specific dimensions of the preform must also be determined. Subtle changes in the preform dimensions mark the difference between successful and unsuccessful forging. This observation is true in preform design for nearly every part.

For more complex shapes, preform design is a major challenge, since a wide variety of deformation modes and combinations may occur. Preform design for connecting rods, for example, requires specification of the preform dimensions for the circular sections at the pin end and the crank end, as well as the beam section connecting these ends. Lateral flow and extrusion occur locally in all three sections, and material flow may occur across the intersections as well. Preform design for this complex case requires careful determination of the weight of material in each section and detailed analysis of the localized metal flow in each section. Unlike most preforms, it is necessary to specify different densities in each of the three sections of the preform to ensure full densification and avoid defects in the finished connecting rods.

The tooling for powder forging is based on the trap die concept, an example of which is shown in Fig. 7. The material is completely trapped between the punches and die, with no flashland. While the trap die concept produces a near-net shape that does not require removal of excess metal in a flashland, it does lead to high stresses on the tooling. Combined with the high temperatures of forging, these stresses and metal flow lead to high wear rates on the punches and die. The most commonly used material for the dies is H13, a hot-working die steel. With proper preform design and cooling of the tools between press strokes, 50,000 to 100,000 parts can be produced before die refurbishment is required.

The temperature of the preform as it enters the forging process influences the mechanical properties of the forged parts as well as the life of the forging tooling. Higher forging temperatures enhance densification of the forged part because plastic flow occurs more easily, as shown in Fig. 9 (Ref 6). On the other hand, higher forging temperatures lead to greater die wear. Typically, steel powder preforms are forged at 980 °C (1800 °F) as the optimum temperature to minimize die wear and ensure part densification.

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