Microstructure

The microstructure of green metal powder compacts reveals the outlines of the original powder particles and the pores. Depending on the particle size distribution of the powder from which the compact is pressed, powder particles and pores that are visible in the microstructure may have narrow or wide size distributions. If the powder particles are polycrystalline, grain boundaries may be observed within the particles. Particles may be flattened and distorted, depending on the nature of the powder and the compacting pressure. Typically, uniaxial compaction creates flattened pores that subsequently contribute to anisotropic dimensional change in sintering.

The microstructure of a green compact pressed from copper powder at a pressure of 180 MPa (26,000 psi) in the unetched condition is shown in Fig. 5. This microstructure indicates that the copper powder had a fairly wide particle size distribution and that some of the particles had interior porosity. Most of the boundaries between particles can be readily discerned. When this compact was sintered 15 min at 705 °C (1300 °F), a temperature too low for adequate sintering, the microstructure shown in Fig. 6 was obtained. The principal difference between the microstructures of the green compact and the one sintered at 705 °C (1300 °F) is in the shape of the pores within the powder particles, which have become rounded during sintering. The sintering temperature is too low to produce adequate bonding between powder particles; however, significant strengthening occurs by this temperature.

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Fig. 5 Microstructure of green compact. Pressed from copper powder at 180 MPa (2,600 psi). Unetched. 450x

Fig. 6 Microstructure of copper powder compact. Pressed at 180 MPa (26,000 psi) and sintered 15 min at 705 °C (1300 °F). Unetched. 450x

Figure 7 shows the microstructure of the copper powder compact sintered 30 min at 980 °C (1800 °F). The sample was etched to reveal the grain boundaries in the microstructure. Boundaries between particles can no longer be discerned; instead, a network of grain boundaries similar to those in wrought and annealed copper is illustrated. Pores of different sizes filled with the medium used to mount the specimen for microstructural examination are seen. These pores are no longer cusp-shaped and irregular, but have become rounded. In compacts made of small powders, complete spheroidization of the pores often is observed.

Fig. 7 Microstructure of copper powder compact. Pressed at 180 MPa (26,000 psi) and sintered 30 min at 980 °C (1800 °F). Potassium dichromate etch. 500x

The transition in microstructure from that of a green compact to that of a well-sintered compact is a function of sintering temperature and time. During annealing of cold worked wrought material, a recrystallization temperature is reached, where nuclei of strain-free grains are formed that grow into the recrystallized structure. Subsequently, grain growth of the recrystallized structure occurs. This temperature for recrystallization and grain growth is a function of the time during which the cold worked material is annealed and the amount of prior cold work. The temperature range in which the typical microstructure of a well-sintered powder compact is developed is much higher than the recrystallization temperature of the wrought material. Compacting pressure frequently has a relatively minor effect on the temperature range at which the sintered structure is developed.

Development of the typical well-sintered structure of a metal powder compact requires that grain growth occur across prior particle boundaries. This grain growth is restricted until material transport that occurs during sintering has progressed to a point where a substantial increase in the contact area between particles has taken place. This increase in contact area is impeded by the network of pores. It is not responsive to the effects of strain hardening during compacting, which are relieved at temperatures below those at which the development of a sintered microstructure is observed. The principal effect of higher compacting pressures on grain growth during sintering is to facilitate extensive contact between particles.

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