Densification of Metal Powders During Sintering, 1973
1973 Series

Sintering of powder metal compacts is one of the basic oper­ ations in powder metallurgy. The useful properties of a machine part are obtained after considerable densification of the sintered material. Although the mechanical properties of the part depend on other structural factors besides porosity, porosity is the main factor. Usually, the practical problem in sintering is to obtain a part with the desired or permissible porosity. Thus, knowledge of the laws governing densification and its final result is neces­ sary to control this process in the production of powder metal parts. The laws governing densification are also important for a more exact physical theory of sintering, which is still in the initial stages of its development. Such processes as the change in the density of lattice defects and the flow of crystalline substances during sintering have not yet received a complete physical inter­ pretation. Analysis of the laws of sintering may provide addition­ al material for more complete phenomenological characteristics of these processes that will be useful for further development of theoretical concepts of the flow of imperfect crystals under small loads. Although a substantial amount of experimental material has been accumulated, generalizations are still difficult.

Notation.- I Laws Governing the Relationship between the Initial and Final Densities of Sintered Bodies.- II Conditions for Observing Densification Process in Pure Form.- III Volume of Pores in Relation to Isothermal Sintering Time.- IV Phenomenological Importance of the Constants of the Kinetic Equation and Their Dependence on Temperature.- V Basic Differences in the Densification Process in Crystalline and Amorphous Bodies.- VI Change in the Surface and Volume of Pores under Various Sintering Conditions.- VII The Flow of Metal under the Influence of Surface Tension at Room Temperature.- VIII Phenomenology of Sintering and Modern Theoretical Concepts.- IX Quantitative Estimate of the Effect of the Geometric Factor.- X Quantitative Estimate of the Effect of the Substructural Factor.- XI Phenomenological Theory of Sintering.- XII Calculating Densification from the Kinetic Constants of the Powder.- XIII Clarification of the Nature of Phenomenologically Elementary Processes and Unresolved Problems of Theory.- XIV Phenomenological Generalizations and Sintering Practice.- Method of Determining the Kinetic Constants of the Powder and Calculating the Reduction in Volume of Pores and Other Densification Characteristics from the Constants of the Powder.- Literature Cited.