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Theoretical Considerations

As sintering usually follows powder conpaction, we discuss the pressure distribution in powder compaction first. Then, we discuss the sintered material density and the various physical, chemical, and metallurgical phenomena that occur during the sintering stage.

Pressure distribution in powder compaction

The pressure distribution along the length of the compact is determined using the slab method of analysis of deformation process.

Let D be the diameter of the compact, L its length, and po the pressure applied by the punch.

The figure below shows an element dx thick with the relevant stresses: the compacting pressure px, die-wall pressure , and the frictional stress .

Balancing the vertical forces and simplifying, one gets:

D dpx + 4 dx = 0

We introduce a measure k of the interparticle friction during compaction. We get a relationship between px and :

= k px

k = 1 when there is no friction between the particles. In this case, the powder behaves like a fluid, i.e., we have a state of hydrostatic pressure.

Substituting for in the first equation, we get an ordinary differential equation in px. Integrating, we get:

px = po e(-4 k x / D)

Thus the pressure within the compact decays as the coefficient of friction, the parameter k, and the length-to-diameter ratio increase.

Sintering

Prior to sintering, the compact is brittle and its strength, known as green strength, is low. Bonding and fusion of the individual particles occur during sintering. The nature and strength of the bond between the particles depend on the mechanisms of diffusion, plastic flow, evaporation of volatile material in the compact, recrystallyzatioin, grain growth, and pore shrinkage.

Sintered Material Density

The sintered density of a part depends on its green density and the sintering conditions in terms of temperature, time, and furnace atmosphere. As the value of this parameters increase, the sintering density increases. Also, the density increases with a less oxidizing type of furnace atmosphere.

One desire to produce a part of high density without allowing much increase in density during sintering. The reasons are the following:

1. For structural parts, a higher sintered density is very desirable, as it leads to better mechanical properties.
2. For better dimensional accuracy, it is preferable to minimize the increase in density during sintering.

Two stages of sintering are distinguished based on time and temperature: conventional and high temperature sintering. Some physical, chemical, and metallurgical phenomena are attributed to each stage. It is important to note that the phenomena attributed to the first stages (conventional) continue during high temperature sintering.

Early Stages:

1. Homogenization: The as-cast, dentritic structure of the atomized particles is removed and microsegration within the particles is eliminated. Diffusion between powder particles begins to occur.
2. Alloying: As the diffusion process continues, admixed additives begin to form alloyed structures with the base ferrous particles. If one element has a lower melting point than the other, it may melt, and its particles then surround the particles that has not melted by surface tension (liquid-phase sintering). Stronger and denser parts can be produced this way such as when cobalt melts in tungsten-carbide parts. For species such as carbon, this takes place early in the sintering process. For elements such as nickel or molybdenum, diffusion is much slower and takes longer times and higher temperatures to achieve a reasonable level of homogeneity.
3. Removal of gases/oxides: Chemical reactions between the sintering atmosphere or admixed additives such as graphite and the surface oxides on the metal particles also begins early in the sintering cycle. This breakdown of oxides and removal of absorbed gases cleanses the metal particle surfaces and promotes the diffusion process.
4. Particle bonding: The formation of solid bridges or necks between individual or clusters of powder particles is the critical result of the early stages of sintering. These particle bonds give the powder mass integrity and mechanical strength.

Advanced Stages:

1. Densification: As the sintering process continues at higher temperatures, the inherent porosity in the powder mass is reduced as pores are eliminated by bulk diffusion to grain boundaries. This reduction in the amount of porosity results in an increase in the density of the powder compact.
2. Porosity shape: The remaining pores in the P/M structure lose their angular, irregular nature and become smooth, tending toward perfect spheres, as eh sintering temperature increases.
3. Grain growth: The individual powder particles lose their identity completely as grain boundaries move across prior particle boundaries. Larger grains replace the original fine particle structure.
4. Liquid phase: Depending on the chemical constituents in the powder mass and the sintering temperature, a transient or permanent liquid phase may be formed. This liquid phase will accelerate particle rearrangement and diffusion, thereby aiding densification and pore elimination. For some additives, such as copper and phosphorus, liquid phase sintering will occur at conventional temperatures, while for silicon iron and tool steels high temperature sintering is required.

On the macro level, as the degree of sintering improves, as indicated by a higher sintering temperature, the properties of the material such as strength, density, ductility, and thermal and electrical conductivities increase.