Sintering in powder metallurgy

 

Sintering in powder metallurgy is defined as the compaction of smaller particles of metal powders by the application of heat below the melting point. In general, powder metallurgy is the science of manufacturing metal powders and finished/semi-finished items out of blended or alloyed powders both with and without nonmetallic elements. Sintering is the process of compacting either a loose powder aggregate or a green compact of the desired composition under controlled temperature and time conditions.

Powder metallurgy has several advantages, including an excellent surface polish, the ability to manufacture complex forms, and the fact that it is environmentally benign and energy-efficient. The increased cost of powder material and tools, as well as the less well-known method and weaker parts, are all negatives.

Now, let’s see about sintering in powder metallurgy.

Types of sintering

a)  Solid-state sintering                 

Consolidation of metal and alloy powders is a common difference in solid-state sintering. Densification happens primarily as a result of atomic diffusion in the solid-state.

Stages in solid-state sintering

First stage:

At the contacts points between the particles, necks are formed which grows continuously. Rapid neck growth takes place during this stage. Pores are irregular in shape and also interconnected.

Second stage:

With enough neck expansion, the pore channels become more cylindrical in the form at this stage. For small neck sizes, the curvature gradient is high, resulting in quicker sintering. The pore ultimately becomes rounded after enough time at the sintering temperature. The curvature gradient lessens as the neck increases, and sintering lowers as well. This means that pore volume does not change, but pore shape does, resulting in pores becoming spherical and isolated. As the sintering process continues, a network of pores and a solid particle skeleton form. Throughout the compact, the pores continue to create a linked phase.

Third stage:

Pore channel closure occurs at this stage, and the pores become isolated and unconnected. Even after long sintering durations, porosity does not change and small pores remain.

Evaporation and condensation mechanism: 

The mechanism's essential premise is that the equilibrium vapour pressure over a concave surface (such as the neck) is lower than that over a convex surface (such as the particle surface). Between the neck region and the particle surface, a vapour pressure gradient is created. The vapour pressure gradient from the neck (concave surface) to the particle surface causes mass movement (convex surface).

Diffusion mechanism: 

The vacancy concentration gradient causes diffusion. A vacancy gradient is created between the two surfaces of two spheres in touch with each other. Surface diffusion, lattice diffusion, vapour transport, grain boundary transport from GB (grain boundary) source, lattice diffusion from sources on GB, and lattice diffusion from dislocation sources all contribute to the formation of the neck.

Viscous flow mechanism: 

Sintering happens due to the presence of lattice vacancies, according to this theory. This is critical for glass sintering. The fluidity of metal particles increases as the temperature rises. During sintering, Balshin hypothesized the following mechanisms as rearrangement of particles, change in particle shape, and grain growth.

Plastic flow mechanism: 

The migration of dislocations in the bulk flow of material has been hypothesized as a possible process for densification during sintering. During the sintering process, it was crucial to locating dislocation sources. During hot pressing, the plastic flow mechanism is dominating.

Mechanism in solid-state sintering

The mechanisms in solid-state sintering are as follows

• Evaporation and condensation mechanism

• Diffusion mechanism

• Viscous flow mechanism

• Plastic flow mechanism

b) Liquid phase sintering                

The use of a modest amount of liquid phase improves densification (1-10 percent vol). At the sintering temperature, the phase of liquid within the particles has some solubility for the solid. Between the solid materials of the compacted sample, a significant amount of liquid is created. The liquid phase crystallizes at the grain boundaries during sintering, bonding the grains together. A rapid rearranging of solid particles occurs at this stage, resulting in an increase in density. Solid-phase sintering happens later in the process, resulting in grain coarsening and a slowing of the densification rate. Used in the sintering of tungsten-copper and copper-tin systems. Covalent compounds such as silicon nitride and silicon carbide, which are difficult to sinter, can also be formed.

Mechanism in liquid phase sintering

The liquid phase created during sintering aids in compact densification. A little amount of a second ingredient with a low melting point is used in liquid phase sintering. This liquid phase aids in the densification of the compact by binding the solid particles together. This method is frequently employed in the production of ceramics, such as porcelain and refractories.

This procedure requires three basic factors:

• The presence of a significant amount of liquid phase

• The solubility of the solid in liquid

• Complete wetting of the solid by liquid.

c)  Activated sintering

In this case, a little quantity of an alloying element called "doping" is applied, which enhances densification by up to 100 times over undoped compact samples. Nickel doping in tungsten compacts is an example.

d)  Reaction sintering

High-temperature materials originate from chemical reactions between specific constituents in this process, resulting in excellent bonding. When two or more components react chemically during sintering to form the final object, this is known as reaction sintering. The interaction of alumina and titania at 1553 K to generate aluminium titanate, which then sinters to form a densified product, is a good example.

Rate controlled sintering, microwave sintering, gas plasma sintering and spark plasma sintering are some of the other techniques that have been developed and used

Sintering theory

Sintering can involve:

1) A single component system, in which self-diffusion is the primary material transport mechanism and the driving force is generated by a chemical potential gradient caused by surface tension and capillary forces between particles.

2) A multi-component system, in which self-diffusion is the primary material transport mechanism and the driving force is generated by a chemical potential gradient caused by surface tension and capillary forces between particles.

In 1922, the first theory was proposed by Sauerwald. There are two stages like adhesion and recrystallization in this theory.  Due to atomic attraction, adhesion occurs during heating, and recrystallization occurs at the recrystallization temperature (over 0.5 Tm). Microstructure changes, phase shifts, grain expansion, and shrinkage all occur during recrystallization.

Properties changes during sintering

• In a single-component system, densification is proportional to shrinkage or the number of pores eliminated.

• Densification occurs when a multi-component system expands rather than shrinks; hence densification cannot be linked to the quantity of porosity removed.

• Densification alters mechanical qualities such as hardness, strength, and toughness, as well as physical properties such as electrical, thermal, and magnetic conductivity. Due to the creation of a solid solution, a change in composition is also expected.

Sintering atmosphere

Functions of sintering atmosphere:

• Avoiding undesired sintering reactions

• Promoting surface oxide reduction

• Facilitating the insertion of other sintering and alloying components that improve sintering rate 

• Assisting with the elimination of lubricants

• Composition control as well as adjusting impurity levels

Example: For sintering atmosphere, Pure hydrogen, ammonia, and reformed hydrocarbons, gases, inert gases, vacuum, nitrogen-based mixes without the addition of carburizing agents and mixtures based on nitrogen with a carburizing component.

Transport type	Material transport mechanism	Driving force
Vapour phase	Evaporation- condensation	Vapour pressure gradient between convex and concave regions
Solid-state	Diffusion- surface diffusion Grain boundary diffusion, volume diffusion Viscous flow, plastic flow	Chemical potential Chemical potential Chemical potential
Liquid phase	Viscous flow Solution precipitation	Surface tension Surface tension

 

The steps in powder metallurgy are the production of powders, compaction, sintering, and secondary operations. In this article, we have seen only sintering in powder metallurgy. Hereafter in upcoming articles, we will see about the remaining parts in powder metallurgy. I hope this article will provide you some ideas on sintering in the powder metallurgy field.