CHAPTER 5: Sintering• process in which an assembly of loose or

compacted particles is metallurgically bonded into a coherent body at elevated temperatures.

• The driving force for solid state sintering is the excess surface free energy that accompanies the reduction in porosity.

• temperature is usually below the melting point of the major constituent of the powder.

• Heating is carried out in a controlled, inert or reducing atmosphere, or in a vacuum to prevent oxidation

• During the process the part shrinks and densify, without losing its pressed or moulded shape.

• Sintering is a complex process and for any given metal and set of sintering conditions there are likely to be different stages, driving forces and material transport mechanisms associated with the process.

• it generally involves 6 distinct stages;– Initial bonding among particles– Neck growth– Pore channel closure– Pore rounding– Densification or pore shrinkage– Pore coarsening

• Material transport mechanisms involved include surface diffusion, volume diffusion, evaporation and condensation.

• Factors that affect sintering include– temperature, – time, – particle size, – compact porosity as well as – pre-alloying.

• Time and temperature however are the most significant factors.

5.1 Mechanism of Sintering• Particles bond with one another due to atomic

diffusion. • The driving force for sintering is the minimization

of the solid-vapour interface area and the elimination of pores.

5.1.1 Particle contact and neck formation• As sintering begins, small necks form and grow

between contacting particles by mass transfer via atomic diffusion.

• In fine powders the driving force of sintering is high because of the larger surface area per unit volume.

• This increases the solid-vapour interfacial energy.

• However some of this solid-vapour interfacial energy is used in creating new grain boundaries at the contact region between particles.

• Thus net energy depends on both surface energy and grain boundary energies

• During this stage mechanisms A, B and F dominate.

• Higher temperatures increase rate of neck growth because the diffusion coefficient, D, increases exponentially with T according to the equation

• Where Do is the pre-expontial factor which depends on atomic movement processes, Q is the activation enrgy, R is the universal gas constant and T the absolute temperature.

5.1.2 Channel closure stage• The particles become individually in-

distinguishable. • Densification is more pronounced as the pore

channels in the powder aggregate gradually and close, thus reducing the pore volume.

• The migration of pore volume becomes possible and pores continue to form a semi-connected phase throughout the aggregate.

• The diffusion and mass transfer mechanism that control this stage is A and E

• The time, t, required to achieve complete homogenization is inversely proportional to the diffusion coefficient D:

• Where l is the average diameter of the dispersed particles and k is a constant depending on alloy system parameters and alloy composition.

5.1.3 Pore isolation• The pores become isolated and are no longer

interconnected. • This occurs in the final stages of sintering. Volume

diffusion becomes the dominant mechanism of transportation.

• Another physical process that occurs in this final stage is pore coarsening, which increases the mean pres size while reducing the number of pores (and percent porosity).

• phenomenon is also called Ostwald ripening: there is competitive growth of larger pores at the expense of smaller pores which lose vacancies to the larger pores and disappear (they get annihilated).

• Generally, fine particles, high sintering temperatures and long sintering times promote better homogenization of the composition and thererefore better quality of product.

5.2 Activated sintering• used when rapid part production with low

energy consumption is required, i.e. T and t must be decreased.

• In this method specific chemicals (activators) are added to the powder during blending.

• The activator helps to form a low melting temperature phase which provides a high difussivity path for rapid sintering and lower activation energy barrier for diffusion to take place

• Ideally the activator must have very low solubility in the main component (base metal) of the powder in order to minimize contamination.

• On the other hand the base metal must have high solubility in the activator in order to lower the liquidus temperature.

• Example is the use of Ni, Pd or Pt as surface activators in tungsten powders

5.3 Liquid Phase Sintering• occurs in powder with a large range of melting

temperatures of metal components. • In such systems a liquid may form as a result of

one phase of the system. • mass transport and sintering kinetics are

improved due if the following conditions are met;– The liquid wets the solid phase and uniformly coats it– The diffusivity of the solid’s atoms in the liquid is large– The solid is soluble in the liquid

• A wetable liquid film enables the surface tension to draw the powders together, thus aiding densification and pore elimination.

• The liquid film also lubricates the solid’s surface and facilitates particle rearrangement.

• These changes contribute to rapid decrease in the compact volume.

• LPS is observed in many metallurgical binary and ternary systems such as the Cu-Co, W-Cu, W-Ni-Fe, W-Ag, Cu-Sn, Fe-Cu and Co-Mo-Fe alloys.

Disadvantages of LPS

• Swelling may occur because the melt penetrates the boundaries in solid phase and the solid disintegrates into smaller particle and separate.

• This reduces the part density. • Swelling can be controlled by selecting fine

powders, low compacting pressures and slow heating rates.

5.4 Sintering Procedure• For production purposes sintering is carried out

in continuous furnaces that comprise of 4 zones

Zone 1: Lubricants burning off (or delubing zone)

• The pressed parts are slightly heated to burn and sweep out lubricant.

• The fumes from this zone are subsequently vented off.

• atmosphere is slightly oxidizing and this helps to convey heat quickly and uniformly.

• For steels the temperature range is 425-700oC

Zone 2 Sintering• The parts are heated to sintering temperature

(700-1120oC for steels). • Neck formation and channel closure occur in

this range. • The atmosphere in this zone must be neutral or

reducing. • For better sintering oxidation must be avoided. • Oxide layers on in coming particles have the

following effects– Prevent wetting of the particle surfaces and this

effectively reduces mass transfer by diffusion

– Reduces the bonding strength between individual particles

– Prevents pore rounding and this affects structural integrity and toughness of the resultant part

• Zone 3: Recarbonisation (for steels)• During slow cooling to 850oC in endorthemic

and neutral or reducing atmosphere, carbon is restored to the surface of the part. The cooling rate is also controlled

Zone 4: Cooling• Thepurpose of this zone is to cool down the

part (from 850oC) to ambient temperature. • The process should however prevent oxidation

of the material and hence the atmosphere is slightly reducing, neutral or slightly oxidizing.

• Once the part gets to room temperature sintering is finished and the next step involves finishing processes prior to selling

References• 1. Edwards L. and Endean M: Manufacturing

with materials• 2. D.K. Singh Fundamentals of Manufacturing

Engineering