sintering
DESCRIPTION
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Sintering
The thermal treatment of a powder or compact at a temperature below the melting point of the main constituent, for the purpose of increasing its strength by bonding together of the particles
What Happens During Sintering?
1. Atomic diffusion takes place and the welded areas formed during compaction grow until eventually they may be lost completely. 2. Recrystallisation and grain growth may follow, and the pores tend to become rounded and the total porosity, as a percentage of the whole volume tends to decrease.
Sintering Atmospheres
The operation is almost invariably carried out under a protective atmosphere, because of the large surface areas involved, and at temperatures between 60 and 90% of the melting-point of the particular metal or alloys.
Particular advantages of the powder technology include:
1.Very high levels of purity and uniformity in starting materials2.Preservation of purity, due to the simpler subsequent fabrication process
(fewer steps) that it makes possible3.Stabilization of the details of repetitive operations, by control of grain size during the input stages4.Absence of binding contact between segregated powder particles – or "inclusions" (called stringering) – as often occurs in melt processes5.No deformation needed to produce directional elongation of grains6.Capability to produce materials of controlled, uniform porosity.7.Capability to produce near net shape objects.8.Capability to produce materials which cannot be produced by any other technology.9.Capability to fabricate high strength material like turbines
Advantages of Powder Technology
1. Solid state sinteringOnly solid phases are present at the
sinter temperature2. Liquid phase sintering
Small amounts of liquid phase are present during sintering3. Reactive sintering
Particles react with each other to new product phases
Types of Sintering
We can divide these parameters into four broad categories
Powder preparation:-- Particle size-- Shape-- Size distribution
Important Parameters in Sintering
As with all processes, sintering is accompanied by an increase in the free energy of the system. The sources that give rise to the amount of free energy are commonly referred to as the driving forces for sintering. The main possible driving forces are
The curvature of the particle surfacesAn externally applied pressureA chemical reaction
Driving Force for Sintering
Three stages are distinguished in sintering
First StageAfter burn out of any organic additives, two things happen to the powder particles when the mobility of the surface atoms has become high enough; initially rough surface of the particles is smoothed and neck formation occurs
Stages of Sintering
Second StageDensification and pore shrinkage. If grain boundaries are formed after the first stage, these are new source of atoms for filling up the concave areas which diminishes the outer surface of the particle
Third StageGrain growth takes place, the pores break up and form closed spherical bubbles
Stages of Sintering
Sintering Mechanism:
Sintering occurs by diffusion of atoms through the microstructure. This diffusion is caused by a gradient of chemical potential – atoms move from an area of higher chemical potential to an area of lower chemical potential. The different paths the atoms take to get from one spot to another are the sintering mechanisms. The six common mechanisms are:
Surface diffusion – Diffusion of atoms along the surface of a particle
Vapor transport – Evaporation of atoms which condense on a different surface
Lattice diffusion from surface – atoms from surface diffuse through lattice
Grain boundary diffusion – atoms diffuse along grain boundary
Plastic deformation – dislocation motion causes flow of matter
Lattice diffusion from grain boundary – atom from grain boundary diffuses
through lattice
The pores in the compact are largely surrounded by the liquid phase and the driving force for sintering is liquid surface energy
With high liquid fractions, full density can be achieved almost entirely by rearrangement
Liquid Phase Sintering
The pores in the compact are largely surrounded by the liquid phase and the driving force for sintering is liquid surface energy
With high liquid fractions, full density can be achieved almost entirely by rearrangement
Liquid Phase Sintering
Grain boundary ‘wetting’ breaks the polycrystalline particle into single crystal particles in the initial stages of liquid phase sintering. These single crystal particles then spheroidize and coarsen
Liquid Phase Sintering
Liquid
Polycrystalline particle
Wetting is a very important phenomena which is happening during LPS
Figure represents the surface tensions of a multi-phase junction as vectors drawn parallel to the respective surfaces
Liquid Phase Sintering
γlv
γsl
γsv θ
The surface energies for different interfaces is given byγsl = solid/liquidγsv = solid/vaporγlv = liquid/vapor
The vectors representing these surface energies must balance at the three phase triple junction. This equation representing this balance is known as ‘Youngs’ equation”
Liquid Phase Sintering
lvslsv )cos(
Liquid Phase Sintering
γsl
γsv
γlv
θ
γsl
γsv
γlvθ
Large γsl, non-wetting θ Large
Large γsv, wetting θ small
Grain boundary wetting during LPS occurs when θA and θB approach zero
Liquid Phase Sintering
Grain A
Grain B
LiquidγBL
γALγAB
θB
θA
)cos()cos( AB ALBLAB
Powder Metallurgy parts-- Copper/Tin alloys -- Iron/Copper structural parts--Tungsten Carbide/Cobalt cemented carbides
Ceramics-- Silicon Nitride with a glassy liquid phase (2wt% alumina + 6wt% yttria)-- SiC with Silicon liquid phase
Examples of LPS
Compact slumping (shape distortion) which occurs when too much liquid is formed during sintering
The same parameters which control the sintered microstructure often control the final properties
Useful application temperature of the material is sometimes limited by the presence of too much low melting point material
Disadvantages of LPS
Two or more constituents in a compact react during sintering to form a new phase or phases
The reaction is normally exothermic and can contribute to an enhancement of sintering
In some cases the reaction is so exothermic that it can generate sufficient heat to cause self-sintering without external heating except that required for initiating the reaction
Reactive Sintering
This is the basis of combustion synthesis which if properly controlled can produce a relatively dense compact of the synthesized reaction product
Example of reaction sintering is
3TiO2 + 4AlN → 2Al2O3 + 2TiN + N2
Reactive Sintering
Ancient sintering techniques for the making of pottery and ceramic art objects remain in wide use to this day but research has also led to more advanced techniques which work for a wider array of ceramics
Sintering Procedure
In a typical sintering procedure
-- Most ceramic materials have a lower affinity for water and a lower plasticity index than clay, requiring organic additives in the stages before sintering
-- A mixture of binder, water and ceramic powder is pressed into a mold to form a green body (unsintered item)
Sintering Procedure
-- The green compact is placed on a mesh belt and moved slowly through the sintering furnace
-- In the preheat zone, the lubricant volatilizes, leaves the part as a vapor, and is carried away by the dynamic atmosphere flow
Sintering Procedure
-- The temperature within the furnace rises slowly in the preheat zone until
reaching the actual sintering temperature
-- It remains essentially constant during the time at that temperature, and proceeds into the cooling zone where the drop in part temperature is controlled
Sintering Procedure
As the parts travel through the furnace, the temperature cycle results in changes in composition and microstructure
In the hot zone metallurgical bonds develop between particles and solid state alloying takes place
The microstructure developed during sintering determines the properties of the part
Sintering Procedure
The operation is almost invariably carried out under a protective atmosphere, because of the large surface areas involved, and at temperatures between 60 and 90% of the melting-point
These are essential for almost all sintering processes, to prevent oxidation and to promote the reduction of surface oxides
Sintering Atmospheres
In practice dry hydrogen, cracked ammonia, and partially combusted hydrocarbons are mainly used
Although the first named is often precluded because of cost. It is however, used for sintering carbides and magnetic materials of the Alnico type
Sintering Atmospheres
It can replace pure hydrogen for many applications at approximately one-third the cost, with the obvious exceptions where reaction with nitrogen cannot be tolerated
It is particularly useful for sintering iron, steel, stainless steel, and copper-base components
Sintering Atmospheres
The most widely used atmospheres primarily because of their lower cost, are produced by partial combustion of hydro-carbons
Sintering Atmospheres
1)Re-Pressing
Even with the best control that is feasible in practice, there will inevitably be some variation in the dimensions of parts produced from a given material in a given die set
Post Sintering Operations
Typically, it is possible for parts 'as-sintered' to be accurate to a tolerance of -0.0508mm per mm, in the direction at right angles to the pressing -direction, and 0.1016mm per mm parallel to the pressing direction
Post Sintering Operations
Dimensional accuracy can be greatly improved by re-pressing the part after sintering. This operation is called “sizing”
Post Sintering Operations
2) Hot Re-PressHot Repressing will give even greater densification, with consequent greater improvement in the mechanical properties, but less accurate control of the final dimensions is to be expected
Post Sintering Operations
3) Hot Isostatic Pressing
HIP is used as a post-sintering operation to eliminate flaws and micro-porosity in cemented carbides
Post Sintering Operations
4) Forging
Forging is a comparatively recent technique in which a blank is hot re-pressed in a closed die which significantly changes the shape of the part, and at the same time can give almost complete density and hence mechanical properties approaching or even surpassing those of traditional wrought parts
Post Sintering Operations
5) InfiltrationAn alternative method of improving the strength of inherently porous sintered parts is to fill the surface connected pores with a liquid metal having a lower melting point. Pressure is not required, capillary action is sufficient
Post Sintering Operations
6) Impregnation
This term is used for a process analogous to infiltration except that the pores are filled with an organic as opposed to a metallic material
Post Sintering Operations
7) Heat TreatmentAlthough many, perhaps the bulk of sintered structural parts are used in the as-sintered or sintered and sized condition, large quantities of iron-based parts, are supplied in the hardened and tempered conditions. Heating should be in a gas atmosphere followed by oil-quenching
Post Sintering Operations
8) Surface-Hardening
Carburizing and carbonitriding of PM parts is extensively used, and again gaseous media are indicated
Post Sintering Operations
9) Steam Treatment
A process peculiar to PM parts is steam-treatment which involves exposing the part at a temperature around 500°C to high pressure steam. This leads to the formation of a layer of magnetite
Post Sintering Operations
10) Blueing
Heating in air at a lower temperature (200-250°C) can also be used to provide a thin magnetite layer that gives some increase in corrosion resistance, but it is much less effective than steam treatment
Post Sintering Operations
11) Plating
Sintered parts may be plated in much the same way as wrought or cast metals, and copper, nickel, cadmium, zinc, and chromium plating are all used
Post Sintering Operations
12) Coatings
A large percentage of hard metal cutting tool inserts are now coated using chemical vapor deposition (CVD) or physical vapor deposition (PVD)
Post Sintering Operations
13) Mechanical Treatments
Although a major attraction of PM parts is that they can be produced accurately to the required dimensions, there are limitations to the geometry that can be pressed in rigid dies, and subsequent machining, for example of transverse holes or re-entrants at an angle to the pressing direction is not uncommon
Post Sintering Operations
14) De-burring
De-burring is done with sintered parts, and is used to remove any 'rag' on edges, resulting from the compacting operation or a machining step
Post Sintering Operations
Particular advantages of this powder technology include:
1. the possibility of very high purity for the starting materials and their great uniformity
2. preservation of purity due to the restricted nature of subsequent fabrication steps
Advantages of Sintering
3. stabilization of the details of repetitive operations by control of grain size in the input stages
4. absence of stringering of segregated particles and inclusions (as often occurs in melt processes)
5. no requirement for deformation to produce directional elongation of grains
Advantages of Sintering