Sintering

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

Schematically it can be shown as

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

The three stages in the dry sintering can be shown as

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

Mechanisms of 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

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


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”


lvslsv )cos(


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


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

Sintering Procedure

Schematically

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


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


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


The most widely used atmospheres primarily because of their lower cost, are produced by partial combustion of hydro-carbons


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


Dimensional accuracy can be greatly improved by re-pressing the part after sintering. This operation is called “sizing”


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


3) Hot Isostatic Pressing

HIP is used as a post-sintering operation to eliminate flaws and micro-porosity in cemented carbides


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


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


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


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


8) Surface-Hardening

Carburizing and carbonitriding of PM parts is extensively used, and again gaseous media are indicated


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


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


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


12) Coatings

A large percentage of hard metal cutting tool inserts are now coated using chemical vapor deposition (CVD) or physical vapor deposition (PVD)


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


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


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

Sintering

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