The principle goal of the compaction process is to apply pressurize and bond the particles to form a cohesion among the powder particles. This is usually termed as the green strength. The compaction exercise imparts the following effects.
1. Reduces voids between the power particles and enhance the density of the consolidated powder,
2. Produces adhesion and bonding of the powder particles to improve green strength in the consolidated powder particles,
3. Facilitates plastic deformation of the powder particles to conform to the final desired shape of the part,
4. Enhances the contact area among the powder particles and facilitates the subsequent sintering process.
Compaction is carried out by pouring a measured amount of metallic powder into the die cavity and applying pressure by means of one or more plungers. To improve uniformity of pressure and reduce porosity in the compacted part, compressive forces from both the top and the bottom sides are necessary. The requisite compacting pressure depends on the specific characteristics and initial shape of the particles, the method of blending and the application of the lubricants. Extremely hard powders are slower and more difficult to press. Some organic binder is usually required to hold the hard particles together after pressing until the sintering process is performed
Compaction is an important step in powder processing as it enables the forming of loose metal powders into required shapes with sufficient strength to withstand till sintering is completed.
• In general, compaction is done without the application of heat. Loose powders are converted into required shape with sufficient strength to withstand ejection from the tools and subsequent sintering process. IN cases like cemented carbide, hot compaction is done followed by sintering. One cannot call this as compaction strictly, as sintering is also involved in this.
Powder compaction methods
Powder compaction techniques can be classified as,
1. Methods without application of pressure – i) loose powder sintering in mould, ii)vibratory compaction, iii) slip casting, iv) slurry casting, v) injection moulding
2. Methods with applied pressure – i) cold die compaction (single action pressing, double action pressing, floating die pressing), ii) isostatic pressing, iii) powder rolling, iv) powder extrusion, v) explosive compaction.
Pressureless compaction techniques
-Used for the production of simple and low density parts such as filters, other parts that are porous in nature; these techniques involve no external force and depend upon gravity for powder packing
I) Loose powder sintering: - Also known as loose powder shaping, gravity sintering, pressureless sintering. In this method, the metal powder is vibrated mechanically into the mould, which is the negative impression of the product and heated to sintering temperature. This is the simplest method and involve low cost equipment. The main reasons for not using this method for part production are, difficulty of part removal from the mould after sintering, & considerable shrinkage during sintering.
- Applications: Amount of porosity ranges from 40 vol% to 90 vol%; Highly porous filter materials made of bronze, stainless steel, and monel, porous nickel membrane for use as electrodes in alkaline storage batteries and fuel cells are typical examples.
II) Slip casting: - Used for compacting metal and ceramic powders to make large & complex shapes for limited production runs - A slip is a suspension of metal or ceramic powder (finer than 5 m) in water or other soluble liquid which is poured into a mould, dried and further sintered.
•Steps in slip casting: i) Preparing assembled plaster mould, ii) filling the mould, iii)absorption of water from the slip into the porous mould, iv) removal of part from the mould,v) trimming of finished parts from the mould • Sometimes mould release agents like oil, graphite can be used.
• Hollow and multiple parts can be produced
• Advantages of slip casting: Products that can not be produced by pressing operation can be made, no expensive equipment is required, works best with finest powder particles
• Disadvantage: slow process, limited commercial applications
• Applications: tubes, boats, crucibles, cones, turbine blades, rocket guidance fins; Also products with excellent surface finish like basins, water closets.
III) Slurry casting: This process is similar to slip casting except that a slurry of metal powders with suitable liquids, various additives, and binders is poured into a mould and dried. The solvent is removed either by absorption into the POP or by evaporation. Very high porous sheet for use as electrodes in fuel cells and nickel cadmium rechargeable batteries are produced by this method.
IV) Tape casting (doctor blade casting): - This is a variation of slurry casting process and is used to produce thin flat sheets.
-This process involves preparing a dispersion of metal or ceramic powder in a suitable solvent with the addition of dispersion agent (to improve the dispersion of the particles). Then a binder is added and fed to a reservoir. Whole mixture is fed on to a moving carrier film from the bottom of the reservoir.
-This slurry layer is deposited on the film by the shearing action of a blade. The slurry should be free of air bubbles, otherwise result in porosity. During sintering, the binder is burnt off first and densification of material occurs.
- In present days, endless stainless steel belt is used instead of carrier tape. This process can be used for making very thin tapes between 50 to 1000 m thickness. This method is used for making electronic substrates, dielectrics for capacitors and piezoelectric actuators.
V) Vibratory compaction: - Vibratory compaction uses vibration energy to compact the powder mass. During this process, smaller voids can be filled with particles of still smaller size and this sequence is carried out till a high packing density of powder is achieved even before consolidation. Mechanical vibration facilitates the formation of nearly closed packed powder by settling particles in the voids present in the powder agglomerate. During vibration, small pneumatic pressure is usually superimposed on the powder mass.
- Brittle powders can be compacted by this method as they develop crack if done by pressure compaction - This method is generally used when, 1) powders have irregular shape, 2) use of plasticizers for forming is not desirable, 3) sintered density is required to be very close theoretical density
- Important variables in vibratory compaction:
1. Inertia of system: larger the system, more the energy required for packing
2. Friction force between particles: more friction results in need of more KE for compaction
3. Particle size distribution: more frequency required if more large particles are present. Vibration cycle is important and not period of vibration.
Pressure compaction techniques
• These techniques involve application of external pressure to compact the loose powder particles; Pressure applied can be unidirectional, bidirectional or hydrostatic in nature.<
> • Die compaction: In this process, loose powder is shaped in a die using a mechanical or hydraulic press giving rise to densification. The mechanisms of densification depend on the material and structural characteristics of powder particles.
• Unidirectional and bidirectional compaction involves same number of stages and are described in this figure. They are, i) charging the powder mix, ii) applying load using a punch (uni-) or double punch (bi-) to compact powders, iii) removal of load by retracting the punch, iv) ejection of green compact. The table gives compaction pressure ranges for metals and ceramics.
Effect of powder characteristics
For a good compaction, 1) irregular shaped particles are preferred as they give better interlocking and hence high green strength,
2) apparent density of powders decides the die fill during compaction. Hence powder size, shape & density affect the apparent density,
3)flow rate affects the die fill time, and once again powder size, shape & density affect theflow rate.
Powder behavior during compaction
- Compaction involves
, 1) flow of powder particles past one another interacting with each other and with die-punch,
2) deformation of particles. In the case of homogeneous compaction, two stages are observed. First stage => rapid densification occurs when pressure is applied due to particle movement and rearrangement resulting in improved packing; Second stage => increase in applied pressure leads to elastic and plastic deformation resulting in locking and cold welding of particles. In the second stage, large increments in pressures are seen to effect a small increase in density.
• Compaction can be done at low and high temperatures. Room temperature compaction employs pressures in the range of 100-700 MPa and produce density in the range of 60- 90% of the theoretical density. At higher temperatures, pressures are kept low within the limits for preventing die damage.
• In single die compaction, powders close to the punch and die walls experience much better force than in center. This results in green density variation across the sample length. Longer the sample more the density difference. This non-uniformity can result in non-uniformity in properties of sintered part.
• This density variation and hence final property variation can be greatly reduced by having double ended die compaction. In this case, powder experiences more uniform pressure from both top and bottom, resulting in minimization of density variation. But this variation will still be considerable if the components have high aspect ratio (length to diameter ratio). This means that long rods and tubes cannot be produced by die compaction. In this case, isostatic pressing can be used.