Transcript powder parts - Dr. Habeeb Al-ani E

HABEEB HATTAB HABEEB
Office: BN-Block, Level-3,
Room-088
Email: [email protected]
Ext. No.: 7292
H/P No.: 0126610058
Nontraditional
Processes and
Powder Metallurgy
NONTRADITIONAL PROCESSES AND POWDER
METALLURGY
NONTRADITIONAL PROCESSES
The machining (cutting) processes, described before, remove material by
chip formation or abrasion. However, there are situations, where these
processes are not satisfactory, economical or even possible. This has led
to the development of other advanced processes called nontraditional
processes to distinguish them from traditional cutting processes. There
are at least four types of these processes:
1 – Mechanical: Ultrasonic Machining UM.
2 – Chemical: Chemical Milling.
3 – Electrochemical: Electrochemical Machining.
4 – Thermoelectric: Laser Beam machining LBM.
Ultrasonic Machining
· Principle: Ultrasonic cutting is done by abrasive grains (boron carbide,
silicon carbide, and Al2O3) forced into the work piece by linear
oscillation of a tool. Abrasives are the cutting edges of the tool and they
carried by a liquid that flows between the work piece and the tool.
A transducer causes the attached tool to oscillate linearly at a high
frequency of 20kHz with a very small amplitude. The tool oscillating
motion is produced by being a part of a sound wave energy transmission
line that causes the tool material to change its length by contraction and
expansion. This will cause the impact forces of abrasives against the
work piece.
# Application: machining hard and brittle materials (glass, ceramic,
tool steel, and carbides).
•# Factors that affect the metal removal rate, roughness and accuracy:
Frequency, amplitude, impact forces, tool material and abrasives.
# Tools are made of brass or soft steel. The tool shape is the mate of the
surface to be machined.
•# (Adv) 1.Absence of thermal stresses.
2. Low cost of tools.
3. No need for employing high skilled people
(Disadv)) 1. Limited to brittle materials.
2. Small material removal rate.
Electrical Discharge Machining EDM
•EDM is the process of removing particles from a conductive metal by
means of an electrical discharge (spark) in the presence of a dielectric.
•Principle: EDM consists of a shaped tool (electrode) and the workpiece
connected to a dc power supply and placed in a dielectric (electrically
non-conducting fluid).
When the potential difference between the tool and the work piece is
sufficiently high (because of the condenser), it overcomes the
dielectric and a spark discharges through the fluid, removing a very
small amount of metal from the work piece.
·
Functions of dielectric fluid:
1. To wash away the product of cutting
2. To maintain a uniform resistance to current flow uniform cutting
conditions.
3. To provide a cooling medium for the tool and work.
Examples of dielectrics: oil and kerosene.
·
The rate of metal removal depends on:
1. The current density.
2. Frequency of sparks.
The rate increases with increasing the current density and decreasing
frequency of sparks..
It does not depend on the hardness or strength of the work since EDM
does not involve any mechanical actions and contact between the tool
and work.
•Tools (electrodes) are made from soft, and easily shaped material since
there is no contact with the work. The tool is shaped to the desired
contour of the work piece.
•Application: die cavities, deep drilling of small diameters, turbine
blades and carbides.
•·
(Adv) Can be used for any soft or hard conductive material.
•·
(Disadv)
1. Limited to conductive materials only.
2. Small rate of metal removal.
Travelling-wire EDM
It It is a variation of EDM.
Principle: a slowly moving wire travels along a
prescribed path, cutting the work piece.
Electro-Discharge Machining (EDM)
- Sparks between electrode-workpiece
- Dielectric flushes the metal powder
- Inexpensive, precise, complex shapes
- Workpiece must be a conductor
Electrode EDM
Wire-cut EDM
This process is used to cut off thick plates, and for machining tools and
dies with complicated shapes and design.
In this process there is no mechanical cutting but sparks are generated
between the wire (electrode) and the workpiece. These sparks cause the
small particles to be removed from the workpiece. The workpiece
together with the wire should be placed in a tank and submerged by a
dielectric fluid.
Electrochemical Machining ECM
ECM is basically the reverse of electroplating.
Principle: An electrolyte as current carrier. The metal ions from the work
piece surface (anode) are discharged using the potential energy between
the tool and the work. Before they have the chance to plate on the tool
(cathode), they are washed away because of the high rate of the
electrolyte movement.
·
The tool is made from brass, copper or bronze.
·
The electrolyte is highly conductive salt solution (sodium chloride)
·
(+) Advantages:
1. Hard or soft conductive metals can be cut.
2. No heating is produced.
3. No metallurgical changes as high temperatures do not occur.
4. Tools can be made from soft and easily shaped metals, as there is no
contact between the tool the work.
5. Good surface finish.
6. No sharp edges on the work ( no need for deburring)
Electrochemical Machining (ECM)
Reverse of electro-plating (workpiece is anode)
Main uses:
- Dies and glass-making molds, turbine and compressor blades, Holes, Deburring
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Laser Beam Machining LBM
· The term laser is an abbreviation of “ Light Amplification by
Stimulated Emission of Radiation”.
The laser in reality is a very strong and intense beam of light that is
highly collimated. This beam can be focused optically onto an area to
heat it to very high temperatures, causing material evaporation.
Types of lasers:
1. Ruby (glass) laser.
2. CO2 laser.
3. Liquid state laser.
4. Semiconductor laser
Laser cutting
Microscope image of laser cut hole
·Application: (Laser machining is a thermoelectric process)
•1. Laser machining is used for small jobs because of the small rate
of material removal. Examples: Drilling microscopic holes and
removing metal in balancing high-speed rotating machinery.
•2. Welding.
•·
(+) Advantages:
•1. Can vaporise any material.
•2. Produces very small heat –affected zones.
•3. Can be used for non-metallic hard materials.
•·
(-) Disadvantages:
•1. High cost of the laser equipment.
•2. Low operating efficiency.
•3. Difficulty in controlling accuracy.
•4.
Used only for small parts.
•5.
Low metal removal rate.
•Electron Beam Machining EBM
EBM is, like LBM, a thermoelectric process.
The source of energy is high-speed electrons, which strike the
surface of the work piece and generate heat, where the beam is focused.
The process is carried out in a vacuum chamber. The EBM utilises high
voltages (50kV – 200kV) to accelerate the electrons to speeds of 50% to
80% of the speed of light.
•·(+) Advantages:
•1.
Accurate cutting (close tolerances).
•2.
Small heat-affected zones.
·(-) Disadvantages:
1. High equipment cost.
2. High skilled people.
3. The work should be shielded because of the X-ray emission.
4. Cannot cut parts of big sizes because of the vacuum chamber.
·Applications: 1. Drilling holes, 2. In semiconductor industry.
POWDER METALLURGY P/M
Metallic powders are formed into parts by pressure and heat (sintering).
Sintering = Heating below the melting point temperature.
Important Characteristics of Metal Powders:
1. The shape of the powder particle: Spherical, flat, and angular.
2. Fineness: It refers to the particle size. Size is defined by passing the
particles through standard sieves
3. Particle size distribution: It refers to the amount of each particle size
in the powder.
4. Flowability: The ability of a powder to flow readily and conform to
the mould cavity.
5. Chemical properties: The existence and percentage of oxides, alloys
and other chemical elements is defined.
6. Compressibility: It is the ratio of the volume of the initial powder to
the volume of the compressed part.
7. Apparent density (kg/m3): It should be constant so that the same
amount of powder can be fed into the die cavity each time without any
shortage or excess.
* Important Characteristics of Metal Powder PARTS (After pressing):
Strength and machinability.
Flowchart (Steps of manufacturing Metal Powder Parts):
The chart shows the sequence that should be followed to manufacture
metal powder parts.
Metals
Processes to manufacture
Powders
powders
Powder preparation ( adding of alloys and die
lubricants)
Finished powder
parts
Finish operations
Finish parts
Sintering
Making of parts
I. Processes to Manufacture Parts:
Purpose: to produce metal in the powder form.
Principal metals used in P/M: Iron, copper, brass, bronze, aluminium,
and nickel.
Types of MFG processes of powders are not the same for all metals.
Mills, grinders, and crushers are used to crush and soften the metal and
convert it into powder.
II. Powder Preparation
1. Prealloyed powders:
Metal powders are prealloyed to impart them better properties
(corrosion and heat resistances, and high strength).
2. Precoated powders:
Metal powder particles are sometimes precoated to provide the powder
with additional properties.
III. Making of Powder Parts:
•- Before pressing, the powder should be carefully selected and its
characteristics should be examined.
•- Lubricants are added to the powder to reduce die wall friction and aid
part ejection.
•- Pressing: The die cavity is filled with powder. The powder is pressed
by means of a punch on hydraulic or mechanical presses.
•- Pressure: P = F/A
• IV. Sintering:
•· Sintering is the process of heating green compacts (pressed powder
parts) to a temperature below the melting point to allow bonding and
fusion of the individual particles.
·Principal variables in sintering:
1. Temperature: must be optimum.
2. The atmosphere of the furnace (oxides should be prevented).
3. Time.
V. Finishing Operations
In order to improve properties of sintered P/M parts or to impart
special characteristics to them, several additional operations may be
carried out after sintering:
1. Impregnation: Products like bearings are impregnated with oil by
immersing them in heated oil. The oil will fill the pores so the bearing
will have a continuous supply of lubricant.
2. Infiltration: is the process of filling the pores of P/M parts by a
molten metal to decrease porosity.
3. Coining and sizing: The products are repressed to impart dimensional
accuracy to them.
4. Plating: to improve appearance and resistance to wear and corrosion
5. Heat treatment: to improve hardness and strength.
6. Machining.
Some Principal Products of P/M
1. Filters: 97% porosity
2. Cemented carbides (inserts) for cutting tools: (tungsten carbide +
cobalt ).
3. Gear and pumps rotors: (iron + graphite)
4. Brushes for motors: (copper + graphite)
5. Bearings: (copper + tin + graphite)
6. Magnets.
Advantages of P/M
1. Some metals that cannot be machined by cutting, may be shaped by
P/M
2. Eliminates most of machining cutting (close tolerances).
3. No metal losses  Cost reduction.
4. No need for skilled people  Low labour cost
5. Porosity can be controlled.
6. Extreme pure products (without impurities).
7. Large-scale production is possible of many small parts.
Disadvantages of P/M
1. P/M parts are difficult to machine because of the high density they
obtain after pressing.
2. High cost of equipment.
3. Metal powders are expensive and difficult to store.
4. Sometimes, intricate designs in products are difficult to attain because
of the poor flowability of the particles during compacting.
5. Some powders (aluminium, magnesium, titanium) present explosive
and fire hazards in a finely divided state.
6. Some powders have thermal difficulties during sintering (oxide
formation).
7.Oxides may be produced during sintering.
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