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 Al 2 O 3 ) 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
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
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. CO 2 laser. 3. Liquid state laser. 4. Semiconductor laser
· 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/m 3 ): 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. Processes to manufacture powders Powder preparation ( adding of alloys and die lubricants) Making of partsSintering Finish operations Finish parts Metals Powders Finished powder 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.