September 28, 2017 | Author: Wai Ying Chong | Category: Welding, Casting (Metalworking), Annealing (Metallurgy), Extrusion, Rolling (Metalworking)
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CHAPTER 1 - METAL FORMING Application of forces exceeding the yield strength and below the ultimate tensile strength Work materials should have, Low yield strength High ductility Advantages 1. Fastest way to change shape of a part 2. Raw materials are cheap. (wrought steels like mild steel are the cheapest structural metals) Metal working temperatures (Page 2) Cold Working Plastic deformation below recrystallization temperature, material is work hardened. Advantages 1. Increased strength in direction of cold working 2. Increased surface hardness and wear resistance 3. Good dimensional tolerances and surface finish 4. Small parts can be shaped quickly 5. No heating required Disadvantages 1. Some metals too brittle to be cold-worked 2. High forces and power required for deformation 3. Work hardening makes subsequent operation difficult or impossible 4. Need annealing to relieve stress 5. Reduce corrosion resistance, increase electrical resistance, change magnetic properties Hot Working Plastic deformation above recrystallization temperature, material is stress relieved (totally annealed). Keep temperature low to minimize oxidation.

Advantages 1. Reduced chance of metal cracking, make bigger sections and more extreme shapes 2. Grain refinement possible a. Grain growth as temp rises b. Recrystallization produce small grains c. Grain growth can be controlled 3. No strain hardening, no annealing (cheaper) 4. Less power needed, smaller machine 5. Repairs casting defects 6. Improves ductility (material more homogenous) a. Better diffusion of alloy constituents b. Breaks up and evenly distributes “ c. Breaks up ad refines cast structure 7. Improves strength if correction of defects (holes and porosity due to casting/powder metallurgy) 8. Can be faster than cold working Disadvantages 1. Some metals cannot be hot-worked 2. Oxide layer on surface 3. Dimensional control difficult a. Metal contraction b. Oxide scales 4. Expensive and difficult to maintain high temp, dangerous to handle hot metal 5. Accurate temp difficult, even heating 6. Difficult to lubricate 7. Decarburization of steel 8. Carbon pick-up 9. Expensive dies ROLLING (page 8) Rotating cylinders to draw and squeeze workpiece. Reduction in thickness normally 1-10% per pass. Reversing Rolling Mill – Saves factory space, suitable for short parts. Much power needed. Heavy forces on rolldrive mechanism.

Planetary Rolling Mill Advantages 1. Cheaper to replace 2. Less contact area with workpiece 3. Spread workpiece less sideways Disadvantages – Small rolls are weaker than large rolls All metals can be hot rolled. Only ductile metals can be cold rolled. Zinc and magnesium are brittle, will crack when cold rolled. Cold working is assumed if desired result is a shining surface and small thickness. EXTRUSION (page 14) Metal is force through an opening. Hot Extrusion Metal preheated to just below melting point to lower yield strength. Applications Long pieces of uniform cross-secion Economical way to make small parts in large quantities. Extrude dies relatively cheap Cold Extrusion (Unlikely) At room temp, complete fast to avoid work hardening Hydrostatic Extrusion Zero shear stress, impossible to fracture. Applications Brittle metals For high reduction of cross-sectional area TENSILE DRAWING (page 19) Secondary process after hot extrusion or hot rolling, economically reduce diameter smaller than 5mm. Cold working only (Steel, Al,Cu and Cu alloys are ductile in tension)

Application 1. Reduce cross-section of bars and tubes 2. Produce high performance bars and seamless tubes of very high strength Reduction in cross-section area per pass below 40%. Work material becomes too hard and brittle(work hardened) must be annealed to draw further. Rolling is generally cheaper than extrusion. Rolling cannot produce hollow sections, extrusion can. FORGING (page 22) Deformation by sudden blows or high pressure to squeeze metal between dies. Best mechanical properties Characteristics 1. Durable, reliable 2. Excellent mechanical properties 3. Dense Hammer Forging – Used for considerable reduction in cross-section Drop Forging – Very common method Produces flash which can be recycled, machining produces even more. Advantages 1. Superior mechanical properties due to fibrous structure 2. High production rate 3. High density product Press Forging – Slow pressure squeeze into shape, give deep penetration. Fast, automated, and large scale. Application 1. For large sections 2. Secondary and finishing operations Advantages 1. Smooth surface 2. Good dimensions 3. Quieter than drop forging 4. More uniform structural quality

Upset Forging – Increase diameter or cross-section area by compression along its length Application – used to shape heads on bolts, nails, etc Roll Forging – Reduce cross-section of lengths while increase length The stock does not completely pass through the rolls to the other side Press forging is generally more expensive than drop forging. SHEET METAL WORKING (page 29) Usually done at room temp unless too thick. Sheet Metal Drawing On mild steel, stainless steel, alloys of Al, Mg, Cu, any ductile material Application 1. Thin-walled seamless metal cup 2. Uniformity and close tolerances Ironing Thinning down the walls of drawn cups Rubber Pad Forming Advantages 1. Saves money of the die 2. Reduce metal springback after forming Shearing of Sheet Metal Advantages 1. Fast 2. Amenable to large scale production 3. No flash

CHAPTER 2 – JOINING PROCESSES WELDING (page 48) Joining by melting the metal to the 2 pieces 1. Fusion Welding 2. Pressure Welding 3. Fusion Welding using filler material Welding defects (page51) 1. Dimensional Defects 2. Structural Defects Advantages 1. Quick and convenient 2. Light weight 3. Easy to make 4. Cheap 5. Does not affect stress-flow design Disadvantages 1. Dependent on hum factors 2. Surface must be clean 3. Fixtures needed 4. Defects are common 5. Heat treatment may be needed 6. Need Quality control methods Low carbon steels are excellent for welding. High carbon steels need special techniques (pre-heating or postheating to prevent cracking) Gas Welding For welding thin sheets below 2mm Advantages 1. Cheap, portable, versatile 2. No electrical supply 3. Can weld thin sheets, lower temp, easier to control

Arc Welding Electric arc generated between electrode and workpiece 1. Joining sections of 2mm thickness or more 2. Faster welding, greater depth of penetration 3. For materials with high heat conductivity Resistance Welding Electrodes press the parts together and carry electricity. Weld sheet metals of roughly the same thickness, bad conductor of heat and electricity. Spot welding Seam welding Advantages 1. Heat is localized 2. Fast 3. No filler metal 4. Easily automated for large-scale production Disadvantages 1. High cost of initial equipment 2. Difficult to join sheets of different thickness Submerged Arc Welding (SAW) For thick sections and sheets. Not for sheets below 8mm. Advantages 1. Automatic feed of electrode and flux 2. Molten flux forms protective coating over weld 3. High current yield high welding speeds 4. Can join thick sections with a single pass Disadvantages 1. Needs lots of space and investment 2. Automation is necessary since joint Metal Inert Gas Welding (MIG) Same as arc welding, with inert gas shield to protect from atmosphere. For large-scale production and weld only 1 type of metal.

Tungsten Inert Gas Welding (TIG) Non-consumable Tungsten electrode. Separate filler rod needed, can weld thin to moderate sections (1-5mm). Application of MIG & TIG TIG needs more space than MIG Both used for welding stainless steels, MG, Al, Cu and Ti alloys TIG slower than MIG Inert Gas Shield will prevent formation of porous chromium oxide layer Friction Welding Advantages 1. Join dissimilar metals 2. Fast Limitation – only for round sections Laser Beam Welding Welding fine wires, thin foils, or jobs requiring high precision Advantages 1. Easily controls energy 2. Tiny (0.1mm) Heat Affected Zone Electronic Beam Welding Advantages 1. Easily controls energy 2. 2 Tiny (0.1) Heat Affected Zone Disadvantage A high vacuum is needed. Much time spent on evacuating the chamber

CHAPTER 3 – CASTING TECHNOLOGY Sand Casting (page 67) Advantages 1. Can produce huge parts 2. Cheap 3. No directional properties 4. Complicated shapes 5. Require only little machining Disadvantages 1. Rough surface 2. Mold can use only once 3. Poor dimensional tolerance, metal shrinks Defects sources 1. Badly prepared pattern 2. Poor design 3. Poor casting technique Die Casting (page 74) Equipment is expensive Good dimensional tolerances Good surface finish Non-expendable molds used for mass production. Ideal for low melting point metals (Al, Zn, Pb, Cu, Mg and Sn alloys). Medium Carbon tool steel molds, water-cooled to prolong life and shorten casting time. High Production rate Hot Chamber Die Casting Only suitable for low melting point metals (Pb, Sn, Zn alloys). Cold Chamber Die Casting Can be used for non-ferrous metals of higher melting point (Al, Cu, Mg alloys). Liquid metal in contact with injection cylinder for short time, less alloying takes place. Fast production rate, cycle time less than 1 minute.

Centrifugal Casting For casting large pipes, grey cast iron cylinder liners Advantages 1. Finger grain size due to fast cooling (tougher) 2. Cleaner casting 3. Highly dense structure, free of defects 4. High production rates 5. Best mechanical properties, very high casting pressure can be achieved 6. Can cast large pipes accurately Continuous Casting Melted metal flows down continuously to allow solidification in a water-cooled mold. Can be hot rolled into products immediately after solidification. Advantages 1. Fully automated 2. Molds/dies are cheap 3. No wastage 4. Reasonable physical properties and surface finish 5. Quick and convenient process 6. Secondary operations make use of existing heat Investment Casting Patterns can only be used once Advantages 1. Extremely good surface finish 2. Complicated shapes 3. No machining required 4. Wide range of alloys is suitable to cast 5. Close tolerances even for high melting point metals Limitations 1. Expensive method 2. Casting cannot be too large



Metal powders are compressed and sintered to form a metal product Advantages (page 85) 1. High melting point metals can be fabricated below their melting points. 2. Non-metallic constituents can be introduced and their contents can be controlled 3. Special structural effects: a. Controlled porosity b. Lamellar structure c. Composite structure d. Controlled density 4. Good dispersion in alloys 5. High purity 6. Low machining costs 7. Close tolerance 8. No waste material 9. High speed production for small parts 10. Uniform composition Disadvantages 1. High cost of raw materials 2. Simple dies 3. Small product size 4. Difficult to store powders 5. Powder size not consistent 6. Product can be brittle 7. Difficult to handle low melting point metals 8. Slight shrinkage on sintering and cooling 9. Cannot be bent or cold-worked 10. Cannot make threads 11. Minimum thickness about 1mm and max 2.5xD

Production methods depend on wall thickness, diameter, length and applications. (page 88) Induction Welding of Cold Rolled Strip To make low cost low strength tubes of various steels, not suitable for 10-15mm below, and not for good conductors of electricity Seam Welding of Skelp Welded seam Hollow Extrusion Mannesmann Process Make reliable tubes and pipes of any size that must not have any weld joint Casting With Central Core For cast iron pipes of any size Centrifugal Casting Roll Bending And Welding For making very thick large pipes Spiral Welding of Metal Strip For extremely large pipes Welded From Separate Metal Sheets For extremely large pipes Drawing And Pressing Of Sheet For very small tubes of thin wall thickness

CHAPTER 6 – UNCONVENTIONAL MACHINING PROCESSES Non-traditional machining processes are very slow which will not normally be employed. Cost a lot more than conventional machines. Electrical Discharge Machining (EDM) (page94) Die-Sinking EDM Applications Machine molds and dies Drill very small holes, orifices, slots Machine very hard materials Machine thin-walled parts that are very weak Advantages 1. Machine any hard materials, so long is conductive 2. Products are completely burr-free 3. Very close tolerances 4. Can machine intricate configurations 5. No mechanical strains induced 6. Produce very sharp corners Limitations 1. Both electrode and workpiece must be electrically conductive 2. Electrode wear 3. Slow metal removal rate, low production rate 4. Recast surface layer has high residual stresses and high roughness 5. Heat treatment causes warping Wire-cut EDM Applications Machining hard conducting materials High precision machining For complicated profiles

CHAPTER 7 – PLASTICS TECHNOLOGY Advantages (page 110) 1. Wide range of colours 2. Good thermal insulation 3. Good electrical insulation 4. Good corrosion resistance 5. Low specific gravity 6. Easy to process 7. Cheap 8. Rigid plastics can be flexible 9. Transparent and translucent Disadvantages 1. Repairs using heat can never be perfect 2. Give off objectionable odours 3. Not for high temp use 4. Creep under any load 5. Week in tension, compression, torsion, bending 6. Subjected to deterioration 7. Low modulus of elasticity and rigidity Thermoplastics (page 111) Soften on heating, re-harden on cooling. Process is reversible and scrap material can be recycled. Easier to process and more amenable to large-scale production. Thermosetting Plastics (page 115) Process is irreversible, will not soften on reheating, rigid, hard, brittle. MANUFACTURING PROCESSES FOR PLASTICS (page 121) Injection Molding Extremely convenient large scale production, usually for thermoplastics. Mold is most expensive component, usually made of Al

Compression Molding For producing thermosetting plastics, require higher pressure to achieve good definition in the grooves Transfer Molding For producing thermosetting plastics only. Advantages over compression molding No flash, less finishing Mold many products simultaneously Can mold small intricate parts which are difficult to compression mold Reaction Injection Molding (RIM)

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