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Fundamentals of metal forming
FUNDAMENTALS OF METAL FORMING INCLUDE: [1] Overview, [2] Material behavior, [3] Temperature, [4] Strain rate sensitivity, [5] Friction and lubrication. >>> OrBiT SeLF: OVERVIEW, BEHAVIOR, TEMPERATURE,,, STRAIN_RATE, FRICTION AND LUBRICATION.
Metal forming
USE OF PLASTIC DEFORMATION TO FORM METAL, using dies to apply stresses over the yield stresses of the worked metal, shaping the metal into the shape of the die, a metal best fitted for this is a one with low yield strength and high ductility, properties quite often achieved by raising the temperature and doing some hot working of metal. Strain rate and friction also affect metal forming. HOT TEMPERATURE DECREASES YIELD STRENGTH AND INCREASES PLASTICITY.
PROCESSES OF METAL FORMING can be grouped into two categories: [I] Bulk deformation, [II] Sheet metalworking. >>> BS: BULK SHEET.
*Bulk deformation processes
SIGNIFICANT DEFORMATION AND MASSIVE SHAPE CHANGES TO BULKY PART with a relatively small surface area to volume ratio. Processes include: [1] Rolling - roll reduce thickness of slab, [2] Forging - slamming workpiece between dies while giving it a desired shape. [3] Extrusion - pushing metal through die to give it desired form. [4] Drawing - pulling round bar through die opening, thus reducing it's diameter. >>> RFED: ROLLING FORGE EXTRUDES DRAWS
STRONG ROLLERS COMPRESS METAL SHEET, reducing its thickness and pulling it forth.
TOUGH DIES FORGE SOFT METAL, giving it form with much striking force, quite often striking the metal while its hot.
MACHINE EXTRUDES METAL THROUGH A DIE VIA COMPRESSION, giving the extruded part the shape of the die opening.
ROUND WIRE REDUCES IN DIAMETER AFTER PASSING THROUGH DIE, the pull exerted on the wire pulls it through the die, reducing its dimensions and making a finer wire.
*Sheet metalworking
Forming and cutting operations on cold, not heated, metal sheets, also called $pressworking$ and $stamping$, processes where the $punch$ and $die$ tools, the positive and the negative parts of the mold, shape and form sheets of metal into shape using: [1] Bending, [2] Drawing - forming a sheet into a hollow and concave form, quite often called cup drawing or deep drawing, and [3] Shearing - cutting off a piece of a metal sheet using a punch and a die.
Punch bends sheet into a certain angle.
Deep drawing
Punch draws sheet into hollow, cup like shape.
Punch and die shear metal sheet, cutting off a portion of it.
PLASTIC BEHAVIOR OF MATERIAL IS OF MOST IMPORTANCE IN METAL FORMING, and in the plastic region the true stress strain curve is well described by the flow curve: $\sigma = K \epsilon^{n}$
Flow stress
Instantaneous value of stress required to continue deforming material in plastic region - $Y_{f}=K\epsilon^{n}$ The $K$ is the strength coefficient, and $n$ is the strain hardening component. INSTANTANEOUS STRESS DEFORMS PLASTIC PART.
Average flow stress
Average value of stress-strain curve, determined by integrating the flow curve equation: $\bar{Y_{f}}=\frac{K\epsilon^{n}}{1+n}$. With $\bar{Y_{f}}$ being the average flow stress, and $\epsilon$ - maximum strain value during deformation process.
HIGH TEMPERATURE REDUCES STRENGTH AND STRAIN HARDENING, $K$, the strength coefficient, and $n$, the strain hardening exponent both decreasing in their size, thus improving ductility of the worked metal and requiring forces needed to shape it. We can distinguish between three different temperature ranges for metal forming: Cold, Warm, and Hot working. BLISTERING HEAT SOFTENS HARD STEE
Cold working
WORKING THE METAL AT ROOM TEMPERATURES, allowing great accuracy, harder and strain hardened parts, and directional properties due to grain flow, yet applying higher forces than in hot working, and having a limited amount of ductility of the worked metal. STRONG AND ACCURATE DIRECTIONAL METAL GRAINS COLDLY SNAP AT LIMITED DUCTILITY
Warm working
WORKING THE METAL AT ABOUT 30% OF THE MELTING POINT ($0.3T_m$), thus applying lesser forces upon it, achieving more intricate geometries, and reducing the need for annealing.
Hot working
WORKING THE METAL AT TEMPERATURES OF 50% TO 75% OF MELTING POINT, $0.5T_m \div 0.75T_m$, greatly increasing the ductility and removing strain hardening, making the metal almost as soft and clay, allowing fragile metals to be worked without cracks or strain hardening, yet all that comes at a decrease of dimensional accuracy, more total energy for the process, oxidation of the work surface, poor surface finish, and shorter tool life. RED HOT METAL FORMS INACCURATELY ROUGH PARTS.
Isothermal forming
HEATING THE TOOL TO SAME TEMPERATURE OF THE PART, cutting the highly alloyed steel or titanium alloy with a heated blade, the heating required to eliminate a thermal gradient, gradient that will increase part's hardness at the area of the tool's contact, a process that reduces the tool's life, the only process for processing some materials. WEAK AND HOT TOOL FORGES HARD AND HOT STEEL
How much the flow stress increases at increased strain rates, meaning that the faster you deform the material, the more it resists deformation, a tendency especially prominent in hot forming operation. >>> HOT METAL RESISTS FAST DEFORMATION.
Strain rate
Ratio between deformation speed and instantanious dimension of part: $$\dot{\epsilon}=\frac{v}{h}\left[\frac{1}{s}\right]$$ Increasing strain rate tends to increase the resistance to deformation. The tendency isespecially prominent in hot forming operations.
Friction causes tool wear and requires to apply more force to the material, thus to avoid this, we apply lubricating oils in cold working processes, and shit like molten glass, oils, and graphite in hot working processes. OILY GRAPHITE GLASS INCREASES EXPENSIVE TOOL LIFE.
Sticking friction
WORK SURFACE STICKS TO THE TOOL, rather than sliding against it, a phenomena occurring when the friction stress is greater than the shear from stress of the metal, causing the metal under the surface of the tool to deform rather than slip underneath, a problem quite prominent in rolling.