Deformation Processing (Meyers & Chawla pp. 87-97) 3 ways of processing into final shape: Casting Powder metallurgy (+sintering) and deformation processing shape change due to plastic deformation structural and property changes essentially an art and only recently mathematical analyses are developed variety of techniques: rolling, forging, extrusion, wire-drawing, metal forming, tube reduction, HERF(high energy rate forming), superplastic forming, HIPping, etc. can classify into 2 classes of processes: Steady-state - plastic deformation zone (size and shape) remains fixed in time (eg rolling, wire-drawing, extrusion and tube reduction) & Non-steady-state - zone shape and size change with time ( e.g., forging, bending and shearing) General Aspects (see Fig. 2.7) illustrates common features of these various techniques: 1. tools and work-piece under load 2. initial and final properties of material 3. plastic zone - where plasticity can be applied 4. friction between blanks and tool (reduce by lubrication) MSE450 page 1
a. rolling (Fig. 2.8) rolls move in opposite directions biaxial stress state can produce plates, sheets, I-Beam, T- beams, etc. * need to go through a number of passes (eg Fig. 2.8b 10 passes) & sometime may need heattreatment in between * For I and T beams - need the rolls properly shaped (not smooth cylinders) b. forging (fig 2.9) (i) open-die (ii) closed-die (i) open-die - need final shaping (ii) can get final shape perhaps in 1step: volume of die (pre-shape) volume of work piece need flash and getter to accommodate excess volume of the materials also, no open spaces left and any voids will be eliminated c. wire-drawing (fig 2.10) wire is pulled in tension through a die of conical shape with hard tool at the nib such WC or diamond can draw into different shapessuch as hexagons, squares, small angles, channels, tubes, etc often lubrication used MSE450 page 2
d. extrusion (fig. 2.11) here, material is under compression 3 different types (i) direct extrusion - ram and workpiece move in the same direction/frictional forces overcome by the ram (ii) indirect extrusion - ram and workpiece move in opposite direction- lower power required than in direct extrusion (iii) impact - excellent for ductile materials such as Al rapid - excellent shapes(final) in 1 operation - materials are softer due to high T generated (no time for temp to conduct away) - for brittle materials, superimpose hydrostatic pressure - eliminate existing voids and reduce void formation during deformation: need to do this in HP chamber (relatively expensive) e. sheet metal forming (fig. 2.12) most common is stamping & is economical - deep drawing is another example (in forming cups etc incorporate draw beads to control the metal flow eliminate wrinkles f. tube reduction (fig 2.13) cast tubes, welded tubes and seamless tubes Pilger or Mannesman processing - for seamless tubing - involves 2 rolls offset by about 10 from horizontal + conical shape & the rolls move in the same direction makes the tubing move axially - generally, a mandrel (piercer) is used and the shape of the mandrel and rolls dictate the deformation strain state (thickness change vs diametral change Q-ratio ln( t / t ) Q = ln( D / ) D o MSE450 page 3
g. High-Energy Rate Fabrication (HERF): (fig. 2.15)-Explosive processing such as welding, cladding, cutting etc very rapid (20-300 m/s) use detonation (explosives) where the high KE is transferred through water in a closed chamber (i) shock-wave from explosion leads to majority (60%) of shape change, and (ii) bubble formed due to prior evacuation completes the final shaping advantage: relatively large parts can be formed with little tooling difficult-to-form metals can formed conveniently h. other advanced operations: superplastic forming - near net shape in a single operation limited to superplastic materials but now many can be processed into materials with superplastic characteristics. HIPping - at high hydrostatic pressure at high temp - voids are eliminated and even very brittle materials can be processed Temperature Effects: Depending on the processing temperature coldworking, warm-working and hot-working (involves temp and strain-rate combinations) CW leads to high workhardening: harder deformed materials (fig. 2. 16) Warm (at intermediate temp) and Hot (high T) lead to softer final product higher the T and lower the strainrate - higher the dynamic recovery or leads to flat σ-ε curves (fig. 2.17) MSE450 page 4
Stress State: depending on the operation generally complex multiaxial stresses develop * Tensile compressive systems (rolling, deep drawing, wire and bar drawing) 1. Biaxial tension uniaxial compression 2. Uniaxial tension uniaxial compression 3. Uniaxial tension biaxial compression * Compressive stress systems (forging, tube extrusion, rolling) 1. Triaxial stresses 2. Biaxial stresses * Tensile stress systems (stretch forming, bulging) 1. Biaxial stress * Shear stress systems (blanking, punch piercing) 1. Shear and possibly tension/compression ------------ Deep-Drawing & Stretch or Cup-Forming: (Dieter, pp. 666-675) R-ratio, LDR and Forming Limit Diagram (FLD), Stretcher Strains Limiting Draw Ratio: D LDR D o p max η e where η is efficiency (<1), D o is the blank diameter and D p is the punch diameter (Eq. 20-13) Fig. 20-14 Stresses in a drawn cup Drawability -- controlled by crystallographic texture that defines the mechanical anisotropy (CSR or R-ratio large R resists wall thinning): ln( w / w R o ) ln( t / t o ) (Eq. 20-14) --- in general ε l Ro + 2R 45 + R90 R = (Eq. 20-16) 4 MSE450 page 5
Higher the R-ratio better the LDR σ I Backofen Formability Parameter (B): B = 2σ Forming Limit Diagrams (Keeler-Goodwin): IV = plane strain stress in region I 2*equibiaxial stress in IV =f(r) Fig. 20-19 (S.S. Hecker, 1977) Defects: Earring, Wall-Thinning, Orange Peeling (large grained matls), etc. Stretcher Strains (or worms mainly in steels) Due to Luders bands minimize by temper-rolling or skin-rolling to eliminate yield points but the yield points may return following aging MSE450 page 6