Metal Forming Processes

Cheatsheet Content

Metal Forming Processes Overview Definition: Manufacturing processes where material is plastically deformed to take the shape of a die geometry. Plastic Deformation: Requires stresses beyond the yield strength of the workpiece material. Categories: Bulk metal forming, Sheet metal forming. Bulk Forming Processes Bulk Forming: Severe deformation process with massive shape change. Surface area-to-volume ratio is relatively small. Mostly done in hot working conditions. Rolling: Workpiece (slab/plate) compressed between rotating rolls to reduce thickness. Forging: Workpiece compressed between two dies containing shaped contours. Extrusion: Workpiece compressed or pushed into a die opening to take the shape of the die hole. Wire or Rod Drawing: Similar to extrusion, but workpiece is pulled through the die opening. Sheet Forming Processes Sheet Forming: Involves forming and cutting operations on metal sheets, strips, and coils. High surface area-to-volume ratio. Bending: Sheet material strained by punch to create an angle shape. Deep (or Cup) Drawing: Flat metal sheet formed into a hollow shape by stretching. A blank-holder clamps the blank while a punch pushes into it. Shearing: Cutting of sheets by shearing action. Working Temperatures Cold Working Generally done at room temperature or slightly above. Advantages: Closer tolerances, good surface finish, higher strength/hardness (strain hardening), desirable directional properties (grain flow), minimized heating costs, near net shaped forming. Disadvantages: Higher forces/power, limited deformation due to strain hardening, multi-stage cold forming-annealing cycles may be needed, reduced ductility. Warm Working Performed above room temperature but below recrystallization temperature ($0.3 T_m$, where $T_m$ is melting point). Advantages: Enhanced plastic deformation, lower forces, intricate geometries possible, reduced annealing stages. Hot Working Involves deformation above recrystallization temperature, between $0.5 T_m$ to $0.75 T_m$. Advantages: Significant plastic deformation and shape change, lower forces, materials with premature failure can be hot formed, absence of strengthening due to work hardening. Disadvantages: Shorter tool life, poor surface finish, lower dimensional accuracy, sample surface oxidation. Forging Processes Forging Basics Deformation process where workpiece is compressed between two dies using impact or hydraulic load. Used for high-strength components (crankshafts, connecting rods, gears, aircraft structural components). Categories: Temperature: Cold, warm, hot forging. Presses: Impact load (forging hammer), gradual pressure (forging press). Type of Forming: Open die, impression die, flashless forging. Open Die Forging (Upset Forging) Simplest example: compression of billet between two flat dies. Height decreases, diameter increases. Ideal Conditions (no friction): Homogeneous deformation occurs, diameter increases uniformly. Strain: $\varepsilon = \ln(h_0/h_f)$ Force: $F = \sigma_f A$ (where $\sigma_f$ is flow stress) Actual Forging (with friction): Bulging occurs (barreling effect) due to friction at die-billet interface. Barreling Effect: Significant when diameter-to-height ($D/h$) ratio increases, due to greater contact area. Temperature also affects it. Force Evaluation (actual forging): $F = K_f \sigma_f A$ $K_f = 1 + \frac{0.4\mu D}{h}$ (forging shape factor) $\mu$: coefficient of friction, $D$: workpiece diameter, $h$: workpiece height. Impression Die Forging (Closed Die Forging) Dies have impressions that are imparted to the workpiece. Initial billet partially deforms, forming a bulged shape. Further closure fills the impression and forms "flash" (excess material). Flash is trimmed off later. Often multi-stage: separate die cavities for shape change, uniform property distribution. Force calculation: $F = K_f \sigma_f A$ (same formula as open die, but $K_f$ values differ) $K_f$ values for impression-die forging: Simple shapes with flash: 6.0 Complex shapes with flash: 8.0 Very complex shapes with flash: 10.0 Precision Forging: Developed to produce forgings with thin sections, complex geometries, closer tolerances, and eliminate machining allowances (near-net shape forging). Flashless Forging Workpiece fully restricted within the die, no flash produced. Critical: Workpiece volume must precisely match die cavity volume. Too large billet: excessive pressures, die/press damage. Too small billet: cavity not filled. Suitable for simple, symmetrical geometries and materials like Al, Mg alloys. Coining: Simple application of closed die forging for impressing fine details onto surfaces. Little metal flow, but high pressures required. Forging Equipment Hammers (Drop Hammers) Apply impact loading to workpiece. Upper die strikes workpiece, impact energy forms part to die cavity. Multiple blows may be needed. Gravity Drop Hammers: Energy from falling weight of heavy ram. Force depends on drop height and ram weight. Power Drop Hammers: Ram accelerated by pressurized air or steam. Presses Apply gradual force to forging billet. Mechanical Presses: Convert rotary motion to translation motion (eccentrics, cranks, knuckle joints). High forces at bottom of stroke. Hydraulic Presses: Hydraulically driven piston actuates ram. Low ram speeds. Screw Presses: Screw mechanism drives vertical ram. Low ram speeds. Forging Dies Terminology Parting Line: Divides upper and lower die halves. Affects grain flow, load, flash formation. Draft: Taper given on sides for easy part removal. Draft Angles: 3$^\circ$ for Al/Mg, 5$^\circ$-7$^\circ$ for steel. Webs and Ribs: Thin portions parallel/perpendicular to parting line. More difficult to form as they get thinner. Fillet and Corner Radii: Small radii limit metal flow, increase stresses on die surfaces. Flash: Pressure build-up controlled by proper design of gutter and flash land. Other Forging Operations Upset Forging (Heading) Cylindrical workpiece increased in diameter with length reduction. Done as closed die forging in industry. Widely used for fastener heads (nails, bolts). Maximum length upset in single blow: three times initial wire stock diameter. Swaging Reduces diameter of tube or rod to create a tapered section. Rotating dies hammer workpiece radially inward. Mandrel controls internal diameter for tubular parts. Radial Forging: Similar to swaging, but dies don't rotate; workpiece rotates as it feeds into hammering dies. Roll Forging Reduces cross-section of cylindrical/rectangular rod by passing it through opposing rolls with matching grooves. Combines rolling and forging; classified as forging. Roll-forged parts are stronger with desired grain structure compared to machined parts. Orbital Forging Cone-shaped upper die simultaneously rolls and presses workpiece. Work supported on lower die. Small contact area due to inclined cone axis reduces press load requirement. Isothermal Forging Hot-forging where workpiece and dies are maintained at elevated temperature. Avoids chilling of work, metal flows more readily, reduced force. Expensive, used for difficult-to-forge metals (Ti, superalloys) and complex shapes. Done in vacuum/inert atmosphere to prevent die oxidation. Extrusion Processes Extrusion Basics Bulk forming process: metal forced through a die hole to produce desired cross-section. Advantages: Variety of shapes, enhanced grain structure/strength (cold/warm extrusion), close tolerances (cold extrusion). Types: Direct (forward), Indirect (backward). Direct Extrusion Metal billet loaded into container with die holes. Ram compresses material, forcing it through die holes. Significant friction between billet surface and container walls, increasing ram force. Hot direct extrusion: oxide layer on billet surface increases friction and can cause defects. Dummy block used to leave oxide layer in container, producing oxide-free product. Butt: extra portion of billet that cannot be extruded, separated from product. Indirect Extrusion Die mounted to ram, not container. Ram compresses metal, which flows through die hole in opposite direction to ram movement. No relative motion between billet and container, so no friction at interface, lower ram force. Limitations: Lower rigidity of hollow ram, difficulty supporting extruded product. Hollow Shapes (Direct Extrusion) Made by preparing billet with hole parallel to axis. Material flows through gap between mandrel and die opening. Simple Analysis of Extrusion Extrusion Ratio ($r_e$): $r_e = A_0 / A_f$ (where $A_0$ is initial CSA, $A_f$ is extruded CSA). True Strain (ideal deformation): $\varepsilon = \ln(r_e) = \ln(A_0 / A_f)$. Ram Pressure (ideal deformation): $p = Y_f \ln(r_e) = Y_f \ln(A_0 / A_f)$, where $Y_f$ is average flow stress. Actual Pressure: Greater than ideal due to friction between billet/die and billet/container wall. Johnson Formula (actual true strain): $\varepsilon_x = a + b \ln r_e$. (Typical $a=0.8, b=1.2-1.5$). Ram Pressure (with friction): $p = Y_f \varepsilon_x$. Billet-Container Friction Force: Additional ram force to overcome friction. Sliding friction: $\mu p_e \pi D_0 L = \frac{p_f \pi D_0^2}{4}$ (where $p_e$ is pressure against container wall). Sticking friction: $K \pi D_0 L = \frac{p_f \pi D_0^2}{4}$ (where $K$ is shear yield strength). Additional Pressure for Friction: $p_f = Y_f \frac{2L}{D_0}$ (for direct extrusion). Actual Ram Pressure (direct extrusion): $p = Y_f (\varepsilon_x + \frac{2L}{D_0})$. Pressure decreases as billet length $L$ decreases. Higher die angles cause steeper pressure buildups. Extrusion Dies Factors: Die angle, orifice shape. Die Angle Effect: Low angles: large surface area, increased friction, higher ram force. Large angles: more turbulence, increased ram force. Optimal die angle exists (U-shaped ram force function). Orifice Shape Effect: Affects ram pressure, determines amount of metal squeezing. Die Shape Factor ($k_x$): $k_x = 0.98 + 0.02 (C_x / C_c)^{2.25}$ $C_x$: perimeter of extruded cross section. $C_c$: perimeter of circle with same area as extruded shape. $C_x / C_c$ typically varies from 1 to 6. Die Materials Hot Extrusion: Tool and alloy steels. High wear resistance, high thermal conductivity. Cold Extrusion: Tool steels and cemented carbides. Used for high production rates, long die life, dimensional control. Other Extrusion Processes Impact Extrusion: Higher speeds, shorter strokes. Billet extruded by impact pressure. Can be forward, backward, or combined. Cold forming, produces thin walls (e.g., toothpaste tubes). Advantages: large reductions, high production rates. Hydrostatic Extrusion: Billet surrounded by fluid, pressurized by ram. No friction inside container, minimized at die opening. Hydrostatic pressure increases ductility, usable for brittle metals. High reduction ratios possible. Billet tapered to act as seal, preventing fluid leakage. Extrusion Defects Centerburst (Chevron Cracking): Internal crack from tensile stresses along workpiece center. Caused by high die angles, low extrusion ratios, impurities. Piping: Sink hole at billet end. Minimized by using a dummy block. Surface Cracking: Cracks on surface due to high workpiece temperatures, high extrusion speeds, high strain rates, heat generation, friction, surface chilling. Wire, Rod, Bar Drawing Process where wire, rod, bar are pulled through a die hole to reduce cross-section. Difference: Bar drawing for large diameter stock, wire drawing for small diameter stock (e.g., 0.03 mm). Operating Stages: Bar Drawing: Single stage, straight inlet bars, batch operation. Wire Drawing: From coils, series of dies (4-12), "continuous drawing" with butt-welded segments. Simple Analysis of Wire Drawing True Strain (ideal deformation): $\varepsilon = \ln(A_0 / A_f) = \ln(1/(1-r))$, where $r = (A_0 - A_f) / A_0$. Stress Required (ideal deformation): $\sigma_d = \bar{Y}_f \ln(A_0 / A_f)$. Schey's Equation (with die angle and friction): $\sigma_d = \bar{Y}_f (1 + \frac{\mu}{\tan \alpha}) \ln(A_0 / A_f)$. Drawing Force: $F = A_f \sigma_d$. Maximum Reduction per Pass: $r_{max} = 0.632$ (theoretical limit for perfectly plastic material, no friction/redundant work). Practical industrial reductions are 0.5-0.3 per pass due to friction and strain hardening. Drawing Dies Entry Region: Bell-shaped, contains lubricant, prevents wear. Approach Region: Cone-shaped (half-angle 6$^\circ$-20$^\circ$), where drawing occurs. Bearing Surface (Land): Determines final drawn workpiece size. Back Relief: Exit zone (half-angle 25$^\circ$-30$^\circ$). Tube Drawing Reduces diameter or wall thickness of seamless tubes/pipes. Can be with or without mandrel. Without Mandrel (Tube Sinking): Simplest, only diameter reduction. Cannot control ID or wall thickness. With Fixed Mandrel: Mandrel attached to support bar, controls ID and wall thickness. Support bar length restricts tube length. With Floating Plug: Mandrel floats, shape designed for suitable position in reduction zone. No length restriction. Rolling Processes Flat Rolling Thickness reduced by compressive forces from two rotating rolls. Workpiece width increases (spreading), significant with low width-to-thickness ratio and low friction. Thickness Reduction (draft): $d = t_0 - t_f$. Reduction ($r$): $r = d / t_0$. True Strain: $\varepsilon = \ln(t_0 / t_f)$. Roll Speed ($v_r$): Work velocity continuously increases from entry ($v_0$) to exit ($v_f$). $v_f > v_0$. No-Slip Point (Neutral Point): Point where work velocity equals roll surface velocity. Forward Slip ($S$): $S = (v_f - v_r) / v_r$. Rolling Forces & Power Friction coefficient depends on lubrication, material, temperature. Cold rolling: $\approx 0.1$. Warm rolling: $\approx 0.2$. Hot rolling: $\approx 0.4$, up to $0.7$ (sticking friction). Roll Force ($F$): $F = \bar{Y}_f w L$ (where $wL$ is contact area). Contact Length ($L$): $L = \sqrt{R(t_0 - t_f)}$. Rolling Power ($P$): $P = (2\pi N) F L$ (for two powered rolls). Maximum Possible Draft: $d_{max} = \mu^2 R$. If friction is zero, no rolling is possible. Rolling Mills Two-High Rolling Mill: Two rolls rotating in opposite directions. Roll diameters: 0.6 to 1.4 m. Types: reversing or non-reversing. Non-reversing: rolls rotate one direction, slab moves entry to exit. Reversing: roll rotation reverses, slab passes back and forth. Three-High Rolling Mill: Three rolls. Two used per pass. Slab shifted for reduction. Four-High Rolling Mill: Two small rolls for thickness reduction, two large backing rolls for support. Reduces roll force, prevents elastic deflection. Cluster Rolling Mill: Uses smaller rolls for rolling. Tandem Rolling Mill: Series of 8-10 rolling stations. Thickness reduction at each station, work velocity increases. Used with continuous casting. Other Rolling Operations Thread Rolling: Creates threads on cylindrical parts by rolling between two dies. Used for mass production of bolts/screws. Ring Rolling: Thick-walled ring rolled into thin-walled, larger diameter ring. Used for bearing races, tires, rings for pipes. Rolling Defects Waviness: Undulations on the rolled strip. Cracking: Longitudinal or transverse cracks. Edge Defect: Irregularities at the edges of the strip. Alligatoring: Splitting of the workpiece ends. Sheet Bending Straining metal around a straight axis. Inner side compressed, outer side stretched. No change in thickness. V-bending: Sheet bent between V-shaped punch and die. Included angles can vary. Edge Bending: Cantilever loading. Pressure pad holds sheet against die, punch bends it over die edge. Deformation During Bending Original length $l_0$ becomes $l_s = \rho\theta$. Axial strain: $\varepsilon_x = \ln(l_s/l_0) = \varepsilon_a + \varepsilon_b$. Bending strain approximation: $\varepsilon_b \approx y/\rho$. Material Models for Bending Elastic, Perfectly Plastic Model: Strain hardening neglected if radius of curvature/thickness ($\rho/t$) is about 50. $\sigma_1 = E' \varepsilon_1$ (elastic range, $E'$ is plane strain modulus). $\sigma_1 = S$ (plastic range, $S$ is plane strain yield stress). Rigid, Perfectly Plastic Model: Elastic strains and strain hardening neglected for small radius bends. $\sigma_1 = S$. Strain Hardening Model: For large strains, elastic strains neglected. Power hardening law: $\sigma_1 = K' \varepsilon_1^n$. Springback Occurs due to variation in bending stresses across thickness. Tensile stresses decrease to zero at neutral axis. Outer surface cracks if tensile stress exceeds ultimate tensile strength. Metal closer to neutral axis stressed below elastic limit, farther regions undergo plastic deformation. After load removal, elastic band tries to return to original flat condition, but plastic regions restrict it. Quantification: $SB = (\alpha' - \alpha_{tool}) / \alpha_{tool}$. Minimization: Overbending, bottoming (squeezing part at end of stroke), stretch forming. Stretch Forming Sheet metal intentionally stretched and simultaneously bent. Sheet held by jaws or drawbeads, stretched by punch above yield strength. Combined stretching and bending results in less springback. Forming Limit Diagram (FLD) Plots major strain vs. minor strain. Forming Limit Curve: Boundary between safe and failure regions. Strain Paths: Deep drawing (negative minor strain), plane strain, bi-axial stretching (positive minor strain). Plastic Anisotropy Preferred orientation of grains due to mechanical forming. Plastic Strain Ratio ($R$): Ratio of true plastic strain in width direction to thickness direction ($\varepsilon_w / \varepsilon_t$). Higher $R$ indicates higher resistance to thinning. Isotropic materials: $R=1$. Anisotropic: $R \neq 1$. Average Plastic Strain Ratio ($\bar{R}$): $\bar{R} = (R_0 + 2R_{45} + R_{90}) / 4$. Planar Anisotropy ($\Delta R$): $\Delta R = (R_0 + R_{90} - 2R_{45}) / 2$. Measures difference from symmetry axes. Deep Drawing Defects Wrinkling in Flange and Cup Wall: Ups and downs due to compression. If clearance too small, ironing occurs. Tearing: Crack near base due to high tensile stresses (thinning). Can also occur due to sharp die corner. Earing: Peaks and valleys on drawn cup walls. Results from planar anisotropy ($\Delta R$). Surface Scratches: Caused by rough punch/dies or poor lubrication. Cup Deep Drawing Parameters Clearance ($c$): $c = 1.1t$ (where $t$ is sheet thickness). Drawing Ratio ($DR$): $DR = D_b / D_p$ (blank diameter / punch diameter). Higher DR means more severe operation. Limiting Values: $DR \le 2$, $R \le 0.5$. Reduction ($R$): $R = (D_b - D_p) / D_b$. Thickness to Diameter Ratio ($t/D_b$): If $>1\%$, wrinkling tendency decreases. Maximum Drawing Force ($F$): $F = \pi D_p t_{UTS} (D_b / D_p - 0.7)$. Holding Force ($F_h$): $F_h = 0.015 \sigma_{ys} \pi \{D_b^2 - (D_p + 2.2t + 2R_d)^2 \}$. (Approx. $F_h = F/3$). Redrawing Multiple deep drawing steps for severe shape changes. Guidelines: First draw 40-45% reduction, second 30%, third 16%. Reverse Redrawing: Sheet part faces down, drawing completed in direction of initial bend. Drawing Without Blank Holder Possible for large thickness-to-blank diameter ratio ($t/D_b$). Die must have funnel/cone shape. Limiting Value: $D_b - D_p = 5t$.