Sheet-Metal Forming Processes

CHAPTER 7

Sheet-Metal Forming Processes(판재성형가공)

•Shearing•Bending

•Stretch Forming•Bulging

•Rubber Forming•Spinning

•High-Energy-Rate Forming•Superplastic Forming

•Deep Drawing

Outline of Sheet-Metal Forming Processes

VIDEO

Stamping Press and

Press Frames

Figure extra (a) and (b) Schematic illustration of types of press frames for sheet-forming operations. Each type has its own characteristics of stiffness, capacity, and accessibility. Source: Engineer's Handbook, VEB Fachbuchverlag, 1965. (c) A large stamping press. Source: Verson AllsteelCompany.

Characteristics of Sheet-Metal Forming Processes

TABLE extra Process Characteristics Roll forming Long parts with constant complex cross-sections; good surface finish; high production rates; high

tooling costs. Stretch forming Large parts with shallow contours; suitable for low-quantity production; high labor costs; tooling

and equipment costs depend on part size. Drawing Shallow or deep parts with relatively simple shapes; high production rates; high tooling and

equipment costs. Stamping Includes a variety of operations, such as punching, blanking, embossing, bending, flanging, and

coining; simple or complex shapes formed at high production rates; tooling and equipment costs can be high, but labor cost is low.

Rubber forming Drawing and embossing of simple or complex shapes; sheet surface protected by rubber membranes; flexibility of operation; low tooling costs.

Spinning Small or large axisymmetric parts; good surface finish; low tooling costs, but labor costs can be high unless operations are automated.

Superplastic forming

Complex shapes, fine detail and close tolerances; forming times are long, hence production rates are low; parts not suitable for high-temperature use.

Peen forming Shallow contours on large sheets; flexibility of operation; equipment costs can be high; process is also used for straightening parts.

Explosive forming

Very large sheets with relatively complex shapes, although usually axisymmetric; low tooling costs, but high labor cost; suitable for low-quantity production; long cycle times.

Magnetic-pulse forming

Shallow forming, bulging, and embossing operations on relatively low-strength sheets; most suitable for tubular shapes; high production rates; requires special tooling.

Figure 7.1 (a) Localized necking in a sheet specimen under tension. (b) Determination of the angle of neck from Mohr’s circle for strain. (c) Schematic illustrations for diffuse andlocalized necking, respectively. (d) Localized necking in an aluminum strip stretched in tension. Note the double neck.

nwhenneckingK n =→= εεσNecking: localized/diffused strain rate sensitivity m mCεσ &=↔

Sheet Metal Elongation

Yield-Point Elongation

Figure 7.2 (a) Yield-point elongation in a sheet-metal specimen. (b) Lueder's bands (also called stretcher strain marks or worms) in a low-carbon steel sheet. Source: Courtesy of Caterpillar Inc. (c) Stretcher strains at the bottom of a steel can for household products.

(a) (b) (c)

Figure 7.3 Stress-corrosion cracking in a deep-drawn brass part for a light fixture. The cracks developed over a period of time. Brass and austenitic (300 series) stainless steels are among metals that are susceptible to stress-corrosion cracking.Tensile residual stress Stress-corrosion cracking

Sheet metal characteristics:

•Anisotropy

•Grain size

•Residual stresses

•Springback

•Wrinking

•Coated sheet matal

Shearing

Figure 7.4 Schematic illustration of the shearing process with a punch and die. This is a common method of producing various openings in sheet metals.

Figure 7.5 Characteristic features of (b) a punched hole and (c) the slug. Note that the slug has been sealed as compared with the hole.

VIDEO

Cutting sheet metal by shear stresses

Clearance

Figure 7.6 (a) Effect of the clearance, c, between punch and die on the deformation zone in shearing. As the clearance increases, the material tends to be pulled into the die rather than be sheared. In practice, clearances usually range between 2% and 10% of the thickness of the sheet. (b) Microhardness (HV) contours for a 6.4-mm (0.25-in) thick AISI 1020 hot-rolled steel in the sheared region. Source: H. P. Weaver and K. J. Weinmann.

Figure 7.7 Deformation and temperature rise in the shearing zone. The temperature was measured by thermocouples. Punching at (a) slow speed and (b) high speed. Note that the deformation is confined to a narrow zone in high-speed shearing and that temperature is higher than in slow-speed shearing. Source : After N. Yanagihara, H. Saito, and T.Nakagawa. Numbers above the figures indicate punch penetration. (c) Fracture zone in shearing with static and dynamic loading.

Figure 7.8 Typical punch-penetration curve in shearing. The area under the curve is the work done in shearing. The shape of the curve depends on process parameters and material properties.

Maximum punch force Pmax ~ 0.7(UTS)(t)(L)

Pmax

Shearing Operations

Figure 7.9 (a) Punching (piercing) and blanking. (b) Examples of various shearing operations on sheet metal.

Fine Blanking

(a) (b)

Figure 7.10 (a) Comparison of sheared edges produced by conventional (left) and by fine-blanking (right) techniques. (b) Schematic illustration of one setup for fine blanking. Source: Feintool U.S. Operations.

very smooth and square edges

Slitting

Figure 7.11 Slitting with rotary knives. This process is similar to opening cans.

A pair of circular blades

Shaving and Shear Angles

Figure 7.12 Schematic illustrations of the shaving of a sheared edge. (a) Shaving a sheared edge. (b) Shearing and shaving, combined in one stroke.

Figure 7.13 Examples of the use of shear angles on punches and dies.

trimmed by cutting

continuous shear

Compound and Progressive Die

Figure 7.14 Schematic illustrations: (a) before and (b) after blanking a common washer in a compound die. Note the separate movements of the die (for blanking) and the punch (for punching the hole in the washer). (c) Schematic illustration of making a washer in a progressive die. (d) Forming of the top piece of an aerosol spray can in a progressive die. Note that the part is attached to the strip until the last operation is completed.

(a) (b) (c)

(d)

Characteristics of Metals Important in Sheet FormingTABLE extra Characteristic Importance Elongation Determines the capability of the sheet metal to stretch without necking and failure; high

strain-hardening exponent (n)and strain-rate sensitivity exponent (m)desirable. Yield-point elongation Observed with mild-steel sheets; also called Lueder’s bands and stretcher strains; causes

flamelike depressions on the sheet surfaces; can be eliminated by temper rolling, but sheet must be formed within a certain time after rolling.

Anisotropy (planar) Exhibits different behavior in different planar directions; present in cold-rolled sheets because of preferred orientation or mechanical fibering; causes earing in drawing; can be reduced or eliminated by annealing but at lowered strength.

Anisotropy (normal) Determines thinning behavior of sheet metals during stretching; important in deep-drawing operations.

Grain size Determines surface roughness on stretched sheet metal; the coarser the grain, the rougher the appearance (orange peel); also affects material strength.

Residual stresses Caused by nonuniform deformation during forming; causes part distortion when sectioned and can lead to stress-corrosion cracking; reduced or eliminated by stress relieving.

Springback Caused by elastic recovery of the plastically deformed sheet after unloading; causes distortion of part and loss of dimensional accuracy; can be controlled by techniques such as overbending and bottoming of the punch.

Wrinkling Caused by compressive stresses in the plane of the sheet; can be objectionable or can be useful in imparting stiffness to parts; can be controlled by proper tool and die design.

Quality of sheared edges Depends on process used; edges can be rough, not square, and contain cracks, residual stresses, and a work-hardened layer, which are all detrimental to the formability of the sheet; quality can be improved by control of clearance, tool and die design, fine blanking, shaving, and lubrication.

Surface condition of sheet Depends on rolling practice; important in sheet forming as it can cause tearing and poor surface quality; see also Section 13.3.

Bending

Figure 7.15 Bending terminology. Note that the bend radius is measured to the inner surface of the bent part.

VIDEO

R/T Ratio versus % Area Reduction

160

−=rT

R

100: ×−

=o

fo

AAA

r

Figure 7.16 Relationship between R/T ratio and tensile reduction of area for sheet metals. Note that sheet metal with a 50% tensile reduction of area can be bent over itself, in a process like the folding of a piece of paper, without cracking. Source: After J. Datsko and C. T. Yang.

160−=

rTR

Figure 7.17 The effect of length of bend and edge condition on bend radius-thickness ratio of 7075-T aluminum. Source: After G. Sachs and G. Espey.

⎟⎠⎞

⎜⎝⎛

TL

⎟⎠⎞

⎜⎝⎛

TR

Minimum Bend Radius for Various Materials at Room Temperature

TABLE 7.1 Condition Material Soft Hard Aluminum alloys Beryllium copper Brass, low-leaded Magnesium Steels Austenitic stainless Low-carbon, low-alloy, and HSLA Titanium Titanium alloys

0 0 0

5T

0.5T 0.5T 0.7T 2.6T

6T 4T 2T

13T

6T 4T 3T 4T

Bending

Figure 7.18 (a) and (b) The effect of elongated inclusions (stringers) on cracking, as a function of the direction of bending with respect to the original rolling direction of the sheet. (c) Cracks on the outer surface of an aluminum strip bent to an angle of 90o. Note the narrowing of the top surface due to the Poisson effect.

(a) (b)

(c)

Springback

Figure 7.19 Springback in bending. The part tends to recover elastically after ending, and its bend radius becomes larger. Under certain conditions, it is possible for the final bend angle to be smaller than the original angle (negative springback).

Figure 7.23 Methods of reducing or eliminating springback in bending operations. Source: V. Cupka, T. Nakagawa, and H. Tyamoto.

Figure 7.20 Springback factor Ks for various materials: (a) 2024-0 and 7075-0 aluminum; (b) austenitic stainless steels; (c) 2024-T aluminum; (d) ¼ hard austenitic steels; (c) 2024-T aluminum; (d) ¼ hard austenitic stainless steels; (e) ½hard to full-hard austenitic stainless steels. Source: After G. Sachs, Principles and Methods of Sheet-Metal Fabricating. Reinhold, 1951, p.100

( )( ) 12

12++

==TRTRK

f

i

i

fs α

α

Figure 7.21 Schematic illustration of the stages in bending round wire in a V-die. This type of bending can lead to negative springback, which does not occur in air bending (shown in Fig. 7.26a). Source: After K.S. Turke and S.Kalpakjian, Proc. NAMRC III, 1975, pp. 246-262.

contact

Figure 7.22 Range of positive and negative springback for various materials (with the same modulus of elasticity) as a function of the ratio of bend radius to wire diameter. Source: After K.S. Turke and S.Kalpakjian, Proc. NAMRC III, 1975, pp. 246-262

⎟⎠⎞

⎜⎝⎛

DR

Bending Operations

Figure 7.24 Common die-bending operations, showing the die-opening dimension, W, used in calculating bending forces.

Figure 7.26 Examples of various bending operations.

WLTUTSkP

2

max)(

=

Bending in a Press Brake

Figure 7.25 (a) through (e) Schematic illustrations of various bending operations in a press brake. (f) Schematic illustration of a press brake. Source: Verson Allsteel Company.

Bead Forming

Figure 7.27 (a) Bead forming with a single die. (b) Bead forming with two dies, in a press brake.

Flanging

Figure 7.28 Various flanging operations. (a) Flanges on a flat sheet. (b) Dimpling. (c) The piercing of sheet metal to form a flange. In this operation, a hole does not have to be prepunched before the bunch descends. Note, however, the rough edges along the circumference of the flange. (d) The flanging of a tube; note the thinning of the edges of the flange.

Roll Forming

Figure 7.29 Schematic illustration of the roll-forming process.

•Continuous lengths of sheet metals

•Large production runs

Figure 7.30 Stages in roll forming of sheet-metal door frame. In stage , the rolls may be shaped as in A or B. Source: G. Oehler.

Tube Bending

Figure 7.31 Methods of bending tubes. Internal mandrels, or the filling of tubes with particulate materials such as sand, are often necessary to prevent collapse of the tubes during bending. Solid rods and structural shapes can also be bent by these techniques.

Figure 7.32 A method of forming a tube with sharp angles using axial compressive forces. Compressive stresses are beneficial in forming operations because they delay fracture. Note that the tube is supported internally with rubber or fluid to avoid collapsing during forming. Source: After J.L. Remmerswaal and A. Verkaik, lnt. Conf. Manufacturing Technology, ASME, 1967, pp. 1171-1181

Stretch Forming

Figure 7.33 Schematic illustration of a stretch-forming process. Aluminumskins for aircraft can be made by this method. Source: Cyril Bath Co.

BulgingFigure 7.34 (a) The bulging of a tubular part with a flexible plug. Water pitchers can be made by this method. (b) Production of fittings for plumbing, by expanding tubular blanks under internal pressure. The bottom of the piece is then punched out to produce a "T." Source: J. A. Schey, Introduction to Manufacturing Processes (2d ed.) New York: McGraw-Hill Publishing Company, 1987.

Manufacturing of Bellows

Figure 7.34 Steps in manufacturing a bellows.

Embossing

Figure extra An embossing operation with two dies. Letters, numbers, and designs on sheet-metal parts and thin ash trays can be produced by this process.

Figure 7.35 Examples of the bending and the embossing of sheet metal with a metal punch and with a flexible pad serving as the female die. Source: Polyurethane Products Corporation.

Rubber Forming

Hydroform Process

Figure 7.36 The hydroform (or fluid forming) process. Note that, in contrast to the ordinary deep-drawing process, the pressure in the dome forces the cup walls against the punch. The cup travels with the punch; in this way, deep drawability is improved.

Conventional Spinning

Figure 7.37 and 7.39 (a) Schematic illustration of the conventional spinning process. (b) Types of parts conventionally spun. All parts are axisymmetric.

Figure 7.38 Stages in conventional spinning of a tubular component from a flat, circular metal disk. This operation requires considerable skill to prevent the part form collapsing or buckling during spinning.

Shear and Tube Spinning

Figure 7.37 and 7.44 (a) Schematic illustration of the shear spinning process for making conical parts. The mandrel can be shaped so that curvilinear parts can be spun. (b) Schematic illustration of the tube spinning process.

constant diameter

reduction of thickness

Figure 7.40 Schematic illustration of a shear-spinnability test. As the roller advances, the part thickness is reduced. The reduction in thickness at fracture is called the maximum spinning reduction per pass.Source: After R.L Kegg, J. Eng. Ind., vol. 83, pp. 119-124

100

sin

×−

=−

−=−

o

fo

f

o

ttt

reductionMaximum

tatfracturett α

Spinnability test

Figure 7.41 Experimental data showing the relationship between maximum spinning reduction per pass and the tensile reduction of area of the original material. Note that once a material has about 50% reduction of area in a tension test, further increase in the ductility of the original material does not improve its spinnability. Source: S. Kalpakjian, J. Eng. Ind., vol. 8, 1964, pp. 49-54.

Figure 7.42 Examples of external and internal tube spinning and the variables involved.

Spinning of a Compressor Shaft

Figure 7.43 Steps in tube and shear spinning of a compressor shaft for the Olympus jet engine of the supersonic Concorde aircraft.

Zigeunerweisen, Sarasate

From Tukla(4600m) to Lhobuche(4900m), Khumbu, Nepal

Explosive Forming

Figure 7.45 and 47 (a) Schematic illustration of the explosive forming process. (b) Illustration of the confined method of explosive bulging of tubes.

High-Energy Rate Forming

Figure 7.46 Influence of the standoff distance and type of energy-transmitting medium on the peak pressure-transmitting medium should have high density and low compressibility. In practice, water is a commonly used medium.

Figure 7.48 Schematic illustration of the electrohydraulic forming process.

•Underwater-spark

•Electric-dischargePlasma generation

•Lower level of energy

•Smaller workpieceexplosive forming

Magnetic-Pulse Forming

Figure 7.49 (a) Schematic illustration of the magnetic-pulse forming process used to form a tube over a plug. (b) Aluminum tube collapsed over a hexagonal plug by the magnetic-pulse forming process.

(a) (b)

Magnetic field of coil current + Magnetic field of eddy current Repelling

Diffusion Bonding and Superplastic Forming

Figure 7.50 Types of structures made by diffusion bonding and superplasticforming of sheet metal. Such structures have a high stiffness-to-weight ratio. Source: Rockwell International Corp.

Diffusion-bonded

air-pressure

•High ductility

•Low strength

•Ti-6Al-4V (Ti alloy)

•7475-T6 (Al alloy)

Figure 7.51 Peen-forming machine to form a large sheet-metal part, such as an aircraft-skin panel. The sheet is stationary and the machine traverses it. Source: Metal Improvement Company.

Various Forming Methods

Honeycomb Structures

Figure 7.52 Methods of manufacturing honeycomb structures: (a) Expansion process; (b) Corrugation process; (c) Assembling a honeycomb structure into a laminate.

Deep Drawing

Figure 7.53 (a) Schematic illustration of the deep-drawing process on a circular sheet-metal blank. The stripper ring facilitates the removal of the formed cup from the punch. (b) Process variables in deep drawing. Except for the punch force, F, all the parameters indicated in the figure are independent variables.

: a punch to press the blank into the die

Figure 7.54 Deformation of elements in the flange (a) and the cup wall (b) in deep drawing of a cylindrical cup.

•Radial tensile stress: to pull the blank•Compressive hoop stress: reduction in circumferential direction wrinkling

Drawing force

Figure 7.55 Examples of drawing operations: (a) pure drawing and (b) pure stretching. The bead prevents the sheet metal from flowing freely into the die cavity. (c) Possibility of wrinkling in the unsupported region of a sheet in drawing. Source: After W.F. Hosford and R.M. Caddell.

Drawbeads

Figure 7.56 (a) Schematic illustration of a draw bead. (b) Metal flow during the drawing of a box- shaped part, while using beads to control the movement of the material. (c) Deformation of circular grids in the flange in deep drawing.

Figure 7.57 Schematic illustration of the ironing process. Note that the cup wall is thinner than its bottom. All beverage cans without seams (known as 2-piece cans) are ironed, generally in three steps, after being drawn into a cup by deep drawing. (Can with separate tops and bottoms are known as 3-piece cans.)

sheet thickness > clearance

AnisotropyFigure 7.58 Strains on a tensile-test specimen removed from a piece of sheet metal. These strains are used in determining the normal and planar anisotropy of the sheet metal.

Figure 7.61 The relationship between average normal anisotropy and the limiting drawing ratio for various sheet metals. Source: M. Atkinson.

t

wRεε= : normal anisotropy

imump

o

DDLDR

max

=

Typical Range of Average Normal Anisotropy, R, for Various Sheet Metals

TABLE 7.2 Zinc alloys Hot-rolled steel Cold-rolled rimmed steel Cold-rolled aluminum-killed steel Aluminum alloys Copper and brass Titanium alloys (a) Stainless steels High-strength low-alloy steels

0.4–0.6 0.8–1.0 1.0–1.4 1.4–1.8 0.6–0.8 0.6–0.9 3.0–5.0 0.9–1.2 0.9–1.2

Anisotropy and Elastic Modulus

Figure 7.60 Relationship between average normal anisotropy R and the average modulus of elasticity E for steel sheet. Source: After P.R. Mould and T.R. Johnson, Jr., Sheet Met. Lnd., vol. 50, 1973, p. 328.

42 90450 RRRR ++

=

Earing

Figure 7.62 Earing in a drawn steel cup, caused by the planar anisotropy of the sheet metal.

Wavy edge

Figure 7.63 Schematic illustration of the variation of punch force with stroke in deep drawing. Note that ironing does not begin until after the punch has traveled a certain distance and the cup is formed partially. Arrows indicate the start of ironing.

⎟⎟⎠

⎞⎜⎜⎝

⎛−= 7.0)(max

p

oop D

DUTStDP π

Figure 7.64 Effect of die and punch radii in deep drawing on fracture of a cylindrical cup. (a) Die radius too small. The die radius should generally be 5 to 10 times the sheet thickness. (b) Punch corner radius too small Because friction between the cup and the punch aids in the drawing operation, excessive lubrication of the punch is detrimental to drawability.

Figure 7.66 Deep drawing without a blankholder, using a tractrix die profile. The tractrix is a speed curve, the construction for which can be found in texts on analytical geometry or in handbooks.

opo tDD 5<−

Drawing practice: clearance = (1.07 ~ 1.14) to

corner radius fracturedraw beads: to control the blank flowredrawing: multi-stage drawing reverse redrawinglubrication………

Erichsen and Bulge-Tests

Figure 7.67 and 7.69 (a) A cupping test (the Erichsen test) to determine the formability of sheet metals. (b) Bulge-test results on steel sheets of various widths. The specimen farthest left is subjected to, basically, simple tension. The specimen farthest right is subjected to equal biaxial stretching. Source: Inland Steel Company.

(a)

(b)

Formability

Figure 7.68 Schematic illustration of the punch-stretch test on sheet specimens with different widths and clamped at the edges. The narrower the specimen, the more uniaxial is the stretching. A large square specimen stretches biaxially under the hemispherical punch (see also Fig. 7.69).

Major and Minor Strain

Figure 7.70 (a) Strains in deformed circular grid patterns. (b) Forming-limit diagrams (FLD) for various sheet metals. Although the major strain is always positive (stretching), the minor strain may be either positive or negative. In the lower left of the diagram, R is the normal anisotropy of the sheet. Source: S. S. Hecker and A. K. Ghosh.

Tearing

Figure 7.71 The deformation of the grid pattern and the tearing of sheet metal during forming. The major and minor axes of the circles are used to determine the coordinates on the forming-limit diagram of Fig. 7.70. Source: S. P. Keeler.

Figure 7.72 Major and minor strains in various regions of an automobile body.

1, 3, 5, …: frequency of occurence

Figure 7.73 Deformation of a square mesh in computer simulation of forming a sheet-metal part. Source: J.L Duncan, R. Sowerby, and E. Chu.

Modeling

Laser WeldingFigure 7.74 Production of an outer side panel of a car body, by laser butt-welding and stamping. Source: After M. Geiger and T. Nakagawa.

tailored blank

Examples of Laser Welded Parts

Figure 7.75 Examples of laser butt-welded and stamped automotive body components. Source: After M. Geiger and T. Nakagawa.

Cost Comparison for Spinning and Deep Drawing

Figure 7.76 Cost comparison for manufacturing a round sheet-metal container either by conventional spinning or by deep drawing. Note that for small quantities, spinning is more economical.

Economics

Figure 7.77 (a) Scale fender with wheel arch, AKDQ steel. (b) Scale fender without wheel arch, HSLA steel. (c) Scale fender schematic (Trial #1), (d) Forming performance (Trial #2), (e) Scale fender schematic (Trial #3), (f) Forming performance (Trial #3)

•AKDO: aluminum-killed, deep-drawing •HSLA: high-strength, low-alloy

•Elimination of crack and wrinkle load change: Fig.7.78

Figure 7.78 Wrinkle behavior of scale fender forming with constant and step-down binder force control (Trial #4).

Steps in Manufacturing an Aluminum

Can

Figure 7.65 The metal-forming processes involved in manufacturing a two-piece aluminumbeverage can

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