Metal Forming CourseCharacteristics of Metals Important in Sheet Forming

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.

Metal Forming Course

Localized Necking in Sheet Metal

• (a) Localized necking in a sheet specimen under tension. (b) Determination of the angle of neck from the Mohr’s circle for strain. (c) Schematic illustration for diffuse and localized necking. (d) Localized necking in an aluminum strip stretched in tension. Note the double neck.

Metal Forming CourseYield Point Elongation

• (a) Yield point elongation and Lueder’s bands in tension testing. (b) Lueder’s bands in annealed low-carbon steel sheet. (c) Stretcher strains at the bottom of a steel can for household products.

Metal Forming CourseShearing

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

Metal Forming Course

Characteristics of Hole and Slug

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

Metal Forming CourseShearing

• (a) Effect of clearance c between the punch and die on the deformation zone in shearing. As clearance increases, the material tends to be pulled into the die, rather than being 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-thick AISI 1020 hot-rolled steel in the sheared region.

• 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.

Metal Forming Course

Punching, Blanking and Shearing Operations

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

Metal Forming CourseFine Blanking

• (a) Comparison of sheared edges by conventional (left) and fine-blanking (right) techniques. (b) Schematic illustration of the setup for fine blanking.

Metal Forming CourseBending

• (a) Bending terminology. The bend radius is measured to the inner surface of the bend. Note that the length of the bend is the width of the sheet. Also note that the bend angle and the bend radius (sharpness of the bend) are two different variables. (b) Relationship between the ratio of bend radius to sheet thickness and tensile reduction of area for various materials. Note that sheet metal with a reduction of area of about 50% can be bent and flattened over itself without cracking.

Metal Forming CourseThe Effect of Elongated Inclusions

• (a) and (b) The effect of elongated inclusions on cracking as a function of the direction of bending with respect to the original rolling direction. This example shows the importance of the direction of cutting from large sheets in workpieces that are subsequently bent to make a product. (c) Cracks on the outer radius of an aluminum strip bent to 90˚.

Metal Forming CourseSpringback in Bending

• Terminology for springback in bending. It is caused by the elastic recovery of the material upon unloading. In this example, the material tends to recover toward its originally flat shape. However, there are situations where the material bends further upon unloading (negative springback).

Metal Forming CourseNegative Springback

• 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 free bending.

Metal Forming Course

Methods of Reducing or Eliminating Springback

• Methods of reducing or eliminating springback in bending operations.

Metal Forming CourseCommon Die-Bending Operations

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

Metal Forming CourseRoll-Forming Process

• Stages in roll forming of a sheet-metal door frame. In Stage 6, the rolls may be shaped as in A or B.

• The roll-forming process.

Metal Forming CourseTube Forming

• 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.

Metal Forming CourseStretch-Forming Process

• Schematic illustration of a stretch-forming process. Aluminum skins for aircraft can be made by this process.

Metal Forming CourseBulging Of A Tubular Part

• (a) 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 with internal pressure. The bottom of the piece is then punched out to produce a “T.” (c) Manufacturing of Bellows.

Metal Forming Course

Hydroform Process

• The hydroform, or fluid-forming, process. Note that, unlike in the ordinary deep-drawing process, the dome pressure forces the cup walls against the punch. The cup travels with the punch, and thus deep drawability is improved.

Metal Forming CourseTube-Hydroforming Process

• (a) Schematic illustration of the tube-hydroforming process. (b) Example of tube-hydroformed parts. Automotive exhaust and structural components, bicycle frames, and hydraulic and pneumatic fittings are produced through tube hydroforming.

Metal Forming CourseSpinning Processes

• Schematic illustration of spinning processes: (a) conventional spinning and (b) shear spinning. Note that in shear spinning, the diameter of the spun part, unlike in conventional spinning, is the same as that of the blank. The quantity f is the feed in mm/rev.

Metal Forming CourseExplosive Forming Process

• Schematic illustration of the explosive-forming process. Although explosives are generally used for destructive purposes, their energy can be controlled and employed in forming large parts that would otherwise be difficult or expensive to produce by other methods.

Metal Forming CourseDeep-drawing Process

• (a) Schematic illustration of the deep-drawing process. The stripper ring facilitates the removal of the formed cup from the punch. (b) Variables in deep drawing of a cylindrical cup. Only the punch force in this illustration is a dependent variable; all others are independent variables, including the blankholder force.

Metal Forming CourseDeformation in Flange and Wall in

Deep Drawing

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

Metal Forming CourseDraw Bead

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

Metal Forming CourseIroning Process • Schematic illustration of the ironing process. The cup wall is

thinner than its bottom. All beverage cans without seams (known as two-piece cans) are ironed, generally in three steps, after being deep drawn into a cup. (Cans with separate tops and bottoms are known as three-piece cans.)

• Definition of the normal anisotropy ratio, R, in terms of width and thickness strains in a tensile-test specimen cut from a rolled sheet. The specimen can be cut in different directions with respect to the length, or rolling direction, of the sheet.

Normal Anisotropy

Metal Forming Course

Average Normal Anisotropy

• Typical range of the average normal anisotropy ratio, R, for various sheet metals.

Zinc alloys Hot-rolled steel Cold-rolled rimmed steel Cold-rolled aluminum-killed steel Aluminum alloys Copper and brass Titanium alloys () 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

Metal Forming Course

Effect of Average Normal Anisotropy

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

Metal Forming CourseDeep Drawing

• Effect of die and punch corner radii in deep drawing on fracture of a cylindrical cup. (a) Die corner radius too small. The die corner 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.

Metal Forming CourseRedrawing Operations

• Reducing the diameter of drawn cups by redrawing operations: (a) conventional redrawing and (b) reverse redrawing. Small-diameter deep containers undergo many drawing and redrawing operations.

Metal Forming Course

• (a) Forming-limit diagram (FLD) for various sheet metals. The major strain is always positive. The region above the curves is the failure zone; hence, the state of strain in forming must be such that it falls below the curve for a particular material; R is the normal anisotropy. (b) Note the definition of positive and negative minor strains. If the area of the deformed circle is larger than the area of the original circle, the sheet is thinner than the original, because the volume remains constant during plastic deformation.

Forming-Limit Diagram (FLD)

Metal Forming Course

The End