Bulk Deformation Processes

Bulk-Deformation Processes TABLE 6.1 General characteristics of bulk deformation processes.

[1] Forging Operations Forging can be carried out as: a. cold working (T < 0.3Tm) b. warm working (0.3Tm < T < 0.5Tm ) c. hot working (T > 0.6Tm) Three basic types: open-die forging (also known as upsetting) Impression die forging Closed-die forging

Ideal (homogeneous) Deformation FIGURE 6.1 (a) Ideal deformation of a solid cylindrical specimen compressed between flat frictionless dies. This process is known as upsetting. (b) Deformation in upsetting with friction at the die-workpiece interfaces.

Equations for Open-Die Forging Reduction in height =   and

Grain Flow Lines FIGURE 6.2 Grain flow lines in upsetting a solid steel cylinder at elevated temperatures. Note the highly inhomogeneous deformation and barreling. The different shape of the bottom section of the specimen (as compared with the top) results from the hot specimen resting on the lower, cool die before deformation proceeded. The bottom surface was chilled; thus it exhibits greater strength and hence deforms less than the top surface. Source: J. A. Schey et al., IIT Research Institute.

Deformation force and work calculations Assumptions: no friction at the die-workpiece interface, and material is perfectly plastic with a yield stress (Y), initial height (h0), initial area (A0), instantaneous height (h1), and instantaneous area (A1).   Average Flow Stress

Impression-Die Forging FIGURE 6.14 Schematic illustration of stages in impression-die forging. Note the formation of flash, or excess material that is subsequently trimmed off. Analysis F = (Kp)(Yf)(A) TABLE 6.2 Range of Kp values in Eq. (6.21) for impression-die forging.

Orbital Forging Process FIGURE 6.16 Schematic illustration of the orbital-forging process. Note that the die is in contact with only a portion of the workpiece surface. This process is also called rotary forging, swing forging, and rocking-die forging and can be used for forming bevel gears, wheels, and bearing rings.

Heading Piercing Operations FIGURE 6.17 Forging heads on fasteners such as bolts and rivets. These processes are called heading. FIGURE 6.18 Examples of piercing operations.

Cogging Operation FIGURE 6.19 Schematic illustration of a cogging operation on a rectangular bar. With simple tools, the thickness and cross-section of a bar can be reduced by multiple cogging operations. Note the barreling after cogging. Blacksmiths use a similar procedure to reduce the thickness of parts in small increments by heating the workpiece and hammering it numerous times.

Roll Forging Operation FIGURE 6.20 Schematic illustration of a roll forging (cross-rolling) operation. Tapered leaf springs and knives can be made by this process with specially designed rolls. Source: After J. Holub.

Manufacture of Spherical Blanks FIGURE 6.21 Production of steel balls for bearings by the skew-rolling process. Balls for bearings can also be made by the forging process shown in Fig. 6.22. FIGURE 6.22 Production of steel balls by upsetting of a cylindrical blank. Note the formation of flash. The balls are subsequently ground and polished for use as ball bearings and in other mechanical components.

Defects in Forging Surface Cracking: due to excessive stresses and poor die design Internal Cracks: due to oversized billets Cold Shuts: due to small corner radii in the mold Exposed grains: poor material flow and original blank orientation Forgings are generally anisotropic due to various directions in the metal flow.

Defect Formation In Forging FIGURE 6.25 Effect of fillet radius on defect formation in forging. Small fillets (right side of drawings) cause the defects. Source: Aluminum Company of America.

Forging A Connecting Rod FIGURE 6.26 Stages in forging a connecting rod for an internal combustion engine. Note the amount of flash that is necessary to fill the die cavities properly.

[2] Rolling Operations Rolling is the process of reducing the thickness (flat rolling) or changing the cross-section (shape rolling) of a long workpiece by compressive forces applied through a set of rolls. Rolling Operations Flat Rolling Shape Rolling Plates: thickness>0.25 Sheets: 0.004<thickness<0.25 Foils: thickness<0.004 Ingot rolling is replaced by continuous casting/rolling with a higher efficiency and lower cost. Rolling can be performed as hot, warm, or cold forming operations.

Flat-And-Shape-Rolling Processes FIGURE 6.29 Schematic outline of various flat-and-shape-rolling processes. Source: American Iron and Steel Institute.

Mechanics of Flat-Rolling wo and ho : initial width and thickness wf and hf : final width and thickness Vo and Vf : entry and exit velocities Vr : tangential velocity of roll R: roll radius L: roll gap FIGURE 6.31 Schematic illustration of the flat-rolling process. A greater volume of metal is formed by rolling than by any other metalworking process.

Neutral (No-Slip) Point Vo : entry velocity of the workpiece Vf : exit velocity of the workpiece Vr : tangential velocity of roll : angle of acceptance Forward Slip = Notes At the “no-slip” point: Vr = Vworkpiece (neutral point) Before the neutral point: Vr > Vworkpiece After the neutral point: Vr < Vworkpiece FIGURE 6.32 Relative velocity distribution between roll and strip surfaces. Note the difference in the direction of frictional forces. The arrows represent the frictional forces acting on the strip.

Forces and Power Draft = h = ho – hf = 2 . R µ = tan  (angle of acceptance)

Notes on Force Calculations As friction increases, the draft, forces, power and the damage to surface finish increase. Roll forces can be reduced by: reducing friction Reducing rolls size Reducing draft Applying tension forces (back, front, or both) Diverse effects of roll forces: Rolls deflection Rolls flattening

Problems in Rolling Operations Rolls Deflection: forces bend rolls elastically, this results in thicker parts at the center (CROWN), which can be reduced by: Grinding rolls thicker at the center (CAMBER)~0.01“, or Bending rolls by applying moments at the end Roll Flattening: results in larger roll radius, and hence larger contact area, which results in larger rolling forces for the same rolling draft. Plastic deformation of rolls (Thermal Camber) Stretch of roll stand (low rigidity) Spreading, especially with high thickness/width ratio. Solution: edger mills.

Roll Deflection and Spreading FIGURE 6.37 (a) Bending of straight cylindrical rolls (exaggerated) because of the roll force. (b) Bending of rolls, ground with camber, that produce a sheet of uniform thickness during rolling. FIGURE 6.38 Increase in the width of a strip (spreading) in flat rolling. Spreading can be similarly observed when dough is rolled with a rolling pin.

Workpiece Defects Wavy edges: due to roll bending Surface cracks: due to poor ductility and/or low rolling temeperature Alligatoring: due to defects in the original cast Residual stresses (only when not desired): the type of these stresses are: Tensile in the middle/compressive at surface when using small rolls or draft Compressive in the middle and tensile on the surface when using large rolls or draft.

Workpiece Defects In Flat Rolling FIGURE 6.39 Schematic illustration of typical defects in flat rolling: (a)wavy edges; (b) zipper cracks in the center of strip; (c) edge cracks; (d) alligatoring.

Shape Rolling FIGURE 6.44 Stages in shape rolling of an H-section part. Various other structural sections, such as channels and I-beams, are also rolled by this process.

Ring-Rolling FIGURE 6.45 (a) Schematic illustration of a ring-rolling operation. Reducing the thickness results in an increase in the part’s diameter. (b) Examples of cross-sections that can be formed by ring rolling.

Thread-Rolling Processes FIGURE 6.46 Thread-rolling processes: (a) flat dies and (b) two-roller dies. These processes are used extensively in making threaded fasteners at high rates of production.

Machined And Rolled Threads FIGURE 6.47 (a) Schematic illustration of machined or rolled threads. (b) Grain-flow lines in machined and rolled threads. Unlike machined threads, which are cut through the grains of the metal, rolled threads follow the grains and are stronger, because of the cold working involved.

Mannesmann Process FIGURE 6.48 Cavity formation by secondary tensile stresses in a solid round bar and its use in the rotary-tube-piercing process. This procedure uses the principle of the Mannesmann mill for seamless tube making. The mandrel is held in place by the long rod, although techniques have been developed in which the mandrel remains in place without the rod.

[3] Extrusion Processes 1.     Direct Extrusion 2.     Indirect Extrusion 3.     Hydrostatic Extrusion 4.     Impact Extrusion 5.     Lateral Extrusion

Types Of Extrusion FIGURE 6.49 Types of extrusion. (a) direct; (b) indirect; (c) hydrostatic; (d) impact.

Extrusion Ideal: no-friction at the billet-container-die interfaces FIGURE 6.51 Schematic illustration of three different types of metal flow in direct extrusion. Ideal: no-friction at the billet-container-die interfaces Typical: friction at the billet-container-die interfaces High container wall-billet friction FIGURE 6.50 Extrusion and examples of products made by sectioning off extrusions. Source: Kaiser Aluminum.

Extrusion Parameters 1. die angle () 2. extrusion-ratio (R) 3.     circumscribing-circle-diameter (CCD): the diameter of the smallest circle into which the extruded cross-section will fit. 4.     shape factor (ratio of the perimeter to the cross- sectional area) 5.     billet temperature 6.     ram speed 7.     type of lubricant

Extrusion Force/Power Calculations Extrusion Ratio   Absolute value of the true strain Energy dissipation per unit volume (for perfectly plastic materials) Work supplied by the ram force (F), which travels a distance Lo Extrusion pressure (ideal – no friction). For strain-hardening materials, use average flow stress Y Extrusion pressure (friction between die and billet only)

Extrusion Constant P=Ke*ln(R) FIGURE 6.55 Extrusion constant, Ke, for various materials as a function of temperature. Source: After P. Loewenstein, ASTME Paper SP63-89.

Cold and Impact Extrusion FIGURE 6.56 Examples of cold extrusion. Arrows indicate the direction of material flow. These parts may also be considered as forgings. FIGURE 6.57 (a) Impact extrusion of a collapsible tube (Hooker process).(b) Two examples of products made by impact extrusion, these parts may also be made by casting, forging, and machining, depending on the dimensions and materials involved and the properties desired. Economic considerations are also important in final process selection.

Notes on Extrusion Practices Cold Extrusion has the following advantages over hot extrusion: better mechanical properties (due to work hardening), better dimensional accuracy (little finishing operations) and surface finish (no oxide film), and high production rates at relatively low cost. However cold extrusion requires more expensive tooling (high hardness, strength, toughness, and fatigue strength), complex lubrication and cooling process, and high capacity presses. Hydrostatic extrusion requires much lower forces due to the increased ductility of the material (suitable for brittle materials), and the low friction. Also, it is possible to use low die angles as well as high extrusion ratios. Metal flow in the die influences the quality and mechanical properties of the parts. Extrusion ratio (R) can go up to 400:1 Coaxial extrusion (cladding) is possible when the strength and ductility of both metals are compatible. Hot extrusion: low forces, excessive die wear (can be reduced with preheated dies), non-uniform deformation (can be reduced with preheated dies), oxide film on the billet (reduced by a smaller ram than the container), poor surface finish due to surface oxidation. Dead-metal zones produce extrusions with bright finishes Die materials are generally hot-work die steels, coated with Zicronia to extend the die life.

Defects in Extrusion Three principal defects in extrusion: Surface Cracking Cause: too high extrusion temperature, friction or speed (intergranular) or at low temperature due to periodic sticking of the extrudate and the die land (knows as bamboo defect) especially during hydrostatic extrusion. Can be reduced by increasing the extrusion speed. Extrusion defects (pipe, tailpipe, and fishtailing) Cause: metal flow on the container wall draw surface oxides and impurities towards the center of the billet. Can be reduced by modifying the flow pattern to more homogeneous by reducing friction and minimizing the temperature gradient. Internal Cracking (chevron cracking or centerburst) Cause: hydrostatic (secondary) tensile stresses at the centerline of the deformation zone. Can be reduced reduced by adjusting the extrusion parameters (die angle, extrusion ratio, friction) so the the deformation zones around the die overlap.

Chevron Cracking FIGURE 6.59 (a) Deformation zone in extrusion, showing rigid and plastic zones. Note that the plastic zones do not meet, leading to chevron cracking. The same observations are also made in drawing round bars through conical dies and drawing flat sheet plate through wedge-shaped dies. Source: After B. Avizur. (b) Chevron cracking in round steel bars during extrusion. Unless the part is inspected properly, such internal detects may remain undetected and possibly cause failure of the part in service.

Extrusion of Seamless Tube FIGURE 6.60 Extrusion of a seamless tube. The hole in the billet may be prepunched or pierced, or it may be generated during extrusion.;

[4] Drawing Operations Drawing is similar to extrusion, except “pulling” through a die is used instead of pushing. It can be used to make wires as small as 0.001 in diameter. Process Parameters Ao , Af initial and final diameters = the die angle F = Drawing Force d = Drawing Stress  = coefficient of friction FIGURE 6.62 Variables in drawing round rod or wire.

Maximum Reduction per pass The maximum allowed drawing stress is the yield stress of the existing material. Therefore, there is a maximum strain (reduction) per pass. For a perfectly plastic material, the maximum reduction in cross sectional area is 63% (where ε1=1). For a strain hardening material, the maximum reduction in cross sectional area can be calculated as:

Tube Drawing FIGURE 6.67 Various methods of tube drawing.

Defects is Drawing Operations Center cracking is caused by inhomogeneous plastic deformation which increases by increasing the die angle, decreasing the reduction per pass, increasing friction, and the presence of inclusions. Seams, longitudinal scratches/folds in the material which can open up during subsequent forming operations. Residual stresses (transverse, longitudinal, radial) due to inhomogeneous deformation. Very light reductions leaves compressive residual stresses on the workpiece surface which improves fatigue strength.

Rotary Swaging Also known as Rotary Forging, where a solid rod or a tube is reduced in diameter by the reciprocating radial movement of two or four dies driven by a set of rollers in a cage. FIGURE 6.71 Schematic illustration of the swaging process: (a) side view and (b) front view. (c) Schematic illustration of roller arrangement, curvature on the four radial hammers (that give motion to the dies), and the radial movement of a hammer as it rotates over the rolls.

Rotary Swaging (continued) Mandrels are used to control the internal diameter and/or shape (example gun/rifle barrels). The workpiece diameter is limited to 2 inches, while the length is limited to the length of the mandrel. Generally performed at room temperature. FIGURE 6.72 Reduction of outer and inner diameters of tubes by swaging. (a) Free sinking without a mandrel. The ends of solid bars and wire are tapered (pointing) by this process in order to feed the material into the conical die. (b) Sinking on a mandrel. Coaxial tubes of different materials can also be swaged in one operation.

Cross-Sections Produced By Swaging FIGURE 6.73 (a) Typical cross-sections produced by swaging tube blanks with a constant wall thickness on shaped mandrels. Rifling of small gun barrels can also be made by swaging, using a specially shaped mandrel. The formed tube is then removed by slipping it out of the mandrel. (b) These parts can also be made by swaging.

Die Failures Failure of dies in metal forming operations results from one or more of the following causes: Improper die design Defective die materials Improper heat treatment and finishing operations Improper installation, assembly, and alignment Overheating and heat checking Excessive wear Overloading, misuse, and improper handling Dies can fail by cracking, chipping, wear, heat checking (from thermal cycling), or deformation (especially in hot working)