Lecture 9: Metal Forming Large group of manufacturing processes in which plastic deformation is used to change the shape of metal workpieces The tool, usually called a die, applies stresses that exceed the yield strength of the metal The metal takes a shape determined by the geometry of the die

Material Properties in Metal Forming Desirable material properties: Low yield strength High ductility These properties are affected by temperature: Ductility increases and yield strength decreases when work temperature is raised Other factors: Strain rate and friction

Basic Types of Deformation Processes Bulk deformation Rolling Forging Extrusion Wire and bar drawing Sheet metalworking (Lecture 10) Bending Deep drawing Cutting Miscellaneous processes

Bulk Deformation Processes Characterized by significant deformations and massive shape changes "Bulk" refers to workparts with relatively low surface area‑to‑volume ratios Starting work shapes include cylindrical billets and rectangular bars

Material Behavior in Metal Forming Plastic region of stress-strain curve is primary interest because material is plastically deformed In plastic region, metal's behavior is expressed by the flow curve: Where s = stress (MPa,psi), K = strength coefficient (MPa,psi), e = true strain, and n = strain hardening exponent Flow curve based on true stress and true strain

Flow Stress For most metals at room temperature, strength increases when deformed due to strain hardening Flow stress = instantaneous value of stress required to continue deforming the material where Yf = flow stress (MPa,psi), that is, the yield strength as a function of strain. But to estimate the force and power in rolling the average flow stress is used Where = average flow stress (MPa,psi)

Example Problem 1 (SI units) Austenitic stainless steel has a flow curve with strength coefficient = 1200 MPa and strain-hardening exponent = 0.40. A tensile test specimen with gage length = 100 mm is stretched to a length = 145 mm. Determine the flow stress and average flow stress that the metal experienced at this strain. Solution:  = ln (145/100) = ln 1.45 = 0.372 Flow stress Yf = 1200(0.372)0.40 = 808 MPa Average flow stress = 1200(0.372)0.40/1.40 = 577 MPa

Temperature in Metal Forming For any metal, K and n in the flow curve depend on temperature Both strength (K) and strain hardening (n) are reduced at higher temperatures In addition, ductility is increased at higher temperatures

Temperature in Metal Forming Any deformation operation can be accomplished with lower forces and power at elevated temperature Three temperature ranges in metal forming: Cold working  Room Temp.<T<0.3Tm, where Tm = melting point (absolute temperature) for metal If it is lower than room Temp., use room Temperature Warm working  0.3Tm<T<0.5Tm Hot working  0.5Tm<T Note that Tm must be in degree Rankine or Kelvin. oK = oC + 273 oR = oF + 460

Advantages of Cold Forming Better accuracy, closer tolerances Better surface finish Strain hardening increases strength and hardness Grain flow during deformation can cause desirable directional properties in product No heating of work required

Disadvantages of Cold Forming Higher forces and power required in the deformation operation Surfaces of starting workpiece must be free of scale and dirt Ductility and strain hardening limit the amount of forming that can be done In some cases, metal must be annealed to allow further deformation In other cases, metal is simply not ductile enough to be cold worked

Disadvantages of Hot Working Lower dimensional accuracy Higher total energy required (due to the thermal energy to heat the workpiece) Work surface oxidation (scale), poorer surface finish Shorter tool life

What is Strain Rate? Strain rate in forming is directly related to speed of deformation v Deformation speed v = velocity of the ram or other movement of the equipment (m/s, ft/s) Strain rate is defined: h = instantaneous height of workpiece being deformed (m, ft)

Effect of Strain Rate on Flow Stress Flow stress is a function of temperature At hot working temperatures, flow stress also depends on strain rate As strain rate increases, resistance to deformation increases This effect is known as strain‑rate sensitivity

Effect of Temperature on Flow Stress Figure 18.6 Effect of temperature on flow stress for a typical metal. The constant C, as indicated by the intersection of each plot with the vertical dashed line at strain rate = 1.0, decreases, and m (slope of each plot) increases with increasing temperature.

Figure 19.1 The rolling process (specifically, flat rolling). Deformation process in which work thickness is reduced by compressive forces exerted by two opposing rolls Figure 19.1 The rolling process (specifically, flat rolling).

Diagram of Flat Rolling Figure 19.3 Side view of flat rolling, indicating before and after thicknesses, work velocities, angle of contact with rolls, and other features.

Types of Rolling Based on workpiece geometry : Flat rolling - used to reduce thickness of a rectangular cross section Shape rolling - square cross section is formed into a shape such as an I‑beam Based on work temperature : Hot Rolling – most common due to the large amount of deformation required Cold rolling – produces finished sheet and plate stock

Rolled Products Made of Steel Figure 19.2 Some of the steel products made in a rolling mill.

Flat Rolling Terminology Draft = amount of thickness reduction where d = draft; to = starting thickness (in, or mm); and tf = final thickness (in, or mm) Reduction = draft expressed as a fraction of starting stock thickness: r = d/to where r = reduction

Conservation of matter Volume of metal exiting the rolls equal the volume entering. towovo=tf wf vf to and tf are the before and after work thickness. wo and wf are the before and after work widths. vo and vf are the entering and exiting velocities.

Contact length Contact length can be approximated by Where to and tf are the before and after work thickness. and R = roll radius (inch or mm)

Rolling force F Can be calculated based on the average flow stress experienced by the work material in the roll gap. That is, F = w L where L is contact length, w is the width of work material. and = average flow stress (psi, or MPa); Force unit is pound or (N in metric system)

Required Power Power is calculated based on the fact that rolling mill consists of two powered rolls (in.lb/min or Watt). T = Torque (lb.in or N.m) N = rotational speed (rev/min) F = rolling force ( lb, or N) L = contact length (inch or mm) Watt = N.m/s = N. mm/min × 1/60,000 HP = 396,000 in.lb/min

Example problem 2 (USCS units) A 4-in-thick aluminum slab is 10 in wide and 100 in long. It is reduced in one pass in a two‑high rolling mill to a thickness = 3.5 in. The roll rotates at a speed = 12 rev/min and has a radius = 20 in. The aluminum has a strength coefficient = 25,000 lb/in2 and a strain hardening exponent = 0.20 (Table 3.4). Determine (a) roll force, (b) roll torque, and (c) power required to accomplish this operation. Solution: (a) Draft d = 4.00 – 3.5 = 0.5 in, Contact length L = (20 x 0.5)0.5 = 3.16 in True strain  = ln(4.0/3.5) = ln 1.143 = 0.1335 = 25,000(0.1335)0.20/1.20 = 13,928 lb/in2 Rolling force F = wL = 13,928(10)(3.16) = 440,125 lb (b) Torque T = 0.5FL = 0.5(440,125)(3.16) = 789,700 lb-in. (c) N = 12 rev/min Power P = 2(12)(440,125)(3.16) = 104,863,473 in-lb/min HP = (104,863,473 in-lb/min)/(396,000) = 265 hp

Two-High Rolling Mill Figure 19.5 Various configurations of rolling mills: (a) 2‑high rolling mill.

Figure 19.5 Various configurations of rolling mills: (d) cluster mill Multiple backing rolls allow even smaller roll diameters Figure 19.5 Various configurations of rolling mills: (d) cluster mill

Thread Rolling Bulk deformation process used to form threads on cylindrical parts by rolling them between two dies Important commercial process for mass producing bolts and screws Performed by cold working in thread rolling machines Advantages over thread cutting (machining): Higher production rates Better material utilization Stronger threads and better fatigue resistance due to work hardening

Thread Rolling Figure 19.6 Thread rolling with flat dies: (1) start of cycle, and (2) end of cycle.

Ring Rolling Deformation process in which a thick‑walled ring of smaller diameter is rolled into a thin‑walled ring of larger diameter As thick‑walled ring is compressed, deformed metal elongates, causing diameter of ring to be enlarged Hot working process for large rings and cold working process for smaller rings Applications: ball and roller bearing races, steel tires for railroad wheels, and rings for pipes, pressure vessels, and rotating machinery Advantages: material savings, ideal grain orientation, strengthening through cold working

Ring Rolling Figure 19.7 Ring rolling used to reduce the wall thickness and increase the diameter of a ring: (1) start, and (2) completion of process.

Forging Deformation process in which work is compressed between two dies Oldest of the metal forming operations, dating from about 5000 B C Components: engine crankshafts, connecting rods, gears, aircraft structural components, jet engine turbine parts Also, basic metals industries use forging to establish basic form of large parts that are subsequently machined to final shape and size

Classification of Forging Operations Cold vs. hot forging: Hot or warm forging – most common, due to the significant deformation and the need to reduce strength and increase ductility of work metal Cold forging – advantage: increased strength that results from strain hardening Impact vs. press forging: Forge hammer - applies an impact load Forge press - applies gradual pressure

Types of Forging Dies Open‑die forging - work is compressed between two flat dies, allowing metal to flow laterally with minimum constraint Impression‑die forging - die contains cavity or impression that is imparted to workpart Metal flow is constrained so that flash is created Flashless forging - workpart is completely constrained in die No excess flash is created

Open-Die Forging Figure 19.10 Homogeneous deformation of a cylindrical workpart under ideal conditions in an open‑die forging operation: (1) start of process with workpiece at its original length and diameter, (2) partial compression, and (3) final size.

Required Force for Open-Die Forging

Example problem 3 Solution: (USCS units) A cylindrical work part has a diameter = 2.5 in and a height = 4.0 in. It is upset forged to a height = 2.75 in. Coefficient of friction at the die‑work interface = 0.10. The work material has a flow curve with strength coefficient = 25,000 lb/in2 and strain hardening exponent = 0.22. Determine the force needed to perform this operation. Solution: Volume of cylinder V = D2h/4 = (2.5)2(4.0)/4 = 19.635 in3 At h =2.75,  = ln(4.0/2.75) = ln 1.4545 = 0.3747 Yf = 25,000(0.3747)0.22 = 20,144 lb/in2 V = 19.635 in3 calculated above. At h = 2.75 in, A = V/h = 19.635/2.75 = 7.140 in2 Corresponding D = 3.015 in (from A = D2/4) Kf = 1 + 0.4(0.1)(3.015)/2.75 = 1.044 F = 1.044(20,144)(7.140) = 150,136 lb

Impression‑Die Forging Compression of workpart by dies with inverse of desired part shape Flash is formed by metal that flows beyond die cavity into small gap between die plates Flash must be later trimmed, but it serves an important function during compression: As flash forms, friction resists continued metal flow into gap, constraining material to fill die cavity In hot forging, metal flow is further restricted by cooling against die plates

Impression-Die Forging Figure 19.14 Sequence in impression‑die forging: (1) just prior to initial contact with raw workpiece, (2) partial compression, and (3) final die closure, causing flash to form in gap between die plates.

Advantages and Limitations Advantages of impression-die forging compared to machining from solid stock: Higher production rates Less waste of metal Greater strength Favorable grain orientation in the metal Limitations: Not capable of close tolerances Machining often required to achieve accuracies and features needed

Flashless Forging Compression of work in punch and die tooling whose cavity does not allow for flash Starting workpart volume must equal die cavity volume within very close tolerance Process control more demanding than impression‑die forging Best suited to part geometries that are simple and symmetrical Often classified as a precision forging process

Flashless Forging Figure 19.17 Flashless forging: (1) just before initial contact with workpiece, (2) partial compression, and (3) final punch and die closure.

Forging Hammers (Drop Hammers) Apply impact load against workpart Two types: Gravity drop hammers - impact energy from falling weight of a heavy ram Power drop hammers - accelerate the ram by pressurized air or steam Disadvantage: impact energy transmitted through anvil into floor of building Commonly used for impression-die forging

Forging Presses Apply gradual pressure to accomplish compression operation Types: Mechanical press - converts rotation of drive motor into linear motion of ram Hydraulic press - hydraulic piston actuates ram Screw press - screw mechanism drives ram

Upsetting and Heading Forging process used to form heads on nails, bolts, and similar hardware products More parts produced by upsetting than any other forging operation Performed cold, warm, or hot on machines called headers or formers Wire or bar stock is fed into machine, end is headed, then piece is cut to length For bolts and screws, thread rolling is then used to form threads

Upset Forging Figure 19.22 An upset forging operation to form a head on a bolt or similar hardware item The cycle consists of: (1) wire stock is fed to the stop, (2) gripping dies close on the stock and the stop is retracted, (3) punch moves forward, (4) bottoms to form the head.

Swaging Accomplished by rotating dies that hammer a workpiece radially inward to taper it as the piece is fed into the dies Used to reduce diameter of tube or solid rod stock Mandrel sometimes required to control shape and size of internal diameter of tubular parts

Swaging Figure 19.24 Swaging process to reduce solid rod stock; the dies rotate as they hammer the work In radial forging, the workpiece rotates while the dies remain in a fixed orientation as they hammer the work.

Trimming After Impression-Die Forging Figure 19.29 Trimming operation (shearing process) to remove the flash after impression‑die forging.

Extrusion Compression forming process in which work metal is forced to flow through a die opening to produce a desired cross‑sectional shape Process is similar to squeezing toothpaste out of a toothpaste tube In general, extrusion is used to produce long parts of uniform cross sections Two basic types: Direct extrusion Indirect extrusion

Figure 19.30 Direct extrusion.

Hollow and Semi-Hollow Shapes Figure 19.31 (a) Direct extrusion to produce a hollow or semi‑hollow cross sections; (b) hollow and (c) semi‑hollow cross sections.

Indirect Extrusion Figure 19.32 Indirect extrusion to produce (a) a solid cross section and (b) a hollow cross section.

Advantages of Extrusion Variety of shapes possible, especially in hot extrusion Limitation: part cross section must be uniform throughout length Grain structure and strength enhanced in cold and warm extrusion Close tolerances possible, especially in cold extrusion In some operations, little or no waste of material

Wire and Bar Drawing Cross‑section of a bar, rod, or wire is reduced by pulling it through a die opening Similar to extrusion except work is pulled through die in drawing (it is pushed through in extrusion) Although drawing applies tensile stress, compression also plays a significant role since metal is squeezed as it passes through die opening

Figure 19.40 Drawing of bar, rod, or wire. Wire and Bar Drawing Figure 19.40 Drawing of bar, rod, or wire.

Figure 19.43 Draw die for drawing of round rod or wire. Draw Die Details Figure 19.43 Draw die for drawing of round rod or wire.

Preparation of Work for Drawing Annealing – to increase ductility of stock Cleaning - to prevent damage to work surface and draw die Pointing – to reduce diameter of starting end to allow insertion through draw die

1- Reading Assignment 2- Written Assignment Week1 – Rolling Chapters 17, and 18 2- Written Assignment Week1 – Rolling What are the differences between bulk deformation processes and sheet metal processes? How does increasing temperature affect the parameters in the flow curve equation? Name the four basic bulk deformation processes. What is rolling in the context of the bulk deformation processes? Besides flat rolling and shape rolling, identify some additional bulk forming processes that use rolls to effect the deformation. (USCS units) Annealed low-carbon steel has a flow curve with strength coefficient = 75,000 lb/in2 and strain-hardening exponent = 0.25. A tensile test specimen with gage length = 2.0 in is stretched to a length = 3.3 in. Determine the flow stress and average flow stress that the metal experienced during this deformation. (SI units) A low-carbon steel plate is 300 mm wide and 25 mm thick. It is reduced in one pass in a two‑high rolling mill to a thickness of 20 mm. Roll radius = 600 mm, and roll speed = 8 rev/min. Strength coefficient = 500 MPa, and strain hardening exponent = 0.25 (Table 3.4). Determine (a) roll force, (b) roll torque, and (c) power required to perform the operation.

1- Reading Assignment 2- Written Assignment Week2 – Forging Chapters 17, and 18 2- Written Assignment Week2 – Forging What is forging? Why is flash desirable in impression die forging? What are the two basic types of forging equipment? What is extrusion? What are wire drawing and bar drawing? (USCS units) A cylindrical work part with diameter = 2.5 in, and height = 2.5 in is upset forged in an open die to a height = 1.5 in. Coefficient of friction at the die work interface = 0.10. The work metal has a strength coefficient = 40,000 lb/in2 and strain hardening exponent = 0.15. Yield strength = 15,750 lb/in2. Determine the instantaneous force in the operation just as the height is reached to h = 2.3 in.