Chapter 17, 18, 19
Types of Rolling
based on work piece geometry: flat rolling, shape rolling Based on work temperature: hot rolling, cold rolling
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
Sheet metalworking terminology
• Punch‑and‑die - tooling to perform cutting, bending, and drawing • Stamping press - machine tool that performs most sheet metal operations • Stampings - sheet metal products
Material behavior in metal forming
plastic region of stress-strain curve is primary interest because material plastically deformed In plastic region metal's behavior is expressed by sigma=k(e)^n Flow curve is based on true stress and true strain
Upset Forging
(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
Evaluation of strain rate
-by Workpart geometry -Variations in strain rate in different regions of the part Strain rate can reach 1000/s or more for some metal forming operations
Bar Drawing
Accomplished as a single‑draft operation ‑ the stock is pulled through one die opening Beginning stock has large diameter and is a straight cylinder Requires a batch type operation
Advantages and Limitations (Impression-Die Forging)
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
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
Blanking and Punching
Blanking-sheet metal cutting to separate piece from surrounding stock Punching-similar to blanking except cut piece is scrap, called slug
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
Hot working
Deformation at temperatures above recrystallization temperature Recrystallization temperature= about one-half of melting point on absolute scale in practice, ot working usually performed somewhat above 0.5 Tm Metal continues to soften as temperature increases above 0.5 Tm, enhancing advantage of hot working above this level
Forging
Deformation process in which work is compressed between two dies
Cold working
Performed at room temperature or slightly above many cold forming processes are important mass production operations Minimum or no machining usually required These operations are near net shape or net shape processes
Why hot working?
Capability for substantial plastic deformation of the metal - far more than possible with cold working or warm working Why? -strength coefficient (K) is substantially less than at room temperature -strain hardening exponent (n) is zero theoretically Ductility is significantly increased
Bulk Deformation Processes
Characterized by significant deformations and massive shape changes Bulk refers to work parts with relatively low surface area to volume ratios Starting work shapes include cylindrical billets and rectangular bars
Flashless Forging
Does not allow for flash Starting workpart volume must equal die cavity volume within very close tolerance Control more demanding than impression‑die forging Best suited to geometries that are simple and symmetrical Often classified as a precision forging process
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
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 (cf. 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
Basic Types of Sheet metal processes
Cutting Shearing to cut Bending Straining sheet around a straight axis Drawing Forming of sheet into convex or concave shapes
Trimming
Cutting operation to remove flash from workpart in impression‑die forging Usually done while work is still hot, so a separate trimming press is included at the forging station Trimming can also be done by alternative methods, such as grinding or sawing
Drawing Practice and Products
Drawing practice: § Usually performed as cold working § Most frequently used for round cross sections Products: § Wire: electrical wire; wire stock for fences, coat hangers, and shopping carts § Rod stock for nails, screws, rivets, and springs § Bar stock: metal bars for machining, forging, and other processes
Warm working
Performed at temperatures above room temperature but below recrystallization temperature Dividing line between cold working and warm working 0.3 T_m-0.5 Tm, where Tm= melting point (in absolute temp. scale) for metal
Material Properties in Metal forming
Desirable material properties -Low YS -High Ductility These Properties are affected by temperature -Ductility increases and yield strength decreases when T is raised Other factors: strain rate and friction
Average flow stress
Determined by integrating the flow curve equation between zero and final strain value defining the range of interest Yf=Ke^n Average flow stress: Yf=Ke^n/(1+n)
Wire Drawing vs. Bar Drawing
Difference between bar drawing and wire drawing is stock size § Bar drawing - large diameter bar and rod stock § Wire drawing - small diameter stock - wire sizes down to 0.03 mm (0.001 in.) are possible
Rolling Mills
Equipment is massive and expensive Rolling mill configuratiosn: two-high-two opposing rolls Three high-work passes through rolls in both directions four high backing rolls support smaller work rolls Cluster mill-multiple backing rolls on smaller rolls Tandem rolling mill-sequence of two-high mills
Impression-Die Forging
Flash is formed by metal that flows beyond die cavity into small gap between die plates
Effect of strain rate on flow stress
Flow stress is a function of temperature At hot working temperatures, flow stress also depends on the strain rate as strain rate increases, resistance to deformation increases This effect is known as strain-rate sensitivity
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
Sheet metalworking
Forming and related operations performed on metal sheets, strips, and coils High surface area to volume ratio of starting metal, which distinguishes these from bulk deformation Often called pressworking because presses perform these operations parts are called stampings usual tooling: punch and die
Comments on Die Angle
Low die angle - surface area is large, which increases friction at die‑billet interface § Higher friction results in larger ram force Large die angle - more turbulence in metal flow during reduction § Turbulence increases ram force required Optimum angle depends on work material, billet temperature, and lubrication
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
Advantages of warm working
Lower forces and power than in cold working More intricate work geometries possible Need for annealing may be reduced or eliminated
Lubrication in Metal forming
Metalworking lubricants are applied to tool-work interface in many forming operations to reduce harmful effects of friction Benefits: reduced sticking, forces, power, tool wear, better surface finish, removes heat from the tooling
Type of forgings
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- work part is completely constrained in die
What is strain rate
Rate in which material is strained e=V/h where h is instantaneous height of work piece being deformed, V is the velocity of the ram or other movement of the equipment
Diagram of Flat Rolling
Review on the slide * important
Shearing
Sheet metal cutting operation along a straight line between two cutting edges Typically used to cut large sheets
Sheet-metal drawing
Sheet metal forming to make cup‑shaped, box‑shaped, or other complex‑curved, hollow‑shaped parts • Sheet metal blank is positioned over die cavity and then punch pushes metal into opening • Products: beverage cans, ammunition shells, automobile body panels • Also known as deep drawing (to distinguish it from wire and bar drawing
Orifice Shape of Extrusion Die
Simplest cross section shape is circular die orifice Shape of die orifice affects ram pressure As cross section becomes more complex, higher pressure and greater force are required Effect of cross-sectional shape on pressure can be assessed by means of the die shape factor Kx
Sheet-Metal bending
Straining Sheetmetal around a straight axis to take a permanent bend
Strain Rate Sensitivity
Theoretically, a metal in hot working behaves like a perfectly plastic material, with strain hardening exponent n=0 The metal should continue to flow at the same flow stress once that stress is reached However an additional phenomenon occurs during deformation, especially at elevated temperatures: Strain rate sensitivity
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
Shape Rolling
Work is deformed into a contoured cross section rather than flat (rectangular) Accomplished by passing work through rolls that have the reverse of desired shape
Advantages of hot working
Workpart shape can be significantly altered Lower forces and power required Metals that usually fracture in cold working can be hot formed Strength properties of product are generally isotropic No strengthening of part occurs from work hardening -advantageous in cases when part is to be subsequently processed by cold forming
Wire Drawing
continuous drawing machines consisting of multiple draw dies separated by accumulating drums
Open-die forging
deformation operation reduces height and increases diameter
Ring Rolling
deformation process in which a thick-walled ring of smaller diameter is rolled into a thin-walled ring of larger diameter 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
Rolling
deformation process in which work thickness is reduced by compressive forces exerted by two opposing rolls
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 Any deformation operation can be accomplished with lower forces and power at elevated temp Three temperature ranges in metal forming Cold working; warm working; hot working
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 (i.e., to keep it flowing) Y_f=Ke^n
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
Friction in metal forming
in most metal forming processes, friction is undesirable: Metal flow is retarded Forces and power are increased Tooling wears faster Friction and tool wear are more severe in hot working
The rolls
rotating rolls perform two main functions -pull the work into the gap between them by friction between workpart and rolls simultaneously squeeze the work to reduce its cross section