FUNDAMENTALS OF METAL FORMING SME Video - Forming Processes Forging (vts_14) Overview of Metal Forming Material Behavior in Metal Forming Flow Stress Average Flow Stress In-class Assignment ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Metal Forming Plastic deformation Tool (die) applies stresses that exceed the yield strength of the metal Metal takes a shape determined by the geometry of the die Cold working Warm working Hot working ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Classification of Metal Forming Processes Compressive stresses Tensile stresses Shear stresses ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Material Properties in Metal Forming Desirable material properties: Low yield strength High ductility Properties are affected by temperature: Ductility increases and yield strength decreases when work temperature is raised Other factors: Strain rate and friction ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Basic Types of Deformation Processes Bulk deformation Rolling Forging Extrusion Wire and bar drawing Sheet metalworking Bending Deep drawing Cutting ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Bulk Deformation Processes - Rolling Significant deformations and massive shape changes "Bulk" refers to workparts with relatively low surface area‑to‑volume ratios Starting work shapes Cylindrical billets Rectangular bars rolling ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Forging forging ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Extrusion extrusion ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Wire and Bar Drawing drawing ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Sheet Metalworking Operations performed on metal sheets High surface area‑to‑volume ratio Often called pressworking Presses perform these operations Parts are called stampings punch and die tooling ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Sheet Metal Bending bending ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Deep Drawing drawing ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Shearing of Sheet Metal ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Material Behavior in Metal Forming Plastic region of stress-strain curve Material is plastically deformed In plastic region, metal's behavior is expressed by the flow curve: K = strength coefficient n = strain hardening exponent Flow curve based on true stress and true strain ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Flow Stress Metals at room temperature Strength increases when deforming strain hardening Flow stress = instantaneous value of stress required to continue deforming the material Yield strength as a function of strain Yf = flow stress ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Average Flow Stress where = average flow stress  = maximum strain during deformation process Stress-strain curve ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

In-class Example A metal has a flow curve with parameters: strength coefficient = 850 MPa and strain-hardening exponent = 0.30. A tensile specimen of the metal with gage length = 100 mm is stretched to a length = 157 mm. Determine the flow stress at the new length and the average flow stress that the metal has been subjected to during the deformation. ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

SME Video ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

In-class Assignment A particular metal has a flow curve with parameters: strength coefficient = 35,000 lb/in2 and strain-hardening exponent = 0.26. A tensile specimen of the metal with gage length = 2.0 in is stretched to a length = 3.3 in. Determine the flow stress at this new length and the average flow stress that the metal has been subjected to during deformation. ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Production Scheduling Machine-Machine Diagrams ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Time Permitting Content ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Temperature in Metal Forming K and n in the flow curve depend on temperature Strength (K) and strain hardening (n) are reduced at higher temperatures Ductility is increased at higher temperatures Deformation operation can be accomplished with lower forces at elevated temperature Three temperature ranges in metal forming: Cold working Warm working Hot working ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Cold Working Performed at room temperature Mass production operations Near net shape or net shape processes Advantages of Cold Forming Better accuracy, closer tolerances Better surface finish Strain hardening and directional properties Disadvantages of Cold Forming High forces Ductility and strain hardening limit process Annealing for further deformation ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Warm Working Above room temperature but below recrystallization temperature 0.3Tm, where Tm = melting point (absolute temperature) for metal Advantages of Warm Working Lower forces than in cold working More intricate work geometries possible Need for annealing may be reduced ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Hot Working Deformation at temperatures above the recrystallization temperature Recrystallization temperature = about one‑half of melting point on absolute scale (0.5Tm) Capability for substantial plastic deformation Why? Strength coefficient (K) is substantially less Strain hardening exponent (n) is zero (theoretically) Ductility is significantly increased ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Hot Working Advantages of Hot Working Workpart shape can be significantly altered Lower forces and power required Use for metals that fracture in cold working Strength properties are generally isotropic No strengthening due to work hardening Disadvantages of Hot Working Lower dimensional accuracy Higher energy (due to the thermal energy) Work surface oxidation (scale) Shorter tool life ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Strain Rate Sensitivity Theoretically, metal in hot working behaves like a perfectly plastic material Strain hardening exponent n = 0 Additional phenomenon “Strain rate sensitivity” occurs at elevated temperatures = true strain rate v = velocity of the ram h = instantaneous height of workpiece ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Strain Rate Sensitivity linear scale log-log scale ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Observations about Strain Rate Sensitivity Increasing temperature decreases C and increases m At room temperature, effect of strain rate is almost negligible Flow curve is a good representation of material behavior As temperature increases, strain rate becomes increasingly important in determining flow stress ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

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 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

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 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e