Fundamentals of Metal Forming Example A metal obeys the Hollomon relationship and has a UTS of 300 MPa. To reach maximum load requires an elongation of 35%. Find K and n. Outline 9 Mechanical Properties - Example 9Overview of Metal Forming 9 Cold working - Strain Hardening 9 Annealing - Recrystallization True stress-strain curve plotted on log-log scale 9 Temperature in Metal Forming 9 Friction and Lubrication in Metal Forming Dr. M. Medraj Mech. Eng. Dept. - Concordia University Mech 421/6511 lecture 3/1 Dr. M. Medraj Mech. Eng. Dept. - Concordia University Mech 421/6511 lecture 3/2 Overview of Metal Forming Bulk Deformation Rolling Performed as cold, warm, and hot working Forging Bulk Deformation Extrusion rolling extrusion Wire and bar drawing Metal Forming Mainly cold working Large group of mfg Bending processes in which Sheet plastic deformation is Shearing used to change the shape Metalworking of metal workpieces Deep and cup Wire/bar drawing drawing forging Dr. M. Medraj Mech. Eng. Dept. - Concordia University Mech 421/6511 lecture 3/3 Dr. M. Medraj Mech. Eng. Dept. - Concordia University Mech 421/6511 lecture 3/4 Sheet Metalworking Formability (workability) Formability of the material depends on: (1) process variables - ……………… Desirable material properties in metal forming: - ……………… – Low yield strength and high ductility - ……………… (2) Metallurgical changes during deformation bending Deep/cup drawing - formation of voids, composition, inclusions, precipitation, .... etc. Ductility increases and yield strength decreases when work temperature is raised → Any deformation operation can be accomplished with lower forces and power at elevated temperature shearing Dr. M. Medraj Mech. Eng. Dept. - Concordia University Mech 421/6511 lecture 3/5 Dr. M. Medraj Mech. Eng. Dept. - Concordia University Mech 421/6511 lecture 3/6 Homologous Temp. Ranges for Various Processes Strain or Work Hardening • Strain hardening (work hardening) is where a material becomes less ductile, harder and stronger with plastic deformation •T:working temperature Process T/Tm Cold working < 0.3 • Encountered during cold working •T: melting point of metal (based m Warm working 0.3 to 0.5 • percentage cold work can be expressed as: on absolute temperature scale) Hot working > 0.6 Ao − Ad Ao = original cross-sectional area • e.g. lead %CW = ×100 Ao Ad = deformed cross-sectional area –Tm = 327 °C – Formed at room temperature (20 °C), …………………………………. 9 Most metals strain harden at room temperature according to the • Ductility ……...…. flow curve (n > 0) with cold work 9 But if heated to sufficiently high temperature and deformed, strain • yield and tensile hardening does not occur strength …………… - Instead, new grains are formed that are free of strain - The metal behaves as a perfectly plastic material; that is, n = …. Dr. M. Medraj Mech. Eng. Dept. - Concordia University Mech 421/6511 lecture 3/7 Dr. M. Medraj Mech. Eng. Dept. - Concordia University Mech 421/6511 lecture 3/8 Strain or Work Hardening Example: Cold Work Analysis • Yield strength (σ ) increases. Copper y • What is the tensile strength & • Tensile strength (UTS) increases. ductility after cold working? Cold • Ductility (%EL or %AR) decreases. work → • dislocation density increases with CW • motion of dislocations is hindered as Do=15.2mm Dd=12.2mm their density increases • stress required to cause further Stress deformation is increased σ (MPa) UTS (MPa) ductility (%EL) y 60 • strain hardening is used commercially to 800 improve the yield and tensile properties 700 40 – cold-rolled low-carbon steel sheet 500 600 % co – aluminum sheet Cu ld 300 400 20 wo • strain hardening exponent n indicates Cu rk rain St the response to cold work (i.e. larger n Cu 100 200 0 The influence of cold work on the means greater strain hardening for a 0406020 0 20 40 60 0 20 40 60 stress–strain behavior for a low- given amount of plastic strain) % Cold Work % Cold Work % Cold Work carbon steel. Dr. M. Medraj Mech. Eng. Dept. - Concordia University Mech 421/6511 lecture 3/9 Dr. M. Medraj Mech. Eng. Dept. - Concordia University Mech 421/6511 lecture 3/10 Cold Working Cold Working Disadvantages of Cold Forming: • Performed at room temperature or slightly above • Many cold forming processes are important mass production • Equipment of higher forces and power required operations • Surfaces of starting workpiece must be free of scale and dirt • Minimum or no machining usually required • Ductility and strain hardening limit the amount of forming – These operations are near net shape or net shape processes that can be done – In some operations, metal must be annealed to allow further Advantages of Cold Forming vs. Hot Working: deformation •Betteraccuracy, closer tolerances – In other cases, metal is simply not ductile enough to be cold worked •Bettersurface finish • Strain hardening increases strength and hardness Purposes of annealing: • Grain flow during deformation can cause desirable directional - ………….. properties in product - …………………… • No heating of work required (less total energy) - ………………………….. Involves three steps Dr. M. Medraj Mech. Eng. Dept. - Concordia University Mech 421/6511 lecture 3/11 Dr. M. Medraj Mech. Eng. Dept. - Concordia University Mech 421/6511 lecture 3/12 Annealing-Recrystallization in Metals Recovery, Recrystallization and Grain Growth Recovery • Formation of new strain-free grains is called recrystallization • occurs during heating at elevated temperatures below the Residual recrystallization temperature Stresses • Recrystallization takes time - the recrystallization temperature • dislocations reconfigure due to diffusion and relieve the is specified as the temperature at lattice strain energy which new grains are formed in Ductility • electrical and thermal properties are recovered to their pre- Strength about ………… cold worked state • Recrystallization can be exploited in manufacturing Recrystallization • Heating a metal to its • recrystallization results in the nucleation and growth of recrystallization temperature prior new strain-free, equiaxed grains Grain to deformation allows a greater • contain low dislocation density equivalent to the pre-cold growth amount of straining, and lower forces and power are required to worked condition → annealed state Schematic illustration of the effects of recovery, perform the process • restoration of mechanical properties → softening recrystallization, and grain growth on mechanical properties and on the shape and size of grains. Dr. M. Medraj Mech. Eng. Dept. - Concordia University Mech 421/6511 lecture 3/13 Dr. M. Medraj Mech. Eng. Dept. - Concordia University Mech 421/6511 lecture 3/14 Recrystallization in Metals Recrystallization in Metals • Rate of recrystallization increases with amount of cold work • require a critical amount of cold- work to cause recrystallization (2- 20%) Initial stage of rec. after Partial replacement of cw Cold-worked (33%CW) heating 3 s at 580oC grains by rec. ones 4 s. • recrystallization is easier in pure metals than alloys and occurs at lower temperature –0.3Tm versus ~0.7Tm • hot-working involves deformation The variation of recrystallization temperature with percent cold work for iron. For and concurrent recrystallization at deformations less than the critical (about high temperature 5%CW), recrystallization will not occur. Complete recrystallization Grain growth after 15 Grain growth after 10 (8 s at 580oC). min at 580oC min at 700oC Dr. M. Medraj Mech. Eng. Dept. - Concordia University Mech 421/6511 lecture 3/15 Dr. M. Medraj Mech. Eng. Dept. - Concordia University Mech 421/6511 lecture 3/16 Grain Growth Grain Growth Kinetics • Variation of grain size (d) with • Growth of new grains will time is: continue at high temperature n n • does not require recovery and d − do = Kt recrystallization where do = initial grain size at t = 0, • occurs in both metals and and K and n are time-independent ceramics at elevated temperature constants, n is ≥ 2 • involves the migration of grain boundaries • log d versus log t plots give • large grains grow at expense of linearity at low temperatures small ones The logarithm of grain diameter versus • grain size increases with • reduction of grain boundary area the logarithm of time for grain growth in brass at several temperatures. temperature (driving force) Schematic representation of grain growth via atomic diffusion. • toughness and strength are superior in fine grained materials Dr. M. Medraj Mech. Eng. Dept. - Concordia University Mech 421/6511 lecture 3/17 Dr. M. Medraj Mech. Eng. Dept. - Concordia University Mech 421/6511 lecture 3/18 Warm Working Hot Working • Deformation at temperatures above recrystallization temperature • Performed at temperatures above room temperature but – In practice, hot working usually performed somewhat above 0.5Tm below recrystallization temperature – Metal continues to soften as temperature increases above 0.5Tm, enhancing advantage of hot working above this level • Warm working: T/Tm from 0.3 to 0.5 Advantages of Warm Working: Why Hot Working? • Lower forces and power than in cold working Capability for substantial plastic deformation of the metal - far • More intricate work geometries possible more than possible with cold working or warm working • Need for annealing may be reduced or eliminated •Why? – Strength coefficient is substantially less than at room temp. – Strain hardening exponent is zero (theoretically) – Ductility is significantly increased Dr. M. Medraj Mech. Eng. Dept. - Concordia University Mech 421/6511 lecture 3/19 Dr. M.