BULK DEFORMATION PROCESSES in METALWORKING Introduction 1
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Introduction BULK DEFORMATION • Input: bulk materials in a form of cylindrical bars PROCESSES IN and billets, rectangular billets and slabs or METALWORKING elementary shapes • Process: large plastic deformation - Rolling, 1. Rolling Forging, Extrusion and Wire and Bar drawing 2. Forging under cold, warm and hot working conditions 3. Extrusion • Output: work materials for subsequent processes or final products (net shaping) 4. Wire and Bar Drawing 1 2 1. Rolling Process Information • Thickness of a work material is reduced by the • Cold Rolling compressive forces exerted by two opposing rolls. • Ingot casting – Input: Molten metal – tight tolerance, better – plates (>6mm or 1/4 in) - ship hull, bridge surface and mechanical – sheets (<6mm) - car bodies, appliance – Output: Ingot properties – foil (<0.1mm) - aluminum foil • Soaking • Hot Rolling • Flat (typical) and shape rolling – Input: Ingot – above recrystallization temp. • Equipment: roll mills (expansive) – Output: heated Ingot (450C for Al alloy, 1250C for steel alloy and 1650C for • Hot rolling – large deformation, low force, no residual • Rolling refractory alloy) converts the stress and isotropic properties but problems with – Input: Heated Ingot cast structure to a wrought tolerance and surface finish – Output: bloom, billet or structure slab • Cold Rolling - strengthen, tight tolerance, better surface – Heavy scale forms on the surface. 3 4 Flat rolling Spreading: Conservation of Mass Rolling Analysis I • Friction at the entrance controls the maximum possible draft. towo Lo = t f wf L f dmax = maximum draft (mm); vr or towovo = t f wf v f 2 µ = the coefficient of friction; dmax = µ R where R R = Roll Radius (mm) Draft: d = to − t f θ d • It however depending on lubrication, work and roller materials Reduction:r = and temperature. to ⎡ 0.1 ⎤ forcoldworking to to tf True strain: ε = ln ⎢ ⎥ n µ = 0.2 for warmworking t f Kε ⎢ ⎥ ⎢ ⎥ v Average Flow stress: Yf = ⎣0.3− 0.4⎦ for hotworking v f 1+ n o Slipping and friction on the contact arc except on no-slip • When sticking occurs, µ can be as high as 0.7 or neutral point - One point along the arc where work • Higher the friction and larger the roll radius, the greater the vr velocity equals roll velocity. maximum draft and reduction in thickness become. Amount of slip between the work and the rolls: Forward slip v f − vr s = 5 6 vr 1 Rolling Analysis II Problem 19.11 L A single-pass rolling operation reduced a 20mm thick plate to • Roll force: F = w pdL =Y wL 18mm. The starting plate is 200mm wide. Roll radius=250mm ∫ f and rotational speed=12rev/min. The work material has a 0 strength=600MPa and a strain hardening coefficient =0.22. Determine (a) roll force (b) roll toque and (c) power required for – Newton or lb, L is in meter or ft and N is in rpm this operation. • Contact length: L = R()to − t f (a) Assuming a typical friction coefficient for cold rolling of 0.1 2 Draft d=20-18=2mm < dmax=0.1 (250)=2.5mm • Torque for each roll: T = 0.5FL Contact length L=(250x2).5=11.18mm=0.0112m True Strain ε=ln(20/18)=0.1054 Average flow stress Y =600(0.1054)0.22/1.22=300MPa P = 2πNFL f Rolling force F=YfwL=300(0.0112)(0.2)=0.672MN •Power: = 2πNFL / 60,000kW (b) Torque T=0.5(672,000)(0.0112)=3,720 N.m (c) Power P=2π(12/60)(672,000)(0.0112)=37,697W = 2πNFL / 33000hp 7 8 Intermediate & final rolled form Rolling • Rolling mills – Two-high (basic) – nonreversing and reversing – Three-high – Four-high Bloom –Cluster I-beam Railroad track – Tandem • Shape Rolling – I-beam, L-beam and U-channels, railroad track, Slab Plates, sheets Coil round and square bars. – Roll-pass design • Designing the intermediate shapes and rolls Billet Bars Rods 9 10 Other Rolling Operations 2. Forging • Ring rolling • Dating from about 5000 B C • A deformation process achieved by gradual and certain movement of two dies. (forging press or • Thread rolling – cold thread-making process – higher production, better strength, fatigue resistance forging hammer) and less material waste • Hot or warm forging and sometime cold forging • Three types • Gear rolling – Open-die forging – no lateral constraints – higher production, better strength, fatigue resistance – Impression-die forging – the die surface contains the and less material waste material but not completely • Roll piercing – making seamless thick-walled – Flashless (closed-die) forging – the material is tubes completely constrained within the die 11 12 2 Impression-die & Flashless Analysis on Open-die forging h • The true strain for the process: ε = ln o h • Force required: F = K f Yf A 0.4µD where K =1+ f h Yf is the flow stress, not average flow stress Impression-die forging Flashless (closed-die) forging Ideal Actual 13 14 Impression and Flashless Impression-die Forging • Impression-die forging can achieve Close • Force required: F = K f Yf A tolerance Impression • Advantages compared to machining from solid Simple w/ flash 6.0 stock: Complex w/ flash 8.0 – Higher production rates, less waste, Greater strength – Favorable grain orientation in the metal Very complex w/ flash 10.0 • Limitations: Flashless – Not capable of close tolerances Coining 6.0 – Machining often required to achieve accuracies and features needed, such as holes, threads, and mating Complex shape 8.0 surfaces that fit with other components Need machining afterward 15 16 Problem 19.14 Flashless forging A cylindrical part is warm upset forged in an open die. Do=50mm and ho=40mm. Coefficient of friction =0.2. The work materials has a flow curve • Flashless (closed-die) forging defined by K=600MPa and n=0.12. Determine the force in the operation (a) just as the yield point reached(0.002), and (b) h=20mm. –A precision forging process – Process control more demanding than 2 2 3 (a) V=πD L/4=π (50) 40/4=78,540mm impression-die forging For a small strain, ε=e=∆h/ho=ho-hf/ho=40-hf/40=0.002 0.12 Therefore, hf=39.92mm and Yf=600(0.002) =284.5MPa – Best suited to part geometries that are simple A=V/h =78,540/39.92=1963.5mm2; D =50.0000637mm f f and symmetrical Kf=1+0.4(0.2)(50)/39.92=1.1 F=1.1(284.6)(1963.5)=614,693N • Forging dies (b) hf=20mm; ε=ln(40/20)=0.693 • Gravity or Power drop hammers 0.12 Yf=600(0.693) =574.2MPa Because the volume stays constant, Df=70.7mm • Mechanical, Hydraulic & Screw Presses Kf=1+0.4(0.2)(70.7)/20=1.283 F=1.283(574.2)(3927)=2,892,661N 17 18 3 Sequence in impression-die forging Other Deformation Processes F F • Upset and Heading Upper die • Swaging and Radial Forging Flash Lower die • Roll Forging F • Orbital Forging F • Hobbing Punch • Isothermal and Hot Die Forging •Trimming Die 19 20 Upsetting and Heading Swaging Trimming F die die Punch Starting Work Final diameter diameter Flash feed Die Die • Used to reduce diameter of Cutting operation to remove flash tube or solid rod stock from a work part while work is still • Mandrel sometimes required hot in impression-die forging. Alternatively grinding or sawing to control shape and size of operation is used. internal diameter of tubular parts 21 22 4.