Experiment (4) Extrusin Operatins Dr. Mhammad Al-tahat Department f Industrial Engineering. University f Jrdan. Lab. Of Manufacturing Prcesses. Curse N: 90641, 090641 1. Objective: The main bjective f this experiment is t study the prcess f extrusin f metals and t examine the deferent factrs influence the prcess.. Backgrund: Fr mre infrmatin abut the subject f the experiments, it is recmmended fr the student t review sectin 6.3 f chapter six f the text. 3. Thery A material (may be rund billet r f ther varius shapes) is placed in a chamber and frced thrugh a die pening by a ram. The die als, may be rund r f varius ther shapes see figure 1. Figure 1: The Prcess f Extrusin. The prcess can be carried ut ht r cld. It is a batch prcess. Because each billet is extruded individually. Varius shaped parts culd be extruded as shwn in the figure bellw. Figure illustrates extrusins and examples f prducts made by sectining ff extrusins. Figure : Examples f prducts made by Extrusin. Page 1 f 9
Frictin, Thermal Gradients, and Metal Flw In Extrusin. Fr investigating the flw pattern is t halve the rund billet lengthwise and mark ne face with a grid pattern. The tw halves are then placed tgether in the cntainer (they may als be fastened tgether r brazed t keep the tw halves intact) and extruded. They are then taken apart and inspected. Figure 3 shws three typical results in direct extrusin with square dies. The cnditins under which these different flw patterns are btained are as fllws. Figure 3: Types f metal flw in direct Extrusin. a. The mst hmgeneus flw pattern is btained when there is n frictin at the billet cntainer-die interfaces (Fig. 3 a). This type f flw ccurs when the lubricant is very effective r with indirect extrusin. b. When frictin alng all interfaces is high, a dead-metal zne develps (Fig. 3 b). Nte the high-shear area as the material flws int the die exit, smewhat like a funnel. This cnfiguratin may indicate that the billet surfaces (with their xide layer and lubricant) culd enter this high-shear zne and be extruded, causing defects in the extruded prduct. c. The high-shear zne extends farther back (Fig. 3 c). This extensin can result frm high cntainer-wall frictin, which retards the flw f the billet, r materials in which the flw stress drps rapidly with increasing temperature. In ht wrking, the material near the cntainer walls cls rapidly and hence increases in strength. Thus the material in the central regins flws tward the die mre easily than that at the uter regins. As a result, a large dead-metal zne frms and the flw is inhmgeneus. This flw pattern leads t a defect knwn as a pipe r extrusin defect. Thus the tw factrs that greatly influence metal flw in extrusin are the frictinal cnditins at billet-cntainer-die interfaces and thermal gradients in the billet. Page f 9
1. Miscellaneus Extrusin Operatins..1 Direct (Frward) Extrusin In direct extrusin (frward extrusin) the billet slides relative t the cntainer wall; the wall frictin increases the ram frce cnsiderably. That is similar t frcing the paste thrugh the pening f a tthpaste tube as seen in figure 64 a. Figure 4: (a) Direct extrusin, (b) indirect extrusin.. Indirect (Reverse) Extrusin In indirect extrusin (reverse, inverted, r backward extrusin), the die mves tward the billet figure (4 b); thus, except at the die, there is n relative mtin at the billet-cntainer interface..3 Impact Extrusin Is a frm f indirect extrusin and is particularly suitable fr hllw shapes. The prcess ften - included in the categry f cld extrusin. As seen in the figure 5 a, the punch descends at a high speed and strikes the blank, extruding it upward. The prcess prduces tubular sectins having wall thickness that are small in relatin t their diameters. Example f impact extrusin is the prductin f cllapsible tubes, such as fr tthpaste as seen in 5 b. Figure 5: Impact Extrusin Prcess, a) extruding a blank upward. b) Extrusin f cllapsible tubes.4 Hydrstatic Extrusin In hydrstatic extrusin figure 6, the chamber is filled with a fluid that transmits the pressure t the billet, which is then extruded thrugh the die. There is n frictin alng the cntainer walls. The high pressure (1400 Mpa) in the chamber Page 3 f 9
transmits sme f the fluid t the die surfaces, thus significantly reducing frictin and frces. Figure 6: Hydrstatic extrusin. 3 Mechanics f Extrusin The fllwing different situatins will be cnsidered fr the calculatins f the ram frce in direct extrusin. 3.1 Ideal Defrmatin The extrusin rati R and The abslute value f the true strain defined as in (1), () respectively A R = (1) A f A L f ε 1 = ln = ln = ln R A f L () A p = u = Y ln = Y ln R (3) A f Fr strain-hardening materials, Y shuld be replaced by the average flw stress. A p = u = Yf ln = Yf ln R (4) A f 3. Ideal Defrmatin and frictin Based n the slab methd f analysis, when frictin at the die-billet interface is included (but nt the cntainer wall frictin) and fr small die angles, the pressure p is tanα p = Y ct 1 + [ R µ α 1] (5) µ Based n the assumptin that the frictinal stresses is equal t the shear yield stress k, and that because f the dead zne frmed, the material flws alng a 45 die angle, an estimate f p can be given as: Page 4 f 9
L p = Y 1.7 ln R + (6) D Where L is the length f the billet remaining in the cntainer. Fr strainhardening materials, Yin these expressins shuld be replaced by the average flw stress as: L p = Y f 1.7 ln R + (7) D 3.3 Optimum Die Angle The die angle has an imprtant effect n frces in extrusin. Its relatinship t wrk is as fllws. 1. The ideal wrk f defrmatin is independent f the die angle (Fig. 7), because it is a functin nly f the extrusin rati.. The frictinal wrk increases with decreasing die angle because the length f cntact at the billet-die interface increases, thus requiring mre wrk. 3. The redundant wrk caused by inhmgeneus defrmatin increases with die angle. Because the ttal ram frce is the sum f these three cmpnents, there is an angle where this frce is a minimum (Fig. 7). Unless the behavir f each cmpnent as a functin f the die angle is knwn, determinatin f this ptimum angle is difficult. FIGURE 7 Schematic illustratin f extrusin frce as a functin f die angle: (a) Ttal frce; (b) Ideal frce; (c) Frce required fr redundant Defrmatin; and (d) Frce required vercming Frictin. Nte that there is an ptimum die angle where the ttal extrusin frce is a minimum. 3.4 Frces in Ht Extrusin. Because f the strain-rate sensitivity f metals at elevated temperatures, frces in ht extrusin are difficult t calculate. The average true strain rate is 6V D tanα ε = ln R (8) 3 3 D D f Where V is the ram velcity. Nte frm this equatin that fr high extrusin ratis (D >> D r ) and fr a = 45, as may be the case with a square die (thus develping a dead zne) and pr lubricatin, the strain rate reduces t Page 5 f 9
6V ε = ln R (8) D The effect f ram speed and temperature n extrusin pressure is shwn in Fig. 8. As expected, pressure increases rapidly with ram speed, especially at elevated temperatures. As extrusin speed increases, the rate f wrk dne per unit time als increases. FIGURE8 Schematic illustratin f the effect f temperature and ram speed n extrusin pressure. The subsequent rise in temperature can cause early melting f the wrkpiece material and cause defects. Circumferential surface cracks caused by ht shrtness may als develp; in extrusin this is knwn as speed cracking. Reducing the extrusin speed can eliminate these prblems. A cnvenient parameter that is used t estimate frces in extrusin is an experimentally determined extrusin cnstant K e, which includes varius factrs and can be determined frm p = Ke ln R (9) Figure 9 gives sme typical values f K e fr varius materials. FIGURE 9 Extrusin cnstant K e fr varius materials as a functin f temperature. Nte the ranges f temperature fr varius materials. (a) 1100 aluminum; (b) Cpper; (c) 70-30 brass; (d) Beryllium; (e) Cld-rlled steel; (f) Stainless steel; (g) Mlybdenum; and (h) Chrmium. Page 6 f 9
Example 5.1: Frce in ht extrusin A cpper billet 5 in. in diameter and 10 in. lng is extruded at 1500 F at a speed f 10 in./s. Using square dies and assuming pr lubricatin, estimate the frce required in this peratin if the final diameter is in. Slutin: we have F = p A () 5 πd π A = = = 19.6 in 0in 4 4 6V Square dies and pr lubricatin ε = ln R D ( 5) πd π ln A R = = ln 4 = ln 4 = 6.5 ( ) Af πd f π 4 4 6V 6( 10) = ln R = ln( 6.5) = / secnd D () 5 The effects f strain rate n the strength f materials is generally expressed as ε σ = Cε m, C is m, is fr this material C the strength cefficient ( ) the strain - rate sensitivity expnent = 19000 psi, m = 0.06 0.06 Y = σ = 19000 = 870 psi Recall equatin 5.10 L ( ) ( ) ( 10) p = Y 1.7 ln R + = 870 1.7 ln 6.5 + = 16630 psi D 5 6 F = p A = 16630 0 = 3. 10 ( )( ) Ib 4 Defects In Extrusin 4.1 Surface Cracking. Fir-tree cracking r speed cracking. If the extrusin temperature, frictin, r extrusin speed is t high, surface temperatures rise significantly and can lead t surface cracking and tearing. Is usually the result f ht shrtness, this situatin can be avided by using lwer temperatures and speeds. Stick- slip r bamb defect 4. Extrusin Defects. Extrusin defect, pipe, tailpipe, r fishtailing. Drawing surface xides and impurities tward the center f the billet, much likes a funnel. This defect can be reduced by a. Mdifying the flw pattern t a less inhmgeneus ne, such as by cntrlling frictin and minimizing temperature gradients. b. Anther methd is t machine the surface f the billet prir t extrusin t eliminate scale and impurities. Page 7 f 9
c. The extrusin defect can als be avided by using a dummy blck that is smaller in diameter than the cntainer, thus leaving a thin shell alng the cntainer wall as extrusin prgresses. 4.3 Internal Cracking. Centerburst, center cracking, arrwhead fracture, r chevrn cracking. The center f an extruded prduct can develp cracks as shwn in Fig. 10. These cracks are attributed t a state f hydrstatic tensile stress (als called secndary tensile stresses) at the centerline f the defrmatin zne in the die. This situatin is similar t the necked regin in a uniaxial tensile-test specimen. The majr variables affecting hydrstatic tensin are the die angle, extrusin rati, and the die cntact length. Figure 10: Chervn cracking in rund steel bar 4. Materials: Cmmercial pure lead (98% lead) 5. Equipments: Press, Vernier caliper and measuring instruments, Extrusin die, cntainer and punch. And Set f circular lead billets. 6. Prcedures: 1. Set the extrusin die, cntainer and punch in the press.. Set the press fr peratin. 3. Measure the diameter and length f the circular lead billet. 4. Measure the diameter f the die. 5. Half the billet lengthwise and mark ne face with a grid pattern then, place tw halves tgether again. 6. Perfrm the extrusin peratin by putting the billet, the pressure pad and the punch successively inside the cntainer, and then apply the pressure t extrude the billet t the required shape. Page 8 f 9
7. Repeat the extrusin sequence fr different specimens fr frictin and frictinless cases at the inner surface f the cntainer and the uter surface f the billet. 7. Requirements: 1. Describe the extrusin prcess illustrating with sketch.. Make line diagram shwing the methd f peratin f the press. 3. Sketch the cmpete set f the extrusin die, cntainer and punch. 4. Find the extrusin rati (A / A f ). Where A =Original billet area befre extrusin. A f =final area f extruded sectin. 5. Find the extrusin reductin in area (A - A f )/ A 6. Find the flw pattern with the extrusin billet fr frictin and frictinless cnditins. 8. Questins. 1) Discuss Extrusin methds and Extrusin defects? Page 9 f 9