Forging Dr. B Gharaibeh Production Processes 1

Transcription

1 Forging Dr. B Gharaibeh Production 1

2 Deformation Operations that induce shape changes on the workpiece by plastic deformation under forces applied by various tools and dies - Primary working processes - Secondary working processes Dr. B Gharaibeh Production 2

3 Deformation Primary working processes Operations are those that take a solid piece of metal (ingot) and break it down into shapes like slabs, plates, billets Forging - Rolling Etrusion Secondary working processes The operation which involves further processing of the product from the primary working into final products such as bolts, metal parts, wires, cans. - Deep drawing - blanking Dr. B Gharaibeh Production 3

4 Deformation Classified according to shape and size of workpeice A- Bulk deformation operations 1. The processing of workpeice having a relatively small surface area to volume or (surface area to thickness) ratio 2. In all bulk deformation processing, the thickness or crosssection of the workpiece is changed. B- Sheet forming operations 1. The surface area to the thickness ratio is relatively large 2. Thickness changes are usually undesirable. Dr. B Gharaibeh Production 4

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6 Forging process Plastic deformation by compressive forces, can be done at cold or warm and hot temperatures (homologous temperature) Open-die forging Impression-die forging Closed-die forging Dr. B Gharaibeh Production 6

7 Open Die forging - Upsetting Ideal deformation of a solid cylindrical specimen compressed between flat frictionless dies (platens), an operation known as upsetting. (b) Deformation in upsetting with friction at the die-workpiece interfaces. Note barrelling of the billet caused by friction. Dr. B Gharaibeh Production 7

8 Heading Forming the heads of fasteners, such as bolts and rivets, by the heading process. Dr. B Gharaibeh Production 8

9 Open-Die Forging (a) Cogging operation on a rectangular bar. Blacksmiths use a similar procedure to reduce the thickness of parts in small increments by heating the workpiece and hammering it numerous times along the length of the part. (b) Reducing the diameter of a bar by open-die forging; note the movements of the die and the workpiece. (c) The thickness of a ring being reduced by open-die forging. Dr. B Gharaibeh Production 9

10 Grain Flow Minimize barreling by applying effective lubricants or ultrasonically vibrate the part Grain flow lines in upsetting a solid, steel cylindrical specimen at elevated temperatures between two flat cool dies. Note the highly inhomogeneous deformation and barreling, and the difference in shape of the bottom and top sections of the specimen. The latter results from the hot specimen resting on the lower die before deformation proceeds. The lower portion of the specimen began to cool, thus ehibiting higher strength and hence deforming less than the top surface. Source: After J.A. Schey. Schematic illustration of grid deformation in upsetting: (a) original grid pattern; (b) after deformation, without friction; (c) after deformation, with friction. Such deformation patterns can be used to calculate the strains within a deforming body. Dr. B Gharaibeh Production 10

11 Upsetting Under ideal conditions % Reduction in height=(h 0 -h)/h 0 *100 e 1 =(h 0 -h 1 )/h 0 True strain=ln(h 0 /h 1 ) If v is the velocity between platens (dies): e v h 0 v h 1 True strain rate increases rapidly as height of the specimen approaches zero. Dr. B Gharaibeh Production 11

12 Upsetting under ideal conditions For perfectly plastic material F=Y A 1,A 1 =A 0. h 0 /h 1 For strain hardening material ε u s d ε 1 0 s k ε u u k ε 1 0 n n ε dε n k ε n + 1 Y f ε 1 Y f Work =Volume u = Volume ε 1 is average flow stress F=Y f A 1 Y f = average flow stress (n+1) = kε 1 n Y f is flow stress Y f 12 Dr. B Gharaibeh Production

13 Slab Analysis of Forging From equilibrium: s ds + + ds 2s h h + 2s d s h y d Second equation from the yield criterion 0 y 0 Plain stress compression, between flat dies with friction, horizontal stress is uniform across height (h) Resulting die pressure prediction: Dr. B Gharaibeh Production 13

14 Die Pressure Consider the friction - rectangular cross-section plane strain (no flow perpendicular to the page) p s y Y ' e 2 ( a )/ h Y ' 1.15Y Distribution of die pressure, in dimensionless form of p/y, in plane-strain compression with sliding friction. Note that the pressure at the left and right boundaries is equal to the yield stress of the material in plane strain, Y. Sliding friction means that the frictional stress is directly proportional to the normal stress. Dr. B Gharaibeh Production 14

15 Method of Analysis ( s + ds ) h + 2s d s h y 0 d s 2s + y d h 0 We know from distortion energy criterion for plane strain: s y s 2 3 Y Y' 11/11/2012 Dr. B Gharaibeh Production 15

16 Solving two equations: 1] '[ ' ' ' 0,, ' )/ ( 2 )/ ( 2 / 2 / 2 h a y h a y y h a h y e Y Y e Y p Y a e Y C Ce s s s s s s at 11/11/ Dr. B Gharaibeh Production

17 Upsetting Consider the friction - rectangular cross-section Assume that the deformation is in plane strain (no flow perpendicular to the page) P average can be obtained using: P av Y '( 1 + a ) h For strain hardening material Y f =kε n = average flow stress (n+1) Upsetting force: F=P av *cross-section area F=P av *2a*width This is true for any instantaneous h Y ' is replaced by Y f ' 11/11/2012 Dr. B Gharaibeh Production 17

18 Cylindrical Workpiece Average pressure: Forging force: r is the final radius after upsetting and h is the final value 11/11/2012 Dr. B Gharaibeh Production 18

19 Eample 1 A cylindrical workpeice made of annealed steel (K = 147,000 psi, n=0.17) is 6 in. diameter and 4 in hight. It is upset by open die forging with flat die to a height of 2 in. in plane strain. Assuming that the coefficient of friction is 0.2. Calculate A. the force required at the end of stroke. B. The work consumed to decrease the height to 2 in. Diameter=6 inch Height=4 inch After upsetting=2 inch Coeff. Of friction =0.2 Material is Annealed Steel 11/11/2012 Dr. B Gharaibeh Production 19

20 For annealed steel (r 2 from volume conservation). 10 ) )( ( 000, , 177 ) 2 )( 3 ( ) 2. 0 ( 2 1 [ 000, , 138 ) ( 000, ) 2 4 ln( ) 000, 147 ( 1015 ) ( lb F psi P in r psi y ε kε y n psi MPa k h r Y P av f n f f av ] + + p 20

21 Eample 2 A rectangular workpice has the following original dimensions: 2a = 100 mm, h = 30 mm and width = 20 mm. The metal has a strength coefficient of 400 Mpa and a strain hardening eponent of 0.3. It is being forged in plane strain with µ = 0.2, Calculate A. the force required at the end of stroke. B. The work consumed to decrease the height to 2 in. 11/11/2012 Dr. B Gharaibeh Production 21

22 Impression Die Forging Stages in impression-die forging. Note the formation of a flash, or ecess material that subsequently has to be trimmed off. Range of Kp values for impression-die forging. Forging force: 11/11/2012 Dr. B Gharaibeh Production 22

23 Impression Die Forging Workpiece takes the shape of the die cavity Flashes are created in the radial direction and moving outward High Friction in the Flash Flash: the length to height ratio is large Flash cools faster than the bulk and hence resists deformation - This helps the billet to fill the cavity 11/11/2012 Dr. B Gharaibeh Production 23

24 Closed Die Forging No flash is formed and the workpiece is completely surrounded by the dies Workpiece takes the shape of the die cavity Proper control of the volume of the material is essential to obtain a forging of desired dimensions. 11/11/2012 Dr. B Gharaibeh Production 24

25 Forging Defects Stages in lap formation in a part during forging, due to buckling of the web. Web thickness should be increased to avoid this problem. Stages in internal defect formation in a forging because of an oversized billet. The die cavities are filled prematurely, and the material at the center of the part flows radially outward and past the filled regions as deformation continues. 11/11/2012 Dr. B Gharaibeh Production 25

26 Forging Dies Stages in forging a connecting rod for an internal combustion engine. Note the amount of flash developed, which is important in properly filling die cavities. Standard terminology for various features of a typical forging die. 11/11/2012 Dr. B Gharaibeh Production 26

27 Forging Temperatures Forging temperature ranges for various metals. 11/11/2012 Dr. B Gharaibeh Production 27

28 Metal Working Equipment Various types of presses used in metalworking. The choice of a press is an important consideration in the overall operation and productivity. 11/11/2012 Dr. B Gharaibeh Production 28