Forging Process PDF

Forging Prof. Pasquale Russo Spena  Forging In a forging process, the workpiece is shaped by compressive forces applied through dies and tooling. Forging operations produce discrete parts. Forged parts h...

Forging Prof. Pasquale Russo Spena  Forging In a forging process, the workpiece is shaped by compressive forces applied through dies and tooling. Forging operations produce discrete parts. Forged parts have good strength and toughness, and are reliable for highly stressed and critical applications. Workpiece usually needs heating at high temperatures (warm and hot forging) to improve metal plasticity and to reduce the force and energy involved in the forging process. P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2024-25 2  Forged parts Forging parts include engine crankshafts and connecting rods, gears, aircraft structural components, and jet engine turbine parts, etc... Maching Forging operations produce rough forms, and subsequent operations are required to refine the parts to final geometry and dimensions. P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2024-25 3  Temperature TABLE 6.3 Forging temperature ranges for various metals. P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2024-25 4  Forging Either impact or gradual pressure is used in forging. The distinction derives from the type of equipment used. A forging machine that applies an impact load is called a forging hammer, while one that applies gradual pressure is called a forging press. There are three types of forging operations: a) open-die forging - high temperature* L *cold upset forging of nails (heading) with open dies is an exception b) closed-die forging: impression-die forging - high temperature - c) closed-die forging: cold working and flashless forging - low temperature - - FRERO FLOW complitly coustrain Ex caining mowethe , P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2024-25 5  Open Die Forging In open-die forging, the work is compressed between two flat (or almost flat) dies, thus allowing the metal to flow without constraint in a lateral direction relative to the die surfaces. Open-die forging operations produce rough and simple shapes, and subsequent operations are required to refine the parts to final geometry and dimensions (e.g., closed die forging operation).. P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2024-25 6  Closed Die Forging Closed die forging: hot metal is completely deformed within the closed cavity of two dies, which shapes workpiece. In this type of operation, a portion of the workpiece metal flows outside the die impression to form a flash. Flash is an excess metal that must be trimmed off later. P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2024-25 10  Closed Die Forging: Flashless forging In flashless forging, the work is completely constrained within the die and no excess flash is produced. The volume of the starting workpiece must be controlled very closely so that it matches the volume of the die cavity. Flashless forging is typical for cold working processes (near net shape processes). https://www.youtube.com/watch?v=EKsiT_-swBM P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2024-25 11  Typical forging steps - Preparation of raw material (e.g. billet, ingot) - Heating of raw material (only for hot forging) - Removal of scale from work surface (if necessary) to avoid the embedding of scale in the work - Preheating (only for hot forming) and lubrication of dies - Forging - Other (possible) forming operations - Cleaning - Dimensional control - Machining operations to obtain the final part - Heat treatment(s) (if necessary) - Final control (e.g. final tests, dimensional control) P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2024-25 14 Open Die Forging  Open Die Forging Operations The simplest case of open-die forging involves compression of a cylindrical billet between two flat dies, similarly to a compression test. Upsetting (or upset forging) Cogging Bar Forging Ring Forging P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2024-25 16  Open Die Forging Operations P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2024-25 17  Upsetting or Upset Forging Upsetting (or upset forging) is the simplest open die forging operation, which is performed in order to reduce workpiece height in the direction of action of a forging hammer or press with flat dies or special types of plates, with a simultaneous increase of cross-sectional dimensions. In practical conditions, the billet tends to barrel since there is some friction. Lubrication is required! { real μ≠0 P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2024-25 18  Barreling Problem: frictional forces between metal surface and plates arise and oppose to the lateral enlargment of the material: some energy must be spent to oppose such forces. Further, frictional force is variable: it is minimum at the center of the sample, maximum at the plates (see the grey area). This results in sample barreling. barreling Ideal upsetting An appropriate lubrication helps to reduce notably barrelling; however, it cannot be eliminated enterely. P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2024-25 19  Barreling FEM simulation of upsetting 1 2 3 4 5 6 P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2024-25 20  Ideal Upsetting Under ideal conditions of no friction between the workpiece and die surfaces, open-die forging gives homogeneous deformation, so the radial flow of the material and height is uniform. The workpiece is subjected to a uniaxial state of stress in ideal conditions: σn equals flow stress σp, which allows uniform deformation in the bulk at every stage of compression. frictionless process (μ = 0) Same strees and same deformation P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2024-25 21  Ideal Upsetting Under ideal conditions of no friction between workpiece and die surfaces, open-die forging gives homogeneous deformation, so the radial flow of the material and height are uniform. True strain of workpiece is: ideal reas The upsetting force is calculated as: F = Yf A Yf is the flow stress A is the cross-sectional area At HIGH temperatures (i.e., strain hardening does not occur), flow stress Yf is evaluated from the strain rate sensitivity relationship. At LOW temperatures (i.e., strain hardening occurs), like heading, flow stress Yf is evaluated from the Hollomon relationship relationship. A increases during the whole process as h decreases (and Yf increases at low temperature processes), so the upsetting force reaches the maximum at the end of the forging stroke. P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2024-25 22  F1 Force in Upsetting F2 F3 Compression curre F3 for compression test IDEAl ) F2 Force increases continuously mainly due to the increase of area (strain hardening n = 0 in upsetting) F1 P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2024-25 23  Real Upsetting Due to the action of friction, tangential friction, τp, stresses arise at the metal/die interfaces, which are directed towards the center of the billet. The stresses, dependent on friction coefficient μ, hamper lateral metal flow. Compression strees process with friction (μ > 0) isnit cospert shear spress Baxxeling l n Due presenze of friction Disomogerens Deformation NOTE: 1- the extent of each zone depends on the exerted reduction and friction; 2- there are no distinct borders between these zones (i.e., smooth transition to one another). P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2024-25 26  Real Upsetting The uneven deformation, and hence barreling, can be evaluated by optical microscopy and FEM analysis (i.e. numerical simulation). material Flow better in the middle } - Part alxeedy Cut in the madle î Coldet down so Matevle move resisent so on The lower portion of the specimen began to lass beforretra Deformed mesh, isothermal lines, and cool, thus exhibiting higher strength and level curve for strain rate and strain hence deforming less than the top surface. evaluated for an axialsymmetric Source: After J.A. Schey. upsetting with 47% heigth reduction. P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2024-25 28  Real upsetting EMPIRICAL APPROACH In true conditions the friction opposes the flow of workpiece metal at the die surfaces. The barreling is more pronounced in hot forming than in cold forming (higher friction coefficient in hot le conditions) and, if the diameter/height ratio is high (higher contact surface). This convey to upsetting forces greater than the ideal condition. It is possible to approximate the amount of force using a shape factor to account for the effects of the D/h ratio and friction: F = Kf Yf A shapefacto sf Eseerdefam where Kf is the shape factor defined as: Thin nateonel with μ = friction coefficient; D = workpiece diameter; h = workpiece height P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2024-25 29  F1 Force in Upsetting F2 F3 F3 real F2 ideal F1 P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2024-25 30  HOT open die forging - Ex. A hot upset forging operation is performed in an open die. The initial size of the workpart is: D0 = 25 mm, and h0 = 50 mm. The part is upset to a diameter Df = 50 mm. The press speed is v = 20 mm/s. The work metal at this elevated temperature shows strain rate sensitivity and the related coefficients are C = 85 MPa and m = 0.151. The friction coefficient at the die- work interface μ = 0.40. Determine the final height of the part, and the maximum force in the operation. Solution. The workpiece volume is Without barreling, the final height is The strain rate is needed to evaluate the flow stress where H is the final height of the workpiece, H = Hf P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2024-25 35  blommis b Frinæx İ o C ssipa n = somm =6,957 s - casm  HOT open die forging - Ex. So The flow stress, Yf Kf is The final area The maximum force, at the end of the upsetting P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2024-25 36  COLD open die forging - Ex. A cylindrical workpiece is cold upset. The starting piece has a height of 75 mm and a diameter of 50 mm. It is reduced in the operation to a height of 36 mm. The material has a flow curve defined by K = 350MPa and n = 0.17. Assume a coefficient of friction of 0.1. Determine the force as the process begins, at intermediate heights of 62 mm, 49 mm, and at the final height of 36 mm. Solution. The upsetting force is So, Kf, Yf, and A are requested at different heights. The workpiece volume V is: At the beginning of the process, the initial plastic deformation condition (e = 0.2% = 0.002) is P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2024-25 37  COLD open die forging - Ex. Calculating Kf The initial area of the workpiece is So, the upsetting force at the beginning is At the height 62 mm, the deformation is So, Yf is P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2024-25 38  COLD open die forging - Ex. Without considering the barreling effect, the new cross-section area of the workpiece by the conservation of the volume So, the diameter becomes Thus, P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2024-25 39  COLD open die forging - Ex. h = 49 mm P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2024-25 40  COLD open die forging - Ex. h = 36mm P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2024-25 41  Upsetting Theoretical approach - slab method CARTESIAN ref. system Hypothesis: plane strain condition (i.e., strain only in the xy plane) vexiation Vexietion in Leught in high based on volume constancy thus The finite average deformations can be obtained by integration eöte Şk P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2023-24 42  Upsetting Theoretical approach - slab method CYLINDRICAL ref. system In an axisymmetric body, a cylindrical reference system (r, , z) has to be considered to simplify the analytical computation. cont We use Infintesina body Then plain strain caditu cause the direction chages in all dixectiva NOTE: plane strain condition does not occur in this case - the body deforms in all the 3-spatial directions! P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2023-24  Upsetting Theoretical approach - slab method CYLINDRICAL ref. system P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2023-24  Upsetting Theoretical approach - slab method CYLINDRICAL ref. system -6 d Tea $2110 o 22 2o: = = 220 àn so based on volume constancy Caşz ) g 2 o o =- EdEz dEx dEO -ęEE. = P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2023-24  Upsetting Theoretical approach - slab method CYLINDRICAL ref. system The finite average deformations can be obtained by integration P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2023-24  Upsetting Theoretical approach - slab method Slab method analysis for plastic deformation processes The slab method analysis assumes that metal deforms uniformly throughout the metal part. Therefore, the state of stress and strain in a infinitesimal region (with all the geometrical characteristics of the deformed body) is rappresentative of the overall state of stress and strain. This method is based on the static equilibrium of forces acting on the infinitesimal region. The differential equation arising from the static equilibrium equation can be integrated by using proper boundary conditions to find the solution of the problem. The slab method analysis is an approximated approach useful to understand several processes, such as forging, extrusion, rolling, drawing etc… P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2023-24 47  Upsetting Theoretical approach - slab method Assumptions Chages walume - i o - isotropic and incompressible body - elastic deformations of the workpiece are unimportant - no inertial forces MEx E beformation no Ez. 4 - plane strain condition (i.e. no deformation perpendicular to the image below) - 5 negligible friction stresses (i.e. the upsetting pressure is considered as principal fxction Gefficia vaxe low stresses; friction stresses do not affect internal stresses and deformation field) importart - - friction coefficient is constant during the process l and uniform at the workpiece/dies interfaces bace - at elthex o cosscunt - the dies are rigid P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2023-24 48  Upsetting Theoretical approach - slab method Considering the convention of principal stresses metod p slap Under Plane Strain condition: T 2 σ  σ1  σ 3   Y  1.15 Y  Y´ Uighers lowest Von Mises 3 shear strength at yielding k = Y’ / 2 k = Mex sheek forces oce at 450 Tresca σ  σ1  σ 3  Y shear strength at yielding k = Y / 2 P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2023-24 49  Upsetting Theoretical approach - slab method p = 100 MPa σ = -100 MPa İK d E b p = 100 MPa cottispanding pressure of σ = -100 MPa NOTE: p is the pressure on the metal. Even though it is compressive stress, it can be expressed as a positive value (e.g., 2 bar, 500 MPa, 10 atm). Therefore, for solid mechanics (compressive stress is negative in sign): p=-σ P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2023-24 50  Upsetting Theoretical approach - slab method LEFT side RIGHT side i infitessinnal bofy with seme leketnexistic of the some bodye " h stress bit ittle L fifferent only raxial stressjænd nrno sheex stress - - stress distribention } of the infiuitescule botp no Princirel Stress. couse sheet stress But Up is sull s... b = depth , prrinsipel h = height stress l , l = width P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2023-24 51  Upsetting wtite hoxizatdl foxces Fx RIGHT side eqp =Ø y q İb couse aze neglete we imextial forces F ( body isn accellerate 't The ão ) =8A) L , P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2023-24 52  Upsetting LEFT side 48 : - - Note: dx (i.e. the movement of material during upsetting) has a negative sign at the left side! o b ine ate mooning towards negative directiem P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2023-24 53  Upsetting Theoretical approach - slab method RIGHT side: 1 equation (for each side) with 2 unknown (p and σx) LEFT side: Another equation relates the contact pressure p to σx through the Von Mises plasticity criterion (it is more proper for ductile metals than Tresca). Under plane strain condition: all hen Pises sans principal stress 2. I σ  σ σ 1 3   Y  1.15 Y  Y´ and p=-σ 3 dloxtplzdø k î Y’ is constant at high temperatures, so dY’ = 0 P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2023-24 54  Upsetting - =0 RIGHT side ndp-eupdx 6 C’ can be found by the following boundary condition: At the right lateral surface, x = a, the stress σx = 0 x=a σx = 0 combining both Eqs. P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2023-24 55  Upsetting LEFT side C’ can be found by the following boundary condition: At the left lateral surface, x = -a, the stress σx = 0 Boudery condition x = -a σx = 0 combining both Eqs. P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2023-24 56  Upsetting Theoretical approach - slab method RIGHT side LEFT side î increose fuction coefficient Being l = 2a x=-a x=a P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2023-24 57  Upsetting Theoretical approach - slab method In the middle plane (x = 0), the pressure exhibits the maximum value: pmax 8 At x = a and x= -a (lateral surfaces) Trends for a cort issont 1 of tmne x=-a x=a At the lateral surfaces, the pressure equals the flow stress of the material in plane strain condition, Y’. P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2023-24 58  Upsetting Average Pressure and Force If the friction coefficient is small the upsetting pressure can be simplified by developing the exponential function using Taylor’s series Taylor’s series For the symmetry of the upsetting pressure diagram, the average upsetting pressure pav can be evaluated by integrating p between x = 0 and x = a (right side) or approximately Taylor’s series The upsetting force is P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2023-24 59  Upsetting Average Pressure P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2023-24 60  Upsetting Average Pressure Being l = 2a l P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2023-24 61  Upsetting 3D body The material flows along the direction with the minor friction resistance (shear stress). The 3D upsetting pressure exhibits a peak in the middle of the workpiece, and with a trend similar to that of parallel planes in respect to lateral surfaces of the workpiece. P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2023-24 62  Upsetting - STICKING model Theoretical approach - slab method If friction becomes large enough, there is a tendency for the the workpiece and the tool surfaces in relative motion to adhere to each other rather than slide (i.e. no relative motion between them). This condition is called sticking (or sticking friction). It means that the friction stress between the surfaces exceeds the shear flow stress of the work metal, thus causing the metal to deform by a shear process beneath the surface rather than slip at the surface. In sticking condition, the resistance to the relative movement can be better described by a fraction m of the shear stress at yielding k of the workpiece Interface shear strength m = 0 perfect sliding (i.e. no friction) m=τ/k= m= 1 fully adhesion (sticking) Flow stress in shear Typical adopted values of m: 0.05-0.15, cold working of steel, aluminum alloys, copper alloys , using lubricants; 0.2-0.4, hot working of steel, aluminum alloys, copper alloys, using lubricants; 0.1-0.3, hot working of titanium alloys, using lubricants; 0.7-1, in the absence of lubrication (e.g. some rolling processes, extrusion of Al alloys) P. Russo Spena - Manufacturing and Assembly Technologies - a.a. 2023-24 63  Upsetting - STICKING model Theoretical approach - slab method τ=mk m: shear factor (0