Selec%on fo the Manufacturing Processes

Transcription

1 Selecon fo te Manufacturing Processes How would we manufacture a mountain bike? Te selec7on of a specific manufacturing process requires te knowledge of te following: (a) How te material performance is influence by te manufacturing process; (b) How te materials selec7on influence te decision of te manufacturing process. (c) Wic criteria are more important. (d) Produc7on volume. (e) Component size (f) Features (oles, undercuts, uniform walls, cavi7es). Rear Brake Seat Post Rear Derailleur Saddle Down Tube Pedal Top Tube (Courtesy of Trek Bicycle, 2002) Handle Bar Fork Front Brake

2 Types of Manufacturing Processes Manufacturing Processes Deformation Casting Seet Metal Polymer Processes Macining Finising Assembly Extrusion Forging Rolling Bar drawing Wire drawing Centrifugal Die casting Investment Permanent mold Sand casting Bending Blanking Drawing Puncing Searing Spinning Blow molding Casting Compression molding Extrusion Injection Molding Termoforming Transfer molding Boring Drilling Facing Grinding Milling Planing Turning Sawing ECM, EDM Anodizing Honing Painting Plating Polising Automated Bonding Brazing Manual Riveting Soldering Welding

3 Forging Classifica(on by Process: Working of metals to a useful sape by ammering or pressing. Also known as blacksmit. Most forging opera7ons are ot working opera7ons. Tey produce discrete parts. Open Die Forging Impression Die Forging Closed- die Forging

4 Forging Dies - Types Open die forging Two flat dies are used to compress te work- piece, allowing te metal to flow laterally witout constraint Impression die forging Te work- piece sape is imparted by te cavity or te impression on te die surface. Te constrain in te metal flow causes te forma7on of a flas. Flasless forging - workpiece is completely constrained in die and no excess flas is produced

5 Open- die Forging ε = ln o Te solid work- piece is placed between two flat dies and reduced in eigt by compression. It is also known as upse<ng, upset forging or flat die forging. If tere is no fricon ten te work- piece will be deformed uniformly. As sown above, omogeneous deforma7on occurs only if tere is no fric7on between work and die surfaces. Ten, te radial flow is uniform trougout work- piece eigt and true strain is given by: were o = star7ng eigt; and = eigt at some point during compression At = final value f, true strain is maximum value Fric7on between work and die surfaces causes lateral constraints to te metal flow of te work- piece, inducing a barreling effect. Te barreling effect is more pronounced in ot working processes because of te cooling effect on te material in contact wit te dies increasing its local resistance to deforma7on.

6 Barreling: Wen ig fric7onal forces develop at te die interface, tey oppose te outward flow of te work- piece. It can be minimized by using an effec7ve lubricant. Cogging: Opera7on were tickness of a bar is progressively reduced by successive forging steps at specific intervals Eng Strain = = e ε 1 = ln o 1 ( = True strain

7 Wit a rela7ve velocity v between te plates, te work- piece is subjected to a strain- rate of e = v o e T = v 1

8 Open- die forging Closed- die forging Impression Die Forging

9 Deformaon work and Forces - Open- die Process - No Fricon We will assume te following: a. tere is no fric7on at te interface between te work- piece, b. te die and tat te material is perfectly plas7c (yield stress = σ y ), c. Constant volume is maintained in a cylindrical work- piece te forging load is terefore te compressive force (P) ac7ng on a round metal bar. P = Compressive Force σ y = Yield Stress A = Area P = σ y A σ compression = 4P π D 2 D o 2 o = D 2 σ comp = 4P ( = σ A y ) π D 2 o o A o o = σ y

10 Deformaon work and Forces - Open- die Process - No Fricon Consider ideal condi7ons, i.e., no fric7on at te interface between plates and work- piece; wit strain ardening material (wit yield strengt Yf), constant volume, cylindrical work- piece F = Y f A 1 Constant volume Te force at any eigt 1 A 0 0 = A 1 1 Te total work of deforma7on is te product of te volume for te specific energy W = V σ = kε n ε 1 0 σ δε W = VYε 1 Y = k ε 1 0 ε n dε n = kε 1 ε 1 n +1

11 Rectangular sec(on Work- piece wit low fric(on Condi(ons Plain Strain Plane- strain condions: Te deforma7on is confined in te x- y plane. As te work- piece is reduced in eigt (y- direc7on), it expands laterally (x- direc7on). Tere is no expansion in te z- direc7on. Assump7ons: 1. Material is perfectly plas7c (no elas7city, no strain ardening). 2. Te fric7on coefficient is constant 3. Plane strain condi7ons 4. In any tin slab, stresses are uniform

12 σ x σ Te orizontal stress is assumed to be uniformly y distributed along te eigt of te element. µσ y Tere is a stress in te z- direc7on equal to te average of te stresses in te x and y- direc7ons. σ x +δσ Please note tat σy=pressure =p x Equilibrium of orizontal forces. ( σ x +δσ x ) + 2 µσ y σ x = 0 µσ y δσ x + 2µ σ yδx = 0 σ y Assuming tat te stresses in x and y are principal stresses (acceptable for low values of te coefficient of fric7on) and using distor7on energy criterium for plane- strain. Boundary condi7ons: x=a and σx=0 and ence σy=y σ y σ x = 2 3 Y = Y δσ y = δσ x C = Y e! " 2µa p = σ y = Y e ( σ x = σ y Y = Y e 2µ(a x) + 1 )*,- ( )! 2µ a x " δσ y = δ p σ y p = 2µ δx Y δ p a = 2µ px p δ x 0 σ y = Ce 2µx ) (

13 Determina7on of te p- average for a given. p average = 1 a p average = p avg = Y flow 0 Y flow a pδ x = 1 a exp 2µ a ( a 0 a Y flow 0 a exp 2µ exp 2µ exp 2µ a ( exp 2µ x (( ( a 2µ ( p avg = Y flow 2µ exp 2µ a a ( 1 ( ( a x ) ( δ x ( x ) ( δ x x=a ( x=0 Using Taylor s Series Expansion and for a small µa/: 2µ! exp 2µ " a =1+ 2µ ) a + ( a * 2µ, + a * ), ( + + 2! 3!! 2 2µ p avg Y ) flow 2µ 2µ a a + ( a *, ! " p avg Y flow F forging = Y flow 2 3 2µ a * ), ( + + 4!! 1+ µ " a F forging = p avg widt dept! 1+ µ " a 2a dept

14 Slab- Die Interface Condi7ons for sliding à τ fricon < τ flow Condi7ons for s7cking à τ fricon > τ flow Assump7ons: Te material is perfectly plas7c (stresses greater tan te flow stress are not possible) and te deforma7on occurs in a sub- layer Te transi7on between sliding and s7cking occurs wen τ fricon = τ flow at a distance xk p = σ y = Y e µ p = Y " a x k ( ) " 2µ a x ( ) " 2µ a x 2 = µy e = 1 2µ ln " 1 2µ

15 For te s7cking region à δσ y σ y δ p = Y = δ p p = 2µ δx δ p = 2µ pδx µ p = Y δx p x = p xk + Y pxk δ p = Y xk px δ x p x xk p x = Y x k x ( ) p x = Y x k x 2µ + Y p x = 1 Y 2µ + * a 1 2µ ln 1 -, )/ x + 2µ (. ( ) p x = 1 * 1 -, 1 ln ) Y 2µ + 2µ (. /+ a x x k x 2 ( ) ( ) x k = a 2µ ln 1 ) 2µ (

16 All S7cking Approxima7on à xk=a δσ y σ y = δ p p = 2µ δx δ p = 2µ pδx µ p = Y 2 δ p = Y δx p = Y x x + C x = a p = Y C = Y +Y a p x = a x Y +1 (

17 Hig Fricon or Scky Condions: Condi7ons for sliding fric7on: Recall tat te sear yield stress for te Von Mises criterion is as follows: τ flow = Y flow 3 Te average pressure under non- s7cky condi7ons, using te Taylor s Series Expansion and for µa/ small Equa7on for te fric7on- ill under non- s7cky condi7ons. p > 2τ flow At te interface, tere is an upper limit for te sear stress: µ p τ flow p 2τ flow µ ( 2τ ) flow τ flow µ 0.5 If sliding fric7on is to occur.

18 If bot are present S7cking and Sliding, tey need to be included in te calcula7on of te total force. F forging = F sliding + F sticking F forging = p aver A ( ) sliding + p aver A ( ) sticking Example: A lead bar is forged to a final eigt of 0.25in, wit a fric7on coefficient of Te Flow Stress is 1000psi If te eigt is reduced to 0.25in ten te widt is increased to 4in (mass conserva7on). Te transi7on from s7cking to sliding occurs at: " a x k = 1 2µ ln " 1 " 2 x k 2µ 0.25 a x k = x k =1.653 = " ( ) ln 1 2( 0.25) =

19 Sliding region: p x = σ y = Y e ( ) " 2µ a x x = e " 0.25 ( ) =1150 e 2 2 x ( ) S7cky Region: " a x p x = Y +1 " 2 x = = x ( ) Sliding S7cking Sliding

20 Calculate te area per unit dept: A sliding = 2 * = 0.69 Calculate te forces (separately for sliding and s7cking regions): F forging per unit dept = 2 xk 2 p x δ x) ( 2 Y 0 sticking xk 0 p x δ x) ( sticking xk a x = 2 Y +1 )δ x ( + ( a + ) x k x 2. k - 0 = , 2 / ,- a 2 p x δ x) ( xk sliding 0 = e a + 2 p x δ x) ( xk = 2 Y A sticking = 2 *1.653 = 3.31 xk 0 sliding ( a + x)δ x ( ( )( 1.653) ) 2 2 ( 2 x ) δ x F forg unit dept = = 22800lb a xk = ( 2 x) e2. 0 / 0 = 21648lb =1152lb

21 If we assume all s7cking situa7on: p average =1.15Y! a +1 " 2 f! 2 p average = " = 5750 psi ( ) F per unit dept = p average Area = 5750 psi 4 F = 23000lb Fric7on is very important for te es7ma7on of te forces. For coefficients of fric7on greater tan 0.1, one sould expect bot sliding and s7cking regions. For µ lower tan 0.15 sliding will prevail and for µ greater tan 0.4 only s7cking condi7ons will be present.

22 Wen te coefficient of fric7on is greater tan 0.5, ten µp must be replaced by te yield sear stress. Te maximum value tat µp can reac is te yield sear stress. Te equilibrium equa7ons are as follows: σ y = p δσ y σ y = 2µ δx δ p p = 2µ δ p = 2τ flow δx δ p = 2µ p δx δx δ p = 2Y flow 3 δx = Y flow dx p = Y flow x + C p = Y flow C = Y flow 1+ a ( for x = a µ 0.5 µ p = τ flow p = Y flow p avg = Y flow " a x +1 " a 2 +1

23 Example: A block of lead of dimensions 25x25x150mm 3 is pressed between flat dies to a final size of 6.25x100x150mm 3. Determine te total forging load and te pressure distribu7on along te 100mm dimension. Te uniaxial flow stress Y=6.9MPa and a coefficient of fric(on of Plane strain condi7ons (te 150mm dimension does not cange) p = ( 1.15Y ) exp 2µ ( a x ) ( = ( ) exp 2 ( 0.25 ) ( 50 x) ( 6.25 x=0mm σmax=435mpa x=25mm σ=58.9mpa x=50mm σ=8.0mpa p avg Y flow " 1+ µ a " 0.25 = = ( ) Force = P avg A = F = 35.71kN! a p average =1.15Y +1 " 2 f! 50 p average = " = 39.8MPa F = p average Area = 39.8MPa x10 6 F = 597kN ( ) Te maximum value occurs at te center line. Te average stress under scky fricon condions:

24 Te transi7on from s7cking to sliding occurs at: " a x k = 1 2µ ln " 1 " 50 x k 2µ 6.25 = " ( ) ln 1 2( 0.25) = x = k

25 Axisymmetric Compression (Cylinder) For a constant value of te coefficient of fric7on µ and equilibrium of forces along te radial direc7on. σ r rδθ + 2µ prδθδr + 2σ δθδr θ 2 2µ prδr +σ θ δr σ r δr rδσ r = 0 Wit axisymmetric flow and Yielding: ( σ r +δσ r )δθ ( r +δr) = 0 ε θ = ε r σ θ = σ r σ r + p = Y Neglec7ng iger orders. δσ r = δ p Integra7ng between te boundary condi7ons: r=0 à σr=0 and r=r à p=y δ p p = 2µ δr " p = Y exp 2µ ( R r )

26 Te average pressure can be obtained by te expression: p average = 1 π R 2 p average = 1 ( 2 µr * p average = Y 1+ 2µR - +, 3. / 0 R 2 p 2π r δr * Y exp 2µR, ( 2µR /. Using a Taylor s series expansion. Good approxima7on for µ small. Under s7cking condi7ons, we replace µp by τ flow. Using Von Mises criterion µ p = τ flow 2τ flow rδr +σ θ δr σ r δr rδσ r = 0 2τ flow δr = δ p p = Y + 2τ flow p avg = Y + 2τ flowr =1.15Y 1+ R 3 3 ( ( R r) ε θ = ε r σ θ = σ r σ r + p = Y δσ r = δ p τ flow = Y 3

27 Example: A cylindrical block of lead of dimensions 28mm diameter and 25mm ig is pressed between flat dies to a final size of 6.25mm ig. Determine te total forging load and te pressure distribu7on along te radius. Te uniaxial flow stress Y=6.9MPa and a coefficient of fric(on of Assuming constant volume te final radius is 56mm " p = Y exp 2µ ( R r " ) = exp 6.25 p average = Y 1+ 2µR!! " 3 = * 6.25 " F = π D2 4 p avg = 24.4kN ( 56 r) ( ) Te average stress under scky fricon condions: = 9.9MPa ! p avg =1.15Y 1+ R! " 3 = ( 56 ) 2 " =19.78MPa F = π D2 4 p avg = 48.7kN r=0mm σ max =700MPa r=28mm σ=75mpa r=56mm σ=8.0mpa

28 Impression- die and Closed- die Forging Impression- die forging: Te die as a pre- determined sape cavity. Te work- piece takes te sape of te die cavity. Closed- die Forging is similar to impression forging, but witout te forma7on of a flas, as te work- piece fills up all te cavity

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30 Forging Force for Impression Dies Te required force in impression dies is given by te equa7on: F = ky f A Were : k = mul7plying factor (see Table) and Y f = flow stress of te material for te strain rate and at te forging temperature Te flas as a iger deforma7on resistance tan in te die due to te iger a/ ra7o, ence, te material completely fill te cavity rater tan being extruded sideways.

31 Forging Operaons: Fullering: Only a part of te sec7on of te work- piece is reduced. Te metal flows away from te center of te fuller. Swaging: Part of te sec7on of a bar is reduced to a smaller diameter. It is deforma7on takes place by successive rapid ammer blows. Tube Swaging: Te inside diameter, te tickness and or sape of te tube is reduced wit or witout te use of internal mandrels Isotermal Forging : Te die is eated to te same temperature as te workpiece. Complex parts can be produced wit good dimensional accuracy. Also known as ot- die forging.

32 Heading or Upset Forging: It is performed at one or bot of te ends of te work- piece in order to increase te cross- sec7on. Example: nails, bolts, rivets, screws, fasteners, etc.. (a) eading a nail using open dies (b) round ead formed by punc (c) and (d) two common ead styles for screws formed by die (e) carriage bolt ead formed by punc and die

33 Coining: It is used for te min7ng of coins, medallions and jewelry. As te quality of te coins depends on te quality of te impression, te pressure applied is 5 to 6 7mes te pressure required for te material to flow. Piercing: Te work- piece surface is indented wit a punc in order to produce a cavity or an impression. Forging of a cluster- gear blank in a four step opera7on.

34 Trimming: Cunng opera7on to remove te flas from te work- piece in impression die forging. Orbital Forging: Te die moves along an orbital pat and it is in contact wit a por7on of te work surface at one 7me. Example (inside of te bearing rings).

35 Forging Macines: Tere are four basic types of forging macines.

36 Forging Macine Mecanical Hydraulic Hammers Caracteriscs Mecanical presses are stroke limited (te ram stroke is sorter). Very ig forces can be applied. Tey are preferred for ig precision work Two types crank or eccentric Works at a constant speed and te contact 7me is longer. Tey are load limited, but te full load is available at any point in te stroke.tey are slower Board or drop ammer. Te energy supplied by te blow is equal to te poten7al energy of te weigt of te ram and te eigt of te fall.

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38 Effect of Forging in te Microstructure From ASM Metals Handbook Vol.9 Te grain structure of te work- piece produced by forging, macining and cas7ng are very different. Te grain structure in forging as a fibrous appearance (at low magnifica7ons). Te work- piece as superior proper7es along te fibrous appearance, but te proper7es perpendicular to tem will be inferior. Te work- piece as anisotropic proper7es. Te grains in te forging are usually recrystallized, ence, it will ave superior proper7es tan cas7ngs.

39 Origen of te Apparent Fibrous Structure: Te apparent fibrous structure is due to te redistribu7on of te metal structure eiter by te redistribu7on of inclusions and/or te redistribu7on of te crystallograpic orienta7on of te grains. Redistribu7on of soo and ard inclusions. Any columnar or dendri7c structure is broken down during forging.

40 Forgeability of Metals Te ability of a material to undergo deforma7on witout breaking is known as Forgeability. Two simple tests are used to measure forgeability, namely te upse>ng test and te ot- twist test. Upse<ng test: greater te deforma7on prior to cracking, te greater te forgeability of te metal Hot- twist test: te work- piece is twisted along a longitudinal axis. Te maximum number of turns occurs at te forging temperature for maximum forgeability.

41 Die Materials: Te materials for te dies must ave termal sock resistance, ig temperature strengt, ig tougness and duc7lity, ig fa7gue resistance, ig dimensional stability wit temperature, ig wear resistance, ig ardenability and ig macinability. Typical materials include ig carbon steels ( C0 use in small tools and flat impressions coining); medium alloy tool steels used for example in ammer dies and ig alloyed tool steels, used for example in dies for presses.