Manufacturing Processes METAL FORMING AND SHAPING PROCESSES Dr Jamil Renno CEng FIMechE FHEA What is Metal Forming? The metal is formed through mechanical deformation – there is neither addition, nor removal of material; the mass of the workpiece remains the same. • Plastic deformation is used to change the shape of the metal – stresses will exceed the yield stress of the material • Usually, the stresses used are compressive (e.g., rolling, forging, extrusion) • However, some forming processes: • Stretch the material (tensile stresses) • Bend the material (tensile and compressive stresses) • Apply shear stresses © Dr Jamil Renno CEng FIMechE FHEA 2 Relevant Mechanical Properties a) A standard tensile-test specimen before and after pulling, showing original and final gage lengths. b) Stages in specimen behavior in a tension test. © Dr Jamil Renno CEng FIMechE FHEA 3 © Dr Jamil Renno CEng FIMechE FHEA 4 What Happens to the Material? • Forming results in desirable material properties: • Lower yield stress • Higher ductility • These properties are affected by temperature: • Ductility increases and yield stress decrease when work temperature is raised This figure shows the effect of temperature on mechanical properties of a carbon steel; most materials display similar temperature sensitivity for elastic modulus, yield strength, ultimate strength, and ductility. © Dr Jamil Renno CEng FIMechE FHEA 5 Basic Types of Metal Forming • Bulk Deformation – significant deformation and massive shape changes; workpiece has low-surface-to-volume ratio • Rolling • Forging • Extrusion • Wire Drawing • Sheet metalworking • Bending • Deep or cut drawing • Shearing © Dr Jamil Renno CEng FIMechE FHEA 6 Forming Temperature • Generally in metal forming, the process is carried out in one of three temperature ranges: cold, warm or hot working as shown below: ππ΄ Cold Working 0.3ππ Warm Working 0.5ππ Hot Working 0.75ππ where ππ΄ is the ambient (room) temperature and ππ is the workpiece melting temperature. © Dr Jamil Renno CEng FIMechE FHEA 7 Metal Rolling © Dr Jamil Renno CEng FIMechE FHEA 8 What is metal rolling? • Rolling is the process of reducing the thickness or changing the cross-section of a long workpiece by compressive forces applied through a set of rolls. • Rolling accounts for 90% of all metals produced by metal working; it was first developed in the late 1500s. © Dr Jamil Renno CEng FIMechE FHEA 9 What is produced? © Dr Jamil Renno CEng FIMechE FHEA 10 Steel Rolled Products © Dr Jamil Renno CEng FIMechE FHEA 11 Plates • Plates generally have a thickness greater than 6 mm (1/4 in) • Examples of plates in industry • 300 mm for support of a large boiler • 150 mm for reactor vessel • 100-125 mm for battleships/tanks Small reactor Industrial boiler, 55 ton of steam per hour, operate at 400°C and 42 bar pressure; gas fired © Dr Jamil Renno CEng FIMechE FHEA 12 Sheets • Sheets have a thickness less than 6 mm (1/4 in) • Used in automobile bodies, containers for food and beverages • Commercial aircraft fuselage are usually made of 1mm aluminum sheet • Beverage cans are 0.28 mm thick • Aluminum foil is 0.008 mm thick © Dr Jamil Renno CEng FIMechE FHEA 13 Rolling Process © Dr Jamil Renno CEng FIMechE FHEA 14 Rolling Processes – Based on Geometry • Flat Rolling • Used to reduced the thickness of a rectangular cross-section • Shape Rolling • Used to form a square cross-section into a shape such as an I-beam or rail tracks • Ring Rolling • Used to manufacture seamless rings (e.g., bearings rings, turbine disks, gear blanks, • Thread Rolling • Used to produced threads • Skew Rolling • Used to produced balls for ball-bearings © Dr Jamil Renno CEng FIMechE FHEA 15 Rolling Processes – Based on Temperature • Hot Rolling (Forming) • Can achieve significant deformations, mostly carried out before cold rolling • Deformation at/above recrystallization temperature (approximately 50% of the melting temperature) • Above 50% of the melting temperature, the metals continue to soften enhancing the advantage of hot rolling • Cold Rolling (Forming) • Produces sheet and plate stock, product has higher strength, hardness and better surface finish © Dr Jamil Renno CEng FIMechE FHEA 16 Hot Rolling • The metal is heated above its recrystallization temperature • This converts the workpiece from a cast structure to a wrought product • This structure has finer grains and enhanced ductility resulting from breaking up brittle grain boundaries and closing-up internal defects, such as porosity. • Temperature ranges for hot rolling are typically about 450oC for aluminum alloys, up to 1250oC for alloy steels, and up to 1650oC for refractory alloys © Dr Jamil Renno CEng FIMechE FHEA 17 Cold Rolling • Performed at room temperature or slightly above • Many cold forming processes are important mass production operations • Minimum or no machining is required • Starting work surface must be free of scale/dirt • Ductility will limit the amount of forming that can be done © Dr Jamil Renno CEng FIMechE FHEA 18 Hot vs Cold Rolling Hot Rolling Cold Rolling • Larger deformation • Less accuracy • Lower rolling energy • Scaling needed for good surface finish • Less deformation • Tight tolerance • Higher rolling energy • Better surface finish © Dr Jamil Renno CEng FIMechE FHEA 19 Flat Rolling • Friction forces act on the strip surfaces • Roll force, πΉ, and torque, π, act on the rolls • The reduction in thickness is draft π = β0 − βπ • Reduction ratio is π = πΤβ0 • The maximum possible draft is ππππ₯ = π2 π· Τ2 where π is the coefficient of friction and π· is the diameter of the roll © Dr Jamil Renno CEng FIMechE FHEA 20 Conservation of Volume • Conservation of matter is preserved so the volume exiting the gap between the rolls equals the volume entering β0 π€0 πΏ0 = βπ π€π πΏπ • Similarly, before and after volume rates of material flow must be the same, so the before and after velocities are related β0 π€0 π0 = βπ π€π ππ • The roll velocity ππ is greater than the entering velocity and less than the exiting velocity. © Dr Jamil Renno CEng FIMechE FHEA 21 © Dr Jamil Renno CEng FIMechE FHEA 22 Friction and Velocity • Typical values for the coefficient of friction π are • For cold-rolling with lubrication, π~0.05 − 0.10 • For hot-rolling, π~0.20 • Forward slip in rolling is defined as ππ − ππ π= ππ • Low values of π indicate better surface finish π· • Note that ππ = π 2 © Dr Jamil Renno CEng FIMechE FHEA 23 Force and Power The roll force in flat rolling is πΉ = πΏπ€πππ£π where πΏ = π· 2 β0 − βπ , π€ is the width of the strip and πππ£π is the average true stress of the strip in the roll gap. The power in kW can then be obtained as 2ππΉπΏπ π= 60,000 where π is the revolutions per minute of the roll. © Dr Jamil Renno CEng FIMechE FHEA 24 Average True Stress The average true stress can be obtained from πΎπ π πππ£π = 1+π where π = ln β0 βπ is the true strain, πΎ is the strength coefficient and π is the strain-hardening exponent. Note that the strain here is compressive strain. © Dr Jamil Renno CEng FIMechE FHEA 25 © Dr Jamil Renno CEng FIMechE FHEA 26 © Dr Jamil Renno CEng FIMechE FHEA 27 © Dr Jamil Renno CEng FIMechE FHEA 28 Example An annealed copper strip is 228 mm wide and 25 mm thick. The strip is rolled to a thickness of 20 mm in one pass. The roll’s diameter is 600 mm, and the rolls rotate at 100 rpm. Calculate the roll force and the power required in this operation. © Dr Jamil Renno CEng FIMechE FHEA 29 Solution To find the force, use πΉ = πΏπ€πππ£π , where πΏ can be obtained form the geometry as πΏ= π· 2 600 2 β0 − βπ = 25 − 20 = 38.7mm The maximum strain during this process is β0 25 π = ln = ln = 0.223 βπ 20 which allows finding the average flow stress as πΎππ 315 0.223 0.54 πππ£π = 1+π = = 90.96 MPa 1+0.54 Finally, the force and the power are πΉ = πΏπ€πππ£π = 38.7 × 228 × 90.96 = 803536.9 N which is 803.5 kN – the weight of a Land Cruiser is around 26 kN (2600 kg) 2ππΉπΏπ 38.7 1 The power is π = 60000 = 2π × 803536.9 × 103 × 100 × 60,000 = 325.9 kW © Dr Jamil Renno CEng FIMechE FHEA 30 Rolling Force • Roll forces can cause deflection and roll flattening, which in turn, adversely affect the rolling operation. • Roll forces can be reduced by • Reducing friction • Using smaller diameter rolls to reduce contact area • Taking smaller reductions per pass to reduce contact area • Rolling at elevated temperature to reduce the strength of material • Applying longitudinal tensions to the strip during rolling (elaboration) © Dr Jamil Renno CEng FIMechE FHEA 32 Roll Deflection • Rolls can deflect under transverse load; this can produce strips that do not have the same thickness along the width • In this case, the rolls are ground such that they have a smaller diameter at the edges © Dr Jamil Renno CEng FIMechE FHEA 33 What are the rolls made from? • The main requirement for roll material is strength and resistance to wear • Three common roll materials are cast iron, cast steel, and forged steel • Roll for cold rolling are ground to a fine finish and for special applications, are polished © Dr Jamil Renno CEng FIMechE FHEA 34 Most Common Configurations (a) Two-high rolling mill – can be reversing or non-reversing (b) Reversing mill in three-high configuration (c) Four-high rolling mill – using two smaller diameter rolls (in contact with the workpiece) and two backing rolls Reversing adds complication and cost © Dr Jamil Renno CEng FIMechE FHEA 35 Additional Rolling Mill Configurations (d) Clustering rolling mill – achieves higher throughput rates in standard products (e) Tandem rolling mill – often used following a continuous casting process of slabs © Dr Jamil Renno CEng FIMechE FHEA 36 Rolling © Dr Jamil Renno CEng FIMechE FHEA 37 Shape Rolling Straight and long structural shapes (such as channels, I-beams, railroad rails and solid bars) are formed by shape rolling (profile rolling), in which the heated stock passes through pairs of specially designed rolls. Steps in the shape rolling of an I-beam. Various other structural sections, such as channels and rails, also are rolled by this process. © Dr Jamil Renno CEng FIMechE FHEA 39 Rail Tracks © Dr Jamil Renno CEng FIMechE FHEA 40 Ring Rolling (a) schematic illustration of a ring-rolling operation: thickness reduction results in an increase in the part diameter; (b) through (d) examples of cross-sections that can be formed by ring rolling. © Dr Jamil Renno CEng FIMechE FHEA 41 Ring Rolling Example Products obtained using ring rolling: 1. Ball and roller bearing races 2. Steel tires for railroad wheels 3. Pressure vessel components 4. Housing for rotating machinery © Dr Jamil Renno CEng FIMechE FHEA 42 Thread Rolling Thread rolling is a cold forming process to form straight or tapered threads on round rods or wires. © Dr Jamil Renno CEng FIMechE FHEA 43 Machined vs Roll Threading Unlike machining, which cuts through the grains of metal, the rolling of threads imparts improved strength because of the favorable grain flow. a) Features of a machined or rolled thread. b) Grain flow in machined thread c) Grain flow in rolled thread. © Dr Jamil Renno CEng FIMechE FHEA 44 Thread Cutting © Dr Jamil Renno CEng FIMechE FHEA 45 Thread Rolling Process © Dr Jamil Renno CEng FIMechE FHEA 46 Thread Rolling – Another Video © Dr Jamil Renno CEng FIMechE FHEA 47 Seamless Tube and Pipe Production Cavity formation in a solid round bar and its utilization in the rotary tube piercing process for making seamless pipe and tubing. When compressive forces are applied radially, tensile stresses develop at the center. © Dr Jamil Renno CEng FIMechE FHEA 48 Seamless Tube Manufacturing © Dr Jamil Renno CEng FIMechE FHEA 49 © Dr Jamil Renno CEng FIMechE FHEA 50 Skew Rolling – Steel Balls (a) Production of steel balls by the skew-rolling process. (b) Production of steel balls by upsetting a cylindrical blank. Note the formation of flash. The balls made by these processes are subsequently ground and polished for use in ball bearings © Dr Jamil Renno CEng FIMechE FHEA 51 Why do we need steel balls? © Dr Jamil Renno CEng FIMechE FHEA 52 How does skew rolling work? © Dr Jamil Renno CEng FIMechE FHEA 55 How does skew rolling work? © Dr Jamil Renno CEng FIMechE FHEA 56 Forging © Dr Jamil Renno CEng FIMechE FHEA 57 Example of Forging © Dr Jamil Renno CEng FIMechE FHEA 58 What is Forging? • The workpiece is shaped by compressive forces applied through various dies and tools. • One of the oldest metal working operations (8,000 B.C.). • Simple forging can be done by hand hammers and an anvil – traditional blacksmith. • Nowadays, forging requires heavy equipment, dies, presses, etc. • Forging produces discrete parts. • Forged parts have good strength and toughness due to the favorable grain flow. © Dr Jamil Renno CEng FIMechE FHEA 59 What do you make with forging? Forging is used to make products that are used reliably for high stressed and critical applications (e.g., landing gear, jet engine shafts and disks). Landing gear of Airbus A350 XWB Landing gear of C5A transport aircraft © Dr Jamil Renno CEng FIMechE FHEA 60 Other typically forged parts Crack Shaft Link Shaft Bevel Pinion Half Link Bearing Fittings © Dr Jamil Renno CEng FIMechE FHEA 61 5/6 Arch Doha All the links are very likely to be made by forging – they have to be constant under tension (stress). © Dr Jamil Renno CEng FIMechE FHEA 62 What is the crankshaft? © Dr Jamil Renno CEng FIMechE FHEA 63 Forging a crankshaft © Dr Jamil Renno CEng FIMechE FHEA 64 Hot Forging vs Cold Forging • Forging may be done at room temperature (cold forging) or at elevated temperatures (warm or hot forging, depending on the temperature) • Cold forging requires greater forces, and the workpiece material must have sufficient ductility at room temperature • Cold forged parts have good surface finish and dimensional accuracy. • Hot forging requires smaller forces, but dimensional accuracy and surface finish are not as good. • Forgings generally require additional finishing operations, such as heat treating to modify properties and machining for accurate finished dimensions © Dr Jamil Renno CEng FIMechE FHEA 65 Impact Forging vs Press Forging Impact forging – forge hammer applies impact force Press forging – forging force is applied gradually © Dr Jamil Renno CEng FIMechE FHEA 66 Types of Forging • Open Die Forging – workpiece is compressed between two flat dies, allowing metal to flow laterally with minimum constraint • Closed Die Forging (also known as Impression-die forging) – die contains cavity or impression that is imparted to the workpiece/blank. • Metal flow is constrained so flash is created • Flash-less forging – metal is completely constrained and no flash is created. © Dr Jamil Renno CEng FIMechE FHEA 67 Open-Die Forging – Upsetting (a) Solid cylindrical billet upset between two flat dies. (b) Uniform deformation of the billet without friction. (c) Deformation with friction. Note barreling of the billet caused by friction forces at the billet-die interfaces. Hight is reduced and diameter increases. © Dr Jamil Renno CEng FIMechE FHEA 68 Comments about Open-Die Forging • The barreling problem could be minimized if an effective lubricant is used • Simple forging can be made by the open-die process. • Open die forgings generally weigh 15-500 Kg, but forgings as high as 275 tons have been made. • Sizes may range from very small to shafts up to 23 m long in the case of ship shaft. © Dr Jamil Renno CEng FIMechE FHEA 69 Open Die Forging – Cogging Cogging is basically an open-die forging operation in which thickness of the bar is reduced by successive forging steps at certain intervals. Two views of a cogging operation on a rectangular and circular bars. Blacksmiths use this process to reduce the thickness of bars by hammering the part on an anvil. © Dr Jamil Renno CEng FIMechE FHEA 70 How much force is needed? The forging force, πΉ, in an open-die forging operation on a solid cylindrical workpiece can be estimated as ππ· πΉ = ππ π΄ 1 + 3β π 2 π΄ = π· is the area of the cylinder, π is the coefficient of 4 friction. The flow stress of the material is ππ = πΎπ © Dr Jamil Renno CEng FIMechE FHEA π β0 and π = ln . β 71 © Dr Jamil Renno CEng FIMechE FHEA 72 © Dr Jamil Renno CEng FIMechE FHEA 73 Example A solid cylindrical workpiece made of 304 stainless steel is 150 mm in diameter and 100 mm high. It is reduced by 50%, at room temperature, by open die forging with flat dies. Assume that the coefficient of friction is 0.2. Calculate the forging force at the end of the stroke. © Dr Jamil Renno CEng FIMechE FHEA 74 Solution – 1/2 We use the final dimensions to find the forging force at the end of the stroke. 100 The final height is given βπ = = 50 mm 2 The final diameter can be obtained knowing that the volume will remain constant π 4 π 2 150 100 = π·π 50 ⇒ π·π = 212.13 mm 4 β0 The true strain is π = ln = ln 100/50 = 0.69 βπ 2 The flow stress is ππ = πΎπ π = 1275 0.69 0.45 = 1079 MPa © Dr Jamil Renno CEng FIMechE FHEA 75 Solution – 2/2 The forging force is then ππ· πΉ = ππ π΄ 1 + 3β π 0.2 212.13 2 = 1079 212.13 1+ 4 3 50 = 48920175.1 N Thus, the force is 48.9 MN. If we know the velocity of the upper die, can we calculate the power needed to perform this forging process? © Dr Jamil Renno CEng FIMechE FHEA 76 Impression-Die and Closed-Die Forging • The work-piece acquires the shape of the die cavities (impressions) • During forging, some of the material flows outward and forms a flash © Dr Jamil Renno CEng FIMechE FHEA 77 Forging Force in Impression Forging • The forging force πΉ required in an impression-die forging operation can be estimated as πΉ = πππ π΄ where π is a multiplying factor and A is the projected area of the forging, including the flash area. Range of π values for closed-die forging © Dr Jamil Renno CEng FIMechE FHEA 78 Stages of Closed-Die Forging Forging a connecting rod: • Edging – define the dimensions of the connecting rod • Blocking – reserve the material for the connecting rod • Finishing – imprint the fine details of the connecting rod • Trimming – remove the flash © Dr Jamil Renno CEng FIMechE FHEA 79 So, how does forging work? © Dr Jamil Renno CEng FIMechE FHEA 80 Closed-Die Forging & Flash © Dr Jamil Renno CEng FIMechE FHEA 81 Flashless Forging • In flashless forging, flash does not form, and the workpiece completely fills the die cavity • Undersized blanks prevent the complete filling of the die cavity (a) closed-die forging with flash (b) precision or flashless forging of a round billet. © Dr Jamil Renno CEng FIMechE FHEA 82 Advantages and Limitations of Forging • Advantages compared to machining • Higher production rate • Less waste of material • Greater strength • Favorable grain orientation • Limitations • Not capable of close tolerances • Machining is often required afterwards © Dr Jamil Renno CEng FIMechE FHEA 83 Forging Operations – Coining • A closed-die forging process used in the minting of coins, medallions and jewelry • Marking parts with letters and numbers can be done rapidly through coining © Dr Jamil Renno CEng FIMechE FHEA 84 Forging Operations – Piercing • A process of indenting the surface of a workpiece with a punch in order to produce a cavity or an impression Examples of piercing operations © Dr Jamil Renno CEng FIMechE FHEA A pierced round billet, showing grain flow pattern. 85 Other forging – Swaging (a)Schematic illustration of the rotary-swaging process. (b)Forming internal profiles on a tubular workpiece by swaging. (c) A die-closing type swaging machine, showing forming of a stepped shaft. (d)Typical parts made by swaging. © Dr Jamil Renno CEng FIMechE FHEA 86 Swaging with or without Mandrel (a) Swaging of tubes without a mandrel; note the increase in wall thickness in the die gap. (b) Swaging with a mandrel; note that the final wall thickness of the tube depends on the mandrel diameter. (c) Examples of cross-sections of tubes produced by swaging on shaped mandrels. © Dr Jamil Renno CEng FIMechE FHEA 87 Animation of Swaging © Dr Jamil Renno CEng FIMechE FHEA 88 Metal Forgeability Forgeability is defined as the capability of a material to undergo deformation without cracking © Dr Jamil Renno CEng FIMechE FHEA 89 Design of Forging Die • Die distortion can be an important consideration, particularly if close tolerances are required. • The most important rule in die design is that the part or work piece material will flow in the direction of least resistance. • For most forgings the parting line is usually at the largest cross-section of the part. © Dr Jamil Renno CEng FIMechE FHEA 90 Draft Angle is a Must © Dr Jamil Renno CEng FIMechE FHEA 91 Forging Defects – Insufficient Material If there is not enough material to fill the die, the web may buckle during forging and develop laps. © Dr Jamil Renno CEng FIMechE FHEA 92 Forging Defects – Too much Material • If the web is thick, the excess material flows past the already forged portions of the forging and develops internal cracks. • Internal defects may also develop from non-uniform deformation of the material in the die cavity and temperature variations throughout the work-piece during forging. © Dr Jamil Renno CEng FIMechE FHEA 93 Other Forging Defects • Surface cracking • Inconsistent material flow patterns • Temperature gradients in the workpiece may result in no uniform deformations and non uniform metallurgy • Grain flow might not also be consistent throughout the workpiece or even hindered • Forging defects can cause fatigue failures, corrosion and wear during service. © Dr Jamil Renno CEng FIMechE FHEA 94 © Dr Jamil Renno CEng FIMechE FHEA 95 Economics of Forging The setup and tooling costs per piece decrease as the number of pieces forged increases, if all pieces use the same die. © Dr Jamil Renno CEng FIMechE FHEA 96 Relative Cost Per Piece © Dr Jamil Renno CEng FIMechE FHEA 97 Extrusion © Dr Jamil Renno CEng FIMechE FHEA 98 What is extrusion? In the extrusion process, material is forced through a die, which is like squeezing a toothpaste © Dr Jamil Renno CEng FIMechE FHEA 99 Extrusion Properties Extrusion is a compression driver process in which the work metal is forced to flow through a die opening to produce a desired cross-sectional shape. • Advantages • a variety of shapes are possible, especially with hot extrusion; • grain structure and strength properties are enhanced in cold and warm extrusion; • fairly close tolerances are possible, especially in cold extrusion; • little/no wasted material; • long workpiece with fixed cross-section. • Limitations • cross-section must be uniform throughout the length; • high initial cost setup. © Dr Jamil Renno CEng FIMechE FHEA 100 Example of Hot Extrusion © Dr Jamil Renno CEng FIMechE FHEA 101 Another Example of Hot Extrusion © Dr Jamil Renno CEng FIMechE FHEA 102 Extruded Products • Almost any solid or hollow cross-section may be produced by extrusion. • Since the die geometry remains the same throughout the operation, extruded products have a constant cross-section. • Depending on the ductility of the material, extrusion may be carried out at room or elevated temperatures. • Typical products made by extrusion are door and window frames, railing for sliding doors, tubing and structural and architectural shapes. © Dr Jamil Renno CEng FIMechE FHEA 103 Extrusion Techniques Extrusion Direct Indirect © Dr Jamil Renno CEng FIMechE FHEA Hydrostatic 104 Direct Extrusion • Direct/forward extrusion: a round billet is placed in a chamber (or container) and forced through a die opening by a hydraulically driven ram – can produce solid or hollow products. • Friction between the billet and container is a problem causing higher extrusion forces. © Dr Jamil Renno CEng FIMechE FHEA 105 © Dr Jamil Renno CEng FIMechE FHEA 106 Direct Extrusion for Hollow Shapes • (a) Direct extrusion to produce hollow or semi-hollow cross sections; (b) hollow and (c) semi-hollow cross sections © Dr Jamil Renno CEng FIMechE FHEA 107 Indirect Extrusion • In indirect Extrusion (reverse, inverted, or backward extrusion), the dies moves towards the billet – there is no friction between the billet and the lining (less extrusion force is needed) • However, supporting the extruded product as it exits is difficult © Dr Jamil Renno CEng FIMechE FHEA 108 Hydrostatic Extrusion • In hydrostatic extrusion, the billet is smaller in diameter than the chamber, which is filled with a fluid, and the pressure is transmitted to the billet by a ram. Unlike direct extrusion, there is no friction to overcome along the container walls. • Holding the pressure is tough and the method can only be used for cold extrusion. © Dr Jamil Renno CEng FIMechE FHEA 110 Hot vs Cold Extrusion • Extrusion can be performed either cold, warm or hot depending on the work metal and the amount of strain to which it is subjected during deformation. • Aluminum, Copper, Magnesium, Zinc, Tin and their alloys are typically extruded cold or warm/hot. • Steel alloys are usually extruded hot although the softer more ductile grades are sometimes cold extruded (e.g., low carbon steels and stainless steel). • Aluminum is probably the most ideal metal for extrusion (cold/warm/hot) and many commercial aluminum products are made by this process (structural shapes, window frames, etc.) © Dr Jamil Renno CEng FIMechE FHEA 111 Advantages of Extrusion – 1 • Variety of shapes possible, especially in hot extrusion • Grain structure and strength enhanced in cold and hot extrusion • Close tolerance possible, especially in cold extrusion • Little/no material waste (a) Aluminum extrusion used as a heat sink for a printed circuit board. (b) Extrusion die and extruded heat sinks. © Dr Jamil Renno CEng FIMechE FHEA 112 Advantages of Extrusion – 2 • Extrusion can be used to produce part geometries that allow assembly of the extruded sections: (a) Lap joints; (b) lap-lock joints; (c) cylindrical sliding fits; (d) cylindrical sliding lock joints; (e) snap fit and (f) keyed assembly. © Dr Jamil Renno CEng FIMechE FHEA 113 Disadvantages of Extrusion • Disadvantages • Part cross-section must be uniform throughout the length © Dr Jamil Renno CEng FIMechE FHEA 114 Analysis of Extrusion – 1 • Assume round billet extrusion to round product • The extrusion ratio also called the reduction ratio is π΄ defined as π = 0 π΄π • The true strain in extrusion is given as π΄0 π = ln π = ln π΄π • Assuming ideal deformation (without friction), the pressure on the ram is π΄0 π = πππ£π ln π΄π © Dr Jamil Renno CEng FIMechE FHEA 116 Analysis of Extrusion – 2 • W. Johnson argued that the above is not correct (particularly for the case of direct extrusion due to the presence of friction). • The actual pressure is higher than what is given in the previous equation – he suggested the following: ππ₯ = π + ππ where π and π are empirical constants for a given die angle • In direct extrusion, the effect of friction between the container and the billet increase ram pressure: ππ·02 ππ = πππ ππ·0 πΏ 4 where ππ is the additional pressure required to overcome friction, π is the coefficient of friction, ππ is the pressure of the billet against the container wall and ππ·0 πΏ is the area of the interface between the billet and the container wall © Dr Jamil Renno CEng FIMechE FHEA 117 Analysis of Extrusion – 3 ππ·02 ππ = πππ ππ·0 πΏ 4 • The right hand side of this equation indicates the billetcontainer friction force and the left hand side gives the additional ram force to overcome that friction. • In the worst case, sticking occurs at the container wall so that the friction stress equals the shear yield strength of the work metal: πππ ππ·0 πΏ = ππ ππ·0 πΏ where ππ is the shear yield strength; thus ππ·02 ππ = ππ ππ·0 πΏ 4 © Dr Jamil Renno CEng FIMechE FHEA 118 Analysis of Extrusion – 4 • Assuming that ππ = πππ£π Τ2 (maximum-shear-stress theory of yielding), then ππ becomes 2πΏ ππ = πππ£π π·0 Based on this, the following formula can be used to compute the ram pressure in direct extrusion: Without friction π = πππ£π ππ₯ Taking friction into account 2πΏ π = πππ£π ππ₯ + π·0 © Dr Jamil Renno CEng FIMechE FHEA 119 Analysis of Extrusion – 5 • Ram force (in N) in direct or indirect extrusion can be obtained as πΉ = ππ΄0 • Power (in W) required to carry out the extrusion is simply π = πΉπ£ where π£ is the ram velocity in m/s © Dr Jamil Renno CEng FIMechE FHEA 120 Example A billet 75 mm long and 25 mm diameter is to be extruded in a direct extrusion operation with an extrusion ratio π = 4. The extruded part has a round cross-section. The die angle is 90o. The work metal has a strength coefficient of 415 MPa and strain hardening exponent of 0.18. Use the Johnson formula with π = 0.8 and π = 1.5 to estimate the extrusion strain. Determine the pressure applied to the end of the billet as the ram moves forward. © Dr Jamil Renno CEng FIMechE FHEA 121 Extrusion Dies • Die angle and orifice shape – most important about the die. • Die angle (more precisely die half-angle) shown as πΌ. • Square dies (i.e., with πΌ = 90π ) are usually used for nonferrous metals. © Dr Jamil Renno CEng FIMechE FHEA 122 Variation of Extrusion Force • Dies with low angles have increased friction because of the larger area – this results in higher extrusion force. • On the other hand, large die angle causes “turbulence” in the metal flow during reduction, increasing the ram force. • The optimum angle depends on: work material, billet temperature and lubrication. Metal flow in square dies: (a) low friction or in indirect extrusion (b) high friction at the billet– chamber interfaces. (c) high friction or with cooling of the outer regions of the hot billet in the chamber (observed in metals whose strength increases rapidly with decreasing temperature) © Dr Jamil Renno CEng FIMechE FHEA 123 Extrusion Force for any Shape • The shape of the die orifice affects the ram pressure. • A complex cross-section requires higher pressure ο higher force ο higher power. • The effect of the shape of the die orifice can be included by the die shape factor which is expressed as 2.25 πΆπ₯ πΎπ₯ = 0.98 + 0.02 πΆπ where πΎπ₯ is the die shape factor in extrusion, πΆπ₯ is the perimeter of the extruded cross-section and πΆπ is perimeter of the circle of the same area as the extruded shape. This shape factor is valid for 1 ≤ π ≤ 6 © Dr Jamil Renno CEng FIMechE FHEA 124 © Dr Jamil Renno CEng FIMechE FHEA 125 © Dr Jamil Renno CEng FIMechE FHEA 126 © Dr Jamil Renno CEng FIMechE FHEA 127 © Dr Jamil Renno CEng FIMechE FHEA 128 © Dr Jamil Renno CEng FIMechE FHEA 129 © Dr Jamil Renno CEng FIMechE FHEA 130 © Dr Jamil Renno CEng FIMechE FHEA 131 Extrusion Force For indirect extrusion, π = πΎπ₯ πππ£π ππ₯ and for direct extrusion π = πΎπ₯ πππ£π 2πΏ ππ₯ + π·0 Expressions of the force and power remain the same. © Dr Jamil Renno CEng FIMechE FHEA 132 © Dr Jamil Renno CEng FIMechE FHEA 133 © Dr Jamil Renno CEng FIMechE FHEA 134 Die Materials • Die materials used for hot extrusion include • Tool and alloy steels • Important properties • High wear resistance, • High hot hardness and higher thermal conductivity • For cold extrusion, tool steels and cemented carbides are preferred • Carbides are used when high production rates, long operational life and good dimensional control are required © Dr Jamil Renno CEng FIMechE FHEA 135 Problem A direct extrusion process starts with an aluminum billet with diameter 20 cm and length of 50 cm. The extruded product is a solid square of 8 cm side. Calculate the forging force. If you leave 3 cm as a butt in this process, what is the length of extruded product? © Dr Jamil Renno CEng FIMechE FHEA 136 Design for Extrusion Examples of poor and good design practices for extrusion; note the importance of eliminating sharp corners and keeping section thicknesses uniform. © Dr Jamil Renno CEng FIMechE FHEA 137 Impact Extrusion • Impact Extrusion is like indirect extrusion and is often included in the cold extrusion category. • The punch descends rapidly on the blank, which is extruded backward – this produced discrete parts unlike standard extrusion © Dr Jamil Renno CEng FIMechE FHEA 138 Example of Impact Extrusion © Dr Jamil Renno CEng FIMechE FHEA 139 Another Example of Impact Extrusion © Dr Jamil Renno CEng FIMechE FHEA 140 Drawing © Dr Jamil Renno CEng FIMechE FHEA 141 What is drawing? • In drawing, the cross-section of a round rod or wire is typically reduced or changed by pulling it through the die – the process features are similar to those of extrusion. • The major variables in drawing are like those in extrusion, that is, • reduction in cross-sectional area, • die angle, • friction along the die-workpiece interface, and • speed © Dr Jamil Renno CEng FIMechE FHEA 142 The Drawing Process • In the case of yielding, the product will undergo further deformation after it leaves the die, which is not acceptable. • Ideally, the maximum reduction in cross-section area per pass is 63%. • Thus, for example, a 10 mm diameter rod can at most be reduced to a diameter of 6.1 mm in one pass © Dr Jamil Renno CEng FIMechE FHEA 143 Bar vs Wire Drawing – 1 • Although drawing is accomplished through tensile stresses, compression also is present and plays a significant role since the area is squeezed as it passes through the die opening. • For this reason, the deformation that occurs in drawing is sometimes referred to as indirect compression. • Bar drawing is the term used for large diameter bar and rod workpiece. • Wire drawing applies to small diameter stock – down to 0.03 mm are possible in drawing. • The mechanics of bar drawing and wire drawing are the same but the machinery used is different. © Dr Jamil Renno CEng FIMechE FHEA 144 Bar vs Wire Drawing – 2 • Bar drawing is generally is generally accomplished as a single-draft process. The stock is pulled through one die opening. • Bar drawing is a batch-type operation because the length of the workpiece before and after drawing is limited. • In contrast, wire is drawn from coils consisting of several hundred (or even thousands) of meters of wire and is passed through a series of draw dies. The number of dies varies between 4 and 12. • This is a continuous process (and sometimes wires are butt-welded to keep the process running). © Dr Jamil Renno CEng FIMechE FHEA 145 Continuity of Mass in Drawing • Mass continuity should be maintained so throughout the dies, you have π΄π π£π = Constant © Dr Jamil Renno CEng FIMechE FHEA 146 Preparation of Work for Drawing • Annealing – to increase ductility of stock • Cleaning - to prevent damage to work surface and draw die • Pointing – to reduce diameter of starting end to allow insertion through draw die opening at beginning of process © Dr Jamil Renno CEng FIMechE FHEA 147 Typical drawing equipment • A multistage wire-drawing machine that is typically used in the making of copper wire for electrical wiring. • Very long rod and wire (many kilometers) and smaller crosssections, usually less than 13 mm, are drawn by a series of rotating drums. © Dr Jamil Renno CEng FIMechE FHEA 148 Wire Drawing in Action © Dr Jamil Renno CEng FIMechE FHEA 149 Wire Drawing Analysis • In a drawing operation, the change in size of the workpiece is usually given by the reduction in area defined as π΄0 − π΄π π= π΄0 The area reduction is usually expressed as a percentage. • The draft is simply the difference between the diameter of the original and final stock π = π·0 − π·π © Dr Jamil Renno CEng FIMechE FHEA 150 Mechanics of Drawing • The true strain could be determined as π΄0 1 π = ln = ln π΄π 1−π • The true stress π΄0 ππ‘ = πππ£π π = πππ£π ln π΄π where πΎπ π πππ£π = 1+π © Dr Jamil Renno CEng FIMechE FHEA 151 Adding the Effect of Friction • A number of methods have been proposed for predicting the drawing stress π π΄0 ππ = πππ£π 1 + π ln tan πΌ π΄π where ππ is the draw stress, π is the coefficient of friction between the die and the workpiece, πΌ is the die angle © Dr Jamil Renno CEng FIMechE FHEA 152 Adding Correction Factor • The drawing stress includes a correction factor π π π΄0 ππ = πππ£π 1 + π ln tan πΌ π΄π where π·ππ£π π = 0.88 + 0.12 πΏπ © Dr Jamil Renno CEng FIMechE FHEA 153 Geometric Consideration • π·ππ£π is the average diameter between the initial and final diameters of the rod π·0 + π·π π·ππ£π = 2 • πΏπ is the contact length between the die and the workpiece π·0 − π·π πΏπ = 2 sin πΌ © Dr Jamil Renno CEng FIMechE FHEA 154 Drawing Force and Drawing Power • The corresponding draw force is then the area of the drawn cross-section multiplied by the draw stress π π΄0 πΉ = π΄π ππ = π΄π πππ£π 1 + π ln tan πΌ π΄π • The drawing power is simply π = πΉπ£ where π£ is the drawing velocity. © Dr Jamil Renno CEng FIMechE FHEA 155 Alternative Expressions of Drawing Force An alternative formula that has been proposed to calculate the drawing force is π π΄0 2 πΉ = π΄π πππ£π 1 + ln + πΌ πΌ π΄π 3 where πΌ is the die angle in radians. This equation is more conservative – the force predicted using this equation is higher than the previous equation. © Dr Jamil Renno CEng FIMechE FHEA 156 Example A wire is drawn through a die with an angle of 15π . The initial diameter is 2.5 mm and the final diameter is 2 mm. The coefficient of friction at the work-die interface is 0.07. The metal has a strength coefficient of 2.5 MPa and a strain hardening exponent of 0.2. Determine the draw stress and draw force. © Dr Jamil Renno CEng FIMechE FHEA 157 Sheet Metal Working © Dr Jamil Renno CEng FIMechE FHEA 158 © Dr Jamil Renno CEng FIMechE FHEA 159
