Lecture 5- Metal Forming Processes.pdf

1 Workshop Practice (ME F112) Metal Forming Dr. Amit Kumar Department of Mechanical Engineering, Birla Institute of Technology and Science, Pilani 333031 (Rajasthan) INDIA...

1 Workshop Practice (ME F112) Metal Forming Dr. Amit Kumar Department of Mechanical Engineering, Birla Institute of Technology and Science, Pilani 333031 (Rajasthan) INDIA Department of Mechanical Engineering, BITS Pilani  Introduction What is a Metal Forming Process? Process in which the desired shape and size are obtained through the plastic deformation of a metal through various states of the forces. Classification? Fig. 1: General classification of metal forming operation 2 Department of Mechanical Engineering, BITS Pilani  Different forming processes Bulk deformation: characterized by significant deformations and massive shape changes, and the surface area-to-volume of the work is relatively small. Rolling Extrusion Wire Forging drawing Fig. 2: Basic bulk deformation processes Products of Bulk Deformation Processes Department of Mechanical Engineering, BITS Pilani  Different forming processes Sheet metalworking: forming and cutting operations performed on metal sheets, strips, and coils. The surface area to volume of the work is relatively high. Deep drawing Bending (1) (2) Shearing Products of Sheet metal working Fig. 3: Sheet metalworking processes Department of Mechanical Engineering, BITS Pilani Products Made by Metal Forming Processes Typical examples of the products made by metal forming processes. Body of vehicles Tooth paste tubes Utensils Rails etc. 5 Department of Mechanical Engineering, BITS Pilani  Metal forming Advantages Flow Stress Higher productivity: Hundred of tonnes of end Instantaneous value of products per working shift stress required to continue Close control on mechanical properties deforming the material— to keep the metal ‘‘flowing.’’ Minimum waste of metal Flow stress of the metal as a High dimensional accuracy and surface function of strain finish Production of many products (Extra thin foil, wires, sheets etc.) started with the development of mechanical working Department of Mechanical Engineering, BITS Pilani  Temperature in Metal Working Cold Working (also known as cold forming) is metal forming performed at room temperature or slightly above. Benefits Limitations Greater accuracy Higher forces and power are required Better surface finish Strain hardening Higher strength and hardness Need of high capacity No heating of the work is presses required Department of Mechanical Engineering, BITS Pilani  Temperature in Metal Working Hot working (also called hot forming) involves deformation at temperatures above the recrystallization temperature. Benefits Limitations Lower forces and power are Lower dimensional accuracy required Work surface oxidation Shape of the workpiece can be Poor surface finish significantly altered Shorter tool life No Strain hardening Department of Mechanical Engineering, BITS Pilani  Rolling It is a process in which the thickness of the work is reduced by compressive forces exerted by two opposing rolls. Fig. 4: Flat rolling operation Department of Mechanical Engineering, BITS Pilani  Terminology Used In Rolling Ingot: cast product with circular, square or any other convenient cross-section. Bloom:  obtained by hot rolling of an ingot.  First product by rolling down of the ingot.  Square cross section with a cross-section area above 225 cm2. 10 Department of Mechanical Engineering, BITS Pilani  Billet: The minimum cross sectional area of a billet is about 4 cm by 4 cm. Slab: A slab refers to a hot rolled ingot with a cross sectional area greater than 100 cm2 and with a width at least twice the thickness. 11 Department of Mechanical Engineering, BITS Pilani  Plate and Sheet: These are intermediate products obtained by rolling.  Plate (t > 6 mm)  Sheet (t< 6 mm) Sheet and Strip: Both have thickness < 6 mm.  Strip (w300 mm) 12 Department of Mechanical Engineering, BITS Pilani Typical products made by rolling process 13 Department of Mechanical Engineering, BITS Pilani  Hot Rolling Initial break down of an ingot is Typical temperature ranges for done by hot rolling. hot rolling are about  450 °C for aluminum alloys Hot rolling converts cast  up to 1250 °C for alloy structure to wrought structure. steels  up to 1650 °C for refractory alloys Scale developed during hot rolling may be removed by pickling or by mechanical methods. Fig. 5: Changes in the grain structure of cast during hot rolling Fig. 6: Hot rolled sheet 14 Department of Mechanical Engineering, BITS Pilani  Cold Rolling The hot rolled sheets are cold rolled to produce final sheet with desired thickness and surface finish. During cold rolling amount of thickness reduction is low. Intermediate annealing is done to control mechanical properties. Final annealing is done to produce sheets with good ductility. 15 Department of Mechanical Engineering, BITS Pilani  Rolling Analysis Draft Reduction True strain Average flow stress Fig. 7: Side view of flat rolling 16 Department of Mechanical Engineering, BITS Pilani  Rolling Analysis Limit of max Draft For cold rolling = 0.1 For hot rolling = 0.4 Power requirement Rolling force Contact length Fig. 7: Side view of flat rolling Numerical: A 300 mm wide strip 25 mm thick is fed through a rolling mill with two powered rolls each of radius = 250 mm. The work thickness is to be reduced to 22 mm in one pass at a roll speed of 50 rev/min. The work material has a flow curve defined by K=275 MPa and n=0.15, and the coefficient of friction between the rolls and the work is assumed to be 0.12. Calculate the roll force and rolling power. 17 Department of Mechanical Engineering, BITS Pilani  Types of Rolling Mill Rolling Two High rolling mill Three High rolling mill mills Rolling mill consist of Three Rolling mill consists of two opposing rolls rolls in a vertical column Diameters in the range of 0.6 to 1.4 m Direction of rotation of each roll Two-high configuration can be either remains unchanged reversing or non-reversing Fig. 9: 3 high rolling mill Fig. 8: 2 high rolling mill Department of Mechanical Engineering, BITS Pilani  Types of Rolling Mill Four High rolling mill Lower roll radius, leads to lower forces, torque, and power. Four-high rolling mill uses two Supporting main smaller-diameter rolls to contact the rolls work and two backing rolls behind them Larger backing rolls prevent the elastic deflection of small rolls. Fig. 10: Four high rolling mill Department of Mechanical Engineering, BITS Pilani  Types of Rolling Mill Rolling Tandem rolling mill Cluster rolling mill mills To achieve higher throughput rates in standard products, a tandem rolling mill Based on the principle that small- is often used diameter rolls will provide lower roll forces and power requirements Consists of a series of rolling stands May have eight or ten stands, each making a reduction in thickness Fig. 11: Cluster rolling mill Fig. 12: Tandem rolling mill Department of Mechanical Engineering, BITS Pilani  Rolling Defects Surface defects Scale, Rust , Scratches, Cracks  These defects occur due to impurity and inclusions in the original cast material Roll bending Long Strains Internal structural defects edges adjustment Wavy edges, Zipper cracks, Edge cracks, Alligatoring  Defects due to bending of rolls Wavy edge Zipper cracks  Defect due to in homogenous deformation Fig. 15: Consequences of roll bending Department of Mechanical Engineering, BITS Pilani  Metal forming Rolling Defect due to inhomogeneous Defects deformation (across the width) Rounding Edge cracks Center split Fig.17 : Defect resulting from lateral spread Department of Mechanical Engineering, BITS Pilani Metal Forming-Extrusion Department of Mechanical Engineering, BITS Pilani  Extrusion Extrusion is a compression process in which the work metal is forced to flow through a die opening to produce a desired cross-sectional shape Type of extrusion process: Advantage: Depending upon method Variety of shapes are possible Direct extrusion Indirect extrusion Grain structure and strength properties are enhanced Depending upon working temperature Hot extrusion Fairly close tolerance are possible Cold extrusion Little or no wastage of material Depending upon final product Continuous process Discrete process Department of Mechanical Engineering, BITS Pilani  Direct Extrusion It is also known as forward extrusion A metal billet is loaded into a container A ram compresses the material, forcing it to flow through one or more openings in a die As the ram approaches the die, a small Fig.1: Direct extrusion portion of the billet remains that cannot be forced through the die opening Largest dimension of the die opening must be smaller This extra portion, is separated from than the diameter of the billet the product by cutting it just beyond the exit of the die Department of Mechanical Engineering, BITS Pilani  Problems in Direct Extrusion Significant friction exists between the work surface and the walls of the container as the billet is forced to slide toward the die opening. Friction causes a substantial increase in the ram force required in direct extrusion. In hot extrusion, the friction problem is aggravated by the presence of an oxide layer on the surface of the billet. This oxide layer can cause defects in the extruded product. To address these problems, a dummy block is often used between the ram and the work billet. Department of Mechanical Engineering, BITS Pilani Hollow Section in Direct Extrusion Direct extrusion for hollow section Hollow section Fig.2: Direct extrusion to produce hollow section Department of Mechanical Engineering, BITS Pilani  Indirect Extrusion It is also known as backward extrusion The die is mounted to the ram rather than at the opposite end of the container The metal is forced to flow through the clearance in a direction opposite to the motion of the ram Fig.3: Indirect extrusion (a solid cross section) Limitation: There is no friction at the container walls, and the ram force is therefore Lower rigidity of the hollow ram lower than in direct extrusion Difficulty in supporting the extruded product as it exits the die Department of Mechanical Engineering, BITS Pilani Analysis of Extrusion Fig.4: Ram pressure v/s ram stroke Department of Mechanical Engineering, BITS Pilani  Hot Extrusion v/s Cold Extrusion Hot extrusion involves prior heating of Cold extrusion and warm extrusion the billet to a temperature above its are generally used to produce recrystallization temperature. discrete parts, often in finished (or Permitting more extreme size near finished) form. reductions and more complex shapes Magnitude of the stresses on the to be achieved in the process tooling in cold extrusion is very high Advantage of reduced ram force and Punch hardness usually ranges increased ram speed between 60 and 65 HRC is required Cooling of the billet as it contacts the Punches must possess sufficient container walls is a problem strength, sufficient toughness, resistance to wear and fatigue Isothermal extrusion is sometimes used failure to overcome this problem. Department of Mechanical Engineering, BITS Pilani  Analysis of Extrusion Actual true strain Actual pressure in direct extrusion Direct extrusion Extrusion ratio Pressure applied on ram Where εx = extrusion strain; and a and b are empirical constants for a given die True strain angle. Values of a and b tend to increase Average flow stress with increasing die angle. Department of Mechanical Engineering, BITS Pilani  Impact Extrusion Impact extrusion is performed at higher speeds and shorter strokes than conventional extrusion. It is used to make individual components. Fig.6: Example of forward extrusion Fig.7: Example of backward extrusion punch impacts the work part rather than simply applying pressure to it. Impacting can be carried out as forward extrusion, backward extrusion, or combinations of these. Ex: toothpaste tubes and battery cases. Department of Mechanical Engineering, BITS Pilani  Hydrostatic Extrusion Problems in direct extrusion Friction along the billet– container interface Solution: Fig.8: Hydrostatic extrusion Surrounding the billet with fluid inside the container and Pressurizing the fluid by the Hydrostatic pressure on the work increases the forward motion of the ram material’s ductility Advantage to use this process on metals that Defects in extrusion: Piping, center would be too brittle for conventional extrusion burst, and surface crack. operations. High reduction ratios are possible Department of Mechanical Engineering, BITS Pilani Thank You 34 Department of Mechanical Engineering, BITS Pilani