Manufacturing Process - I UNIT II Metal Forming Processes Prepared By Prof. Shinde Vishal Vasant Assistant Professor Dept. of Mechanical Engg. NDMVP S Karmaveer Baburao Thakare College of Engg. Nashik Contact No- 8928461713 E mail:- nilvasant22@gmail.com Website:- www.vishalshindeblog.wordpress.com
Introduction Practically all metals, which are not used in cast form are reduced to some standard shapes for subsequent processing. Manufacturing companies producing metals supply metals in form of ingots which are obtained by casting liquid metal into a square cross section. Slab (500-1800 mm wide and 50-300 mm thick) Billets (40 to 150 sq mm) Blooms (150 to 400 sq mm) Sometimes continuous casting methods are also used to cast the liquid metal into slabs, billets or blooms. These shapes are further processed through hot rolling, forging or extrusion, to produce materials in standard form such as plates, sheets, rods, tubes and structural sections.
Sequence of operations for obtaining different shapes
Metal forming processes Large group of manufacturing processes in which plastic deformation is used to change the shape of metal work pieces The tool, usually called a die, applies stresses that exceed the yield strength of the metal The metal takes a shape determined by the geometry of the die
Basic Types of Metal Forming Processes 1. Bulk deformation Rolling processes Forging processes Extrusion processes Wire and bar drawing 2. Sheet metalworking Bending operations Deep or cup drawing Shearing processes
Temperature in Metal Forming Any deformation operation can be accomplished with lower forces and power at elevated temperature Three temperature ranges in metal forming: Cold working Warm working Hot working
Hot and cold working processes
Cold Working Performed at room temperature or slightly above Many cold forming processes are important mass production operations Minimum or no machining usually required These operations are near net shape or net shape processes
Advantages of Cold Forming Better accuracy, closer tolerances Better surface finish Strain hardening increases strength and hardness Grain flow during deformation can cause desirable directional properties in product No heating of work required
Disadvantages of Cold Forming Higher forces and power required for deformation Surfaces of starting work must be free of scale and dirt Ductility and strain hardening limit the amount of forming that can be done In some cases, metal must be annealed before further deformation can be accomplished In other cases, metal is simply not ductile enough to be cold worked
Hot Working Deformation at temperatures above the re crystallization temperature. The process of formation of new grains is called recrystallisation process and corresponding temperature is called recrystallisation temperature Recrystallization temperature = about one-half of melting point on absolute scale In practice, hot working usually performed somewhat above 0.5T m Metal continues to soften as temperature increases above 0.5T m, enhancing advantage of hot working above this level
Advantages of Hot Working Work piece shape can be significantly altered Lower forces and power required Metals that usually fracture in cold working can be hot formed Strength properties of product are generally isotropic No strengthening of part occurs from work hardening Advantageous in cases when part is to be subsequently processed by cold forming
Disadvantages of Hot Working Lower dimensional accuracy Higher total energy required, which is the sum of The thermal energy needed to heat the workpiece Energy to deform the metal Work surface oxidation (scale) Thus, poorer surface finish Shorter tool life Dies and rolls in bulk deformation
Hot working operations Forging Rolling Welding Extrusion Spinning Hot piercing and rolling
Cold working operations Cold rolling Extrusion Pressing Deep drawing Squeezing Bending Shearing
Bulk Deformation Processes Rolling: Compressive deformation process in which the thickness of a plate is reduced by squeezing it through two rotating cylindrical rolls. Forging: The work piece is compressed between two opposing dies so that the die shapes are imparted to the work. Extrusion: The work material is forced to flow through a die opening taking its shape Drawing: The diameter of a wire or bar is reduced by pulling it through a die opening (bar drawing) or a series of die openings (wire drawing)
Basic bulk deformation processes: (a) rolling (b) Forging
Basic bulk deformation processes: (c) extrusion (d) Drawing
Sheet Metal working Forming and related operations performed on metal sheets, strips, and coils High surface area-to-volume ratio of starting metal, which distinguishes these from bulk deformation Often called press working because presses perform these operations Parts are called stampings Usual tooling: punch and die
Basic sheet metalworking operations: (a) bending (b) Drawing
Basic sheet metalworking operations: (c) shearing
Sr. No Comparison of Hot and Cold Working Hot Working 1 Working above recrystallization temperature Cold Working Working below recrystallization temperature 2 Formation of new crystals (Grains) No crystal formation (Grains) 3 Surface finish not good Surface finish is good 4 No stress formation Internal Stress developed 5 Improved Mechanical Property due to grain formation 6 Closed dimensional tolerances cannot be maintained 7 Improves some mechanical properties like impact strength and elongation Leads to distortion of metal grains Superior dimensions can be obtained During process, strength and elongation decreases
Friction in Metal Forming In most metal forming processes, friction is undesirable: Metal flow is retarded Forces and power are increased Wears tooling faster Friction and tool wear are more severe in hot working if the co efficient of friction is very high, that condition is called as sticking or sticking friction
Lubrication in Metal Forming Metal working lubricants are applied to tool-work interface in many forming operations to reduce harmful effects of friction Lubricant for cold working processes are mineral oil, fats and fatty oils, soaps Lubricants for hot working processes are mineral oils, graphite, moltan glass etc Benefits: Reduced sticking, forces, power, tool wear Better surface finish Removes heat from the tooling
Hot rolling Deformation process in which work thickness is reduced by compressive forces exerted by two opposing rolls
The cross section of the work piece is reduced by the process. The material gets squeezed between a pair of rolls, as a result of which the thickness gets reduced and the length gets increased. Mostly, rolling is done at high temperature, called hot rolling because of requirement of large deformations. Hot rolling results in residual stress-free product. Bloom is has a square cross section, with area more than 230 cm2. A slab, also from ingot, has rectangular cross-section, with area of at least 100 cm2 and width at least three times the thickness. A billet is rolled out of bloom, has at least 40 mm X 40 mm cross-section. Blooms are used for rolling structural products such as I-sections, channels, rails etc. Billets are rolled into bars, rods. Bars and rods are raw materials for extrusion, drawing, forging, machining etc. Slabs are meant for rolling sheets, strips, plates etc.
Rolling Mills Types of rolling mills Equipment is massive and expensive Rolling mill configurations: Two-high two opposing rolls Three-high work passes through rolls in both directions Four-high backing rolls support smaller work rolls Cluster mill multiple backing rolls on smaller rolls Tandem rolling mill sequence of two-high mills
ARRANGEMENTS OF ROLLERS USED IN ROLLING MILLS TWO HIGH MILL The stock is returned to the entrance for further reduction. THREE HIGH MILL Consist of upper and lower driven rolls and a middle roll, which rotates by friction.
FOUR HIGH MILL Small-diameter rolls (less strength & rigidity) are supported by larger-diameter backup rolls CLUSTER ROLLING MILLS Each of the work rolls is supported by two backing rolls.
Tandem rolling mill sequence of two-high mills A series of rolling stands in sequence.
Forging process Forging is a process in which the work piece is shaped by compressive forces applied through various dies and tools. It is one of the oldest metalworking operations. Most forgings require a set of dies and a press or a forging hammer. Unlike rolling operations, which generally produce continuous plates, sheets, strip, or various structural cross-sections, forging operations produce discrete parts. Typical forged products are bolts and rivets, connecting rods, shafts for turbines, gears, hand tools, and structural components for machinery, aircraft, railroads and a variety of other transportation equipment.
Depending upon complexity of the part forging is carried out as open die forging and closed die forging. In open die forging, the metal is compressed by repeated blows by a mechanical hammer and shape is manipulated manually. In closed die forging, the desired configuration is obtained by squeezing the work piece between two shaped and closed dies. On squeezing the die cavity gets completely filled and excess material comes out around the periphery of the die as flash which is later trimmed. Press forging and drop forging are two popular methods in closed die forging. In press forging the metal is squeezed slowly by a hydraulic or mechanical press and component is produced in a single closing of die, hence the dimensional accuracy is much better than drop forging. Both open and closed die forging processes are carried out in hot as well as in cold state.
Open-Die Forging Compression of work piece between two flat dies Deformation operation reduces height and increases diameter of work Common names include upsetting or upset forging or cogging
Open-Die Forging with No Friction If no friction occurs between work and die surfaces, then homogeneous deformation occurs, so that radial flow is uniform throughout work piece height Fig:- Homogeneous deformation of a cylindrical work part 1. start of process with work piece at its original length and diameter, 2. partial compression, 3. final size.
Open-Die Forging with Friction Friction between work and die surfaces constrains lateral flow of work, resulting in barreling effect. In hot open-die forging, effect is even more pronounced due to heat transfer at and near die surfaces, which cools the metal and increases its resistance to deformation
Impression-Die Forging or closed die forging Compression of work part by dies with inverse of desired part shape Flash is formed by metal that flows beyond die cavity into small gap between die plates Flash serves an important function: As flash forms, friction resists continued metal flow into gap, constraining material to fill die cavity In hot forging, metal flow is further restricted by cooling against die plates
Trimming operation (shearing process) to remove the flash after impression-die forging.
Sr. No Open Die forging 1 In this method, the work piece is compressed between two flat dies 2 Cost of dies are low Higher cost Closed die forging Work piece is compressed between two impressed dies 3 The process is simple process is complex 4 No stress formation Residual stress formation takes place 5 During process, poor utilization of metals 6 After the process, machining of component is required Better utilization Machining of component is not required 7 Dimensional accuracy is not good Dimensional accuracy is good 8 Suitable only for production of simple component Simple and complex component
Extrusion A plastic deformation process in which metal is forced under pressure to flow through a single, or series of dies until the desired shape is produced. Process is similar to squeezing toothpaste out of a toothpaste tube In general, extrusion is used to produce long parts of uniform cross sections Typical products made by extrusion are railings for sliding doors, tubing having carious cross-sections, structural and architectural shapes, door and windows frames.
Types (1)Direct Extrusion (Forward Extrusion) (2)Indirect Extrusion (Backward Extrusion) (3)Cold Extrusion (4)Hot Extrusion Direct Extrusion Billet is placed in a chamber and forced through a die opening by a hydraulically-driven ram or pressing stem. Dies are machined to the desired cross-section
Indirect Extrusion Metal is forced to flow through the die in an opposite direction to the ram s motion. Lower extrusion force as the work billet metal is not moving relative to the container wall. Limitations Lower rigidity of hollow ram Difficulty in supporting extruded product as it exits die
Drawing Commonly used to make wires from round bars Large quantities of wires, rods, tubes and other sections are produced by drawing process which is basically a cold working process. In this process the material is pulled through a die in order to reduce it to the desired shape and size. In a typical wire drawing operation, once end of the wire is reduced and passed through the opening of the die, gripped and pulled to reduce its diameter.
Drawing is an operation in which the cross-section of solid rod, wire or tubing is reduced or changed in shape by pulling it through a die. Drawn rods are used for shafts, spindles, and small pistons and as the raw material for fasteners such as rivets, bolts, screws. Drawing also improves strength and hardness when these properties are to be developed by cold work and not by subsequent heat treatment.
Wire drawing machines consisting of multiple draw dies (typically 4 to 12) separated by accumulating drums Each drum (capstan) provides proper force to draw wire stock through upstream die Each die provides a small reduction, so desired total reduction is achieved by the series
Die Materials Commonly used materials are Tool Steels and Carbides Diamond dies are used for fine wire. For improved wear resistance, steel dies may be chromium plated, and carbide dies may be coated with titanium nitride For Hot drawing, cast-steel dies are used
Tube drawing Tube drawing is also similar to wire drawing, except that a mandrel of appropriate diameter is required to form the internal hole. The process reduces the diameter and thickness of the tube. Tube drawing is very similar to bar drawing, except the beginning stock is a tube. It is used to decrease the diameter, improve surface finish and improve dimensional accuracy. A mandrel may or may not be used depending on the specific process used. Here two arrangements are shown in figure (a) with a floating plug and (b) with a moving mandrel
Tube Drawing