Group 93 / 1 / 2006Ch. 15: Extrusion and Drawing of MetalCh. 16: Sheet-Metal Forming Process David Ortegel Steven Neidigk Tim Ngo Jason Maestas
CHAPTER 15 Extrusion and Drawing Of Metals
15.1 Introduction Extrusion • Rooted From Latin Term “Extrudere”(To Force out). • A Cylindrical Billet is Forced Through A Die • Same Concept as forming Play-Doe • Die Geometry remains constant throughout the operation • Forms a constant extruded cross-section • Aluminum, copper, Steel Magnesium, lead are the most common material used in the Extruding process
Products Made by Extrusions • Railings for Sliding Doors • Window Frames • Aluminum Ladders • Extrusions can be cut into desired lengths, allowing for discreet parts such to be produced as Gears, and Brackets
Economics of Extrusions • Extrusions can be economical for Large productions runs as well as short ones. • Tool cost is generally low • Manufactures are able to produce quantity because of the ability to cut individual parts from a single extrusion
Drawing • Drawing – Cross-sections of solid rod, wire, or tubing is reduced or changed in shape by pulling through a die. • Developed between 1000 -1500 AD • Used to from shafts, automobile components, spindles, fasteners.
Depending on the ductility pf the material used extrusions can be caries out various ways: • Cold Extrusion – Extrusion carried out a ambient temperature. Often combined with forging operations • Hot Extrusions – Extrusions carries out at elevated temperatures • Impact extrusion – punch is rapidly descends on billet material • Hydrostatic extrusion – pressure is applied by a piston through incompressible fluid medium surrounding the billet
15.2 The Extrusion Process • 3 Basics Types of Extrusion: • Direct or Forward Extrusion: A billet is placed in a chamber and forced hydraulically through a die by a ram. Fig a • Indirect Extrusion (Reverse, Inverted, Backward Extrusion): The Die Moves Toward the billet material. fig b
Hydrostatic Extrusion: • A Billet that is smaller that the chamber is used. • The Chamber is filled with a fluid. Pressure is then applied to the pressing stem • There is no friction to overcome
Variables • Die Angle • A ۪ – Initial cross-sectional Area • A – Final cross-sectional Area
Extrusion Force • The force required for extrusions F = A*k*ln( A۪/ A ) k = Constant determined experimentally • The force required depends on: • Strength of Billet Material • Extrusion Ratio: A ۪/A • Friction between the billet material and the chamber • Temperature • Speed of the Ram • Type of lubricants used
Metal Flow • Flow patterns influence the quality and mechanical properties of the extruded product • Material flows longitudinally (ex. Fluid flowing in a channel) • The extruded product has a elongated grain structure
A common technique in investigating a flow is to section the round billet in half • Dead-Metal Zone – The corner of the material stays stationary. Bright finishes result due to material flowing past the die angle • Fig a – flow patterns attained with low friction • Fig b – flow patterns attained with high friction • Fig c – flow pattern attained with high friction or cooling of the billets outer regions in the chamber
15.3 Hot Extrusion • Extrusions carried out at elevated temperatures • Used for metals that do not have sufficient ductility at room temperature. • Used when wanting to reduce forces on billet material
Special Requirements • Due to excessive heat die wear becomes excessive and rapid cooling of the product after extrusion causes deformations. • Pre-heated dies prolong the life of the die done my hot forging. This also reduces rapid cooling if the material • Billet develops an oxide film. Oxides can result in a finished product that is unacceptable. • Placing a dummy block, a little smaller than the chamber ahead of the ram, results in a thin shell of the oxide layer left in the container.
Die Design Main types of die designs: 1) Square dies: Used in extruding nonferrous metals - Develops dead metal zones that form the dies angle 2) Solid or Hollow Dies: Used in extruding tubing products - The ram is fitted with a mandrel that pierces a hole in the billet -Wall thickness is limited to 1 mm for aluminum. 3mm for stainless steels, 5mm to stainless steels
3) Spider, porthole dies ,and bridge dies: Used in creating hollow cross-section extrusions -Created by welding chamber methods -Metal divides and flows around the supports for the internal mandrel -The extruded strands become welded back together by the high internal pressure existing in the chamber only suitable for aluminum and its alloys because of their capacity to create strong welds.
(a) • (c) • (b)
Die Materials • Hot Worked dies Steels are used for hot extrusion • Coatings such as zirconia used to extend die life • Not suitable for dies used for complex shapes because of the sever stress gradients that develop in the die.
Lubrication • Lubrication Is important in hot extrusions because of major effects on: • Material flow • Surface finish on extrusion • Product quality • Extrusion forces
Sejournet process was developed in the 1940s uses Glass as a lubricant on high temperature steels. • A circular glass pad is placed in the chamber at the die entrance • The head conducted cause the glass to melt around the extruded product acting as a lubricant
Jacketing: also known as canning, uses softer and lower strength metals metals such as copper or a mild steel. • Used for materials that have a tendency to stick to the container and die • Also used or toxic and radioactive material. The jacket that is formed over the extruded material acts as a protected barrier
15.4 Cold Extrusion • Developed in the 1940’s, are extrusion produced at ambient temperature. • Denotes combinations of operations, that consist of indirect extrusions and forgings
Cold extrusions are widely used Automobile products Appliances Fasteners
Advantages cold extrusion has over hot extrusion - Improved mechanical properties - Control over dimensional tolerances - Improved surface finishes - production rates and cost are competitive - magnitude of stress is extremely high
15.5 Extrusion Defects • Surface Cracking: Cracking on billet materials occurs due to temperature, friction, punch speed. • High Temperatures • Crack from along the grain boundaries. Typically occur in aluminum, magnesium, zinc alloys • Cold Temperatures • Caused by sticking of billet material at the die land • Known has the “Bamboo Defect” because of its similar appearance to bamboo
Pipe: The metal-flow pattern tends to draw oxides and impurities toward the center of the billet • Internal Cracking: Center of extruded product develops cracks. • Attributed to a state of hydrostatic tinsel stress • Cracks increase with increasing die angle, impurities, and decreasing extrusion ratio and friction
15.6 Extrusion Equipment • Basic horizontal press: Suitable because the stroke and speed of the press can be controlled • Vertical Hydraulic press: used for cold extrusions. Take up less floor space
15.7 The Drawing Process • A long rod or wire is reduces or changes by pulling through a die called a “draw die” • Rod and wire is used in many applications such as drive shafts, machine and structural components, welding electrodes, spokes.
Drawing Force: used under ideal and frictionless conditions F = YA * ln(A ۪/ A) • Y- average true stress • A ۪ – initial cross–sectional area • A – Final cross-sectional area
Drawing Force used under frictional forces: F = Y*A* [(1 + u/s) * ln(A ۪ / A) + (2/3)*s] • S – Die angle in radians • U – Frictional forces
15.8 Drawing Practice • Bundle Drawing: Drawing many wires simultaneously • Produces wire in polygonal cross-sections • Theses types of wire are cut into many shapes. Used in medical implant, electrically conductive material, filter media • Reduces cost of production by producing 100 or more.
Die Design: • Die angles range from 6 -15 degrees • Die designs go through many period of trial and error • Die Materials: • Die are made out of typical tool steels and carbides • Cast steels are used for hot extrusions • Diamond dies are used for drawing extremely fine wire • Insert and nibs are used to support carbide and diamond dies because of their lack of tensile strength
Lubrication: • Proper lubrication is essential to improve die life and product surface finish • Particular in tube drawing, because of its difficulty to maintain for a thick lubrication film at the mandrel • Wet drawing – Dies and rod are completely immersed in lubricant • Dry Drawing – Rod is coated in a lubricant by passing through a box filled with lubricant
15.9 Drawing Defect and residual Stresses • Typical defects in a drawn rod or wire are similar to extrusion • Internal Cracking with are longitudinal • Seam may open • Cold–drawn products have residual stresses, this causes stress-corrosion cracking over a period of time • Quality control becomes a major problem in drawn defects and residual stresses
15.10 Drawing Equipment • Draw Bench: • Contains a single die, design is long • Pulling for is supplied by a chain drive that is hydraulically activated
Rotating Drum: • Used in producing long rods and wires of smaller cross-sections less that 13mm • A large drum is rotated
Introduction • We utilize sheet metal for products such as • Cans • Cookware • Appliances • Car bodies • Low carbon steel is the most commonly used sheet metal for its strength and formability • The most general sheet forming operations • Press working • Press forming
Shearing • Shearing metal is like punching holes in paper with a hole punch. • It can also be compared to using a cookie cutter • The punch must be harder and stronger that the metal being sheared
Shearing • Typical features of sheared edges are not smooth nor perpendicular to the cutting plane • Edge quality can be improved by increasing punch speed • Shearing is most commonly done with a punch and die
Having the correct between the punch and die is important Too much and the sheared edge can become rough Too little and tools can be damaged Burr is a thin edge or ridge an the punched out piece of metal Can be caused by dull tool edges The size and shape of the burr can change subsequent forming operations Shearing
Shearing Operations • Die cutting- perforating, parting, notching, lancing • Parts produced by die cutting are commonly used in assembly with other components • Fine blanking- used to produce smooth and square edges • Nibbling- moves a small straight punch up an down rapidly into a die. One advantage of this technique is intricate slots and notches.
Shearing Operations • Slitting- a pair of circular blades follow a line or path cutting as they go. • much like a can opener • Scrap in shearing- The sheets of metal that are not used. • Can be as large as 30% of large stampings • Increases manufacturing cost
Shearing Operations • Tailor-Welded Blanks- Can be used of sheet metal with different shapes and thicknesses (often used in automotive industry) • Reduction in scrap • Eliminate need for subsequent spot welding • Better control of dimensions • Improved productivity
Punches and Dies • Compound dies-Limited to simple shapes • Several operations on the same sheet may be performed in one stroke at one station with a compound die
Punches and Dies • Progressive dies- good for making parts that require multiple operations to produce • Use a series of punches • Other operations include • Laser beam cutting • Water-jet cutting • Friction sawing • Flame cutting
Sheet-Metal Characteristics and Formability • After a blank is cut into a sheet it is formed into various shapes • Elongation- Stretching the metal • High elongation is desirable for good formability • Yield-Point Elongation- Happens in low carbon steel and aluminum-magnesium alloys. Causes stretcher-strain marks, or worms • Elongations, depressions along the surface
Sheet-Metal Characteristics and Formability • Dent-Resistance of sheet metal- refers to the dynamic yield or yield stress under high rate of deformation • Ways to improve dent-resistance is to increase sheet thickness of decrease the metals elastic modulus
Formability Tests for Sheet-Metals • Formability- The ability to of the sheet-metal to undergo the desired shape without failure • May undergo two basic modes of deformation- stretching and drawing • Cupping Tests- earliest tests to determine formability • Erichsen test- Sheet clamped between two circular dies then a steel ball of round punch is forced into the sheet until it cracks