Traditional Manufacturing Processes Casting Forming Sheet metal processing Cutting Joining Powder- and Ceramics Processing Plastics processing Surface treatment
FUNDAMENTALS OF METAL FORMING Overview of Metal Forming Material Behavior in Metal Forming Temperature in Metal Forming Strain Rate Sensitivity Friction and Lubrication in Metal Forming
Metal Forming Large group of manufacturing processes in which plastic deformation is used to change the shape of metal workpieces The tool, usually called a die, applies stresses that exceed yield strength of metal The metal takes a shape determined by the geometry of the die
Stresses in Metal Forming Stresses to plastically deform the metal are usually compressive –Examples: rolling, forging, extrusion However, some forming processes –Stretch the metal (tensile stresses) –Others bend the metal (tensile and compressive) –Still others apply shear stresses
Material Properties in Metal Forming Desirable material properties: –Low yield strength and high ductility These properties are affected by temperature: –Ductility increases and yield strength decreases when work temperature is raised Other factors: –Strain rate and friction
Bulk Deformation Processes Characterized by significant deformations and massive shape changes "Bulk" refers to workparts with relatively low surface area ‑ to ‑ volume ratios Starting work shapes include cylindrical billets and rectangular bars
Basic bulk deformation processes: (a) rolling
Basic bulk deformation processes: (b) forging
Basic bulk deformation processes: (c) extrusion
Basic bulk deformation processes: (d) drawing
Material Behavior in Metal Forming Plastic region of stress-strain curve is primary interest because material is plastically deformed In plastic region, metal's behavior is expressed by the flow curve: where K = strength coefficient; and n = strain hardening exponent Stress and strain in flow curve are true stress and true strain
Flow Stress For most metals at room temperature, strength increases when deformed due to strain hardening Flow stress = instantaneous value of stress required to continue deforming the material where Y f = flow stress, that is, the yield strength as a function of strain
Temperature in Metal Forming For any metal, K and n in the flow curve depend on temperature –Both strength and strain hardening are reduced at higher temperatures –In addition, ductility is increased at higher temperatures
Cold working is metal forming performed at room temperature. Advantages: better accuracy, better surface finish, high strength and hardness of the part, no heating is required. Disadvantages: higher forces and power, limitations to the amount of forming, additional annealing for some material is required, and some material are not capable of cold working. Warm working is metal forming at temperatures above the room temperature but bellow the recrystallization one. Advantages: lower forces and power, more complex part shapes, no annealing is required. Disadvantages: some investment in furnaces is needed. Hot working involves deformation of preheated material at temperatures above the re crystallization temperature. Advantages: big amount of forming is possible, lower forces and power are required, forming of materials with low ductility, no work hardening and therefore, no additional annealing is required. Disadvantages: lower accuracy and surface finish, higher production cost, and shorter tool life.
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
Lubrication in Metal Forming Metalworking lubricants are applied to tool ‑ work interface in many forming operations to reduce harmful effects of friction Benefits: –Reduced sticking, forces, power, tool wear –Better surface finish –Removes heat from the tooling
Considerations in Choosing a Lubricant Type of forming process (rolling, forging, sheet metal drawing, etc.) Hot working or cold working Work material Chemical reactivity with tool and work metals Ease of application Cost
Definitions Plastic Deformation Processes Operations that induce shape changes on the work piece by plastic deformation under forces applied by various tools and dies. Bulk Deformation Processes These processes involve large amount of plastic deformation. The cross-section of workpiece changes without volume change. The ratio cross-section area/volume is small. For most operations, hot or warm working conditions are preferred although some operations are carried out at room temperature. Sheet-Forming Processes In sheet metalworking operations, the cross-section of work piece does not change—the material is only subjected to shape changes. The ratio cross-section area/volume is very high. Sheet metalworking operations are performed on thin (less than 6 mm) sheets, strips or coils of metal by means of a set of tools called punch and die on machine tools called stamping presses. They are always performed as cold working operations.
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 workpiece 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)
Rolling Definition Rolling is a Bulk Deformation Process in which the thickness of the work is reduced by compressive forces exerted by two opposing rolls
22
Rolling
Important Applications: Steel Plants, Raw stock production (sheets, tubes, Rods, etc.) Screw manufacture
Rolling Basics Sheets are rolled in multiple stages (why ?) Screw manufacture:
Forging Definition Forging is a Bulk Deformation Process in which the work is compressed between two dies. According to the degree to which the flow of the metal is constrained by the dies there are three types of forging: Open-die forging Impression-die forging Flash less forging
Forging
Stages in Open-Die Forging (a) forge hot billet to max diameter (b) “fuller: tool to mark step-locations (c) forge right side (d) reverse part, forge left side (e) finish (dimension control) [source:
Stages in Impression-die (Closed-Die) Forging [source:Kalpakjian & Schmid]
Stages in Impression-die (Closed-Die) Forging
Flash less forging
Forging grain flow
Quality of forged parts Stronger/tougher than cast/machined parts of same material Surface finish/Dimensional control: Better than casting (typically) [source:
A material is pushed or drawn through a die of the desired cross- section.Any solid or hollow cross-section may be produced by extrusion, which can create essentially semi-finished parts. The metal can forcing through a die in the same direction or opposite direction.
Extrusion Typical use: ductile metals (Cu, Steel, Al, Mg), Plastics, Rubbers Common products: Al frames of white-boards, doors, windows, …
Extrusion: Schematic, Dies Exercise: how can we get hollow parts?
The cross-sections that can be produced vary from solid round, rectangular, to L shapes, T shapes. Extrusion may be continuous (theoretically producing indefinitely long material) or semi-continuous (producing many pieces). Extrusions can be done with the material hot or cold. Commonly extruded materials include metals, polymers, ceramics, and foodstuffs.
Extruded products Typical products made by extrusion are railings for sliding doors, tubing having carious cross-sections, structural and architectural shapes, and door and windows frames. Extruded products
Direct extrusion: A metal billet is located into a container, and a ram compresses the material, forcing it to flow through one or more openings in a die at the opposite end of the container. Indirect extrusion: The die is mounted to the ram rather than at the opposite end of the container. One advantage of the indirect extrusion process is that there is no friction, during the process, between the billet and the container liner.
Drawing Commonly used to make wires from round bars Similar to extrusion, except: pulling force is applied
WHAT is DRAWING? 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. The principle of this procedure consist of reducing the thickness of a pointed,tapered wire by drawing it through a conical opening in a tool made of a hard material.The wire will take shape of the hole.
Drawing improves strength and hardness when these properties are to be developed by cold work and not by subsequent heat treatment Where is it used? This process is widely used for the production of thicker walled seamless tubes and cylinders therefore; shafts, spindles, and small pistons and as the raw material for fasteners such as rivets, bolts, screws.
DRAWING TOOLS The most important tool in the drawing process is without doubt the drawplate.This consist of a plate of high grade steel into which similar shaped holes have been placed whose size in evenly reduced from one hole to another. The most common drawplate have round holes and are used to reduce the size of round wire. Drawing wire with the draw tongsdrawbench
How such a drawplate hole is made