LECTURE 3: METAL FORMING/METALWORKING
What is Metal Forming/Metalworking?
Metalworking covers a wide range of processes, all of which involve shaping of the part by the use of plastic deformation (exploit the plasticity of engineering materials). It comes under the category of deformation processes. Requires high force.
Typical Processes Subgroup Typical Processes
Forging Rolling Mill Hot Bulk Rolling Forming Tandem Rolling Mills Extrusion Deformation Processes Wire Drawing
Swaging Cold Forming Roll Forming
Deep Drawing
What is plasticity?
Ability of a material to flow as a solid without deterioration of properties.
What categories of metalworking are there?
• Bulk deformation processes (Rolling, forging, extrusion, drawing) o Thickness/Cross section area is reduced • Sheet-forming/Sheet metal processes (Shearing - piercing & blanking ¬ching & trimming, Bending, deep drawing,) o No cross sectional area change
What is the Basic Rolling Process?
• Metal is passed between two rolls that rotate in opposite directions • Friction propels material forward (required!) • Metal is squeezed and elongates to compensate for decrease in cross sectional area
What Properties does Rolling have?
• Simplest bulk deformation process • Often the first process that is used to convert into a finished wrought product • Thick stock can be rolled into blooms, billets or slabs
How can Rolling be adjusted?
• Smaller or larger diameter rolls • Smaller diameter process less length contact for a full rotation, and therefore requires less force to produce a given change in shape (P = F/A) • However, smaller diameter rolls have a reduced stiffness o Prone to flexing as only supported on the ends o à Use cluster mills to overcome stiffness problem
How do Continuous (Tandem) Rolling Mills work?
• Billets, blooms and slabs are heated and fed through an integrated series of nonreversing rolling mills • Can make moderately complex shapes • Progressive plastic deformation of metal through each rolling mill o Otherwise possible fracture of material
What is Forging?
Step up from rolling. Processes that induce plastic deformation through localized compressive forces applied through dies. It is the oldest known metalworking process. It can produce parts of various size. It has the best quality in terms of mechanical properties of all manufacturing processes. This due to the compressive forces squeezing out defects or porosity.
What is Open-die Hammer Forging?
• Same as forging done by blacksmith, but by equipment • Impact delivered by mechanical hammer • Can be as simple as gravity drop up to computer controlled hammers • Friction must be minimized, unlike rolling
What is Impression-die Hammer Forging?
• Two shaped dies are brought together, and the metal fills the cavity • Upper die is attached to hammer • Lower die attached to anvil • Excess metal (flash) may squeeze out of the die o Ensures die gets filled completely o Forms a ring as it cools and forces the metal back to fill the die • Flashless forging can be done that provides total confinement • Many products done through a series of cavities o First impression is called edging o Intermediate impressions to approximately final shape o Final impressions gives final shape • Also has a parting line • Important design details o Draft required o Number of intermediate steps o Shape of each step o Amount of excess metal o Dimensions of flash o Good dimensional accuracy
What is Extrusion?
Metal is compressed by a ram and forced through shaped die to form a product with a constant cross section. Can create fairly complex shapes, and can be performed hot or cold. Commonly extruded metals are aluminium, magnesium, copper and lead.
Advantages Disadvantages
Many shapes possible that can be produced that Have to do everything in one step, otherwise material aren’t possible with rolling will crumple
Complex in 2D, uniform in 1 direction Complex in 2D, uniform in 1 direction
No draft required
Amount of reduction in single step is only limited by equipment, not material or design
Dies relatively inexpensive
Small quantities can be produced economically
What is Wire/Rod/Tube Drawing?
Similar to extrusion, but force is tensile. Reduction of cross section by pulling it through a die. This is process is most commonly used for wire, and must be careful of maximum tensile force of the material. The process is very quick and can have a continuous process, very good for high volume production.
What is Sheet Metal Forming?
Processes that involve plane stress loadings and lower forces than bulk forming. Almost all are considered secondary processing, as bulk forming is used to initially form the sheet. By definition sheet metal forming is for materials <5mm. Most operations involve a press, punch, or a ram and a set of dies.