Flashcards in Forming Deck (22):
What is cold working?
When the temperature is below around 30% of the absolute melting temperature of the material
Benefits of cold working
Improve strength and hardness of materials but reduces the ductility.
Gives a better dimensional accuracy and surface finish than hot working
What is hot working?
Occurs at a temperature at 60% above the melting point where the material is able to recrystallise
Benefits of hot working
Give improves mechanical properties as the grain structure is refined. Reduces the flow stress as molten metal flows more easily
What is warm working
working between 30% and 60% of the absolute melting temperature
How is metal forging carried out?
It involves the compression of metal between two surfaces. It can be done at room or elevated temperatures, depending on the mechanical properties and forces
What is open die forging?
Using a process named upsetting, a workpiece is placed between two die surfaces. The shape that results depends on the friction that occurs. Low friction results in homogeneous deformation
What is barrelling?
It is a result of high friction between the dies. It can also occur as a result of using a hot work piece between cold dies.
Why is closed die forging used?
It is more accurate and gives a better surface finish than open die forging
A flash is produced under pressure giving rise to a large resistance
Precision die forging
The amount of material processed is precisely to illl the cavity. The volume of metal is accurately controlled. It requires higher loads and harder tools, but reduces subsequent machining.
Softer metals such as Au and Mg alloys are used. complex deformation can occur,
Surface cracks are where materials have either cooled too quickly or strained too much
laps arise from buckling of sections
cold shuts are where the die design allows the metal to fold back on itself
Design rules for forging
Appropriate fillet and corner radii
reasonable draft angles
all forging components will have anisotropic materials caused by the flow of grains
parting line should always pass through the maximum area possible
Minimum recommended thickness for ribs and webs
Punchout holes can reduce weight and provide clearance
What is rolling used for?
Generating plates, bars, rods, pipes
How does rolling occur?
warm/hot metal with finish normally carried out cold. In rolling operation the speed of the slab at entry must be less than the speed at exit.
Deformation has two zones where the metal or the roller is moving faster,there is slip in each zone. Inbetween is the no slip point this is at the highest point of pressure
Four common types of rolling mills
Two high - simplest
Three high - more productivity per roll
Four high - small roll to do the work reducing the force and power requirements
Sendzimir - lots of rolls, very rigid and accurate. normally for cold rolling
How does metal extrusion work?
Simply the pressure is applied to a metal billet and forced through a die (which has a smaller area than the billet) the metal emerging with the shape of the die.
Which ways increase the pressure in reality of metal extrusion?
Friction - if it is high then it is easy for the material to shear than to flow so this can be reduced by using an angled or tapered die
Redundant work is plastic work used in unnecessary shearing of the material
surface cracking caused by high speed
skin inclusion caused by too much friction
Design rules for metal extrusion
Solid shapes and cheap and easy
uniform wall thickness
fillet radii on corners
complicated shapes can be produced by joining two simpler shapes
What is metal drawing?
Same as forward extrusion but the work is pulled rather than pushedIt is done cold so the material has improved properties as a result
used for small area, tubes, wire and rod