Metal forming Flashcards
Where is the sheet thinest when drawing and where is it thickest?
If blankholder force too high:
- Risk of to much stretching
- Fracture
At the top the material is compressed and that creates thicker material
The blankholder prevents wrinkling
the material is the thinnest at the bottom.
What i smultiple step formning?
The drwing is done in several steps, important if large depth and smal diameter,
What is anisotrophy ?
- Single grains and oxides are eleganted in the rolling direction thus resulting in a “mechanical fiber sturctur”
- The crystallographic direction is rotated within each single grain in the rolling drection resulting in “crystal texture”
-> The “ defomation resistance” in the plate material varies depending on direction od deformation e.i. anisotrophy normal to the plane and in the plane
What is earing?
Its created when its not enough blank holder force which creted uneven end of the draw.
What is work hardening?
In the nech (local thinning) strain hardening is larger than in the sourrounding material
-> surronding material is less strain hardened and will therefore deofrm more easily.
high strain hardenign gives a wide neck, low strain hardening createds a small necking.
What is strech formning? and what is drawing beads for?
first the materil is streach out then its lays over a form that bends is after the form.
Drawing beads
- To control the material flow during a press operation
- In drawing to be able to control degree of streching or material flow
What is bening? important factors? Bottoming and crakc initiation?
Important factors:
- Springback (the material goes back a bit)
- Crack initiation at the outer edges
- Minimun bend radius
- Workpiece length
- Bending near holes
Bottomning:
Have a form under that the bening is done in so both tool over and under.
- For minimizing spring bakc
- Full plastical deformation in bottom
Crack initiation:
- Avoid bending lines in rolling direction beacuse of anisotrophy.
- Avoid to have burr side on the tensile side, burr side are different changes in the surface
What to types of shearings is it? How does the tool wear and burr hight affects with the number of operations?
- Parallel blades
- Punvh and die (closed conture)
The tool waer increase by muber of operations
The burr hight increases with the number of operations
What is formability? And what properties affect formability ?
Formability is the possiblity to form plastically wothout fracture (or other errors: wrinkling, springback).
- strain hadening ability-
- at beginning of necking, the neck is strengthed (by strain hardening) and deformeation continues outside of the neck, thuse effecting the forming possibility
- Anisotrophy-
- If the material will deform more easily in the plane than in the thickness direction this leads to better formability
- The stress strain-
- Where brittleness is naturally unwanted sould bemantioned that within a material tyoe say steel- the material strength is an indocotor. typically more high strength materials are more brittle
- Planar anisotrophy-
- Which causes earing, material deforms moreeasiliy in different directions in the plane, This does not cause failure typically so mostly it is not considered.
What is the limitation curve?
When testing a material you get a limitation curve, this helps to see if the test for a specific component is in the right place for the materials imitation curve.
If it is over it is a risk of failure. If it is right under the tip its perfect. It is under but to the left it is a wrinkle risk.
How does hydroforming work? why is the pressure important?
- The press i sloseing and
- filled with pressure medium.
- Application of horizontal cylinders, water pressure control, control of counter pressure punch
- Press opening, ejection of part
The pressure is important:
If it is to high it will couse cracks
If it is to low it will couse wrinkles
What is a typicalle process chain for hydroforming parts?
- Pre- forming: normally radial or/and axial bending
- Hydroforming
- Post-processing: for eample additional hole making
What are the advantages of hydroforing?
Economy
- Elimination of welding operations
- Dimished number of parts
- Lower tooling costs
Design aspects
- Relatively high fleibility/ freedom of design
- Good for narrow spaces “packing” of comples parts
- More uniform (less localised) wall thinning
- Strain is more uniform work- hadening
Quality
- Geometrical tolerance (outher surface), tight fit ti die
Weight reduction
- No welding flanges
- Optimised design
What are de disadvantages with hydroformning?
- Futher joining of hydroformed parts maybe more difficult
- Uses dry lubricants which are costly
- Dry lubricants can be difficult to remove
- Relatively long cycle time for the hydroforming
- Handling time realitively long
- Still- there are restictions in size and shape
- Deformed mittle part on tube hard.
What is the principle of rollforming?
Consecutive bending in between rools multiple steps nr depending on: thickness and strength, complexity of shape and tolerance
