Introduction to advanced and flexible sheet forming Flashcards
What are the trends that move towards new manufacturing solutions?
Demographic, regulatory, competitive
What are the different common materials for sheet metal?
Traditional steel, hot-formed steel, aluminum, magnesium, composites
Advantages of traditional steel metal sheet?
-Proven material, in use for decades
- Good forming capabilities
- Good availability
Disadvantages of traditional steel metal sheet?
- High specific weight
- Large number of process steps required (incl. tooling)
Adavantages of hot-formed steel sheet?
- very high strength with good formability
- attractive cost-benefit ratio for weight saving
- variable strength through sophisticated processing (eg. tailored tempering)
Disadvantages of hot-formed steel
- Higher investment, sourcing and operation costs
- Use limited to structural BIW components
Rank the materials according to the specific stiffness (MN.m/kg against price). steels, aluminum alloys, mg alloys and CFRP (Carbon fiber reinforced polymer)
From Lowest to highest price: steels, aluminum alloys, mg alloys and CFRP (Carbon fiber reinforced polymer)
Steels, al alloy and mg alloys have relatively the same specific stiffness (except for some Al alloys which have slightly higher specific stiffness). While the CFRP has potentially more than 3 times the specific stiffness of the steels.
Why do we need advanced and flexible forming processes as an alternative to conventional?
Conventional rigid tools are expensive as they require a long production lead time and are meant to medium to large production batches.
It is difficult to make minor changes to the part design.
Heavy, difficult to handle manually.
Rigid tools may also induce aesthetical defects.
Difficult to work on materials with low ductility (HSS) and with complex geometries.
What are the principal defects in sheet metal forming?
-Excessive thinning/necking/fracture
-Excessive thickening/buckling/wrinkling
Biaxial streching state of stress and biaxial stretching state of stress will fall in which quadrant of the FLD diagram?
1st quadrant
Conventional deep drawing will fall in which quadrant of the FLD diagram?
second quadrant. On this quadrant, larger deformations are possible before fracture.
In which condition do we see the lowest amount of plastic deformation at fracture?
Plane strain condition
By looking at the FLD diagram, we know that our process will not have defects if?
Our map falls below the FLD limit (red line)
How can the FLD be obtained?
- Experimentally, by etching a regular grid on the sheet before deformation and then measuring the deformation of each square of the grid.
- by FEM
Elastic springback is higher for:
- Smaller values of Young’s modulus E
-Higher values of the stress hardening coefficient K - larger values of the ratio p (ro) / t
Advanced sheet forming can be categorized in two “big” categories.
High temperature forming and Flexible forming.
Advanced sheet metal forming subcategory “High temperature forming”, can be divided in turn in two subcategories:
- Hot forming and Warm forming
What is the difference between hot and warm forming?
Both hot and warm forming improve formability, while Hot forming reduces forces and warm forming reduce springback
Flexible forming can be divided in 3 categories:
- Rapid tools
- Flexible media
- CNC forming
Define semi-rigid rapid tools
Rapid or “semi-rigid” tools are made with hard plastics (generally thermosetting if the are machined or thermoplastics if 3D printed).
They do not require thermal or surface tratments nor any grinding operation.
Define flexible tools
The tool is compliant, as in rubber pad forming or fluid forming (e.g. hydroforming)
One of the tools, either the die or the punch is replaced by a flexible medium (rubber or water) and the shaping action is provided by a conventional rigid tool
Define CNC forming
Dieless forming (air forming, air bending).
Part of the tooling setup is not present as in air bending.
- Single point incremental, explosive, electro-magnetic are other processes that do not have significant industrial applications.
Define punch
Convex tool that penetrates the material
Define die
Concave tool that determines the final shape of the part
