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A Finite Element Study of the Deformability of Steel
Jingyi Wang
Qi Rui
Jiadi Fan
Background
punch
dieholder
A real-life problem
Use finite element analysis software to simulate the stamping process of bakeware
Background
Improper forming condition will lead to defect.
The wrinkling and fracture defect during deep drawing process
Manufacturing and fixing stamping mold is expensive
Simulation needed to test whether certain mold and forming condition is reasonable before manufacturing
Method
Experimental and Empirical Analysis
3D model with different parts under dynamic loading– Use ABAQUS
Different forming conditions
Temperatures
Strain rates
Holding force
Compare, design and optimize forming condition to avoid possible defect
ABAQUS
Wrinkling
Why wrinkling happens?
During the deep drawing process, metal flows inside. From large perimeter area to small perimeter area.
Under minimum principal stress, the blank will be thickened. Uneven thickening will lead to wrinkling.
We need a reasonable holding force to provide a restriction.
Why fracture happens?
The friction between holder and blank, die
and blank will block metal from flowing.
If the friction is too big, the metal at the corner will fracture because of over-thinning.
If the thickness after deformation reduces to 70% of the original thickness or less, we treat it as fracture.
Fracture
As temperature rises, the deformability of metal will improve.
However, good deformability may lead to the over-thinning at the corner.
Higher temperature is also more energy expensive. So, forming temperature is a parameter that need to be balanced.
Suggested temperature (dependent on holding force, material, etc): 550-850 ℃
Temperature effect
Temperature ℃
Maxim
um
str
etc
h d
epth
(m
m)
SimulationExperiment
Under large strain rate, the deformability of metal is poor. It will be more likely to generate fracture.
Small strain rate decreases the productivity.
In industrial process, strain rate is also a design parameter.
Strain rate effect
Oversize holding force can lead to fracture defect
An undersize holding force can lead to wrinkling defect
Dependent on details of the object
Holding force effect
Holding Force (MPa)
Maxim
um
str
etc
h d
epth
(m
m)
SimulationExperiment
Element type S4R: 4-node general-purpose shell, reduced integration
with hourglass control, finite membrane strains
Membrane theory
Target geometry after deep drawing
A realistic geometry2D and 3D modelAnimation
One quarter of the entire model
holder punch die
Simulation model
42CrMo high-strength steel
Young’s moldus: 210Gpa
Poison's ratio: 0.31
Density: 7,830 kg/m^3
True stress-strain curve
Material property
Result—different temperatures
The thickness at the corner is smaller under higher temperature. As the thickness ratio are both lower than 70%. It’s unnecessary to simulate a higher temperature.
Strain rate: 1, holding force: 10,000NTemperature: 600 ℃ and 650 ℃
T=600 ℃ T=650 ℃
The thickness at the corner is smaller under higher strain rate
Temperature: 600 ℃, holding force:10,000N Compare strain rate : 0.1 and 1
=0.1 =1
Result—different strain rates
Result–different holding forces
Reasonable holding force range: 7,000~13,000N
F=5000N F=10,000N F= 30,000N
Refine the size of blank
After the deep drawing process, the extra blank needs to be cut off.
The former blank is 400*400mm, it will cause a huge waste of material.
Refine it to 300*300mm and 240*240mm.
400*400mm 300*300mm 240*240mm
Result–size of the blank
240*240mm good enough for our bakeware
Conclusion
In our particular case, the best forming conditions are
Temperature: T=600℃
Strain rate: =0.1
Holding load: F=7,000-13,000N
Original Blank size: 240*240mm
This is just a simple FE application. ABAQUS is able to do very complicated problems. In our case, the geometry of production is simple. However, in more complicated cases, we need to consider much more.
Thank you for your listening