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