Deep drawing of fiber metal laminates for automotive lightweight structures

Abstract :
At present challenge in automobile industry is to reduce the consumption of fuel and reduce CO2
emissions into the atmosphere. Reduction in weight of the car is the primary focus as of now. Body
weight of the car contributes large amount to the weight. Weight can be highly reduced by the usage of
composite materials. On the positive part composites are light weight , high stiffness. But, on the negative
part composites are costly and labour intensive, not suitable for high production. A development to this is
made by combination of high-strength steel alloys and CFRP pre-pregs in a special hybrid material/fiber
metal laminate (FML).

Introduction :
A solution to light vehicle cars with high stiffness to weight ratio is the combination of high strength steel
alloys and CFRP pre-pregs in a hybrid material/fiber metal laminate. The CFRP patches are enclosed by the
top layers and do not come into contact with the tool surfaces. Compared to forming purely composite
materials, the forming of fiber metal laminates is more economic. The forming process introduces complex
tension-pressure stresses and multiaxial tension stresses into the semi-finished parts. Direct replacement of
sheet metal parts by fiber metal laminates is not possible for the following reasons : metal and CFRP have
dissimilar material behavior. Hence research has to be conducted into the process and material design.
In manufacturing processes such as deep drawing, combined pressure-tension stresses and tension-tension
stresses are induced in the flange, which is the main forming zone.

Literature review :
[1] The analysis of the obtained thermal measurements (temperature, heat flux) shows the required
sensitivity threshold of sensitivity of thermal sensors to be able to detect on-line the rate of thermal reaction.
Thus, an adapted sensor body with an optimized signal conditioning has to be developed.
[2] A sensor system will be introduced that allows the measurement of the global draw-in amount of a drawn
panel. Further, an FEM model is presented that allows the determination of the effective range of these
actuators.
[3] The embossing of grooves itself is a bulk metal forming process and, therefore requires volume elements.
The first model, which has integrated volume elements, simulates the folding of single folding cells
targeting to detect the interaction of the embossing and the subsequent bending process. In the second
model, shell elements are used, with a focus on the effect of the grooves’ run rather than the effects of the
embossing itself.

Methods:
Experimental setup :
1. Tests were carried out at LUF for developing an advanced tool and process design with which to
examine the forming behavior of fiber metal laminate sheets during the forming process and the
resulting shape and dimensional accuracy of fiber metal laminate parts. The effects of combined tensile
 pressure stresses and bending stresses on semi-finished parts consisting of sheet metal and CFRP were
investigated using special cup-design fiber metal laminate specimens.

2. The hybrid specimen consists of two sheet blanks (HCT490X, s = 0.55 mm) having initial diameter of
180mm and two layers of CFRP pre-pregs (initial diameter of 180mm).

3. A temperature stable deep-drawing film was used to reduce friction between tool and the specimen
surfaces.

4. Setup consists of stamp,blank-holder, and die.

5. Tool elements can be heated using heat catridge.

6. The cylindrical stamp has diameter of 100mm and a radius of 10mm.

7. The die has an inner diameter of 105mm and a blanking radius of 10mm.

8. The drawing depth is 38mm.

9. The tools were heated upto 1700 C .

10. After placing hybrid sheets in the forming device and the forming procedure was started.

11. After forming, parts were cured at 1700 C for 7 minutes.

13. The semi-finished component used for the first experiments consisted of two top layer circular sheet-
metal blanks and two layers of CFRP prepregs.

14. The single layers of the CFRP prepregs were arranged with 0° and 90° fiber orientation between the top
layer sheets, giving the hybrid sheet an orthotropic two-layer design.

Defects found :
1. Buckling , wrinkling ,breaking of rovings and fibers were identified.
 2. Delamination of flange areas were seen.

3. High variation of CFRP thickness at the bottom of the part were noticed.

4. High local thinning of CFRP wall thickness.

Analysis :
1. A method was found to reduce the above occurring defects such as buckling ,and wrinkling of the
rovings.
2. Aim of this method is to place and align the CFRP patches in oriented directions so that only tensile
stresses affect the rovings during forming process.
3. The CFRP patches were arranged between the two circular top layer sheet metal plates .
4. The orientation of the fiber axis varies between 0° and 135°, so the CFRP prepreg has an orthotropic,
radial design.

5. Largest area has an orientation of 00 . Next largest area is the two 900 orientation . corners were
covered with triangular CFRP patches of 450 and 1350 orientation .

6. The second design is same except for rotated through 900 compared to the first layer.

7. Due to this radial fiber orientation, the rovings are not compressed in the fiber direction by tangential
pressure stresses in the main forming zone during the deep drawing process.

8. Finally, wrinkling and buckling of the CFRP patches were highly reduced.
9. The height of the wrinkles and hence the fiber defects , like the buckling and breaking of rovings,
were scaled down by 62% from 1.59 mm to 0.68 mm. The delamination of the flange was
significantly reduced in this way.

10. To reduce local thinning two methods were used.

11. The first approach focuses on the direct force transmission between the metal sheets with a constant
gap. With this approach, no pressure is applied to the uncured resin. Feeler gauge stocks with
different thicknesses were thus used. In this way, it is possible to achieve constant thicknesses in the
radii areas but the bonding between the CFRP patches and the metal sheets attains only a fairly low
strength due to the large-area surface contact between the feeler gauge stock and the metal sheets,
which means that the bonding area is significantly reduced.

12. The second approach focuses on the use of filler materials to achieve a defined gap between the
metal sheets and thus homogenous thickness distributions of the CFRP patches in the radii areas.
Glass beads were therefore used. Glass beads have only punctual contact with the top layers of the
fiber metal laminate, without having a major negative impact on the bonding area. No further cutting
operations for the CFRP prepregs are necessary. By using glass beads with different diameters,
specific wall thicknesses can be achieved in the CFRP. With this approach, the absolute thickness
deviation has been scaled down by 51% from 0.13 mm to 0.06 mm.

13. An approach to reduce the inhomogeneous wall thickness distributions in flat and bottom areas of the
deep drawn components is the local pre-curing of the inner CFRP patches inside the fiber metal
laminate sheet.
 14. A pre-curing device was thus installed inside a tensile test machine in order to apply a defined
pressure for a certain amount of time until 80% of the cross-linking degree of the matrix material is
reached.

15. After pre-curing and forming, the scatter of the wall thickness at the bottom area has been reduced by
73%, from 0.84 mm to 0.10 mm.

Conclusion :

1. The primary focus of this paper was to reduce failures like buckling, wrinkling of the rovings during
forming process.

2. The wrinkle height inside the deep drawn hybrid cup was reduced by almost 65%, from 1.59 mm
down to 0.68 mm.

3. The shape and dimensional accuracy of deep drawn parts has been highly improved by introducing
pre-curing processes before the actual combined forming and curing process is conducted.

4. The thickness distributions of the radii areas have been improved by defining the gap between the
metal-sheet top layers with filler material. With these strategies, the thickness scatter was reduced by
73% and the thickness deviation in radii areas of the FML cups was reduced by 51%.

References :

1. Thermal sensors to control polymer forming. Challenge and solutions