In general, the cold roll-bonding process made clad metal that has many advantages, especially accurate dimension control and straight bonding layers.
However, because the clad metals in this research are produced at room temperature, the residual stresses generated in the rolling process cannot be released by the conventional annealing process because the melting temperatures of the individual layer sheets are often different from each other. Therefore, the effect of residual stresses on the secondary formability of the clad metal sheet is significant.
In this section, the specimens of Al/Cu clad metal sheets were obtained through a cold roll-bonding process. Second, the mechanical properties of clad metal sheets with different initial thicknesses were measured by tensile tests. Then, the punch stretching test was carried out to determine the forming limit data of Al/Cu clad metal sheets with different initial thickness ratios. After that, deep drawing tests of clad metal sheets using a square punch were carried out. The deep drawing tests were also simulated with ABAQUS to validate the possibility and accuracy of using FE models.
During this verification process, fracture initiations during the deep drawing of the clad metal sheets were carefully inspected.
3. 1 Cold rolling process and mechanical property test 3.1.1 Experimental procedure for roll-bonding
Specimens of Al/Cu clad metal sheets were prepared in the following manner.
The base materials were aluminum sheets with thicknesses of 2.0, 1.5, and 1.0 mm and copper sheets 1.0mm thick. The two-ply clad metal sheets were arranged as illustrated in Fig. 3.1. Before the roll-bonding process, all contact surfaces of the base
‐ 30 ‐
order to enhance the bonding performance of the Al/Cu clad metal sheets. The rolls were set to a small roll-gap opening, resulting in a nominal deformation. However, the final thickness of the clad metal sheet was always larger than the initial roll-gap opening, which is attributed to elastic recovery of the blank. Table 3.1 shows the list of thickness combinations of the clad metal sheets. All Al/Cu clad metal sheets were made by the cold rolling process, as shown in Figure 1.1.
Table 3.1 Different thickness combinations of clad metal sheets
Sample Initial thickness (mm)A1050
Initial thickness (mm) C1100
Final bonded thickness
(mm)
Stages of Rolling
1 2.0 1.0 1.3 1
2 1.5 1.0 1.3 1
3 1.0 1.0 1.3 1
4 2.0 1.0 0.97 2
5 1.5 1.0 0.97 2
3.1.2 Tensile test of clad metal sheets
In order to measure the mechanical properties of the Al/Cu clad metal sheets, tensile tests were carried out on a MTS-810 tensile machine. In this research, all specimens of Al/Cu clad metal sheets were produced at room temperature, and thus the residual stresses of two base materials cannot be released by conventional annealing process. After the roll-bonding process, the specimens were very thin, and it was difficult to obtain the flow rule of each of the two materials. Based on the iso-strain deformation behaviors of Al/Cu clad metal sheets, these sheets were considered to be equivalent to a single material in the FE simulation. Therefore, the fracture of Al/Cu clad metal sheets was predicted according to the forming limit data of the equivalent
sing ets with thr
mm/Cu 1.0 Cu clad met ritical site ar the locus o ration can b greater elon area of the
.
1 shows the ree different tal sheets. I re plotted at of the strain be drawn. T ngation. If t
ellipse afte
e true stress t initial thic Al 1.0 mm/C
These tens
stress–strai
est (formin
g tests were In the formi t the onset o
combinatio The major a the area of t
g limit test)
e carried ou ing limit di of visible, l ons that will axes of the e the original ion, the thic
ves obtaine binations (A ) and two di
ties were u
f the Al/Cu c
)
ut to evaluat agram, the ocalized ne l produce fa ellipses are l circle befo ckness of th
d for the A Al 2.0 mm/
ifferent bon used in the
clad metal s
te the form major and m ecking in a d
ailures in an parallel to ore deforma he sheet has
Al/Cu clad m /Cu 1.0 mm nded thickne e following
sheets
ming limit of minor strain deformed sh n actual form the directio tion is less s changed a
metal
‐ 32 ‐
methods were used to construct the diagram. First, the clad metal sheets were clamped at their edges and stretched by a 50 mm diameter hemispherical punch, as shown in Figure 3.3. Specimens 100 mm long and of various widths from 10 mm to 100 mm were prepared to cover various stretch modes. Talcum powder was used for lubrication; the blank holding force was set to 160 KN. The aluminum side of the Al/Cu clad metal sheets was in contact with the punch. Before these tests, the surfaces of the copper side of the Al/Cu clad metal sheets were etched with circular meshes so that the major and minor strains could be measured after the stretching test.
Figure 3.2 (a) Tooling dimension; (b) 50 ton universal material test machine (a)
(b)
Blank
3.1. tching test wn in Figur obviously h ing the form m has a low
ated to the nd of the FL ing the roll-nificant fac ondary form
igure 3.3 FL ults
al thickness
ed condition are shown re 3.3(b). Th higher than ming test. O wer ductility reduction r LDs of the
-bonding pr ctor for rai ming proces
Major strain (%)
combinatio
ns of the Al in Figure 3 he trend of t
those in ot On the other y limit. Acc
rate becaus clad metal rocess. Ther ising the fo
s.
Al/Cu clad m
0 ther cases r hand, the cording to e the final sheets; this refore, the h forming lim g limit curve
and thus in clad metal this result, thickness i s is due to t higher reduc mit of the
s for differe
6
Al 2.0mm Cu 1.0m
Al 1.5mm Cu 1.0m
Al 1.0mm Cu 1.0m
m, and 1.0 m
clad metal fferent thick e of Al 2.0 m ndicates be
sheet of Al the initial s the same) the work-ha ction rate of clad metal