Damaging of Al-Mg alloys severely plastic deformed by equal channel angular pressing.
Comaneci, Radu ; Comanici, Adrian
1. INTRODUCTION
ECAP is one of the main processing techniques to produce
ultrafine-grained materials. In ECAP, a billet is pressed through a die
that contains two equal cross-sectional channels meeting at an angle
[phi], with or without fillet corner corresponding to an angle [psi]
(see Fig. 1). Because the cross-section of the billet remains the same
during extrusion, the process can be repeated until the accumulated
deformation reached a desired level.
Obviously, achieving a large amount of strain during ECAP is
essentially for grain refinement. The material must withstand repeated
high strains, without cracking. Unfortunately there are no criteria
which ensure a guaranteed successful SPD process. A good workability of
the material is a necessary condition but not sufficient. Inherent
failures of ECAP if not made a correct process design were reported,
especially billets damages due to the cracking on their upper surfaces
(Kim, 2006). As the microstructure and the mechanical properties of the
severely plastic-deformed material are directly related to the
deformation, most researches are focused on deformation behaviour in
terms of strain distribution or working load level (Li et al., 2004).
There is lack of systematic studies to understand damaging in ECAP
[FIGURE 1 OMITTED]
In this paper, a FEA is performed to evaluate the damage during
ECAP, depending on die geometry and process parameters. The results of
damage prediction were confirmed by experimental data for AA5052. In the
future, we must optimise the die designs using damaging analysis in
order to accurately predict the material behaviour in SPD processes.
2. MATERIAL AND PROCEDURE
A commercial non heat-treatable AA5052 with a composition, in wt.%,
of Al-2.8%Mg-0.2%Cr was used in experiments. Specimens with dimensions
of 10x10x60mm were machined from as-received alloy. The experiments were
conducted at room temperature with a ram speed of 8.75mm/s, using dies
with channel angle ^ of 90[degrees] with and without fillet corner of
the channels.
Commercial finite element code DEFORM 3D was used for the
simulations. The workpiece was discretized in 8000 tethraedral elements.
The friction coefficient 0.12, Poisson's ratio 0.33, and
Young's modulus 69GPa together with isotropic strain hardening of
the material were assumed. The tolerance, positioning of the workpiece
and top/bottom die, convergence criteria, re-meshing conditions, and
boundary conditions were specified before the execution of the
simulation process.
Three design scenarios are analyzed by FEA to reveal the
deformation behaviors and their relationship with the design
configuration (90_R_r, where R and r are outer and inner corner radius
of the two die channels, respectively):
A--die with no arc transition R, r = 0 mm; (90_0_0)
B--die with outer arc transition R = 4 mm; r = 0 mm (90_4_0)
C--die with inner arc transition R = 0 mm; r = 2 mm (90_0_2).
To estimate the damage, the Cockcroft-Latham model was used.
According to this model, a damage factor ([D.sub.f]) is defined by the
following relationship (Figueiredo et al., 2007):
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)
where [[sigma].sub.T] is the maximum principal tensile stress,
[[sigma].sub.- ] is the effective stress, [d[epsilon].sup.-] is the
effective strain increment and the integral is evaluated from zero
strain to the final effective strain, [[epsilon].sup.-.sub.f]. The
effective strain for one ECAP pass is (Iwahashi et al., 1998):
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)
where the significance of terms is revealed in Fig. 1.
This form of the Cockcroft-Latham relationship is generally
considered to provide a good prediction of the fracture of metals during
ECAP.
To determine the critical damage [D.sup.*.sub.f] corresponding to
the failure of the material, a compression test was performed for
AA5052. It reveals that accumulated damage at failure was
[D.sup.*.sub.f] = 0.310. When [D.sub.f] exceeds [D.sup.*.sub.f], the
cracking takes place.
3. RESULTS AND DISCUSSIONS
Using damage factor ([D.sub.f]), as defined in Eq. (1), Fig.2 shows
damage distributions for the three scenarios. The highest level of
damage (0.850) corresponds to 90_0_0 die. Outer fillet corner of the die
channels (90_4_0 die) determines only a small decreasing of maximum
damage (0.654). A significant change takes place for inner fillet corner
of the die channels (90_0_2 die) when the level of damage down to the
value of 0.230, due to increasing compressive deformation component
(Yoon & Kim, 2008).
In accord with failure condition ([D.sub.f] > [D.sup.*.sub.f]),
the damage should appear for the first two cases (A and B). Indeed,
experimental data confirm this hypothesis. Fig. 2 shows massive billets
segmentation on upper surfaces of the billets for 90_0_0 and 90_4_0
scenario.
The nature of cracking on upper surfaces of the billets can be
depicted from stress distribution. As it is shown in Fig.3, the maximum
principal stress was registered on the upper surface of the billet.
Furthermore, stress distribution gives an understanding of
Cockcroft-Latham model. As we can see from Fig.3, the outer arc
transition between channels increases [[sigma].sub.T] (187MPa) but the
damage factor slowly decreases from 0.850 to 0.654. In opposition, inner
arc transition between channels both decreases [[sigma].sub.T] (91.4MPa)
and [D.sub.f] (0.230). Regarding [[sigma].sub.T], between 90_0_0 and
90_0_2 scenario, small differences are registered, but material behavior
is dramatically changed. This means that [[sigma].sub.T] itself is not
an absolute damage criterion, while normalized
[[sigma].sub.T]/[[sigma].sup.-] gives the real measure of the failure in
ECAP. In this way, the maximum principal stress distribution gives the
explanation of cracks on upper surfaces of the billets (Figueiredo et
al., 2007).
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
4. CONCLUSIONS
The evolution of damage during ECAP of 5052 Al-Mg alloy was
investigated by FE simulation. The damage was studied for different tool
designs.
It was found that inner fillet corner of the die channels has the
most significant influence on damaging in ECAP. The results were
confirmed by experiments. It is also explained the cracking of the
billets by stress distribution on upper surfaces of the workpieces. The
paper leads to more accurate design of the dies taking into account the
material behaviour according to the predict damage. The next step in our
research plan will be to correlate the results of damaging prediction
with process design to ensure the success of severe plastic deformation.
5. REFERENCES
Figueiredo, R.B.; Cetlin, P.R. & Langdon. T.G. (2007). The
processing of difficult-to-work alloys by ECAP with an emphasis on
magnesium alloys, Acta Materialia, Vol.55, 2007, pp. 4769-4779, ISSN 1359-6454
Iwahashi, Y.; Horita, Z.; Nemoto, M. & Langdon, T. G. (1998).
The process of grain refinement in equal-channel angular pressing, Acta
Materialia, Vol. 46, 1998, pp. 3317-3326, ISSN 1359-6454
Kim, H.S. (2006). Analytical and numerical modeling of strain and
strain rate in ECAP, Key Engineering Materials, Vol. 306-308, 2006, pp.
965-970, ISSN 1013-9826
Li, S.; Bourke, M.A.M.; Beyerlein, I.J.; Alexander, D.J. &
Clausen, B. (2004). Finite element analysis of the plastic deformation
zone and working load in equal channel angular extrusion, Materials
Science and Engineering A, Vol. 382, 2004, pp. 217-236, ISSN 0921-5093
Yoon, S.C. & Kim, H.S. (2008). Finite element analysis of the
effect of the inner corner angle in equal channel angular pressing,
Materials Science and Engineering A, Vol. 490, 2008, pp. 438-444, ISSN
0921-5093