Corrosion studies of laser-formed metallic alloy sheets
Z Liu
Corrosion and Protection Centre, School of Materials, University of
Manchester, Manchester, M60 1QD, UK
zhu.liu@manchester.ac.uk
Abstract Previous studies of laser forming have been focused on the variation of
bending angle/shape with laser operating conditions and modelling of the forming
process. Although microstructures and mechanical properties have received some
attentions, no work has addressed corrosion behaviours of laser-formed materials.
However, laser forming is a thermal process, sensitisation of microstructures to
corrosion may be anticipated, depending upon the forming conditions. The aim of the
present work is to investigate the corrosion behaviour of various laser-formed metallic
alloy sheets, including AA 2024-T3, AA 7075-T6, and AA 5083-O aluminium alloys,
304 austenitic and 430 ferritic stainless steels. The results showed that laser forming
process induced different degrees of sensitization within irradiated zones and heataffected zones (HAZs), leading to significant intergranular corrosion attacks for most
of the alloys. However, an improved resistance to intergranular corrosion after laser
forming was observed for AA 5083-O alloy.
1. Introduction
Laser forming is a technique for modifying the curvature of sheet metal by thermal residual
stresses generated by laser-assisted heating without any externally applied mechanical
forces [1-2]. The advantages of laser forming over conventional bending include flexibility of
non-contact processing, amenability to materials with diverse shape/geometry, and high
precision/productivity. Despite a substantial amount of literature concerning the
understanding/modeling of the laser forming process [3-4], the influences of operating
parameters on bending angle/shape [5], and the changes of microstrurctural and mechanical
properties [6-10], no work has been reported on the corrosion performance of laser-formed
components. In many cases, laser-formed components are used in corrosive environments.
Therefore, it is important to understand how applications of the laser-processed components
under those conditions could be affected.
This paper reports the corrosion behaviour following laser forming of AA 2024-T351,
AA7075-T6 and AA 5083-O aluminium alloys, 304 austenitic and 430 ferritic stainless steel
sheets. Materials characterisation, in terms of chemical segregation, phase transformation
and precipitate distributions, within the laser-bent zones and HAZs were carried out to
correlate microstructural evolution with corrosion performance to gain a better understanding
of corrosion mechanisms of various metallic alloys after laser forming.
2. Experimental procedure
AA 2024-T351, AA 7075-T6 and AA 5083-O alloy sheets in dimensions of 75 mm x 30 mm x
1.5 mm, and hot-rolled and solution-treated 304 austenitic and 430 ferritic stainless steel
sheets in dimensions of 100 mm x 30 mm x 1.5 mm, with their chemical composition given in
Table 1, were investigated. The samples to be treated were supported in an aluminium
clamp by one edge, leaving the other suspended in the air. A straight-line forming process
without additional waiting between scans was conducted using a 2 kW CW CO2 laser. The
laser beam power was kept as 250 W for the aluminium alloys and 400 W for the stainless
steels, with a beam spot size of 3 mm. The scanning velocity and the number of scans were
varied from 10 to 40 mm/s and from 5 to 180, respectively. For all the alloys, the bending
angles increased with decreased scanning velocity and increased with increased number of
scans under the same laser power and beam spot size. For three aluminium alloys, the
bending angles in the range of 5° to 72.5° were achieved without visual observation of
surface melting. For 304 SS, the maximum bending angle without surface melting was 19°
while with surface melting, it reached 29°; For 430 SS, the maximum bending angle without
surface melting was 16° while with surface melting, it reached up to 22°. Laser bent-zones
and HAZs were investigated, in terms of materials characterisation using scanning electron
microscopy (SEM) and transmission electron microscopy (TEM) and dispersive energy of Xray spectroscopy (EDS), and various corrosion tests.
Table 1: Chemical
Materials Cu
2024
4.5
7075
1.2-2.0
5083
0.1
Materials
304
430
C
0.045
0.028
composition of the various alloys studied in the experiments, wt%.
Mg
Si
Zn
Mn
Ti
Fe
Al
1.4
0.5
0.25
0.3-0.9 0.15
0.5
balance
2.1-2.9 0.4
5.1-6.1 0.3
0.2
0.5
balance
4.5
0.4
0.25
0.6
0.15
0.4
balance
Cr
19.00
17.00
Ni
9.25
-
Si
1.00
1.00
Mn
2.00
1.00
P
0.045
0.04
S
0.03
0.03
Fe
balance
balance
2. Results and Discussion
2.1 AA 2024 Alloy
Cross sections of the laser-formed samples, as shown in figure 1, reveal the locations of
corrosion attacks following 48 h exposure in 40% of the standard ASTM G-34 EXCO
solution, containing 4 M (234 g/l) NaCl, 0.5 M (45 g/l) KNO3 and 0.1 M (6.3 g/l) HNO3. Under
this time of exposure, the as-received AA 2024-T351 alloy did not show evidence of
corrosive attack. For the laser-formed samples, corrosion damage was mainly localised in
the HAZs, but also within the laser-bent regions. In the laser-bent regions, different degrees
of the susceptibility to localised corrosion were observed. Close to the top surface, θ-Al2Cu
intermetallics were formed while magnesium remained in the -Al solution. Localised attack
at the location of such intermetallics occurred due to the cathodic nature of θ-Al2Cu phase
with the respect to the -Al matrix, but the susceptibility was relatively low. Next to this
region, much server localised corrosion attack took place due to the formation of S-Al2MgCu
phase particles at the grain boundaries. Since S-phase is anodic with respect to the -Al
matrix, some of the S-phase particles were severely attacked; other particles fell out or were
entirely dissolved. Further away from the surface, the susceptibility to localised corrosion was
reduced. Towards the bottom layer, for the sample with bending angle of 42.5, the
susceptibility to intergranular corrosion became remarkably enhanced (figure 1b), leading to
exfoliation corrosion attack; while for the sample of 72.5, the bottom layer remained
unattacked (figure 1a). For the sample of 7, the laser irradiated surface was only slightly
attacked with feature of intergranular corrosion (figure 1c). Further away from the surface, it
was unattacked until reaching the bottom layer that was again seriously attacked
intergranularly (figure 1c).
HAZ(f)
72.5
BZ
HAZ(c)
(a)
42.5
(b)
7
(c)
Figure 1 Cross sections of laser-formed samples, after immersion tests, with bending angles
of (a) 72.5, (b) 42.5 and (c) 7. Note: HAZ(c)-HAZ close to the clamp; HAZ(f)-HAZ at the
far end.
Figure 2 also reveals a more severe attack in the HAZ(f)s of the samples than that in the
HAZ(c)s due to the higher temperature and slower cooling rate within HAZ(f)s. Both HAZs
presented a typical morphology of intergranular corrosion resulted from the dissolution of Sphase particles along the grain boundaries within the HAZs. Such intergranular attack, in
both HAZs, developed into exfoliation corrosion, reducing the sample thickness considerably.
For the samples of 72.5 and 42.5, the remaining thickness of the sheets within HAZs(f) was
down to 0.86 mm compared with the original thickness of 1.5 mm.
2.2 AA 7075 alloy
Immersion tests in 40% of the standard ASTM G34 EXCO solution for 1 day, 3 days, 4 days
and 5 days, showed that corrosion sites were distributed non-uniformly. The most
susceptible region to attack was within the HAZs. With increasing the immersion time, the
corrosion attack progressed toward and finally included the bent zones. The depth of the
attack within the HAZs was much deeper than that within the bent zone as shown in figure 2.
The corrosion attack within the bent-zone was a typical intergranular corrosion while the one
within the HAZs might be more accurately described as localised corrosion. Such corrosion
attacks were caused by the depletion of copper within the grain boundaries due to the
precipitation of Mg(Zn2,AlCu).
Clamped side
Clamped side
1 day
(a)
3 days
4 days
(b)
5 days
(c)
150 m
(d)
150 m
Figure 2 Corrosion appearance of laser-formed AA 7075 alloy (a) and (b) and cross sections
of bent-zone (c) and HAZ (d) after immersion tests.
2.3 AA 5083 alloy
Observation of surface morphology (figure 3) after the immersion tests in the solution of
concentrated nitric acid at 30 C for 24 h based on ASTM G67 shows that the as-received
alloy (I) exhibited corroded surface (figure 3a) with cross-section indicating a feature of
localised corrosion attack (figure 3b). This is due to the existence of certain amount of βMg3Al2 precipitates at and within the grain boundaries, as the β-phase precipitate is
electrochemically more active than the aluminum matrix. For Samples II-IV, the laser-bent
zones and the HAZs exhibited shining surface after the immersion tests, with the crosssections indicating much improved resistance to localised corrosion. With an increase in
bending angle, HAZs caused by laser forming became enlarged. Weight loss measurement
shows that the weight loss for the as-received alloy and the HAZ for the sample with the
biggest bending angle of 58 was 10.67 mg/cm2, and 1.73 mg/cm2, respectively. This finding
suggested that the heat-treatment induced by laser forming within the bent-zones and the
HAZs dissolved/redistributed the β-precipitates, leading to an improved resistance to
localised corrosion.
I
II
III
IV
I
40 m
40 m
Figure 3 Corrosion Appearance of AA 5083 alloy (a) and cross-section without (b) and with
(c) laser bending after immersion tests.
2.4 304 austenitic stainless steel
Hot-rolled and solution-treated 304 austenitic stainless steel consisted of single-phased
austenite. It was possible to control the laser forming conditions to avoid melting of the top
surfaces, but when large bending angles were required, a small degree of melting became
inevitable. A typical microstructure of laser-formed component without surface melting
consisted of HAZ (I) and HAZ (II) as shown in figure 4. HAZ (I) shows the effect of graincoarsening, while no significant change in the grain size was found in HAZ (II) though
deformation, i.e. slightly elongated grains, due to thermal stress, is present. Within the HAZ
(I) chromium-rich carbides were unlikely to be formed, while the HAZ (II) was considered to
experience the temperature range between 450 C and 850 C approximately, resulting in
the formation of chromium-rich carbides along the grain boundaries, depending on the
duration, i.e. scanning velocity and number of laser passes.
After the corrosion tests in 10% oxalic acid at 1 A/cm2 for 90 s based on ASTM A262, the
laser-formed 304 stainless steel sample of bending angle of 15, without superficial melting.
resulted from scanning velocity of 30 mm/s and number of passes of 20 using laser power of
400 W and beam spot size of 3 mm, revealed a significant feature of intergranular corrosion,
as ditch structure, within HAZ (II), while no intergranular corrosion took place within HAZ (I).
The results of double-loop electrochemical potentiokinetic reactivation (DL-EPR) test in
deaerated 0.5M H2SO4+0.01M KSCN showed that the laser-formed 304 stainless steel sheet
presented a larger reactivation current than the as-received. Greater values of Ir/Ip of 5.06%
for the laser-formed sample with the same treatment conditions as described above than
2.83% for the as-received suggested that laser forming of 304 stainless steel reduced the
resistance to intergranular corrosion within particular regions of HAZs, due to the
sensitization, i.e. the formation of chromium-carbides along the grain boundaries by
multiscan forming process.
HAZ(II)
HAZ(I)
25 m
20 m
25 m
Figure 4 Corrosion morphology of a typical laser-bent 304 stainless steel after oxalic acid
tests. (a) as-received, (b) HAZ(I) and (II) and (c) HAZ(II).
2.5 430 ferritic stainless steel
Hot-rolled and solution-treated 430 ferritic stainless steel consisted of an elongated
morphology of ferrite due to the rolling texture inherited from the rolling process. Similar to
304 stainless steel, when large bending angles were required, a small degree of surface
melting was inevitable. When the surface was melted, the microstructure of the bent zone
consisted of melted zone with much coarser grains and carbides within and along the grain
boundaries, retained austenite and/or martensite along the grain boundaries; and followed by
HAZ with coarsened grains. In addition, precipitation of cementite within the ferritic grains
and precipitate-free-zones (PFZs) near the grain boundaries were also found. When the
surface was not melted, only HAZs appeared with much growth of ferritic grains and
formation of martensite along the ferritic grains.
After the corrosion tests in 10% oxalic acid at 1 A/cm2 for 90 s based on ASTM A763,
laser-formed samples revealed a completely change of corrosion morphology. The bentzones presented much more pronounced intergranular corrosion attacks, compared with the
as-received condition (figure 5), indicating that the sensitization took place during laser
forming. Observation of the corrosion morphology showed that regardless the surface
melting or not, the sensitization occurred not only within the melted zone, but also the HAZs.
The melted zone also showed corrosion attacks taking place within the grains, while the
HAZs exhibited intergranular corrosion. DL-EPR further confirmed that the laser-formed
samples demonstrated a much higher susceptibility to intergranular corrosion by increasing
values of Ir/Ip from 20.2% to 50.2%.
30 m
40 m
30 m
Figure 5 Corrosion morphology of a typical laser-bent 430 stainless steel after oxalic acid
tests. (a) as-received, (b) melt-zone and (c) HAZ.
4 Conclusions
During laser forming of various metallic metal sheets, different thermal conditions, in terms of
temperature, duration and cooling rate, through the laser bent-zones to HAZs, introduced a
corresponding variation of the microstructure and elemental segregations. Such
microstructural changes significantly affected their corrosion performance. For AA 2024T351, AA 7075-T6 alloys, 304 austentic and 430 ferritic stainless steels, HAZs present
severe intergranular corrosion due to the laser-induced sensitisation, by formation of various
precipitates at the grain boundaries. Different degrees of localised corrosion also took place
within the laser bent-zones. For aluminium alloys, the use of the clamp led to asymmetrical
degrees of intergranular corrosion within the HAZs on both sides due to different thermal
conditions. However, for AA 5083-O aluminium alloy sheets, an improved resistance to
intergranular corrosion within the laser-bent zones and HAZs was observed.
References
[1] L.W. Zhang, Q. Zhong and J.B. Pei, Journal of Physique IV, 120 (2004) 507-512.
[2] S.P. Edwardson, E. Abed, K. Bartkowiak G. Dearden and K.G. Watkins,
Journal of Physics D-Applied Physics, 39 (206) 382-389.
[3] J. Magee, K.G. Watkins, W.M. Steen, N.J. Calder, J. Sidhu and J. Kirby, Journal of
Laser Applications, 10 (1998) 149.
[4] Y. Fan, P. Cheng and Y.L. Yao, Journal of Applied Physics, 98 (2005) Art. No.
013518.
[5] J.D. Majumdar, A.K. Nath and I. Manna, Materials Science and Engineering A, 385
(2004) 113-122.
[6] M. Merklein, T. Hennige, M. Geiger, Journal of Materials Processing Technology, 115
(2001) 159-165.
[7] J.A. Ramos, J. Magee and K.G. Watkins, Journal of Laser Applications, 13 (2001) 3240.