5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014, IIT

Guwahati, Assam, India

395-1

Improvement of Corrosion Resistance by Laser Surface Melting of

7075 Aluminum Alloy

A.C.Umamaheshwer Rao1*, V.Vasu2, S.M. Shariff3, K.V. Sai Srinadh4

1*Department of Mechanical Engineering, National Institute of Technology Warangal, Warangal, India, 506004, E-mail: umamaheswar4@gmail.com

2Department of Mechanical Engineering, National Institute of Technology Warangal,Warangal, India, 506004,. E-mail: vvvasu@gmail.com

3International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Hyderabad, India, 500005, E-mail: shariff@arci.res.in

4Department of Mechanical Engineering, National Institute of Technology Warangal, Warangal, 506004, India, E-mail: kvsai.srinadh@yahoo.com

Abstract

In the present work, a high power diode laser (HPDL) has been used for surface melting of a 7075-T651

aluminum alloy under a nitrogen atmosphere to induce microstructural changes on the surface to improve corrosion

resistance. The re-solidified laser-melted layer got refined with elimination of detrimental constituent particles and

grain boundary network present in wrought structure. The compositional and microstructural analysis of the laser

melted layer carried out by SEM, XRD and EDS indicated complete absence of coarse precipitates. The comparative

corrosion study determined by Potentiodynamic Polarization measurements in 3.5 % sodium chloride solution

showed corrosion current reduced by 5 times in laser melted surface compared to un-treated substrate. The refined

microstructure of laser melted layer with presence of aluminum nitride phase, that can plausibly enhance electrical

insulation, could be attributed to its vast improvement in corrosion resistance.

Keywords: Aluminum 7075, Laser surface melting, corrosion

1 Introduction

High strength Aluminum alloys (HSAL) such as

7075-T6 is widely used for aircraft structural materials

as it is high strength and a low density material J.K.

Park (1988). Because of the Navys unique service

requirements, this alloy is subjected to aggressive

conditions where it often encounters salt water spray

and/or salt fog environments. This alloy is susceptible to

pitting corrosion and intergranular corrosion cracking

when interacted with the corrosion environments such

as Cl+ and br – environments. The main alloying

elements of the 7075 Al alloy are Zn, Mg, Cu and minor

Fe and trace of other intermetallic elements such as Fe,

Cr, Si and Mn. This alloyM. Gaoet al. (1998) contains

two types of constituent particles which are contributing

for high strength and SCC behavior. Those are (i)

particles such as Al, Mg and Zn which are anodic with

respect to the matrix and readily dissolve (ii) particles

acts as cathodic to the matrix and tends to promote

dissolution of the adjacent matrix such particle are Fe,

Cu, Mn. Due to the presence of these two distinct

categorized constituent particles tends to arise galvanic

effect and hence corrosion pits can readily develop at

these particle interfaces. Even though wide varieties of

heat treatments are available for reducing the corrosion

rates one of them is over aging which has ability to

resist the hydrogen transportation along the grain

boundaries by forming a coarse precipitates and

aggregated along the grain boundaries. But in

comparison with T6 condition, over aging is

accompanied by a loss in strength of about 10 to 15 %

and observed low hardness values due to coarsening of

the precipitates as it is subjected to prolonged heating.

To overcome this penalties one of the non – traditional

surface engineering method namely Laser Surface

melting has profound interest in recent research for its

ability to improve corrosion resistance of aluminum

alloys. This technique has ability to change the surface

properties of the alloy without altering the bulk

properties of the material. Laser treatment provides

rapid melting of the surface and has fast solidification,

which results in a homogeneous and fine surface which

is exempt from unwanted intermetallic particles. Most

Improvement of Corrosion Resistance by Laser Surface Melting of 7075 Aluminum Alloy

395-2

of the research has been done with CO2, EXCIMER and

Nd-YAG lasers. This work intended to pertinence of

high power diode lasers (HPDLs) in corrosion studies

and characterization before and after laser treatment.

2 Experimentation

The aerospace 7075- T6 aluminum alloy was

investigated with the nominal chemical composition of

the alloy is given in the table 1. The alloy was in peak

aged condition and in the form of plate with the

thickness of 6.35 mm.

Table 1. Chemical composition (wt%) of 7075-T6

Aluminum alloy

Zn Mg Cu Fe Si Cr Mn Ti Al

5.5 2.4 1.3 0.16 0.05 0.23 0.03 0.071 bal

The sample surface was initially polished with SiC

grit paper, was surface treated by employing continuous

wave high power fiber-coupled diode laser with 3 kW

by varying transverse speed of 5 mm/s. This laser has

emission wavelength of 890 – 980 nm, laser spot size of

20 mm X 5 mm. surface treatment was carried out my

maintaining working distance of 300mm and in the

presence of N2 Shroud gas at 10 bar pressure from off-

axial shroud Nozzle. The laser treatment was carried out

along the transverse direction of the plate that is

perpendicular to the rolling direction. Electrochemical

measurement was performed using a conventional three-

electrode system in 3.5 % NaCl solution. A 10 mm X

10mm exposed area is used as working electrode and

Ag/AgCl electrode was employed as the reference

electrode. The IM6e, ZAHNER, GmbH Model was

employed for the potentiodynamic polarization test, and

an initial delay time of 20 min was employed.

Electrochemical measurement was performed using a

conventional three- electrode system in 3.5 % NaCl

solution. The polarization scan was started at 0.25 V

below the steady open-circuit potential (OCP) at a

scanning rate of 10mV/s.

Table 2 Summary of laser parameters used for laser

processing

Laser power 3 kW

Liner speed 5 mm/s

Size of focused laser spot 20 mm X 5 mm

Working distance 300 mm

Laser energy density 120 J/mm2

Shrouding gas N2 at 10 bar

Before and after laser melting, the microstructure and

phase composition of the surface were characterized by

light microscopy (LOM). Keller’s reagent was used to

etch for the microstuctural study. Scanning electron

microscope (SEM) (Test Scan, Czech Republic)

equipped with energy dispersive spectrometry (EDS)

was used and X-ray diffraction (XRD; Panalytical,

X’Pert PRO model) was employed to study the phases

present before and after LSM.

3 Results and discussions

3.1 Microstructure

Figure 1 shows the cross sectional microstructure of

the as received material a large amount of coarser

precipitates was found to be present and these particles

identified as Al-Cu-Fe and Al-Cu-Mg phases P. S. Paoet

al. (2000). The Al-Cu-Fe was cathodic relative to the

matrix material and Al-Cu-Mg act as anodic. Figure 2

shows the stereographic cross sectional image of laser

surface melted Al 7075 Alloy with an applied energy

density of 120 J/mm2. The microstructure consists of the

melted zone, interface and the heat affected zone. It was

observed that no surface defects or cracks were found

after the Laser surface treatment. It has been observed

that the melt depth reported about 1.85mm and the

interface thickness about 400 µm. It has been reported

that the a high cooling rate in LSM results in a fine

solidified microstructure, which may contain non-

equilibrium phases, new precipitates and extended solid

solubility. The variation in solidification structure

occurs due to the compositional change and due to

variation in the growth rate and cooling rates Z. Liu et

al.(2006). In the present work a planar growth region

was observed at the base of modified layer which is

shown in figure 3, whereas the rest of the layer had a

segregated cellular structure and the microstructure at

5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12

Guwahati, Assam, India

the surface of the laser melted region is observed to be

fine and dendrite.

Figure 1. Cross-sectional microstructure of

untreated material

Figure 2. Stereographic image of the cross

of laser treated 7075 Al sample

Figure 3. Microstructure at the interface

treated sample at 200X

Figure 4 and 5 shows the SEM surface images of the as

received and laser treated surface respectively

All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12

the surface of the laser melted region is observed to be

sectional microstructure of

Stereographic image of the cross – section

of laser treated 7075 Al sample

Figure 3. Microstructure at the interface of the laser

treated sample at 200X

shows the SEM surface images of the as

respectively. A large

sized and number of coarse constituents

present in the as received alloy, also precipitates in the

grain and along the grain boundary have been observed.

From the SEM-EDS analysis it was observed that these

phases are rich in iron and copper. These Cu and Fe

phases are cathodic to the matrix and tend to promote

the surrounding matrix dissolution

After laser surface melting no such coarse

present and observed that fine structure about 3

microns dispersed uniformly throughout the surface

can be seen in figure 5. From the EDS analysis the

dispersed particles confirmed as they are rich of Cu, Mg

and Zn. The EDS analysis of laser treated sample shown

in table 3which consolidate the wt % elem

of untreated and laser treated samples

present and on the matrix respectively

concluded that laser melting has profound effect

surface modification that can offer the potential to

reduce to eliminate such detrimental inter

phases from the surface without affecting the bulkand

could leads to its vast improvement in corrosion

resistance.

Figure 4. shows the SEM image at 2000X

magnifaction of surface of untreated sample.where A

and B are the EDS sopt copositional

coarsed phase and on the matrix of the alloy

All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014, IIT

395-3

sized and number of coarse constituentsphases are

also precipitates in the

grain and along the grain boundary have been observed.

EDS analysis it was observed that these

h in iron and copper. These Cu and Fe

are cathodic to the matrix and tend to promote

the surrounding matrix dissolution chanet al (2003).

After laser surface melting no such coarse phases were

present and observed that fine structure about 3-5

ns dispersed uniformly throughout the surface, it

From the EDS analysis the

dispersed particles confirmed as they are rich of Cu, Mg

and Zn. The EDS analysis of laser treated sample shown

consolidate the wt % elements analysis

of untreated and laser treated samples on the phases

present and on the matrix respectively. It can be

concluded that laser melting has profound effect on the

surface modification that can offer the potential to

mental inter-metallic

phases from the surface without affecting the bulkand

its vast improvement in corrosion

Figure 4. shows the SEM image at 2000X

magnifaction of surface of untreated sample.where A

copositional analysis on the

coarsed phase and on the matrix of the alloy.

Improvement of Corrosion Resistance by Laser Surface Melting of 7075 Aluminum Alloy

395-4

Figure 5. shows the SEM image at 5000X

magnification of surface of laser treated

sample.where C and D are the EDS sopt analysis on

the dispersed phase and on the matrix of the alloy.

Table 3 EDS elemental analysis of un-treated and

laser treated samples

Sl

no.

Elemen

ts

spot A

figure :4

spot B

figure :4

spot C

figure :5

spot D

figure

:5

1 Mg 0.26 2.90 7.76 1.67

2 Al 53.23 87.69 82.03 96.36

3 Cr 0.18 0.15 0.10 0.13

4 Fe 13.97 0.14 0.02 0.03

5 Cu 31.16 1.68 3.58 0.11

6 Zn 1.25 7.43 14.02 1.71

3.3 X- Ray Diffraction Analysis

Figure6 shows the results of low angle X-ray

diffraction of the AA 7075-T6 alloy before and after

LSM. It can be seen that in as-received alloys large

number of second phase particles such as MgZn2,

AlCuMg4, AlCuFe and AlSi were present. These

particles have major contribution towards the corrosion

initiation. After the laser treatment no significantly high

peaks from large second phases were observed. All the

constituent particles such as, Mg2Si, AlCuMg and

Al7Cu2Fe were absent. There absence might be due to

this particle re-dissolve in to solution during the laser

melting and stayed in solution during solidification.

Except MgZn2 phase has been observed after LSM this

was also reported by Benedetti et al (2011) and it was

also observed the AlN phase also formed after the LSM

as the treatment was carried out in Nitrogen atmosphere.

Figure 6: XRD analysis for both as-received and

LSM 7075-T6 alloy treated

3.4. Potentiodynamic polarization test

Figure 7 shows thepotentiodynamic polarization

curves for the laser-treated and the as-received AA

7075-T6 alloy in 3.5% NaCl solution. It has been

observed that the corrosion current (icorr) of the

untreated and laser-treated specimens was reported to be

20.1µA and, 3.84 µA respectively. This means that a

five -fold decrease in corrosion current was obtained

after the laser treatment. This was considered to be due

to the absence of coarse second phase particles on the

surface of the laser treated sample which allows a

continuous passivation film to form thus providing

better overall corrosion resistance. The surface

morphologies of the as received and the laser-treated

specimens after the polarization test are shown in Figure

8.It has been observed that the untreated specimen

suffered from extensive pitting corrosion attacks shown

in figure 8 (a) whereas the situation greatly improved

after laser treatment shown in figure 8 (c).Table 5,

Figure 8 (c) and (d) shows the EDS analysis of

untreated and laser treated sample after polarization test.

5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014, IIT

Guwahati, Assam, India

395-5

It has observed that the composition of the corrosion

affected and unaffected area were almost same in the as

received and in laser treated sample respectively. Apart

from the absence of coarse constituent phases, in XRD

analysis an AlN phasewas observed which act as an

electrical insulator, behaves as a barrier to prevent the

flow of electrons, thus reducing the rate of chemical

reaction and hence corrosion rate Yueet al.(2006).Table

3 shows the corrosion rates of untreated and laser

treated samples as per ASTM G 102 – 89.

Table 4 showing the corrosion current, corrosion

potential and corrosion rate of untreated and laser

treated sample

Sample Corrosion

current(Icor)

µA

Corrosion

potiential(Ecorr)

mv

corrosion

rate

( mm/yr)

untreated 20.1 -910.1 0.165

Laser

treated 3.84 -500.6 0.031

Figure 7:Potentiodynamic polarization curves for

the laser-treated and as-received AA 7075-T6 alloy

in 3.5% NaCl solution.

(a)

(b)

(c)

Figure 8: Surface morphology of the untreated

specimen after the Potentiodynamic polarization test

(a) Untreated at 200X magnification (b) EDS

analysis at corrosion pit and at matrix of the

untreated sample(c) EDS analysis at corrosion pit

and at matrix of the laser treated sample.

Improvement of Corrosion Resistance by Laser Surface Melting of 7075 Aluminum Alloy

395-6

Table 5 EDS elemental analysis of un-treated and

laser treated samples after

Potentiodynamicpolarization test

Sl

n

o.

Eleme

nts

EDS

analysis

on

Untreated

unaffected

area spot P

EDS

analysi

s on

Untrea

ted

Corros

ion pit

Q

EDS

analysi

s on

LSM

unaffec

ted area

R

EDS

analysi

s on

LSM

Corros

ion pit

S

1 Na 0.29 - - 0.89

2 Mg 2.10 2.23 2.47 1.95

3 Al 90.67 89.56 89.73 87.13

4 Cl 0.12 - - 1.48

5 Cr 0.18 0.33 0.22 0.17

6 Fe 0.06 0.19 0.17 0.24

7 Cu 1.26 3.29 1.37 2.39

8 Zn 5.32 4.39 6.04 5.75

4 Conclusions

Laser surface treatment has significantlyimproved the

corrosion resistance of 7075 alloy. In general,the

improvement in corrosion pitting resistance ofthe alloy

is considered to be due to the reduction ofconstituent

particles in the laser modified layer as wellas the

chemical homogenisation of the matrix material.The re-

solidified laser-melted layer got refined with elimination

of detrimental constituent particles and grain boundary

network present in wrought structure.From SEM-EDS

analysis after LSM observed that fine structure about 3-

5 microns dispersed uniformly throughout the surface

which is rich in Cu and Mg content.After Laser surface

melting all the constituent particles such as Mg2Si,

AlCuMg and Al7Cu2Fe were absent. The comparative

corrosion study determined by Potentiodynamic

Polarization measurements in 3.5 % sodium chloride

solution showed corrosion current reduced by 5 times in

laser melted surface compared to un-treated substrate.

The corrosion rateis drastically decreased after LSM. It

shows that LSM has a profound effect on the corrosion.

References

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