Journal of Materials Science and Engineering B 1 (2011) 200-205
Formerly part of Journal of Materials Science and Engineering, ISSN 1934-8959
Microhardness Evaluation of Pure Aluminum Substrate
after Laser Surface Alloying with Iron and Copper
Yusef Ab. Alwafi, Noriah Bidin, Rosli bin Hussin, Muhammad Shakhawat Hussain and Dwi Gustiono
Department of Physics, Faculty of Science, Universiti Teknologi Malaysia (UTM), Skudai 81310, Johor, Malaysia
Received: January 12, 2011 / Accepted: January 29, 2011 / Published: July 25, 2011.
Abstract: A low level energy of Q-switched Nd-YAG laser was used to produce thin layers of Fe-Cu alloy on pure aluminum
substrates. Two-step laser deposition (2SLD) method was employed. Pure aluminum substrates were pre-coated by a mixture of Fe and
Cu powder (with the rate 1:2) using a suitable binder and then irradiated by partially overlapped laser pulses. Different parameters were
conducted in order to obtain the suitable treatment condition. X-ray diffractometer (XRD), glow discharge spectrometer (GDS), optical
microscope and microhardness tester were used to analyze the treated surfaces. According to XRD results, copper-iron phase
(Cu9.9Fe0.1) was formed. GDS analysis showed that the weight percentage of Fe is higher than that of Cu within the modified layer. The
average micro hardness of the modified surface was found to be four times higher than that of untreated surface.
Key words: Laser alloyed, Fe-Cu powder, microhardness, laser plasma.
1. Introduction
Among the various materials, aluminum has
attracted the attention of many researchers because of
its unique properties. Aluminum is the world’s most
abundant metal and the third most common element
comprising 8% of the earth’s crust. Aluminum has
excellent technological properties; it can be easily
rolled, forged, drawn and extruded practically to any
required complicated shape [1]. However, its hardness,
wear resistance and mechanical properties are poor in
comparison to steel but such properties can be
substantially improved by alloying and heat treating.
One feature of aluminum is its ability to form many
intermetallic phases with most common elements, such
as Fe, Ni, Co, Cr, Ti, or Cu. The nickel and iron
aluminides intermetallics, in particular, have received
the most attention in recent studies [2-7]. The alloying
process can be done by several techniques but laser
surface alloying is considered to be the most effective
technique for surface modification. The most important
Corresponding author: Noriah Bidin, professor, research
field: laser technology. E-mail: Noriah@utm.my.
advantage of laser surface modification processes is the
possibility of modifying the properties and composition within a thin surface layer without affecting the
properties of the bulk material. In the present paper, the
Fe-Cu laser alloyed on aluminum substrate is reported
by using low energy laser. The microstructure analysis
and microhardness of the modified surface is the
subject of discussion.
2. Experimental Details
The main purpose of this study was to increase the
surface hardness of aluminum using a low power laser.
Pure aluminum plate was used as a substrate. The plate
was cut into small pieces; each piece has a surface area
of about 2 cm2. These pieces were chemically cleaned
and then coated by a mixture of Fe and Cu with the
ratio 1:2 respectively using suitable glue. A Q-switched
HY200 Nd: YAG laser manufactured by LUMONIC
was employed as a source of energy. The laser was
operated in repetitive mode. The pulse duration of laser
is 10 nanosecond with variable output energy of within
10-100 mJ per pulse. Fig. 1 shows a schematic diagram
of the experimental set-up. The laser beam was focused
Microhardness Evaluation of Pure Aluminum Substrate after Laser Surface Alloying with Iron
and Copper
Trigger
unit
Power supply and
cooling system
201
CCD video camera
Filter
He-Ne
laser
Nd:YAG laser
Laser surface alloyed
material
Plasma formation
Photodetector
Translation stage
Powermeter
Fig. 1
Experimental set-up.
using a convex lens with focal length of 30 cm. The
power density at the focal point was estimated to be as
5 × 1015 W/m2. The energetic source at the focal point
can be visualized by the appearance of optical
breakdown and the plasma formation. The temperature
of the plasma region is almost 1 eV (each eV is
equivalent with 11,600 K) [8]. The plasma can expands
in the free surface within 300 to 500 μs depend on the
energy delivered at the focal point [9]. In this case the
quench rate induced by plasma was estimated to be as
20 × 106-40 × 106 K/s.
In order to avoid direct photodisruption, the samples
were exposed at defocused distances. Experiments
were carried out with different defocused distances to
determine the suitable position of the samples with
respect to the focal point. The precision location and
the plasma formation were visualized and recorded by
a charge couple device (CCD) video camera. A
Helium-Neon (He-Ne) laser of 633 nm (red color) was
also coaxial with Nd:YAG laser to guide the invisible
light targeted on the specimen.
After laser alloying process, all the treated samples
were analyzed for evaluating the surface properties.
Glow discharge spectrometer (GDS) was used to
obtain a quantitative analysis of the alloyed layer. A
Reichert Polyvar 2 Met optical microscope was utilized
to evaluate the microstructure. The strength of
modified surface was measured by means of HMV-2
Shimadzo microhardness tester. At least, 10 indents
were made on each sample. The average of the
indentations was calculated to find the hardness for
each individual sample. The composition of the treated
surface was also analyzed by using X-ray diffraction
(XRD).
In this work, two-step laser deposition (2SLD)
method was used. Mixture of Fe and Cu powder was
pre-placed on pure aluminum substrates. The thickness
of mixture layer was about 0.8 mm. The alloying
process was carried out in free space.
3. Results and Discussion
A typical plasma formation induced by focusing the
Q-switched Nd:YAG laser is shown in Fig. 2. The laser
beam direction was from the right to the left of the
figure. The target material was located at the left hand
side of the picture. The image was captured via a high
speed photographic system. As shown in Fig. 2, the
plasma formation was observed attached to the target.
Although in the reality the target was placed at a
defocused distance of 5 mm away from the focal point.
However, the plasma touching the target is relatively in
large area. This means the power density is not that
strong to damage the surface, but potentially enough to
alloy the mixture into the aluminum base. In addition,
the surface is also subjected to a thermal shock wave
induced associated with the optical breakdown. It is
202
Microhardness Evaluation of Pure Aluminum Substrate after Laser Surface Alloying with Iron
and Copper
Fig. 2 The plume of plasma formation at defocus point.
Table 1
Physical properties of tested materials.
Properties
Melting temperature (°C)
Boiling temperature (°C)
Thermal conductivity (Wm-1·K-1)
Specific heat (Jg-1·K-1)
Thermal diffusivity (cm2·sec-1)
Mass density (g·cm-3)
Aluminum
660.2
2480
237
0.900
0.7
2.7
Copper
1083
2595
401
0.386
0.9
8.9
Iron
1536
2861
80.2
0.451
0.12
7.8
better to note that the plasma temperature (11,000 K) is
much higher than the melting and boiling points of
each element deposited on the aluminum surface
(Table 1). Since the short lifetime of the plasma (within
nanosecond to microsecond region), the cooling period
or the quench process is fast enough to control further
heat dissipation in the bulk of aluminum base.
However in the plasma region the heat transfer is
enough to melt the metals mixture as well as the
aluminum within a skin depth layer appropriate with
transient time of plasma existence. As a result the alloy
process stops immediately and forms a skin depth layer
of Fe-Cu-Al on the aluminum base.
(a)
Fig. 3 shows the typical microstructure of the laser
Fe-Cu alloyed surface. Fig. 3a shows the aluminum
based before expose to laser pulse for comparison
purposes. The thermal skin depth effect on cross
section of modified surface can be seen clearly in Fig.
3b. Since pure aluminum was used, the solidified
surface became irregular after heat treatment. The
white portion on the top surface in Fig. 3b indicates
the solidified region.
The laser alloyed surface of the same specimen was
further quantified based on the Glow Discharge
Spectroscopy (GDS) analysis. The result obtained
from GDS analysis is shown in Fig. 4. GDS analysis
showed that Fe atoms have diffused into the Al base to
a depth down to 22 µm. Meanwhile the Cu atoms have
penetrated less (12 μm) into the Al bulk. On the other
hand, Al atoms have diffused up and appeared on the
upper portion of the alloyed layer. The weight
percentage of Fe (55%) available on the laser surface
alloyed almost 5 times greater than the Cu atoms
(10%). It is better to remember that the initial
composition of mixture Cu was twice greater than Fe.
Why iron atoms are more than cu on the surface layer
and why they diffused further down from the alloyed
surface? In order to explain this phenomenon, we
have to investigate their thermal properties.
According to Table 1, iron has very low thermal
conductivity (80 Wm-1·K-1) in contrast with high
conductivity of copper (401 Wm-1·K-1). This means
that iron quenched 5 times faster than Cu. That is why
(b)
Fig. 3 Optical micrograph of polished cross section of: (a)- untreated aluminum surface and, (b)-the alloyed surface of
Fe-Cu on Aluminum substrate (magnification X50).
Microhardness Evaluation of Pure Aluminum Substrate after Laser Surface Alloying with Iron
and Copper
203
Cu
Fig. 4
The weight percentage versus the depth within the modified surface obtained by GDS analysis.
the weight percentage of Fe is 5 times greater than that
of Cu from the GDS result. Why Fe can be found
deeper than Cu? This can be described base on the
mobility each of the atom.
However, the diffusion of Fe and Cu atom are almost
the same trends. Their weight percentage are
decreasing with depth. In contrast, Al has shown
entirely different profile with the other two previous
elements. Instead of decreasing, the weight percentage
of Al was increasing with the depth as shown in Fig. 4.
Initially, the amount of Al component is less than Fe
because the surface has been covered by coating layer.
As mentioned earlier the direction of weight
percentage is different, instead going down for Fe and
Cu atoms, the Al atoms move upwards. It is not
surprise for the Al to achieve 100% concentration
because it is the substrates. In fact Al is a noticeble to
have the lowest melting temperature (660 °C) almost
two times lower compared to Cu (1,083 °C) or Fe
(1,536 °C). Therefore during alloyed, Al was melted
earlier and diffused upward, as demonstrated in Fig. 4.
Furthermore, due to its small mass density, the
conductivity process slow down the Al to achieve its
boiling point hence, controlling its transformation into
other phases (such as vaporize). But the advantage of
this slow heat conduction gives high potential for
mixing and creating others compound.
The XRD analysis can give us the real composition
of element existing under the modified surface. The
typical result obtained from this experiment is shown in
Fig. 5. As expected from the GDS results, that Fe has
formed binary alloyed with Al i.e., AlFe and Al13Fe4.
These compositions are in good agreement with the
result obtained by other researchers [10]. Although
initially Fe:Cu was prepared with a ratio of 1:2, Cu
actually did not take part in the alloying process with
Al. However, some alloying between Fe and Cu was
evident (Cu9.9Fe0.1) in the XRD analysis. It can be
obviously seen that Fe (310) has shown the majority
line from this XRD analysis beside the Al as a
basement.
Finally the modified surface was examined by using
microhardness test. The typical results obtained from
this experiment is presented in Fig. 6. Initially, the
hardness of the laser Fe-Cu alloyed surface is found to
be linearly increases with the number of pulses
irradiating the same area up to 7 pulses. The average
hardness within this range is 4.057 HV/pulses. This
means the more energy absorbed on the coated surface
the more Fe and Cu will be melted and more metastable
phases will be formed. As a result, the surface becomes
harder than the untreated surface. The hardness of the
treated surface is found to be 4 times higher than
untreated surface.
204
Microhardness Evaluation of Pure Aluminum Substrate after Laser Surface Alloying with Iron
and Copper
Al(220)
Fe(310)
Cu9.9 Fe0.1(400)
Al(111)
AlFe(110)
Al13Fe4
Al(311)
Al(331)
Fe(110)
Fe(211)
The XRD analysis of Cu-Fe alloyed by laser on aluminum substrate.
Fig. 6
Number of pulses
Microhardness of Fe+Cu-coated surfaces treated with different number of pulses.
Micro hardness (HV)
Fig. 5
However, when the surface was treated more than 7
pulses at the same area, the average microhardness is
drastically dropped and remained the same hardness of
the aluminum base. It means that irradiating the same
area by 7 pulses was enough to melt all the powder
material at the surface. But more than that, the coated
surface or the alloyed surface was vaporized and
destroyed. Consequently the original surface of
aluminum base was exposed. As laser surface alloyed
treated continue with greater number of pulses, the
hardness turn back to be the same as original aluminum
base. In fact, it became worse because undergoes
surface damage.
4. Conclusions
A method to harden a surface was introduced by
using a relatively low average of energy laser. This was
achieved by focusing a Q-switched Nd: YAG laser.
The quenching rate introduced by the laser plasma
expansion is the mechanism responsible to alloy the
surface. To avoid photodisruption of the surface, the
alloying process was carried out at a defocused
distance and less number of pulses. The plasma heat
treatment was enough to melt and create some new
metaphases including copper-iron Cu9.9Fe0.1,
aluminum-iron, AlFe and Al13Fe4 phases. These
phases were responsible to make the modified surface
Microhardness Evaluation of Pure Aluminum Substrate after Laser Surface Alloying with Iron
and Copper
became harder in comparison to untreated surface.
However, the maximum laser alloyed treatment was
allowed up to 7 pulses only at the same area. Greater
than that limitation, the surface alloyed was
destroyed and original surface was exposed for
damaging.
Acknowledgment
The authors gratefully acknowledge to financial
support from Malaysia government through Escience
fund vote 73989 and UTM through RMC.
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