5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014, IIT
Guwahati, Assam, India
SURFACE ALLOYING OF ALUMINUM WITH COPPER USING CO2
LASER
Woldetinsay G.Jiru1*, Mamilla R. Sankar2 and Uday S. Dixit3
Department of Mechanical Engineering, Indian Institute of Technology Guwahati–781 039
1*
2
3
Emails: [email protected], [email protected], [email protected]
Abstract
Aluminium and its alloys have high demand in industries and service applications due to their light weight
compared to their strength when alloyed with different metals like Cu, Mg, Ni, Cr, Zn and others for enhanced
longer service life. In this work, commercially available 99% pure aluminium was alloyed with copper powder of
10µm particles size, which was melted by CO2 laser. Three different methods were used for uniform placing of
95% copper powder and 5% aluminium powder on aluminium substrate. The result was examined by Vickers
hardness test. SEM and FESEM were used for studying surface and subsurface defects. Defect free aluminium
alloy with improved microstructure and enhanced mechanical property was achieved.
Keywords:Surface alloying, CO2 laser, Copper powder, Aluminium
1 Introduction
Nowadays materials with special surface properties
such as high corrosion, high wear resistance and enhanced
hardness are highly demanded for advanced applications.
Producing such alloys is not easy and their demand
drastically increases the cost of the parts. Failure or
degradation of engineering components due to
mechanical, chemical or electrochemical interaction with
the surrounding environment is most likely to initiate at
the surface due to the high intensity of external stress and
environmental attack at the surface. Surface engineering
is a method of altering the properties of the surface in
order to reduce the degradation over time. Laser surface
alloying is one method to modify the surface properties of
a material without affecting its bulk property [Elijahet al.
(2009)].
Generally, laser surface modification is used to
change either the surface composition or microstructure
of a material to give it certain desired properties. The
benefits of rapid solidification by laser power for surface
modification are as follows:
• Fine grain structure due to fast solidification,
• Reduced micro segregation,
• Extended solubility of alloying elements.
The intimate contact between the melt and the solid
substrate causes a very fast heat extraction during
solidification resulting in very high cooling rates of the
order of 105 to 108 K/s. The high cooling rates to which
this surface layer is subjected result in the formation of
different microstructures from bulk metal leading to
improved
surface
properties
[Dubourget.al.
(2002)].Figure 1 shows the schematic diagram of alloying
used for this experiment.
Figure 1 Schematic of laser alloying
In view of the importance of aluminum for
automobile, aerospace and many other sectors, this paper
investigates the surface alloying of aluminum by copper.
Three methods of alloying have been used. The
mechanical properties and microstructures of surface
alloyed samples have been compared.
2 Materials and Methods
In the present work, commercially pure aluminum of
99% purity was alloyed with copper powder of 10 µm
particle size and melting was done by continuous CO2
laser power. During alloying 50% overlap is provided
between consequent beads. After cooling, re-melting was
carried out at orthogonal direction to alloyed layers.
Sample dimension was 110 mm x 50 mm x 6 mm. Table1
shows substrate material composition tested by EDX.
Table 1 Chemical composition of substrate material
Element
Cu
Si
Fe
Al
Composition (wt %) 0.12 0.14 0.59 Balance
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SURFACE ALLOYING OF ALUMINUM WITH COPPER USING CO2 LASER
Three methods used for depositing the copper powder are
as follows:
a.
b.
c.
Stirring of metal powder with Fevicol and then
painting it on substrate to achieve 0.5 mm
coating for samples 12, 13, 14, 15.
First painting the substrate by Fevicol, then
applying powder uniformly and finally
compacting it to achieve total coating thickness
of 1 mm for samples 4, 5, 6, and 9.
First painting the substrate by Fevicol, then
applying powder and blowing out excess powder
to achieve total coating thickness of 0.5 mm for
samples 1, 2, 3, 7, 8, 10 and 11.Table 2 shows
the process parameters in different experiments.
Where β is the full width at half maximum, K is a
constant, λ is the X-ray wave length of Cukα radiation (λ =
1.5418 Å), D is average crystal size and η is lattice strain.
The strain in the sample was 0.002 as observed in inset of
Figure 2 and average size of crystallite is 152.06 nm. The
crystal is cubic in structure as justified from ICDD#:021254, confirming the formation of Al4Cu9 phase.
3 Results and Discussion
3.1 Crystal size and lattice strain
The crystal size and lattice strain developed due to
thermal effect was examined for Sample 4 by XRD and
the result is shown in Figure 2.Strain was calculated from
[Williamsonetal.(1953)]
Kλ
β cosθ =
+ 4η sinθ
(1)
D
Figure 2 X- ray diffraction pattern of Al4Cu9
Table 2 Process parameters
Process parameters for alloying
S.
No.
Laser
power
(kW)
Stand off
distance
(SD)(mm)
Scan speed
(V)
(mm/min)
S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
S11
S12
S13
S14
S15
1.7
1.7
1.7
1.7
1.7
1.6
2.0
1.6
1.8
1.7
1.8
1.7
1.8
1.6
1.7
30
30
30
40
30
30
40
30
30
30
30
40
40
30
30
600
500
500
500
400
500
400
400
400
600
500
600
600
600
400
Laser
spot
diameter
(mm)
4.05
4.05
4.05
5.39
4.05
4.05
5.39
4.05
4.05
4.05
4.05
5.39
5.39
4.05
4.05
Process parameters forre-melting
Laser
Power
(kW)
1.6
1.6
1.6
1.6
1.6
1.7
1.6
1.7
1.5
1.5
1.6
1.7
Stand
off
distance
(mm)
30
30
40
40
40
30
40
30
40
40
40
30
Scan
Speed
(V)(mm/
min)
600
600
800
800
800
500
800
500
400
800
800
500
Laser
spot
diameter
(mm)
4.05
4.05
5.39
5.39
5.39
4.05
5.39
4.05
5.39
5.39
5.39
4.05
Remelting
Yes
No
Yes
No
Yes
No
Yes
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SURFACE ALLOYING OF ALUMINUM WITH COPPER USING CO2 LASER
12.5
10
7.5
5
2.5
0
-2.5
-5
-7.5
-10
-12.5
(a)
-15
0
100
200
300
400
500
600
700
800 µm
700
800 µm
4
3
2
1
0
-1
-2
-3
-4
-5
(b)
0
100
200
300
400
500
600
Figure 3 Surface topography and corresponding line surface roughness of alloyed samples: (a) non uniform
coated surface (b) uniform coated surface
Figure 4 Vickers hardness variation towards the alloyed area on the specimen
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SURFACE ALLOYING OF ALUMINUM WITH COPPER USING CO2 LASER
3.2. Surface Roughness
The surfaces of the alloyed layers were tested
without polishing the surface. Taylor Hobson make CCI
light, non-contact optical profilometer was used to check
surface quality of alloyed samples. Figure 3 (a) shows
Sample 2, where area of non-uniform coating resulted in
void surface with larger peaks and valleys and Fig. 3 (b)
is Sample 9, with uniform deposited and alloyed surface
with average roughness value (Ra) up to 1.9µm.
3.3 Micro hardness Analysis
Micro-hardness test was conducted on thickness side of
the samples by Vickers hardness test using 500gf and the
results are plotted as Fig. 4. Maximum hardness
achieved is 156HV at laser power of 1.7 kW laser beam
diameter 5.39 mm and scanning speed of 500 mm/min.
The re-melting was done in orthogonal direction to the
alloyed direction at laser power of 1.6 kW and scanning
speed of 800 mm/min. The re-melting was done after
alloying improved hardness Pinto et al.[2003]. Samples
4 and 9 with 1 mm deposition thickness of Cu powder at
resulted in improved hardness compared to other
samples.
3.4 Microstructure and Morphology Analysis
A pure metal normally solidifies at a constant
temperature with the inter face between the solid and
liquid media being planar for an alloy. Figure 5 shows
morphology in solidified regions at different
magnification analysed by SEM. It shows Cu growth in
Al4Cu9 inter metallic phase with a fine microstructure
region [Porter and Easterling (1992)]. After initial
cellular growth from the bottom of the molten pool, the
(a)
solidifying area developed lateral instability favourable
for forming homogeneous structure.
Figure 6 (a) shows uniformity of Cu growth into
white Al primary grains and dark eutectic well
distributed intermetallicAl4Cu9phase. Figure6 (b) shows
the grain boundary cracking that contain appreciable
amounts of copper, when the cooling rate is insufficient
to prevent precipitation on grain boundaries at higher
temperature and hence is susceptible to stress corrosion
cracking (SCC) and micro-crack initiation [Otterlooet al.
(1995)].
Atoms of Cu lower their free energy when they
migrate to lattice defects such as grain boundaries, phase
boundaries and dislocation. In Figure 7 (a) marked
region ‘A’ is un-melted region. A regular growth of
binary Al4Cu9 eutectic morphology as a result of
uniform and fast cooling time resulted in fine
microstructure as shown by arrow in Figure 7 (b) at
higher magnification. Segregation to grain boundaries
affects the mobility of the boundary and has pronounced
effects on re-crystallization, texture and grain growth.
Figure 8 shows the exsistance of gas pores or
pockets within the alloyed area for samples first painted
by adhessive and then copper powder deposition. The
voids are formed due to gas entrapment during
solidification. It is likely to occure when the rate of
solidification is fast and there is no adequate time for
the evolved gas to escape.
(b)
Figure 5 Scanning electron microscopic morphology on Cu alloyed Al: (a) Sample 4 (b) Sample 9
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SURFACE ALLOYING OF ALUMINUM WITH COPPER USING CO2 LASER
Figure 6 SEM result: (a) Sample 9 (P =1.8 kW,
P =1.6 kW,
(a) SD = 30 mm, V = 400 mm/min and re-melting(b)
SD=40mm and V = 800 mm/min). (b) Sample 13 (P = 1.8 kW, SD = 40 mm, V= 600 mm/min, re-melting = 1.5
kW, SD = 40 mm, V = 800 mm/min).
A
(b)
Figure 7Laser parameters used for Sample 5: P = 1.7 kW, SD = 30 mm, V = 400 mm, no re-melting
Porosity in the alloying
Figure 8 SEM Morphology for Sample 11 showing porosity in alloyed area (P =1.8 kW, SD = 30 mm,V = 500
mm/min Remelting,P = 1.5kW SD = 40 mm,V = 400 mm/min)
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SURFACE ALLOYING OF ALUMINUM WITH COPPER USING CO2 LASER
(a)
(b)
Figure 9 FESEM fractography test from powder of alloyed area: (a) sample 4 (b) sample 9
Powder of alloyed region was tested by FESEM to
investigate crack and defect at subsurface of alloyed
regions. No defects were observed as a result of fine
microstructure of Cu doped in aluminium forming
Al4Cu9 phase matrix with uniform and fine
microstructure shown in Figure 9 for Samples 4 and 9.
The alloyed region is free of defects.
The authors acknowledge for the facilities procured
under Department of Science and Technology FIST
project number: SR/FST/ETI-244/2008.The authors also
acknowledge for getting access to XRD Xpert facility
procured under Ministry of Steels sponsored project
number ME/P/SKJ/04 (PI: Dr. S Kanagaraj).
4 Conclusions
References
Surface alloying of commercially pure aluminium by
10µm copper powder resulted these findings;
• Uniformly deposited Cu powder by
compacting resulted in uniformly surface
alloyed layer with average surface roughness
of 1.9 µm.
• At laser power of 1.7kw and scanning speed of
500mm/min alloying and re-melting done at
lower power of 1.6kw orthogonal direction to
alloyed layer resulted in improved micro
hardness from 60HVto 156 HV.
• Formation of fine microstructure with Al4Cu9
intermetallic phase shown figure 5(a) and (b) ,
figure 6 (a) developed due to fast solidification
and uniform deposition (allying) of cu powder
on substrate material.
• First painting the substrate material and
applying cu powder at higher laser power of
1.8kw resulted grain bounder cracking of
alloyed regions which is favorable for initiation
of stress corrosion cracking figure 8(b).
• Surface allying pure aluminum with 10µm cu
powder resulted in defect free at subsurface
regions of alloyed layers as investigated by
FESEM figure 9 (a) and (b).
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Acknowledgments
634 - 6