The document summarizes research on laser surface alloying of aluminum alloy AA1200 with nickel powder. Key findings: 1. Laser surface alloying was used to successfully harden the surface of aluminum alloy AA1200, improving hardness from 24HV to over 646HV, over 20 times harder than the substrate. 2. The best results were obtained at the lowest laser scan speed of 0.6 m/min. Higher scan speeds produced less hardened surfaces. 3. Microstructure analysis found the alloyed surfaces contained dendritic structures and intermetallic phases including Al4Ni3, Al3Ni2, and Al1.1Ni0.9, which contributed to the high hardness.

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1. Department of Chemical and Metallurgical Engineering, Tshwane University of Technology
2. Center for Scientific and Industrial Research – National Laser Centre
IINNTTRROODDUUCCTTIIOONN
As the automotive industries addresses environmental concerns, the
problem of fuel consumption and weight reduction has come to fore.
Various test works have been done to introduce alternative aluminum
alloy due to its low cost, high strength- weight ratio, low density and
good corrosion resistance. Aluminum exhibit weak interatomic bonds,
low abrasion resistance and low melting temperature does not suit the
element in some engineering applications.
An intermediary element (Nickel) is introduced as an alloying substance
to form a metallic layer on the surface of the alloy. Pure Ni is ductile,
tough and possesses a face-centred cube crystal structure up to its
melting point [1, 2, 6]. The alloying process was done by the use of high
power Rofin Sinar ND: YAG laser machine with Ni as a coating element.
Laser surface alloying is particularly efficient for producing surface layers
with improved wear resistance. It combines the controlled modification of
the microstructure and chemical composition to tailor surface properties
[1, 2,5,].
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The following work relates to surface hardening of aluminum alloy
AA1200 of known micro hardness of ~24HV. The test work is to be
performed at four different laser scan speeds (0.6, 0.8, 1.0, 1.2m/min) to
produced the best hardened alloy. The chemical composition of the
aluminum substrate to be used, is indicated in table 1.
Table 1: Chemical composition of the alloy AA1200
Table 2: Laser processing parameters on all four samples produced
Particle size analyses of the Nickel powder determined using the Malvern
Mastersizer 2000 indicated an average particle size of 30mm. Figure 1
indicates size distribution of Nickel particles against the bulk volume.
Figure 1: Particle size distribution of Ni powder
Figure2: Scanning electron micrograph and EDS of Ni powder
The EDS indicated high peaks of Nickel, which indicates that the powder to
be used for alloying is pure. Figure 3 indicates the X-diffractograph of the Ni
powder used for alloying. The XRD analysis on the Nickel powder showed
their patterns with only Nickel, powder purity confirmed.
Figure 3: Xray Diffractograph of the Nickel powder
10 20 30 40 50 60 70 80
3000
2000
2500
2000
1500
1000
500
1 = Al
Figure 5: Microhardness distribution of laser-alloyed surface under
different laser beam energy.
900
800
700
600
500
400
300
200
100
A crackless, uniformly alloyed surface and good bonding was obtained
from sample A as illustrated in figure 6. The depth of the alloyed surfaces
decreases with an increase in laser scan speed. For sample A and
sample D, the depth is 1.46mm and 1.20mm respectively.
Figure 6: Stereo micrograph of the laser-surfaced AA1200 with Nickel
(sample A) at 0.6m/min laser scan speed.
The X-Ray diffraction results of sample A shown in figure 8 reveal that the
laser treated surface is composed mainly of pure aluminum and secondary
phases (Al4Ni3, Al3Ni2 and Al1.1Ni0.9). Some of the phases have
overlapping peaks. The EDS analysis of the samples confirmed the
presence of Ni, Al and O. Very slight trace of C elements were shown to
be present in the EDS analysis of samples A.
Figure 8: X-ray diffractograph of a typical alloy layer; laser processing
parameter: laser power 4 kW, scan speed 0.6 m/min and powder feed
rate 2rpm.
20 30 40 50 60 70 80 90 100 110
22500
10000
2500
AACCKKNNOOWWLLEEDDGGEEMMEENNTTSS
The author would like to thank Tshwane University of Technology and CSIR-National
Laser Centre for financial support of this work.
RREEFFEERREENNCCEESS
[1]S.L. Pityana, Hardfacing of aluminium alloys by means of Metal Matrix Composites
produced by laser surfacing alloying, Proceedings of LIM-2009, Munich, Germany, 439-
444.
[2] M.H. Staia, M. Cruz, N.B. Dahotre, Microstructural and tribological characterization of an
A-356 aluminium alloy superficially modified by laser alloying, Thin Solid Films 377-378,
(2000) 665-674.
[3] C. Tassin, F. Laroudie, M. Pons, L. Lelait, Improvement of the wear resistance of 316L
stainless steel by laser surface alloying, Surface ad Coatings Technology 80, (1996) 207-
210.
[4] H.C. Man, S. Zhang, T.M. Yue, F.T. Cheng, Laser surface alloying of NiCrSiB on Al6061
aluminium alloy, Surface and Coatings Technology 148, (2001) 136-142.
[5] R.J. Davis, J. R. Davis & Associates, Aluminum and aluminum alloys, ASTM
International. Handbook Committee, (1993) 574-579.
Position [°2Theta]
Counts
0
1000
Ni
Ni
Ni
NIPOW.RD
The X-ray diffractograph of the AA1200 alloy can be seen in Figure 4. This
shows the identified phases present in the Al; only aluminum peaks can be
seen, an evidence of the purity of the substrate. The micro hardness value
of the AA 1200 is 24.0 ± 0.4.
0
20 40 60 80 100 120
2 Theta (degrees)
Intensity (A.U)
1
1
1
1
1 1
1 1
Figure 4: Xray diffractograph of AA1200 aluminum alloy
Sample A is the best improved sample on hardness value (~646.9Hv). The
dispersion of HV obtained is related to the heterogenouity of the surface
microstructure. The lowering of the hardness can be attributed to melting
and mixing of more aluminium substrate into the coating, which is clearly
demonstrated. The improvement in hardness (over 20 times the
microhardness of substrate) was attributed to the formation of intermetallic
phases/dendritic microstructure formed by alloying.
Hardness profile with depth of each sample
0
0 500 1000 1500 2000
Position (microns)
Hardness (Hv)
sample A
(0.6m/min)
sample B
(0.8m/min)
sample C
(1.0m/min)
sample D
(1.2m/min)
Figure6: (a) Scanning electron micrograph and EDS of Sample A
(0.6m/min), (b)Optical micrograph of sample A
The optical micrograph, the scanning electron micrographs with the EDS of
the cross section of the polished samples laser alloyed can be seen in
Figures 6 and 7. The microstructure is dendritic in nature for both samples
alloyed.
Figure 7: (a) scanning electron micrograph and EDS of sample D
(1.2m/min) (b) optical micrograph of sample D
Single track lines were made on the AA 1200. Cross-sections of the alloyed
layers were cut and polished. The polished surfaces were etched using
Keller’s reagent. The microstructures of the new phases were
characterized by optical and SEM. The characteristics of the phases were
studied by means of X-ray diffraction. It was analysed with a PANalytical
X’Pert Pro powder diffractometer with X’Celerator detector and variable
divergence- and receiving slits with Fe filtered Co-Kα radiation. The phases
were identified using X’Pert Highscore plus software.
Hardness was determined using the Vickers hardness tester.100 μm
spacing between corresponding indentations with an applied load of 200 g
and a holding time of 5 seconds was used. The hardness/depth profiles
were plotted for all samples.
This indicated high possibilities of a uniform reactivity between the powder
and aluminum substrate during laser alloying. Figure 2 indicates the
scanning electron microscopy and the EDS of Nickel powder. The shapes
of the Ni particles are round and irregular, which assist in providing
constant, frictionless movement when fed onto the molten surface of the
substrate during alloying.
Sample label A B C D
System composition Al-Ni Al-Ni Al-Ni Al-Ni
Laser Power (kW) 4.0 4.0 4.0 4.0
Beam diameter (mm) 3.0 3.0 3.0 3.0
Scan speed (m/min) 0.6 0.8 1.0 1.2
Powder feed rate (rpm) 2.0 2.0 2.0 2.0
Shielding gas Argon Argon Argon Argon
Shielding gas flow
4.0 4.0 4.0 4.0
(l/min)
RREESSUULLTTSS
Position [°2Theta] (Cobalt (Co))
Counts
0
PopoolaP_Al-Ni_7
Peak List
Al; Aluminum, syn; Cubic; Fm-3m
Al1.1 Ni0.9; Cubic; Pm-3m
Al4 Ni3; Cubic; Ia-3d
Al3 Ni2; Hexagonal; P-3m1
CCOONNCCLLUUSSIIOONN
1. Laser surface alloying of AA 1200 pure aluminium with Nickel
reinforcement using a Rofin Sinar continuous wave Nd: YAG solid-state
laser was successfully carried out. The best alloyed surface with the
highest HV was obtained at the lowest Laser scan speed. The hardness
of the newly produced surface is 20 times higher than the original
substrate. The average HV of the alloy was improved from ~24 to
~646HV.
2. The alloyed surfaces microstructure formed consisted of dendritic
structure and different intermetallic phases which includes: Al4Ni3,
Al3Ni2, Al1.1Ni0.9. A crackless surface was achieved.
Element Al Fe Cu Si
Composition (wt.%) Balance 0.59 0.12 0.13
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The substrate material, was cut to dimensions 100 x 100 x 6 mm, sand
blasted to clean the surface of substrate and to improve absorptivity of the
laser beam.
The Malvern Mastersizer was used for the analysis of the Ni powder
particle size distribution. The powder particle morphology and size
distribution were analyzed using a scanning electron microscope SEM. A
Philips PW 1713 X-ray diffractometer fitted with a monochromatic Cu Kα
radiation set at 40 kV and 20 mA was used to determine the phase
composition of powder. The scan was taken between 10 and 80 two theta
(2Θ) with a step size of 0.02 degree. Phase identification was done using
Philips Analytical X’Pert HighScore® software with an in-built International
Centre for Diffraction Data (ICSD) database.
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Laser surface alloying was carried out with a high power Rofin Sinar Nd:
YAG solid-state laser fitted with off-axes nozzle used for powder feeding.
The laser is delivered to the substrate through fibre optics. A Kuka robot is
used to move the alloying head. Argon gas was used to shroud the molten
pool from the atmosphere to prevent oxidation during the alloying process.
Laser parameters are shown on table 2.