1

Laser alloying of Al with mixed Ni, Ti and SiC powders

Luyolo Mabhali1,2

, Sisa Pityana1, Natasha Sacks

2

1Council for Scientific and Industrial Research - National Laser Centre, PO Box 375, Building 46F,

Pretoria, 0001, South Africa

2School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Private Bag 3,

Wits, 2050, South Africa

Abstract: Laser alloying of aluminium AA1200 was performed with a 4.4kW Rofin Sinar Nd:YAG

laser to improve the surface hardness. Alloying was carried out by depositing Ni, Ti and SiC powders

of different weight ratios on the aluminium substrate. The aim was to form surfaces on aluminium

reinforced with metal matrix composites and intermetallic phases. The laser parameters used were

4kW power and 10mm/s laser scanning speed. The microstructures formed were examined using

optical and scanning electron microscopes and the phases were identified with XRD and EDX

analytical techniques. Preliminary results showed that the microstructure of the alloyed layer depends

on the content of Ni, Ti and SiC in the deposited powder mixture. The distribution of the SiC particles

was not homogeneous. Intermetallic phases were formed between Al-Ni-Ti-SiC during laser

alloying. An increase in surface hardness was achieved after laser alloying. A surface hardness of

approximately 356.8±43.4 HV0.1 was achieved after alloying with a powder containing 70wt%Ni,

20wt%Ti and 10wt%SiC. The increase in hardness was attributed to the formation of the intermetallic

phases.

Key words: Laser alloying; Aluminium; Nickel; Titanium; Silicon carbide; Metal matrix composite;

Intermetallic phases.

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Introduction

Aluminium is widely used in industry due to its low cost, light weight and excellent workability.

However its low wear resistance and hardness are limiting factors in many applications. Laser

alloying may be used to improve the aluminium surface properties such as hardness by modifying

the composition and microstructure of the surface without affecting the bulk properties of the

material [1,2,3]. This process involves melting the substrate surface and injecting powder of the

alloying material into the melt pool. Process parameters such as laser power, laser beam spot size,

laser scanning speed and powder feed rate have to be controlled to achieve the desired surface

properties [4].

The surface properties can be modified by adding elemental powders, thereby forming intermetallic

compounds. An intermetallic compound is a solid phase consisting of two or more metallic

elements in definite proportions. The intermetallic compounds generally have superior properties

compared to the base aluminium such as high hardness and high wear and corrosion resistance but

have low ductility [2,4,5]. Hard particles such as Al3O2, SiC, TiC and WC can also be injected into

the surface of a metal matrix to improve surface properties [6-8]. In this way a surface metal matrix

composite (MMC) is formed. The MMC layer has excellent hardness and wear resistance compared

to the base alloy [9-13].

Man et al. [14] used a high power continuous wave Nd:YAG laser to alloy aluminium AA 6061 with

preplaced NiTi (54 wt% Ni & 46 wt% Ti) powder to improve its hardness and wear resistance. A

laser alloyed surface free of cracks and pores was achieved and SEM micrographs showed a

dendritic microstructure. XRD patterns confirmed the intermetallics formed as TiAl3 and Ni3Al. The

wear tests were performed on a pin on disc setup. A hardness increase of 200HV and wear

resistance of about 5.5 times that of the virgin substrate was achieved for the modified layer.

Ternary intermetallic phases and the wear mechanisms were not reported by the authors.

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Ravi et al. [2] laser alloyed aluminium with preplaced Ni and Cr powders. The alloying was

performed with a 3kW continuous wave CO2 laser. The hardness of the laser alloyed surface

increased with decreasing Ni and a maximum of 490HV was achieved with the composition of

20wt%Ni-80wt%Cr. The microstructure showed needle type structures for alloys with a high Ni

composition which disappear as the Cr content is increased. The microstructure undergoes a phase

transformation when the Cr concentration is increased. The intermetallic phases formed were AlNi,

Al3Ni2, AlNi3, AlCr2, Cr9Al17 and Cr3Ni2. The wear mechanisms of these alloys were not

investigated.

Man et al. [15] synthesised TiC in situ on AA6061 aluminium surface by alloying with SiC and Ti

powders. A Nd:YAG laser was used during the alloying at 1 to 1.5kW of power. The laser scanning

speed was between 5 and 25mm/s and the track overlap was 50%. The optimum powder

composition for a high quality surface metal matrix composite was achieved with 40wt% SiC and

60wt%Ti. The average size of the TiC particles that formed after alloying was less than 3μm and

these were uniformly dispersed in the matrix. XRD analysis of the alloyed layer revealed TiC, TiAl,

Ti3Al, SiC, Al and Si phases. The hardness increased from 75HV to 650HV due to the formation of

the TiC particles and TiAl and Ti3Al intermetallics.

The laser cladding of Al-12wt%Si with a powder containing SiC and Al-12wt%SiC in a 3:1 volume

ratio were performed by Su and Lei [9]. A CO2 laser was used with 2-4kW laser power, 2-10mm/s

laser scanning speed and a 3mm defocus laser beam spot size. The aim of the study was to form a

surface MMC layer on the aluminium matrix. It was reported that the laser melting of SiC particles

onto an aluminium substrate produces aluminium carbides. The presence of the Al4C3 phase in

MMC is not desirable as it is brittle and hydroscopic. The addition of Al-12wt%Si was found to

suppress or eliminate the aluminium carbides in the MMC layer. A good distribution of injected SiC

particles was achieved near the surface. The microhardness of the coating was between 220 and 280

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HV0.1.

The aim of this work is to improve the surface properties of aluminium AA1200 by forming metal

matrix composites and intermetallic compounds into the surface layer during laser alloying. The

alloying powder was a mixture of Ni, Ti and SiC particles. The microstructure, hardness and in situ

products were studied.

Experimental Method

The aluminium AA1200 was cut into 100x100mm2 plates with a thickness of 6mm. These plates

were sand blasted and cleaned with acetone prior to alloying. The chemical composition of the

aluminium AA1200 base was 0.12wt%Cu, 0.13wt%Si, 0.59wt%Fe and the balance was Al. The

laser surface alloying-particle injection was carried out with a 4.4kW Rofin Sinar Nd:YAG laser

using the experimental setup shown in Figure 1. The processing parameters used were 4kW laser

power, 10mm/s laser scanning speed and 4mm defocus laser beam spot size. An off-axis nozzle

placed 12mm from the aluminium plate was used for powder injection. This arrangement assured

that the powder stream coincided with the laser beam at the interaction zone. Argon was used as

the carrier and shielding gas to prevent oxidation during the alloying process and the flow rate was

4L/min. The incident specific laser energy density of 100MJ/m2 was calculated from the following

equation:

E (MJ/m2) = P/(dBv) (1)

where P (W) is the laser power, dB (m) is the diameter of the laser beam spot and v (m/s) is the

laser scanning speed.

It should be noted that the absorbed energy is less than the incident specific laser energy as much of

incident radiation is reflected by the base material and is dispersed away from the interaction zone

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during laser beam irradiation.

Figure 1: Experimental setup using Nd:YAG laser.

The alloying powders used were Ni, Ti and SiC powder particles of different weight ratios. The

powder compositions used were 33.3wt%Ni + 33.3wt%Ti + 33.3wt%SiC, 50wt%Ni + 20wt%Ti +

30wt%SiC and 70wt%Ni + 20wt%Ti + 10wt%SiC. These compositions were chosen since the

effect of increasing Ni content and reducing SiC content could be studied as the Ti content was not

significantly varied. Al-Ni intermetallic phases are known to increase the surface hardness of

aluminium alloys [1,2]. The powder particle morphology and size distribution were analyzed using

a scanning electron microscope (SEM) and Malvern Mastersizer 2000 image analyzer. Laser

alloying with powders containing Ni only (100wt%Ni), Ti only (100wt%Ti) and 50wt%Ni +

50wt%Ti was also performed for comparison.

Cross-sections of the alloyed surface layers were cut, mounted and polished to a 1μm finish. The

polished surfaces were etched using Keller’s reagent. The microstructure and chemical composition

of deposited layers were studied using optical and scanning electron microscopy with energy

dispersive x-ray (EDX) analysis. X-ray diffraction (XRD) was used for identifying the phases

KUKA articulated

arm robot

Off axis nozzle

Cooling water

Aluminium sample

Powder feeding tube

Alloying head

Nd:YAG fibre optics

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formed.

The hardness measurements of the specimens were performed on polished cross-sections using a

Vickers microhardness tester with a load of 100g. Hardness profiles were performed for each

alloying composition depicting the hardness from the alloyed surface through to the base aluminium.

For the profiles the spacing between two consecutive hardness indents was 100μm.

Results and Discussion

The Ni, Ti and SiC powder particle morphology was investigated. The shape of the particles was

determined from SEM micrographs and the size distribution was determined using a Malvern

Mastersizer 2000 image analyzer. The results are given in Table I.

Table I: Sizes and shapes of the Ni, Ti and SiC powder particles.

Powder Ni Ti SiC

Size (μm) 7-200 5-158 14-800

Shape

Spherical &

irregular

Irregular

agglomerates

Irregular

Alloyed layers

Figure 2 shows a laser alloyed surface with a 33.3wt%Ni, 33.3wt%Ti and 33.3wt%SiC powder. The

time taken to complete a single laser track was calculated from the length of a single laser track

(100mm) and the laser scanning speed (10mm/s) as 10s. The amount of powder added to the

alloying surfaces was approximately 0.3g after 10s giving a feed rate of 0.03g/s. The depth of the

laser alloyed surface layer was 1.81mm. SiC particles can be observed in the alloyed surface layer

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showing that a metal matrix composite was formed.

Figure 2: The alloyed surface layer formed after laser alloying with a powder containing

33.3wt%Ni, 33.3wt%Ti and 33.3wt%SiC.

Figure 3 shows a typical microstructure of this alloyed surface (33.3wt%Ni, 33.3wt%Ti and

33.3wt%SiC). The SiC particles are not homogeneously distributed within the alloyed layer. A large

variation in size of the SiC particles is observed in the alloyed surface layer due to the wide particle

size distribution of the SiC particles (Table I).

SiC particle

SiC particles

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Figure 3: Optical micrograph of an alloyed surface layer showing SiC particles distributed within

the alloyed layer.

The shape of the alloyed surface layer for specimen alloyed with 50wt%Ni + 20wt%Ti + 30wt%SiC

and 70wt%Ni + 20wt%Ti + 10wt%SiC were similar to that observed with 33.3wt%Ni + 33.3wt%Ti

+ 33.3wt%SiC shown in Figure 2. The depth of the laser alloyed surface layer was approximately

2.23mm for 50wt%Ni + 20wt%Ti + 30wt%SiC and 1.64mm for 70wt%Ni + 20wt%Ti + 10wt%SiC

which were close to 1.81mm achieved with 33.3wt%Ni + 33.3wt%Ti + 33.3wt%SiC.

Microstructure

Laser alloying Al AA1200 with Ni only resulted in the formation of Al3Ni and Al3Ni2 phases while

alloying with Ti resulted in the formation of an Al3Ti phase. Al3Ni, Al3Ni2 and Al3Ti phases were

formed when laser alloying with 50wt%Ni + 50wt%Ti. A SEM micrograph of a surface laser

alloyed with a powder containing 33.3wt%Ni, 33.3wt%Ti and 33.3wt%SiC is shown in Figure 4.

The phases were identified using XRD and EDX. The reaction of Al with Ni resulted in the

formation of Al3Ni and Al3Ni2. The reaction of Al with Ti resulted in the formation of Al3Ti. The

SiC particles act as reinforcements resulting in the formation of an aluminium matrix composite.

Some of the SiC particles dissociate due to high laser alloying temperatures to formed Si and C. The

C reacts with either Ti or Al to form TiC or Al4C3. The changes in Gibbs free energy ΔG for the

reactions are given by [15]:

4/3 Al + C → 1/3 Al4C3

ΔG (kJ/mol) = 1/3 ΔGAl4C3 = -89.611 + 0.0328T

Ti + C → TiC

ΔG (kJ/mol) = -205.351 + 0.0291T

The Gibbs free energy is more negative for the formation of TiC and thus there was a higher

tendency for the formation of TiC than of Al4C3. The C then reacted with Ti to form TiC dendrites.

The SiC also reacted with Al to form Al4SiC4 intermetallic phases.

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Figure 4: SEM cross-sectional micrograph (near surface region) of a surface alloyed with a powder

containing 33.3wt%Ni, 33.3wt%Ti and 33.3wt%SiC.

The XRD diffractograph of the surface laser alloyed with a powder containing 33.3wt%Ni,

33.3wt%Ti and 33.3wt%SiC at 10mm/s laser scanning speed is shown in Figure 5. The phases

observed were Al, SiC, TiC, Al3Ni, Al3Ni2, Al3Ti and Al4SiC4. The XRD results together with the

results from the EDX were used to identify the phases in the microstructure.

SiC

Al3Ni

Al3Ni2

Al3Ti

TiC

10

33.3 wt% Ni + 33.3 wt% Ti + 33.3 wt% SiC (0.01m/s)

0

100

200

300

400

500

600

700

800

20 40 60 80 100 120

2 Theta (degrees)

Inte

ns

ity

(A

.U)

Figure 5: XRD diffractograph of surface laser alloyed with a powder containing 33.3wt%Ni,

33.3wt%Ti and 33.3wt%SiC at 10mm/s laser scanning speed.

Figure 6 shows a microstructure of an aluminium surface alloyed with a powder containing

50wt%Ni, 20wt%Ti and 30wt%SiC. The phases observed are SiC, Al3Ni, Al3Ni2 and Al4C3. Due to

low Ti content in the alloying powder, the formation of the brittle Al4C3 phase was not avoided.

This Al4C3 phase was formed according to the following equation.

4Al + 3SiC → Al4C3 + 3Si

The phases in the microstructure were identified using XRD and EDC. An Al3Ti phase was also

detected due to the reaction of Al with Ti. The interfacial TiC phases were formed around the SiC

particles due to adsorption of Ti on the SiC particle surface.

1 = Al

2 = SiC

3 = TiC

4 = Al3Ni

5 = Al3Ni2

6 = Al3Ti

7 = Al4SiC4

4

1

1 2,3 2,7 7 4 2

1,5

1,6 3

1

1 5

1,5

2,3

4 1 6 3 2

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Figure 6: SEM cross-sectional micrograph (near surface region) of a surface alloyed with a powder

containing 50wt%Ni, 20wt%Ti and 30wt%SiC.

Figure 7 shows a microstructure of an aluminium surface alloyed with a powder containing

70wt%Ni, 20wt%Ti and 10wt%SiC. Since Ni was dominant in the alloying powder, Al3Ni phase

dominated the microstructure of the alloyed layer. An Al3Ti phase was also observed due to the

reaction of Al with Ti. Other phases identified from XRD analysis were SiC and Al3Ni2.

Al4C3

SiC

Al3Ni

Al3Ni2

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Figure 7: SEM cross-sectional micrograph of a surface alloyed with a powder containing 70wt%Ni,

20wt%Ti and 10wt%SiC.

Hardness

The hardness profiles of surfaces laser alloyed with Ni, Ti and SiC powders of different ratios are

shown in Figure 8. The average hardness values are given in Table II. Amongst the surfaces laser

alloyed with different ratios of Ni, Ti and SiC, the highest hardness of 362.5±31.9HV was achieved

when alloying with 70wt%Ni + 20wt%Ti + 30wt%SiC. Laser alloying with 33.3wt%Ni+

33.3wt%Ti + 33.3wt%SiC and 50wt%Ni+ 20wt%Ti + 30wt%SiC powders results in a similar

hardness profiles. Formation of Al3Ni and Al3Ni2 phases result in very high hardness of

766.9±38.5HV as was achieved when alloying with Ni only. The reaction of Al with Ti to form an

Al3Ti phase reduced the hardness to 220±14.7HV when alloying with 50wt%Ni + 50wt%Ti and

109±17HV when alloying with Ti only. The dominance of the Al-Ni intermetallic phases (Al3Ni and

Al3Ni2) in the microstructure of the surfaces laser alloyed with 70wt%Ni + 20wt%Ti + 30wt%SiC

resulted in the high hardness observed.

Al3Ni

Al3Ti

13

0.0

50.0

100.0

150.0

200.0

250.0

300.0

350.0

400.0

450.0

0 500 1000 1500 2000 2500 3000

Position (µm)

Vic

ke

rs H

ard

ne

ss

(H

V)

33.3wt%Ni + 33.3wt%Ti + 33.3wt%SiC 50wt%Ni + 20wt%Ti + 30wt%SiC 70wt%Ni + 20wt%Ti + 10wt%SiC

Figure 8: Hardness profile of the alloyed surface layers.

Table II: Average hardness of the alloyed surface layers.

Powder content Hardness (HV0.1)

100wt%Ni 766.9±38.5

100wt%Ti 109±17

50wt%Ni + 50wt%Ti 220±14.7

33.3wt%Ni + 33.3wt%Ti + 33.3wt%SiC 95.5±16.9

50wt%Ni + 20wt%Ti + 30wt%SiC 94.4±16.0

70wt%Ni + 20wt%Ti + 10wt%SiC 362.5±31.9

Conclusion

Laser surface alloying of aluminium AA1200 with powders containing Ni, Ti and SiC result in an

increase in hardness. The increase is 4 times when alloying with 33.3wt%Ni + 33.3wt%Ti +

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33.3wt%SiC and 50wt%Ni + 20wt%Ti + 30wt%SiC and 14 times when alloying with 70wt%Ni +

20wt%Ti + 30wt%SiC. The increase in hardness is attributed to the formation of the hard and brittle

Al3Ni, Al3Ni2 intermetallic phases. Laser alloying with Ni only resulted in a very high surface

hardness of 766.9±38.5HV. Therefore, for surface hardness measurements only, laser surface

alloying Al with Ni only is more effective than combining MMC and intermetallic phases. Al3Ti

phase is not as affective in improving the surface hardness as the Al-Ni intermetallic phases. The

retained SiC particles were not homogeneously distributed in the alloyed layers. The TiC was

formed in situ during the alloying. If the SiC content is greater than the Ti content in the alloying

powder mixture, then the Al4C3 phase is formed from the reaction of the excess C (from

dissociation of SiC) with Al.

Acknowledgements

The Department of Science and Technology and the Council for Scientific and Industrial Research

are acknowledged for financial support.

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