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Applied Surface Science 253 (2007) 4947–4950

Laser surface alloying of Ni-plated steel with CO2 laser

A. Hussain, I. Ahmad, A.H. Hamdani, A. Nussair, S. Shahdin *

Pakistan Institute of Lasers and Optics, P.O. Box 505, Rawalpindi, Pakistan

Received 30 May 2006; received in revised form 31 October 2006; accepted 31 October 2006

Available online 18 December 2006

Abstract

Laser surface alloying of low carbon steel electroplated with thin (10 mm) Ni using an 850 W CW CO2 laser is reported for the first time. Fe–Ni

binary alloys of different concentrations are formed by varying laser traverse speed from 0.5 to 5 m/min. The phase transformation from a to a + g

is discussed as a function of Ni contents. Development of microstructure in the modified zone is analysed in terms of solidification rate and Ni

concentration. A three-fold increase in the microhardness of the binary alloy is observed. Formation of homogenous, adherent and crack free

surface alloys is reported.

# 2006 Elsevier B.V. All rights reserved.

PACS : 42.62.Cf; 81.65.�b; 68.35.Rh

Keywords: Laser; Surface melting; Microstructure; Martensite transformation; Binary alloy Fe–Ni; Solidification rate

1. Introduction

Laser surface alloying (LSA) is an important technique

used to enhance surface properties of materials [1,2]. It can

lead to reduction in the consumption of costly elements such

as Ni, Cr, Mo, Co, etc., as cheaper substrates can be surface

alloyed for different applications. Extensive work has been

carried out to study surface alloying of electroplated thick

layer of Ni–P and duplex layers of Ni and Cr on mild steel [3–

6]. Surface modification of low alloy steel and formation of

Fe–Ni alloys of different compositions have been reported

[7]. LSA of Ni-plated Al for enhancing surface properties has

also been reported [8]. Most of these works involve thick

layers (20–150 mm) of Ni and duplex layers of Ni and Cr.

However, to the best of our knowledge, no study with thin

(10 mm) layer of Ni on low carbon steel AISI 1010 has been

reported.

The objective of this work is to study phase transformation

from a to a + g as a function of Ni concentrations in Fe–Ni

alloy. The aim is to produce an alloy with 5% Ni as this

concentration lies on the transition line separating a and a + g

* Corresponding author. Tel.: +92 51 9268158/9; fax: +92 51 9268144.

E-mail address: pilo786@yahoo.com (S. Shahdin).

0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2006.10.067

phases. Alloys in a and a + g zones corresponding to 3 and 8%

Ni are also produced for comparison.

2. Experiment

Samples of mild steel (AISI 1010), 100 mm � 40 mm �5 mm, were prepared for laser treatment. The sample surfaces

were ground and polished to �1 mm, so that the electroplated

Ni should deposit uniformly over the whole surface area. A thin

layer of 10 mm Ni was electroplated on steel substrates and

melted with an 850 W CO2 laser. The experimental setup for

laser treatment is shown in Fig. 1. The laser beam was focused

to Ø 0.6 mm by a focusing mirror of FL = 100 mm onto the

work piece. For melting, the samples were mounted on CNC

XY table and moved under the laser beam with speeds from 0.5

to 5 m/min for single tracks. Nitrogen was used as shielding gas

to prevent surface oxidation and contamination.

Laser treated samples were cut and mounted along the cross-

section. Optical microscope was used to study the micro-

structure of the modified zone. Scanning electron microscopy

(SEM) was used to determine the concentration of Ni and other

elements using EDX analysis method. The samples were etched

in Velilla solution to resolve the microstructure. The micro-

hardness of the laser treated samples was measured using

Vicker’s hardness testing machine.

Fig. 1. Experimental setup for laser surface melting of steel.

A. Hussain et al. / Applied Surface Science 253 (2007) 4947–49504948

3. Results and discussions

3.1. Surface temperature and optimum working speed

The surface temperature ‘T’ beneath the centre of the laser

beam can be estimated by using thermo-physical parameters of

Ni [8]:

T ¼ Aq

rl

�0:147� 0:054 ln

�vr

4a

��

where q, r and v represent the laser power (W), radius of laser

spot (m) onto work piece and working speed (m s�1), respec-

tively. ‘a’ and ‘l’ are the thermal diffusivity (m2 s�1) and

thermal conductivity (W m�1 K�1) of material. The absorp-

tivity ‘A’ of polished nickel surface at CO2 laser wavelength

may vary with surface conditions and is reported �0.2 [8].

Fig. 2 is a theoretical plots of surface temperature of Ni layer

beneath the laser beam as a function of traverse speed for

three different values of ‘A’: 0.2, 0.25 and 0.3. As indicated by

Fig. 2. Theoretical plots of surface temperature vs. speed for different values of

‘‘A’’.

horizontal line, for proper melting and intermixing to form

new alloys, the surface temperature must be >1800 K. It

should ideally fall between the melting and boiling points

of Ni.

Experiments reported here indicate that samples melt and

intermix uniformly with substrate at traverse speed of

�2.0 m/min whereas at higher speeds non-uniform melting/

mixing is observed. Subsequently an optimum working range

of 0.5–2.0 m/min has been maintained through out the

experiment.

3.2. Variation of Ni concentration with speed

Ni concentration of new alloys was studied as a function of

traverse speed. Alloys with average values of 3, 5 and 8% Ni

contents were formed at traverse speeds of 0.5, 1.0 and 2.0 m/

min, respectively. The measurement error in the value of Ni

concentration through out the experiment is about �0.5%.

These alloys are characterized by dotted lines ‘a’, ‘b’, ‘c’ in the

relevant part of phase diagram (Fig. 3) and correspond to a-

phase, transition line and a + g phase, respectively [9]. The

decrease in Ni content at decreasing speed is due to longer

interaction time of the laser with the sample resulting in

increased melting/mixing of the substrate with the same amount

of Ni.

Phase transformation from a to a+g, generally, takes place

at 5% Ni content [9]. This alloy was studied a bit more closely.

Fig. 4 gives EDX analysis of the sample and confirms the

formation of Fe–Ni alloy on the surface. SEM based profile in

Fig. 5 shows the compositional variation of Ni and Fe along the

depth of the treated zone. It further confirms uniform mixing of

Ni and Fe and subsequent formation of new steel in the area.

The case depth and width of the laser treated zone is found to

Fig. 3. Relevant part of Fe–Ni phase diagram. Dotted lines represent 3, 5 and

8% Ni in Fe–Ni alloys.

Fig. 4. EDX of modified zone confirming the formation of Fe–Ni alloy.

Fig. 6. Case depth/width vs. working speed.

A. Hussain et al. / Applied Surface Science 253 (2007) 4947–4950 4949

decrease with increasing working speed (Fig. 6) due to decrease

in laser interaction time with work piece.

3.3. Microhardness measurements

Microhardness of the alloy with 5% Ni was measured both

along the surface as well as along the depth of the modified

zone. Fig. 7 shows a plot of depth versus microhardness for

different working speeds. The microhardness of the base metal

was 125 Hv where as the hardness of the melted zones was

measured in the range of 360–420 Hv, confirming a three-fold

increase. Measurements carried out across the 600 mm surface

of the modified zone indicated similar values for hardness

within experimental errors. This observation once again

confirms the excellent quality of mixing of Ni and Fe in the

newly developed alloy. Similar values of hardness were

measured for the alloy with 3% Ni while the alloy with 8% Ni

showed lower hardness due to larger grain size.

3.4. Microstructure analysis

As already indicated in Fig. 3, the alloys with 3, 5 and 8% Ni

are characterized by dotted lines ‘a’, ‘b’, ‘c’ in the phase

diagram and correspond to a-phase, transition line and a + g

Fig. 5. Composition profile of Fe and Ni in the melted zone.

phase, respectively. Ni contents stabilize the austenite to lower

transformation temperature and promote the formation of

coarse-grained coalesced bainite and martensite. This also

lowers the thermal conductivity of Fe and reduces the

transformation rate [10,11].

Fig. 8a shows the microstructure of the alloy with 3% Ni.

This lies within the a-phase having bcc structure. The

microstructure follows the melting of ferrite and pearlite and

results in transformation to martensite with dendritic structure.

The fine lathes and values of hardness confirm that the laser

treated zone is transformed to martensite.

Fig. 8b represents the microstructure of 5% Ni alloy. In this

case, the modified zone was transformed to martensite along

with some retained austenite. At this concentration, the retained

austenite starts to appear, leading to increase in grain size as

compared to the 3% Ni alloy. The measured values of hardness

for the alloy are in reasonable agreement with the values of

hardness related to the percentage of martensite for a

conventionally treated sample with similar Ni contents [12].

This confirms that the modified zone is transformed to

martensite phase with some retained austenite.

Fig. 8c shows the microstructure of 8% Ni alloy. In this alloy,

the grain size is larger as compared to 3 and 5% Ni alloys. The

larger grain size shows that the modified zone is transformed to

Fig. 7. Microhardness vs. case depth at different speeds.

Fig. 8. The development of microstructure as a function of Ni contents in Fe–Ni

alloys: (a) 3%, (b) 5% and (c) 8% Ni.

A. Hussain et al. / Applied Surface Science 253 (2007) 4947–49504950

a dual phase alloy containing martensite and austenite resulting

in reduced hardness. The development of microstructure is

dominated by Ni contents instead of cooling rate.

Cracks have been reported in alloys with Ni content �21%

[7]. This is due to the production of larger volume fractions of

fcc g-phase in the modified zone. Thermal stresses generated

during rapid solidification and ductility of materials at higher

temperature also play a critical role in the development of these

cracks [2]. The observation of crack free modified zones in the

present experiment is attributed to lower Ni contents

confirming the viability of the process for the formation of

new alloys.

4. Conclusions

Laser surface alloying of thin (10 mm) Ni-plated steel using

a CO2 laser was carried out for the first time. Homogeneous,

adherent and crack free surface alloys were obtained. The alloy

with 5% Ni, lying at the a to a + g transition line, was

successfully produced and analysed. Approximately, three-fold

increase in the microhardness was observed. The microstruc-

ture analysis confirmed that the melted zone was transformed to

martensite due to high cooling rate. Extension of the present

work to Ni and Cr and Ni–Co and Cr instead of pure Ni is in

progress and would be reported elsewhere.

Acknowledgments

Continued help and encouragement of Director General,

Pakistan Institute of Lasers and Optics, through out this work is

gratefully acknowledged. Technical discussions with the

members of the Department of Metallurgy and Materials

Engineering, GIKI (Topi) are also acknowledged.

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