D. JANICKI: IN-SITU SYNTHESIS OF TITANIUM CARBIDE PARTICLES IN AN IRON MATRIX DURING ...
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IN-SITU SYNTHESIS OF TITANIUM CARBIDE PARTICLES IN AN
IRON MATRIX DURING DIODE-LASER SURFACE ALLOYING OF
DUCTILE CAST IRON
IN SITU SINTEZA DELCEV TITANOVEGA KARBIDA V OSNOVI
@ELEZA MED POVR[INSKIM LEGIRANJEM LITEGA @ELEZA Z
DIODNIM LASERJEM
Damian Janicki
Silesian University of Technology, Faculty of Mechanical Engineering, Welding Department, 18a Konarskiego Street,
44-100 Gliwice, Poland
damian.janicki@polsl.pl
Prejem rokopisa – received: 2015-07-02; sprejem za objavo – accepted for publication: 2016-05-03
doi:10.17222/mit.2015.198
An in-situ formation of titanium carbide (TiC) particles in an iron matrix during diode-laser surface alloying of ductile cast iron
(DCI) with direct injection of titanium powder into a molten pool was investigated. The microstructure of the in-situ
TiC-reinforced surface alloying layers (SALs) was assessed with scanning electron microscopy (SEM), energy-dispersive
spectroscopy (EDS) and X-ray diffraction (XRD). Comparative erosion tests of the as-received DCI and the SALs were
performed following the ASTM G 76 standard test method. It was found that the morphology and volume fraction of the TiC
particles directly depend on the amount of the titanium powder introduced into the molten pool, which, in turn, is strongly
affected by the laser-power level. An increase in the titanium concentration in the molten pool results in a change in the TiC
morphology, form cubic (> 2 μm in size) to dendritic (up to 10 μm in size). The maximum volume fraction of the TiC particles
in the SALs was 12 %. The SALs exhibited a noticeable increase in the erosion resistance in comparison to the as-received DCI,
especially at steep angles.
Keywords: in-situ composite, composite surface layer, TiC, laser surface alloying, ductile cast iron, erosive wear
Preiskovan je bil in situ nastanek delcev titanovega karbida (TiC) v osnovi `eleza med legiranjem povr{ine duktilnega litega
~eleza (DCI) z diodnim laserjem, z vpihovanjem titanovega prahu v staljeno kopel. Mikrostruktura povr{inskega legiranega sloja
(SAL), oja~anega z in situ nastalim TiC je bila ocenjena z vrsti~nim elektronskim mikroskopom (SEM), z energijsko
disperzijsko spektroskopijo (EDS) in z rentgensko difrakcijo (XRD). Primerjalni erozijski preizkusi med dobavljenim DCI in
SAL so bili izvedeni po ASTM G 76 standardni preizkusni metodi. Ugotovljeno je, da sta morfologija in volumski dele` TiC
delcev neposredno odvisna od koli~ine prahu titana, ki je doveden v talino, kar je posledi~no mo~no odvisno od mo~i laserja.
Pove~anje koncentracije titana v staljeni kopeli se odra`a na spremembi morfologije TiC iz kubi~ne (pri velikosti > 2 μm) v
dendritno (do velikosti 10 μm). Najve~ji volumski dele` delcev TiC v SAL je bil 12 %. SAL je pokazal ob~utno pove~anje
odpornosti na erozijo v primerjavi z dobavljenim DCI, {e posebno pri ve~jih kotih.
Klju~ne besede: in situ kompozit, kompozitna povr{inska plast. TiC, legiranje povr{ine z laserjem, duktilno lito `elezo, erozijska
obraba
1 INTRODUCTION
Ductile cast iron (DCI) is extensively used for manu-
facturing machine parts in many industries such as auto-
motive, power generation, mining, military and agricul-
tural industry.1,2 However, in some applications, there is a
need to increase the durability of machine parts by
improving their resistance to erosion.3–7 The enhance-
ment of erosion resistance of DCI can be achieved by
reinforcing its surface layer with ceramic particles (the
reinforcing phase), in particular, with an in-situ synthesis
of titanium carbide (TiC). During the in-situ fabrication
of composite surface layers (CSLs), reinforcing phases
(RPs) are formed throughout the matrix (from the melt)
due to a chemical reaction.8 As a result, the RPs are free
of contaminants and have a high coherency and a strong
interface with the metal matrix. The difficulty with the
in-situ fabrication of CSLs is related to the fact that the
distribution homogeneity and the average particle size of
an RP depend on the solidification conditions and the
fluid flow in the molten pool, so they are difficult to
control.8–10
Laser surface alloying is particularly well suited to
the needs of the in-situ fabrication of the CSLs, mainly
due to an accurate control of the molten-pool solidifi-
cation.11 Moreover, the molten pool generated during
laser processing undergoes a rapid solidification, which
provides a unique opportunity to synthesize non-equili-
brium phases.10 To date, a number of studies have
focused on the laser surface alloying of both grey and
ductile cast irons in order to improve the wear resistance.
Some studies also attempted to investigate the in-situ
formation of CSLs on a cast-iron substrate.8,9 Most of the
research, however, have been conducted using CO2 and
various types of Nd:YAG lasers. A high-power direct-
diode laser (HPDDL) with a uniform intensity distri-
Materiali in tehnologije / Materials and technology 51 (2017) 2, 317–321 317
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Professional article/Strokovni ~lanek MTAEC9, 51(2)317(2017)
bution over the rectangular beam spot (called a flat-top
beam) is especially suited for the surface-alloying
process with direct injection of alloying powder into the
molten pool. In the above-mentioned surface-alloying
process, the flat-top beam of HPDDL and the powder
shape (the powder spot size) match the laser-beam spot
result in reducing the heat input and providing a better
control of the molten-pool solidification.12,13
The present study investigated an in-situ formation of
TiC particles during laser surface alloying of DCI with
direct injection of titanium powder into the molten pool,
using a high-power direct-diode laser (HPDDL) with a
rectangular laser beam spot and a uniform distribution of
the laser power (the flat-top beam). This study was
specifically concerned with establishing the effect of
processing parameters on the precipitation morphology
of TiC particles and its distribution throughout the iron
matrix.
2 EXPERIMENTAL PROCEDURE
The substrate material used in this investigation was a
ductile cast iron (DCI) with its chemical composition
shown in Table 1. Microstructurally, the DCI consisted
of graphite spheres with the average diameter of 35 μm
in a pearlite matrix. The substrate specimens, in the form
of 10-mm discs with a diameter of 50 mm, were ground
to a surface finish of 0.5 μm Ra and cleaned with acetone
prior to the alloying process. A commercially available
titanium powder (99.6 % purity) having a particle size
range of 45–75 μm was used as the alloying material.
The laser surface alloying (LSA) was carried out
using a high-power direct-diode laser (HPDDL) Rofin-
Sinar DL020 with the maximum output power of 2.0 kW.
The rectangular HPDDL beam spot with a size of 1.8 ×
6.8 mm and a focal length of 82 mm was focused on the
top surface of the substrate and the long beam axis was
set transversely to the traverse direction (6.8 mm wide).
The titanium powder was injected into the molten pool
using an off-axis powder injection nozzle. The powder
feed-rate accuracy of the used feeder was within 1 %.
The molten pool was protected by a shielding gas –
argon – at a flow rate of 10 L/min. The cylindrical
shielding-gas nozzle with a diameter of 20 mm was set
coaxially with the powder feeding nozzle.
The investigation of the LSA process was performed
based on the amount of titanium powder (the powder
feed rate) provided per unit length of a single-pass alloy
bead (SAB) in a range of 0.0010–0.0050 g/mm. To study
the impact of the laser-power level and the traverse speed
on the SAB geometry, microstructure and hardness, the
experiments were conducted at three laser-power levels,
1000 W, 1200 W and 1400 W, and different traverse
speeds, ranging from 0.1 m/min to 0.25 m/min. The
surface-alloyed layers (SALs) were produced with the
multi-pass overlapping alloying with an overlap ratio of
40 %. The experiments were performed without pre-
heating the substrate.
Fluorescent dry penetrant testing was used to detect
cracks of the SABs and SALs. The structure and mor-
phology of the SALs were investigated with scanning
electron microscopy (SEM) and energy-dispersive spec-
troscopy (EDS). The volume fraction of TiC was
measured using a Nikon NIS-Elements quantitative
image analysis system. The measurements were con-
ducted on cross-sections from under the surface and in
the middle areas of the SALs, over a total area of 10 mm2
for each SAL. Geometrical parameters of the SABs were
measured using an optical microscope and the Image
Analyzer software. The cross-sectional microhardness
distribution of the SALs was measured by means of a
Wilson Wolpert 401 MVD Vickers hardness tester at a
load of 200 g for a dwell time of 10 s. The X-ray
diffraction analysis of the as-received DCI and the SALs
was conducted using a PANalytical X’Pert PRO MPD
X-ray diffractometer equipped with an X’Celerator
detector and a Co-K ( = 0.179 nm) source. The X-ray
tube was operated at 40 kV and 30 mA.
Erosion-resistance tests of the as-received DCI and
SALs were performed at impact angles of 30° and 90° in
accordance with standard ASTM G76-95. Angular
alumina powder (Al2O3) with the average particle size of
50 μm was used as the abrasive material. The impact
velocity of the abrasive particles was kept in a range of
30±2 m/s. The abrasive-particle flow rate and test period
were 2.0±0.5 g/min and 10 min, respectively. After the
erosive wear tests, the mass loss of the specimens was
measured with an accuracy of 0.01 mg.
3 RESULTS AND DISCUSSION
Cross-sectional macrographs of the single-pass
alloyed beads (SABs) produced at a traverse speed of 0.2
m/min, a powder feed rate of 0.0016 g/mm and two
levels of laser power are presented in Figure 1. Fusion-
zone cross-sectional shapes indicate that, during the
investigated LSA process, the surface-tension tempera-
ture coefficient on the molten-pool surface was positive.
In this case, the surface tension is highest at the center of
the molten pool, producing an inward fluid flow along
the surface of the molten pool.14 As a result, the SABs
have a hemispherical shape in their cross-sections. Fur-
D. JANICKI: IN SITU SYNTHESIS OF TITANIUM CARBIDE PARTICLES IN AN IRON MATRIX DURING ...
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Table 1: Chemical composition of the investigated ductile cast iron
Tabela 1: Kemijska sestava preiskovanega duktilnega litega `eleza
C Si Mn Cu Cr Ni Ti Al Mg P S Fe
3.21 2.58 0.23 0.83 0.07 0.05 0.03 0.01 0.04 0.017 0.008 bal.
thermore, an increase in the laser-power level, leading to
a higher fluid-flow velocity in the molten pool driven by
the surface-tension gradient, results in a concave bead
profile and deeper fusion at the center. Typical dimen-
sions of a SAB were a width of approx. 5.5 mm and a
depth of fusion in a range of 0.7–1.2 mm. The fluid-flow
velocity and its pattern are also the major factors, which
determine the transfer of the alloying material in the
molten pool and thus directly affect the TiC distribution
in the alloyed layers.
Figure 2 presents the EDS line-scan analysis made at
the cross-section of a SAB produced at a laser power of
1400 W, traverse speed of 0.25 m/min and powder feed
rate of 0.0042 g/mm. The composition profiles, taken at
the mid-depth of the bead along the line parallel to the
bead surface (Figure 2a), indicated a slight titanium (Ti)
enrichment in the centre of the alloyed bead, whereas the
depth profile revealed an almost homogeneous distri-
bution of Ti (Figure 2b). It follows that this fluid-flow
pattern transfers the alloying elements quite efficiently
throughout the fusion zone, leading only to a slightly
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Figure: 3: SEM micrographs taken at the mid-sections of the SALs
produced at a laser power of 1400 W, traverse speed of 0.15 m/min
and powder feed rates of: a) 0.0010 g/mm, b) 0.0046 g/mm
Slika 3: SEM-posnetek iz sredine preseka SAL, nastalega pri mo~i
laserja 1400 W, pre~ni hitrosti 0,15 m/min in hitrosti dodajanja prahu:
a) 0,0010 g/mm, b) 0,0046 g/mm
Figure 1: Macrographs of SABs produced at a traverse speed of
0.2 m/min, powder feed rate of 0.0016 g/mm and laser-power levels
of: a) 1000 W, b) 1200 W
Slika 1: Makroposnetek SAB-ov, nastalih pri pre~ni hitrosti 0,2 m/min,
hitrosti dodajanja prahu 0,0016 g/mm in pri mo~i laserja: a) 1000 W,
b) 1200 W
Figure 4: Typical EDS spectrum of a TiC particle
Slika 4: Zna~ilen EDS-spekter delcev TiC
Figure 2: EDS line-scan analysis of the cross-section of a SAB
produced at a laser power of 1400 W, traverse speed of 0.25 m/min
and powder feed rate of 0.0042 g/mm: a) profile taken at the
mid-depth of the bead along the line parallel to the bead surface, b)
depth profile in the centre of the bead
Slika 2: EDS-linijska analiza na preseku SAB nastalem pri laserju
mo~i 1400 W, pre~ni hitrosti 0,25 m/min in hitrosti dodajanja prahu
0,0042 g/mm: a) profil, posnet na srednji globini kopeli, vzdol` linije
vzporedne s povr{ino kopeli, b) profil v globino na sredini kopeli
higher fraction of the TiC precipitation in the central area
of the alloyed bead.
Generally, the laser-power level determines the maxi-
mum amount of titanium powder that can be introduced
to a SAB, thus the concentration of titanium in the mol-
ten pool and, in consequence, the morphology and frac-
tion of TiC particles. In the cases of laser-power levels of
1000 and 1400 W, the maximum powder feed rate, which
allows the production of SABs with no internal defects,
such as porosity or microvoids, was 0.0016 and 0.0046
g/mm, respectively.
Figure 3 shows SEM micrographs of the surface
alloyed layers (SALs) produced at powder feed rates of
0.0010 and 0.0046 g/mm. Based on the EDS analysis,
the approximate concentration of Ti in these SALs was
estimated to be 3 % and 7 % of the mass fractions, res-
pectively. In turn, a quantitative analysis of the micro-
graphs of the above-mentioned SALs indicated that the
volume fraction of the TiC particles was approx. 4 % and
12 %, respectively.
Figure 4 presents a typical EDS spectrum of the TiC
particles (dark grey particles). As can be seen in Fig-
ure 3, the concentration of Ti strongly affects the shape
and size of TiC particles. With an increasing Ti concen-
tration, the morphology of TiC changes form cubic
(>2 μm in size) to dendritic (up to 10 μm in size). Fur-
thermore, the size of the TiC particles varies from the
fusion boundary to the surface of a SAL, due to the
changes in the local solidification conditions. The
smaller TiC particles observed in the fusion boundary
area (Figure 5) result primarily from the highest cooling
rate; however, as indicated by the EDS line-scan analysis
(Figure 2b), they can also be attributed to a lower
concentration of Ti in this area.
The presence of TiC particles in the SALs was con-
firmed with the X-ray diffraction analysis (Figure 6).
The X-ray diffraction pattern of a SAL, containing the
highest volume fraction of TiC, indicated that martensite
is the dominant phase. Apart from martensite, austenite
and cementite were also detected. Furthermore, a me-
tallographic examination and X-ray analysis revealed
that an increasing titanium concentration in the SALs
increases both the volume fractions of TiC and austenite,
leading simultaneously to a lower fraction of cementite.
All the SALs were free of porosity and microcracks.
The results of fluorescent dry penetrant testing
showed that the SABs were free of cracks; however,
transverse cracks were revealed in all the SALs. The
cracks propagate mainly perpendicularly to the fusion
boundary and do not affect the integrity of the SALs.
Figure 7 shows the microhardness distribution on the
cross-sections of the SALs produced at a laser power of
1400 W, traverse speed of 0.2 m/min and different pow-
der feed rates. As can be seen, the increase in the powder
feed rate slightly reduced the hardness of the SAL,
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Figure: 6: XRD pattern of a SAL produced at a laser power of 1400
W, traverse speed of 0.15 m/min and powder feed rate of 0.0046 g/mm
Slika 6: Rentgenogram SAL, nastalega pri laserju mo~i 1400 W,
pre~ni hitrosti 0,15 m/min in hitrosti dodajanja prahu 0,0046 g/min
Figure 7: Microhardness distribution on the cross-sections of the SALs
produced at a laser power of 1400 W, traverse speed of 0.2 m/min and
different powder feed rates
Slika 7: Razporeditev mikrotrdote na preseku SAL, nastalega pri mo~i
laserja 1400 W, pre~ni hitrosti 0,2 m/min in pri razli~ni hitrosti
dodajanja prahu
Figure 5: SEM micrograph of the fusion boundary area of a SAL
produced at a laser power of 1400 W, traverse speed of 0.15 m/min
and powder feed rate of 0.0046 g/mm
Slika 5: SEM-posnetek meje zlivanja podro~ja SAL, nastalega pri
laserju mo~i 1400 W, pre~ni hitrosti 0,15 m/min in hitrosti dodajanja
prahu 0,0046 g/mm
which is connected with the above-mentioned change in
the volume fractions of austenite and cementite.
The solid-particle erosion-test results showed that the
HPDD laser surface alloying of DCI, involving an in-situ
synthesis of TiC, provides a significant improvement in
the erosion resistance of the investigated DCI for both
30° and 90° impact angles. Erosion values of the SALs
at an impact angle of 90° were approx. 40 % lower than
those of the as-received DCI. In turn, for the impact
angle of 30°, the SALs exhibited a three-time higher ero-
sion resistance than for the as-received DCI. A detailed
analysis of the erosion-degradation mechanism of the
investigated in-situ composite layers is still in progress.
4 CONCLUSIONS
The HPDD laser surface alloying of DCI with direct
injection of the titanium powder into the molten pool
enables us to produce in-situ TiC-reinforced surface
alloying layers with quite a uniform distribution of TiC
throughout the matrix and the TiC volume fraction of up
to 12 %. The morphology and fraction of TiC particles
depend directly on the amount of the titanium powder
introduced into the molten pool, which, in turn, is
strongly affected by the laser-power level. With an in-
creasing titanium (Ti) concentration in the molten pool,
the morphology of the TiC particles changes form cubic
(> 2 μm in size) to dendritic (up to 10 μm in size).
Moreover, the increase in the Ti concentration increases
the volume fraction of both TiC and austenite and,
simultaneously, results in a lower fraction of cementite.
The SALs exhibited a noticeable increase in the erosion
resistance in comparison to the as-received DCI,
especially at steep angles.
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