Analysis of the Wire Melting Behavior Depending on Wire Design

published at the ITSC 2002, Essen, D, March 4th - 6th 2002
Analysis of the Wire Melting Behavior Depending on Wire Design and Process Characteristics
J. Wilden, B. Wielage, T. Schnick, A. Wank, Inst. of Composite Materials, Chemnitz Univ. of Techn., Chemnitz, D
P. Fronteddu, Olympus Optical Co. GmbH (Europe), Hamburg, D
Die Mikrostruktur thermisch gespritzter Schichten hängt von den Charakteristika des angewendeten Prozesses ab.
Neben den Maschineneinstellgrößen nimmt der Spritzzusatz, insbesondere beim Einsatz von Drähten, einen wesentlichen Einfluss. In der Regel erlaubt der Einsatz von Drähten höhere Auftragleistungen und -wirkungsgrade im
Vergleich zu Pulvern. Allerdings ist die Auswahl der Werkstoffe eingeschränkt. Fülldrähte erweitern das Spektrum
anwendbarer Werkstoffe wesentlich. Dabei hängt die Spritzbarkeit stark von der Qualität des Drahtes ab. Das Abschmelzverhalten unterschiedlicher Massiv- und Fülldrähte wird mit Hilfe einer Hochgeschwindigkeits CCD Kamera untersucht. Die Analysen schließen das Lichtbogenspritzen und Hochgeschwindigkeitsflammdrahtspritzen
(HVCW) ein. Die Schichtmorphologie wird mit dem Abschmelzverhalten korreliert und Richtlinien für die Herstellung optimierter Spritzdrähte sowie Verarbeitungsbedingungen werden abgeleitet.
1
Introduction
The international competitiveness depends significantly on innovation in the field of material and process technology. Time and cost pressure demand new
solutions in coating technology with the objective to
manufacture functional surfaces with economical
benefit. The application of wire feedstock in thermal
processes is usually accompanied by an increased
deposition rate and efficiency as well as by an improved process efficiency and decreased feedstock
costs. The main application field of wire feedstock
thermal spraying is the manufacturing of in the first
place corrosion and in the second place wear protective coatings [1].
Conventional wire feedstock processes are arc spraying and wire flame spraying. In the arc spraying process the arc burns between wire electrodes fed onto
each other, while in the wire flame spraying process
generally an axially fed single wire is continuously
melted off by a concentric flame. The insight, that
increasing particle velocities improve the coating adhesion and density, led to the development of the high
velocity combustion wire spraying process (HVCW).
An optimized nozzle design permits supersonic jet
velocities. Single wire arc spraying processes with an
arc between the axially fed wire and a nonconsumable ring anode are not yet applied industrially
as well as wire flame spraying processes with radial
wire feeding.
Most commonly compact wires are used and the melting off behavior of pure metals is well known since the
early 60´s [2]. A disadvantage of wire spraying is, that
the spectrum of applicable materials is much smaller
compared to powder feedstock. Cored wires expand
this spectrum significantly. There are different methods to produce cored wires. On the one hand grooved
cored wires are produced by inclosing a filler material
into a velum sheet with an overlap of the velum after
wrapping. On the other hand tube cored wires are
manufactured by filling a tube with filler material and a
subsequent forming process to reduce the diameter to
a specific value. Because of the superior mechanical
stability of tube cored wires the filler content can be
higher in comparison to grooved cored wires.
Up to now the optimisation of wire spraying processes
has only been done by extensive factorial design. The
interaction between wire and flame or arc and atomization gas flow, which take direct influence on the
particle parameters and thereby on the coating properties, have only been studied in scientific research
work.
The onhand work uses a high speed CCD camera for
investigations on the process stability and melting off
behavior of wires with different design in HVCW and
arc spraying processes. The analysed process characteristics are correlated to the resulting coating
properties.
2
Experimental
For arc spraying a OSU G30/2 system with open nozzle configuration LD/U2 (OSU Maschinenbau GmbH,
Duisburg) is applied. In order to determine the influence of oxygen in the atomizing gas compressed air
and pure nitrogen are use for atomization. The Praxair
system type 216 (Praxair Services GmbH, Wiggensbach) with ethylene as fuel gas is used for HVCW
spraying. This commonly manually controlled gun is
moved by a robot and the wire feed rate is controlled
by the wire feed pressure. The process parameters
for arc and HVCW spraying are comprised in Table 1
and 2 respectively.
Table 1. Arc-spraying parameters.
Tabelle 1. Lichtbogenspritzparameter.
voltage U:
current I:
wire feed pressure pV:
atomization pressure pat.:
atomization gas:
spraying distance:
28V
150 A
1.5 – 3.0 atm.
1.5 – 3.5 atm.
compr. air, N2
100 – 150 mm
In addition to different compact wire materials grooved
and tube cored wires with an outer diameter of 1.6
mm are applied. The composition of the used wires is
given along with the wire design and the manufacturer
in Table 3.
Table 2. HVCW spraying parameters.
Tabelle 2. HVCW Spritzparameter.
ethylene flow rate:
oxygen flow rate:
cooling air pressure:
Wire feed pressure pV:
spraying distance :
20 slpm
100 slpm
5.0 atm.
0.4 – 1.5 atm.
80 – 160 mm
Table 3. Applied wires.
Tabelle 3. Verwendete Drähte.
wire
composition
design
manufacturer
OSU 65
13T
01S
06C
74MXC
AS 751
AS 754
Megafil A864M
Megafil A760M
Mesalox 701
110MnCrTi8
Mo
Al6Si
Ni20Cr
Ni-Al-Mo
Ni-B-FTC
Ni-Cr-B-Si
Fe-Ni-Mn-C
Fe-Cr-Mn-Mo-C
Al-Si
compact
compact
compact
compact
grooved
grooved
grooved
tube
tube
tube
OSU*
Tafa**
Tafa**
Tafa**
Tafa**
DURUM***
DURUM***
Drahtzug****
Drahtzug****
Drahtzug****
*
**
***
****
Figure 1. HVCW sprayed molybdenum coating.
Bild 1. HVCW gespritzte Molybdänschicht.
OSU Maschinenbau GmbH, Duisburg, D
Tafa Inc., Concord, NH, USA
DURUM Verschleiss-Schutz GmbH, Krefeld, D
Drahtzug Stein GmbH & Co. KG, Altleiningen, D
For process characterization a PCI-8000S Encore
high speed CCD video camera by Olympus Optical
GmbH, Hamburg, Germany, is used. The computer
based system permits recording of image sequences
with a frequency of 8 kHz and a maximum exposure
frequency of 80 kHz with a resolution of 60 x 68 x 8 bit
Figure 2. HVCW sprayed Al6Si coating.
Bild 2. HVCW gespritzte Al6Si Schicht.
or 160 x 30 x 8 bit per image. To prevent outshining of
the images different filters are applied.
3
Coating properties
Both arc and HVCW spraying permit the manufacturing of coatings without restrictions due to the wire
design. HVCW spraying of molybdenum permits to
tailor the oxygen content in the coatings and thereby
the coating hardness [3]. Depending on the process
parameters up to 1,400 HV0.05 micro hardness is
achievable, Figure 1. For the manufacturing of low
melting metal coatings, like aluminium or zinc, wire
feed systems permitting sufficiently high feed rates
have to be applied. By this way coatings without any
porosity detectable by optical microscopy are manufactured, Figure 2.
The spraying of tube cored wires results in a small
particle jet apex angle comparable to compact wires.
In contrast the apex angle during the spraying of
grooved cored wires can exceed 45°. In Figures 3
and 4 examples of coatings manufactured from tube
and grooved wires are shown respectively. The dark
phases in the cross section of the Fe-Ni-Mn-C coating
are oxides. The micro hardness of this coating
amounts to 820 HV0.05.
Figure 3. HVCW sprayed Fe-Ni-Mn-C coating applying
a tube cored wire.
Bild 3. HVCW gespritzte Fe-Ni-Mn-C Schicht bei Einsatz eines Röhrchenfülldrahts.
Arc Spraying also permits the manufacturing of Al6Si
coatings without porosity detectable by optical microscopy, Figure 5. In contrast to Ni20Cr there is no
significant influence of the atomization gas visible in
cross sections. Both for compressed air and pure
nitrogen the average micro hardness amounts to 125
HV0.05. HVCW sprayed Al6Si coatings show an average micro hardness of 190 HV0.05. While the micro
hardness of arc sprayed Ni20Cr coatings with compressed air as atomization medium amounts to 325
HV0.05, the use of pure nitrogen results in a signifi-
cantly lower micro hardness of 275 HV0.05 on average, Figure 6.
show an excellent interface to the substrate. Depending on the coating material a low porosity (< 5 Vol.-%)
and a homogeneous distribution of hard phases in the
coating is achievable, Figure 7. The average micro
hardness of Ni-Al-Mo coatings produced by arc spraying with nitrogen as atomisation medium is comparable to HVCW sprayed coatings, though the porosity
is significantly higher and amounts to 250 HV0.05.
There is no clear evidence of an improved density for
the use of a special wire design.
Figure 4. HVCW sprayed Ni-Al-Mo coating applying a
grooved cored wire.
Bild 4. HVCW gespritzte Ni-Al-Mo Schicht bei Einsatz
eines gefalzten Fülldrahts.
Figure 7. Arc sprayed Ni-B-FTC coating applying a
grooved cored wire.
Bild 7. Lichtbogengespritzte Ni-B-WSC Schicht bei
Einsatz eines gefalzten Fülldrahts.
Figure 5. Arc sprayed Al6Si coating with pure nitrogen as atomizing gas.
Bild 5. Lichtbogengespritzte Al6Si Schicht bei Einsatz
reinen Stickstoffs als Zerstäubergas.
Figure 8. Arc sprayed Ni-Al-Mo coating applying a
grooved cored wire.
Bild 8. Lichtbogengespritzte Ni-Al-Mo Schicht bei Einsatz eines gefalzten Fülldrahts.
4
Figure 6. Arc sprayed Ni20Cr coating with compressed
air as atomizing gas.
Bild 6. Lichtbogengespritzte Ni20Cr Schicht bei Einsatz von Druckluft als Zerstäubergas.
During the arc spraying process there is no significant
influence of the wire design on the process stability
and the apex angle is comparable for all applied
wires. Both coatings from compact and cored wires
Process diagnostics
The melting off behavior during HVCW spraying depends strongly on the wire material and design. While
the spraying of molybdenum produces a tapered tip,
from which melt droplets are continuously detached,
there is a formation of oxides at the wire tip of Ni20Cr,
which prevents the evolution of a tapered tip and results in a retained melt flow up to a critical droplet
size, Figure 9.
The melting off behavior of iron based tube cored
wires is comparable to that of the compact Ni20Cr
wire. The molten velum material flows to the wire tip,
where it is alloyed by the filler material. After the droplet has grown to a critical size, it is detached. The
melting off behavior of grooved cored wires is significantly different. The velum is molten continuously and
the melt flows over the filler material to the wire tip.
The formed droplets are detached continuously. In
contrast the filler material is heated and large lumps
are detached discontinuously, Figure 9. These lumps
are not completely molten and cannot be atomized by
the high velocity flame like the melt droplets from the
velum material.
Figure 9. Melting off behavior during HVCW spraying with different wire designs (left: Mo compact wire, middle:
tube cored Fe-Cr-Mn-C wire, right: grooved cored Ni-Al-Mo wire).
Bild 9. Abschmelzverhalten beim HVCW Spritzen unterschiedlicher Drähte (links: Mo Massivdraht, Mitte: Fe-CrMn-C Röhrchenfülldraht, right: gefalzter Ni-Al-Mo Fülldraht).
The arc spraying process is basically characterized by
the changing arc length, which can be monitored via
the voltage. Figure 10 shows exemplary sequences
for the use of a compact Ni20Cr wire. The atomizing
gas flow forces the molten droplets to flow over the
wire tip. The melting off for different wire materials and
wire designs is comparable. Therefore the comparable coating microstructure correlates to the observed
melting off behavior for different wire designs. The
high emission intensity of the arc is demanding with
concern to the recording technology, in order to resolve both the wire tips and the detached particles.
5
Summary and Perspectives
The high potential of wire spraying processes for the
manufacturing of high quality coatings has been confirmed. In addition to compact wires tube and grooved
cored wires can be processed by arc and HVCW
spraying. The investigations with concern to the melting off behavior during HVCW spraying show a significantly more continuous melting off for compact and
tube cored wires in comparison to grooved cored
wires. During arc spraying no significant difference
depending on the wire design is observed.
Further investigations with regard to the melting off
behavior will deal with the influence of straightening of
the wire. Additionally the boundary conditions with
respect to fluid dynamics will be optimized.
Figure 10. Melting off behavior of a Ni20Cr compact
wire during arc spraying with pure nitrogen as atomization medium.
Bild 10. Abschmelzverhalten eines Ni20Cr Massivdrahts beim Lichtbogenspritzen mit reinem Stickstoff
als Zerstäubergas.
6
References
[1]
Wilden, J., A. Wank, F. Schreiber: Wires for
arc- and high velocity flame spraying – wire design,
materials and coating properties. Proc. ITSC 2000,
Montreal, Quebec, Canada, 2000, pp. 609-617
[2]
Steffens, H.-D.: Haftung und Schichtaufbau
beim Lichtbogen- und Flammspritzen. Dissertation,
Technische Hochschule Hannover, 1963
[3]
Calla, E., C. Modi, A. Nuki: Characterisation of
molybdenum coatings by a newly developed flame
spray process. Proc. 15th ITSC, Nice, France, 1998,
ISBN 0-87170-659-8, pp. 1455-1459