FIELD OF THE INVENTION
The present invention relates to a method of enhancing cavitation erosion
resistance of Ti6AI4V by laser treatment process on an overlay dual coating
carried out by the Twin Wire Arc Process.
More particularly the invention relates to a surface treatment process to enhance
cavitation erosion resistance of a combination coating with top coat applied on
Ti6AI4V substrate by twin wire are spray (TWAS) procedure.
BACKGROUND OF THE INVENTION
US Patent Number 5,326,362 discloses a method of surface hardening of implant
devices. An implant device made of pure Ti6AI4V or a Ti6AI4V alloy, such as
Ti6AI4V (ELI) is exposed to molecular nitrogen gas at a process temperature and
for a process time duration sufficient to enhance surface hardness and wear
resistance properties, without formation of a measurable TiN layer that tends to
increase surface roughness and diminish wear resistance properties . The
hardened surface of he Ti6AI4V implant occurs primarily due to solid solution
hardening of the Ti6AI4V with nitrogen by dissolution.
US Patent Number 7,985,307 describes a turbine engine fan or compressor blade
material comprising Ti6AI4V with a first portion having a alpha+beta
microstructure and a second portion having martensitic or a bimodal
microstructure. The modified microstructure of the second portion is provided by
selectively heating, and immediately quenching without substantially heating the

first portion. An exemplary method includes providing a near net-shaped article
having a first portion (e.g. an airfoil region) and a second portion (e.g., an
unfinished dovetail region). Initially, the article comprises an alpha+beta
microstructure throughout. Thereafter, the second portion is selectively heated,
followed by immediate quenching, without substantially heating the first portion,
to modify the microstructure of the second portion to a martensitic or bimodal
microstructure without substantially modifying the microstructure of the first
portion. Thereafter, the second potion may be processed to a final body
dimension. This finds application in gas turbine compressor blades.
Patent No.CN101353719 (A) teaches a method for surface treatment of Ti6AI4V
for achieving improved oxidation behaviour at above 500 C. It uses a two/three
step shot peening for the purpose.
US Patent No. 5,068,003 discloses a wear-resistance Ti6AI4V containing Ti6AI4V
carbides which are crystallized and/or precipitated and are dispersed in the beat
phase matrix. The alloy may further comprise alpha.-phase and/or additional
hard particles dispersed in the beta phase matrix.
Twin wire are sprayed (TWAS) SHS 7170 coating is used for application at high
temperature to resist erosion and gives bond strength of more than 450 kg/cm2,
micro hardness values more than 1000 HV30o and has excellent wear and
corrosion resistance. However, it is porous in nature and has a poor cavitation
and slit erosion resistance. The 'as sprayed' coatings contain a high fraction of
two dimensional defect phase boundaries.

As-sprayed coatings give rise to tensile residual stress with increased thickness,
these values exceed the bond strength and failure in the form of detachment
takes place.
Ti6AI4V being used for the advanced class super critical and ultra supercritical
thermal power plans in their blading applications, these are subjected to erosion
due to very high velocity water droplets impacting the low pressure blade tips.
This type of erosion is very similar to the phenomena of cavitation erosion.
Different methods to try and overcome the problem include design modifications,
surface coatings, laser hardening, Plasma nitriding, etc have been attempted.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a surface treatment process
to enhance cavitation erosion resistance of Ti6AI4V.
Another object of the invention is to propose a surface treatment process to
enhance erosion resistance of Ti6AI4V which improves the life of the Ti6AI4V
components of the surface layer which component is susceptible to droplet
erosion and cavitation.
SUMMARY OF THE INVENTION
According to the invention there is provided a surface treatment process to
enhance cavitation erosion resistance of a combination coating with top coat

applied on Ti6AI4V substrate by twin wire arc spray (TWAS) procedure, the
combination coating consisting of a bond coat and a top coat, the TWAS
procedure comprises melting of two electrically opposite charged wire by an
electric arc, atomizing and applying the molten material by compressed air on a
substrate, the process comprising the steps of:
- providing a Ti6AI4V coatable substrate,
- grit blasting of Ti6AI4V base using aluminum grit
at an air pressure between 5 to 5.5 Kg/cm2;
- coating the freshly blasted surface with a bond coat of alpha 1800 alloy
upto a thickness of about 100 micron;
- applying on the bond-coated surface of the substrate SHS 7170 alloy
composition by TWAS procedure; and
- treating the combination-coated surface by laser processing.
According to the invention, SHS 7170 coatings are heat treated above 700°C for
short duration such that carbide and boride type nano hard phases of 66%
volume fraction are formed (Fe-Cr-W-Mo)23C6 and (Fe-Cr-Mo-W)3B, which inter
alia improves the performance.
Thus, a post heat treatment after the spray coating cause devitrification of the
metallic glass contents which results in very low residual stresses, excellent wear
resistance and better impact properties.
SHS 7170 as-sprayed TWAS coatings having a poor adhesion on Ti6AI4V alloy,
and the inventive process overcome this problem by introducing a bond coat of

alpha 1800 material between the top coat and he base. This combination coating
is laser treated using high power diode laser to eliminate the surface porosity as
well as create a nano structure with improved combination of properties
including high bond strength, hardness, wear and corrosion resistance. Surface
coating of SHS 7170 alongwith a bond coat on Ti6AI4V followed by laser
treatment greatly enhances the cavitation erosion resistance.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - photographic view of a TWAS process.
Figure 2 - graphically displays cavitation erosion data for different process
on Ti6AI4V alloy.
Figure 3A - shows fracture properties of the combination coating before laser
treatment according to the invention.
Figure 3B - shows fracture properties of the combination coating after laser
treatment according to the invention.
Figure 4 - shows a flow-chart depicting the inventive process flow.
DETAILED DESCRIPTION OF THE INVENTION
According to the invention, the process consists of grit blasting of Ti6AI4V base
using 24 mesh alumina grit at an air pressure of 5 to 5.5 kg/cm2. The freshly
blasted surface is then coated with a bond coat of alpha 1800 alloy up to a
thickness of around 100 micron followed by a top coat of SHS 7170 alloy by the
twin wire arc process (TWAS). The composition of the powders is given in the
table below:


The TWAS process involves use of two electrically opposite charged wires that
are melted by means of an electric arc. The molten material is atomized by
compressed air and propelled towards a substrate surface which strike the
surface and rapidly solidify to form a coating. The arc spray process is called a
"Cold Process" (relative to the material being coated) as the substrate
temperature can be kept low during the process, thereby avoiding damage,
metallurgical changes and distortion to the substrate material. The twin wire arc
spray process is show in Figure 1.
Manipulation of the samples during coating is done by a six plus two axis robot.
Laser treatment of the combination TWAS coating is then done using a 4.6 kW
continuous wave laser with 20 mm x 2.8 mm optics to ensure that a partial
melting of the sample takes place. These samples were then taken for cavitation
studies.

Cavitation erosion resistance evaluation
The phenomena of cavitation generally occurs in hydro turbines, boilers feed
pumps, ship impellers, etc. and causes excessive damage to the materials in all
these parts. In the laboratory, cavitation erosion is evaluated as per ASTM G-32,
wherein the test equipment consists of a generator, generating high frequency-
high voltage signal which energizes the piezo-electric element in the converter
via the transducer RF cable to oscillate at 20 kHz with amplitude of +/- 50
micron. This mechanical vibration is then transmitted to the horn made of
Ti6AI4V of matching impedance. The sample to be tested is fitted onto the horn
tip and securely tightened to ensure that there is no loss of energy transfer from
the horn tip to the samples. The sample also oscillates at a frequency of 20 kHz
thereby simulating the phenomena of cavitation on the sample surface.
The mass loss measured after carrying out cavitation tests on the samples at
pre-determined intervals. A precision balance was used for measuring the mass
loss after testing. The accuracy and repeatability of the test have been
established on the Ti6AI4V samples and the Nickel reference material. The extent
of erosion damage is calculated in terms of volume loss from the mass loss data
and density of the material and the volume loss was compared with untreated as
well as as-sprayed samples.
The nomenclature used for the experimentation is as follows
1. Untreated Ti6AI4V sample Ti AS
2. Combination coated Ti6AI4V sample Ti+7170
3. Laser treated Ti6AI4V sample Ti+LH
4. Laser treated combination coated Ti6AI4V sample Ti+7170+LH

Results:
Cavitation erosion studies using volume loss was carried out on a number of
samples of Ti6AI4V subjected to the surface treatments including laser hardening
as well as surface coating with 7170 wire spray followed by laser were carried
out. The performance of the SHS 7170 coating on Ti6AI4V peeled off after only 4
hours of cavitation testing. The laser treated Ti6AI4V is better than untreated
Ti6AI4V but the laser treated combination coating on Ti6AI4V gives best results.

WE CLAIM:
1. A surface treatment process to enhance cavitation erosion resistance of a
combination coating with a bond coat and top coat applied on Ti6AI4V
substrate by twin wire arc spray (TWAS) procedure, the (TWAS)
procedure comprises melting of two electrically opposite charged wire by
an electric arc, atomizing and applying the molten material by compressed
air on a substrate, the process comprising the steps of:
- providing a Ti6AI4V coatable substrate;
- grit blasting of Ti6AI4V base on the substrate through an aluminum grit at
an air pressure between 5 to 5.5 Kg/cm2;
- coating the freshly blasted surface with a bond coat of alpha 1800 alloy
by TWAS upto a thickness of about 100 micron;
- applying on the bond-coated surface of the substrate SHS 7170 alloy
composition by TWAS procedure; and
- treating the combination-coated surface by laser processing.

2. The process as claimed in claim 1, wherein the bond coat consists of by
weight percent 6.1 - 9.5 B, 0.02 - 1.0 Si, 0.02 - 1.0 Al, 0.06 0.6c and
90-93 Fe.
3. The process as claimed in claim , wherein the SHS 7170 alloy composition
consists of by weight percent 5.00 B, 3.00 Si, 26.00 Cr, 4.00 Mn, 5/0 Wc,
4.00 M0, 2.00 C and 51.00 Fe.

The invention relates to a surface treatment process to enhance cavitation
erosion resistance of Ti6Al4V by a combination coating with top coat applied on
Ti6Al4V substrate by twin wire arc spray (TWAS) procedure, the combination
coating consisting of a bond coat and a top coat, the (TWAS) procedure
comprises melting of two electrically opposite charged wire by an electric arc,
atomizing and applying the molten material by compressed air on a substrate,
the process comprising the steps of providing a Ti6Al4V coatable substrate; grit
blasting of Ti6Al4V base on the substrate through an aluminum grit at an air
pressure between 5 to 5.6 Kg/cm2; coating the freshly blasted surface with a
bond coat of alpha 1800 alloy upto a thickness of about 100 micro; applying on
the bond-coated surface of the substrate SHS 7170 alloy composition by TWAS
procedure; and treating the combination-coated surface by laser processing.