Effect of Shotpeening on Sliding Wear and Tensile Behavior of Titanium

Implant Alloys

B.K.C. Ganesh 1, W. Sha 2, *, N. Ramanaiah 1, A. Krishnaiah 3

1Department of Mechanical Engineering, Andhra University, Visakhapatnam, India.2School of Planning, Architecture and Civil Engineering, Queen’s University Belfast, U.K.3Department of Mechanical Engineering, Osmania University, Hyderabad, India.

*Corresponding Author. Email: w.sha@qub.ac.uk; Tel. +44 28 90974017.

Abstract

Titanium has good biocompatibility and so its alloys are used as implant materials, but they suffer

from having poor wear resistance. This research aims to improve the wear resistance and the tensile

strength of titanium alloys potentially for implant applications. Titanium alloys Ti-6Al-4V and Ti-

6Al-7Nb were subjected to shotpeening process to study the wear and tensile behavior. An

improvement in the wear resistance has been achieved due to surface hardening of these alloys by the

process of shotpeening. Surface microhardness of shotpeened Ti-6Al-4V and Ti-6Al-7Nb alloys has

increased by 113 and 58 HV(0.5), respectively. After shotpeening, ultimate tensile strength of Ti-6Al-

4V increased from 1000 MPa to 1150 MPa, higher than improvement obtained for heat treated

titanium specimens. The results confirm that shotpeening pre-treatment increased tensile and sliding

wear behavior of Ti-6Al-4V and Ti-6Al-7Nb alloys. In addition, shotpeening increased surface

roughness.

Keywords: Shotpeening, Microhardness, Wear rate, Ultimate tensile strength, Osseointegration

1. Introduction

The introduction of compressive residual stress in the surface layer by surface modification technique

such as shotpeening can mitigate wear and improve mechanical properties. Shotpeening is a method

of cold working in which compressive stresses are induced in the exposed surface layers of metallic

parts by the impingement of stream of shots, directed at the metal surface at high velocity under

controlled conditions. It differs from blast cleaning (shot blasting), the purpose of which is to clean

and remove impurities on the surface. It can also improve the surface roughness to develop the

osseointegration of the materials to be implanted. Although shotpeening cleans the surface being

peened, this function is incidental. A major purpose of shotpeening is to increase the fatigue life of the

components.

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The immediate effect of bombarding high velocity shots onto a metallic target is the creation of a thin

layer of high magnitude compressive residual stress at or near the metal surface, which is balanced by

a small tensile stress in the deeper core, as shown in Fig. 1 [1]. When individual particles of shot in a

high velocity stream contact a metal surface, they produce light and rounded depressions in the

surface, stretching it radially and causing plastic flow of surface metal at the instant of contact. The

effect usually extends to about 0.13 to 0.25 mm, but may extend as much as 0.5 mm below the

surface. The metal beneath the layer is not plastically deformed. In the stress distribution that results,

the surface metal has induced residual compressive stress parallel to the surface, while metal beneath

has reaction induced tensile stress. This compressive stress offsets any service imposed tensile as

encountered in rolling or bending, and improves fatigue life of parts in service markedly.

Peening action improves the distribution of stresses in surfaces that have been caused by grinding,

machining, or heat treating. It is particularly effective on ground or machined surfaces, because it

changes the undesirable residual tensile stress condition that these processes usually impose in a metal

surface to a beneficial compressive stress condition. Shotpeening is especially effective in reducing

the harmful stress concentration effects of notches, fillets, forging pits, surface defects, and the low

strength effects of decarburization, and the heat affected zones of weldments.

The magnitude of this compressive residual stress is a function of the mechanical properties of the

target material and may reach values as high as 50 to 60% of the material’s ultimate tensile strength.

Its depth is largely dependent on the peening intensity and the relative hardness of the impinging shots

and target material. For a relatively soft target material (230-300 HV), it is feasible to produce a

compressive layer of 0.8 to 1 mm, whilst for a harder material (700 HV), it can be difficult to produce

a compressive layer of much more than 0.2 to 0.25 mm. The introduction of this compressive residual

stress at the metal surface layer brings one major benefit. It reduces and can negate any residual or

subsequently imposed tensile stress at the metal surface. It is well known that most fatigue failures

and stress corrosion failures start at or near the surface stressed in tension. Therefore, by reducing the

net tensile stresses at and near the surface of the component, fatigue crack initiation and stress

corrosion can be delayed, improving the fatigue life of the component treated [1].

Media selection plays an important role to obtain the desired properties by the process. Many kinds of

cast steel shots, cut wire shots, glass beads, and zirconium shots are available with various sizes.

Depending upon the amount of pressure exerted through the blast nozzle and the surface being

processed, each type of media can achieve different results. The resultant properties produced by the

application of this process are almost limitless. Change in a few variables can alter various

microstructural and mechanical properties of the peened specimens dramatically. It is important

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therefore to select the optimal variables after the right combination has been found for consistent and

high quality results.

There are many ways to deliver the working medium to the surface being treated including

compressed air, mechanical and water slurry. The most popular way is compressed air. Air blast is

categorized into two methods of media delivery, suction blast and pressure blast. Suction blast

systems are selected for light to medium amounts of production and moderate budgets. Suction is not

as efficient as pressure, so the range of applications is more limited, but it often yields comparable

results. Suction systems have the ability to blast continuously without stopping for media refills. They

are also simpler to use and have fewer wear parts, making them inexpensive and easy to maintain.

Suction systems work on the principle that air passing over an orifice will create vacuum at that point.

This action takes place in the hand held suction gun, with a media hose connected from the vacuum

area to media storage hopper. Compressed air is piped into the back of the gun and causes the lifted

media to be blown out of a nozzle on the front of the gun. Energy is expended indirectly to lift the

media and then mix it with the compressed air, making suction less efficient than a pressure system.

Pressure blasting feeds media into the compressed air stream at a pressurized storage vessel. The

media then accelerates in the air stream as it is routed by a blast hose to the nozzle. Resulting media

velocity is often several times that of a suction system, resulting in a common four-fold increase in

production. Direct pressure uses force, rather than suction, so it offers much more control at very high

and very low operating pressures. Low pressure is used for delicate or fragile substrates, such as

removing carbon from aluminum pistons or flash from integrated circuits. Direct pressure systems are

especially useful for finishing hard-to-reach recessed areas and odd shapes.

The shotpeening process has to be precisely controlled and repeatable for optimum benefit. To

achieve this, all its process variables must be identified and controlled [2]. There are many

fundamental parameters affecting the shotpeening processes. The most common are as follows:

(1) Shot density

(2) Hardness and size of the shots

(3) Nozzle characteristics (diameter, deflection angle, length)

(4) Air pressure

(5) Impact angle

(6) Exposure time

(7) Linear and rotational speed of work piece relative to the nozzle.

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To specify all these variables, every shotpeening job would require time consuming investigations and

industrially impractical procedures. To overcome this problem, a concept was introduced, of peening

intensity measurement based on the curvature induced in a thin test strip, by which most of the listed

process parameters can automatically be incorporated into one process variable called the Almen

peening intensity. With the peening intensity known, one has to only define the shot type and size and

peening coverage desired to fully define the peening process. Despite important progress in

understanding the process, some areas are not totally mastered yet and difficulties are still hard to

avoid. Being able to predict the effect of process in set conditions is indeed the key to gaining

complete control over the process and to making it much more reliable.

Surface hardening by shotpeening is one of the upcoming research areas that requires much attention.

This process of surface hardening is an important application for improving various mechanical

properties which have a poor response to heat treatment process. The application of shotpeening is

very vastly studied in terms of improvement of fatigue life for the components working in a cyclic

loading environment including biomaterials where the compressive residual stress is induced into the

component to prevent crack initiation and propagation. Nowadays this process is also used to improve

the microhardness of the peened surfaces. This process is more elaborately done for improving

surface hardness of 316L stainless steel orthopedic biomaterial and also for improving surface

hardness of various aluminum alloys. Only one paper [3] has studied the effects of shotpeening on

wear behavior of Ti-6Al-4V alloy.

Shotpeening is an effective method of surface treatment for the introduction of residual compressive

stress in the surface and subsurface layers and improving the fatigue strength. Surface modifications

produced by the shotpeening treatment are (a) roughening of the surface, (b) an increased, near

surface, strain hardening and (c) the development of a characteristic profile of residual stress.

Considering fatigue damage, surface roughening will accelerate the nucleation and early propagation

of cracks, strain hardening will retard the propagation of cracks, by increasing the resistance to plastic

deformation and residual stress profile will provide a corresponding crack closure stress that will

reduce the driving force for crack propagation. It also in some cases can introduce large amount of

defects and interfaces into the surface layers and transform the microstructure of surface to include

nano-sized crystals [4].

Shotpeening is one of surface modification processes widely used in industry, inducing severe plastic

deformation on surface region of materials. It has been reported that severe plastic deformation on

surface region leads to surface nano-crystallization and its following improvements of mechanical,

corrosion, wear properties and fatigue strength. It has been suggested that density of dislocations and

density and size of precipitates could influence the surface properties due to shotpeening [5].

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Cvijović-Alagić et al. have recently studied the wear and corrosion behavior of Ti–13Nb–13Zr and

Ti–6Al–4V alloys in simulated physiological solution [5]. The Ti6Al7Nb alloy belongs to the group

of α/β alloys. The microstructure and mechanical behavior of Ti―6Al―7Nb alloy produced by

selective laser melting have been investigated by Chlebus et al. [6]. Geetha et al. reviewed Ti based

biomaterials, the ultimate choice for orthopaedic implants, including Ti–6Al–7Nb Wrought and

titanium–aluminum–vanadium (Ti–6Al–4V) alloy [7]. Wear resistance of experimental titanium

alloys for dental applications has been studied by Faria et al. [8]. Mechanical properties and

biocompatibility of titanium alloys were tested, including α+β alloys (Ti–6Al–7Nb and Ti–6Al–4V).

2. Materials and methods

The Ti-6Al-4V material was procured from South Asia Metal Corporation, Mumbai, India. Ti-6Al-7Nb

was imported from Boaji Litai Corporation, Baoji, Shanxi, China. The chemical compositions of both

alloys are given in Table 1.

Wear test was conducted on the above specimens according to ASTM: G-99 specifications on a pin-

on-disc tribometer. Three tests were made to arrive at a final reading for each condition. A Ducom TR

20LE pin-on-disc wear testing machine was used, with a linear sliding speed of 1 m/sec and a sliding

distance of 500 meters. The titanium alloy pin materials were tested while rotating on a hardened steel

disc which had a hardness of 69 Rc. In this work, a load of 50 N was applied on a pin diameter of 10

mm to obtain a pressure of 0.7 MPa, which was considered to be safe stress acting on the joint during

the loading conditions [9]. The above process parameters, including the fast sliding speed, were

selected as they were considered to be the conditions for the implants to be working in the actual

operating conditions [9,10]. Wear rate was calculated on the basis of volume of material removed

from pin while covering a sliding distance of 500 meters expressed in m3/m.

Hardness was measured by using a Vickers microhardness testing machine under the application of

constant load of 5N. The indentation dwell time was 10 seconds. Surface roughness of the wear track

was measured by using a Mitutoyo SJ 210 surface roughness tester. Surface layer characterization and

particle size analysis was conducted by atomic force microscope (AFM). Images were recorded by a

multimode Scanning Probe (Ntegra Aura, NTMDT Co, Russia) at ambient condition (25±2 °C) using

single crystal silicon N type probes (NSG 03-A) having radius of curvature of 10 nm. The cantilever

with long tips (aspect ratio 3:1), with force constants of 0.35 to 6.06 N/m and resonance frequencies

of 47-150 kHz, was used to image the surface morphology.

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The shotpeening operation was performed according to the SAE AMS2430S [11] standard. The

various shotpeening parameters are: type of shot S230, material of shots steel, angle of projection 90°,

diameter of shots 0.6 mm, duration of peening 60 seconds, coverage area 100%. Pressure blast system

of shotpeening is primarily used in this work for obtaining good control of the operating parameters,

most importantly an appropriate peening intensity for obtaining necessary surface hardening. Various

operating pressures from 3.5 to 5.5 bar were used to conduct the peening operation on the titanium

alloys.

A set of tensile specimens were prepared according to the ASTM: E-8 procedure, as shown in Fig. 2.

Tensile specimens were cut from a plate 3-mm in thickness, with a gauge length of 25 mm and gauge

width of 6 mm. The cut specimen was fixed in the Almen strip holder between two screws. The cut

tensile specimens were first shotpeened when they were rotated with the gripper with the impact of

steel shots supplied from a pipe at the required operating pressure and angle of projection. Only one

side of the tensile specimens was peened until it was ensured that the entire side was covered with

peening action. AFM analysis was conducted on the same, peened side. These specimens were then

tested by using a Dak Ultimate tensile testing machine of 50 kN capacity at a speed of 20 mm/min to

plot the stress-strain curve for both the shotpeened alloys.

3. Results and discussion

3.1. Wear and microhardness

Table 2 shows wear properties of the shotpeened Ti-6Al-4V (Ti64) and Ti-6Al-7Nb (Ti7Nb) alloys at

two different pressures. An increase in the wear resistance of the alloys was obtained together with an

improvement in the hardness. With increasing pressures, from 3.5 to 4.5 and to 5.5 bar, with 20

seconds peening duration, the Almen peening intensities are 0.21, 0.32 and 0.42 A, respectively. It

should be noted that as the peening pressure was increased, the Almen intensity also increased. The

increase in the Almen intensity could result in the improvement of microhardness up to a specific

depth of the peened surface.

The high standard deviation values for the results of the peened specimens were due to the errors

caused by the rougher surface after peening. However, the increase of hardness values of peened

specimens is statistically significant, in all cases. So, it is clear that none of the peened specimens

could have the same hardness as unpeened specimens. The same is the case for wear rates, for both

alloys, despite the apparent high standard deviation values.

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Microhardness profile obtained on the cross section of shotpeened specimens failed to reveal obvious

and statistically significant trend of variations, but we were only able to measure the hardness in the

depth range of 0.1-0.8 mm. It is possible, therefore, that the hardness increase was only significant

within a shallow depth, up to 0.1 mm. This does not contradict the surface hardness measurement data

(Table 2), because, for 5 N loading, indentation has approximately 70 μm depth.

When the Ti64 alloy had been tested at 4.5 bar pressure with increasing time of peening, peening

intensities are as shown in Fig. 3. From the figure, it is evident that, at 20 seconds of peening the alloy

with steel shots, a saturation of intensity is reached. There is no improvement of the Almen intensity

after 20 seconds, with no significant change in the Almen intensity reported. The region where

hardness is increasing with respect to the depth of the specimen in micrometers could be considered

as the region affected by shotpeening and thickness of this layer is proportional to peening pressure.

To establish this phenomena, viz., effect of shotpeening up to certain depth of the specimen, a profile

of hardness data was collected by measuring the microhardness with respect to the depth of surface

layers measured in micrometers. The data collected for shotpeened Ti-6Al-4V specimens at the two

operating pressures did not show varying of microhardness values obtained up to a specific depth

from the surface. The specimen shotpeened at 4.5 bar had values of 346±8 HV(0.5) up to a depth of 0.8

mm, whereas the specimen shotpeened at 3.5 bar showed hardness values of 347±9 HV (0.5) up to 0.8

mm.

While comparing the microhardness values, the alloy Ti7Nb specimens have shown lower

microhardness than Ti64 specimens for the same operating conditions, by 2%, 8% and 20% under

conditions of no peening, and peened at 3.5 and 4.5 bar, respectively (Table 2). There was no much

impact of shotpeening process even with an increase of operating pressure to 4.5 bar. The data

presented in Table 2 also clearly indicates that Ti7Nb alloy has responded less strongly to the peening

process. There are no significant changes in the hardness of Ti7Nb up to 0.8 mm from the surface.

The specimen shotpeened at 4.5 bar has values of 323±8 HV(0.5) up to a depth of 0.8 mm, whereas the

specimen shotpeened at 3.5 bar shows hardness values of 316±16 HV(0.5) up to 0.8 mm.

The increase in the surface microhardness of both alloys is instrumental in higher wear resistance of

the shotpeened alloy. Doni et al. [12] in their experimental work on wear behavior of cobalt-

chromium biomedical alloys have reported that Archard wear rate equation clearly indicates that wear

rate is inversely proportional to hardness of the wearing metal. Improvement of wear resistance of the

peened specimens shown in Table 2 clearly confirms the effect of surface hardening of the treated

alloys due to the application of steel shots at high pressure onto the specimens.

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Table 2 also shows an improvement of higher than 50% of wear resistance of shotpeened Ti64 alloy

as compared to unpeened alloy, whereas smaller improvement of wear resistance is reported for the

Ti7Nb shotpeened alloy. Scanning electron microscope (SEM) image of 4.5 bar Ti64 shotpeened

specimen in Fig. 4 clearly shows presence of thick serrated coarse wear tracks, which indicates that

high hardness of the shotpeened material had developed resistance to abrasive wear when in contact

with the steel disc. In relation to wear mechanism, Hovsepian and Münz discussed scanning electron

microscopy (SEM) image of pin-on-disk wear tracks, the coarse wear debris generated through the

test, and smooth wear track [13]. In contrast, the Ti7Nb alloy peened at the same operating conditions

(Fig. 5) shows finer wear tracks as compared to Ti64 alloy, possibly due to its lower microhardness

(by 20%, Table 2) values. Previous work found that the wear track has formed as a result of fatigue

spallation of individual fine grains [14]. Fatigue spallation contributes to the film wear only in the

final stages of testing, with considerable damage.

3.2. Tensile behavior

The effects of shotpeening on tensile behavior have been studied in the present work. An

improvement in the tensile strength is shown. Shotpeening at 3.5 bar has increased the ultimate tensile

strength (UTS) to 1100 MPa, as compared to the UTS of unpeened Ti64 specimen which was 1000

MPa. When the specimens were shotpeened at 4.5 bar pressure, further improvement in UTS was

observed (1150 MPa). From this, it can be understood that with increasing peening intensities such as

at 4.5 bar pressure, it is possible to improve the UTS. Tensile and percentage of elongation have

improved for the shotpeened specimen to unpeened specimen.

Zhan et al. [15] have reported an improvement of proof stress for a newly developed austenitic steel,

S30432, from 240 MPa to 830 MPa and 940 MPa by the application of conventional and dual

shotpeening, respectively. In terms of microstructure, they stated that shotpeening can refine the

domain size and increase the micro-strain. They further stated that the domain size of 300 nm for

untreated austenitic steel was reduced to 90 nm for both shotpeened and dual shotpeened specimens.

According to the Hall-Petch equation, the yield strength of the materials is related with the domain

size. Smaller domain size gives larger material yield strength [15]. It is also well known that the

interaction of high density dislocations can also improve the yield strength of the material. During

shotpeening, a large amount of small balls repeatedly impact on the material surface.

Cho et al. [16], with their experimental work on AA2024 aluminum alloy, have stated that an

improvement of Vickers hardness was noted after conducting the peening action by zinc shots. Their

experimental results confirm that both the aluminum and zinc underwent severe plastic deformation

8

followed by phase transformation between the two materials which promoted grain refinement. Such

nanocrystallization is instrumental in improving the hardness from 65 to 140 HV. Their experimental

work establishes the fact that surface nanocrystallization is a key factor for improving mechanical

properties. Feng et al. probed the size and density of silicon nanocrystals in nanocrystal memory

device applications, using contact-mode atomic force microscopy (AFM), because they found it hard

to detect Si nanocrystals with electron microscopy [17]. Bachand et al. used atomic force microscopy

(AFM) to further examine the assembly and transport of nanocrystal CdSe quantum dot

nanocomposites using microtubules and kinesin motor proteins [18].

The AFM images of shotpeened tensile specimens at various peening pressures are shown in Fig. 6.

These specimens were tested at gauge length of the treated specimens for a maximum scan size of 10

µm and maximum height of 600 nm. A smooth surface was observed for the unpeened specimen with

surface roughness of 26.4 nm, whereas shotpeened specimens at 3.5 bar and 4.5 bar pressures have

reported surface roughness values of 38.2 nm and 53.2 nm, respectively.

This increase in the UTS can also be possible when the titanium alloy is solution treated above the

beta transus temperature followed by water, air or furnace cooling. Venkatesh et al. [19] have clearly

stated that the type of cooling results in the formation of various bi-modal and lamellar

microstructures, due to which an improvement in low cycle fatigue strength as well as tensile strength

can be obtained. Fig. 7 shows the various stress strain curves obtained for base and solution treated

specimens followed by water quenching (WQ) and aging and air cooling and aging. The figure clearly

explains that tensile strength of the water quenched specimen is higher, by 25%, as compared to

untreated specimen. However, ductility has reduced to significantly with the improvement of tensile

strength.

While comparing the tensile behavior of STA specimens with shotpeened specimens, improvement of

UTS can also be undertaken by selecting suitable shotpeening operating parameters. Further, the

improvement in tensile strength by shotpeening is accompanied by an improvement in the ductility up

to certain boundaries, which is a significant development. This improvement is very significant which

clearly suggests the potential application of shotpeening for load bearing implants which could

improve the fatigue life of the peened specimen but also results in the improvement of the tensile

strength and ductility of the specimen.

So, it has been observed that there is an increase in the tensile strength. Structuring the grain size to

ultra-fine grain size (grain sizes 0.1-0.5 μm) would result in increased yield strength and ductility [20-

22].

9

Koch [20] in his experimental work on copper and various other materials stated that reducing grain

size to ultrafine grain size can obtain better mechanical properties such as higher yield stress with

good ductility. Meyers et al. [22] reported that grain size structuring was possible by certain processes

such as equal channel angular processing (ECAP), where the material is passed between two channels

of a constricted die, through which reduction of the billet diameter is expected to develop plastic

hardening and grain size remodeling to ultra-fine grain size, by passes through the dies of decreasing

diameter. They also suggested that techniques like cryo milling and high pressure torsion (HPT) can

also be considered for grain size remodeling.

The application of the experimental work can also be important for some of the beta titanium alloys

such as Ti-35Nb-7Zr-5Ta (TNZT) alloy which have low modules of elasticity of 55 GPa to

compensate stress shielding effect of implant materials, and also have a low tensile strength of 596

MPa. Another problem with metastable beta titanium alloys is that if the amount of beta stabilizers is

high in the beta alloys it reduces the martensitic start temperature to below the room temperature, due

to which nucleation and growth of alpha phase is restricted and hence metastable beta is retained at

room temperature under rapid cooling. During this stage depending upon the composition and heat

treatment parameters, precipitation of omega (ω) phase is possible. The presence of this phase in

titanium alloys causes embrittlement [23]. In general, for limited beta stabilizer content only and

depending upon cooling conditions, titanium alloys show only alpha and beta phases. However, if the

thermodynamic equilibrium is not reached, metastable phases may be retained at room temperature,

mainly, martensitic and ω phases.

The presently available beta alloys are required to be solution treated for long hours and complex heat

treatment cycles to obtain the desired mechanical properties. Rack and Qazi [24], based on their

experimental work on beta alloys, have reported that strength of the beta titanium alloys increases

with solution treatment and aging. However, this increase in strength is at the expense of ductility and

elastic modulus. In their work on heat treatment of beta titanium alloy such as Ti-29Nb-13Ta-4.6Zr, it

was reported that yield strengths as high as 1100 MPa have been attained after long aging treatments,

at 450°C for 48 hours. However, this increase occurs at the cost of elongation which has decreased to

lower than 3% with an increase in elastic modulus to 85 GPa. Again, the longer aging at higher

temperatures results in the formation of omega phase. Therefore, it can be concluded that, compared

to the problems encountered in the development of tensile strength and hardness by heat treatment,

shotpeening can be the best alternative for enhancing the above properties.

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4. Conclusions

The wear and tensile behavior of the Ti-6Al-4V and Ti-6Al-7Nb titanium alloys, after shotpeening,

has been studied. The main findings are:

(1) Shotpeening has improved the surface hardness and ultimate tensile strength of titanium implant

alloys. Surface hardening in turn greatly enhances their wear resistance. Ti-6Al-4V has responded

better to shotpeening as compared to Ti-6Al-7Nb.

(2) Effect of shotpeening is more pronounced while considering the tensile behavior of the

shotpeened specimens of both alloys. Higher, by 15%, tensile strengths have been obtained, as

compared to same composition unpeened specimens of both alloys.

(3) Shotpeening improves surface roughness.

(4) Shotpeening, as compared to other surface modification techniques for the improvement of

various mechanical properties, can be considered as cost effective, time saving with superior

advantages such as enhanced wear resistance and strength.

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Table and figure captions

Table 1. Chemical compositions, in wt.%, of Ti-6Al-4V and Ti-6Al-7Nb alloys

Table 2. Properties of shotpeened Ti64 and Ti7Nb alloys under different operating pressures

Figure 1. Effects of shotpeening [1].

Figure 2.Tensile specimen according to ASTM: E-8 specification.

Figure 3. Effect of peening time on Almen intensity.

Figure 4. Wear tracks of 4.5 bar shotpeened Ti64 alloy.

Figure 5. Wear tracks of 4.5 bar shotpeened Ti7Nb alloy.

Figure 6. 3D AFM images of (a) unpeened specimen and specimens shotpeened at (b) 3.5 bar and (c)

4.5 bar.

Figure 7. Stress strain curves of various heat treated specimens [19].

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Table 1. Chemical compositions, in wt.%, of Ti-6Al-4V and Ti-6Al-7Nb alloys

Element Ti-6Al-4V Ti-6Al-7Nb

Ti 89.6 87.6

Al 6.29 5.8

V 3.95 -

Nb - 6.5

Fe 0.09 0.037

C 0.029 0.017

Table 2. Properties of shotpeened Ti64 and Ti7Nb alloys under different operating pressures

Alloy ConditionSurface roughness of wear

track (Ra) (µm)

Microhardness

HV(0.5)

Wear rate (×10-11

m3/m)

Ti-6Al-4V No peening 0.016±0.002 326±1 2.016±0.110

Peening 3.5

bar1.67±0.06 412±47 0.976±0.012

Peening 4.5

bar2.27±0.12 439±66 0.907±0.079

Ti-6Al-7Nb No peening 0.609±0.008 319±1 0.974±0.132

Peening 3.5

bar1.73±0.10 377±20 0.807±0.012

Peening 4.5

bar1.96±0.03 352±16 0.786±0.004

15

Figure 1. Effects of shotpeening [1].

Figure 2.Tensile specimen according to ASTM: E-8 specification.

0 10 20 30 40 50 600

0.1

0.2

0.3

0.4

0.5

Time (seconds)

Alm

en In

tens

ity

Figure 3. Effect of peening time on Almen intensity.

16

Figure 4. Wear tracks of 4.5 bar shotpeened Ti64 alloy.

Figure 5. Wear tracks of 4.5 bar shotpeened Ti7Nb alloy.

17

(a) unpeened specimen

(b) 3.5 bar

(c) 4.5 bar

Figure 6. 3D AFM images of (a) unpeened specimen and specimens shotpeened at (b) 3.5 bar and (c) 4.5

bar.

18

Figure 7. Stress strain curves of various heat treated specimens [19].

19