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 UniversityBelfast, U.K.
3Department of Mechanical Engineering,Osmania University,Hyderabad,India.
*Corresponding Author. Email:; 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.
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 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.Toachieve this,all its process variables must be identified and controlled [2].There are many fundamental parameters affecting the shotpeeningprocesses.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.
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 understandingthe 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].
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 testswere 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 1m/sec and a sliding distance of 500 meters. The titanium alloy pin materials were tested while rotating on a hardened steel discwhich had a hardness of 69 Rc. In this work, a load of 50 N was applied on a pin diameter of 10mm 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 ambientcondition (25±2 °C) using single crystal silicon N type probes (NSG 03-A) having radiusof curvature of 10 nm. The cantilever with long tips (aspect ratio 3:1), with forceconstants of 0.35 to 6.06 N/m and resonance frequencies of 47-150 kHz, was used toimage the surface morphology.
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/minto 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 alloyswas obtained together with an improvement in the hardness.With increasing pressures, from 3.5 to 4.5 and to 5.5 bar,with 20seconds peening duration, the Almen peening intensities are0.21, 0.32 and 0.42 A, respectively.It should be noted that as the peening pressure was increased,the Almen intensityalso 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.
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 nosignificant 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 stronglyto the peening process. There are no significant changes in the hardness of Ti7Nb upto 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 isinstrumental in higher wear resistance of the shotpeened alloy. Doni et al. [12] intheir experimental work on wear behavior of cobalt-chromium biomedical alloys have reported that Archard wear rate equation clearly indicates that wear rate is inverselyproportional to hardness of the wearing metal. Improvement of wear resistance of the peenedspecimens shown in Table 2 clearly confirms the effect of surface hardening of the treatedalloys due to the application of steel shots at high pressure onto the specimens.