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Effect of electropulsing treatment and ultrasonic striking treatment on the mechanical propertis and microstructure of biomedical Ti-6Al-4V alloy Xiaoxin Ye, Yongda Ye, Guoyi Tang
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S1751-6161(14)00265-3 http://dx.doi.org/10.1016/j.jmbbm.2014.08.022 JMBBM1251
To appear in: Journal of the Mechanical Behavior of Biomedical Materials
Received date:25 May 2014 Revised date: 19 August 2014 Accepted date: 24 August 2014 Cite this article as: Xiaoxin Ye, Yongda Ye, Guoyi Tang, Effect of electropulsing treatment and ultrasonic striking treatment on the mechanical propertis and microstructure of biomedical Ti-6Al-4V alloy, Journal of the Mechanical Behavior of Biomedical Materials, http://dx.doi.org/10.1016/ j.jmbbm.2014.08.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of electropulsing treatment and ultrasonic striking treatment on the mechanical propertis and microstructure of biomedical Ti-6Al-4V alloy Xiaoxin Ye a,b , Yongda Ye a,b,c, Guoyi Tang a,b,1 a
Advanced Materials Institute, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, PR China b Key Laboratory for Advanced Materials of Ministry of Education, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China c Graduate School at Shenzhen, Harbin Institute of Technology, Shenzhen 518055, PR China
Abstract: The effect of coupling process of high-energy electropulsing treatment and high-frequency ultrasonic striking treatment on the mechanical properties and microstructure evolution of biomedical Ti-6Al-4V alloy is investigated. Results show that the materials ductility under electropulsing treatment is noticeably improved while sacrificing the strength slightly. In this process, refined microstructure is obtained accompanying by recrystallization process and weakened basal texture. Rapid improvement of microstructure and ductility in low-temperature is attributed to accelerated atoms diffusion in recrystallization process with the coupling of thermal and athermal effects. Materials surface microhardness is dramatically enhanced by ultrasonic striking treatment and grain size reaches more than twice of original state. Plastic strain and phase change in the surface layer is contributed in ultrasonic surface strengthening effect.
Keywords: Titanium alloy; Electropulsing treatment; Ultrasonic striking treatment; Mechan-
1
Corresponding author at: Advanced Materials Institute, Graduate School at Shenzhen,
Tsinghua University, Shenzhen, 518055, PR China. E-mail: [email protected] (Prof G.Y . Tang)
ical property; Microstructure.
1 INTRODUCTION Titanium and its alloys, especially dual phase titanium alloys like Ti-6Al-4V alloy, are the most attractive metallic materials used in biomedical field by their outstanding biocompatibility, high specific strength and good corrosion resistance [1]. While the metallic alloy have its shortcoming in the high-cost processing, surface abrasive resistance in multi-functioned working situations. The materials ductility in the usage and processing is very significant. However, the difficulty in the deforming and processing at ambient or low-temperature conditions is the major obstacle for widespread applications of titanium alloys in biomedical field [2, 3]. Gurao et al. [4] found that the lower formability of the cross-rolled commercially pure titanium samples is attributed to grains orientation hardening near the basal texture. Therefore, titanium alloys are known as difficult-to-machine materials. In order to improve the ductility and formability of titanium alloy in the materials processing, many kinds of processing or assisted processing methods including the traditional heat treatment have been introduced to decrease the resistance of deformation in the titanium alloys. Nakai et al. [5] conducted duplex heat treatments as order to improve the ductility of a coil-able thin sheet of (alpha + beta)-type titanium alloy for use in aircraft applications. It’s implicated that the overall materials ductility is improved by increasing the first heat-treatment temperature because the rolling texture was decreased by the treatment. Others like Liu et al. [6] designed subtransus triplex heat treatment on the Ti-5Al-5Mo near titanium alloy samples prepared by laser melting deposition with high strength but low ductility. The triplex heat treatment is designed on the basis of the tradition-
al method significantly reducing the continuous grain boundary phase, and the materials ductility is noticeably improved. What’s regret is that that most of the solutions are not yet well accepted in the industrial domain due to high-temperature or low-efficiency. Therefore, it’s necessary to find a new processing method to improve materials ductility with high-efficiency and low-temperature. On the other hand, many researchers tried to improve materials surface microhardness to meet the demands biomaterials in the human body [7, 8]. Li et al. [9] prepared Zr-N alloying layers on the surface of pure titanium substrates by plasma surface alloying technique to improve the surface microhardness. The surface modification enhanced the microhardness of the surface layers of pure titanium substrates greatly and improves the wear resistance of pure titanium obviously. Tang et al. [10] reported a process combining punching, sandblasting and recovery treatments (PSR) for synthesizing an alloyed Al-Ti nano-crystalline surface layer on an Al alloy to enhance surface microhardness. But what’s regret is that most of surface modification methods is high-cost and low-efficiency, which limits the further development in the materials application. Recently, a novel style of energy input assisted processing technology aiming to improve the materials microstructure and ductility has emerged, that is high-energy electropulsing treatment (EPT). Electro-plasticity can be applied within a short time and lower temperature in this process [11, 12]. Gromov et al. [13] studied the structure, phase composition and dislocation substructure of steel subjected to EPT and found the fatigue strength increases within short time treatment. Zhu et al. [14] carried out tensile deformation of ZA22 alloy with high-energy EPT in studying the effects of the electropulsing on microstructural evolution and
experimental results found that the elongation of the material is increased by 437% at ambient temperature and it’s more important that the process is at a high deformation rate which is high-efficiency tensile process in the materials engineering. Our previous research successfully applied the EPT in processing FGM (functionally graded materials) titanium alloys lowering the processing temperature and persisting time [15]. Another high-efficiency processing method, known as ultrasonic striking treatment, recently has attracted more and more attention in materials surface modification especially improving the surface microhardness by inducing surface severe plastic deformation (SSPD) with high-efficiency and low-cost [16, 17]. Liu et al. [17] conducted this new technique to realize surface nano-crystallization layer on a 316L stainless steel surface. After the ultrasonic treatment, obvious grain refinement was observed and the microhardness enhancement was found. Cherif et al. [18] conducted ultrasonic nanocrystal surface modification on an austenitic steel AISI 304 and studied near-surface microstructures, residual stress states and the surface microhardness in this process. Their results show the ultrasonic striking is better than the deep rolling or shot peening in microhardness improvement and efficiency enhancement. Thus, ultrasonic striking treatment can be used to improve the surface microhardness of the materials due to its high-efficiency. However, coupling of high-energy electropulsing treatment (EPT) and high-frequency ultrasonic striking has not been much reported in improving both of ductility and surface microhardness of Ti-6Al-4V alloy strips in low temperature and ultrafast procedure. Therefore, the effect of coupling process on the mechanical properties and microstructure evolution of biomedical titanium alloy is investigated in this paper.
2 EXPERIMENTAL The furnace-annealing stress relief commercial titanium alloy Ti-6Al-4V (6.66% Al, 5.13% V, 0.21% Fe, 0.03% Mo, balance Ti) was utilized as the original material in this series of experiments. After cold rolling process, the deformed Ti-6-4 strip is 1.6 mm thick without obvious side cracks. The whole on-line EPT process and real-time temperature monitoring device is shown schematically in Fig.1. The cold-worked titanium alloy strip is treated dynamically by two pairs of conductive rollers and obtains different frequency electropulsing treatments in the processing zone between the anode and cathode. Applied current frequency was from 160 to 480Hz and current density was from 16.6 to 31.6 $PP Other EPT electrical parameters were kept in the same condition. The apparent temperature of the strip is from 440 to 730ഒ. The Ti-6Al-4V strips move through two conductive electrodes of EPT system (intermediate processing length is about 225 mm and the moving velocity of the specimen is about 3.6 m/min) and the pressure between the two electrodes and the strips is enough to keep suitable electrical contact without deformation. A self-made electro-pulsing generator supplied multi-positive pulses within the short duration (about 80 Ps). EPT was then applied to the on-line strips in the EPT zone with various EPT frequencies. The electrical parameters of EPT including frequency, root-mean-square current (RMS), amplitude and duration of current pulses were monitored by a Hall effect sensor connected to an oscilloscope in the EPT discharging circuits system. The temperature of strips was measured by a storable K type surface thermocouple in the real-time temperature measuring system and a Raytek MX2 infrared thermoscope. The changing temperature on the material surface could be recorded and analyzed on the temperature monitoring system.
Pre-EPT specimen is then selected for the ultrasonic striking treatment to obtain the surface strengthening layer on the surface of the titanium alloy strips. Ultrasonic surface strengthening modification (USSM) utilizes ultrasonic vibratory energy at a high frequency of 20 kHz and tens of thousands of strikes on the surface of the titanium alloy strips per second. Applied striking number was from 6000 to 96000 times/mm2 and other parameters were kept in the same condition. These high-frequency strikes cause severe plastic deformation on the strip surface. The whole schematic diagrams of the USSM process is shown by Fig.2. The power output of the ultrasonic generator is about 1 kW. The ball tip has a diameter of 2.6 mm and was made of tungsten carbide. The uniaxial tensile tests at room temperature were carried out to measure the mechanical properties of specimens treated by high-energy electropules. The micro-hardness tests were conducted by micro-hardness tester (Struers Duramin 05656242) at ambient condition and each test was measured in 6 repeats to obtain the average microhardness. With the help of scanning electron microscope (SEM) and electron backscatter diffraction (EBSD), the evaluation of the microstructure (including the grain size, grain boundary and characteristic texture) of the treated titanium samples was investigated. Three-dimensional optical stereo-microscope (OSM, HiROX KH-7700 microscope) was applied to observe the cross-section microstructure. X-ray Diffraction (XRD) was utilized to characterize the constitution of materials surface by a Rigaku micro-diffraction goniometer equipped with a one-dimensional detector using Cu KDradiation operated with 40 kV/200 mA. HK21A residual stress tester was utilized to measure the residual stress of the samples after ultrasonic striking treatments. The dislocation density was measured by XRD on a PHILIPS APD-10
X-ray diffractometer. The used techniques in measuring the macroscopic residual stress: In measuring the macroscopic residual stress, there’re two kinds of measurements: quantitative measurement method and qualitative measurement method. It’s widely applied in the quantitative measurement of macroscopic residual stress as the drilling blind hole method due to precision and repeatability. This measuring method is drilling a small-sized hole in the measured point on the materials to release the residual stress, whose strain variable is measured by strainometer (strain gage) adherent to the neighboring area. Then, the macroscopic residual stress is calculated by the theory of elastic mechanics based on the measured strain variable. The used techniques in measuring the dislocation density: Accurate dislocation density measurement can be obtained by the close relation between the dislocation density and residual strain. Thus, XRD measurement is used in measuring the residual strain and further obtained the dislocation density through relevant calculating models, which is more statistical and accurate than other methods. The measurement and calculation of the dislocation density: The value of the dislocation density U was calculated from the average value of the crystal size D and microstrain H 3 by the following relationship: 1/2
3 2S H 3
1/ 2
U
Db
(1)
where b is the Burgers vector which is relative to the materials crystal structure and lattice parameter.
The crystal size and microstrain are obtained by XRD diffraction peaks data through Williamson hall plot method. A series of FW S , that is full width at half maximum diffraction peaks, were selected to obtain the grain size and microstrain from the curve of -
FW (S) cos T
O
sin T
O
sin T
O
. The slope of the curve is twice of the microstrain H and intercept in
the vertical coordinates is reciprocal of the crystal size D according to the least square fitting method. Therefore, the dislocation density value is obtained by substituting the obtained crystal size D and microstrain H into the above dislocations density equation.
3 RESULTS ͵Ǥͳ Ǧ The dependence of the ultimate tensile strength (UTS) and elongation to failure (EL) of the treated titanium alloy strips by the high-energy EPT frequency is presented by Fig.3. With the help of electropulsing treatment, the ductility of the cold-rolled (CR) sample is enhanced with increasing frequency of EPT. The whole process of ductility evolution could be divided into three parts: (a) preliminary part (exerting frequency is less than 260 Hz), in this period the materials ductility changed slightly, (b) intermediate part (the frequency of EPT is from 260 Hz to 360 Hz), ductility was greatly improved and reached the maximum value (32.5%) at the frequency of 360 Hz, which is enhanced by 550% from CR-state level (5%)
and (c) decline part, the elongation to failure is lowered gradually. In the meantime, the strength decreases slightly by 16.2% (from 1160 MPa to 972 MPa). Therefore, high-energy EPT brings in noticeable ductility enhancement only sacrificing a little strength of the material. Fig.4a showed the microhardness distribution form the sample surface under ultrasonic striking. The microhardness maintains at 260 HV uniformly as the pre-EPT sample before striking. Applying USSM brings in a strengthened surface layer and higher striking number (2#-6#) improves both of effective depth and surface microhardness. Maximum microhardess equals to 426s6 HV in the striking number of 5#-specimen (72000 times/mm2) and further increasing striking number takes no effect in increasing additional microhardness. Fig.4b concluded the effect of ultrasonic striking on the effective depth of surface strengthening and surface microhardness. In the increasing shocking intensity on the samples surface, these two values gradually enhance and reach celling value 426HV at 72000 times/mm2. Therefore, the strengthened depth and surface microhardness are both enhanced with the help of high-frequency ultrasonic striking, and celling strengthening effect occurs at 72000 times/mm2. What’s more, residual compressive stress is introduced by ultrasonic striking as shown by Fig.5. Weak striking (striking number is less than 30000) gradually enhances residual compressive stress. Medium intensity (striking number is from 30000-72000) induces a rapid increment of compressive stress value. Further increasing striking number keeps the stress value at 26MPa constantly. In our novel method of electropulsing treatment and ultrasonic striking treatment, the macroscopic stress value is relatively small due the coupling effect of
electropulses and weak ultrasonic striking effect [19]. On the one hand, electropulses decreased the dislocations piling up and entangling, which weakens the residual stress in some extent. On the other hand, the applied energy of ultrasonic vibration is much lower than the laser shock peening and deep rolling. Fortunately, the processing cost and efficiency is advantageous of ultrasonic vibration.
͵Ǥʹ Fig.6 shows the microstructure and grain size distribution of treated titanium materials by electropulsing with the help of EBSD characterization. Fig.6a presents many elongated grains and irregular grains induced by cold-rolled deformation, which causes poor ductility of titanium alloy strips. The deformed grains were about 5-15 m shown by Fig.6b. 260Hz-EPT induces large amounts of recrystallized grains newly formed at deformed bands where has the enough EPT energy input and storable energy of deformation, which can be seen from denotation of white arrows in Fig.6c. The newly formed grains were about 5-45 m in even distribution and the microstructure is effectively refined with smaller grain size, shown by Fig.6d. Refined microstructure facilitates the ductility enhancement of titanium alloy strips. If the frequency of EPT processing is further increased to 480Hz, the normal grains growth gradually occurs in the Fig.6e and Fig.6f. It’s implicated that the suitable parameter of EPT is important to control the microstructure. The pole figures of the cold-rolled sample and electropulsing treated sample are shown by Fig.7. Fig.7a showed the strong basal texture of the samples in the deformed state with the
maximum texture intensity of 17.12, which explains the poor plasticity due to scarce slip systems under strong basal texture. Additionally, the 10 10 crystal orientation is relatively concentrated. While the ^0001` basal texture of 260Hz-EPT sample is weakened (maximum intensity is decreased to 10.56) that improves elongation to the failure of the titanium materials, as can be seen be Fig.7b.
͵Ǥ͵ Ǧ The microstructure of the treated titanium alloys by ultrasonic striking is illustrated by Fig.8. Fig.8a presents the uniform microstructure of pre-EPT sample before ultrasonic striking. Weak striking (3# sample with striking number of 36000) introduces a thin surface layer (less than 100m) of plastic strain, as shown by Fig.8b. Enhancing-intensity ultrasonic striking (6# sample with striking number of 96000) increases the thickness of plastic straining layer to about 120m in Fig.8c. High-resolution SEM observation (Fig.8d) demonstrates beta-Ti phase and twins in the surface strengthening layer induced by high-intensity striking treatment. Fig.9 shows the XRD patterns of the specimen surface at various intensity ultrasonic striking. Increasing intensity of beta-Ti phase peak demonstrates that striking facilitates the beta phase transformation and higher striking intensity increases the transition trend, which is correlated with surface strengthening by inducing phase change. The results of increasing dislocation density of ultrasonic striking sample demonstrate that surface strain strengthening and increased interior defects is another factor leading to the
enhancement of surface microhardness (Fig.10).
4 DISCUSSION ͶǤͳ Ǧ Ǧ In the process of recrystallization of the titanium alloy strips, the energy for recrystallization nuclei was mainly from the stored energy in the cold-rolled deformation and the energy input of electropulsing. For the traditional heat treatment, the external energy input was just from the thermal source and heating efficiency was low due to non-direction heat transfer feature of the furnace facilities and low heat transfer coefficient. The energy transferred from heat source to the object materials can be expressed by the Fourier’s law [20]. The energy transfer efficiency of the traditional method was low which is not only relative to the temperature gradient but concerned with the thermal conductivity and heating time. Therefore, low thermal conductivity and decreasing temperature gradient in the heating process prolongs the heating duration for enough energy input for recrystallization nucleation and the following growth rate of the recrystallization nuclei. While thanks to high-energy electropulsing technique, rapid energy transmission through accelerating electrons in the interior material supplies the enough energy to overcome the energy obstacle to the nucleation process in an ultrafast procedure, which is demonstrated by Fig.6c the recrystallization process occurring in
several second on-line treatment. In the process of EPT, the peak current effect ( J m ) was made as the athermal effect due to short duration of current effect and the effect of root-mean-square value of current densities in EPT ( J e ) was called thermal effect. During the EPT process, atomic diffusions are greatly accelerated by the coupling of the thermal and athermal effects [21]. In the EPT process, athermal effect of electropulsing accelerates the motional atoms participating in nucleation and growth of recrystallization lowering the necessary temperature of recrystallization. On the other hand, it’s implicated that the atomic diffusion for the recrystallization process and lattice diffusion coefficient are both relative to the temperature by Zhu et al [12]. The total energy input of electropulsing is consisting of thermal and athermal effects. Thermal effect is Joule heat positively proportional to the applied frequency. On the other hand, athermal effect is also relative to the temperature originated from thermal effect. Therefore, high frequency electropulsing brings in more energy input to the materials than the traditional heat treatment with only Joule heat. Therefore, higher frequency electropulsing brings in more noticeable thermal effect and athermal effect accelerating the recrystallization process and lowering apparent temperature. Refined microstructure (Fig.6d) induced by recrystallization is facilitated in enhancing ductility. Small grains accommodates tensile deformation through grains rotation reducing concentrated stress and incubation of microvoids [22] so that the elongation to the failure of 260Hz-EPT sample is effective improved. On the other hand, disappearance of deformation microstructure and interior defects are positive in putting off the microcracks development in the tensile fracture. Additionally, the basal texture is also weakened by rotating the
near-normal crystal direction to the transverse direction to lower the difficulty of deforming titanium alloy strip as shown by Fig.7. Yanbin et al. [11] also pointed out the weakened basal texture induced by the recrystallization process facilitates and ductility improvement of alloy strips. Scarce slipping system in the basal plane is the leading factor in restricting the deformation ability. Thus, weakened basal texture by electropulsing is positive in enhancement of ductility. However, too high-frequency EPT causes grain coarsening shown by Fig.6e and Fig.6f. Accommodated deformation comes to be difficult for big sized microstructure by lacking of small grains rotating deformation mode. Therefore, ductility is decreased gradually when applied frequency is more than 360Hz, as can be seen by Fig.3. Therefore, it’s important to control the electropulsing parameters in improving the microstructure and ductility. Additionally, ultimate strength of specimen decreases slightly with increasing frequency is concerned with weakening of strain strengthening induced by recrystallization process and grain growth under electropulsing treatment.
ͶǤʹ Ǧ Ǧ The total energy ( Pt ) applying on the pre-EPT titanium alloy strip from the ultrasonic vibratory device is relative to striking frequency and numbers [23]. In this series of striking treatments, the applied frequency is keeping unchanged at 20 kHz. Thus total energy on the surface strengthened layer is positive to striking numbers. The high-frequency collision of shocking ball tip on the surface of the titanium alloy
strips brings in severe plastic deformation. Moreover, the time interval between two shocks is so short that the elastic recovery is restricted in such a short duration. Thus, overlap of multi-elastic deformation leads to the plastic compressive deformation even if the stress value is less than the stress yield strength as shown by Fig.8b and Fig.8c, which causes strain strengthening effect improving surface microhardness. Intensified striking with higher striking numbers induces severe plastic deformation introducing large amounts of dislocations demonstrated by Fig.10. When the density of dislocations reaches the critical value dislocation cell is originated by dislocation annihilation and arrangement. With the reproduction of the dislocations, when the cross-slip of many dislocations is impeded there is concentrated stress forming in this process accompanying by the mechanical twinning induced by the increment of strain extent and strain rate, as shown by Fig.8d. Thus this strengthening effect including the surface microhardness and effective depth is enhanced with increasing striking numbers. Moreover, this plastic deformation in the materials surface can be accomplished in an ultrafast procedure thanks to the high-frequency energy input. In addition, there is a phase change phenomenon in applying the ultrasonic striking on the materials surface, as can be seen from intensifies beta phase peaks in Fig.9. The beta phase in the two-phase Ti-6Al-4V alloy strip is the unstable phase in low temperature so the beta strips and colony start to transform from matrix alpha phase near the sample surface under striking [24]. Beta phase as strengthening phase in the two-phase titanium alloy is also positive in improving the surface microhardness.
ͶǤ͵ Ǧ In fact, coupling of electropulsing treatment and ultrasonic striking are both high-efficient processing methods. The on-line processing rate of electropulsing can be 3.6m/min so that the time of processing 1 meter long titanium alloy strip is only 16.7 seconds; on the other hand, the feed speed of ultrasonic striking can be 3 minutes for 1 meter long strip material. Therefore, the whole coupling process is just less than 4 minutes for one meter strip material. The ductility of the treated material can be dramatically improved to 32.5% by 550% increment and the surface microhardness can be increased to 426 HV more than twice of original material. The process enhances the surface microhardness so that wear resistance enhancement is thus deduced. Therefore, this high-efficiency coupling technique is an abstracting high-efficient, green and energy saving materials processing method to prepare outstanding biomedical titanium alloy materials with high ductility and surface microhardness.
5 CONCLUSIONS Coupling process of high-energy electropulsing treatment (EPT) and high-frequency ultrasonic striking has been innovatively designed in improving both of ductility and surface microhardness of Ti-6Al-4V alloy strips in low temperature and ultrafast procedure. The effect of coupling process on the mechanical properties and microstructure evolution of bio-
medical titanium alloy is investigated with the help of uniaxial tensile test machine, three-dimensional optical stereo-microscope, scanning electron microscope and electron backscatter diffraction, microhardness tester and X-ray Diffraction. Results show that the materials ductility under EPT is noticeably improved by 550% increment to 32.5% while sacrificing the strength slightly. In this process, refined microstructure is obtained accompanying by recrystallization process. In addition, basal texture is effectively weakened and distracted. Rapid improvement of microstructure and ductility in low-temperature is attributed to accelerated atoms diffusion in recrystallization process with the coupling of thermal and athermal effects. On the other hand, surface microhardness and effective depth are both enhanced by ultrasonic striking and celling value of microhardness is 426 HV more than twice of original material. Surface layer with plastic strain and phase change in the process of high-frequency ultrasonic striking is contributed in surface strengthening. Therefore, this high-efficiency coupling technique is an abstracting high-efficient, green and energy saving materials processing method to prepare outstanding biomedical titanium alloy materials with high ductility and surface microhardness.
ACKNOWLEGMENTS The work is supported by National Natural Science Foundation of China (No. 50571048) and Shenzhen science and technology research funding project of China (No. SGLH20121008144756946)
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Figure Captions Fig.1. Schematic view of pre-EPT process and real-time temperature monitoring system.
Fig.2. Schematic diagrams of the ultrasonic surface strengthening modification (USSM) on the pre-EPT titanium alloy strips.
Fig.3. The dependence of the mechanical properties ultimate tensile strength (UTS) and elongation to failure (EL) of the treated Ti-6Al-4V alloy strips on the high-energy EPT experimental frequency. CR show values for a cold rolled specimen.
Fig.4. (a) The dependence of the microhardness distribution along the depth from the sample surface on different USSM parameters. Samples number: 1#-pre-EPT sample without ultrasonic striking; 2#-6# mean samples at striking number of 6000, 36000, 60000, 72000 and 96000 times/mm2. (b) The concluded dependence of the effective depth of striking and top surface micro-hardness of samples on the striking number of ultrasonic striking.
Fig.5. The relationship between the residual macroscopic stress and ultrasonic striking number.
Fig.6. EBSD euler angle orientation map of samples under high-energy EPT and corresponding grain size distribution: (a)-(b) cold-rolled samples; (c)-(d) 260Hz-EPT samples and (e)-(f) 480Hz-EPT samples. Boundaries are defined for misorientations upper than 15°.
Fig.7. The
^0001`
and ^10 10` pole figures of the Ti-6Al-4V alloy strips at different
conditions: (a) cold-rolled samples and (b) 260Hz-EPT samples.
Fig.8. Microstructure near the surface after high-frequency ultrasonic striking treatments: (a) Pre-EPT 1# sample, (b) 3# sample, (c) 6# sample and (d) SEM high-resolution observation of 6# sample.
Fig.9. XRD (X-Ray Diffraction) patterns of specimens surface of pre-EPT 1# sample and 3#-6# ultrasonic striking sample.
Fig.10. Dislocation density of samples treated by ultrasonic striking process.
Highlights z
Coupling of electropulsing and ultrasonic striking treatment on Ti alloy.
z
Ductility and surface microhardness are noticeably improved.
z
Rapid recrystallization is obtained under electropulsing treatment.
z
Plastic strain and phase change occur in the ultrasonic surface strengthening layer.
z
Low-temperature and high-efficiency procedure.
Graphical abstract.tiff
Fig.1.schematic view of EPT.tif
Fig.2.Schematic view of USSM.tif
Fig.3.mechanical property.tiff
Fig.4.Microhardness.tif
Fig.5.Residual stress.tiff
Fig.6.EPT-microstructure.tiff
Fig.7.Pole figure.tiff
Fig.8.USSM microstructure.tiff
Fig.9.XRD.tiff
Fig.10.Dislocation density.tiff
Effect of electropulsing treatment and ultrasonic striking treatment on the mechanical propertis and microstructure of biomedical Ti-6Al-4V alloy Xiaoxin Ye, Yongda Ye, Guoyi Tang
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PII: DOI: Reference:
S1751-6161(14)00265-3 http://dx.doi.org/10.1016/j.jmbbm.2014.08.022 JMBBM1251
To appear in: Journal of the Mechanical Behavior of Biomedical Materials
Received date:25 May 2014 Revised date: 19 August 2014 Accepted date: 24 August 2014 Cite this article as: Xiaoxin Ye, Yongda Ye, Guoyi Tang, Effect of electropulsing treatment and ultrasonic striking treatment on the mechanical propertis and microstructure of biomedical Ti-6Al-4V alloy, Journal of the Mechanical Behavior of Biomedical Materials, http://dx.doi.org/10.1016/ j.jmbbm.2014.08.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of electropulsing treatment and ultrasonic striking treatment on the mechanical propertis and microstructure of biomedical Ti-6Al-4V alloy Xiaoxin Ye a,b , Yongda Ye a,b,c, Guoyi Tang a,b,1 a
Advanced Materials Institute, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, PR China b Key Laboratory for Advanced Materials of Ministry of Education, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China c Graduate School at Shenzhen, Harbin Institute of Technology, Shenzhen 518055, PR China
Abstract: The effect of coupling process of high-energy electropulsing treatment and high-frequency ultrasonic striking treatment on the mechanical properties and microstructure evolution of biomedical Ti-6Al-4V alloy is investigated. Results show that the materials ductility under electropulsing treatment is noticeably improved while sacrificing the strength slightly. In this process, refined microstructure is obtained accompanying by recrystallization process and weakened basal texture. Rapid improvement of microstructure and ductility in low-temperature is attributed to accelerated atoms diffusion in recrystallization process with the coupling of thermal and athermal effects. Materials surface microhardness is dramatically enhanced by ultrasonic striking treatment and grain size reaches more than twice of original state. Plastic strain and phase change in the surface layer is contributed in ultrasonic surface strengthening effect.
Keywords: Titanium alloy; Electropulsing treatment; Ultrasonic striking treatment; Mechan-
1
Corresponding author at: Advanced Materials Institute, Graduate School at Shenzhen,
Tsinghua University, Shenzhen, 518055, PR China. E-mail: [email protected] (Prof G.Y . Tang)
ical property; Microstructure.
1 INTRODUCTION Titanium and its alloys, especially dual phase titanium alloys like Ti-6Al-4V alloy, are the most attractive metallic materials used in biomedical field by their outstanding biocompatibility, high specific strength and good corrosion resistance [1]. While the metallic alloy have its shortcoming in the high-cost processing, surface abrasive resistance in multi-functioned working situations. The materials ductility in the usage and processing is very significant. However, the difficulty in the deforming and processing at ambient or low-temperature conditions is the major obstacle for widespread applications of titanium alloys in biomedical field [2, 3]. Gurao et al. [4] found that the lower formability of the cross-rolled commercially pure titanium samples is attributed to grains orientation hardening near the basal texture. Therefore, titanium alloys are known as difficult-to-machine materials. In order to improve the ductility and formability of titanium alloy in the materials processing, many kinds of processing or assisted processing methods including the traditional heat treatment have been introduced to decrease the resistance of deformation in the titanium alloys. Nakai et al. [5] conducted duplex heat treatments as order to improve the ductility of a coil-able thin sheet of (alpha + beta)-type titanium alloy for use in aircraft applications. It’s implicated that the overall materials ductility is improved by increasing the first heat-treatment temperature because the rolling texture was decreased by the treatment. Others like Liu et al. [6] designed subtransus triplex heat treatment on the Ti-5Al-5Mo near titanium alloy samples prepared by laser melting deposition with high strength but low ductility. The triplex heat treatment is designed on the basis of the tradition-
al method significantly reducing the continuous grain boundary phase, and the materials ductility is noticeably improved. What’s regret is that that most of the solutions are not yet well accepted in the industrial domain due to high-temperature or low-efficiency. Therefore, it’s necessary to find a new processing method to improve materials ductility with high-efficiency and low-temperature. On the other hand, many researchers tried to improve materials surface microhardness to meet the demands biomaterials in the human body [7, 8]. Li et al. [9] prepared Zr-N alloying layers on the surface of pure titanium substrates by plasma surface alloying technique to improve the surface microhardness. The surface modification enhanced the microhardness of the surface layers of pure titanium substrates greatly and improves the wear resistance of pure titanium obviously. Tang et al. [10] reported a process combining punching, sandblasting and recovery treatments (PSR) for synthesizing an alloyed Al-Ti nano-crystalline surface layer on an Al alloy to enhance surface microhardness. But what’s regret is that most of surface modification methods is high-cost and low-efficiency, which limits the further development in the materials application. Recently, a novel style of energy input assisted processing technology aiming to improve the materials microstructure and ductility has emerged, that is high-energy electropulsing treatment (EPT). Electro-plasticity can be applied within a short time and lower temperature in this process [11, 12]. Gromov et al. [13] studied the structure, phase composition and dislocation substructure of steel subjected to EPT and found the fatigue strength increases within short time treatment. Zhu et al. [14] carried out tensile deformation of ZA22 alloy with high-energy EPT in studying the effects of the electropulsing on microstructural evolution and
experimental results found that the elongation of the material is increased by 437% at ambient temperature and it’s more important that the process is at a high deformation rate which is high-efficiency tensile process in the materials engineering. Our previous research successfully applied the EPT in processing FGM (functionally graded materials) titanium alloys lowering the processing temperature and persisting time [15]. Another high-efficiency processing method, known as ultrasonic striking treatment, recently has attracted more and more attention in materials surface modification especially improving the surface microhardness by inducing surface severe plastic deformation (SSPD) with high-efficiency and low-cost [16, 17]. Liu et al. [17] conducted this new technique to realize surface nano-crystallization layer on a 316L stainless steel surface. After the ultrasonic treatment, obvious grain refinement was observed and the microhardness enhancement was found. Cherif et al. [18] conducted ultrasonic nanocrystal surface modification on an austenitic steel AISI 304 and studied near-surface microstructures, residual stress states and the surface microhardness in this process. Their results show the ultrasonic striking is better than the deep rolling or shot peening in microhardness improvement and efficiency enhancement. Thus, ultrasonic striking treatment can be used to improve the surface microhardness of the materials due to its high-efficiency. However, coupling of high-energy electropulsing treatment (EPT) and high-frequency ultrasonic striking has not been much reported in improving both of ductility and surface microhardness of Ti-6Al-4V alloy strips in low temperature and ultrafast procedure. Therefore, the effect of coupling process on the mechanical properties and microstructure evolution of biomedical titanium alloy is investigated in this paper.
2 EXPERIMENTAL The furnace-annealing stress relief commercial titanium alloy Ti-6Al-4V (6.66% Al, 5.13% V, 0.21% Fe, 0.03% Mo, balance Ti) was utilized as the original material in this series of experiments. After cold rolling process, the deformed Ti-6-4 strip is 1.6 mm thick without obvious side cracks. The whole on-line EPT process and real-time temperature monitoring device is shown schematically in Fig.1. The cold-worked titanium alloy strip is treated dynamically by two pairs of conductive rollers and obtains different frequency electropulsing treatments in the processing zone between the anode and cathode. Applied current frequency was from 160 to 480Hz and current density was from 16.6 to 31.6 $PP Other EPT electrical parameters were kept in the same condition. The apparent temperature of the strip is from 440 to 730ഒ. The Ti-6Al-4V strips move through two conductive electrodes of EPT system (intermediate processing length is about 225 mm and the moving velocity of the specimen is about 3.6 m/min) and the pressure between the two electrodes and the strips is enough to keep suitable electrical contact without deformation. A self-made electro-pulsing generator supplied multi-positive pulses within the short duration (about 80 Ps). EPT was then applied to the on-line strips in the EPT zone with various EPT frequencies. The electrical parameters of EPT including frequency, root-mean-square current (RMS), amplitude and duration of current pulses were monitored by a Hall effect sensor connected to an oscilloscope in the EPT discharging circuits system. The temperature of strips was measured by a storable K type surface thermocouple in the real-time temperature measuring system and a Raytek MX2 infrared thermoscope. The changing temperature on the material surface could be recorded and analyzed on the temperature monitoring system.
Pre-EPT specimen is then selected for the ultrasonic striking treatment to obtain the surface strengthening layer on the surface of the titanium alloy strips. Ultrasonic surface strengthening modification (USSM) utilizes ultrasonic vibratory energy at a high frequency of 20 kHz and tens of thousands of strikes on the surface of the titanium alloy strips per second. Applied striking number was from 6000 to 96000 times/mm2 and other parameters were kept in the same condition. These high-frequency strikes cause severe plastic deformation on the strip surface. The whole schematic diagrams of the USSM process is shown by Fig.2. The power output of the ultrasonic generator is about 1 kW. The ball tip has a diameter of 2.6 mm and was made of tungsten carbide. The uniaxial tensile tests at room temperature were carried out to measure the mechanical properties of specimens treated by high-energy electropules. The micro-hardness tests were conducted by micro-hardness tester (Struers Duramin 05656242) at ambient condition and each test was measured in 6 repeats to obtain the average microhardness. With the help of scanning electron microscope (SEM) and electron backscatter diffraction (EBSD), the evaluation of the microstructure (including the grain size, grain boundary and characteristic texture) of the treated titanium samples was investigated. Three-dimensional optical stereo-microscope (OSM, HiROX KH-7700 microscope) was applied to observe the cross-section microstructure. X-ray Diffraction (XRD) was utilized to characterize the constitution of materials surface by a Rigaku micro-diffraction goniometer equipped with a one-dimensional detector using Cu KDradiation operated with 40 kV/200 mA. HK21A residual stress tester was utilized to measure the residual stress of the samples after ultrasonic striking treatments. The dislocation density was measured by XRD on a PHILIPS APD-10
X-ray diffractometer. The used techniques in measuring the macroscopic residual stress: In measuring the macroscopic residual stress, there’re two kinds of measurements: quantitative measurement method and qualitative measurement method. It’s widely applied in the quantitative measurement of macroscopic residual stress as the drilling blind hole method due to precision and repeatability. This measuring method is drilling a small-sized hole in the measured point on the materials to release the residual stress, whose strain variable is measured by strainometer (strain gage) adherent to the neighboring area. Then, the macroscopic residual stress is calculated by the theory of elastic mechanics based on the measured strain variable. The used techniques in measuring the dislocation density: Accurate dislocation density measurement can be obtained by the close relation between the dislocation density and residual strain. Thus, XRD measurement is used in measuring the residual strain and further obtained the dislocation density through relevant calculating models, which is more statistical and accurate than other methods. The measurement and calculation of the dislocation density: The value of the dislocation density U was calculated from the average value of the crystal size D and microstrain H 3 by the following relationship: 1/2
3 2S H 3
1/ 2
U
Db
(1)
where b is the Burgers vector which is relative to the materials crystal structure and lattice parameter.
The crystal size and microstrain are obtained by XRD diffraction peaks data through Williamson hall plot method. A series of FW S , that is full width at half maximum diffraction peaks, were selected to obtain the grain size and microstrain from the curve of -
FW (S) cos T
O
sin T
O
sin T
O
. The slope of the curve is twice of the microstrain H and intercept in
the vertical coordinates is reciprocal of the crystal size D according to the least square fitting method. Therefore, the dislocation density value is obtained by substituting the obtained crystal size D and microstrain H into the above dislocations density equation.
3 RESULTS ͵Ǥͳ Ǧ The dependence of the ultimate tensile strength (UTS) and elongation to failure (EL) of the treated titanium alloy strips by the high-energy EPT frequency is presented by Fig.3. With the help of electropulsing treatment, the ductility of the cold-rolled (CR) sample is enhanced with increasing frequency of EPT. The whole process of ductility evolution could be divided into three parts: (a) preliminary part (exerting frequency is less than 260 Hz), in this period the materials ductility changed slightly, (b) intermediate part (the frequency of EPT is from 260 Hz to 360 Hz), ductility was greatly improved and reached the maximum value (32.5%) at the frequency of 360 Hz, which is enhanced by 550% from CR-state level (5%)
and (c) decline part, the elongation to failure is lowered gradually. In the meantime, the strength decreases slightly by 16.2% (from 1160 MPa to 972 MPa). Therefore, high-energy EPT brings in noticeable ductility enhancement only sacrificing a little strength of the material. Fig.4a showed the microhardness distribution form the sample surface under ultrasonic striking. The microhardness maintains at 260 HV uniformly as the pre-EPT sample before striking. Applying USSM brings in a strengthened surface layer and higher striking number (2#-6#) improves both of effective depth and surface microhardness. Maximum microhardess equals to 426s6 HV in the striking number of 5#-specimen (72000 times/mm2) and further increasing striking number takes no effect in increasing additional microhardness. Fig.4b concluded the effect of ultrasonic striking on the effective depth of surface strengthening and surface microhardness. In the increasing shocking intensity on the samples surface, these two values gradually enhance and reach celling value 426HV at 72000 times/mm2. Therefore, the strengthened depth and surface microhardness are both enhanced with the help of high-frequency ultrasonic striking, and celling strengthening effect occurs at 72000 times/mm2. What’s more, residual compressive stress is introduced by ultrasonic striking as shown by Fig.5. Weak striking (striking number is less than 30000) gradually enhances residual compressive stress. Medium intensity (striking number is from 30000-72000) induces a rapid increment of compressive stress value. Further increasing striking number keeps the stress value at 26MPa constantly. In our novel method of electropulsing treatment and ultrasonic striking treatment, the macroscopic stress value is relatively small due the coupling effect of
electropulses and weak ultrasonic striking effect [19]. On the one hand, electropulses decreased the dislocations piling up and entangling, which weakens the residual stress in some extent. On the other hand, the applied energy of ultrasonic vibration is much lower than the laser shock peening and deep rolling. Fortunately, the processing cost and efficiency is advantageous of ultrasonic vibration.
͵Ǥʹ Fig.6 shows the microstructure and grain size distribution of treated titanium materials by electropulsing with the help of EBSD characterization. Fig.6a presents many elongated grains and irregular grains induced by cold-rolled deformation, which causes poor ductility of titanium alloy strips. The deformed grains were about 5-15 m shown by Fig.6b. 260Hz-EPT induces large amounts of recrystallized grains newly formed at deformed bands where has the enough EPT energy input and storable energy of deformation, which can be seen from denotation of white arrows in Fig.6c. The newly formed grains were about 5-45 m in even distribution and the microstructure is effectively refined with smaller grain size, shown by Fig.6d. Refined microstructure facilitates the ductility enhancement of titanium alloy strips. If the frequency of EPT processing is further increased to 480Hz, the normal grains growth gradually occurs in the Fig.6e and Fig.6f. It’s implicated that the suitable parameter of EPT is important to control the microstructure. The pole figures of the cold-rolled sample and electropulsing treated sample are shown by Fig.7. Fig.7a showed the strong basal texture of the samples in the deformed state with the
maximum texture intensity of 17.12, which explains the poor plasticity due to scarce slip systems under strong basal texture. Additionally, the 10 10 crystal orientation is relatively concentrated. While the ^0001` basal texture of 260Hz-EPT sample is weakened (maximum intensity is decreased to 10.56) that improves elongation to the failure of the titanium materials, as can be seen be Fig.7b.
͵Ǥ͵ Ǧ The microstructure of the treated titanium alloys by ultrasonic striking is illustrated by Fig.8. Fig.8a presents the uniform microstructure of pre-EPT sample before ultrasonic striking. Weak striking (3# sample with striking number of 36000) introduces a thin surface layer (less than 100m) of plastic strain, as shown by Fig.8b. Enhancing-intensity ultrasonic striking (6# sample with striking number of 96000) increases the thickness of plastic straining layer to about 120m in Fig.8c. High-resolution SEM observation (Fig.8d) demonstrates beta-Ti phase and twins in the surface strengthening layer induced by high-intensity striking treatment. Fig.9 shows the XRD patterns of the specimen surface at various intensity ultrasonic striking. Increasing intensity of beta-Ti phase peak demonstrates that striking facilitates the beta phase transformation and higher striking intensity increases the transition trend, which is correlated with surface strengthening by inducing phase change. The results of increasing dislocation density of ultrasonic striking sample demonstrate that surface strain strengthening and increased interior defects is another factor leading to the
enhancement of surface microhardness (Fig.10).
4 DISCUSSION ͶǤͳ Ǧ Ǧ In the process of recrystallization of the titanium alloy strips, the energy for recrystallization nuclei was mainly from the stored energy in the cold-rolled deformation and the energy input of electropulsing. For the traditional heat treatment, the external energy input was just from the thermal source and heating efficiency was low due to non-direction heat transfer feature of the furnace facilities and low heat transfer coefficient. The energy transferred from heat source to the object materials can be expressed by the Fourier’s law [20]. The energy transfer efficiency of the traditional method was low which is not only relative to the temperature gradient but concerned with the thermal conductivity and heating time. Therefore, low thermal conductivity and decreasing temperature gradient in the heating process prolongs the heating duration for enough energy input for recrystallization nucleation and the following growth rate of the recrystallization nuclei. While thanks to high-energy electropulsing technique, rapid energy transmission through accelerating electrons in the interior material supplies the enough energy to overcome the energy obstacle to the nucleation process in an ultrafast procedure, which is demonstrated by Fig.6c the recrystallization process occurring in
several second on-line treatment. In the process of EPT, the peak current effect ( J m ) was made as the athermal effect due to short duration of current effect and the effect of root-mean-square value of current densities in EPT ( J e ) was called thermal effect. During the EPT process, atomic diffusions are greatly accelerated by the coupling of the thermal and athermal effects [21]. In the EPT process, athermal effect of electropulsing accelerates the motional atoms participating in nucleation and growth of recrystallization lowering the necessary temperature of recrystallization. On the other hand, it’s implicated that the atomic diffusion for the recrystallization process and lattice diffusion coefficient are both relative to the temperature by Zhu et al [12]. The total energy input of electropulsing is consisting of thermal and athermal effects. Thermal effect is Joule heat positively proportional to the applied frequency. On the other hand, athermal effect is also relative to the temperature originated from thermal effect. Therefore, high frequency electropulsing brings in more energy input to the materials than the traditional heat treatment with only Joule heat. Therefore, higher frequency electropulsing brings in more noticeable thermal effect and athermal effect accelerating the recrystallization process and lowering apparent temperature. Refined microstructure (Fig.6d) induced by recrystallization is facilitated in enhancing ductility. Small grains accommodates tensile deformation through grains rotation reducing concentrated stress and incubation of microvoids [22] so that the elongation to the failure of 260Hz-EPT sample is effective improved. On the other hand, disappearance of deformation microstructure and interior defects are positive in putting off the microcracks development in the tensile fracture. Additionally, the basal texture is also weakened by rotating the
near-normal crystal direction to the transverse direction to lower the difficulty of deforming titanium alloy strip as shown by Fig.7. Yanbin et al. [11] also pointed out the weakened basal texture induced by the recrystallization process facilitates and ductility improvement of alloy strips. Scarce slipping system in the basal plane is the leading factor in restricting the deformation ability. Thus, weakened basal texture by electropulsing is positive in enhancement of ductility. However, too high-frequency EPT causes grain coarsening shown by Fig.6e and Fig.6f. Accommodated deformation comes to be difficult for big sized microstructure by lacking of small grains rotating deformation mode. Therefore, ductility is decreased gradually when applied frequency is more than 360Hz, as can be seen by Fig.3. Therefore, it’s important to control the electropulsing parameters in improving the microstructure and ductility. Additionally, ultimate strength of specimen decreases slightly with increasing frequency is concerned with weakening of strain strengthening induced by recrystallization process and grain growth under electropulsing treatment.
ͶǤʹ Ǧ Ǧ The total energy ( Pt ) applying on the pre-EPT titanium alloy strip from the ultrasonic vibratory device is relative to striking frequency and numbers [23]. In this series of striking treatments, the applied frequency is keeping unchanged at 20 kHz. Thus total energy on the surface strengthened layer is positive to striking numbers. The high-frequency collision of shocking ball tip on the surface of the titanium alloy
strips brings in severe plastic deformation. Moreover, the time interval between two shocks is so short that the elastic recovery is restricted in such a short duration. Thus, overlap of multi-elastic deformation leads to the plastic compressive deformation even if the stress value is less than the stress yield strength as shown by Fig.8b and Fig.8c, which causes strain strengthening effect improving surface microhardness. Intensified striking with higher striking numbers induces severe plastic deformation introducing large amounts of dislocations demonstrated by Fig.10. When the density of dislocations reaches the critical value dislocation cell is originated by dislocation annihilation and arrangement. With the reproduction of the dislocations, when the cross-slip of many dislocations is impeded there is concentrated stress forming in this process accompanying by the mechanical twinning induced by the increment of strain extent and strain rate, as shown by Fig.8d. Thus this strengthening effect including the surface microhardness and effective depth is enhanced with increasing striking numbers. Moreover, this plastic deformation in the materials surface can be accomplished in an ultrafast procedure thanks to the high-frequency energy input. In addition, there is a phase change phenomenon in applying the ultrasonic striking on the materials surface, as can be seen from intensifies beta phase peaks in Fig.9. The beta phase in the two-phase Ti-6Al-4V alloy strip is the unstable phase in low temperature so the beta strips and colony start to transform from matrix alpha phase near the sample surface under striking [24]. Beta phase as strengthening phase in the two-phase titanium alloy is also positive in improving the surface microhardness.
ͶǤ͵ Ǧ In fact, coupling of electropulsing treatment and ultrasonic striking are both high-efficient processing methods. The on-line processing rate of electropulsing can be 3.6m/min so that the time of processing 1 meter long titanium alloy strip is only 16.7 seconds; on the other hand, the feed speed of ultrasonic striking can be 3 minutes for 1 meter long strip material. Therefore, the whole coupling process is just less than 4 minutes for one meter strip material. The ductility of the treated material can be dramatically improved to 32.5% by 550% increment and the surface microhardness can be increased to 426 HV more than twice of original material. The process enhances the surface microhardness so that wear resistance enhancement is thus deduced. Therefore, this high-efficiency coupling technique is an abstracting high-efficient, green and energy saving materials processing method to prepare outstanding biomedical titanium alloy materials with high ductility and surface microhardness.
5 CONCLUSIONS Coupling process of high-energy electropulsing treatment (EPT) and high-frequency ultrasonic striking has been innovatively designed in improving both of ductility and surface microhardness of Ti-6Al-4V alloy strips in low temperature and ultrafast procedure. The effect of coupling process on the mechanical properties and microstructure evolution of bio-
medical titanium alloy is investigated with the help of uniaxial tensile test machine, three-dimensional optical stereo-microscope, scanning electron microscope and electron backscatter diffraction, microhardness tester and X-ray Diffraction. Results show that the materials ductility under EPT is noticeably improved by 550% increment to 32.5% while sacrificing the strength slightly. In this process, refined microstructure is obtained accompanying by recrystallization process. In addition, basal texture is effectively weakened and distracted. Rapid improvement of microstructure and ductility in low-temperature is attributed to accelerated atoms diffusion in recrystallization process with the coupling of thermal and athermal effects. On the other hand, surface microhardness and effective depth are both enhanced by ultrasonic striking and celling value of microhardness is 426 HV more than twice of original material. Surface layer with plastic strain and phase change in the process of high-frequency ultrasonic striking is contributed in surface strengthening. Therefore, this high-efficiency coupling technique is an abstracting high-efficient, green and energy saving materials processing method to prepare outstanding biomedical titanium alloy materials with high ductility and surface microhardness.
ACKNOWLEGMENTS The work is supported by National Natural Science Foundation of China (No. 50571048) and Shenzhen science and technology research funding project of China (No. SGLH20121008144756946)
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Figure Captions Fig.1. Schematic view of pre-EPT process and real-time temperature monitoring system.
Fig.2. Schematic diagrams of the ultrasonic surface strengthening modification (USSM) on the pre-EPT titanium alloy strips.
Fig.3. The dependence of the mechanical properties ultimate tensile strength (UTS) and elongation to failure (EL) of the treated Ti-6Al-4V alloy strips on the high-energy EPT experimental frequency. CR show values for a cold rolled specimen.
Fig.4. (a) The dependence of the microhardness distribution along the depth from the sample surface on different USSM parameters. Samples number: 1#-pre-EPT sample without ultrasonic striking; 2#-6# mean samples at striking number of 6000, 36000, 60000, 72000 and 96000 times/mm2. (b) The concluded dependence of the effective depth of striking and top surface micro-hardness of samples on the striking number of ultrasonic striking.
Fig.5. The relationship between the residual macroscopic stress and ultrasonic striking number.
Fig.6. EBSD euler angle orientation map of samples under high-energy EPT and corresponding grain size distribution: (a)-(b) cold-rolled samples; (c)-(d) 260Hz-EPT samples and (e)-(f) 480Hz-EPT samples. Boundaries are defined for misorientations upper than 15°.
Fig.7. The
^0001`
and ^10 10` pole figures of the Ti-6Al-4V alloy strips at different
conditions: (a) cold-rolled samples and (b) 260Hz-EPT samples.
Fig.8. Microstructure near the surface after high-frequency ultrasonic striking treatments: (a) Pre-EPT 1# sample, (b) 3# sample, (c) 6# sample and (d) SEM high-resolution observation of 6# sample.
Fig.9. XRD (X-Ray Diffraction) patterns of specimens surface of pre-EPT 1# sample and 3#-6# ultrasonic striking sample.
Fig.10. Dislocation density of samples treated by ultrasonic striking process.
Highlights z
Coupling of electropulsing and ultrasonic striking treatment on Ti alloy.
z
Ductility and surface microhardness are noticeably improved.
z
Rapid recrystallization is obtained under electropulsing treatment.
z
Plastic strain and phase change occur in the ultrasonic surface strengthening layer.
z
Low-temperature and high-efficiency procedure.
Graphical abstract.tiff
Fig.1.schematic view of EPT.tif
Fig.2.Schematic view of USSM.tif
Fig.3.mechanical property.tiff
Fig.4.Microhardness.tif
Fig.5.Residual stress.tiff
Fig.6.EPT-microstructure.tiff
Fig.7.Pole figure.tiff
Fig.8.USSM microstructure.tiff
Fig.9.XRD.tiff
Fig.10.Dislocation density.tiff
