Nondestructive Magnetic Monitoring of Residual Stresses in a Medical Ti–6Al–4V–ELI Alloy Using a Fluxgate Sensor

Attempts to improve the osteointegration, fixation and stability of Ti-base implants have been focused by producing a significative surface roughness that enhances the surface area available for bone/ implant apposition. In this research study, it shows the experimental data by synchrotron radiation X-ray diffraction (SR-XRD) of the main material surface properties (residual stress and cold work) that changed significantly during the application of different surface treatments such as grit blasting and laser shock peening in a biometallic Ti–6Al–4V alloy. The ratio of residual stress to cold work is primarily determined by the material and the specific surface treatment applied. In order to establish how they alter the recorded magnetic signatures and to validate that the residual stress and cold work effects govern the outcome of the magnetic measurements, we used a nondestructive magnetic method. It was displayed that the magnetic method provides the unique capability of nondestructively sensing the thermomechanical relaxation below the treated surface only calculating the normal and tangential magnetic intensities induced by thermocurrents using a fluxgate magnetometer.


INTRODUCTION
The thermoelectric effects, mainly described by thermoelectric voltage (which is also called Seebeck coefficient), are sensitive to small variations in the kinetics of driving electrons near the surface of Fermi. Such variations may be caused by changes in the local microstructure, chemical composition, heat treatment, hardening of metals and alloys etc., affecting the diffusion of electrons through the volume of the material. The thermoelectric measurements can be carried out in a completely nondestructive way by means of the use of high sensitivity magnetic sensors which detect the thermocurrents produced by various imperfections and inhomogeneities in metals as long as the sample that it is analyzed be submitted to an external directional heating or cooling. A drawing of the selfreferenced noncontact method with magnetic detection is shown in Fig. 1. A thermal gradient is applied externally to the sample resulting in different temperature zones between the host material boundary and the imperfection, therefore also at different thermoelectric potentials. These variations produce thermoelectric fluxes around the zone of host and imperfection. These thermoelectric currents generate a magnetic flux B which can be detected scanning the sample with a magnetometer [1][2][3]. Since the enclosing flawless material acts as the "reference" terminal, the detection sensitivity to material properties changes could be huge assuming that the magnetometer sensitivity is enough to catch the powerless magnetic field developed by the thermocurrents within the sample. The proposed magnetic method can be employed to find several heterogeneities in metals such as alien metallic inclusions, cold work, anisotropy, residual stress etc. [4][5][6].
The distributions of residual stresses (which are normally present in metals and alloys after cold working or deformation) are a mixture of traction and compression, with at least some degree of triaxiality. Under such conditions the changes in Fermi level are very difficult to evaluate on a simple basis. The complexity of the variables and their distributions in polycrystalline metals and alloys are such that their interactions make the separation of their effects extremely complex. In this research analytical-experimental study, we present evidence that suggests that the proposed magnetic method can able to detect and quantitatively assess the weighted average of the residual stress and cold work within the shallow surface layer of two different surface treatments such as grit blasting and laser peening in a biometallic Ti-6Al-4V ELI alloy.

ANALYTICAL SECTION
Consider a slender bar of rectangular cross section with length l much larger than its two other dimensions under the influence of a temperature gradient h 0 . The bar is aligned with the z-direction of a Cartesian coordinate system (x, y, z) as illustrated in Fig. 2. In the simplest first-order approximation of inhomogeneity, the spatial dependence of the material properties can be assumed to follow linear profiles     0 (1 + a x x + a y y + a z z + ...), S  S 0 (1 + c x x + c y y + c z z + ...), where  denotes the electrical conductivity measured at uniform temperature,  is the thermal conductivity for zero electrical field, and S is the absolute thermoelectric power of the material. Here the subscripts 0 refer to the average values of the material properties, while a, b and c are property gradients characterizing the material inhomogeneity. The magnetic field produced by the thermoelectric currents can be calculated using the Biot-Savart law 2 2 2 where x and X are coordinate vectors of the point of observation and the differential volume of the bar respectively. Far away from the ends of the long bar neither the electric current density nor the associated magnetic field depends on z, therefore, without loss of generality, we can evaluate the integral at z = 0. Performing the cross product in Eq. (2) and integration along the length of the bar yields where, after some algebraic simplifications, and

EXPERIMENTAL METHOD
In this section, we will describe the experimental setup used to sense the normal and tangential components of the magnetic field produced by thermocurrents induced in two different surface treatment processes such as grit blasting and laser shock peening in biometallic Ti-6Al-4V ELI alloy specimens when they are subjected to an external temperature gradient. It is well known that surface properties play a major role in deciding the global accomplishment of structural elements. Grit blasting introduces compressive stresses in the surface layers of metallic parts via pro-jecting the surface with a torrent of high-velocity shots. As the plastically deformed surface layer attempts to extend relative to the unharmed core of the specimen, residual compressive stress expands in a parallel direction to the surface at shallow depths, while beneath this layer a reaction-induced tensile stress outcome. In addition to the main residual stress effect, grit blasting also induces other subtle variations in material properties such as surface roughness and raised hardness and texture that are sequela of the important plastic deformation produced by the cold work [7]. On the other hand, laser shock peening is an alternative surface processing technology which introduces residual compressive stress with minimal cold working using shock waves to yield the material [8].

Material
In this research study a Ti-6Al-4V ELI (extra low interstitial) alloy was used. Rectangular specimens of about 8  25 mm 2 and 2 mm thick were machined and grit blasted with alumina (Al 2 O 3 ) particles under a pressure of 350 kPa for 2 min and with a distance between the nozzle and the target surface of 20 cm. A first set of samples, hereafter grit blasting samples, was blasted with Al 2 O 3 angular particles of ~ 750 m. The second set of Ti-6Al-4V ELI alloy specimens were treated with "laser peening" technology with a Spectra Physics Quanta Ray Pro equipment, consisting of a pulsed solid state (Nd:YAG) laser that works with an energy per pulse of up to 1.6 J, producing pulses of 10 ns with a frequency of 30 Hz and a wave length of 1064 nm. Thus, in order to establish how it affects the magnetic TEP measurements, a set of grit blasted and laser peened Ti-6Al-4V samples were annealed at 595 and 710°C for 1 and 2 h, respectively. Such heat treatments are known to induce a partial and a fully release of the residual stresses, respectively.

Magnetic Sensing Technique
The two surface treatment processes (grit blasting and laser shock peening) specimens often exhibit both residual stresses and inhomogeneous cold work (case hardening). Since TEP technique is completely insensitive to surface roughness, therefore the measured magnetic signature is due to a combination of both effects. Whether the actual signature is dominated by residual stress or cold work can be established on a case to case basis by comparing the magnetic signatures recorded after polarizing the magnetic sensor tangentially or normally. Figure 3 shows the experi- mental set up used to study the magnetic signatures produced by the two different surface treatment processes (grit blasting and laser shock peening) in biometallic Ti-6Al-4V ELI alloy specimens. The fluxgate magnetometer can be polarized either tangentially or normally with respect to the sample surface in order to measure the x and y components of the magnetic field. The biometallic Ti-6Al-4V ELI alloy samples were seated on two copper blocks that also served as heat exchangers to aid an efficient heating and cooling process and the entire assembly was seated on a nonmagnetic translation table for scanning. The copper heat exchangers were heated and cooled to temperatures of 73 and 6°C, respectively. The real temperatures of the Ti-6Al-4V ELI samples were supervised by thermocouples. Finally, the real temperature gradient along the Ti-6Al-4V ELI samples were held at 4.0°C/mm.
The biometallic specimen was scanned with a 2axis magnetic field sensor (Mag-03 Bartington) that has a sensitivity of 10 T/V. The two fluxgate magnetometers were arranged in a differential set up to catch the thermocurrents signals from the surface treatment samples. The primary magnetometer, which is nearby the titanium sample, senses a stronger signal than the secondary magnetometer, while the two magnetometers display an equal sensitivity for external magnetic sources, which in our experiments are rejected. The total strength of the magnetic field was measured at the primary and the secondary positions of the magnetometers and then subtracted. The distance between the primary magnetometer and the sample surface was 1.5 mm. In order to ensure the highest sensitivity in the experimental data, the magnetic signatures were recorded with a minimum distortion. The ac magnetic interference signals were efficiently canceled using a low-pass filter of 20 Hz cut-off frequency. However, these basic procedures stand on the frequency spec- trum of the spurious magnetic signals cannot be capable to eliminate nonstationary external magnetic signals in the frequency spectrum where the thermocurrents signals are registered from 0.1 to 20 Hz. In these frequency spectra, it can isolate the needed magnetic signals from undesired intrinsic background signals "noise" based on their spatial rather than temporal dependence [9,10]. Figure 4 displays cross sectional views of the grit blasted and laser peened Ti-6Al-4V samples. Comparative analysis at zones beneath the blasted and laser peened samples reveals two important differences in the subsurface microstructures. Figure 4 denotes a severely deformed region about 2-4 m thick. This plastic deformation is more evident in the grit blasted specimen (Fig. 4a), than in the laser peened one (Fig. 4b). In each case, grit blasting produced the highest degree of cold work due to the high number of impacts. Further cold working increases the dislocation density and the range of microstress. The degree of cold work as a function of depth is maximum at the top of the deformed surface.

RESULTS AND DISCUSSION
In the next step, all the specimens were inspected by the magnetic thermoelectric technique. Since the generated magnetic field is perpendicular to the heat flux inside the specimen (parallel to the surface) and the gradient of the material property (normal to the surface), the fluxgate magnetometers were polarized in the tangential direction for residual stress data and the normal direction for cold work data as shown in Fig. 3. The characteristic magnetic profiles were obtained from each polarized sensor direction. For the case of tangential direction (Fig. 5a) a unipolar magnetic signal shape is obtained while for the normal direction ( Fig. 5b) a bipolar magnetic signal shape is registered.
In order to separate the individual effects of residual stress and cold-work induced texture in the measured thermoelectric signature, the in-depth distribution of residual stresses was measured on selective specimens by using high energy X-rays from a synchrotron radiation source at the EDDI beam line of BESSY II, which operates in the range of 10-150 keV. Measurements were carried out in the reflection mode using an angle of 2 = 16°. The incoming beam was defined by slits of 1 mm height and 1 mm width, while the diffracted beam size was adjusted by slit of 30 m (gauge volume 1  1  0.03 mm 3 ). The so-called sin (2) method was used and a biaxial RS state approach was assumed, i.e.  i 3 (z = 0) = 0 (i = 1, 2, 3). The residual stress distributions of the two sets of grit blasting and laser shock peening specimens are presented in Figs. 6 and 7, respectively. The residual stress data from the grit blasted and laser-peened specimens without stress release show that the maximum compressive layer can be always found slightly below the surface. The residual stress data taken after a 595°C partial stress release show a substantial reduction in residual stress at both surface treatment conditions. Finally, after the full stress release (710°C), the compressive stresses are essentially gone.
In order to further investigate the parallel decay of residual stress and cold work and their combined effect on the measured thermoelectric signature, we conducted a number of additional experiments. Figure 8 shows the peak to peak amplitudes of the magnetic signatures recorded on laser shock peening and grit blasting specimens as function of the sensor polarization at different stages of stress release development at 1.5-mm lift-off distance and 4.0°C/mm temperature gradient. On the laser shock peening specimen with a tangential sensor polarization (i.e., before stress release) the peak to peak value of the measured magnetic flux density increased to ~8 nT. Then the magnetic flux density decreased gradually after the first stress relaxation treatment (595°C, 1 h). Finally, on the second stress relaxation treatment (710°C, 2 h), the flux density was reduced after full recrystallization as shown in Fig. 8a. Therefore, the variation in the related magnetic flux density exclusively correlates to the relaxation of the compressive residual stresses [4,5]. Figure 8a also shows the laser shock peening specimen data at a normal sensor polarization, the peak to peak value of the measured magnetic flux density increased to ~ 4 nT. The partial stress relaxation at 595°C resulted in only modest 25% drops in the magnetic signal, while the repeated annealing 760°C further reduced the amplitude.
On the other hand, Fig. 8a shows the peak to peak amplitudes of the magnetic signatures recorded on grit blasting specimens as function of two sensor polarizations at different stages of the stress release development. Analysis of the magnetic TEP values (Fig. 8b) reveals that the peak to peak magnetic flux density increased significantly ~10 nT for the grit blasting specimen. Then the flux density decreased gradually after the first stress relaxation treatment (595°C, 1 h) and on the second stress relaxation (710°C, 2 h), respectively. During the annealing processes at normal sensor polarization, magnetic flux intensity values of blasted specimens reached a peak to peak value of 5 nT. After the partial and total stress relief annealing, magnetic flux intensity values gradually decay, respectively.
In general, the magnetic flux density values of grit blasted specimens at the two sensor polarization positions are much higher than those of laser peened one. This fact is corroborated from the XRD data indicates that the maximum value of compressive residual stresses ~ -1000 MPa is presented in grit blasted sur-faces. From the literature [11,12], laser shock peening induces remarkably little cold work of the surface compare to conventional grit blasting because only a single, or few, deformation cycles are required.

CONCLUSIONS
In the current research, magnetic flux density measurements were applied as an assessment technique to detect subtle material variations produced by the manufacturing process of grit blasting and laser peening in a medical Ti-6Al-4V alloy. It has been shown that magnetic flux density tangential and normal polarization measurements are strongly influenced by the microstructural changes induced by residual stress and cold work respectively. Finally, it has been found that the magnetic thermoelectric method can be used to characterize the relaxation of the prevailing residual stress in a treated surface.