This research work was focused on the laser peening surface process in a metallic Ti-6Al-4V biomaterial. The Ti-6Al- 4V samples were surface treated at different laser conditions varying parameters such as pulse density and wave length. Laser peening induced effects were evaluated by synchrotron radiation X-ray diffraction (SR-XRD) to determine the residual stress state; scanning electron microscopy (SEM) to assess microstructural changes and thermoelectric testing (TEP) to sense the subtle material variations such as local texture, increased dislocation density, hardening and residual stresses degree. The TEP measurements demonstrate that the non-contact technique is very sensitive to the compressive residual stresses with increasing the severity of the laser treatment parameters, while the TEP contact results are closely related to grain size, cracks, anisotropy, and work hardening.
In general the ultrasonic techniques have been used to determine the mechanical properties of materials on based of their relationship with metallurgical characteristics. In this research work, the relationship between ultrasonic velocity and phased array and the microstructure of steel pipeline welded joints is investigated. Measurements of ultrasonic wave velocity were made as a function of the location across the weld. Hardness measurements were performated in an attempt to correlate with ultrasonic response. In addition, the coarse and dendritic grain structure of the weld material is extreme and unpredictably anisotropic. Thus, due to the acoustic anisotropy of the crystal itself weld material of studied joints is anisotropic, too. Such structure is no longer direction-independent to the ultrasonic wave propagation; therefore, the ultrasonic beam deflects and redirects and the wave front becomes distorted. Thus, the use of conventional ultrasonic testing techniques using fixed beam angles is very limited and the application of conventional ultrasonic phased array techniques becomes desirable.
Laser peening has recently emerged as a useful technique to overcome detrimental effects associated with other well-known surface modification processes such as shot peening or grit blasting used in the biomedical field. It is worthwhile to notice that besides the primary residual stress effect, thermally induced effects might also cause subtle surface and subsurface microstructural changes that might influence corrosion resistance and fatigue strength of structural components. In this work, plates of Ti-6Al-4V alloy of 7 mm in thickness were modified by laser peening without using a sacrificial outer layer. Irradiation by a Q-switched Nd-YAG laser (9.4-ns pulse length) working at the fundamental 1064-nm wavelength at 2.8 J/pulse and with water as a confining medium was used. Laser pulses with a 1.5-mm diameter at an equivalent overlapping density of 5000 cm−2 were applied. Attempts to analyze the global-induced effects after laser peening were addressed by using the contacting and noncontacting thermoelectric power techniques.
Laser peening has recently emerged as a useful technique to overcome detrimental effects associated to another wellknown surface modification processes such as shot peening or grit blasting used in the biomedical field. It is worth to notice that besides the primary residual stress effect, thermally induced effects might also cause subtle surface and subsurface microstructural changes that might influence corrosion resistance. Moreover, since maximum loads use to occur at the surface, they could also play a critical role in the fatigue strength. In this work, plates of Ti-6Al-4V alloy of 7 mm in thickness were modified by laser peening without using a sacrificial outer layer. Irradiation by a Q-switched Nd-YAG laser (9.4 ns pulse length) working in fundamental harmonic at 2.8 J/pulse and with water as confining medium was used. Laser pulses with a 1.5 mm diameter at an equivalent overlapping density (EOD) of 5000 cm-2 were applied. Attempts to analyze the global induced effects after laser peening were addressed by using the contacting and non-contacting thermoelectric power (TEP) techniques. It was demonstrated that the thermoelectric method is entirely insensitive to surface topography while it is uniquely sensitive to subtle variations in thermoelectric properties, which are associated with the different material effects induced by different surface modification treatments. These results indicate that the stress-dependence of the thermoelectric power in metals produces sufficient contrast to detect and quantitatively characterize regions under compressive residual stress based on their thermoelectric power contrast with respect to the surrounding intact material. However, further research is needed to better separate residual stress effects from secondary material effects, especially in the case of low-conductivity engineering materials like titanium alloys.
Fretting is a wear phenomenon that occurs when cyclic loading causes two surfaces in intimate contact to undergo small oscillatory motions with respect to each other. During fretting, high points or asperities of the mating surfaces adhere to each other and small particles are pulled out, leaving minute, shallow pits and powdery debris. Sometimes these surface conditions are neglected, but they are important in some application such as the aerospace industry. In this research work, non-contacting and contacting thermoelectric power techniques are performed in fretted 7075-T6 and Ti-6Al-4V samples. It has been found that the contacting and non-contacting thermoelectric power measurements are associated directly with the subtle material variations such as work hardening and residual stresses due to plastic deformation produced in the fretting zone but surface topography. Therefore, both techniques could be used for a global characterization of the most relevant fretting induced effects. Potential of these techniques to monitor subsurface changes in other severe surface plastic deformation processes are clearly envisaged.