Microstructural response and grain refinement mechanism of commercially pure titanium subjected to multiple laser shock peening impacts

Microstructure and high temperature oxidation resistance of in-situ synthesized TiN/Ti3Al intermetallic composite coatings on Ti6Al4V alloy by laser cladding process

The gradient nanostructure evolution and the mechanism governing this evolution of α phase in Ti-6Al-4V alloy induced by ultrasonic surface rolling process were investigated. A gradient nanostructure consisting of a roughly equiaxed nanograin layer, an elongated nano-lamellar layer, an elongated ultrafine lamellar layer, a refined grain layer, and a low-strain coarse-grained layer was formed with a thickness of more than 400 µm. The formation of gradient nanostructure of α phase was dominated by complex dislocation activities in hcp grains without twins occurring, supplemented by hexagonal close-packed (hcp) titanium (Ti) to face-centered cubic (fcc) Ti phase transformation. During the microstructural evolution, the coarse hcp-Ti grains were first elongated into lamellae. Then, these sub-micron lamellae were gradually transformed into roughly equiaxed nanograins via two deformation modes of longitudinal splitting and transverse breakdown, accompanied by dynamic recovery. The fcc-Ti grains were deformed mainly via twin-twin intersections and twin-dislocation interactions, accompanied by longitudinal splitting and transverse breakdown, resulted in refinement of the micron-scale fcc-Ti grains to roughly equiaxed nanograins. The interaction of hcp and fcc phases influenced and synergistically promoted the microstructural evolution process. In addition, the microhardness improvement in the surface layer of Ti-6Al-4V alloy was attributed to the increase of dislocation density, grain refinement and the occurrence of deformation twinning in fcc-Ti grains.

Gradient nanostructure evolution and phase transformation of α phase in Ti-6Al-4V alloy induced by ultrasonic surface rolling process

Effect of rare-earth on friction and wear properties of laser cladding Ni-based coatings on 6063Al

An ultrasonic surface rolling process (USRP) is a novel mechanical surface treatment technique for enhancing the fatigue performance of metallic materials. In this work, USRP with different repeated processing numbers was employed for enhancing the fatigue performance of a Ti-6Al-4V alloy. The effect of USRP on their surface integrity (including microstructure, surface quality, microhardness, and residual stress) were investigated, which were characterized by means of scanning electron microscope, transmission electron microscope, confocal laser scanning microscope, microhardness tester, and X-ray diffraction residual stress tester. Especially, a refined microstructure (grain size: ~100–400 nm) was formed on the topmost surface of twelve-repeat USRP specimen. Subsequently, the fatigue behavior of the specimens was investigated via rotating-bending fatigue tests, and the results suggested that USRP could effectively enhance the fatigue performance of the Ti-6Al-4V alloy. The USRP-induced enhancement mechanism of the fatigue performance can be ascribed to the synergistic effect of the compressive residual stress, microstructure, work hardening, and improved surface quality. The best synergistic effect and, correspondingly, the greatest improvement in the fatigue performance were realized for the one-repeat USRP specimen.

On the influence of ultrasonic surface rolling process on surface integrity and fatigue performance of Ti-6Al-4V alloy

Laser shock peening (LSP) or Laser shock processing is a novel surface treatment technique for strengthening metal materials. In this work, several investigations were performed to evaluate the effect of LSP on some mechanical properties of a near-α titanium alloy Ti834. Micro-hardness measurements of the untreated and LSP treated specimens were carried out by using a Vickers indenter. The depth profiles of subsurface micro-hardness of the shocked region were determined through the electro-polishing material removal in steps of 0.1 mm. It is observed that the micro-hardness of Ti834 alloy can be improved by LSP, and repeated shocks have a very beneficial effect on surface hardening. Residual stress distribution as a function of depth was determined by X-ray diffraction (XRD) with the sin 2 ψ method. The high-cycle fatigue performance of the alloy was investigated and the fractographs of fatigue specimens were observed by SEM. Results reveal that the high-cycle fatigue life of Ti834 alloy increases after laser shock peening due to the introduction of compressive stress which can delay the initiation and growth of the fatigue crack.

Quan Hong

Papers

Effect of laser shock peening on the mechanical properties of a near-α titanium alloy

Research on the thermal stability of a near α titanium alloy before and after laser shock peening