A novel local heating technique utilizing fiber-laser spot heating system has been developed and adopted to produce the agehardening 2024 aluminum alloy sheets which possess anisotropy in bending properties. Cold rolled sheets of 2024 aluminum alloy of 1.0 mm thickness (80% in reduction) and T3 treated sheets of 0.7mm thickness were prepared and subjected to the laser spot heating in ambient atmosphere at various energy densities varying from 63.7 to 111.4kW/cm2 and heating time of 1, 3 and 5s. The beam size on the focal plane or sample surface was fixed to 200μm in diameter. The spot heating produced recrystallized microstructure, the size of which depends on the irradiation conditions. In the case of 111.4kW/cm2 for 3s, recrystallizes grains appeared in the whole part of sample through the thickness below the irradiation point while part of sample in the vicinity of surface melts yielding a couple of pores. Striped patterns composed of an alternative array of soft and hard region in 0.7mm thick sheets were produced with the present laser spot heating system, and anisotropy of 33.5% in the maximum bending load was obtained when the bending test was performed on the sheets prepared by two types of laser spot heating treatment: one was parallel and the other was perpendicular to the bending axis.
Femtosecond lasers can be used for forming the nano-sized periodic structure on the surface of fused silica. However, the processing control range is narrow and unstable due to the nonlinearity of the multiphoton absorption process. By coating the surface with a thin metallic film, we found that the periodic structure can be formed easily below the ordinary ablation threshold for fused silica. But, the periodic structure processing mechanism was not understood. We proved the thing that the thin metallic film on the fused silica contributed to a nano-sized periodic structure formation. When fused silica coated copper was irradiated with femtosecond laser, micro-sized Copper particles were formed on the surface of fused silica and the priodic structure of 800nm size was formed around micro-sized Copper particles. 800nm periodic structure formed around there by interference of the diffraction light of laser. Copper remains convex section of the 800nm periodic structure, and that is localized surface plasmon polariton (SPP). The nano-sized periodic structures is formed by SPP around nano-sized copper particles. We found the processing condition to determine nano-sized periodic structure. The pitch length of nano-periodic structure is determined by the volume of copper and the laser energy density. The depth of the structure is determined by the number of pulses of laser irradiation.
The applications of Ti and its alloys are limited to high-performance products because of expensive cost and poor plastic formability. In this study, pure Ti powder was consolidated by using spark plasma sintering (SPS), warm compaction and cold compaction, respectively. The mechanical properties of the hot-extruded powder metallurgy (P/M) pure Ti materials were evaluated in order to develop a cost-effective processing. The specimen prepared by warm compaction showed a higher tensile strength of 973.7 MPa and a better elongation of 26.0 % comparing with other materials. Orientation, grain refinement and solid solution strengthening mechanism were discussed respectively. Solution strengthening of oxygen was considered as the main strengthening factor in this study. The strengthening effect of solid solution of oxygen was calculated as 370.6 MPa/at.%[O] on 0.2 %YS.
Titanium-nickel (TiNi) alloys of near-equiatomic composition are the most popularly commercialized shape memory alloys (SMAs) with excellent characteristics, such as shape memory effect, superelasticity, a high strength, good corrosion resistance and good biocompatibility. Recently, TiNi SMAs with superelastic behavior are used in advanced medical devices, such as guide wire, catheter and stent to restore a damaged blood vessel. Improvement of mechanical and shape memory properties of TiNi SMAs cause a remarkable reduction of wire diameter, and results in a reduction of surgical invasion and an improvement of quality of life (QOL). In this study, microstructural, mechanical and shape memory properties of the extruded and heat-treated TiNi alloys by sintering the mixture of TiNi pre-alloyed powder with titanium dioxide (TiO2) particles were investigated. Ti-51.19 at.% Ni pre-alloyed powder with TiO2 particles were consolidated by spark plasma sintering (SPS) at 1173 K in vacuum. SPSed TiNi alloy powder compacts were extruded at 1373 K and shape memory heat-treated at 773 K. Ti4Ni2O phase was formed during SPS by reaction between TiNi matrix and oxygen atoms originated from additive TiO2 particles. As a result, the soluted Ni content in TiNi matrix of the TiNi alloy with 1.0 vol.% TiO2 particles was increased by 0.78 at.% compared to the original TiNi alloy with no TiO2 particle. The tensile and shape memory properties of the heat-treated TiNi alloy with 1.0 vol.% TiO2 particles were obtained as follows; an average value of plateau stress: 524 MPa, ultimate tensile strength (UTS): 1298 MPa, and shape recovery rate when 8% strain: 89.3%. The heat-treated TiNi alloy with 1.0 vol.% TiO2 particles revealed the high strength and good shape memory properties. The high strengthening mechanism of the TiNi alloys with TiO2 particles was mainly due to a decrease martensitic transformation temperature by an increase soluted Ni content in TiNi matrix after reaction between TiNi and TiO2.
Magnesium (Mg) and it alloys are highly corrosive due to their low standard electrode potential (SEP). The conventional methods to improve corrosion resistance of Mg alloys are anodic oxide film formation and chemical conversion treatment. However, these methods are expensive and also make it hard to recycle the Mg products with these treatments. In this study, unique oxide layer formed by radical reaction between pure Mg surface and radical species generated under X-ray irradiation was investigated. Pure Mg surface was modified by X-ray irradiation under atmospheric condition. XPS analysis indicated MgO layer was formed at the surface of X-ray irradiated pure Mg specimen. Cross-sectional observation by TEM showed the MgO layer was dense compared to the natural oxidation coating films. The thickness of surface modification layer by X-ray irradiation was increased with the increase of irradiation time. After salt water immersion test, X-ray irradiated pure Mg for 24 hours showed about four times corrosion resistance of the as-polished original pure Mg. These results suggest that dense MgO layer formed by X-ray irradiation on pure Mg surface were functioning as a protective film.