H.C Man

Corrosion properties of laser surface melted NiTi shape memory alloy

A Nd-YAG laser was used to melt the surface of NiTi shape memory alloy in air and argon environment respectively. The laser surface melted (LSM) layer is free of porosity and crack. The corrosion resistance of the LSM specimen in 3%NaCl solution has been improved significantly and is attributed to the increased amount of TiO2 and Ti/Ni ratio on the surface.

Study on the formation of an apatite layer on NiTi shape memory alloy using a chemical treatment method

A chemical treatment method has been employed to prepare a bioactive layer on the surface of NiTi alloy. Prior to forming an apatite layer, all samples were dipped in 32.5% HNO3 aqueous solution and boiled in 1.2 M NaOH aqueous solution. Then, they were soaked in simulated body fluid (SBF) to form an apatite layer on the surface. The surfaces of chemically treated NiTi alloys were characterized by X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS) and environment scanning electron microscopy (ESEM). The results showed that a passive oxide film had been formed on NiTi treated with nitric acid, which mainly consisted of TiO2 and TiO. A sodium titanate hydrogel layer and Ni2O3 were formed on the surface treated by NaOH. After the above chemical treatment, an apatite layer was spontaneously deposited onto the surface when the samples were soaked in SBF. This revealed that a bioactive layer could be obtained by chemical treatment.

Excimer laser surface treatment of Ti-6Al-4V alloy for corrosion resistance enhancement

Abstract Excimer laser surface treatment of Ti–6Al–4V alloy was conducted with the aim of improving the pitting corrosion resistance of the alloy. Laser surface treatment was performed under two different gas environments, argon and nitrogen. The microstructure, phase and composition of the modified surface structure were analysed using TEM, XRD and EDX; the electrochemical behaviours of the untreated and the laser-treated specimens were evaluated by electrochemical polarization tests. Excimer laser surface treatment significantly increased the pitting potential of the Ti alloy, especially when the material was treated in argon gas, while a seven-fold reduction in corrosion current was obtained when the material was treated in nitrogen gas. These improvements are considered to be primarily due to the reduction of solute partitioning effect of detrimental Al segregated to the α phase. The N 2 -treated specimen, although it had the lowest corrosion current, had a pitting potential some 100 mV lower than that of the Ar-treated specimen. This is considered to be due to TiN precipitates acting as galvanic cathodes at high corrosion potentials.

Excimer laser surface treatment of aluminum alloy AA7075 to improve corrosion resistance

Abstract Excimer laser surface treatment was found to be an effective method for improving the pitting corrosion resistance of the aluminum alloy 7075. The results of the TEM study showed that laser surface melting of the alloy at an intensity of 10.3 J/cm2 resulted in the elimination of coarse second-phase particles in the laser-melted zone. More importantly, two compact layers containing aluminum oxide were formed on top of the laser-melted surface. Potentiodynamic polarization tests showed that as a result of the laser treatment, the corrosion current can be reduced by as much as six times, and a passive region was obtained. Besides, the analysis of the electrochemical impedance measurements showed that at an open-circuit potential (OCP), the polarization resistance and double-layer capacitance of the film/electrolyte interface of the laser-treated specimen were one order of magnitude higher and six times lower than that of the untreated specimen, respectively. Furthermore, when tested at OCP+50 mV, the untreated specimen suffered serious pitting corrosion, while a passive film had formed on the laser-treated specimen, which served as an effective barrier for reducing anodic dissolution.

Microstructure and wear properties of laser surface-cladded Mo–WC MMC on AA6061 aluminum alloy

Abstract The creation of a wear-resistant surface of metal matrix composite on AA6061 aluminum alloy by laser surface melting of pre-pasted Mo–WC powder was successfully achieved using a 2-kW CW Nd-YAG laser. A high-quality coating without porosity or cracking was obtained. Excellent bonding between the coating and the aluminum alloy substrate was ensured by the strong metallurgical interface. Different ratios of Mo and WC powders were investigated. The chemical composition, microstructure and surface morphology of the clad layer were analyzed using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffractometry (XRD). The surface hardness of the Al specimens clad with Mo/WC is increased seven-fold. The samples have been characterized with abrasive pin-on-disc wear tests. The test results show that the composite coating with a WC hard phase has improved abrasive wear resistance when compared with the as-received Al substrate. The wear mechanism of the specimens is discussed based on microscopic observation of the worn surface.

Improvement of cavitation erosion resistance of AISI 316 stainless steel by laser surface alloying using fine WC powder

Abstract Fine WC powder of approximately 1 μm size was employed as a convenient source of tungsten and carbon in the laser surface alloying of AISI 316 stainless steel for improving the cavitation erosion resistance. A slurry containing WC powder was preplaced on the substrate by pasting and processed with a high-power CW Nd:YAG laser to achieve surface alloying. The composition and microstructure of the alloyed layer and the phases formed were investigated by energy-dispersive X-ray spectroscopy, optical microscopy, scanning electron microscopy, and X-ray diffractometry, respectively. The cavitation erosion behavior of the laser surface-alloyed samples in 3.5% NaCl solution was studied with a vibratory cavitation erosion tester. The microhardness of the alloyed layer increases with the total W content in the layer. By employing proper processing parameters, an alloyed layer that is hard but not too brittle can be formed, with a cavitation erosion resistance that may reach more than 30 times that of the as-received 316. The improvement in cavitation erosion resistance may be attributed to the increase of W in solid solution and to the precipitation of dendritic carbides, both resulting from the dissociation of the fine WC powder during laser processing.

Laser surfacing of S31603 stainless steel with engineering ceramics for cavitation erosion resistance

Abstract As a preliminary study on incorporating ceramic particles into stainless steels for improving cavitation erosion resistance, ceramic-reinforced coatings were fabricated on austenitic stainless steel UNS S31603 using a laser surfacing technique. Ceramic powders, including WC, Cr 3 C 2 , SiC, TiC, CrB 2 and Cr 2 O 3 , were used to produce surface metal matrix composites (MMCs) with high volume fractions of reinforcement. The powders were pasted on the surface of UNS S31603 first, followed by surface melting using a high power laser. The microstructures and structures of the surfaces were analyzed. The cavitation erosion characteristics of the surface-modified specimens in 3.5% NaC1 solution at 23°C were studied by means of a 20-kHz ultrasonic vibrator at a peak-to-peak amplitude of 30 μm. The cavitation erosion resistance R e of all the laser-modified specimens was improved, except for the case of Cr 2 O 3 , with the improvement depending on the microstructure formed. The R e of surface MMCs reinforced with CrB 2 , WC and TiC increased to 9.4, 8.5 and 1.7 times that of monolithic S31603, respectively. For the specimens modified with Cr 3 C 2 and SiC, a eutectic mixture of austenite and carbides or silicide was formed and the R e was increased by a factor of 4.8 and 2, respectively.

Cavitation erosion behavior of laser gas nitrided Ti and Ti6Al4V alloy

Abstract A CW 2 kW neodymium-doped yttrium–aluminum–garnet (Nd-YAG) laser was used to carry out the gas nitriding process on commercial pure titanium and Ti6Al4V alloy. The microstructure, surface appearance, hardness depth profile and the cavitation erosion behavior of the nitrided samples in 3.5% NaCl solution were examined. The microstructure of the nitrided layer is mainly composed of TiN dendrites, of which the density gradually decreases towards the interface between the nitrided layer and the substrate. The cavitation erosion resistance of the nitrided samples was significantly improved as compared with the untreated samples. Increase in the surface hardness and the fine dendritic structure of the TiN layer contribute to the significantly enhanced cavitation erosion performance of Ti and Ti6Al4V alloy.

Laser transformation hardening of AISI 440C martensitic stainless steel for higher cavitation erosion resistance

Abstract Surface hardening of AISI 440C martensitic stainless steel was achieved by laser transformation hardening (LTH) using a high-power CW Nd:YAG laser. A hardened layer of a few 100 μm thick, composed of martensite, retained austenite and fine carbides was formed. The microstructure and hardness of the laser-treated layer were dependent on the laser processing parameters, with the hardness values reaching the range 600–800 HV. Compared with conventionally heat-treated samples, the laser-treated samples contained more retained austenite and finer carbides due to a higher degree of carbide dissolution. The cavitation erosion resistance of AISI 440C was significantly improved after laser treatment, and was approximately three-fold higher compared with that achieved by conventional heat treatment. The high erosion resistance was attributable to a desirable microstructure, which exhibited a favorable combination of hardness and toughness, and contained fewer weak sites for erosion attack.

Characterization of the laser gas nitrided surface of NiTi shape memory alloy

Abstract Owing to its unique properties such as shape memory effects, superelasticity and radiopacity, NiTi alloy is a valuable biomaterial for fabricating implants. The major concern of this alloy for biological applications is the high atomic percentage of nickel in the alloy and the deleterious effects to the body by the corrosion and/or wears products. In this study, a continuous wave Nd-YAG laser was used to conduct laser gas nitriding on the substrate of NiTi alloy. The results show that a continuous and crack-free thin TiN layer was produced in situ on the NiTi substrate. The characteristics of the nitrided surface layer were investigated using SEM, XRD, XPS and AAS. No nickel signal was detected on the top surface of the laser gas nitrided layer. As compared with the mechanical polished NiTi alloy, the nickel ion release rate out of the nitrided NiTi alloy decreased significantly in Hanks’ solution at 37xa0°C, especially the initial release rate.