Keizoh Honda

Properties of In-situ Nitride Reinforced Titanium-aluminide Layers Formed by Reactive Low Pressure Plasma Spraying with Nitrogen Gas

Abstract In-situ nitride dispersed TiAl layers were produced from pre-alloyed TiAl powder by a reactive low pressure plasma spraying (RLPPS) process with nitrogen as a powder carrier gas. RLPPS TiAl layers had fine grain structures whose grain sizes ranged from approximately 50 to 150 nm and included ternary nitride, Ti2AlN. The volume fraction of nitride decreased with increasing spray distance from 300 to 600 mm, whereas no significant change in the nitrogen contents of sprayed layers occurred. Nitride precipitation may be suppressed by rapid cooling in sprayed layers formed at a long spray distance. The maximum hardness of 709 Hv was obtained at room temperature in a RLPPS TiAl layer formed with an appropriate spray distance, 400 mm in the present case. However, a RLPPS TiAl layer formed with shorter spray distance, 300 mm, was superior in hardness at temperatures above 873 K because of higher volume fraction of nitride. Although the hardness values of RLPPS TiAl layers dropped to approximately 450 Hv after being exposed to a temperature of 1273 K for 24 h, the hardness value was higher by approximately 100 Hv than that of a LPPS TiAl layer formed with argon carrier gas. This is caused by effects of both the fine grain structure and nitride precipitation hardening.

Properties of titanmm-aluminide layer formed by low pressure plasma spraying

Abstract TiAl alloyed powder was sprayed onto a substrate surface by the low pressure plasma spraying (LPPS) process. The microstructure and properties of sprayed TiAl layers were examined and compared with those of a TiAl bulk plate. As-sprayed TiAl layer consisted of γ fine grains whose average grain size was 90 nm including a small amount of α2. The sprayed layer had a hardness value of 589 Hv at room temperature and above 400 Hv even at 1073 K. Although the hardness value dropped to 225 Hv at 1173 K, it was still comparable to that of a TiAl bulk plate at room temperature. The microstructure and mechanical properties of the sprayed TiAl layers changed only slightly when exposed to a high temperature of up to 973 K for 48 h. Although the hardness values of sprayed TiAl layers decreased to 350 Hv when annealed at 1273 K for 24 h and more because of the occurrence of grain growth, the hardness values were significantly higher than that of a TiAl bulk plate. Tensile strength above 400 MPa, which was almost equal to that of a TiAl bulk plate, was obtained for sprayed TiAl layers both in as-sprayed and annealed (at 1273 K for 24 h) conditions. Therefore, sprayed TiAl layers were found to be a feasible hardfacing layer at elevated temperatures.

High-temperature mechanical properties of laser welds in Co-base superalloy and its improvement by laser surface melting

Summary The high-power CO2 laser is extensively used for practical applications such as cutting and welding of metals. Laser welding of superalloys requiring high-quality and high-efficiency welding, however, is seldom employed in heavy industry. Conventional laser welding of metals with filler wire produces the columnar structure which meets at the weld bead centre and leads to deterioration of creep properties. To improve the creep rupture behaviour of joints within this context, this paper describes the development of a laser surface melting technique involving the weld bead surface being remelted by laser. In the experiments described, Haynes 188 Co-base superalloy with a thickness of less than 3 mm is used as the base metal. The spot size of the laser beam used to irradiate the first layer of the weld bead surface is around 3 mm. The depth of the surface melting layer is varied by the travelling speed. The results suggest that both the elongation at fracture and creep rupture life after laser surface...

NITRIDE REINFORCED TIAL FORMED BY REACTIVE LOW PRESSURE PLASMA SPRAYING

Low pressure plasma spraying (LPPS) with reactive gas, which is referred to as RLPPS, is one of feasible in-situ fabrication processes for intermetallic-matrix composites (IMCs). In the present work, nitride dispersed TiAl based IMC layers were successfully produced from pre-alloyed TiAl powder by RLPPS process using nitrogen as a powder carrier gas. Ternary nitride precipitates Ti 2 AlN formed at the inter-layer boundaries of the splayed TiAl layers. The RLPPS TiAl layer formed with an appropriate spray distance of 400mm had the maximum hardness of 709Hv at room temperature, which was significantly higher than hardness of a non-reinforced LPPS TiAl layer formed with argon carrier gas. Although the hardness values of RLPPS TiAl layers dropped to approximately 450Hv after the high-temperature exposure at 1273K for 24h, the hardness value was higher by approximately 100Hv than that of the LPPS TiAl layer. This mainly results from the nitride precipitation hardening.

Effects of preheating on the coating process in low‐pressure plasma spraying

The effect of preheating on coating process was investigated in low pressure plasma spraying. In this study, microstructure, bonding strength and blast erosion property were examined for the coating which consists of Ni based superalloy as a substrate and CoNiCrAlY spraying material. The results are summarized as follows.(1) High bonding strength over 72 MPa of coating to the substrate by low pressure plasma spraying, is derived from the diffusion zone formed adjacent to the coating interface.(2) 1073 K preheating coupled with the plasma spraying causes short-time diffusion adjacent to the coating interface that is equivalent to the vacuum-furnace heating at approximately 1373K.(3) The blast erosion property of low pressure plasma sprayed CoNiCrAlY is close to the stainless steel rather than the atomospheric plasma sprayed coating.(4) Low pressure plasma sprayed coating which is preheated over 873 K, consists of the top layer with higher blast erosion rate and excellent layers with low blast erosion rate under it. It is considered that in-situ sintering phenomenon caused by sufficient preheating and the subsequent spraying would improve combining strength between sprayed particles.(5) Plasma flame treatment just after spraying improves the blast erosion property of the top layer in low pressure plasma coating.