Papers by Author: Takehiko Watanabe

Abstract: This study focuses on understanding the effect of Ni and Co additions to filler metals on Ag-brazed joints of cemented carbide and martensitic stainless steel. Ni and Co added braze alloys were processed based on Ag-Cu-Zn ternary alloy, and joint strength and microstructure of the brazed layer has been investigated. The joint strength increased by the 2.0mass%Ni and 0.5mass%Co addition into braze alloy. This trend is remarkable in the Co added alloy, and the brazed joint increased by 141% compared to that in no-added alloy. The joint strength was closely related to the suppression effect of Co dissolution from cemented carbide into filler layer and Fe diffusion from the stainless steel to the brazed layer. In the brazed microstructure, Co-depleted zone caused by dissolution of Co in the cemented carbide was observed near the interface between the cemented carbide and the steel. Width of the Co-depleted zone significantly decreased in the Co added alloy. However, the joint strength decreased in the multiple addition compared to that in the single addition of Ni or Co.

Abstract: Diffusion treatments of TiAl-based alloys (49.1 at% Al) aluminum coated by thermal spray were carried out at the temperature range of 700°C-1100°C. The influence of the diffusion condition for the formation of intermetallic phases in the coating has been investigated. In the initial stage of diffusion treatment, TiAl₃ was formed on the outermost surface by the diffusion between liquid aluminum and the substrate. In addition, an intermediate layer comprised of Ti₂Al₅ (at 1100°C), TiAl₂ and Al diffused layer (Al-rich TiAl) was confirmed under the outermost layer. The maximum thickness of TiAl₃ during the initial stage increases as the diffusion temperature decreases. In addition, the shape of TiAl₃ layer was dependent on the diffusion temperature; the outermost layer without pores was confirmed at the temperature of 700°C. TiAl₂ and Al-rich TiAl developed by solid-state diffusion from TiAl₃ layer following a parabolic low. The activation energies for growth have been calculated to be 194 kJ/mol for TiAl₂ and 292 kJ/mol for Al-rich TiAl.

Abstract: Cemented carbides have been widely used for cutting tools because of their high hardness and abrasion resistance. Since the cemented carbides are so expensive, it is desirable to reuse a tool shank made of cemented carbides. For the reason, so far, a new blade of a tool has been brazed to used shanks. However, when cemented carbides are heated for brazing, heating inevitably causes the deterioration in the mechanical properties. This study was carried out to braze the cemented carbides at lower temperatures for reducing the deterioration of the shank. First of all, authors developed a new Ag-based brazing filler metal with a low melting point of about 605°C, and investigated the effects of the new Ag filler metal on the properties of a brazed joint. Moreover, Co element or Ni element was added into the Ag filler metal to make the bending strength of a brazed joint improved. The addition of Co element increased the bending strength of a joint and the strength was equivalent to that of a joint brazed at 750°C using a conventional Ag filler metal, but the addition of Ni element decreased the bending strength of a brazed joint.

Abstract: Al-rich aluminide coating on Ti-49.1at%Al alloys has been performed by two-step process of thermal aluminum spray and diffusion treatment. Effect of the diffusion time on the oxidation resistance, and the change of microstructures in the coating during the oxidation test in air at 900°C for 100 h were investigated. In the aluminized coating before the oxidation tests, formations of TiAl₃ on the outer layer and an intermediate layer consisting of Ti₂Al₅, TiAl₂, and Al-rich TiAl were observed. The intermediate layer developed by the diffusion treatment for 3600s. In the oxidation test over 50 h, the specimen diffusion treated for 3600 s exhibited the lower oxidation rate than that diffusion treated for 900 s. Multi-oxide scales of TiO₂ and Al₂O₃ were observed on the both surfaces of aluminized specimens. Large TiO₂ particles were observed on the specimen of t_D = 900 s. It was found that the TiAl₂ layer developed during the oxidized specimen, while the thicknesses of Al-rich TiAl and TiAl₃ layer decreased by the growth of TiAl₂.

Abstract: Aluminizing the surface of a TiAl-based alloy (49.1 at% Al) was carried out by thermally spraying a pure aluminum coating and subsequent diffusion treatment at 1100°C. The influence of the diffusion time for the formation of Ti-Al intermetallic phases in the coating layers and the oxidation resistance of the aluminized TiAl-based alloys were investigated. The layer formed on the outermost surface was comprised of the Al-rich intermetallic T2iAl5 and contained pores. On the other hand, an intermediate layer consisting of TiAl2 and TiAl was formed between the outermost layer and the substrate. The thickness of the outermost layer decreased as the diffusion time increased, while the thickness of the intermediate layer increased. In addition, the coating/substrate interface changed from a wavy to a linear form with the growth of the intermediate layer. The aluminized coating showed good oxidation resistance at 900°C for all diffusion times.

Abstract: Authors ultrasonically welded A6061 aluminum alloy sheet using two types of weld tips with the different contact face geometry, and investigated the effect of the weld tip geometry on the performance and the interface structure of welds. One type of tip has a cylindrical contact face without knurl, which is called C-tip in this study. The other type of tip has flat contact face with knurl, which is called K-tip in this study. The strength of the joints welded using C-tip was higher than that welded using K-tip and the C-tip could stably produce the higher strength joint.

Abstract: Ultrasonic welding between SS400 mild steel sheet and aluminum alloy A5052 sheet containing magnesium was conducted. In this study, authors investigated the influence of ultrasonic welding condition on the mechanical properties and the interface microstructure of a joint, and examined the effect of an insert metal to improve the joint strength. The main results obtained in this study are as follows. It was possible to ultrasonically weld SS400 mild steel sheet and A5052 sheet. When the clamping force was varied keeping the welding time constant for 1s, the joint strength showed the maximum at the clamping force of 588N and it decreased with increasing the clamping force because the frictional action at the interface decreased with it. When the clamping force was kept constant at 588N and the welding time was varied, the joint strength reached the maximum at the welding time of 2.5s, followed by decreasing in the joint strength welded at 3s welding time due to the formation of Fe2Al5 intermetallic compound at the interface. Using insert metal of commercially pure aluminum successfully improved the joint strength and the strength of the joint welded at 3s welding time was about three times larger than that without the insert metal.

Abstract: In this study, authors investigated the effect of horn tip geometry on the mechanical properties of a joint ultrasonically welded between a mild steel sheet and an aluminum alloy sheet. Two types of weld tips with the different contact face geometries were employed. One type of a tip has a cylindrical contact face without knurl. The other type of a tip has flat contact face with knurl. The main results obtained in this study are as follows. Authors successfully welded the mild steel sheet to the aluminum alloy sheet using both of the weld tips. However, the strength of the joint welded by the tip with cylindrical geometry was higher than that welded by the flat tip with knurl under the same welding condition, and the strength of the joint by the tip with cylindrical geometry was twice larger than that by the flat tip with knurl under the optimal welding condition. The strength of the joint increased with increasing the welding time and the clamping force, and showed the maximum under the reasonable welding condition. Excessively long welding time and large clamping force damaged the workpiece surface contacted the horn side tip, resulting in the decrease of the joint strength. It was possible to improve the joint strength using commercially pure aluminum foil with suitable thickness as insert metal.

Abstract: This paper described an experimental study on the friction stir welding of dissimilar metal between commercially pure titanium (CPTi) and magnesium alloy AZ31B (Mg). Butt-welds produced at various tool rotation speeds were evaluated by the observation of the weld surface appearance, X-ray radiographic test, tensile test, SEM and EDX analysis. The main results obtained are as follows. Butt-joint welding of the CPTi plate to the Mg plate was easily and successfully achieved. The ignition of Mg occurred during welding at the tool rotation speeds over 1200rpm. The fragments of CPTi existed in a continuous form in Mg matrix. The tool rotation speed of 1200rpm attained the maximum tensile strength of a joint, which was about 75% of that of Mg base metal. Fracture occurred at the weld interface in most joint. EDX analysis revealed that Al in the Mg diffused into CPTi through the weld interface and MgO existed at the interface. It seems that the decrease in Al concentration in the Mg and MgO formation around the weld interface caused the low tensile strength of the joints.

Abstract: This study was carried out to newly develop the fluxes and filler metals for brazing magnesium alloy AZ31B more easily at lower temperatures. Furthermore, surface preparation was developed to improve the brazeability of magnesium alloy. The main results obtained are as follows. We could successfully develop the fluxes that consisted of chlorides containing Ca ion and Li ion, which made the faying surface of the magnesium alloy active at around 450°C. In addition, we succeeded in developing the filler metals with the melting temperatures lower than 490°C which were Mg-Sn-In system containing a small amount of Al to lower the melting temperature. Surface preparation for magnesium alloy by immersion in aqueous solution containing halogen ion improved remarkably the brazeability of the magnesium alloy. Using the surface preparation together, the fluxes and filler metals could achieve the brazed joints with a high strength equivalent to that of the base metal.