Papers by Author: Tomo Ogura

Abstract: The hot cracking (solidification cracking) susceptibility in the weld metals of duplex stainless steels were quantitatively evaluated by Transverse-Varestraint test with gas tungsten arc welding (GTAW) and laser beam welding (LBW). Three kinds of duplex stainless steels (lean, standard and super duplex stainless steels) were used for evaluation. The solidification brittle temperature ranges (BTR) of duplex stainless steels were 58K, 60K and 76K for standard, lean and super duplex stainless steels, respectively, and were comparable to those of austenitic stainless steels with FA solidification mode. The BTRs in LBW were 10-15K lower than those in GTAW for any steels. In order to clarify the governing factors of solidification cracking in duplex stainless steels, the solidification segregation behaviours of alloying and impurity elements were numerically analysed during GTAW and LBW. Although the harmful elements to solidification cracking such as P, S and C were segregated in the residual liquid phase in any joints, the solidification segregation of P, S and C in LBW was inhibited compared with GTAW due to the rapid cooling rate in LBW. It followed that the decreased solidification cracking susceptibility of duplex stainless steels in LBW would be mainly attributed to the suppression of solidification segregation of P, S and C.

Abstract: The microstructure and mechanical properties of a joint produced by laser brazing between A5052 and AZ31 with AZ61, AZ91 and AZ125 filler metal was investigated. The effects of filler metals on joint characteristics are also discussed. Measurement of microstructural factors in the laser brazed joint revealed that increasing the laser power results in a decrease in the weld toe angle and an increase in the bead width, which indicates superior wettability. A high strength laser brazed joint can be achieved through the combination of good wettability and a thin intermetallic layer produced by a laser power of 590 W in a brazed joint with AZ125 filler metal Any further increase in power, however, results in a rapid increase in the thickness of the intermetallic compound (IMC) reaction layer. The superiority of the brazed joint with AZ125 filler metal is due to its lower melting point than that of AZ61 and AZ91 filler metal.

Abstract: Effects of high-speed deformation on age hardening and microstructural evolution behavior of 6061 aluminum alloys were studied. By affecting the high-speed impact compression (about 5 GPa) to the 6061 aluminum alloy plate in the state of quenching after the solution heat treatment, the maximum hardness became twice as high as the original hardness. Even after the impact compression, age-hardening was clearly identified both at 175 °C and 100 °C. TEM observation revealed that point defect clusters were distributed densely inside grains after the impact compression, possibly due to the effect of high-speed deformation. The point defect clusters observed were assumed to be stacking fault tetrahedra on the basis of high resolution TEM analysis. The point defect clusters and precipitates were both visible even after the peak-aged condition at 175 °C. The 6061 aluminum alloy specimen after the solution heat treatment, followed by the impact compression (8.0 GPa) and the peak-aged condition showed the highest hardness value (154 Hv) among the testing conditions selected in the present study.

Abstract: A friction stir welded A3003 aluminum alloy /SUS304 stainless steel dissimilar lap joint was successfully produced. A sound joint that fractured at the base metal was obtained in the center region of the joint through the reaction layer of aluminum-rich intermetallic compounds with nanoorder thickness. The microstructural changes at the interface of the joint was examined by studying the hole left by the extracted welding tool produced at the end of the friction stir welding (FSW) bead using transmission electron microscopy (TEM). Mixed layers consisted of ultra-fined intermetallic compounds and stainless steel were observed. The stirred aluminum alloy flows onto the mixed layer after the tool transit and the joining was achieved. Based on the TEM observations, the joining process of the lap joint was also discussed.

Abstract: The effects of pre-aging before natural aging on the bendability of an asymmetric-rolled Al-Mg-Si alloy were investigated. In the specimen without pre-aging, hardness increased with natural aging time due to the formation of nanoscale cluster (nanocluster; Cluster (1)). The suppression of Cluster (1) during natural aging is clearly seen in the pre-aged specimens though the formation of Cluster (2). It was found that tensile properties were not so affected by the types of clusters. Bending test clearly showed that the pre-aging improves the bendability of this alloy. This effectiveness of pre-aging means Cluster (2) well contributes the deformability of this alloy compared with Cluster (1). Such an improvement of bendability is considered to be derived from the structure of nanoclusters and its interactions with the factor of plastically deformation of alloys.

Abstract: The effect of PFZ and grain boundary precipitates formed in aging processes on the macroscopic mechanical properties in Al-Zn-Mg(-Ag) alloys were evaluated using TEM, SEM, tensile test and nanoindentation. Decreases in width of PFZ and smaller size of grain boundary precipitates aging at lower temperatures and/or the addition of Ag processes improved the tensile properties, and the presence of PFZ was found to be harmful to the fracture. Nanoindentation hardness results clarified that the hardness within PFZ is smaller than that in grain interiors, indicating that, in the alloy with large width of PFZ, preferential deformation occurs within PFZ in the initial stage of deformation, and this causes lower elongation regardless of the same level of proof stress and the same sizes of grain boundary precipitates. From the quantitative correlation between precipitate microstructures in the vicinity of grain boundaries, mechanical properties and fracture morphologies, the deformation process of the alloys is considered to divided by three types; i.e. in case of the alloys with the small width of PFZ and the small size of grain boundary precipitates, in case of the alloys with the large width of PFZ and the small size of grain boundary precipitates and in case of the alloys with the large width of PFZ and the large size of grain boundary precipitates.