Papers by Author: Keiyu Nakagawa

Abstract: In this study, the effect of the surface structure and hydrogen on the fatigue strength of electroless Ni-P plated Al-2%Cu alloy was investigated. As the results, the following points were clarified. Large precipitates were recognized near the specimen surface of the furnace-cooled Al-Cu alloy, but these were not recognized in the aged Al-Cu alloy. Fatigue strength of the Al-Cu alloy specimen subjected to Ni-P plating after a furnace cooling treatment was overall reduced rather than one of the non-processed specimens. Fatigue strength of the Al-Cu alloy specimen subjected to Ni-P plating after the aging treatment showed a clear increase in comparison to one of non-processed materials. In the Al-2%Cu alloy specimens subjected to Ni-P plating after the furnace cooling treatment or aging treatment, a clear hydrogen desorption was recognized. On the other hand, there was only hydrogen desorption from a few of the non-processed specimens. It is considered that the poor fatigue strength of the plating materials is mainly due to the interaction between the surface precipitates and hydrogen gas.

Abstract: Age-hardening of Al-Zn alloy after quenching develops inhomogeneously due to the effect of surface as a vacancy sink and grain boundary as an easy path. In this study, reversion of the age-hardened Al-Zn alloys, in which ellipsoidal GP zones were formed, was investigated by Vickers micro-hardness test. Ellipsoidal GP zones were reverted more quickly near the surface and grain boundary than in the interior, as spherical GP zones in Al-10%alloy did. It is considered that the surface and grain boundary plays a role of effective source for vacancies, in addition to the interior source such as dislocations, as in the case of the reversion of spherical GP zones.

Abstract: In this study, the effect of heat treatment conditions and small addition of Cu on occurrence of serration in Al-Si alloys was investigated. Specimens were aged for various times up to 87ks at 273K or 473K after quenching from 853K, and tensile-tested at room temperature. In the binary alloy, serration was observed even after aging for 87ks at 273K, while in the case of aging at 473K, serration did not occur under aging conditions at aging time, t_A≧20s. On the other hand, serration was observed even after aging for 72ks at 473K in the Cu-added alloy. In both alloys, serration was also recognized when the specimens were furnace-cooled from 853K to room temperature. Thus, for aging at 473K of the binary alloy serration was observed only when the aging period was short enough, but addition of Cu to the binary alloy prolonged the aging period where serration could be recognized. Aging rate of both alloys measured by tensile strength was almost the same. The size of precipitates in the Cu-added alloy was smaller than that in binary alloy. Moreover, the number of the precipitates at the grain boundary in the Cu-added alloy was smaller than that in the binary alloy. It is considered that serration occurs for Al-Si alloys when the specimen is heat-treated so that small precipitates may be formed. Now the details of the effect of Cu addition are not clear.

Abstract: The effect of heat treatment conditions and small addition of Fe on occurrence of serration in Al-Zn alloys was investigated. Specimens were aged for various times up to 5Ms at 293K or 273K after quenching from various temperatures (T_Q), 398K to 853K, and tensile-tested at room temperature. Serration occurred more easily according as T_Q became lower and the aging time became shorter: in the case that T_Q=473K serration was observed even after aging for 2.6Ms, while in the case that T_Q=773K serration did not occur irrespective of aging conditions. Serration was also recognized when the specimens were furnace-cooled from 773K to room temperature. Thus, for the binary alloy serration was observed only when the aging period was short enough, but addition of Fe to the binary alloy prolonged the aging period where serration could be recognized. Aging rate measured by hardness was remarkably retarded with the increase of Fe addition. These results together with those obtained by the electrical resistometry suggest that the serration in Al-Zn alloys occurs in the early stage of aging where small GP zones or solute clusters are formed.

Abstract: In this paper, we investigated effects of aging at 473K on the relationship between microstructure in the vicinity of the grain boundaries and fatigue strength for Al-1.2%Si alloy. Results obtained show the following features. (1) As aging time, tA increase, the tensile strength (σB) and 0.2% proof stress (σ0.2) increase slowly, but gradually decrease after reaching a maximum at around 18 ks. On the other hand, fracture elongation shows an opposite trend, suggesting that at aging times above 18ks, over aging occurs. (2) The fatigue strength lowers with increasing aging time, however, when the aging time is more than 18 ks at 473K, the fatigue strength remains almost the same. (3) When the aging time is more than 6 ks, grain boundary precipitates with a size greater than several 10s of nm are observed. (4) When the aging time is 18 ks, an accumulation of dislocations are observed at the grain boundaries and in the vicinity of grain boundary precipitates, and dislocations increase with the number of stress cycles. (5) When the aging time is more than 6 ks, the fatigue fracture surface is mainly intergranular. These results suggest that reduction of fatigue strength results from propagation of micro-cracks which are initiated at the large precipitates on the grain boundaries.

Abstract: The surface layer of binary Al-Zn alloy specimen, ever after a long time aging, remains softer than the interior when the specimens are age-hardened at around room temperature after quenching from high temperature. In this study, effects of the soft surface layer on the fatigue strength of Al-Zn alloys were studied under repeated tensile loading. Vickers microharhness test revealed that there existed less hardened region in the vicinity of grain boundary and specimens surface, and that the region extends 50 to 100μm from the surface inward. From the plot of the stress amplitude against the number of cycles to failure, it is concluded that the presence of less hardened surface layer increases fatigue resistance of age-hardened Al-Zn alloys containing 8 to 16 mass%Zn under the repeated tensile loading.

Abstract: The aging of Al-Zn alloys has been vastly studied for decades. In the previous paper, 0hta et al. studied carefully the hardness of the alloy during aging and revealed the existence of softer regions near the surface and the grain boundary than the interior of the specimen even after aging for a long time. Electrical resistivity measurement and X-ray small angle scattering experiment together with hardness test suggested that in these regions vacancy decay to the surface and grain boundaries was severe, thus the growth of GP zones were suppressed and therefore age hardening was retarded. Also, it is well-known that an addition of a small amount of Ag raised solvus temperature of GP zones. In this paper, soft surface layer formed in an Al-12mass%Zn alloy is studied by adding small amount of Ag by means of hardness test and resistometry. Addition of Ag more than 0.1% decreases the thickness of soft surface layer as well as accelerates age hardening rate and suppresses the formation of soft region near the grain boundaries. Higher quenching temperature also reduces the thickness of soft surface layer. Together with the behavior of aging curves of the specimen with various thicknesses, the origin of the soft surface layer is confirmed to be the effective role of surface as sinks for vacancies.

Abstract: In this study, we investigated the effects of a small addition of Cu (0.1%) or Ge (0.1%) on the microstructure and mechanical property of Al-Si alloys. The results are as follows. The size of precipitates in the Cu-added or Ge-added alloys is smaller than that in the binary alloy. The size of the precipitates at the grain boundary in the Cu-added or Ge-added alloys is larger than the size of the precipitates in the binary alloy. However, the number of the precipitates at the grain boundary in the Cu-added and Ge-added alloys is smaller than that in the binary alloy. It was found that a small addition of Cu or Ge has the effect of raising the fatigue strength under repeated tensile loading as well as the tensile strength.