Yasuo Takaki

Effects of Pre-Aging and Natural Aging on Bake Hardening Behavior in Al-Mg-Si Alloys

The effects of pre-aging and natural aging on the bake hardening behavior of Al-0.62Mg-0.93Si (mass%) alloy with multi-step aging process were investigated by means of Vickers hardness test, tensile test, differential scanning calorimetry analysis (DSC) and transmission electron microscopy (TEM). The characteristics of nanoclusters (nano scale solute atom clusters) formed during pre-aging and natural aging were also investigated using the three dimensional atom probe (3DAP) analysis. The results revealed the occurrence of natural age hardening and that the bake hardening response was decreased after the extended natural aging even though the pre-aging was conducted before natural aging. Since the 3DAP results exhibited the Si-rich clusters were newly formed during extended natural aging, it was assumed that the Si-rich clusters caused the natural age hardening and the reduced bake hardening response corresponding to Cluster(1). The decrease of the bake hardening response was markedly higher in the later stage of bake hardening than in the early stage. The size of the β’’ precipitates were reduced with increasing the natural aging time. Exothermic peaks of Peak 2 and Peak 2’ were observed in the DSC curves for the alloys pre-aged at 363K. Peak 2’ became larger with the natural aging time. This is well understood by the following model. The transition from Cluster(2) to the β’’ phase occurs preferentially at the early stage of the bake hardening. Then the growth of the β’’ phase is inhibited by the presence of Cluster(1) at the later stage of bake hardening. The combined formation of Cluster(1) and Cluster(2) by the multi-step aging essentially affects the bake hardening response and the β’’ precipitates in the Al-Mg-Si alloys.

Effects of Pre-Aging Condition on Multi-Step Aging Behavior in Al-Mg-Si Alloys

The effects of pre-aging temperature on natural aging and following final aging (bake hardening) behavior in the Al-0.6mass%Mg-0.6mass%Si alloy were investigated by means of hardness test, tensile test, differential scanning calorimetry analysis (DSC) and transmission electron microscopy (TEM). As the results of hardness change, lower pre-aging temperature increase the natural age hardening, and decrease the bake harden-ability with natural aging time. By comparing DSC measurements and TEM observations, it is found that Cluster(1) formed during natural aging after the pre-aging, and the β” decomposition temperature is moved higher with natural aging time. The relationship of natural age hardening and bake hardening response suggested that the Cluster(1) suppressed the β” formation even if Cluster(2) coexistent condition.

Effects of Sn Addition on Clustering and Age-Hardening Behavior in a Pre-Aged Al-Mg-Si Alloy

The effects of Sn addition on clustering and age-hardening behavior in an Al-0.6Mg-1.0Si (mass %) alloy were investigated. Addition of Sn delayed the age-hardening in single aging at 170 ̊C. On the other hand, Sn promoted the age-hardening response in 3-step aging process which comprises a pre-aging (PA) at 90 ̊C for 18ks followed by natural aging (NA) for 604.8ks and artificial aging (AA) at 170 ̊C. The characteristics of clusters formed during PA and NA were evaluated by differential scanning calorimetry (DSC) analysis and atom probe tomography (APT). The DSC results show that the endothermic peak at around 160 ̊C to 200 ̊C was observed in the Sn-free alloy. On the other hand, in the Sn-added alloy, endothermic peak was not observed. It is suggested that Sn addition suppresses the formation of the clusters formed during NA. The APT results show that the Sn addition decreases the number density of clusters, especially smaller clusters. No Sn precipitates were found in Mg-Si precipitates formed during AA at 170 ̊C for 3.6ks. It is speculated that suppression of smaller cluster formation by addition of Sn promotes the age-hardening response

Characterization of Roping on AA6xxx Alloy Sheet by Numerical Analysis for Surface Topography

AA6xxx alloys are known to suffer from a phenomenon called roping, which is a ridge and valley pattern appearing after stamping is treated as a fatal surface defect for the automotive skin panels. Although the roping is the important factor to determine the exterior quality, it is commonly evaluated by visual check done by inspecting staff, which is very subjective and difficult to measure quantitatively. In this study, to quantify the roping level in detail and easily, a new calculation method based on Fourier transform and using simple device to measure topography is proposed. As a result of analyses for several different AA6022 alloy sheets, it is found that the strong roping surface has the intensive directional pattern along the rolling direction with a specific wavelength. The roping level can be expressed numerically as the ratio between the amplitude of the rolling directional pattern and the average amplitude of each directional pattern. This value shows good agreement with the roping level determined by human observation. Moreover, it is found that the proposed method can be applied to the surface appearance obtained by digital camera without a 3D profiler.