Chin-Sung Chung

Optimization of strength and ductility of 2024 Al by equal channel angular pressing (ECAP) and post-ECAP aging

Abstract Equal channel angular pressing (ECAP) in the solid solution state with post-ECAP low-temperature aging treatment was found to be quite effective in enhancing the mechanical properties of 2024 Al alloy. Yield stress (YS) as high as 630 MPa could be achieved. Moreover, the ductility of ECAPed alloy was improved after the aging treatment.

Improvement of high-cycle fatigue life in a 6061 Al alloy produced by equal channel angular pressing

In the present study, mechanical properties and fatigue behavior of solid solution treated 6061 Al alloy fabricated by equal channel angular pressing (ECAP) process were evaluated. Yield stress and ultimate tensile stress significantly increased after ECAP. A remarkably large enhancement in fatigue life, by a factor of about 10, compared to a T6 treated commercial 6061 Al alloy was recognized to occur in low and high cycle regimes after a single pass. Further deformation by ECAP, however, virtually eliminated this improvement especially in the high cycle regime. Fine-grained microstructure with low-grain boundary misorientation angles was proposed to yield the best result in fatigue performance in a 6061 Al alloy. The current result suggests that one may pay attention to a single passed material rather than multi-passed material if improvement in fatigue life is a primary concern for successful engineering applications.

Fatigue properties of ultrafine grained low carbon steel produced by equal channel angular pressing

Abstract Ultrafine grained low carbon (0.15 wt.% C) steel produced by equal channel angular pressing (ECAP) was tested for investigating fatigue properties, including cyclic softening and crack growth rate. Emphasis was placed on investigating the effect of load ratio on the fatigue crack growth rates of ultrafine grained microstructure. The ECAPed steel exhibited cyclic softening. After the first cycle, the tension and compression peak stresses decreased gradually with the number of cycles. Fatigue crack growth resistance and the threshold of ECAPed ultrafine grained steel were lower than that of an as-received coarse grained steel. This was attributed to a less tortuous crack path. The ECAPed steel exhibited slightly higher crack growth rates and a lower Δ K th with an increase in R ratio. The R ratio effect on growth rates and Δ K th was basically indistinguishable at a lower load ratio ( R >0.3) compared with other alloys, indicating that the contribution of the crack closure vanished. This was explained by the fact that finer grained materials produce a lower opening load P op due to a relatively less serrated crack path. Consequently, K min can reach K op readily with a smaller increment of load ratio. The crack growth rate curve for the ECAPed ultrafine grained steel exhibited a linear extension to the lower growth rate regime than that for the coarse grained as-received steel. This behavior can be explained by a reverse crack tip plastic zone size ( r p ) that is always larger than the grain size.

Effects of redistributing residual stress on the fatigue behavior of SS330 weldment

Residual stress redistribution of a weldment during crack growth and its effect on the fatigue crack propagation were investigated. Fatigue tests were conducted using center notched specimens machined from welded plates. The residual stress and its redistribution during crack growth were measured by a magnetizing stress indication method and hole drilling method. Fatigue crack propagation was estimated using specimens containing residual stresses. Crack growth rates were predicted and compared with the experimental results. It has been found that the predicted crack growth rates, using the effective stress ratio (determined from the residual stress redistribution), are in good agreement with the experimental results, compared with those from the initial residual stress distribution. A crack closure concept was adopted for predicting the influence of residual stress on fatigue crack propagation. It has found that under the current testing conditions the crack growth rates predicted by the redistributing residual stress is more accurate, compared with those by the crack closure concept.

Fatigue properties of fine grained magnesium alloys after severe plastic deformation

Fine grained AZ31 and AZ61 magnesium alloys produced by equal channel angular pressing (ECAP) were tested for investigating tensile and fatigue properties, including microstructure, monotonic tensile flow, fatigue life and crack growth rate. For the two alloys, the yield stress of the ECAPed sample was lower than that of the unECAPed (=as received) sample, because of the fact that the softening effect due to texture anisotropy overwhelmed the strengthening effect due to grain refinement. Grain refinement of the AZ31 and AZ61 alloys through ECAP was found not to be significantly effective in increasing fatigue strength.

Fatigue Crack Growth Behavior in Ultrafine Grained Low Carbon Steel

Ultrafine grained (UFG) low carbon (0.15 wt.% C) steel produced by equal channel angular pressing (ECAP) was tested for investigating the effect of load ratio on the fatigue crack growth rate. Fatigue crack growth resistance and threshold of UFG steel were lower than that of as-received coarse grained steel. It was attributed to the less tortuous crack path. The UFG steel exhibited slightly higher crack growth rates and a lower ΔKth with an increase of R ratio. The R ratio effect on crack growth rates and ΔKth was basically indistinguishable at lower load ratio (R>0.3), compared to other alloys, which indicates that contribution of the crack closure vanishes. The crack growth rate curve for UFG steel exhibited a longer linear extension to the lower growth rate regime than that for the coarse grained as-received steel.

OPTIMUM JOINING CONDITIONS IN A MECHANICAL PRESS JOINT

Mechanical press joining has been used in sheet metal work because it is a simple process and offers the possibility of joining dissimilar sheet metals, such as steel and aluminum alloy sheets. The mechanical press joint strength was found to vary with joining conditions, such as sheet thickness and punch diameter. The optimum joining conditions of the mechanical press joint under complex loading can be determined by correlating strength ratio with diameter ratio and sheet thickness ratio. The failure mode was considered during estimation of the joining strength. Under this experimental condition, the optimum strength ratio was acquired at a sheet thickness ratio of 1.0 and a diameter ratio of 1.683.

Steady-state creep behavior of a Ti-25al-10Nb-3v-lMo alloy

Creep tests were conducted on Ti-25Al-10Nb-3V-1Mo alloy in the the temperature range of 913 - 1093 K at stresses ranging from 40 to 600 MPa. The creep behavior of the Ti3Al alloy under these testing conditions revealed three different stress exponent regimes. In the temperature range of 1033 to 1093 K at low applied stress levels, the stress exponent was equal to 1.5. At the intermediate stress range (103<σ/E<3x10-3), a stress exponent of 3.3 was exhibited indicating that the creep deformation was controlled by a viscous dislocation glide process As the applied stress increase, the stress exponent changed from 3.3 to 4.4 The activation energy for creep was equal to 288 kJ/mole in the region where viscous dislocation glide was the dominant deformation mechanism (n=3.3) In view of the diffusion data, the rate-controlling species in the viscous glide region was assumed to be Ti lattice diffusion