Papers by Author: Sang Shik Kim

Abstract: In order to study the effect of trace impurities on high-cycle fatigue damage of ultrafine grained (UFG) copper, fatigue tests were carried out for two samples: oxygen-free copper (OFC, 99.99 wt% Cu) and deoxidized low-phosphorous copper (DLP, 99.95 wt% Cu). After the processing by equal channel angular pressing (ECAP) using eight passes, equiaxed grains with an average size of 250 nm were formed for both the samples. Fatigue strength of UFG copper was enhanced by the impurities. The formation behavior of surface damage and the change in surface hardness was monitored. A close relationship was observed between the change in hardness and the formation behavior of surface damage. The physical background of the effect of trace impurities on the fatigue damage was discussed from the viewpoints of surface damage formation behavior.

Abstract: The effects of strain rate in rolling on microstructures and mechanical properties of a nano-grained high purity copper processed by accumulative roll bonding (ARB) were studied. The rolling during ARB was conducted with two kinds of strain rates (2.6sec-1 and 37sec-1). The microstructural evolution of the copper with ARB proceeding was somewhat different in both methods. However, the variation of mechanical properties with ARB was very similar to each other.

Abstract: Oxygen-free copper was processed by equal channel angular pressing with different numbers of ECAP process cycles, NP. Tensile strength was increased with an increase in NP, but it tended to saturate after NP = 4. Conversely, elongation was dramatically decreased by first pressing, but it tended to saturate up to NP = 3, followed by slight increasing trend after NP = 4. Fatigue tests of specimens processed with NP = 4 and 8 were performed. The change in surface morphologies during fatigue was monitored successively. In addition to this, the change in surface hardness was measured. Significant decrease in surface hardness due to cyclic stresses was measured. The physical basis of fatigue damage of UFG copper was discussed based on the experimental results.

Abstract: The objective of this study is to quantify and understand the effectiveness of a hexavalent chrome replacement ion to inhibit environmentally assisted fatigue crack propagation (EFCP) in high strength aluminum alloys. Addition of molybdate (MoO4 2-) to bulk-low chloride solution effectively inhibits EFCP in peak aged 7075, comparable to that of CrO4 2-. The effectiveness of inhibition depends strongly on loading variables: .K, R, and frequency as explained qualitatively by mechanical instability of a crack tip passive film that otherwise hinders production and uptake of embrittling hydrogen. The critical loading frequency (and crack tip strain rate), below which film stability and inhibition occur, increases with increasing inhibitor concentration, but only for low stress ratio loading, perhaps due to occluded crack transport and reaction considerations. Molybdate could be a beneficial replacement for chromate and a candidate for inhibitor release from a tailored coating.

Abstract: The effect of prior T4 heat treatment and subsequent microstructural evolution on tensile behavior of friction stir welded (FSWed) AZ31B-H24 alloy was examined in this study. Selected AZ31B-H24 plates were prior T4 heat treated at 400
for 24 hours and subsequently friction stir welded. The tensile properties, optical micrographs and SEM fractographs for FSWed AZ31B-T4 specimens were compared with those for the H24 counterparts. Prior T4 heat treatment tended to decrease the tensile ductility reduction in FSWed AZ31B-H24 specimen. The tensile ductility reduction mechanism as associated with prior T4 heat treatment is discussed based on detailed micrographic and fractographic observations.

Abstract: Fatigue crack propagation (FCP) behavior of friction stir welded (FSWed) 6061-T651 Al alloy was examined with the fatigue crack growing either along the dynamically recrystallized zone (DXZ) at variable
K or perpendicular to the DXZ at a constant
K value of 7.5, 10 and 15 MPa√m, respectively. The FCP rates in the DXZ tended to be significantly lower than those in the PM particularly in low
K regime. Compressive residual stress reducing effective
K cannot be solely responsible for the enhanced FCP resistance. The fine recrystallized grains in the DXZ causing intergranular fatigue failure appeared to be detrimental to the FCP behavior of FSWed 6061-T651 specimen. The constant
K fatigue test across the weld zone showed that substantial crack retardation occurred far beyond the heat affected zone (HAZ) at low
K regime. The FCP behavior of FSWed 6061-T651 is discussed based on residual stress measurement and fractographic observation.

Abstract: The mechanical behavior of nano grain-sized pure copper produced by various SPD (severe plastic deformation) processes such as ECAP (equal channel angular pressing) and ARB (accumulated roll bonding) was investigated in relation to the microstructural evolution. These processes promoted the formation of equiaxed nanoscale grains in pure copper. The present observation suggested that the tensile behavior of the specimens prepared by the current SPD processes was influenced by several mechanisms involving strain hardening and dynamic recovery. The heat flow was measured by using a DSC (differential scanning calorimeter) to elucidate the relationship between the dislocation density and the tensile behavior in the specimens.