Journal of the Japan Institute of Metals and Materials

Exhaust gas recirculation (EGR) coolers are standard accessory items to reduce internal combustion engine emissions and fuel consumption. These coolers are composed of stainless steel brazed with Ni-based brazing filler metals that can internally withstand high exhaust gas temperatures and corrosive environments. In this study, the corrosion behavior of the SUS444 joint brazed with MBF67 foil was examined by an anodic polarization measurement in hydrochloric acid and was compared with that of the SUS444 joint brazed with MBF20 foil. The result indicated that the SUS444 joint brazed with MBF67 showed corrosion with a much higher potential in the polarization curve than the SUS444 joint brazed with MBF20. The reason for the high potential was concluded that the Cr-depleted zone was not formed at the boundary layer between the SUS444 and MBF67 because of low B in MBF67, and the high phosphorus-containing (Ni, Cr)-P eutectic phase with high corrosion resistance remained at the brazed layer because of high P in MBF67.

Fig. 6　Comparison of optical micrographs between (a) before and (b) after polarization for SUS444 joint brazed with MBF67 foil in 0.025 mol/L HCl solution. Fullsize Image

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Hot-extruded AZX611Mg alloy was multi-directionally forged under decreasing-temperature conditions (dMDFed) and followed by room-temperature MDFing (rMDFing). Coarse initial grains were gradually fragmented by dynamic recrystallization during dMDFing and ultrafine-grained structure was homogeneously developed over the cumulative strain of ΣΔε = 4.8. Additional rMDFing contributed to further grain fragmentation due to kinking and mechanical twinning. Nevertheless, coarse precipitates in the as-hot-extruded samples and newly formed fine precipitates during dMDFing impeded twinning. The achieved grain size appeared rather larger than that of the dMDFed AZ61Mg alloy. While mechanical properties were drastically improved by simple dMDFing or rMDFing, the combined processes of dMDFing and rMDFing were not so effective to induce additional strengthening. This was attributed to complicatedly combined effects of impediment of twinning by precipitates to grain fragmentation and fracture caused by stress concentration at around precipitates/matrices.

Fig. 9　Change in hardness during dMDFing to various cumulative strains and followed by rMDFing of a fixed cumulative strain of ΣΔε = 1.0 (i.e., 10 passes of rMDFing). The arrows indicate the change in the hardness of the dMDFed and additionally rMDFed samples. Fullsize Image

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