Inhomogeneity of Microstructure and Damping Capacity of a FC25 Disc-Brake Rotor and Their Interrelationship
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A disc-brake system of a vehicle consists of a rotor, a caliper, and two pads.[1] The mechanical friction between the rotor and the pads causes significant noise, vibration, and harshness (NVH). For ride comfort, therefore, the reduction of NVH occurring in the brake system has received a lot of attention. One way to alleviate undesirable NVH is to use a rotor fabricated with a high damping alloy, such as gray cast iron (GCI),[2] Fe-Mn alloy,[3] Cu-Al-Mn alloy,[4] and Mg-Zr alloy.[5] Of these high damping alloys, the GCI has been extensively investigated and widely utilized due to both its low material cost and high productivity.[6] Visnapuu et al.[7] investigated the change in specific damping capacity (SDC) with the morphology of graphite, and found that the SDC increased, as the shape
JONGBIN PARK, Graduate Student, SEUNG-JOON LEE, Postdoctoral Research Associate, and YOUNG-KOOK LEE, Professor, are with the Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea. Contact e-mail: [email protected] JEONGHO HAN, Postdoctoral Research Associate, formerly with the Department of Materials Science and Engineering, Yonsei University, is now with the Department of Microstructure Physics and Alloy Design, Max-Planck-Institut fu¨r Eisenforschung, 40237 Du¨sseldorf, Germany. KYOUNGDON YI and CHELWOONG KWON, Researchers, are with the Chassis Design 2 Team, Seohan Industry, Hwaseong, Gyeonggi-do 445-170, Republic of Korea. Manuscript submitted October 10, 2015. Article published online June 15, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
of graphite changed from nodule to flake because the sharp edges of flake graphite are effective in dissipating vibrational energy. Murakami et al.[8] reported that the damping capacity of Fe-(2.6-4.6)C-2.0Si (wt pct) GCI became greater with increasing carbon content due to the increase of the volume fraction of flake graphite. These results imply that the damping capacity of GCI is closely related to both the morphology and volume fraction of flake graphite. Until now, the damping by flake graphite has been explained in two different mechanisms: damping at the interphase boundaries between graphite particles and the metallic matrix and damping within graphite particles. Regarding the damping at the interphase boundaries, Sugimoto[9] and Zhang et al.[10] reported that the GCI has high damping capacity due to the viscous flow (or plastic flow) at the interphase boundaries between graphite particles and the metallic matrix. Golovin et al.[11] provided more detailed results that the local strain concentration occurred in the matrix next to flake graphite particles, which are oriented to the direction in which the maximum shear stresses act, i.e., at an angle of p/4 to the direction in which the macro-stresses act. Regarding the damping within graphite, Millet et al.[12–14] observed a rapid increase of the internal friction of GCI at low temperatures ranging from approximately 180 K to 270 K (93 C to 3 C), regardless of the vibrational frequency
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