Effects of Different Graphite Types on the Thermal Fatigue Behavior of Bionic Laser-Processed Gray Cast Iron

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BECAUSE of the existence of graphite, gray cast iron has many excellent properties, such as shock absorption, wear resistance, and good cast-ability. It is often used to manufacture brake drums, brake disks, and cylinders.[1] Graphite, however, is a double-edged sword that not only provides excellent performance to gray cast iron but also exerts a splitting effect on the substrate, which eventually results in stress concentration. Many gray cast iron devices often work in hot and cold alternating environments, giving rise to thermal fatigue cracking failure, resulting in a great waste of resources or even accidents.[2,3] Figure 1 shows some failed gray cast iron brake drums and the thermal cracks appeared on the HAIQIU LU, MING LIU, DAHUI YU, HONG ZHOU, PENG ZHANG, HAIFENG BO, and WEI SU are with the Key Laboratory of Automobile Materials, Ministry of Education, Jilin University, Changchun 130025, P.R. China and also with the School of Material Science and Technology, Jilin University, Changchun 130025, P.R. China. TI ZHOU is with the Key Laboratory of Automobile Materials, Ministry of Education, Jilin University and also with the School of Mechanical Science and Engineering, Jilin University, Changchun 130025, P.R. China. Contact e-mail: [email protected] ZHIHUI ZHANG is with the Key Laboratory of Bionic Engineering, Ministry of Education, Jilin University, Changchun 130025, P.R. China. HENG BAO is with the Security Department of 9588 Troops, No. 1827 Yingbin Road, Changchun 130062, P.R. China. Manuscript submitted April 15, 2018.

METALLURGICAL AND MATERIALS TRANSACTIONS A

friction surface after repeated braking cycles. Numerous studies have shown that the microstructure determines the material performance.[4–6] Because of different solidification conditions and chemical compositions, the graphite type of gray cast iron is quite different and directly affects the thermal fatigue property of gray cast iron.[7,8] Yi et al. reported the thermal mechanical coupling characteristics of graphite morphologies, indicating that the stress of graphite tip is proportional to its length. Moreover, the stress interaction between the two ends of the sufficiently short graphite would increase the stress concentration.[9] Goo et al. found that the fatigue life of gray cast iron could be improved by adjusting the composition and microstructure.[10] Studying the effect of graphite on the thermal fatigue properties and improving the fatigue resistance of gray cast iron, in order to achieve the high-quality and lightweight iron parts is an important research direction of gray cast iron at present. Improving the thermal fatigue resistance of gray cast iron had been extensively studied, and a bionic idea has emerged. Wegst et al. believed that natural structural materials usually comprise hard and soft phases. The resulting materials are lightweight and often display unique combinations of strength and toughness. Further investigation revealed that the toughening mechanism of the biological structure of soft and hard phase shows the

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