Fatigue microstructure and weld penetration for the structural materials

• Main Authors: Ruei-Siang Chen, 陳瑞祥
• Other Authors: Shing-Hoa Wang
• Format: Others
• Language: en_US
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/53666904969200344830

碩士 === 國立臺灣海洋大學 === 機械與機電工程學系 === 97 === Part.I Steel has been widely used in various applications. Particularly in automobile application, a low cost, lightweight steel with a high specific strength is required to save energy and reduce carbon emissions to combat global warming. The microstructure of this ferrite-pearlite (FP) high strength low alloy steel is composed of ferrite, pearlite, and nano TiC precipitates. The yield strength and tensile strength of this FP high strength, low alloy steel rise above the conventional levels of high strength, low alloy steels. Furthermore, its fracture strain can reach about 33% under a slow strain rate of 10-4 s-1. The material undergoes a cyclic strain hardening behavior under a relatively higher strain amplitude in a low cycle fatigue. In contrast, cyclic softening is exhibited under a lower strain amplitude. The cyclic hardening results from tangled dislocations and embryonic subgrain formation, which obstruct the movement of dislocations. The low density of dislocations and larger mean free paths for dislocation motion explain the cyclic softening phenomena under low strain amplitude. Meanwhile, the number and length of secondary cracks grow in quantity under higher strain amplitude with slow strain rate, leading to premature failure. Part.II The purpose of this study is to improve the penetration and strength of gas tungsten arc welding (GTAW) in Ti-6Al-4V alloy welds. The components TiO2, NaF, and nano-particle D with different ratios were blended together intoseveral kinds of fluxes. The activating effects of those fluxes were compared to commercial ActivaTec 860 (ACT 860). The fluxes consisting of NaF with 1wt% either TiO2 or nano-particle D effectively yielded refined grain size, deep weld penetration, and a fine transformed needle structure. The strength of the weldment was just slightly higher than that of the base metal, leading to a desirable uniform mechanical property in the weld design.