| Summary: | 碩士 === 國立臺灣師範大學 === 機電科技研究所 === 98 === The main purpose of this study focuses on 2024 aluminum alloys. it includes two parts:
The first part aims to investigate the hot cracking susceptibility of 2024 aluminum alloys. The specimens were experimented with tungsten Inert gas welding (TIG) at first under the condition of no filler metal used. Then, the spot varestraint testing was used to evaluate the effect resulting from the times of thermal cycles and augment strain over hot cracking susceptibility of 2024-T351 and 2024-T6 aluminum alloys. Meanwhile, the causes and patterns of hot cracking were observed and analyzed with Metallographic test and scanning electron microscope (SEM). The experimental result shows that the times of thermal cycles is irrelevant to hot cracking susceptibility in the weld fusion zone of 2024-T351 and 2024-T6. But the times of thermal cycles does affect hot cracking susceptibility in weld metal heat affected zone (W. M. HAZ) of both 2024-T351 and 2024-T6. As the times of thermal cycles increases, the length of hot cracks grows. Besides, the length of hot cracks in base metal heat affected zone (B. M. HAZ) of 2024-T351. However, this phenomenon does not appear in base metal heat affected zone (B. M. HAZ) of 2024-T6.on the other hand, as augment strain given increases, the total crack length (TCL) of these two materials both remains constant. With Metallographic test, the hot cracking occurrence was seen lying along the crystalline grain boundary in fusion zone and HAZ. Hot cracks yielded in fusion zone and in HAZ are categorized to solidification cracking and liquation one respectively. The types of hot cracking can be further identified with SEM.
The second part focuses on the research over mechanical property of dissimilar metal welding with 2024-T351 and 7050-T7451 through metallography and tensile test. TIG was adopted. ER2319 and ER5356 serve as individual filler during welding. After welding, specimens were conducted with respectively different heat treatment, natural aging treatment (T1) and solution heat treatment (T4). The experimental result indicates: (1)The micro-hardness in fusion zone of dissimilar metal welding enormously after T4 is enormously bigger than that after T1. The difference can achieve 20 to 30 Hv. (2) The specimens using ER2319 as filler have higher ultimate tensile strength (UTS), yield strength (YS) and elongation (El) than those using ER5356 after T1. But the above-mentioned parameters of specimens with these two fillers show no difference after T4.
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