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Cyclic Loading and Dissimilar Laser Welding of High Strength Titanium Alloys
|Authors: ||Cheng-Ta Hsieh|
|Contributors: ||NTOU:Institute of Materials Engineering|
cyclic loading;notched tensile strength;post-weld heat treatment;laser welding;fusion zone
|Issue Date: ||2013-10-07T02:46:14Z
|Abstract: ||循環負載的試驗中，先對Ti-6Al-4V圓棒施以固溶熱處理、循環負載，再進行拉伸試驗、缺口拉伸試驗，以探討熱處理、循環負載對機械特性之影響。實驗結果顯示，Ti-6Al-4V圓棒母材經熱處理後硬度下降且抗拉強度變低，而在900至1000℃的熱處理區間中，隨著熱處理溫度越高，材料延性下降與硬度增加，但抗拉強度無顯明差異。缺口拉伸強度變化趨勢依缺口造型而有所不同，但整體而言V槽缺口拉伸強度高，而圓槽缺口強度延性較佳。循環負載為173MPa、應力比R為0.1的情況下，原始棒材受負載次數所影響，產生延性降低後又再次提升之現象。其他材料除受1000℃熱處理之固溶V槽試片有發生延性大幅下降外，均無明顯改變。 鈦合金異種金屬銲接試驗中，採用5KW二氧化碳雷射，Ti-15V-3Cr-3Sn-3Al板材分別與Ti-6Al-4V、Ti-6Al-6V-2Sn及SP-700等三種α-β雙相鈦合金進行接合，並探討銲道熔融區經時效後之變化情形。實驗結果顯示，所有銲件熔融區於銲後原始狀態下硬度皆與Ti-15V-3Cr-3Sn-3Al熱影響區硬度相近，且經426℃時效後硬度均大幅提升，此外銲件經XRD分析後，發現均含有β相、α”相，故推論所有銲件之熔融區屬α-β雙相鈦合金，時效後硬度大幅提升應與β晶粒內部微細α相的析出或α”相的分解有關。|
The first part of this study was to investigate the effects of heat treatments and cyclic loading on the mechanical properties of Ti-6Al-4V. Tensile tests and notch tensile tests were performed on 10 mm Ti-6Al-4V round bars and notched bars in this work. In the notch tensile test, there are two types of notch, semicircular notch and v-notch. The results showed that solution heat treatment at 900 ℃ and 950℃ reduced the hardness and tensile strength of the as-received Ti-6Al-4V round bars. In the regime of solution heat treatment from 900 ℃ to 1000℃, hardness increased and ductility decreased with increasing the solution temperature. However, there was not much difference in tensile strengths among the solution treated round bars. The type of notch resulted in different trends for notch tensile strength. V-notched specimens were higher in NTS and lower in ductility, compared to semicircular notched ones. Under the cyclic loading conditions, cyclic tensile stress of 173MPa and stress ratio of 0.1, the as-received round bars cyclically strain hardened at 3x104 cycles of cyclic loading, and then cyclically strain softened at 6x104 cycles. The ductility of V-notched round bar, 1000℃ solution heat treated, decreased noticeably after 6x104 cycles of cyclic loading, and the rest of notched and heat treated specimens had little variation in ductility and NTS. The second part of this work was to investigate dissimilar laser welding of α/β and β titanium alloys. Dissimilar welding of Ti-6Al-4V (Ti-6-4) to Ti-15V-3Cr- 3Sn-3Al(Ti-15-3), Ti-6Al-6V-2Sn (Ti-6-6-2) to Ti-15V-3Cr-3Sn and Ti-4.5Al-3V- 2Mo-2Fe (SP-700) alloys was performed by CO2 laser in this work. The results indicated that the hardness values in the fusion zone (FZ) of all the as-welded specimens were close to the heat affected zone (HAZ) of Ti-15V-3Cr-Sn, and these values were hugely increased by aging at 426℃. Moreover, orthorhombic α” phase and β phase were detected by XRD in all welds. These observations concluded that fusion zones of all the welds could be considered as β titanium alloys and that the increase in hardness of aged fusion zone could be attributed to theα precipitation withinβ phase or decomposition ofα” phase.
|Appears in Collections:||[材料工程研究所] 博碩士論文|
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