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The Investigation of Mechanical Properties and Characteristics for The Ti-6Al-4V and SP700 Welds
|Authors: ||S.Y. Ding|
|Contributors: ||NTOU:Institute of Materials Engineering|
|Issue Date: ||2011-06-30T07:22:55Z
|Abstract: ||研究中使用5KW二氧化碳雷射對SP700及Ti-6Al-4V合金板材進行銲接，比較此兩種鈦合金母材與銲件其機械性質之差異性。 對SP700、Ti-6Al-4V母材及銲件進行缺口拉伸實驗，結果顯示，SP700銲件雖在482℃時效後有最高硬化度，但卻會引發缺口脆化，而在更高的時效溫度(593℃)，破裂模式將由穿晶破裂轉變為沿晶破裂。而Ti-6Al-4V銲件缺口強度則顯示，受時效溫度影響不顯著，缺口拉伸試件破壞面顯示，粗大柱狀晶易呈現穿晶脆性破壞，此特質可能和鈦合金六方最密堆積結構有較少之滑移系統有關，但沿柱狀晶邊界則顯示窩穴狀延性破壞，此延性區域經EDX分析後，具有高Al含量，推測應為α-Ti在柱狀晶邊界析出。比較母材缺口拉伸之破斷面，SP700比Ti-64具較微細之窩穴狀破裂。 對SP700、Ti-6Al-4V母材及銲件進行疲勞裂縫成長試驗，SP700銲件雖具較粗糙的表面，有較曲折的裂縫成長路徑。但結果顯示，母材與銲件呈現一致的疲勞裂縫成長特性，母材其破裂特徵為穿晶破壞，而銲件之破裂特徵則顯示了受顯微組織的影響，裂縫在不同晶面上成長形成穿晶破壞。對SP700銲件進行定值ΔK試驗，在高ΔK下，其破裂會由穿晶轉變成沿晶。探討Ti-64之疲勞裂縫成長試驗，銲件比母材有較佳之裂縫阻擋效果，銲件其破斷面主要為混晶破壞特徵，除了穿晶疲勞破壞之外，裂縫在部份區域亦會沿特定晶面成長，這些特定晶面上並呈現麻田散體針狀組織。|
Abstract The effect of coarse welding structures, which were made by a 5 kW CO2 laser, on the notched tensile strength (NTS) and fatigue crack growth characteristics of two α+β titanium alloys (Ti-6Al-4V and SP-700 alloys) was investigated in this study. The microstructures of the as-received SP-700 were similar to those of Ti-6-4 plate; both consisted of grain boundary β distributed in the elongated α phase. The difference between them was SP-700 contained a greater amount of β phase relative to Ti-6-4. However, such a difference in microstructure did not make a great distinction in tensile properties between them. In case of the SP 700 base plate solution-treated at 980oC and followed by water quench, a rapid increase in hardness occurred after aging at 482oC for an hour. Additionally, the hardness can be as high as Hv 540 in the peak aged condition. In contrast, the hardness of air-cooled specimens showed a very low response to aging treatment. For the specimens solution-treated at 870oC, the trend of variation in aged hardness was similar to that of the 980oC treated ones. However, the former had much lower aged hardness than the latter. SP700 welds also showed the same trend of a quick response to age-hardening as the base plate. Moreover, Ti-6-4 weld showed a sluggish reaction in aged hardness during aging over the temperature range 500 to 600oC. Aged SP-700 welds were found to be suffered from notch embrittlement especially in the peak aged condition, but not for Ti-6-4 welds. Coarser solidified structure and higher aging temperature were more likely to induce intergranular dimple fracture in SP-700 welds. All Ti-6-4 weld showed nearly same fracture features; transgranular dimple fracture with the crack growth on specific plane. Fatigue crack growth tests revealed that SP-700 alloy had a slightly higher fatigue crack growth rate (FCGR) than the Ti-6-4. The zigzag crack path in the FZ of Ti-6-4 laser welds resulted in a reduced FCGR. In contrast, enhanced FCGR of the SP-700 laser welds was associated with extensive cleavage fracture in peak-aged welds whereas intergranular fracture in 593oC aged welds. The change of fatigue-fractured morphology from transgranular to intergranular fracture occurred with increasing the stress intensity factor range, which was associated with the plastic zone size ahead of crack front. Smaller plastic zone size implied the slip was more likely to occur within single grain on particular slip plane, hence, transgranular fatigue was observed. As the plastic zone size reached the columnar boundaries of the weld, the separation of weak solidified boundaries was responsible for intergranular fracture. Keywords: Laser welding; Notch tensile test, Fatigue crack growth; Ti-6Al-4V; SP-700.
|Appears in Collections:||[材料工程研究所] 博碩士論文|
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