| 摘要: | 在本次研究中進行拉伸測試和氣壓成形一系列實驗，來探討細晶AZ31B-O和AZ3B-H24鎂合金薄板的金屬流動行為和快速氣壓成形性。在AZ31B-O材料為POSCO公司所生產，厚度為 0.6mm；而本次實驗所使用的AZ31B-H24鎂合金厚度為0.5mm的H24條件，是由美國MEL所生產提供。拉伸測試試片沿滾壓方向取用，初始應變速率為4×10−3 to 1×10−1 s−1溫度條件為 250℃，300℃，370℃和420℃。本次研究分析細晶AZ31B和AZ31-H24鎂合金薄板片在拉伸時流變形為和相關微結構變化。自由鼓脹測試成形是讓壓縮氬氣來使板片變形於圓柱狀腔體。壓成形進行在不同溫度條件，與其溫度分別為 200、250、300、370和420℃的利用各種加壓程序方式。減少成形時間於氣壓成形時使用階梯漸進式和恆定壓力式加壓成形，非恆定加壓程序氣壓成形來調查快速氣壓成形性。 結果顯示，應用拉伸條件使得流變應力行為變化可能是相關顯微結構產生變化結果。熱變形期間，有些冶金現象例如應變硬化、動態回復和動態再結晶(DRX)可能會同時出現，改變微結構與機械性質結果。由應力應變速率的數值結果顯示，細晶AZ31B薄板片高溫拉伸測試，在應變速率4×10‒3 到2×10‒2 s‒1時應變速率敏感指數約0.27，表示差排潛變會是一個可減少氣壓成形時間的變形機制，以拉伸測試AZ31B-H24鎂合金薄板片結果顯示，可提升了低溫氣壓成形性。動態再結晶應該是增加成形性的重要因素，因此低溫氣壓成形應該是在H24狀態下的鎂合金AZ31B(AZ31B-H24)可適用之製程。 AZ31B-O和AZ31B-H24薄板片在不同溫度下採用各種加壓程序狀態變形成半球。在較低的溫度以階梯漸進式加壓狀態應該是較適合的製程，然而，較高溫度下，在成形過程期間以恆定或者是近似恆定加壓則應該是較好的方式。細晶AZ31B-O薄板材可採用氣壓成形方式來應用在快速成形(QPF)。有鑑於實驗結果發現，AZ31B-H24在低溫環境氣壓成形成形性優於細晶AZ31B-O。鎂合金AZ31在快速氣壓成形時產生的空孔量小於鋁合金5083。成形一個較淺的長方盒狀來證明使用QPF製程可能性，採用傳統母模成形成形在高度10mm，達成成形在減少時間，成形時間低於163s。盒狀成形中彎角半徑是影響成形時間的關鍵因素。在本次研究中改良式公模氣壓成形是有預成形條件下，使成形時間低於160秒，並且適合成形於淺的長方形有高表面品質。 In this study, a series of experiments of tensile tests and gas blow forming was performed to explore the flow behaviors and rapid gas blow formability of the fine-grained AZ31B and AZ31B-H24 Mg alloy sheets. The AZ31B-O of material was supplied by POSCO Ltd, with a thickness of 0.6mm. The Mg alloy AZ31B-H24 sheet used in this work, with a thickness of 0.5mm in H24 temper condition, was provided by Magnesium Elektron North American Inc. (USA). Tensile tests were carried out on specimens in the rolling direction, using initial strain rates in the range of 4×10−3 to 1×10−1 s−1 at temperatures of 250, 300, 370 and 420 °C. The flow behaviors and associated microstructure changes of fined-grained wrought AZ31B-O and AZ31B-24 Mg alloy sheets deformed in tension were analyzed in this work. Free bulging tests were performed by deforming the sheet into a right cylindrical die cavity by compressed argon gas. Gas blow forming was carried out at temperatures of 200, 250, 300, 370 and 420 °C using various pressurization profiles. Decreasing the forming time in gas blow forming using stepwise pressurization profiles and constant gas blow forming, which were non-constant gas blow fotming, were conducted to investigate the rapid gas blow formability. Results showed that variations in flow behavior under tension could be related to the changes in microstructure resulting from applied tensile conditions. During hot deformation, some metallurgical phenomena such as strain-hardening, dynamic recovery, and dynamic recrystallization (DRX) may occur simultaneously, resulting in the changes in the microstructure and mechanical properties. The stress-strain rate data showed that fine-grained AZ31B thin sheet on testing at higher temperatures exhibited strain rate sensitivity exponent values of approximately 0.27 in a strain rate ranging from 4×10‒3 and 2×10‒2 s‒1, indicating that the dislocation creep would be a possible deformation mechanism to reduce forming time in gas blow forming. The tensile test results of the AZ31B-H24 alloy sheet suggested that gas blow formability could be enhanced at lower temperatures. DRX should play an important role in enhancing formability. Thus, low temperature gas blow forming would be a possible process by using Mg alloy AZ31B in H24 condition at lower temperatures. The AZ31B-O and AZ31B-h24 alloy sheets were successfully deformed into hemispherical domes at different temperatures under various pressurization profiles. A stepwise pressurization profile should be a suitable process at lower temperatures, whereas a constant or near constant pressure imposed during forming would be a better method at higher temperatures. The fine-grained AZ31B-O alloy sheet could be gas blow formed using Quick plastic forming (QPF) process. Whereas, the formability of AZ31B-H24 was better than that of fine-grained AZ31B-O during low temperature gas blow forming. Forming a shallow rectangular pan demonstrated the possibility of using QPF process. Cavitition level in Mg alloy AZ31B was lower than that in Al alloy 5083 during fast gas bloe forming. A significant reduction in forming time was achieved using traditional female die forming, in which a rectangular pan was formed with a height of 10 mm in less than 163 s. Fillet radius of the rectangular pan should be one of the key factors influencing forming time. An innovated male die gas forming with pre-deformation was also proposed in this study. It is feasible to form a rectangular pan with high surface quality in less than 160 s. |
