含鈧鋁合金銲接熱裂性質研究

Abstract

本研究主要探討針對M6、M7E、M7B、M9H4 之含鈧鋁合金，施以不同次數之惰氣鎢極電弧銲，在不使用填料之情況下，利用點銲可調應變試驗(Spot Varestraint Test) 機，來探討在不同之熱循環(number of thermal cycles) 次數與不同之外加應變量下其銲接熱裂縫敏感性。利用立體顯微鏡觀察熱裂紋與影像擷取軟體計算裂紋長度，配合光學顯微鏡觀察與掃描式電子顯微鏡(SEM) 觀察裂縫形成之微觀組織與不同含鈧(Sc) 量之含鈧鋁合金銲接熱裂縫敏感性之比較。 分析結果顯示M6、M7B、M9H4 含鈧鋁合金在不同之外加應變量下，其熔融區之熱裂縫總長度不會隨著熱循環次數之增加而增加，但會隨著應變量之加大而增加。熱裂敏感性比較，在不同外加應變量及熱循環次數下之量測結果，依序是M6 裂縫總長度最長，M7B與M7E 在一次及二次熱循環中熱裂敏感性相近，M9H4 則為最小。即M9H4＞M7E＞M7B＞M6。 裂縫破斷面微結構組織上，在熔融區均觀察到三個明顯區域，樹枝狀區(D區)、樹枝-平滑轉換區(D-F區)與平滑區(F區)，此可證明熔融區均屬於凝固熱裂機構；在熱影響區均為沿晶脆性破壞之液化熱裂機構。部分熔融區處經EDS分析，發現M7B、M7E 含鈧鋁合金並未出現含有成份Cu、Mg 之偏析現象，屬單純的晶界液化。M6與M9H4二種合金金屬在晶界附近均有相當大的Cu偏析現象，且隨著熱循環次數增加而加劇。因此，M6與M9H4二種合金在部分熔融區均為偏析熱裂。

The “Spot-Varestraint Test” was applied to assess the sensitivity of four scandium–aluminum alloys –M6, M7E, M7B and M9H4 – to hot cracking from welding. In these experiments we applied Gaseous Tungsten Arc Welding (GTAW) without an added feeder. Samples with varied thermal cycles and with one or two welding energy inputs on the same welding seam were prepared, The stereoscopic microscope is used to observe hot cracking and the software for computer image acquisition and analysis measurement are used to measure and to analyze the length of hot cracks in the fusion and the heat-affected zones with varied augmented applied strains and thermal cycles. The optical microscopy (OM) and scanning electron microscopy (SEM) to observe the microstructure of crack formation with different amount of scandium the containing scandium aluminum for welding hot crack sensitivity. The results indicate that the number of cracks increases with increasing augmented strain. This phenomenon occurs in both the fusion and the heat-affected zones. The number of thermal cycles also has a significant influence on the heat-affected zone; the number of hot cracks increases, especially in the heat-affected zone of the metal weld, with increasing number of thermal cycles. The hot cracking sensitivity under different augmented strain and number of the thermal cycles shows that the crack length of M6 is the length, M7B and M7E an similar in the hot cracking sensitivity, the M9H4 is the shortest. The compositions of these four alloys show that M6, M7B and M7E have similar tendencies to be subject to hot cracking, greater than M9H4. With increasing number of thermal cycles, the hot cracks show the same tendency, M9H4 > M7E> M7B> M6. On cracking fracture surface, the experiment results show three distinct regions in the fusion zone, dendritic area (D area), dendritic-flat area(D-F area) and the flat area (F area), to confirmed the fusion zone belong to the solidification cracking. In the heat affected zone, the results indicate the brittleness fracture phenomenon along grain boundary on the fracture surface, and it is presented in the form of liquefied hot cracking. The EDS analysis for the partial melting zone shows M7B and M7E does not produce Cu and Mg segregation increases. These two kinds of alloys are purely grain boundary liquation.M6 and M9H4 have the Cu element segregation in the vicinity of grain boundaries, and are the Cu segregation increases with the number of thermal cycles. So that the M6 and M9H4 are segregation-induced liquation mechanism in the partially melted zone.