The Study Of Hot Cracking On The Scandium–Aluminum Alloys

• Main Authors: Yeh, Sheng-Wen, 葉聖文
• Other Authors: Chou, Chang-Pin
• Format: Others
• Language: zh-TW
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/69186729921134943071

碩士 === 國立交通大學 === 工學院精密與自動化工程學程 === 100 === 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.