Browsing by Author "Cheng, C. M."

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Characterization of Hot Cracking Due to Welding of High-Strength Aluminum Alloys
(2012)
Chang, C. C.
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Chen, C. L.
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Wen, J. Y.
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Cheng, C. M.
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Chou, C. P.
The "Spot-Varestraint Test'' was applied to assess the sensitivity of three aluminum alloys-A2024-T351, A2219-T87, and A7050-T6 to hot cracking from welding. 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 compositions of these three alloys show that A2024 and A7050 have similar tendencies to be subject to hot cracking, greater than A2219. With increasing number of thermal cycles, the hot cracks show the same tendency, A2024 > A7050 > 2219.
Hot Cracking in AZ31 and AZ61 Magnesium Alloy
(2011-07-01)
Huang, C. J.
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Cheng, C. M.
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Chou, C. P.
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Chen, F. H.
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機械工程學系
This paper examined the impact of the number of thermal cycles and augmented strain on hot cracking in AZ31 and AZ61 magnesium alloy. Statistical analyses were performed. Following observation using a scanning electron microscope (SEM), an energy dispersive spectrometer (EDS) was used for component analysis. Results showed that Al content in magnesium alloy has an effect on hot cracking susceptibility. In addition, the nonequilibrium solidification process produced segregation in Al content, causing higher liquid Mg-alloy rich Al content at grain boundaries, and resulting into liquefied grain boundaries of partially melted zone (PMZ). In summary, under multiple thermal cycles AZ61 produced serious liquation cracking. AZ61 has higher (6 wt%) Al content and produced much liquefied Mg(17)Al(12) at grain boundaries under multiple thermal cycles. The liquefied Mg(17)Al(12) were pulled apart and hot cracks formed at weld metal HAZ due to the augmented strain. Since AZ31 had half the Al content of AZ61, its hot-cracking susceptibility was lower than AZ61. In addition, AZ61 showed longer total crack length (TCL) in one thermal cycle compared to that in three thermal cycles. This phenomenon was possibly due to high-temperature gasification of Al during the welding process, which resulted in lower overall Al content. Consequently, shorter hot cracks exhibited in three thermal cycles. It was found the Al content of AZ31 and AZ61 can be used to assess the hot-cracking susceptibility.
The Influence of Aluminum Content of AZ61 and AZ80 Magnesium Alloys on Hot Cracking
(2011)
Huang, C. J.
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Cheng, C. M.
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Chou, C. P.
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機械工程學系
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Department of Mechanical Engineering
This study aims to investigate how aluminum content in magnesium alloys AZ61 and AZ80 impacts the hot cracking susceptibility of magnesium alloys. Differences in aluminum content are known to influence the total crack length of hot cracking. Magnesium alloy AZ61's total crack length was the longest in one thermal cycle, while AZ80's total crack length increased as the number of thermal cycles increased. The most significant difference between AZ61 and AZ80 was the hot crack at the heat-affected zone (HAZ). As the number of heat inputs increased, the grain would coarsen in the HAZ and precipitation started, which resulted in the accumulation of hot cracks at weld metal HAZ (W. M. HAZ). During the solidification of AZ80, which has higher aluminum content, the segregation of aluminum at the grain boundary caused Mg17Al12 to liquefy, increasing the length of hot cracks. Augmented strain caused miniature cracks between Mg17Al12 and grains. Therefore, aluminum content and augmented strain were found causes of hot cracking susceptibility in magnesium alloys.