PSI - Issue 14

Premkumar Manda et al. / Procedia Structural Integrity 14 (2019) 467–474 Author name / Structural Integrity Procedia 00 (2018) 000–000

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These particles appear to be Fe-Mn-Al (Fe, Mn)Al 6 (large)) and Al-Mn-Si compounds. The presence of elements in these particles has been confirmed by EPMA line scans. The microstructures of cooling duct are very fine and exhibit the presence of above mentioned insoluble particles. The hardness values at different locations within damaged area of cooling duct reflect nearly same and consistent. The low hardness values of the alloy also reveal that the cooling duct is manufactured from wrought and annealed sheet material. The fracture surfaces of the cooling duct shows the different features such as micro-cracks, flat, mud cracks, fatigue and featureless features. It appears that the fracture surface is fully covered with oxide layer. The analyzed hydrogen content (Table 2) in cooling duct is significantly higher (  12 ppm) than the specified one. It is reported in literature that the solubility of hydrogen in aluminium at room temperature is  1 ppm [Morton, 1976; Polmear, 2006]. However, the EBSD microstructure does not show the presence of hydrogen induced blisters.

Fig. 8. Fracture surfaces of the damaged cooling duct: (a), (b) rubbed marks; (c) mud cracks; (d) flat surface / featureless features.

Hydrogen embrittlement or hydrogen dame occurs occasionally in aluminium and its alloys (mostly in high strength alloys such as 2xxx, 6xxx and 7xxx series). However, it is not as serious as in high strength steels. Hydrogen embrittlement of aluminium alloys can result in intergranular or transgranular cracking along with little / small branching. In addition, this phenomenon occurs in aluminium alloys during solidification due to formation of gas filled voids. These voids can affect the properties of both the cast as well as wrought products. It is to be noted that the present alloy does not belongs to the series of above mentioned aluminium alloys but it does show the presence of high content of hydrogen i.e. 12 ppm (Table 2). The high hydrogen in cooling duct of aircraft has probably been ascribed to moisture or water vapour present in environment. In addition, fracture surfaces also exhibit transgranular feature along cracks with little branching. This indicates that the presence of large amount of hydrogen appears to be responsible for failure of the cooling duct along with vibrational fatigue.