In this work the Weibull method was used to analyze the tensile test results from an engineered development casting used as a test source for mechanical test samples. Both melt conditions, sample placement, testing method and chill po- tency were investigated.
Results
Figure 3a and 3b shows the detailed Light Optical Micrographs (LOM) orientated perpendicular to the fracture surface, and a Scanning Electron Micrograph in Secondary Electron (SEM/ SE) mode image positioned directly on the fracture surface before sectioning. From the LOM images, the fracture sur- face is seen to traverse specifi cally through the interdendritic regions which contain the Al-Si eutectic and other secondary phase constituents such as the Al15
(see Figure 3b). The preferred fracture path is governed by the stress concentration effects of brittle secondary phase constitu- ents and shrinkage pores, and the work hardening of the α-Al
(Fe,Mn,Cr)3 Si2 script phase
matrix, ahead of the advancing crack. The LOM images also reveals similar depth of the ridge and valley type features gov- erned by the 3-D distribution of the most stress concentrated points in the cast structure. However, as seen in the SEM/SE mode images, a duller and more relatively featureless fracture surface is revealed for the fi ner microstructure. The small box- es located on the lower left of the SEM/SE images are shown in higher magnifi cation in Figure 4a, revealing the valley and ridge morphology. Higher magnifi cations seen in Figures 4b and 4c show the size of the dimples left behind in the α-Al ma- trix structure, the remnant of the coalesced micro-voids during the tensile test. These micro-voids can be initiated by de-cohe- sion of a secondary phase constituent from the α-Al matrix,2,5,6 or by the cracking of the secondary phase constituent itself.5
As
loading increases the micro-voids grow in size, coalescing with neighboring micro-voids, eventually leading to a growing crack size and crack advancement. Also seen in Figures 4b and 4c is the presence of a 525µm and 485µm shrinkage pores, respec- tively, in the coarse microstructure fracture surface.
Chill #1, Iteration #2
See 4b
See 4c (4a) (4b)
See 4c
(4c) Figure 4a-d. SEM/SE images of tensile fracture. International Journal of Metalcasting/Winter 10 37
(4d)
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