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Table 3. High Cycle Fatigue Strength of 351 and A356+0.5%Cu Alloys Alloy


mentioned in previous section. Te high cycle fatigue strength of


the 351 alloy and the A356+0.5Cu alloy is shown in Table 3. In com- parison with the A356+0.5%Cu, the 351 alloy does not show clear supe- riority in fatigue as it does in tensile. It is generally accepted that fatigue strength is controlled mainly by defect size while tensile properties are more related to volume fraction of defects. In aluminum castings, the size of defects depends more upon melt quality, hydrogen level, solidification rate, and other casting process variables than upon alloy composition. Figure 7 shows the S-N data of the


351 alloy samples taken from cylinder head deck face and high pressure oil line areas. Te samples from the high pressure oil line area tend to be slightly superior to those from the deck face area, even though the deck face samples have a finer DAS and lower volume fraction of porosity (measured metal-


351-T6 A356+0.5%Cu-T6


lographically) compared to the high pressure oil line samples. Fractographic analysis of the fractured samples shows that in the deck face samples, the fatigue crack initiation was mainly through the quick linkage among multiple small pores by either shearing the dendrites or debonding the semi- columnar grain boundaries (Figure 8). Tese multiple small pores together with shear bands acted as a large pore, initiating a fatigue crack, and reducing the fatigue strength. While in the high pressure oil line samples, the individual pores are large and far apart due to slow solidification rate, and the fatigue crack initiated from just one single pore in each specimen (Figure 9). Following the evaluation of the


microstructure and mechanical proper- ties of the 351 alloy in the cylinder head


91.2


Fatigue Strength (MPa@10^7 cycles, 150C) Deck Face 83.4


HPOL 95.0


70.0


castings, several conclusions were drawn: • In general, the castability of the 351 alloy is good. Sound cylinder head castings were made using the same tools as for A356+0.5%Cu alloy.


• Tensile strength of the 351 alloy are remarkably superior to A356+0.5%Cu alloy at both room temperature and elevated temperatures.


• Creep resistance of the 351 alloy is significantly better than that of A356+0.5%Cu alloy.


• Fatigue performance of the 351 alloy is similar or slightly superior to the A356+0.5%Cu alloy.


Tis article is based on the paper “Evaluation of a New High Temperature Cast Aluminum Alloy for Cylinder Head Applications” (18-067) originally presented at the 122nd Metalcasting Congress.


July 2018 MODERN CASTING | 41


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