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semi-permanent mold cast cylinder heads. In the study, the 351 alloy showed a significant improvement in high temperature tensile proper- ties, particularly in creep resistance in comparison with the commonly used cylinder head aluminum alloy A356+0.5%Cu. Aluminum 351 behaved similarly with regard to fatigue performance. In the study, 80 cylinder heads


were made with the 351 alloy using the semi-permanent mold casting process. Forty of the cylinder heads were also grain refined. Table 1 shows a comparison of the chemical compo- sition between the 351 alloy and the A356+0.5%Cu alloy. Te 351 alloy contains a small amount of zircon and vanadium in addition to an increased copper and silicon content as com- pared to the A356+0.5%Cu alloy. Te cylinder heads were heat-


treated and then samples were taken from both the deck face and high- pressure oil line locations for micro- structure and mechanical property evaluation. Microstructure character- ization included quantitative mea- surement of secondary dendrite arm spacing (DAS) and porosity using an image analyzer. Mechanical property evaluation consisted of tensile, creep, and fatigue. Tensile properties were measured at room temperature, 302F (150C), 392F, 482F, and 572F (300C). Creep testing was conducted at 572F for up to 300 hours under a constant tensile stress of 20MPa and 22MPa. Fatigue testing was carried out under fully reversed uniaxial loading (R=-1) at room temperature and 302F. Figure 2 shows the typical micro-


Fig. 4. Tensile properties from the deck face of the cylinder head were recorded for yield strength (a), ultimate tensile strength (b), and elongation (c).


38 | MODERN CASTING July 2018


structure of the cylinder heads made with the 351 alloy. Te microstructure fineness varies within the head from a fine DAS of ~30µm in the deck face area to relatively coarse DAS of ~45µm in the high pressure oil line area. A detailed quantitative analysis of the microstructure in terms of DAS and porosity is shown in Figure 3. As expected, a fine microstructure results in low porosity. However, grain refine- ment increased porosity in the cylinder heads even though refining the grains appeared to be beneficial to the DAS. Te increased porosity in the grain


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