Aluminum 351 Can Take the Heat, Study Says
The higher demand for lightweighting and fuel efficiency in combustion engines has significantly increased engine power density, exhaust temperatures, and peak cylinder pressures. Cast aluminum alloys have been increasingly used in the automotive industry to replace cast iron in applications such as engine blocks and cylinder heads to reduce mass. With increasing demand for fuel economy, high temperature properties (including tensile, creep, and fatigue strength) of the cast aluminum alloys become critical. This poses a significant challenge on existing cast aluminum alloys for high temperature performance.
Over the past decade, the maximum operating temperature of components like cylinder heads increased from approximately 338F (170C) to temperatures exceeding 392F (200C). The higher operating temperatures result in more severe high cycle fatigue and more low cycle fatigue and/or thermo-mechanical fatigue damage in areas of cylinder heads exposed to high thermal gradients, where the complex out-of-phase transient thermo-mechanical fatigue loading is produced.
In today’s cylinder head designs, the most commonly used cast aluminum alloys are A356, 319 and AS7GU (A356+0.5%Cu). A356 is a primary aluminum alloy with good ductility and fatigue properties at low to intermediate temperatures. However, above approximately 392F, creep resistance and tensile strength of this alloy are rapidly degraded due to the rapid coarsening of Mg/Si precipitates in the alloy.
The 319 alloy is a secondary aluminum alloy representing a lower cost alternative to the A356. The copper-bearing 319 alloy has the advantage of better tensile and creep strength at intermediate temperatures because the Al/Cu precipitates are stable to a higher temperature than the Mg/Si precipitates in A356. However, this alloy is prone to shrinkage porosity due to the high iron and copper content and low ductility at room temperature.
The recently developed AS7GU alloy is a variant of A356, strengthened with 0.5%Cu. Like A356, the AS7GU alloy has good castability while the small copper addition improves creep resistance and tensile strength at intermediate temperatures.
Both Mg/Si and Al/Cu precipitates are thermally unstable, thus all three alloys have poor mechanical properties above 482F (250C) due to the rapid coarsening of these precipitates. Figure 1 shows that, at room temperature, the AS7GU-T64 alloy is superior to “W” and “E” 319-T7 alloys. However, at 482F, all the alloys evaluated show equivalent and significantly reduced fatigue properties compared to the room temperature data. This indicates that the beneficial effects of precipitation hardening on fatigue resistance completely disappear in the typical operating temperature range desired for engine efficiency.
A new high-temperature cast aluminum alloy 351 has been registered with the Aluminum Association by Alcoa and recently was studied for semi-permanent mold cast cylinder heads. In the study, the 351 alloy showed a significant improvement in high temperature tensile properties, 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 composition between the 351 alloy and the A356+0.5%Cu alloy. The 351 alloy contains a small amount of zircon and vanadium in addition to an increased copper and silicon content as compared to the A356+0.5%Cu alloy.
The cylinder heads were heat-treated and then samples were taken from both the deck face and high-pressure oil line locations for microstructure and mechanical property evaluation. Microstructure characterization included quantitative measurement 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 microstructure of the cylinder heads made with the 351 alloy. The 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 refinement increased porosity in the cylinder heads even though refining the grains appeared to be beneficial to the DAS. The increased porosity in the grain refined cylinder heads may be attributed to an increase in the amount of oxides generated when grain refiner was introduced into the melt. The grain size remained large in both grain refined and non-grain refined heads, above 500µm, in both fast and slowly solidified locations.
Figure 4 and Table 2 show a comparison of the tensile properties between the 351 and A356+0.5%Cu alloys at various test conditions, for specimens taken from cylinder head deck face locations. In general, the tensile strength of 351 alloy is higher than that of A356+0.5%Cu alloy.
The elongation of 351 alloy is, however, lower than that of A356+0.5%Cu alloy. The improved tensile strength may be attributed to an increased amount of Q precipitates in the aluminum matrix, and zirconium- and vanadium-containing dispersoids in the aluminum matrix and at the grain boundaries, as shown in Figure 5.
Figure 6 shows the creep strain as a function of exposure time for both 351 alloy and A356+0.5%Cu alloy tested at 300C and 22MPa. At the stress and temperature tested, 351 alloy is clearly superior to A356+0.5%Cu alloy, particularly with the increase of exposure time. The drastic improvement of creep resistance of the 351 alloy is attributed to the presence of fine, semi-coherent and thermally extremely stable zirconium- and vanadium-containing dispersoids formed during solution treatment as mentioned in previous section.
The high cycle fatigue strength of the 351 alloy and the A356+0.5Cu alloy is shown in Table 3. In comparison with the A356+0.5%Cu, the 351 alloy does not show clear superiority 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. The 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 metallographically) 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). These 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 properties of the 351 alloy in the cylinder head 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.
This 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.
Click here to see this story as it appears in the July 2018 issue of Modern Casting