Researchers aim to develop a new alloy to maximize performance of cylinder heads in diesel engines.
A MODERN CASTING Staff Report
(Click here to see the story as it appears in the March issue of Modern Casting.)
Two major incentives exist for the automotive manufacturing industry to improve efficiency. The first is customer pull: Who doesn’t want more power while simultaneously minimizing gasoline consumption? Secondly, increasing environmental regulation from governments around the world aim to reduce the environmental impact of passenger car transportation. These two competing forces have led car manufacturers to reduce engine displacement and weight while maintaining performance, mainly through the use of direct fuel injection systems and turbochargers.
These technological improvements, however, mean higher operating temperatures and higher pressures, leading to increased thermo-mechanical stress, particularly on the valve bridge between inlet and exhaust ports on the combustion face of the cylinder heads. The development of a new aluminum alloy, one that could increase performance without associated problems related to hot tearing, could be an avenue that addresses the two, often competing demands faced by automotive manufacturers. Such an alloy also may lead to uses in other applications in other industries.
Researchers Bruno Bourassa and Danny Jean, Rio Tinto Alcan, Arvida Research and Development Center, Jonquiere, Quebec, Canada; and J. Fred Major, Kingston, Ontario, Canada, investigated a possible alloy based on the 200 series and published their findings in the paper, “Alloy Development for Highly Stressed Diesel Engine Cylinder Heads.” Like the 201 or A/B206 families, this alloy provides high levels of strength and ductility. The addition of vanadium and zirconium increased the alloy’s creep resistance. Also, while low levels of titanium in the B206 alloy has improved castability and hot tear resistance, titanium was used in a high temperature alloy variant to help increase creep resistance.
Can a new generation of aluminum alloy based on the 200 series be developed to improve strength, ductility and creep resistance?
Since manufacturers began using aluminum for diesel engine cylinder heads, four generations of alloy have been developed. The primary A356 or A357 was pressed into service when thermal fatigue cracks were encountered in gasoline engines. This first generation was an alternative to the common 319-320 alloys.
The second generation began with the development of the A356+0.5% copper alloy. Currently this is the standard for diesel engine cylinder heads in Europe and has made inroads in the North American market. The addition of 0.5% copper increases the high temperature strength without notably decreasing the ductility or thermal conductivity.
Next, zirconium and manganese were added to the A356+0.5% copper alloy as dispersoid formers to inhibit creep. This development tested well but is considered evolutionary rather than revolutionary. It may help with existing designs, but it is not seen as something that allows for a significant leap in design capabilities.
From the opposite end of the spectrum, a fourth generation alloy has been developed based on A319/A320 alloys to improve performance through the expedients of working in a prime composition to eliminate brittle phases and raise the ductility to withstand hot deformation. Zirconium, manganese and vanadium were added to inhibit creep. During testing, very small levels of magnesium increased strength and fatigue performance, even though the elimination of magnesium allowed for higher solutionizing temperatures with better recovery to solution of the copper.
Shown in Table 1, six versions of alloy 224 were prepared in a 77.2-lb. (35-kg) electrical resistance tilting furnace, according to the following specifications:
- Silicon: ≤0.10%
- Iron: 0.12-0.15%
- Copper: 3.5-4.7%
- Manganese: 0.20-0.50%
- Titanium: 0.2-0.25%
- Zirconium: 0.15%
- Vanadium: 0.16-0.22%
- Magnesium: 0%, 0.1% and 0.15% (three levels were investigated)
Before casting, the molten metal was degassed using a graphite rotary impeller degasser. Argon flowing at 1.3 gal/minute (5 L/minute) was used for 10 minutes with 1.1 oz. (30 g) of cleaning salt. The cleaning salt was a 60/40 mixture of MgCl2 and KCl. The Rio Tinto Alcan small bar mold (Fig. 1) was used to make separately cast tensile bars. This mold design was based on the Stahl mold design, included as an example in the ASTM B-108 standard, but at a reduced scale. The four bars produced per shot have a 0.25 in. (0.63 cm) gauge diameter. The small size ensures a high cooling rate while also reducing restraint and lowering the odds of hot tearing. Typical Secondary Dendrite Arm Spacing (SDAS) values obtained with this mold are 20-25 μm.
Molten metal temperature in the furnace was 1,508F (820C) and casting temperature measured in the pouring ladle was 1,328F (720C). The mold temperature remained at 662F (350C) during casting. For each composition, seven pieces were cast for a total of 28 bars per alloy.
For heat treatment, the incipient melting temperature was determined by means of differential scanning calorimetry. All bars were T7 heat treated following the parameters shown in Table 2. Solutionizing included a plateau at 923F (495C) for two hours to dissolve the lower temperature copper phases and avoid incipient melting during the ramp up in temperature. After the T7 treatment, the bars destined for tensile testing at 482F (250C) were maintained at 482F (250C) for 100 hours. A similar treatment but at 572F (300C) was given to the bars destined for testing at 572F (300C).
Tensile tests on small bars were conducted on a 10 KN mechanical test frame. Parameters for tensile testing at ambient temperature, 482F (250C), and 572F (300C) are presented in Table 3. As these alloys operate at high temperature, tensile tests were carried out at three different temperatures, i.e. ambient, 482F (250C) and 572F (300C). Six bars per condition were pulled. For elevated temperature testing, a pretest soak at test temperature for 100 hours was applied. Since the operation temperature of these alloys is usually higher than the T7 temperature and the alloy is not stable, customers generally ask for properties from pieces which have been exposed to an elevated temperature for a certain time. In the present case, a 100-hour exposure time was used to allow comparison with other alloys previously designed for cylinder head applications. Bars showing inclusions in the fracture face were excluded from averages.
3. Results and Conclusions
The actual chemical compositions obtained after batching the alloy in the electrical furnace are presented in Table 1. The vanadium level of the 224+VZrMgCu3.6 versions is slightly lower than the target, which could be due to the settling of vanadium containing phases during batch preparation.
Tensile results are presented in Table 4. Alloys with 0.10% and 0.15% Mg at 4.6% Cu and alloys with 0.35% Mg at 3.6% Cu surpass all other alloys at all temperatures evaluated. The only alloy that has yield strength properties at elevated temperature, similar to those of the 224 variants, is the AlCu5NiCoZr_T7 (AA 203).
An interesting point is the wide range of yield strengths shown between the lower level of magnesium and the highest one. This significant difference is observed at both 3.6% Cu and 4.6% Cu, but tends to be more pronounced at higher copper level. The researchers believe magnesium accelerates the rate of Al-Cu precipitation hardening reaction in Al-Si-Cu(Mg) and Al-Cu(Mg) systems at these levels, which might mean the two elements behave synergistically. 224+VZr4.6Cu_T7 and 224+VZr0.1Mg3.6Cu_T7 show similar trends for all temperatures. Both alloys present a lower yield than that of the A356 family at ambient temperature, but higher at elevated temperature. 224+VZr0.15Mg3.6Cu_T7 shows the same trend than this lower magnesium version but with higher properties. Ambient properties are equivalent to those of the A356 family and superior at elevated temperatures.
The same pattern is observed in ultimate tensile strength (UTS). At an elevated temperature, the 224 variants show higher properties than other alloys, with the exception of the AlCu5NiCoZr_T7. Also, a general trend is that the higher the level of copper and magnesium, the higher the mechanical properties. The same is true for the yield strength; a significant difference is observed between the lower and the highest level of magnesium for each level of copper. Again, 224 alloys with 3.6% Cu have an ambient temperature UTS in the same range of the A356 family, while the higher copper versions have higher properties for all temperatures.
There is a tradeoff between strength and elongation at break. The 224 alloys are not an exception to this rule and do not perform as well as other alloys. Figure 2 presents the evolution of elongation at break in function of the temperature. At room temperature, 224 alloys are not totally outclassed but, contrary to other alloy behaviors, elongation at break does not increase when the temperature augments. AlCu5NiCoZr_T7 and AlSi5Cu3Mg_F have elongation at break lower or equivalent to that of the 224 family. Elongation of the first alloy can be explained by the fact that it has high strength like 224 alloys while the low elongation of the second alloy is a result of its high iron level.
Figure 3 shows the evolution of the quality index as a function of temperature. Lower Cu versions of 224 show a quality index superior to other engine alloys for 0.15% and 0.35 % versions, but similar for the lower magnesium version (0.10%). For all test temperatures, a significant improvement in the quality index is visible when the copper level increases from 3.6% to 4.6%.
Other conclusions from this research include:
- The 224 alloy variant with 0.15% Mg has the highest strength across all temperature ranges in comparison to the other alloys.
- Al-Cu based alloys provide higher performance in general terms compared to conventional Al-Si based alloys.
- The higher strength of the Al-Cu alloys does limit the ductility.
- The quality index shows that the 224 alloys studied in this work give the best results considering both strength and ductility.
- The 224 alloys with added dispersoids resulted in superior creep strength to similar 319 alloys under comparable loading and temperature.
This article is based on paper 14-008 that was presented at 2014 AFS Metalcasting Congress.