Handling 200-Series Aluminum
Cast aluminum alloys are used in many automotive applications to reduce weight and improve efficiency and vehicle performance. Some major uses for aluminum alloys include gasoline engine block and cylinder head castings, suspension components and turbine compressor wheels. The approximate temperature limit for 354 aluminum heat treated to the T61 temper is 347F (175C). The maximum operating temperature of most structural aluminum alloys, both cast and forged, is about 392F (200C). Historically, aluminum alloys have scarcely been used above that temperature.
The need for aluminum alloys with improved creep resistance at increasing temperatures has intensified the search for alloying elements that produce stable precipitates. David Weiss, Eck Industries, Manitowoc, Wis., Gerald Gegel, Material & Process Consultancy, Morton, Ill., and Kumar Sadayappan, Materials Technology Laboratory, Ottawa, Canada, recently set out to study those alloying elements.
Question: How can 200-series aluminum be used to produce quality castings for high temperature applications?
Demands from industrial and military sectors require lightweight alloys that can be used in the 482-572F (250-300C) temperature range. For example, the need to reduce the exhaust emissions of medium- and heavy-duty diesel engines has led to the use of two-stage series turbocharger air system designs. Single stage compressors run at an outlet temperature of about 347F (175C) at sea level, which is the approximate temperature limit of the 354-T61 aluminum alloy used for cast impellers. Second-stage outlet air temperatures are predicted to reach 500F (260C) or higher at sea level conditions, and this temperature will increase with operational altitude.
The present research effort and paper, “Development of Cast Al Alloys for Elevated Temperature (250C) Service,” intended to find an aluminum-copper-scandium alloy for elevated temperature applications and determine thermal treatment and optimal casting parameters for the alloy. Ultimately, the alloy will be used to produce a metal matrix composite.
An induction furnace was used to prepare five experimental alloys. Either A206 ingot or pure aluminum was the starting material for preparing the alloys. Alloying additions were made as master alloys or pure metals. In each experiment, 44 lbs. (20 kg) of the alloy was prepared. The composition of the alloy was tested using optical emission spectrograph before casting. Most of the melts were designed as split melts with extra alloy additions and carried out after the first set of experiments. The final composition was evaluated by wet chemical analysis. Permanent mold cast plates (6 x 4 x 0.5 in. [150 x 100 x 12.5 mm]) or rods (0.7-in. [19-mm] diameter) were produced to obtain test coupons.
The test coupons were subjected to thermal treatments designed to facilitate the precipitation of Al3Sc. The solution treatment temperature varied from 977F to 1,094F (525C to 590C). The coupons were aged at 572F (300C) for several different durations. Hardness was measured on the test coupons to assess strength for each alloy chemistry and heat treatment. After optimization, some of the selected alloys were soaked at 482F (250C) for up to 1,000 hours. Their response was measured using hardness testing. After heat treatment, the alloys were subjected to long-term exposure testing at 482F (250C).
To establish casting process parameters and evaluate the castability of one of the alloys, a 1,000-lb. lot was commercially prepared and tested. In preparation for determining tensile and fatigue properties of the alloy at elevated temperatures, a study was performed to establish the heat treatment parameters that would maximize the properties at 482F (250C).
3. Results and Conclusions
The compositions of five alloys tested in this investigation are presented in Table 1. The first three alloys (10, 11 and 12) were solution treated at 977F (525C) and aged at 572F (300C). The hardness results (Fig. 1) indicated the following:
- Alloys 10 and 11 have low hardness in the as-cast condition. Alloy 12 has high hardness. This is due to the effect of magnesium, which provides some solid solution strengthening. After the solution treatment, all three alloys exhibit softening, but alloy 12 remained stronger than the other two.
- The aging treatment does not significantly improve the hardness of alloys 10 and 11. Alloy 12 exhibits a peak hardness value at a shorter duration, which rapidly decreases as the aging time is increased. The final hardness of all the alloys is similar to or lower than that of the as-cast base alloy.
The best performance of the three was that of the aluminum-copper-magnesium-scandium alloy. However, its final hardness was low, possibly due to the lack of scandium for precipitation during aging treatment. The solubility of scandium in aluminum increases with solution temperature and reaches 0.2% at about 1,112F (600C). The heat treatment had been restricted to 977F (525C) due to the high copper content of alloys 10, 11 and 12. Therefore, the solution temperature was increased and the copper content held to less than 2% for further testing.
Alloys 13 and 15 contain 2% copper but were solution treated at 1,040 and 1,094F (560 and 590C) instead of 977F (525C). The alloys treated at 1,040F were designated as 13 and 15, and those solution treated at 1,094F were designated as 13a and 15a. After this high temperature solution treatment, these alloys were aged at 572F (300C). The hardness test results (Table 2) indicated the following:
- Solution treatment at 1,094F results in alloys with a softer matrix compared to those solution treated at 1,040F.
- The hardness of alloy 13 increases after aging treatment. The increase is not significant for alloy 15.
- Hardness reaches a peak just after 15 minutes of aging treatment for alloy 13. Longer holding times result in reduced hardness.
- Alloy 13a is more stable than the other alloys. Although the initial hardness of alloy 13a is lower compared to other alloys, the hardness is higher after six hours of aging, and the material stabilizes more rapidly.
After heat treatment, the alloys were subjected to long-term exposure at 482F (250C) for 1,000 hours. The hardness values of the samples are reported in Table 3. The results indicate alloy 13a retains its hardness. All other alloys exhibit softening to various degrees. The higher stability of the alloy can be attributed to the presence of scandium trialuminides.
In order to assess the castability of alloy 13, an air-cooled cylinder head with section sizes varying from 3 to 0.6 in. (15 mm) was cast (Fig. 2). This part is currently produced from alloy A242, which exhibits good high temperature strength and moderate castability.
The experimental alloy showed improved elevated temperature strength with similar casting characteristics to alloy A242 and improved castability over 206-type alloys. It is anticipated casting practice can be adjusted to avoid filling problems and yield good castings consistently with this alloy composition.
This article was adapted from a paper that appeared in the 2011 American Foundry Society Transactions.