Table 2. Effect of Solution Heat Treat Temperature on Hardness Alloy


Solution Temp. (F)


13


13a 15


15a 1,040 Brinell Hardness


As Cast


83


1,094 83 1,040 1,094


79 79


3


Solution Treated


77 56 83 81


Aged for


15 min. 2 hr. 6 hr. 113


90 77


96 90 84 76 82


79 79


77 80


Results and Conclusions T e compositions of fi ve alloys tested in this


investigation are presented in Table 1. T e fi rst three alloys (10, 11 and 12) were solution treated at 977F (525C) and aged at 572F (300C). T e


hardness results (Fig. 1) indicated the following: Alloys 10 and 11 have low hardness in the as-cast condi-


tion. Alloy 12 has high hardness. T is is due to the eff ect 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. T e aging treatment does not signifi cantly improve the


hardness of alloys 10 and 11. Alloy 12 exhibits a peak hard- ness value at a shorter duration, which rapidly decreases as the aging time is increased. T e fi nal hardness of all the alloys is similar to or lower than that of the as-cast base alloy.


Alloy


10 11


12 13


13a 14


14a


As Cast


62 59 81


83 83


79 79


Brinnel Hardness


Solution Treated


48 53 68 77 56


79 79


Aged, 6 hr.


63 60 69 77 84 76 80


Exposed, 1000 hr.


62 57 60 71


80 75 75


T e best performance of the three was that of the aluminum- copper-magnesium-scandium alloy. However, its fi nal hardness was low, possibly due to the lack of scandium for precipitation during aging treatment. T e solubility of scandium in aluminum increases with solution temperature and reaches 0.2% at about 1,112F (600C). T e heat treatment had been restricted to 977F (525C) due to the high copper content of alloys 10, 11 and 12. T erefore, 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). T e 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 treat- ment, these alloys were aged at 572F (300C). T e 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.


• T e hardness of alloy 13 increases after aging treatment. T e increase is not signifi cant 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 ag- ing, and the material stabilizes more rapidly.


After heat treatment, the alloys were subjected to long-


term exposure at 482F (250C) for 1,000 hours. T e hardness values of the samples are reported in Table 3. T e results indicate alloy 13a retains its hardness. All other alloys exhibit softening to various degrees. T e 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 cylin-


der head with section sizes varying from 3 to 0.6 in. (15 mm) was cast (Fig. 2). T is part is currently produced from alloy A242, which exhibits good high temperature strength and moderate castability. T e experimental alloy showed improved elevated temperature


castings consistently with this alloy composition.  This article was adapted from a paper that appeared in the 2011 American Foundry Society Transactions.


44 | METAL CASTING DESIGN & PURCHASING | May/Jun 2012


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 fi lling problems and yield good


Table 3. Effect of 1,000-Hour Exposure at 482F on Alloy Hardness


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