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Table 1. Impact of Height Above the Chill on the Properties of 6061 0 mm


Ultimate tensile strength (MPa) Yield strength (MPa) Elongation (%)


Table 2. Impact of Homogenizing Time on the Properties of 6061


Ultimate tensile strength (MPa)


Yield strength (MPa) Elongation (%)


6 hrs 369


334 7.2


12 hrs 370


331 7.7


385 340 9.6


10 mm 373


332 8.9


18 mm 364


335 5.6


Table 3. Impact of Iron level on the Properties of 6061


Ultimate tensile strength (MPa)


Yield strength (MPa) Elongation (%)


Table 4. Impact of Height Above the Chill on the Properties of 6082 0 mm


Ultimate tensile strength (MPa) Yield strength (MPa) Elongation (%)


Table 5. Impact of Homogenizing Time on the Properties of 6082


Ultimate tensile strength (MPa)


Yield strength (MPa) Elongation (%)


nificant differences between the iron phases encountered in 6061 and those in 6082, with respect to their distance from a chill (a metal insert used in a casting mold to induce rapid solidi- fication of certain pockets of metal). In this study, the chill was positioned at the bottom of the copper mold. Because the mold sides are composed of sand, a majority of the heat is drained through the bottom. Te dif- ferences still were significant after six hours of heat treating at 986F (530C) and 1,058F (570C) (Figs. 1 and 2). A comparative image analy-


3


sis was performed between 0.1 and 0.15% iron content in the alloys, with homogenizing time for the 6061 variants. Te researchers said the plots showed little advantage was gained


Results and Conclusions


Te study of the as-cast


microstructure of the two alloy families showed sig-


6 hrs 372


343 4.6


12 hrs 375


340 5.8


390.52 346.15 8.85


10 mm 373.77 340.09 5.43


0.10% 368


331 7.2


0.15% 372


339 7.5


25 mm 354.36


321.79 5.7


via heat treatment may be possible, the researchers said, and the casting process used is critical to the success of the alloys’ production. Emerging casting processes like the ablation process (in which water is used to strip a sand mold away during cooling to achieve elevated microstructure) have shown little tendency towards hot tearing. Te researchers indicated the


18 mm 370.67 345.01 3.82


Table 6. Impact of Iron Level on the Properties of 6082


Ultimate tensile strength (MPa)


Yield strength (MPa) Elongation (%)


0.10% 374


339 5.8


0.15% 373


344 4.6


from the 24-hour homogenizing time compared to the 12-hour time in terms of mean or maximum iron phase length. Te 24-hour soak also resulted in a phenomenon known to reduce toughness in wrought 6061. Te mechanical property study also


analyzed differences with respect to the chill for both alloy families. Te influence of three different variables is reported in Tables 1-6. In the hot tearing and chemistry


study, the researchers observed that adjustment of the chemical composi- tion and the use of appropriate alloy additions can result in a hot tearing index lower than expected for the alloy. Tey reported semi-solid casting pro- duced a lower hot tear index compared to conventional diecasting for most 6000 series alloys. Te study indicated mechanical


properties exhibiting both high duc- tility and strength can be achieved in a cast product. Further optimization


25 mm 358.88 334.25 2.85


following conclusions based on the results of the procedure: • Wrought alloys of the 6000 series can be cast into intricate shapes if both the chemistry and casting process are optimized to control the incidence of hot tearing.


• The iron phases inherent in 6000 series wrought alloy compositions must be controlled via a combina- tion of freezing rate, iron content and thermal dissolution or break- up of the phases during the heat treat process.


• Alloy 6082 showed a more rapid deterioration in ductility with slower cooling rates than 6061.


• Between the 6061 and 6082 alloys, 6061 was more amenable to microstructural control and high mechanical properties. The alloy’s microstructure was more amenable to control via heat treatment.


• For 6061, six hours at 1,058F (570C) yielded sufficient conver- sion of the microstructure. The 6082 alloy benefited from a longer homogenizing treatment of 12 hours.


• Beyond 12 hours, both alloys showed a reduction in toughness.


• Iron levels between 0.1 and 0.15% seemed equally effective in 6061. The 6082 alloy showed a greater tendency towards deg- radation in properties with an increased iron level.


• Composition and alloy treatment with grain refiners and strontium can affect the tendency towards hot tearing of alloys like 6061.


• Casting process variables also affect the properties achievable in 6061 alloys, with marked differences seen among semi-solid castings, die castings and components cast in the ablation process.


February 2012 MODERN CASTING | 47


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