Grain Refinement Effect on the Hot-Tearing Resistance of Higher-Temperature Al–Cu–Mn–Zr Alloys

Abstract

The hot-tearing resistance of Al-Cu-Mn-Zr (ACMZ) alloys was investigated as a steptoward introducing these new cast alloys for severe duty, higher-temperature applications, such ascylinder heads for down-sized, turbocharged automotive engines. Alloy Cu compositions werevaried from 5 to 8 wt.%. Targeted Ti levels were 0.02, 0.1, and 0.2 wt.% via additions of the Al−5Ti−1B master alloy. Hot-tearing resistance was assessed by visual examination and ranking of thecracking severity in a multi-arm permanent mold casting. It was found that at high impuritycontents (Fe and Si of 0.2 wt.% each), the Al−Cu−Mn−Zr alloy with 4.95 wt.% Cu exhibited thepoorest hot-tearing resistance, irrespective of the grain refining amount. Microstructural analysisindicated an effective reduction in the grain size, as the Ti additions were increased to 0.02 and 0.1wt.% Ti via the Al−Ti−B grain refiner. The finest grain size was attained with a 0.1 wt.% Ti. Basedon the hot-tearing evaluation, it was found that the additional grain refining via the Al−5Ti−1Bmaster alloy at 0.1 wt.% Ti significantly reduces the hot-tearing susceptibility at Cu contents greaterthan 7.3 wt.% for ACMZ alloys with low Fe and Si. These findings indicate that the best hot-tearingresistance was observed at a grain refiner level of 0.1 wt.% Ti and high Cu content (greater than 7.3wt.%). This study to indicates that these Al−Cu−Mn−Zr alloys, which possess excellentmicrostructural stability and mechanical properties at elevated temperatures, can also possessexcellent hot-tearing resistance.

Keywords

• casting, hot-tearing, aluminum, al-cu alloys