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.
Highlights
Hot tearing is a major casting defect that is difficult to overcome, especially for multicomponentAl-Cu alloys, which are well known to be prone to hot tearing in semi-permanent mold castings used for automotive cylinder heads
Hot-tearing resistance has been assessed based on the examination of cracking in castings after they are cooled to room temperature in regions where cross-sectional size changes by employing molds that provide geometrical constraints, in geometries that mimic features from production castings, such as ring [6], “dog-bone or I-beam” [7,8], or “finger-mold” [9,10] in sand molds [6,11,12,13,14] or permanent molds [15]
It was found that the optimum wt.% Ti of 0.1 for ACMZ alloys is within the range recommended for the low-impurity and low-Ti 206 alloy [27] but larger than that recommended for the A206 and B206 alloys [9,33]
Summary
Hot tearing is a major casting defect that is difficult to overcome, especially for multicomponent. The objective of this study was to investigate grain refinement effects on the hot-tearing resistance of a new family of cast Al-Cu alloys containing Mn and Zr additions These alloys have been designed for microstructural stability and retention of mechanical properties at higher temperatures (>300 ◦ C) and are referred to as Al-Cu-Mn-Zr (ACMZ) alloys [1]. This difference could be due to the lower Fe content in the alloy used by Li et al [27] or different pouring temperatures, which were not specified in that study [9] This comparison illustrates that hot-tearing resistance does not necessarily improve after grain refinement of Al–Cu alloys, and suggests significant sensitivity to alloy composition including impurity levels. This effect was explained by the fact that dendrites are smaller, almost globular, and not as highly branched as those in the non-grain-refined alloys, allowing easier grain reorientation and sliding after the coherency point, moving the onset of strength at higher solid packing as solidification proceeds
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