Al-Mn based alloys with high-manganese content are expected to have improved mechanical properties due to solid solution hardening and/or dispersion hardening. However, the increase of Mn solubility of the alloy is difficult by using the conventional DC casting. In order to solve this problem, in the present study, we focused on the twin-roll casting method which is characterized by high cooling rates. Several kinds of high Mn-containing Al-Mn-Si alloy strips were fabricated by using a vertical-type high-speed twin-roll caster equipped with a pair of water-cooled copper rolls. Direct temperature measurement of the liquid melt during the casting was also performed. The alloy strips of various compositions containing up to 4 Mn and 2 Si (wt%) were successfully obtained. By observing the microstructure of the cross section of the strip, we found the characteristic solidified structure. The solidified structure consisted of three layers. Two solidified shells with a columnar dendrite structure grew from the roll surfaces toward the strip center. In the mid-thickness region, the band structure consisting of equiaxed dendrites and globular grains was observed between the solidified shells. Very fine primary particles were observed in the matrix near the strip surface, while, relatively coarse particles with blocky and needle-like shape were observed in the central band of the as-cast strip. The electric conductivity measurement was performed for the as-cast strips. Mn solubility in Al matrix was estimated from the obtained values. The estimated Mn solubility in the Al-2Mn-xSi strips was between 1.5 ~ 1.8wt% Mn. It was over 1.43wt%Mn for the Al-4Mn-xSi strips. We found that the Mn solubility of the as-cast strips was considerably high. The strips were cold-rolled to the sheets and then annealed at various conditions. They were subjected to the tensile tests, and the effects of solid solution hardening and dispersion hardening are discussed.
A color metallography using Weck's reagent was employed to investigate the characteristic microstructure of AlMnSi alloy strip fabricated by high-speed twin-roll casting. The microstructure of the strip consists of two components: solidified shells and a central band. By Weck's reagent etching, the colorful microstructure was obtained, and doughnut-like patterns were observed in the globular grains. Based on the presence of the patterns, the globular grains were divided into two types: Type-I and Type-II. Type-I grains exhibited the core-like structure. On the other hand, Type-II grains had no color contrasts in the grain. SEM-EDS analysis of Type-I grains revealed the high correlation between the obtained color and micro-segregation of Si. TEM and STEM analyses confirmed the formation of an amorphous film on the surface of Al substrate by the etching. The thickness of the film and the roughness of the Al substrate under the film were different from location to location. The local change of the film's features resulted in the different color in the optical microscopic image. Based on the microstructure observation, the origin of globular grains observed in the central band in the AlMnSi alloy strip was discussed in detail.
Vertical-type high-speed twin-roll casting (VT-HSTRC), which is characterized by a high production rate and cooling rate, is a promising method for upgrade recycling of aluminum cast alloy scrap to wrought alloys in the near future. To produce wrought alloy sheets from cast alloy scrap, the strips must be isotropic to achieve good formability. However, in cold-rolled and annealed Al7% Si alloy and A356 alloy sheets fabricated from the HSTRC strips, average elongation is much greater in the rolling direction than in the transverse direction. This elongation anisotropy results from both the morphology and the alignment of eutectic Si particles. In the present study, the effect of homogenization heat treatment on the microstructure and elongation was investigated. Al7% Si and Al11% Si alloy strips were fabricated by HSTRC and were homogenized by heat treatment at 540°C for 10 h and 500°C for 10 h, respectively. The strips were cold rolled at a reduction rate of 50% and annealed. The eutectic Si particles were spheroidized and coarsened by the homogenization heat treatment, and they were uniformly dispersed after cold rolling. There was no significant difference in elongation between the rolling and transverse directions in the Al7% Si and Al11% Si alloys. These results show that the homogenization heat treatment of the strips reduced the elongation anisotropy.
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