Semi-solid process of 2024 wrought aluminum alloy by strain induced melt activation
Surachai Numsarapatnuk and Sukangkana Chayong
pp. 569 - 577
Abstract
The aim of this study is to develop a production process of a fine globular structure feedstock of the 2024 aluminum
alloy suitable for subsequent semi-solid forming. The 2024 wrought aluminum alloy was first annealed to reduce the effect of
work hardening. Then, strain was induced in the alloy by cold compression. After that the microstructural evolution during
partial melting was investigated. The samples were subjected to full annealing at 415°C for 3 hrs prior to cold compression of
40% reduction of area (RA) with 3 mm/min strain rate. After that samples were partially melted at 620°C with varying holding
time from 0 to 60 min followed by water quenching. The grain size and the average grain diameter of solid grains were
measured using the linear intercept method. The globularization was interpreted in terms of shape factor. Liquid fraction and
the distribution of the eutectic liquid was also investigated. It was found that during partial melting, the globular morphology
was formed by the liquid wetting and fragmentation of high angle boundaries of recrystallized grains. The suitable semi-solid
microstructure was obtained from a condition of full annealing, 40% cold working and partial melting at 620°C for 6 min
holding time. The near globular grains obtained in the range of 0-60 min consisted of uniform spheroid grains with an average
grain diameter ranged from 73 to 121 m, quenched liquid fraction was approximately 13–27% and the shape factor was greater
than 0.6. At a holding time of less than 6 min, grain coarsening was dominant by the immigration of high-angle grain
boundaries. At a longer holding time, liquid fraction increased and Ostwald ripening was dominant. The coarsening rate
constant for the 2024 Al alloy was 400.36 mm3
.s-1. At a soaking time of 60 min, it was found that a minimum diameter difference
was 1.06% with coarsening index n=3 in a power law equation. The non-dendritic slug of 2024 alloy was rapid compressed
into a disc with 90%RA and exhibited uniform structure.
