COARSENING KINETICS IN Al ALLOY CAST THROUGH RAPID SLURRY FORMATION (RSF) PROCESS

Robin Gupta,Lorenz Ratke,Upender Pandel,Ashok Sharma

doi:10.12776/ams.v23i1.835

Abstract

In the present investigation, the rate of coarsening kinetics was discussed on conventional diffusional Lifshitz, Slyozov and Wagner (LSW) theory, coarsening by convective mass flow and coalescence by stirring speed. The rate of coarsening kinetics was found to be faster than predicted by diffusional LSW theory, but slower than predicted by the convective mass flow. The effect of flow pattern with baffles in a crucible at different holding time was also examined. The results showed that use of baffles produced fine and more globular primary α-Al phase when compared to sample processed without baffles having the same holding time.

Highlights

The RSF process depends on enthalpy exchange between two alloy systems having different enthalpies levels but with same composition [1]
The results showed that as the stirring speed increased the globularity of the primary α-Al phase increased and as the holding time increased the average grain size of primary α-Al phase increased
The theoretical observations by Takajo et al [17] showed that a combination of Ostwald ripening and coalescence with a size independent collision rate leads to the well-known Ostwald ripening law, but the rate of coarsening kinetics depends on the collision rate

Summary

Introduction

The RSF process depends on enthalpy exchange between two alloy systems having different enthalpies levels but with same composition [1]. The mechanism for Ostwald ripening was given by the Lifshitz, Sloyozoy, and Wagner and known as LSW theory. Birol [3] studied the coarsening kinetics of A357 alloy cast through cooling slope process. Lashkari et al [4] studied the coarsening of the semi-solid A356 alloy at pouring temperature of 595oC Their observation showed that as the holding time increased at that point coarsening of primary α-Al phase mainly occurred due to the two mechanisms, Ostwald ripening and coalescence. Haghdadi et al [6] observed the coarsening rate constant of A356 alloy cast through the accumulative back extrusion (ABE) process. In the present investigation to understand about the coarsening kinetics in RSF process various mechanisms e.q. LSW theory, convective mass flow, and coalescence by shear flow were discussed. Baffles impede rotational flow and diminished unwanted flow and due to this, the samples processed with baffles comprised the fine and more globular primary α-Al phase than the samples processed without the use of baffles

Schematic diagram of different stage involved in the RSF process

Variation of a cube of mean particle size with holding time

Variation of the square of mean particle size with holding time

Conclusions