Research on Grain Refinement in Hypoeutectic Al-Si Alloy during Solidification under an Alternating Electric Current Pulse

Ning Li,Hongjing Wu,Hui Xing,Rong Zhang,Pengfei Yin,Limin Zhang

doi:10.3390/met9050571

Abstract

Experiments on transient directional solidification were carried out to study the columnar to equiaxed transition (CET) in Al-7Si alloy without and with an alternating electric current pulse (AECP). Without AECP, the macrostructure consists of typical columnar and equiaxed zones, separated by a near horizontal plane. As the AECP is applied during solidification, an additional fine equiaxed zone (FEZ) occurs in the as-cast macrostructure. From measured temperature profiles, cooling rate and temperature gradient are determined. It is found that CET occurs for a critical value of the cooling rate, which is observed to be about 0.14 K·s−1 in the present investigation. Furthermore, the macrostructural observation with mold for embedding the mesh plate demonstrates that the major factor responsible for the formation of fine equiaxed grains is the detachment of crystal nuclei from the upper contact surface and the lateral wall. The detachment is in turn ascribed to electric current-associated free energy change (ΔGe)-induced the driving force F.

Highlights

The mechanical properties of cast alloys can be enhanced by grain refinement or microstructure modification
Since the effect of alternating electric current pulse (AECP) on temperature data was found to be qualitatively identical for the present applied current amplitude, it was decided to give the results obtained during solidification under AECP of 400 A priority in the following discussion
Without AECP treatment, the release of latent heat is just visible in the signal of the thermocouple at 85 mm as a plateau, both at primary phase growth at the liquidus temperature with some undercooling, as well as at the eutectic reaction with some undercooling (Figure 2a)

Summary

Introduction

The mechanical properties of cast alloys can be enhanced by grain refinement or microstructure modification. Nakada et al [16] found a transition from columnar to globular grains in Sn-15Pb alloys when a high ECP by discharging a capacitor bank was applied They concluded that the strong local shear force could break off the dendrites. From experimental investigation of the relevance of forced melt flow to the solidification structure of pure Al in the mold for embedding the wire mesh under an alternating electric current pulse (AECP). Since current density is closely related to most of the current effects such as Joule heating, liquid–solid interfacial energy and electric current-associated free energy change (∆Ge ) etc., it should be the dominant ECP parameter for grain refinement during solidification. Under the condition of the negligible convection, we tried to reveal the role of ∆Ge on grain refinement It will provide a better understanding of the complicated refinement mechanism under AECP

Experimental Procedures

Results and Discussion

Conclusions