Effect of Cooling Rates on the Microstructure and Mechanical Property of La Modified Al7SiMg Alloys Processed by Gravity Die Casting and Semi-Solid Die Casting

Longfei Li,Jian Feng,Yongzhong Zhang,Daquan Li,Yonglin Kang

doi:10.3390/met10040549

Abstract

Rare earth (RE) additions are capable of refining the α-Al phase as well as modifying the eutectic Si particles of alloys. The cooling rate in casting process should be carefully concerned when the Al-Si alloys are refined and modified by adding RE elements. In this study, the effect of cooling rates on the microstructure and mechanical properties of La modified Al-7.0Si-0.3Mg alloys was studied in gravity die casting and semi-solid die casting. It is found that in La modified Al-7.0Si-0.3Mg alloys, with increasing the cooling rate from 0.2 to 9 K/s in gravity die casting, the α-Al grains are greatly refined and the Si particles are modified to branching morphology, which evidently increases the UTS and elongation of alloys. In addition, when increasing the cooling rate from 30 to 130 K/s in semi-solid die casting, the α-Al grains are refined from 140 to 47 μm, and the Si particles are modified to fibrous morphology, which increases the UTS from 190 to 230 MPa and elongation from 10% to 11%. However, the 0.4 wt.% La addition results to La-rich phases formed in microstructure, which impairs the mechanical properties of Al-7.0Si-0.3Mg alloys in semi-solid die casting.

Highlights

The Al-Si-Mg alloys, which owe excellent casting characteristics including good fluidity and less solidification shrinkage [1,2], are broadly applied in casting industry [3,4,5]
It is obvious that the refining effect is produced with La additions, the α-Al grains have smaller dendrite arm spacing (DAS) and grain size with increasing cooling rates
It is concluded that the grain size and DAS of the α-Al phase are significantly decreased with increasing cooling rate

Summary

Introduction

The Al-Si-Mg alloys, which owe excellent casting characteristics including good fluidity and less solidification shrinkage [1,2], are broadly applied in casting industry [3,4,5]. The chemical refinement and modification, which is achieved by adding small amount modifying elements (Ti, Sr, rare earth), is a useful method to refine the α-Al grains and improve the Si morphology, and enhance the mechanical properties of alloys [8,9,10]. The RE elements, which are able to refine the α-Al grains and form the flake-to-fiber transition in eutectic Si, are extensively studied in Al-Si-Mg alloys. Kang et al [11] found the synergistic addition of Ce (0.1 wt.%) and Mg (0.7 wt.%) has an obvious refining effect on α-Al grains and modifies the eutectic Si to fibrous morphology. Emamy [12,13] studied the effect of

Methods

Results

Discussion

Conclusion