3D morphological characteristics of shrinkage porosities and their relationship with microstructures in Mg−12Al magnesium alloy

Abstract

The dependence of shrinkage porosities on microstructure characteristics of Mg−12Al alloy was investigated. The distribution, morphology, size, and number density of shrinkage porosities were analyzed under different cooling rates. The relationship between shrinkage porosities and microstructure characteristics was discussed in terms of temperature conditions, feeding channel characteristics, and feeding capacity. Further, the feeding behavior of the residual liquid phase in the solid skeleton was quantified by introducing permeability. Results show a strong correlation between the solid microstructure skeleton and shrinkage porosity characteristics. An increase in permeability corresponds to a declining number density of shrinkage porosities. This study aims to provide a more complete understanding how to reduce shrinkage porosities by controlling microstructure characteristics.

1. Introduction

Magnesium alloys are the lightest metallic structural materials with high specific strength, excellent shock absorption, good electromagnetic shielding, and high thermal conductivity [1−6]. The distinctive advantages lead to extensive applications in various fields, such as automotive, aerospace, electronics, and medical devices [3,7−13]. The cast Mg alloys, as major categories of Mg alloys, are formed by smelting and solidification. Because of the broad solidification temperature range and resultant mushy solidification behavior, the pores formed due to volume contraction are difficult to feed in most cast Mg alloys [14]. Once not fully fed by the liquid, the pores will become shrinkage porosities, one of the most common defects in the castings [15−18], which largely deteriorate mechanical properties, e.g., causing the stress concentration and accelerating the crack initiation and propagation [16,19−24].

The formation of shrinkage porosities involves metal flow, heat transfer, and phase transformation, which are closely related to solidification processes and casting conditions [25]. To reduce and eliminate shrinkage porosity, a large number of studies have been conducted to establish the relationship between casting conditions and shrinkage porosity features. LEE et al [26] compared the influence of the gate speed, intensification pressure, and melt temperature on porosity distribution in high-pressure die-cast AM50 alloys. MI et al [27] simulated the low-pressure die-casting of wheel hubs and found that the rim−spoke connection is susceptible to shrinkage porosities, whereas adding the cooling system can effectively reduce shrinkage porosities. ZHANG et al [28] investigated the effects of vertical centrifugal casting parameters on shrinkage porosities in basin-shaped castings and found that the mold preheating temperature is the most significant parameter. LEE et al [29] studied the effects of process parameters on shrinkage porosities in high-pressure die casting, providing a correlation between process parameters and shrinkage porosities for a given geometry. LI et al [30] found that the porosities exist in thick walls and thick-thin wall junctions of cylinder heads, suggesting that regular shapes and thin walls are advantageous for reducing porosities.

These studies focus on optimizing process parameters...