Influence of pore characteristics and eutectic particles on the tensile properties of Al–Si–Mn–Mg high pressure die casting alloy

Abstract

Heterogeneous microstructure and unpredictable porosity distribution in Al–Si high-pressure die-castings give rise to a significant variation in their mechanical properties. This study investigated the influence of porosity distribution, volume of the largest pore, eutectic phase morphology and Mn-containing intermetallics on the tensile properties of Al–Si–Mn–Mg high-pressure die-castings. The as-cast material has a “network” of fine coralloid eutectic Si as well as blocky AlSiFeMn intermetallics in interdendritic area. The solution treatment step of the T7 heat treatment spheroidized the eutectic silicon. Tensile samples were tested in both the as-cast and T7 conditions. The yield strength (YS), ultimate tensile strength (UTS) and elongation (%E) of as-cast samples were in the ranges of 129–146 MPa, 242–282 MPa and 2.6–4.7%, respectively. After T7 heat treatment the YS, UTS and %E had ranged from 124-142 MPa, 195–207 MPa and 4.6–12.1%, respectively. The T7 heat treatment had little effect on the YS but reduced the UTS considerably. The enhanced %E can be ascribed to the spheroidization of eutectic Si. The largest pore by volume in the sample also plays an important role in determining the ductility according to three-dimensional (3D) pore morphologies obtained via computed tomography (CT). The sample with largest pore by volume has the lowest %E, indicating the %E can be estimated prior to tensile test based on the size of the largest pore by volume in the sample.

Introduction

Al–Si alloys are the most widely used foundry alloys in the automotive and aerospace industries due to a good combination of mechanical properties and castability [1]. Al–Si alloys are commonly used for high-pressure die-casting (HPDC), which is one of the most efficient casting processes for production of complex-shaped thin-walled castings [2]. For HPDC, the metal is injected at high speed into the cavity of a steel die and then solidifies at a high cooling rate, particularly for thin-walled castings. The process produces castings that contain heterogeneous microstructure and unpredictable porosity distribution [3].

The microstructure of a HPDC component generally consists of two distinct layers: a surface skin which has fine microstructure resulting in high integrity and strength due to its high cooling rate, and the core which exhibits a bimodal microstructure which contains coarse externally solidified crystals (ESCs) surrounded by fine grains [2,[4], [5], [6]]. The eutectic Si typically displays a coralloid morphology that is closely interconnected as a spatial network. This microstructural constituent acts as damage initiation sites and has a deleterious effect on mechanical properties, especially ductility. To improve the ductility of these cast alloys, a heat treatment is required to fragment and spheroidize the eutectic Si [7].

Two kinds of pores occur in HPDC, i.e. gas pores caused by gas entrapment during filling the die at high speed, and shrinkage pores caused by solidification contraction [8]. Through in-situ SEM tensile testing, Li et al. [8] found that large pores (e.g. gas-shrinkage pore) and clusters of small pores (e.g. net-shrinkage pore) are the dominant sources for crack initiation, and have negative effects on mechanical properties of high pressure die castings. Significant efforts have been made to understand the influence of the pore characteristics, specifically the area fraction of porosity on a fracture surface, the size of the largest pore on the fracture surface, and location of the pore on the fracture surface, on the mechanical properties, especially ductility [9,10]. Surappa et al. [9] and C$a^{\prime }$ ceres et al. [10] suggested that as the area fraction of pores on fracture surface increases, ductility and ultimate tensile strength of as-cast Al–7Si-0.3 Mg alloy decrease, whereas yield strength remains relatively unaffected. Additionally, the largest pore in the fracture surface (in the paper, the largest pore in the fracture surface is the pore with the largest projected area in the fracture surface), as well as the location of pore, also impacts ductility. X. Teng et al. [11] investigated the relationship between the ductility and the size of the largest pore in the fracture surface for Al–9Si-0.3 Mg alloy castings and revealed that the tensile fracture strain decreases with an increase of the projected area of the largest pore in the fracture surface in an approximately linear way, implying that the largest pore in the fracture surface plays a key role in governing ductility. In addition to the area fraction of the porosity in the fracture surface and the size of the largest pore in the fracture surface, the location of pore is also of importance to the ductility, and the sample with large pore on or close to the edge exhibits a low elongation [12].

Up to now, some models have been developed to predict the ductility of high pressure die castings, which can be classified into two types, i.e. empirical and analytical. Ghosh established a simplified analytical model to estimate ductility in sample with a pre-existing defect, and suggested that the value of %Eⁿ, where %E is the strain to fracture, and n is the strain-hardening exponent, decreases proportionally with the increase in the value of (1-f), where f is the area fraction covered by the defect [13]. It should be noted that the model is based on the assumption that there is only one spherical defect in the sample. However, in fact, there are a great number of defects, especially pores, in high pressure die castings. Typically, a number of defects, rather than just one, are observed in the fracture surface [14]. Therefore, validity and practicability of Ghosh's model may not correlate to actual porosity found in a die-casting.

Empirical models report that there is a clear correlation between ductility and the area fraction of pores in the fracture surface, allowing the ductility to be accurately predicted [15,16]. Nevertheless, in the case of high pressure die castings, it's still a challenge to accurately determine the area fraction of all pores in the fracture surface prior to tensile deformation, and consequently it's essential to explore a method of the estimation of ductility according to the pore characteristics in an untested sample.

Therefore, the aim of this work is to use non-destructive inspection method to quantify the porosity characteristics in a sample prior to testing and determine the impact of heat treatment on eutectic particles. The goal is to develop a method to predict ductility based on the pore characteristics in an untested sample.