Effects of (Al−Ti−La+Nd) modification and heat treatment on microstructure and properties of Al−7Si alloy

Abstract

The effects of adding a novel Al−3Ti−4.35La master alloy and Nd and heat treatment on the microstructure and mechanical properties of Al−7Si alloy were investigated. The results showed that the secondary dendrite arm spacing of α-Al in the as-cast Al−7Si alloy was refined from 18.3 to 11.9 μm after modification with 0.2 wt.% Al−Ti−La and 0.03 wt.% Nd, and the length of eutectic Si was reduced from 8.6 to 5.0 μm. After heat treatment at 535 °C for 3 h followed by 165 °C for 3 h, the morphology of the eutectic Si became more rounded, and the size decreased. The microhardness, ultimate tensile strength, and elongation were HV 66.1, 184.9 MPa, and 24.4%, respectively, which increased by 24.2%, 11.6%, and 194.0% compared to the as-cast state. The addition of Al−3Ti−4.35La master alloy and Nd can reduce the nucleation temperature of eutectic Si in Al−7Si, thereby suppressing its growth. Notably, the Ti2(Al,Si)20(La,Nd) phase formed in the Al−7Si alloy after the addition of Al−Ti−La and Nd adhered to or coexisted near the eutectic Si particles, inhibiting their growth.

1. Introduction

Hypoeutectic Al−Si alloys have garnered extensive application in automotive and aerospace industries due to their advantageous properties, including low density, exceptional castability, minimal thermal expansion, and superior mechanical properties [1,2]. In recent years, there has been an increasing trend of using high-strength aluminum alloys to partially replace structural steel to reduce the weight of automobiles [3,4], thereby imposing strict requirements for the mechanical properties of aluminum alloys.

The solidification of eutectic Al−Si alloys typically results in a microstructure dominated by coarse α-Al grains and needle-like eutectic Si phases. These morphological features are directly responsible for the compromised mechanical properties of as-cast alloys, particularly their low fracture toughness [5]. Modification treatments are routinely applied to molten Al−Si alloys to enhance their mechanical performance. The primary function of these modifiers is to change the morphology of the eutectic Si, transforming it from a long needle-like form to a short rod-like or particulate form. The elements that can modify the eutectic Si phase include Na (sodium), Sr (strontium), P (phosphorus), Ba (barium), Sb (antimony), and RE (rare earth elements), etc [6]. After modification, the as-cast eutectic Al−Si alloy comprises primary dendrites with nonuniform solute distributions and defects such as shrinkage voids [5]. T6 heat treatment is typically implemented to improve the mechanical properties of the Al alloy, which includes solution treatment, water quenching, and artificial aging. Currently, numerous studies have analyzed the impact of various heat treatment processes on the microstructure and mechanical properties of Al−Si alloys, including different solution treatment conditions [7−10], quenching rates [11−13], aging conditions [14−16], and alloying element content [17,18]. The research results indicate that appropriate heat treatment processes can significantly improve the morphology of eutectic Si particles in Al−Si alloys. Eutectic Si, characterized by its finer and more rounded morphology under as-cast conditions, tends to form spheroidized grains during the heat treatment process [19,20], thereby increasing the strength and ductility of the alloy [21,22]. However, in the traditional heat tr