Impact of submicron TiB2 particles on microstructure, casting performance, and mechanical properties of an Al-Cu alloy

Abstract

Abstract: Although the strengthening and grain refinement effects of TiB2 particles on aluminum alloys have been extensively studied, their influence on casting behavior remains relatively underexplored. In this study, the influence of different addition amounts of submicron TiB2 particles on the microstructure, casting performance, and mechanical properties of an Al-Cu (ZL205A) alloy was systematically investigated. The introduction of TiB2 particles leads to significant grain refinement, transforming the microstructure from coarse grains to fine equiaxed grains by providing additional nucleation sites and inhibiting grain growth. SEM and TEM analyses reveal that the added submicron TiB2 particles exhibit minimal effect on the distribution of intermetallic phases or precipitates. Casting performance, as evaluated by spiral fluidity and hot tearing tests, shows notable improvements with TiB2 additions. At a TiB2 content of 3wt.%, the fluidity length increases by 20%, and the hot tearing susceptibility coefficient decreases by 29%. These enhancements are mainly due to the refined grain structure and the formation of interdendritic bridging in TiB2-reinforced alloys. However, the overall enahncement in casting properties shows little variation across the TiB2 additions from 0.2wt.% to 3wt.%. Mechanical testing shows that the highest hardness and strength are achieved with a 1wt.% addition of TiB2 particles, primarily attributed to refined grain size and reinforcement of the aluminum matrix. Based on these findings, a TiB2 particle content of 1wt.% is recommended for optimizing both the casting performance and mechanical properties of the ZL205A alloy.

1. Introduction

To meet growing economic and environmental demands, lightweight design strategies have become essential, driving the increased use of aluminum alloys in critical engine components, such as cylinder heads and engine blocks, that account for a substantial fraction of total engine mass [1]. However, the pursuit of higher combustion temperatures and pressures to improve engine efficiency places severe thermodynamic and mechanical property requirements on conventional aluminum alloys. Among these alloys, Al-Cu alloys are favored in engine manufacturing for their exceptional strength-to-weight ratio, high thermal conductivity, and excellent resistance to high-temperature degradation [2, 3]. Yet, as engine designs push toward more extreme operating conditions, existing Al-Cu alloys face limitations in both mechanical performance and casting behavior. Consequently, further enhancements in strength and castability are essential to meet the rigorous operational standards of high-efficiency engines under extreme thermal and mechanical loads.

Ceramic particles such as TiB2 [4, 5], Al2O3 [6, 7], and SiC [8], known for their high strength and high elastic modulus, have garnered significant attention in materials science for their capability to effectively improve the microstructure and mechanical properties of Al-Cu alloys [5, 8]. Although Al2O3 and SiC reinforcements have proven effective, a notable drawback is their tendency to form intermetallic compounds with the aluminum matrix, which can degrade the mechanical properties and chemical stability of the composites [9]. In contrast, TiB2 particles exhibit a high degree of interface compatibility with the aluminum matrix, facilitating grain refinement during solidification. Additionally, TiB2 particles have a high melting point (approximately 2,790 °C) and superior thermal stability [10], ensuring minimal reaction with the aluminum matrix and preventing the formation of brittle intermetallic phases at the interfaces [11]. These attributes make TiB2 reinforcement a widely adopted approach for enhancing the performance of Al-Cu alloys.