Effect of Al2Ca Addition and Heat Treatment on the Microstructure Modification and Tensile Properties of Hypo-Eutectic Al-Mg-Si Alloys.

Abdul Wahid Shah,Bong-Hwan Kim,Shae K. Kim,Young-Ok Yoon,Seong-Ho Ha,Hyun-Kyu Lim

doi:10.3390/ma14164588

Abdul Wahid Shah, Bong-Hwan Kim + Show 4 more

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https://doi.org/10.3390/ma14164588

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Abstract

The current study investigated the microstructure modification in Al–6Mg–5Si–0.15Ti alloy (in mass %) through the minor addition of Ca using Mg + Al2Ca master alloy and heat treatment to see their impact on mechanical properties. The microstructure of unmodified alloy (without Ca) consisted of primary Al, primary Mg2Si, binary eutectic Al–Mg2Si, ternary eutectic Al–Mg2Si–Si, and iron-bearing phases. The addition of 0.05 wt% Ca resulted in significant microstructure refinement. In addition to refinement, lamellar to fibrous-type modification of binary eutectic Al–Mg2Si phases was also achieved in Ca-added (modified) alloy. This modification was related to increasing Ca-based intermetallics/compounds in the modified alloy that acted as nucleation sites for binary eutectic Al–Mg2Si phases. The dendritic refinement with Ca addition was related to the fact that it improves the efficacy of Ti-based particles (TiAl3 and TiB2) in the melt to act as nucleation sites. In contrast, the occupation of oxide bifilms by Ca-based phases is expected to force the iron-bearing phases (as iron-bearing phases nucleate at oxide films) to solidify at lower temperatures, thus reducing their size. The as-cast microstructure of these alloys was further modified by subjecting them to solution treatment at 540 °C for 6 h, which broke the eutectic structure and redistributed Mg2Si and Si phases in Al-matrix. Subsequent aging treatment caused a dramatic increase in the tensile strength of these alloys, and tensile strength of 291 MPa (with El% of 0.45%) and 327 MPa (with El% of 0.76%) was achieved for the unmodified alloy and modified alloy, respectively. Higher tensile strength and elongation of the modified alloy than unmodified alloy was attributed to refined dendritic structure and modified second phases.

Highlights

Metal matrix composites (MMCs) were recently extensively used as structural materials in various industries, e.g., automotive, aerospace, defense, marine, oil, and electronic [1].Aluminum (Al) and its alloys are widely used as the matrix material for manufacturing these MMCs because of the unique characteristics associated with them, such as low density, good corrosion resistance, and high thermal/electrical conductivity [1,2]
The lamellar eutectic structure (#1 in Figure 1) was the dominant morphology of eutectic binary Al–Mg2 Si phases in the A1 alloy along with rod-type and flake-like morphology (Figure 1c)
The microstructure of the unmodified alloy consisted of primary Al, primary Mg2 Si, binary eutectic Al–Mg2 Si, ternary eutectic Al–Mg2 Si–Si, and iron-bearing- phases

Summary

Introduction

Aluminum (Al) and its alloys are widely used as the matrix material for manufacturing these MMCs because of the unique characteristics associated with them, such as low density, good corrosion resistance, and high thermal/electrical conductivity [1,2]. Due to their better performance and longer life, aluminum matrix composites (AMCs) are considered promising materials to replace their conventional casting alloy counterparts in many applications [1,2,3]. One of the hurdles in achieving the real potential of ex situ AMCs is the processing of ceramic-particle-containing melt as the casting of these ex situ composites is difficult as these solid particles in the melt decreased fluidity of the molten metal [1,2,3]

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