The influence of Li on the tensile properties of extruded in situ Al–15%Mg2Si composite

doi:10.1016/j.msea.2011.10.101

Materials Science and Engineering: A

Volume 532, 15 January 2012, Pages 346-353

https://doi.org/10.1016/j.msea.2011.10.101 Get rights and content

Abstract

This work was carried out to investigate the effect of different Li concentrations (0.15, 0.3, 0.5 and 0.7) as a modifying agent on the microstructure and tensile properties of an in situ Al–15%Mg₂Si composite. Cast, modified and homogenized small ingots were extruded at 480 °C at extrusion ratio of 18:1 and ram speed of 1 mm/s. Various techniques including metallography, tensile testing and scanning electron microscopy (SEM) were used to characterize the mechanical behavior, microstructural observations and fracture mechanisms of this composite. The results showed that 0.5%Li addition and homogenizing treatment were highly effective in modifying Mg₂Si particles. The results also exhibited that the addition of Li up to 0.5 wt.% increases both ultimate tensile strength (UTS) and tensile elongation values. However, the tensile results slightly decrease with the addition of more Li (>0.5 wt.%). The highest UTS and elongation values were found to be 280 MPa and 16% for homogenized and extruded Al–15%Mg₂Si–0.5%Li composite, respectively. Fracture surface examinations revealed a transition from brittle fracture mode in as-cast composite to ductile fracture in homogenized and extruded specimens. This can be attributed to the changes in size and morphology of Mg₂Si intermetallic and porosity content.

Highlights

► Effect of Li contents on the microstructure of homogenized and extruded Al–15%Mg₂Si. The optimum concentration of Li was found to be 0.5 wt.%. ► The highest UTS and %El. values were 280 MPa and 16 for Al–15%Mg₂Si–0.5%Li MMC. ► Li addition changed the fracture behavior of the composite from brittle to ductile.

Introduction

For some applications composite materials are considered to be more suitable than conventional materials as they have desirable mechanical, thermal and wear properties. Recently, particulate reinforced composites (PMMCs) have attracted considerable attention due to their relatively low costs and inherent isotropic properties [1], [2], [3]. Further, an in situ process of fabricating aluminum MMCs possess some merit, such as even distribution of reinforcement, well matrix/reinforcement interface, thermodynamically stable systems and much lower production costs compared to their ex situ process counterparts. The simple technical process and optimized interface of the in situ MMCs have attracted considerable attention recently [4], [5], [6].

The intermetallic compound Mg₂Si exhibits a high melting temperature (1085 °C), low density (1.99 × 10³ kg/m³), high hardness (4500 MN m⁻²), a low coefficient of thermal expansion (7.5 × 10⁻⁶ K⁻¹) and a reasonably high elastic modulus (120 GPa) [7]. In normal as-cast Al–Mg₂Si composites, the primary Mg₂Si phases are usually coarse which lead to the reduced mechanical properties. Therefore, Mg₂Si crystals must be modified to ensure adequate mechanical strength and ductility of the composite [8].

Recently, it was reported that modification of Al–Mg₂Si microstructure has been developed through common gravity casting by additions of rare earth [8], [9], potassium fluotitanate [10], Al–Sr master alloys [11], sodium salt [12] or melt superheating treatment [13]. However, some inherent defects are introduced in these PMMCs. For example, potassium fluotitanate [10] addition decreases the size of Mg₂Si particles, while it has no influence on its morphology. Furthermore, although sodium salt can change the size and morphology of Mg₂Si phases, the required amount of the modifier should be too high (10%) [12].

However, the improper control of some process parameters of this technique can lead to some defects in cast composites such as porosity and non-uniform distribution of the particles. Therefore, thermomechanical procedures such as extrusion, rolling or forging seem to be effective ways for decreasing the porosity and obtaining a more uniform particle distribution in such composites [14], [15], [16], [17], [18], [19]. Metal forming processes can change the structural features, which influence the properties of these materials [20]. The properties of the MMCs are mainly sensitive to the type of reinforcement and the fabrication processing techniques [21]. For example, in extrusion process the tensile elongation and UTS values of the composite increase, as extrusion ratio increases [22].

While there have been a number of studies to tailor the properties of aluminum using different types and amounts of reinforcement, no attempt has been made to study the effects of hot deformation process and chemical modification on the microstructure and tensile properties of Al–Mg₂Si composites.

The aim of this study is to investigate the effect of Li addition on the microstructure and mechanical properties of Al–15%Mg₂Si in situ composite after homogenization and extrusion. The fracture surfaces of this composite were studied by SEM to find out the effect of Li on the fracture mechanisms.

Section snippets

Experimental

Industrially pure Al (>99.8% purity), Mg and Si metals were used to prepare Al–5.5 wt.%Si–9.7 wt.%Mg alloy. Al–15%Mg₂Si composite as primary ingots were prepared in an electrical resistance furnace using a 10 kg SiC crucible. The parent alloy was remelted within a small SiC crucible (with 1 kg capacity) in a resistance furnace in order to prepare alloys with 0, 0.15, 0.3, 0.5 and 0.7 wt.%Li. When the temperature reached 750 °C, pure Li was wrapped in aluminum foil and added in small increments. Due

Microstructural studies

The typical microstructures of the Al–15%Mg₂Si composite after Li addition in two conditions, EX and HEX, are shown in Fig. 2, Fig. 3, respectively.

The dark particles are known as the primary Mg₂Si and the bright phase as α-Al [7], [22]. In Fig. 2a, it is clearly seen that the morphology of primary Mg₂Si particles is irregular. The average size of particles was found to be 56 ± 9 μm in as-cast condition, as seen in Fig. 4.

Similar morphology of primary Mg₂Si particles in Li-free Al–15%Mg₂Si

Conclusions

The influence of different Li contents, homogenizing treatment and extrusion process on the microstructural and tensile properties of in situ Al–15%Mg₂Si composite was studied and the following conclusions can be drawn:

1.
The addition of Li to homogenized and extruded samples changes the morphology of primary Mg₂Si from irregular to spherical shape and also reduces the average size of primary Mg₂Si from 56 ± 9 μm to about 10 ± 2 μm. The morphology of eutectic Mg₂Si phase alters from flake-like to very

Acknowledgments

The authors gratefully acknowledge Imam Khomeini International University and University of Tehran for lab facilities and Iran National Science Foundation for financial support of this work.

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The influence of Li on the tensile properties of extruded in situ Al–15%Mg2Si composite

Abstract

Highlights

Introduction

Section snippets

Experimental

Microstructural studies

Conclusions

Acknowledgments

J. Mater. Process. Technol.

Wear

Mater. Sci. Eng.

Mater. Des.

Compos. Struct.

Mater. Sci. Eng.

Mater. Sci. Eng.

J. Alloys Compd.

Mater. Lett.

J. Alloys Compd.

Mater. Sci. Eng. A

Compos. Sci. Technol.

J. Mater. Process. Technol.

Compos. Sci. Technol.

J. Alloys Compd.

J. Compos. A

Scr. Mater.

The influence of Li on the tensile properties of extruded in situ Al–15%Mg₂Si composite