Pressureless infiltration of liquid aluminum alloy into SiC preforms to form near-net-shape SiC/Al composites

doi:10.1016/j.jallcom.2007.10.055

Journal of Alloys and Compounds

Volume 465, Issues 1–2, 6 October 2008, Pages 239-243

https://doi.org/10.1016/j.jallcom.2007.10.055 Get rights and content

Abstract

SiC/Al composites with uniformly distributed SiC particles were fabricated by pressureless infiltration of liquid AlSi₇Mg alloy into SiC preforms with different porosity derived from SiC powders. The linear expansion of SiC preforms without any deformation was about 4% due to the oxidation of SiC powders during the sintering process. SiC preforms had no change in both shape and dimensions after infiltration so that near-net-shape composites were achieved. As SiC volume fraction varies from 39% to 62%, the thermal conductivity of SiC/Al composites with bending strength beyond 360 MPa and elastic modulus beyond 140 GPa varies from 146 to 118 W m⁻¹ K⁻¹ at room temperature, and the mean linear coefficient of thermal expansion from room temperature to 373 K varies from 10 × 10⁻⁶ to 7.69 × 10⁻⁶ K⁻¹, agreeing better with the Turner's model.

Introduction

SiC/Al composites with high SiC volume fraction have low coefficient of thermal expansion (CTE) and good thermal conductivity (TC). Comparing with W–Cu alloys, SiC/Al composites not only have higher specific strength and elastic modulus, but also have lower cost in raw materials, which made them more suitable for applications such as space structures and electronic heat sinks in motion vehicles [1], [2], [3], [4]. High SiC volume fraction makes SiC/Al composites machined very difficultly owing to the abrasive resistance nature of SiC filler, so near-net-shape processes with little machining become the optimum candidates to lower production cost. The infiltration of liquid Aluminum alloy into SiC preforms with open pores is one of near-net-shape process accepted by most material researchers. Pressure infiltration routine has been successfully applied in industry by several companies [5], [6], [7]. Pressureless infiltration now attracts considerable attention in recently years as it avoids expansive mould bearing heat and pressure impact in pressure infiltration process which usually leads crazing of brittle SiC preforms [8], [9], [10], [11], [12], [13], [14].

Pressureless infiltration to near-net-shape routine includes two steps: the preparation of SiC preforms and liquid metal spontaneous penetrating into porous preforms to form SiC/Al composites [12], [15]. It has significant sense to study the porosity and dimension change during the preparation of SiC preforms for structure–function integrative process because the aluminum alloy content in the composites lies on its porosity, the figuration and tolerance of the composites are both lie on the shape and dimension change of the preforms. Although SiC preforms for pressure infiltration has been investigated by several researchers [16], [17], there are few reports on the preparation of SiC preforms for pressureless routine as the poor wettability between the SiC preforms and liquid aluminum alloy is the main barrier for spontaneous penetration.

In this paper, SiC/Al composites with near-net-shape and different filler volume fraction were manufactured by pressureless infiltration. SiC preforms with different porosity were manufactured by oxidation bonding process at low temperature in atmosphere. Spontaneous infiltration of liquid AlSi₇Mg into the preforms were achieved by the assistance of interface reaction between liquid alloy containing magnesium and solid SiO₂ film in SiC preforms which leads to good wettability of the two phases. The dimension change of the samples in the whole process was investigated. The effects of SiC content on the mechanical and thermal properties of SiC/Al composites were also studied.

Section snippets

Experimental

W28 yellow-green SiC powders in abrasive grade were used as the starting material and W10 super-pure graphite powders were used as pore-forming agents. The weight ratio of graphite and SiC powders initially taken was 0:100, 8:92 and 20:80 respectively. The powder mixtures added with 1 wt% stearic zinc as lubricant and water solution of 20 wt% polyvinyl alcohol as adhesive were ball-milled in tumbling box for 12 h. After drying in atmosphere for 12 h respectively, the mixtures were uniaxially

Results and discussion

The sintered SiC preforms only has SiC and SiO₂ in it according to the XRD patterns shown in Fig. 1. SiO₂ is derived from the oxidation of SiC particles at 1373 K. Adhesive and lubricant were all eliminated by oxidation during the sintering process. Fig. 2 shows the fractograph of sintered SiC preforms. It was found that SiC particles bonded to each other by SiO₂ films on their surface to form a porous skeleton which has enough strength to maintain its shape in succeeding infiltration process.

Summary and conclusion

SiC preforms with different Porosity were obtained by the oxidation process, which used W28 yellow-green SiC powders in abrasive grade as the starting material and W10 super-pure graphite powders as the pore-forming agents. There are two types of pores in SiC preforms. The first type is the clearance between adjacent SiC particles; the second is the voids left by burned out graphite particles. Pores were all connected mutually to form an open pore-net for liquid AlSi₇Mg infiltrating in. Liquid

Acknowledgements

This work has been financially supported by Innovation Groups Foundation (103-037016) and Development Foundation (103-037508) of Hefei University of Technology, China.

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Pressureless infiltration of liquid aluminum alloy into SiC preforms to form near-net-shape SiC/Al composites

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Summary and conclusion

Acknowledgements

J. Mater. Process. Technol.

Compos. Sci. Technol.

Mater. Lett.

JOM

J. Mater. Sci.

Adv. Mater. Proc.

J. Mater. Sci.

Mater. Eng.

Intro. Mater.

Acta Mater.