A novel route for development of Al–Cr–O surface nano-composite by friction stir processing

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Abstract

In this study, friction stir processing (FSP) was used to develop in situ Al–Cr–O hybrid nano-composite on the surface of Al6061 plate. This procedure combines the hot working nature of FSP and the exothermic reaction between aluminum and Cr2O3. A new procedure was introduced for applying the reinforcement particles, in which Cr2O3 powder was applied on Al6061 plate by atmosphere plasma spray (APS), and then the FSP was performed on the plate. During of FSP, due to thermomechanical condition, Cr2O3was reduced by aluminum and as a consequence Cr and Al2O3 were produced. With increasing the number of FSP passes, intermetallic compounds including Al13Cr2 and Al11Cr2 were developed resulting from the reaction between Al and Cr. TEM analysis showed that nano-sized reinforcement particles were distributed in the aluminum matrix. As a result of nano-sized reinforcement particles, the composites possess enhanced hardness and ultimate strength.

Highlights

► Al–Cr–O nano-composites were fabricated in situ by FSP. ► During FSP, Cr2O3 was reduced by Al and then in situ Al–Cr reactions were induced. ► The size of reinforcing particles formed by in situ reaction is ∼100 nm range. ► Nano-composite after six FSP passes has Al2O3, Cr2O3, Cr, Al13Cr2 and Al11Cr2 in Al matrix. ► The nano-composites are fully dense with improved microhardness (147 Hv) and tensile strength (518 MPa).

Introduction

Metal matrix composites (MMCs) are advanced engineering materials for structural and electrical applications. Particulate-reinforced MMCs are of specific interest due to their easy fabrication, low cost, and isotropic properties. Some disadvantages for conventionally processed MMCs are poor interfacial bonding and poor wettability between the reinforcement and the matrix due to surface contamination of the reinforcements. It is widely identified that the mechanical properties of MMCs are controlled by the size and volume fraction of the reinforcements as well as the quality of the matrix–reinforcement interface [1]. Excellent mechanical properties can be obtained when fine and stable reinforcements with proper interfacial bonding are dispersed uniformly in the matrix. A possible choice is to synthesize the reinforcement in situ in the metal matrix [2]. The advantages of in situ MMCs are that they have homogeneous microstructures and are thermodynamically stable. Moreover, they also have strong interfacial bonding between the reinforcements and the matrix. In situ aluminum matrix composites may be fabricated by various techniques such as conventional ingot metallurgy, mechanical alloying (MA), rapid solidification processing (RS) and combustion synthesis. The microstructure produced by conventional ingot metallurgy is rather coarse. Materials produced by MA, RS, or combustion synthesis must be densified by a hot consolidation process such as hot isostatic pressing, hot pressing or hot extrusion in order to obtain the final product. In this research, friction stir processing is used for producing in situ nano-composite. Based on the rules of friction stir welding, Mishra et al. [3] developed friction stir processing (FSP) for the microstructural modification of materials and developing composites [3], [4], [5], [6], [7], [8], [9]. The aim of this study was to produce in situ aluminum nano-composites using the FSP technique. The combination of the hot working behavior of FSP and the exothermic reaction between aluminum and Cr2O3 causes fabrication of nano-composite. During FSP high temperature and severe plastic deformation assist to form intermetallic phases. In this paper, we will present the microstructure and mechanical properties for friction stir processed nano-composites based on Al–Cr–O system.

Section snippets

Experimental

For the FSP experiments, samples of Al6061-T6 alloy rolled plate (13 mm × 50 mm × 200 mm) were used. The chemical composition of Al6061 alloy is given in Table 1.

Cr2O3 powder with 20–40 μm particles and purity of 99.7% were used for producing a layer on the Al samples. Morphology of Cr2O3 powder is shown in Fig. 1. An APS plasma system was used to obtain a 150 μm thick Cr2O3 coating on the substrate (Fig. 2). Details of the spraying parameters are given in Table 2. The FSP tool was made of H13 steel (

Results and discussion

In this study by selecting v = 100 mm/min and ω = 630 rpm maximum measured temperature was 347 °C. It can be said that during FSP in addition to severe plastic deformation and mixing of materials, because of the high temperature, diffusion rate is very high, these conditions accelerate the reactions between materials. The X-ray diffraction patterns for samples after different FSP passes are shown in Fig. 5. From these patterns, it can be concluded that during first three passes of FSP due to

Conclusions

This work has displayed that Al–Cr–O nano-composite can be fabricated in situ by FSP. During FSP, Cr2O3 can be reduced by Al and then in situ Al–Cr reaction can be induced. The size of reinforcing particles formed by in situ reaction is ∼100 nm range, which may be attributed to the short reaction time and severe plastic deformation imposed in FSP. Nano-composite after six FSP passes has Al2O3, Cr2O3, Cr, Al13Cr2 and Al11Cr2 in Al matrix. The nano-composites are fully dense with improved

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