Hybrid reinforced aluminum matrix composites fabricated by selective laser melting
Introduction
Aluminum matrix composites (AMCs) are promising materials for aerospace, defence, automotive, and thermal management areas due to their excellent combination of properties. It combines the properties of the aluminum alloys (high strength and ductility) and the reinforcements (stiffness and high modulus) leading to a superior profile of characteristics [[1], [2], [3]]. Recently, AMCs with multiple reinforcements (hybrid aluminum matrix composites, HAMCs)) are found increasing applications because their mechanical properties and wear resistance are much better than those of the single reinforced composites [4,5]. The multiple reinforcements, like the trace elements in alloys, provide the possibility of diversifying the performance of the composite material. The HAMCs have received considerable research and trials by Toyota Motor Inc. in the early 1980s [6]. Properties of HAMCs can be tailored to the demand of different industrial applications [7]. Till now, there have been a few studies on HAMCs using conventional methods [8,9]. J. David Raja Selvam et al. successfully synthesized AA6061/(TiB2 + Al2O3) HAMCs in situ by casting. The TiB2 and Al2O3 significantly improved mechanical properties such as hardness and tensile strength [10]. Jaswinder Singh et al. also fabricated different HAMCs by stir casting, such as Al/SiC/fly ash, Al/Al2O3/rice husk ash, and found that HAMCs had a dramatic increase in tensile strength compared to a single reinforcement [11]. Furthermore, they revealed that hybrid composites provide more flexibility and reliability in component design. Earlier research on this issue had similar results [[12], [13], [14]]. All the above researches confirmed the promising application of HAMCs.
However, it is well known that conventional methods cannot meet the demand of fabricating parts with complex geometries. Moreover, it is also difficult to machine for high-strength hybrid composites. Selective laser melting (SLM), is one of the additive manufacturing technologies, which can fabricate 3D parts with complex geometries. It has been successfully applied to the fabrication of a variety of metals, including aluminum alloys [15,16], stainless steel [17], superalloys [18], and even composite materials with single reinforcement [[19], [20], [21]]. However, few studies reported the HAMCs processed by SLM. The preparation of hybrid composite powder limits the application of SLMing HAMCs because the powder flowability is vital in SLM processing. Fei Chang et al. found that an in-situ Al4SiC4+SiC hybrid reinforced aluminum matrix composite was produced because of the in-situ reaction between molten aluminum and SiC in the SiC/AlSi10Mg composite during the SLM processing [22]. But the relative density of the HAMCs is only 97.2%. The flow characteristics of the molten pool are susceptible to the addition of large - size SiC during the rapid cooling of SLM, which causes the unfused defects. S. Dadbakhsh et al. investigated SLM behaviour and microstructure of in situ metal matrix composite reinforced with Al2O3 particle and Al–Fe intermetallics [23]. They found a noticeable enhancement of hardness but did not present other mechanical properties because the relative density was less than 80%. In general, the in-situ method is most commonly used in SLMed HAMCs because only one additional component is required. However, the control of the chemical reaction is more difficult in rapid prototyping, because its affected factors are very varied. And the low relative density is a common problem in-situ method. Fortunately, the rise of cermet powders (such as Cr3C2/Ni, WC/Ni, Al2O3/Al) as reinforcement provides an ingenious solution for preparing HAMCs. The low-cost cermet powder with uniform composition replaces the addition of multiple components successfully, which means HAMC can be fabricated by adding one cermet powder directly. It has been reported that a carbide ceramic has a significant impact on the mechanical properties in AMCs [[24], [25], [26]]. The trace elements Cr and Ni can form a variety of precipitation phases with Al, and Cr also has good solubility in Al [27,28].
In this work, Cr3C2–NiCr cermet powders containing trace elements were used to reinforce aluminum alloys. Near full density and crack-free HAMCs reinforced with Cr3C2 ceramic particle and Nr-Cr trace elements were fabricated by SLM, which exhibits good compressive strength and high microhardness. The Cr3C2 particles can improve the strength as a dispersed ceramic phase. The trace elements Nr-Cr contributed to the precipitation strengthening because of the generation of various intermetallic compounds. The phase structure, microstructure, mechanical properties and strengthening mechanism were studied in detail.
Section snippets
Materials
The spherical AlSi10Mg powder with an average particle size (D50) of 33.1 μm, produced by Hengji Powder Technology China, was used as the metal matrix in this experiment. The irregular Cr3C2-25 wt%NiCr cermet powder with an average particle size (D50) of about 10 μm, supplied from the Beijing Metal Powder Research Institute, were used as the multiple reinforcement. The chemical compositions of two starting powders are described in Table .1. The morphologies of the raw materials are shown in
Microstructure and phase structure
In Fig. 4, the microstructures of cross-section of the HAMCs sample obtained at different scanning speeds and different hatching spaces were compared. It shows that a lower scanning speed results in a higher relative density. At the scanning speed of 50 mm/s, a near full density (relative density of 99.99%) sample containing Cr3C2 ceramic particles was obtained. As the scanning speed increases, a few unfused defects and cracks are observed. Those unfused defects arise because the low laser
Conclusion
In this study, we report an HAMCs design concept that can simultaneously provide high compressive strength and good formability. Near full density and crack-free AlSi10Mg/Cr3C2–NiCr hybrid aluminum matrix composites have been successfully fabricated by the SLM process. The phase structure, microstructure, mechanical properties and strengthening mechanism of HAMCs have been investigated in detail. The following conclusions can be drawn:
- 1.
A variety of intermetallic compounds, such as Al13Cr2, Al11Cr
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work is supported by the military industry stability support project (2019KGW.YY4007Tm), the China Postdoctoral Science Foundation (2020M682405), the Shanghai Institute of Aerospace System Engineering (YYHT-F08052018070005) and the Shanghai Foundation for Aerospace Science and Technology Innovation (No. SAST2016044). The authors also thank the Analytical and Testing Center of HUST for the SEM, XRD and EDS analysis.
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