1. Introduction
Aluminium alloy matrix composites are very appealing for human mobility industries due to their high specific properties, allowing the development of faster, more efficient and inexpensive means of transport. Al alloys and particularly their composites are widely used in automotive and aeronautic structural applications [
1,
2]. These metallic matrix composites are usually reinforced with ceramics such as oxides, borides, nitrides and carbides [
3]. The tungsten carbide (WC), which has high hardness and rigidity and Young’s modulus of 670–700 GPa [
4], has attracted some researchers to use this material as a reinforcement of the Al matrix, applying powder metallurgy (PM) [
5], accumulative roll bonding [
6], stir casting [
7] and friction stir processing [
8].
Liu et al. [
6] commented on the increase in hardness, tensile and wear properties of an AA1060-3 vol.% WC composite (particles with an average size of almost 3 μm), produced by 13 accumulative roll bonding cycles. This mechanical response was associated with an increase in the number of dislocations, without the formation of new phases. Ravikumar et al. [
7] highlighted the efficacy of adding 8 wt.% of WC (D
50 = 53–75 μm, mixed by stir casting technique) in the yield and tensile strength of the AA6082-WC composite; however, they mentioned a progressive decrease in density, impact strength and ductility of the composite with increasing reinforcement concentration. These authors did not mention the formation of any transformed phase, though they performed X-ray analysis. Huang et al. [
8] produced AA5083 composite through friction stir processing to distribute WC particles (with 4 μm average size) in the Al matrix. These authors highlighted the positive effect of increasing WC concentration on hardness, yield and tensile strengths but deteriorating the plastic deformation capacity of the Al matrix. The excellent interfacial bonding between WC reinforcements and the Al matrix, without the formation of reaction products, promotes load transferring as the primary strengthening mechanism; besides, these authors mentioned the formation of nanoparticles that can trigger the pinning effect on dislocation movements (Orowan mechanism) during plastic deformation. Borodianskiy and Zinigrad [
9] remarked the influence of WC nanoparticles on grain refinement in pure Al and A356 alloy and the modification effect in the alloy produced by PM and stir casting. These works showed that, as in other metal matrix composites [
10], several strengthening mechanisms can be activated in Al-WC composites depending on the characteristics of the reinforcements and processing history, namely, grain refinement, load transferring, dispersion hardening and thermal/modulus mismatch effects.
Powder metallurgy (PM) processes involve a mixing step. Ball milling is the most promising mixing process in order to obtain homogeneous composites [
5]. However, the production of Al composites through ball milling, as mixing procedure, should be carried out in an inert or reducing atmosphere to prevent Al oxidation. The milling process is long; its duration strongly influences densification and strengthening and, generally, the more hours of milling, the better the improvement [
11].
Razavi and Mobasherpour [
12] produced AA7075-WC composites with different WC contents by ball milling for 20 h and hot pressing at 300 MPa and 430 °C for 15 min. Increasing WC amount reduces the relative density and ductility and increases the hardness; the tensile strength reaches the maximum value for a 20 wt.% of WC. Simon et al. [
13] mixed Al powders with WC powder particles (D
50 = 1 μm) by ball milling and found that the prolongation of the milling time increases the density obtained but decreases the wear resistance and hardness for most composites produced. These authors confirmed the formation of Al
12W during sintering at 580 °C in nitrogen for 20 min. Evirgen and Öveçoglu [
5] remarked the effectiveness of high-energy ball milling in the hardening and densification of an Al–2Cu–7WC (wt.%) composite produced using elemental powders of Al, Cu and WC, with 10, 30 and 3 μm average particle sizes, respectively. The mixed powders were cold pressed at 220 MPa and sintered at 650 °C for 4 h in an Argon atmosphere. These authors attributed the hardening effect to the formation of the Al
12W phase.
This study assesses the effect of processing conditions on the microstructure and mechanical properties of an Al-WC composite produced by powder metallurgy (which involves mixing, compacting and sintering). An innovative aspect of this work is the use of assisted sonification as the mixing method, which allows a significant reduction in time when compared to ball-milling. The use of pure Al allows more precise identification of the hardening mechanisms involved as it avoids possible hardening by solid solution and second phase. The matrix of Al was reinforced with 1 vol.% of ultrafine WC powder particles. This small concentration was used to prevent a considerable increase in density and to benefit the effect of WC particles; the cited works have shown that there is a decrease in the mechanical properties of the composites at high WC concentrations. The optimization of the processing conditions is essential to maximize the properties of the composite and enhance its future application.
2. Materials and Preparation
Alfa Aesar supplied a 99.8 wt.% pure Al powder. This powder consisted of particles with D
50 of 10 µm and very irregular morphology, as shown in
Figure 1a, due to the gas atomization production process. The Al powder particles were composed of fine polycrystalline grains;
Figure 1b shows one of these particles with an average grain size of 0.8 µm. The WC powder, from H.C. Starck Tungsten GmbH, had a purity of 99.8%, a D
50 of 0.33 μm and an angular shape (
Figure 1c). The mixture of Al powders with 1 vol.% WC (5.5 wt.%) was made using the assisted sonication method.
Figure 2a illustrates the three stages of the dispersion procedure, de-agglomerating WC particles in isopropanol using Ultrasonic bath (BANDELIN electronic, Berlin, Germany) for 15 min (
Figure 2(a1)), dispersing Al powder particles also in isopropanol (SIGMA-ALDRICH, St. Louis, MO, USA) at 3000 rpm for 5 min (
Figure 2(a2)) and then mixing these two dispersions and applying simultaneous dispersing by Ultrasonic bath and 11,000 rpm for 5 min (
Figure 2(a3)), after which the mixture was drained and dried in an oven (EHRET, Emmendingen, Germany) at 80 °C for 1 h.
Figure 2b shows that the WC particles were dispersed among the Al powder particles.
The Al-1 vol.% WC powders mixture was cold compacted by uniaxial compression, with 76 or 152 MPa, into discs with diameters of 10 mm and 30 mm and height of 3 mm. These discs were sintered under the conditions indicated in
Table 1 in a horizontal electric furnace (Termolab, Águeda, Portugal) with an alumina ceramic tube. Similar samples of Al powder were prepared with identical conditions and sintered simultaneously with the composite samples for comparison purposes. From sintered samples, specimens for mechanical tests were prepared by electro-discharge machining.
Microstructural characterization involved optical microscopy (OM), using the Leica DM 4000 M equipment (Leica Microsystems, Wetzlar, Germany) and scanning electron microscopy (SEM), with the FEI-Quanta 400 FEG equipment (FEI Company, Hillsboro, OR, USA), through secondary electrons (SEM/SE) and backscattered electron (SEM/BSE) imaging modes. Electron dispersive spectroscopy (SEM/EDS) (Oxford Instrument, Oxfordshire, UK), as a semi-quantitative analysis, was applied to determine the chemical composition of phases. Crystallographic information was obtained by electron backscatter diffraction technique (SEM/EBSD), using TSL-EDAX EBSD Unit and TSL OIM Analysis 5.2 software (EDAX Inc., Mahwah, NJ, USA). Phase identification was performed by X-ray analysis using PANalytical diffractometer (Malvern Panalytical, Malvern, UK) equipped with a CuKα radiation source (1.540598 Å), carried out with a scanning rate of one second per step, using the X’Pert HighScore Plus software (version 2.2b (2.2.2), Malvern Panalytical, Malvern, UK).
The mechanical characterization involved microhardness measurements to determine the strengthening effect of WC in Al matrix, carried out using Struers Duramin equipment (Struers, Ballerup, Denmark), at a 0.490 N load (HV 0.05). Subsequently, the reinforcement effect analysis was complemented by tensile and flexural strength measurements, failure analysis and wear abrasive resistance evaluation of the hardest composite obtained in this study. The tensile and flexural properties were measured by a Shimadzu Table-Top universal tester equipment (SHIMADZU EUROPA GmbH, Duisburg, Germany); the flexural characteristics measured by a three-point bending test. The wear resistance test was performed using a micro ball cratering method (PLINT TE 66 Micro-Scale Abrasion tester equipment) (Phoenix Tribology Ltd, Berkshire, England), with a mixture of ionized water and 2 vol.% of black silicon carbide (with particles of 99.9% purity and average particle size less than 3 µm).
Author Contributions
Conceptualization, O.E.; formal analysis, O.E.; funding acquisition, M.F.V.; investigation, O.E.; methodology, O.E.; resources, M.F.V.; supervision, M.T.V. and M.F.V.; validation, M.T.V. and M.F.V.; visualization, O.E.; writing—original draft, O.E.; writing—review and editing, M.T.V. and M.F.V. All authors have read and agreed to the published version of the manuscript.
Funding
This work was supported by Portuguese funds through UIDB/00285/2020, UIDB/50022/2020 and UIDP/50022/2020 projects funded by FCT (Fundação para a Ciência e a Tecnologia), Portuguese public agency.
Acknowledgments
The authors would like to express thanks from CEMUP (Centro de Materiais da Universidade do Porto) for assistance with SEM, the department of Mechanical Engineering at the Faculty of Engineering—University of Porto for providing the Ball cratering test equipment, and also CINFU (Centro de Formação Profissional da Indústria de Fundição) for cutting aluminium specimens.
Conflicts of Interest
The authors declare no conflict of interest.
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Figure 1.
Microscopic characterization of input materials used in this study: (a) The morphology of Al powder (SEM/SE image), (b) EBSD grain colour map from an individual Al particle and (c) the tungsten carbide (WC) particles (SEM/SE image).
Figure 2.
(a) Mixing procedure: Step 1—dispersion of pristine WC in isopropanol for 15 min in an ultrasonication bath; step 2—dispersion of Al powder in isopropanol at 5000 rpm for 5 min; and step 3—mixing of the two dispersions with simultaneous sonication at 11,000 rpm with the mixing vessel is in the ultrasonication bath; (b) SEM/BSE image of the dry Al-1 vol.% WC mixture.
Figure 3.
SEM/BSE images of Al-1 WC vol.% composites cold compacted at 76 MPa and sintered at 640 °C in high vacuum (~5 × 10−4 Pa): (a) Pores (black spots) dispersed in the microstructure of the specimen sintered for 1 h, (b) pore-free microstructure of the specimen sintered for 2 h.
Figure 4.
(a) One area of the composite in which the two types of particles are observed (white and light gray particles). EDS map analysis illustrating the distribution in that selected zone of (b) Al, (c) W and (d) C.
Figure 5.
Diffractograms of sintered Al and Al-1 vol.% WC. The composite shows Al, Al12W and WC peaks. The Miller indices (hkl) of the diffraction peaks of Al, Al12W and WC are shown in black, green and brown, respectively.
Figure 6.
Phase map (EBSD/SEM) of (a) non-reinforced Al and (b) Al-WC composite; grain size distribution of (c) non-reinforced Al and (d) Al-WC composite.
Figure 7.
Microhardness values as a function of processing conditions for the Al-WC composite and the non-reinforced Al.
Figure 8.
SEM/SE images of fracture surfaces at two magnifications of: (a,b) Non-reinforced Al specimen; (c,d) Al-WC composite.
Figure 9.
Wear abrasive resistance results, performed on specimens produced with 152 MPa/640 °C/2 h/high vacuum (5 × 10−4 Pa), (a) for different loads with a distance of 31.4 m and (b) for different distances with 0.1 N load.
Figure 10.
SEM/SE images showing craters and morphology of the grooves on: (a) Non-reinforced Al and (b) Al-WC composite.
Table 1.
Sintering conditions of non-reinforced Al and of Al-1 WC vol.% composite.
Temperature (°C) | Holding Time (h) | Pressure (Pa) |
---|
630 °C | 2 | ~5 × 10−4 |
640 °C | 0.5, 1 and 2 |
Table 2.
Mechanical properties of non-reinforced Al and of Al-1 vol.% WC composite.
Composition | Ys * (MPa) | UTS (MPa) | Elongation (%) | Flexural Strength (MPa) |
---|
Pure Al | ~63 | 110 | 19 | 336 |
Al-WC | ~110 | 148 | 16 | 532 |
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