1 Introduction

Alumina particles (Al2O3p) have been widely used as reinforcement particulates in an aluminum matrix, due to its low cost, availability, and wide range of commercial grades. Aluminum reinforced with Al2O3p proved their performance over unreinforced Al alloy; they are harder, stronger, tougher, and more wear-resistant [1,2,3,4]. These Al matrix composites (AMCs) are manufactured through different processes such as stir casting [5], infiltration [6], and squeeze casting filtration [7]. Even though these processes gained much attention due to their economic benefit, many limitations have been reported. Particle agglomeration and clustering, gravity segregation in case of nanoparticle-reinforced composites, and interfacial reactions between Al2O3 particles with molten Al have been the major issues [8]. Al2O3 is stable in most Al alloys except for Mg containing Al Alloys, and it may form spinel phase Al2MgO4 or Cu2MgO4 [8,9,10,11,12]. To overcome these issues, rheocasting, semisolid state processing, and powder metallurgy were used; thus, particle clustering was avoided, and interfacial reactions were minimized [13].

Friction stir processing (FSP), one of the solid state processing techniques, is the most industrially feasible process for bulk products used for many technologies such as surface modification, grain refinement, and surface composite manufacturing [14, 15]. When AMCs are processed by FSP, many issues could be solved such as particle clustering and interfacial reactions occurring at particle/matrix interface.

The incorporation of Al2O3p into the AA2024 matrix using FSP enhances the mechanical properties and wear resistance [16,17,18]. Several research works dealt with the effect of post-processing heat treatment (PPHT) on Al composites manufactured by FSP wherein SiC [19], TiB2 [20], and Al2O3 [21] were used as reinforcing particulates. Yet, little is known about the effect of post-processing heat treatment on AA2024/Al2O3p composites produced by FSP. Up to our knowledge, only El-Mahallawi et al. [10] worked on PPHT of this composite system, and they reported improvement in ultimate tensile strength, elongation, and hardness after solution treatment followed by T6 aging treatment on 2024 alloy prepared by semisolid casting. In pursuit of what has been done, the main impetus of this work is devoted to understanding the post-processing heat treatment (PPHT) effect on microstructure, hardness, wear behavior, and precipitation mode of AA2024/Al2O3 FSPed surface composite.

2 Materials and methods

The chemical composition of the Al alloy 2024-T351 sheets used in this study is listed in Table 1. The chemical analysis of the metal sheets was tested using stationary optical emission spectrometer (Oxford, Foundry Master Pro). Before friction stir processing, the sheets were cleaned with acetone and alcohol. The groove with a squared shape of 2.5mm width and 3mm depth was filled with alumina powder (size: 1μm, provider: Sigma Aldrich). A pinless tool was used to seal the groove with the same FSP parameters; then, the sheets were submitted to one and three passes of the tool (pin: circular profile, 6mm diameter, and 3mm length; shoulder: circular profile, 20 mm diameter), at 1400 rpm rotational speed and 40mm transverse speed. The schematic of the working mechanism of setting FSP parameters is shown in Fig. 1. The FSPed composites were heat-treated in an open-air furnace (Nabertherm GmbH, Germany) consisting of solutionizing at 530°C, subsequent quenching in water, followed by aging treatment at 180°C for 8 h.

Table 1 Chemical composition of as-received aluminum alloy 2024-T351
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Fig. 1
figure 1

Schematic of the working mechanism of friction stir process: a pinless and FSP tool dimensions, b working process of Al2O3 powder insertion and sealing the groove, and c–e schematic of the 1st, 2nd, and 3rd pass of FSP, respectively

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The X-ray diffraction (XRD) analysis was done with a Brucker Advance 8 diffractometer with copper radiation. The FSPed samples were cross-sectioned and mechanically grounded with SiC abrasive paper and finished with a diamond solution before the metallographic examination. The Scanning Electron Microscope, SEM (Quanta 650; FEI Netherlands), equipped with an Energy Dispersive X-ray spectrometer, EDS, (Bruker X-Flash 6/10) was used to study the microstructure and the worn tracks of the AA2024-T351/Al2O3p composites.

Vickers microhardness measurements were performed for each sample by applying 100 gf load for a 10s holding time, using a commercial micro-indenter (CSM Instruments, Switzerland) Berkovich diamond tip. The microhardness profiles were done at 1mm and 2mm depth from the sample FSPed surface, and more than three measurements were taken.

Wear tests were done at the top surface of the FSP zone, using a rotary ball-on-disc tribometer (CSM instrument, Switzerland) under 4N load at 15 cm/s sliding speed, for a total distance of 300m. The wear experiments were done at room temperature, under dry sliding by pressing a 6-mm 100Cr6 steel ball, at 40% relative humidity. The load and sliding speed parameters were chosen based on the work by Zhang and Alpas on the wear maps and wear regime transition in AA6061 [22] and in AA6061-reinforced Al2O3 composites [23], sliding against 100Cr6 steel. The present applied load (4N load) falls in the wear regime I (see ref. [23]) for reinforced Al2O3/Al composite and in wear regime II for unreinforced Al alloy, wherein the reinforcement effect (Al2O3p) is relevant [23].

3 Results

3.1 Microstructure of FSPed AA2024/Al2O3p surface composites

Figure 2a shows the XRD pattern of the as-received AA2024-T351 used in this study. The pattern indicates the presence of only Al peaks. The Al alloy 2024 is an age hardenable alloy through the precipitation of Al2Cu and Al2CuMg phases [18]. Herein, these precipitations could not be detected through our XRD analysis. Image analysis by SEM of the AA2024-T351 surface has shown the presence of coarsening Al2Cu or Al2CuMg phases (white particles) identified by EDS (Table 2), where the darker contrast is Al matrix (Fig. 2c). The XRD pattern of Al2O3 powders (Fig. 2b) shows major peaks of two predominantly alumina phases: α Al2O3 and γ Al2O3.

Fig. 2
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X-ray diffraction patterns of a AA2024-T351 and b alumina powder. c SEM-EDS of as-received AA2024-T351

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Table 2 Summary of EDS spots, in at.%, of as-received AA2024-T351 (Fig. 1c). Numbers in first column correspond to point’s number indicated in the figure. The nominal compositions are listed in last column
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SEM microstructure of the AA2024-T351/Al2O3p surface composite is shown in Fig. 3a, after three cycles of the friction stir processing. A defect-free stir zone was partially achieved with nearly homogenous distribution of the Al2O3p particles inside the Al alloy matrix. The reinforcement particle size decreased from micron to submicron size due to powder fragmentation during FSP [13,14,15, 19, 20]. The SEM-EDS elemental mapping is shown in Fig. 3b–e. Oxygen distribution through the Al matrix revealed acceptable dispersion of Al2O3 particles. Multi-pass (three passes) FSP exhibited a microstructure clear from voids and tunnel defect.

Fig. 3
figure 3

a SEM of the AA2024-T351/Al2O3p stir zone composite and SEM-EDS elemental mapping of b oxygen, c aluminum, d copper, and e magnesium

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Figure 4 shows microhardness profiles of the three-pass FSPed AA2024-T351/Al2O3p before and after post-processing heat treatment, which was carried out on a cross-section of the samples. Microhardness values decreased in the heat-affected zone (HAZ) and nugget zone after subsequent FSP (Fig. 4a); this was due to the dissolution of hardening precipitates Al2Cu [21]. The integration of Al2O3p increased the hardness of the processed surface composites to a mean microhardness value of 150HV, as shown in Fig. 4b.

Fig. 4
figure 4

Microhardness profiles of as-received, as friction stir processed areas after and before heat treatment without Al2O3p incorporation (a), AA2024-T351/Al2O3p surface composites before (a) and after heat treatment (c)

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Further tailoring of hardness property could be done by heat treatments after FSP to homogenize the microstructure. In 2024-T351 alloys, heat treatments were extensively studied in many previous scientific reports [24, 25]. The heat treatment was chosen based on the work of Zhang and Alpas [26] on 2024 alloys, consisting of solutionizing at 530°C, subsequent quenching, followed by aging treatment at 180°C for 8 h. The microhardness profiles indicated a slight increment in microhardness average values; the microhardness of heat-affected zone was successfully recovered (Fig. 4c). However, many low values were recorded in the stirred zones (SZs).

After heat treatment, different microstructural features were observed, as shown in Fig. 5. The Al2O3 particles as well as precipitates were agglomerated inside grain boundaries of the Al alloys matrix. Figure 5a shows the interface between the FSPed zone and unprocessed zone, after heat treatment. Higher magnification of the heat-treated FSPed zone (Fig. 5b) shows different microstructural features compared to as-FSPed surface composite (Fig. 3). EDS analysis in this region showed intermixed elements from both Al2O3p particles and AA2024 elements (spots: 3–5, Table 3). The most intriguing phenomenon found herein is in the heat-affected zone (HAZ); it was affected by the PPHT in a way that has never been reported previously. The precipitation mode changed to what is a grain boundary precipitation (Fig. 5c). EDS spots (6–8, Table 3) marked in Fig. 5d show that these precipitations consist of a coarsen Al2Cu and some intermetallic phase mixture.

Fig. 5
figure 5

SEM-EDS of heat-treated AA2024/Al2O3p composites. a Interface between the friction stir processed zone and unprocessed zone. b Higher magnification of red square in a. c Heat-affected zone (unprocessed zone) and d higher magnification of red square in c

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Table 3 Summary of EDS spots, in at.%, of FSPed composites (Fig. 4). Numbers in first column correspond to point’s number indicated in the figure. The nominal composition is listed in last column
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A top view of the surface composite 2024/Al2O3p FSPed at the SZ, after heat treatment, is shown in Fig. 6. Compared to the HAZ (Fig. 5c), the grain size has been reduced due to the stirring action of the FSP tool. Moreover, both these regions underwent precipitation at grain boundaries. It must be noted that swelling behavior was obvious by naked eye at the surface of the FSPed composite as well as extensive pore creation in the SZ.

Fig. 6
figure 6

Stir zone top view of the heat-treated AA2024 /Al2O3p composite’s microstructure

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Further investigation of the heat-treated surface composite in the FSPed zone was done by EDS elemental mapping (Fig. 7). Oxygen, copper, and magnesium distribution revealed that Al2O3 particles were attracted to agglomerate at grain boundaries along with Cu and Mg (Fig. 7b–e), causing their depletion in the Al matrix.

Fig. 7
figure 7

SEM-EDS elemental mapping of heat-treated AA2024-T351/Al2O3p surface composite

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3.2 Friction and wear behavior of AA2024/Al2O3p surface composites

Figure 8 shows the friction coefficient (μ) variation with sliding distance and corresponding wear scar 2D profiles. For unprocessed AA2024 before and after heat treatment, the μ starts at high values and decreases to lower steady-state values. On the other hand, for AA2024/Al2O3p surface composites, the μ decreases with increasing the FSP cycles, and lower μ values were recorded for untreated samples. In means of wear resistance, the material removal rate of the FSPed AA2024/Al2O3p surface composites before heat treatment was lower compared to as-received alloy. However, after heat treatment, the wear resistance of the composites decreased to the same removal rate of the unprocessed 2024 alloy. The highest wear resistance was attributed to the FSPed surface composites after 3 cycles, without heat treatment.

Fig. 8
figure 8

a Evolution of friction coefficient versus sliding distance before and after heat treatment for unreinforced AA2024 and reinforced alumina surface composites. b 2D profiles of as-received AA2024-T351 and reinforced Al2O3p

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Worn track SEM micrographs of unreinforced surface composites (Fig. 9) showed signs of delamination (spalling), pits, and cracks relatively perpendicular to the sliding direction; the latters are signs of adhesive wear, while the grooves parallel to the sliding direction are showing evidence of an abrasive wear mechanism. Plowing wear was highly seen on the unreinforced alloy compared to Al2O3p-reinforced AA2024 surface composites, as shown in Fig. 10. The dominant wear mechanisms in the case of surface composites were less adhesive and more abrasive compared to unreinforced Al 2024 alloy. Comparative EDS analyses on the tribolayers formed on both unreinforced and reinforced AA2024 matrix are both showing the presence of Al, O, and traces of Mn, Cu, and Mg. However, for reinforced composites before and after heat treatment, a small presence of Fe was perceivable in the tribolayer composition.

Fig. 9
figure 9

Worn surfaces of a as-received AA2024-T351 and b heat-treated AA2024 without Al2O3p reinforcements

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Fig. 10
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Worn surfaces of a as-FSPed AA2024-T351/Al2O3p, b higher magnification of red square marked in a, and c heat-treated AA2024/Al2O3p composites

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4 Discussion

Many microstructural studies on different FSP zones were extensively reported in the literature. The heat generation during the friction between the FSP tool and workpiece causes dissolution and partial re-precipitation inside the nugget zone (NZ), grain deformation in the thermo-mechanically affected zone (TMAZ), and precipitation coarsening in the heat-affected zone (HAZ) [14,15,16,17, 20, 21]. As the metal matrix is deformed at a high temperature and individual grains experience diverse stages of straining, this highly transient microstructure type demonstrates different mechanical properties.

Microhardness of AA2024-T351 was enhanced by integrating the Al2O3p compared to FSPed sample. This behavior has been well reported for AMCs reinforced by Al2O3p [11, 16, 27]; besides grain refining due to severe plastic deformation induced by FSP, the alumina particles contribute to further refinement of the microstructure’s grain size during the recrystallization process in the nugget zone. Another factor and most importantly, alumina particles restrict the dislocations’ movement through the Orowan mechanism effect. Hence, in Al/Al2O3p FSPed surface composites both ex situ incorporated particles and Al2Cu precipitates contribute to Orowan strengthening and the strengthening through Hall-Petch effect induced by FSP. The aforementioned effects enhanced the hardness of the Al/Al2O3p surface composites.

In previous works, many researchers reported the effect of heat treating Al-Si alloys reinforced with Al2O3p [9]. However, few research works have been devoted to Mg containing Al alloys reinforced with Al2O3p. It has been reported that the formation of Al2MgO4 spinel deteriorated the mechanical properties of the composites [8, 9] when others reported on its beneficial effect [16]. Tekmen and Cocen [28] solved this issue, and they stated that Al2MgO4 affects positively the AMC if the spinel is formed homogeneously at the particle/matrix interface, where it does the contrary if its formation is discontinuous. Al2MgO4 spinel forms in 2024 alloys because of the higher Mg to Si content [29], and the Si content could be increased in Al/Al2O3 composites to form Mg2Si precipitates rather than the spinel phases.

The elemental distribution shown in Fig. 7 presents probably the formation of spinel phases such as Al2MgO4 or Al2CuO4 through the reaction of Al2O3 and matrix elements. Herein, it is assumed that the complete disappearing of Al2O3 particles is more probable, where the total formation of the spinel phase is expected; future investigations are needed to confirm the above statements.

At 530°C, some liquid Al forms at the grain boundaries [18] and facilitates the O diffusion released during the reaction of Al2O3 and Mg during the formation of the spinel phases [30, 31]. This grain boundary precipitation causes a slight increment of the hardness values after heat treatment. On the other hand, the voids and pores were responsible for hardness drop, explaining such low values in microhardness profiles. It was previously reported that alumina particle incorporation accelerates the precipitation mechanism in precipitation-hardened Al-Si alloys [32]. Based on the totality of these results, it is reasonable to conclude that heat treatment of 530°C for 2h causes an over-aging behavior of AA2024/Al2O3p surface composites, which affects the precipitation mechanisms, in both SZ and HAZ. Any greater attractive forces of the Cu or Mg, such their attraction to Al2O3 particles, should be taken into account or the hardening mechanisms would not be activated. To avoid such issues, it is likely to design 2024 with higher Si/Mg content because of the high affinity of Mg with Si to form Mg2Si compared to Mg with Al2O3p.

Grain boundary precipitation is known to decrease the fracture toughness and formability of aluminum alloys, which could be the reason for deterioration of wear resistance of heat-treated AA2024/Al2O3 [33, 34]. Steele et al. [35] studied the influence of quenching rate on the grain boundary precipitation extent on 6XXX aluminum alloys after solution heat treatment, and it was shown that as the quenching rate decreased, the grain boundary precipitation increased. It is known that Al2O3 particles decrease the thermal conductivity of Al matrix; thus, it may affect the quenching rate of the studied composite [36]. In metals including Al alloys, the conductivity is generally correlated with strength and hardness of the material. A higher strength is usually accompanied with a lower electrical conductivity and vice versa [37]. After PPHT, the hardness values increased; thus, it is expected that the material conductivity decreases [38]. It should be noted that many factors could influence the conductivity such as type and nature of precipitates, grain boundary precipitation, grain size, dislocation density, and crystalline defects [37, 38]. Hence, it could not firmly be confirmed that material conductivity decreased only by strength correlation, without taking all these factors in consideration.

The second reason could be caused by the thermal history induced by FSP. As FSP causes coarsening of precipitates, hardness was frequently seen to decrease in the HAZ due to grain boundary precipitation [39]. The driving force for precipitation is caused by heat and nucleation sites; Liu et al. [40] showed that HAZ presents low dislocation density due to different microstructural changes in 7075AA matrix under the FSW thermal cycle. Reducing the number of dislocations will reduce the nucleation sites at grain interior; hence, grain boundary precipitation is more probable to occur.

Thirdly, oxygen migration from SZ to HAZ could be responsible for grain boundary precipitation as occurred in HAZ; as evidence of EDS point 8, some areas showed oxygen presence. However, EDS should be taken by a grain of salt in quantifying oxygen. Therefore, more works are needed to understand the main factor responsible for this phenomenon in the present FSPed composite system.

The wear scars of AA2024/Al2O3P surface composites before and after heat treatment were characterized by a transition from adhesive to a more dominant abrasive wear mechanism, as shown in Fig. 10. The abrasive wear is due to de-cohesion and de-bonding of the hard Al2O3 particles as third-body abrasive particles. The less adhesive action is due to the hardening of the Al matrix; alumina enhanced the load bearing of the sliding steel ball and minimized the wearing by adhesive wear. It was previously reported [23, 41] that the delamination through adhesive wear is triggered by the crack initiation and growth at the subsurface mechanically mixed layers formed on the wear scars. Heat generation during dry sliding wear decreased the plasticity of the aluminum alloys, which decreases the toughness and ultimate mechanical resistance of the matrix. Thus, it causes a premature failure of this subsurface layer. Another possible effect of the surface composite wear resistance compared to unreinforced alloy is due to the low conductivity of the Al2O3p particles which prevented the heat dissipation during sliding wear, hence delayed the delamination of the mechanically mixed layers (MMLs), resulting in less adhesive wear mechanism.

Contrary to the unreinforced alloy, the EDS analysis revealed the presence of Fe in the tribolayers (or MML) formed on the composite worn surfaces. The iron were transferred from the sliding ball to the composite surface and further oxidized due to the abrasive actions of Al2O3p. It has been reported extensively the beneficial effect of Fe-containing tribolayers on the wear and friction behavior of many metal matrix composites [23, 41, 42]. Fe-containing tribolayers could also be the cause of the wear resistance enhancement.

The heat treatment did not change the wear mechanisms for AA2024/Al2O3p composites, and the low wear resistance of heat-treated AA2024/Al2O3P composite is due probably to the pores and voids that triggered high wear and material removal. Hence, it can be concluded that over-aging heat treatment on AA2024/Al2O3p composites has a detrimental effect on the wear resistance.

5 Conclusion

Herein, AA2024 surface composites were processed by inserting Al2O3p particles as reinforcements through the friction stir processing route. The effect of heat treatment on microstructure and wear behavior was studied; the main conclusions are as follows:

Integrating Al2O3P particles inside the AA2024 matrix enhanced its hardness to an average value of 150 HV as well as wear resistance by a factor of two.

Heat treating the surface composite did not have any beneficial effect on wear resistance or hardness, except recovering the HAZ’s hardness.

After heat treatment, Al2O3P particles reacted with Cu and Mg to form spinel phases and precipitated at the grain boundaries. Moreover, the precipitation mode in the HAZ changed to a precipitation at grain boundaries; this was caused probably by the thermal history induced by FSP.