Wear properties of Al/TiO 2 composites fabricated via combined compo - casting and APB process

: Compo - casting method is one of the popular techniques to produce metal - based matrix composites. But one of the main challenges in this process is un - uniform spreading of reinforced subdivisions ( particles ) inside the metallic matrix and the lack of desirable mechanical prop - erties of the ﬁ nal produced composites due to the low bonding strength among the metal matrix and reinforce - ment particles. To remove these di ﬃ culties and to promote the mechanical properties of these kind of composites, the accumulative press bonding ( WAPB ) technique as a sup - plementary technique to heighten the mechanical and microstructural evolution of the casted Al/TiO 2 composite bars was utilized as a novelty in this study. The micro - structure evolution and mechanical properties of these composites have been compared with various WAPB steps using tensile test, average Vickers micro hardness test, wear test and scanning electron microscopy ( SEM ) . The SEM results revealed that during the higher APB steps, big titanium dioxide ( TiO 2 ) clusters are broken and the TiO 2 particles are distributed uniformly. It was shown that cumulating the forming steps improved the mechan - ical properties of the composites. In general, combined compo - casting and APB process would consent making Al/TiO 2 composites with high consistency and good micro - structural and mechanical properties.


Introduction
Currently, the use of aluminum matrix composites (AMCs) is felt in several productions such as automobile, aerospace, vessels and chemical productions.They possess properties such as high strength, good wear resistance, good chemical resistance, low thermal expansion coefficient, high elastic modulus and lightweight [1][2][3][4][5][6][7][8][9].For the production of metal matrix composites (MMCs) amid the engineering methods, compo-casting is generally popular for its easiness, cost efficiency and its ability for fabricating in large and industrial scales.The compo-casting process is different from the stir casting in which the ceramic or oxide particles are inserted into the molten metal or alloy [10][11][12][13][14][15][16][17][18].Although the compo-casting process is a cost-efficient process for producing MMCs, there are some restrictions in the final produced composites.One of them is the porosity formed during the process which makes low mechanical properties, non-uniform dispersion of reinforced particles and generating zones with free, high amount of particles amount and finally low bonding strength among the reinforced particles and metal matrix due to the low amount of the compression of metal matrix around the particles [17][18][19][20][21][22][23][24][25][26][27].So, it is essential to develop the mechanical properties of these composites with a supplementary forming process with high amount of plastic strain.On the other hand, combining the compo-casting process with a severe plastic deformation (SPD) is a good idea.To produce ultra-fine grain (UFG) materials such as powder metallurgy, there are several SPD processes [11,[28][29][30][31][32][33][34], such as accumulative press bonding (APB) [26], cyclic extrusion compression (CEC) [17,20,27,[35][36][37][38][39][40][41][42], multi-axial forging and so on.Amid these techniques, APB process was proposed by Amirkhanlou et al. in 2013 [37].Also, ARB process is presented as a cumulative forming process to fabricate strip laminated MMCs by Saito in 1998 [38][39][40][41][42][43][44][45][46][47][48][49].Based on all the recent investigations done, there are two main kinds of reinforcement particles used for production of MMCs.The first group is metallic particles such as tungsten and copper [47,[50][51][52][53].The second group is ceramic particles such as titanium dioxide (TiO 2 ), TiC, SiC, SiO 2 , B 4 C and WC [17,20,27,30,35].Also, the main reason for using TiO 2 particles as reinforcement particles in this study is that TiO 2 has a high hardness value in ceramics and does not react with Al matrix [20].To overcome the abovementioned difficulties of the compo-casting process of Al/TiO 2 composites, a novel combined compo-casting and accumulative press bonding (WAPB) process together at 300°C is recommend in this study.Increasing the pressing temperature allows the aluminum matrix to have a better flow around the TiO 2 particles, which improves the bonding among the aluminum matrix and TiO 2 particles as reinforcement.The purpose of this study is to produce Al/TiO 2 composite samples with a highly uniform spreading of TiO 2 particles through Al matrix containing high mechanical properties and wear resistance.
2 Experimental procedure

Materials processing
During this research, AA1060 and nano TiO 2 particles with average size of 50 nm were selected as matrix and reinforcement, respectively.Table 1 illustrates the chemical (elemental) specification of AA1060 used in this research.

Fabrication of cast composites
AA1060/5 wt% TiO 2 composites were formed by compocasting process.Diagram of the experimental sequences utilized in the stir casting is displayed in Figure 1.
In each test, in a graphite crucible of 2 kg capacity, about 1,500 g of AA1060 was melted and the temperature of the molten Al was elevated to 740°C.For having an unchanging temperature state, the molten Al alloy was reserved at the fixed temperature for around 3 m.Then, the melt was enthused at 600 rpm using a graphite propeller with the injection of TiO 2 particles in a pure argon (99.99%) atmosphere.After the end of the injection and after a constant cooling at a cooling rate of 4.5°C/min, the temperature of the final molten alloy is 600°C and then cast hooked on a steel die located under the heater.

APB process
After making stir cast composites, samples with 100 mm length, 50 mm width and 2 mm height were machined.Then, the samples were fully annealed at 450°C for 2 h before the APB process.Then, the samples were degreased in acetone bath for 15 min.In order to eliminate the surface oxide layer, the surfaces of samples were fully brushed to guarantee a successful bonding.There are four theories presented to explain the bonding mechanism, namely, film theory, energy barrier theory, diffusion bonding theory and joint recrystallization theory [18].As can be seen in Figure 2, adsorbed ions, greases, oxides and dust subdivisions surround the surfaces of samples.Using a rotary speed of 2,000 rpm, 95 mm diameter stainless steel with 0.26 mm wire diameter and the sample bars were scratch brushed after degreasing in the acetone bath.So, surface cleaning before each cumulative pressing is essential to generate an acceptable bonding.Then, the two bars were stacked together to achieve 10 mm thickness and press bonded with 50% reduction (plastic strain = 0.8) at 300°C minus any lubrication to acquire 5 mm thickness, Figure 3.
To handle the pressing process, a 100 t hydraulic press machine was used.The composite produced after one step of APB was cut into two parts and preheated at 300°C for 6 min.Then, two bars of MMC were loaded each other after surface cleaning, Figure 3.The fabricated sample was halved into two bars and a pressing process with a 50% of thickness reduction was repeated up to eight steps.The specification of the APB process is proposed in Table 2. Increasing the plastic strain during the cumulative pressing leads to a good scattering of TiO 2 particles.The tensile test specimens with dimensions of 25 and 6 mm along the longitudinal direction were prepared based on the ASTM-E8M standard, This trend is also similar to the yield strength.Two mechanisms can clarify this behavior, (І) strain hardening (dislocation strengthening) at low number of steps and (ІІ) grain boundary strengthening mechanism by increasing the steps due to the creation of UFG aluminum matrix [33,34].In the second stage, limited strain hardening around the TiO 2 subdivisions is a main object for growing the strength hardening [34].TiO 2 as an additive part can initiate slip systems in the aluminum matrix close its together layers which their density and the amount of local strain hardening in them expands by cumulating the plastic strain up to the 8th step.
Figure 6 shows the elongation values against the steps.As can be seen in Figure 6, there is a rapid drop from the annealed Al (23.25%) to step #2 (1.72%).This rapid drop can be ascribed to the strain hardening due to plastic and the less movement of dislocations [39].This trend reverses from step #2 up to step #8 where it reaches its maximum value (7.26%).This improving behavior can  Wear properties of Al/TiO 2 composites  537 be attributed to three mechanisms: (I) growing uniformity of the TiO 2 subdivisions through the matrix, (II) enhancement of the bond strength between the matrix and TiO 2 and (III) breaking of particle clusters and reducing the absorbencies in the structure.Figure 7 shows the SEM micrographs of the agglomerations in the aluminum matrix of composite samples after eight steps of APB.The micrograph for the eighth step fabricated Engineering Strain (-) Step #2 Step #1 Step #6 Step #4 Step #8+ one step of pressing Step #8 sample shows small clusters and an unvarying dispersal of TiO 2 particles in the structure.On the other hand, at higher amounts of plastic strain, the porosities in the clusters are eliminated.Figure 8 illustrates the toughness value of samples vs the APB steps.Matching to Figure 8, tensile toughness value drops severely from the annealed Al (19.2 × 10 4 J m −3 ) up to step #2 (2.36 × 10 4 J.m −3 ) and then begins to grow slightly from step #2 to step #6 (8.34 × 10 4 J m −3 ).But this trend becomes slower than before from step #6 to step #8 (10.02 × 10 4 J m −3 ).Growing values of the strain and strength amplitudes during the APB process is the main reason for the increase in the toughness of compocasted Al/TiO 2 composites.

Hardness test
The tensile toughness and average Vickers microhardness of composite samples vs various APB steps are displayed in Figure 8.According to Figure 8, the average microhardness has an increasing rate from the annealed Al up to step #4 and then remains approximately constant with a minor additional change up to step #8.The initial increasing stage of the average Vickers microhardness is linked to the strain hardening and cumulating the dislocations density inside the crystalline lattice [39].By increasing the number of steps up to eight, the hardness is saturated [35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50].Dislocation saturation occurs at larger plastic strains [39,53].It seems that due to the locking mechanism of dislocations occurring at higher plastic strains creates a unvarying spreading of particles through the alloy matrix [39].Step #2

Wear test
Step #4 Step #6 Step #8 ElongaƟon (%) Strength (MPa)   Wear properties of Al/TiO 2 composites  539 composite samples produced with first and eighth APB steps after the wear test.Also, Figure 9 shows the morphology of the worn surface of composite samples after the first and eighth steps.According to Figure 9, the weight loss of composite in the first step is more than the sample fabricated via eight steps.In other words, debris particles formed at the initial steps cause the increase in the mass loss at higher APB steps, while uniform distribution of TiO 2 particles leads to the reduction in weight losses.The wear resistance of all APB samples increases in comparison with the annealed Al.From steps 1-4, the wear curve increases rapidly.This trend is due to the hardening effect of the Al matrix and dislocation strengthening around TiO 2 particles.Also, from step #4 up to step #8, there is a smooth increasing rate due to the saturation of dislocation strengthening through the Al matrix.Also, by generating a harder situation for cracks propagation by decreasing the porosities between the Al and TiO 2 particles which improves their bonding, the wear resistance improves.For sample produced after cold pressing with 50% reduction in the thickness of the composite sample with eight steps of APB processing, the wear resistance increases rapidly.While all prior accumulative pressing processes were done at 300°C, this behavior is due to the severe strain hardening effects during the applied cold pressing process.There are three mechanisms to clarify the wear phenomenon in metals and composites which are abrasion, adhesion coexists and delamination.But based on Figure 10, delamination was the dominant wear mechanism at higher APB steps [45].During the wear test, the extent of debris particles increases by increasing the APB steps.The study of bond formation between the composite layers is essential to explain this phenomenon.Figure 11 shows the mechanism of bond formation during the APB process.Before the bonding, and as mentioned before, the surface of composite bars is composed of absorbed ions, dust particles and grease.So, to create a successful bond formation, surface preparations including wire brushing and degreasing in the acetone bath is necessary.Surface brushing usually creates a hardened surface which increases the surface roughness.By starting the surface expansion due to the pressure of the pressing, contact of opposite surfaces of layers begins and virgin metals reach each other through the widening fissures and cracks and then create metallic bonding zones.So, when the crystalline structures of layers are same as together, the APB process can generate metallic bonding between them.Increasing the volume of virgin metals at higher plastic strains creates electron sharing between layers and makes bonding on the atomic scale.So, numerous bonds are formed by noticeably bigger, wide areas of the base alloy or metal.These unbonded regions look like small lonely islands where are suitable zones for crack delamination and propagation due to the presence of these minor islands, during the wear test.By increasing the number of layers at higher number of APB steps, the distance between interfaces decreases.These distances are equal to the thickness of layers which are demonstrated in Table 2.

Worn surface morphology
Figure 12 indicates the surface morphology of composite samples after wear testing.According to Figure 12, the wear rate of samples decreases at higher number of steps.Also, small wear tracks are the result of abrasive wear mechanism, Figure 12.Finally, the result shows that the composite sample fabricated after eight steps, has better wear resistance property in comparison with the sample fabricated with one APB step [7,8].
Therefore, it is reasonable that the extent of delamination was increased with the increase in the number of APB steps.Indeed, during the wear test, the nature of laminated structure in the APB process helped more extensive delamination.In Figures 13 and 14, a model describing the wear mechanism is presented for Al-based multilayered composites [47,51].In Figure 13, the second zone describes deformed regions of the base matrix.The elastic-plastic deformation was noted at the interface between zone 1 and 2. Zone 3 is known as tribolayer, which includes worn surface oxides formed and counter face material.Also, the wear debris made between zone 2 and 3 is the results of combination of voids and crack formation.So, the conditions of material, environment and sliding wear have severe effects on the compositional features and extents of these subsurface regions which are established rapidly [47].
Based on the abovementioned discussions, after beginning of wear between the worn surface and the pin, development of the grains happened at deformed regions due to high temperature with more growth in the subsurface region than the middle thickness.So, containing a strain incompatibility regarding the fine grain structure and nonequilibrium UFG grains, a coarse grain structure is formed under the subsurface.In the other words, debris particles were formed when the strain incompatibility produced a delamination of coarse grains.Also, these delamination and recrystallization processes happened in repetitive  rounds.As can be seen in Figure 15, the generation of large particle debris in a shape is the result of this kind of wear mechanism.The initiation and nucleation of cracks on the sliding surfaces are the results of high shear stresses.So, this changes the shape of loss of material from flakes to plates.So, the flow of surface material is toward the sliding direction which generates abrasive grooves under higher applied load [47,[49][50][51].

Fractography
Figure 16a and b clearly shows the SEM fracture surface of samples with two and four steps.In the fracture surface of samples at prior steps, the fracture surface contains long and deep-routed dimples.But by cumulating the steps up to six and especially eight, the basic rapture surfaces do not illustrate long and rooted shape dimples [7,8,51].Figure 16(b) shows that TiO 2 particles have an important effect on the shape of rapture surface.They sit on the walls and ends of dimples and become crack initiation and nucleation colonies [47,49].

Conclusion
1. Combined compo-casting and APB procedure can be an alternative to the conventional technique of achieving high strength AMCs. 2. The TiO 2 particulates are uniformly scattered in the Al matrix as shown by the SEM.The strength, tensile toughness and wear resistance of composites were enhanced due to the presence of additive TiO 2 particles.The other reason is the uniform scattering of these particles through the metallic matrix that generates local strain hardening around these particles.3. SEM results revealed that at higher APB steps, the spreading of particles rises considerably due to the elongation of laminates during cumulative forming steps.Wear properties of Al/TiO 2 composites  543 4. The ultimate tensile strength of samples reaches a maximum value of 162 MPa after the 8th step which is about two times more than that of annealed Al. 5.The maximum elongation of the annealed Al is 23.25% which drops to 1.72% for the sample with two steps.Then, after a certain number of steps, it improves and reaches 7.26% after the eighth step which reveals that the TiO 2 has an improving role in the elongation value.6.The tensile toughness of the annealed Al is 19.2 × 10 40 J m −3 which drops to 2.36 × 10 4 J m −3 for the sample with two APB steps and then it enhances to 19.2 × 10 4 J m −3 for the eight stepped sample.In other words, the TiO 2 particles can improve the tensile toughness of composites with more steps.7. The hardness value improves with the APB steps due to the presence of reinforcement phase.8.The results revealed that the composites with TiO 2 particulates have better wear resistance property compared to annealed base alloy.Particles generate local strain hardening and enhance the wear resistance.9.The investigation of wear resistance of hybrid composites for further investigation is recommended.

Figure 4 .Figures 5
Figures 5 and 6 display the tensile strength values of composite samples vs dissimilar APB steps.The strength of the annealed Al is 81.3 MPa and for the sample with two steps of APB is equal to 161.6 MPa which is a rapid increasing rate.Afterward according to Figure6, the tensile strength remains approximately constant by cumulating the number of steps up to eight (162 MPa).This trend is also similar to the yield strength.Two mechanisms can clarify this behavior, (І) strain hardening (dislocation strengthening) at low number of steps and (ІІ) grain boundary strengthening mechanism by increasing the steps due to the creation of UFG aluminum matrix[33,34].In the second stage, limited strain hardening around the TiO 2 subdivisions is a main object for growing the strength hardening[34].TiO 2 as an additive part can initiate slip systems in the aluminum matrix close its together layers which their density and the amount of local strain hardening in them expands by cumulating the plastic strain up to the 8th step.Figure6shows the elongation values against the steps.As can be seen in Figure6, there is a rapid drop from the annealed Al (23.25%) to step #2 (1.72%).This rapid drop can be ascribed to the strain hardening due to plastic and the less movement of dislocations[39].This trend reverses from step #2 up to step #8 where it reaches its maximum value (7.26%).This improving behavior can

Figure 2 :
Figure 2: The atomic arrangement conditions during the surface preparation.

Figure 3 :
Figure 3: Diagram design of the APB.

Figure 9
Figure9displays the weight loss of Al processed after various steps.Figure8shows the weight loss of

Figure 8 :Figure
Figure 8: The average Vickers microhardness and tensile toughness.

Figure 10 :
Figure 10: Schematic illustration of the delamination during the wear test.

Figure 11 :
Figure 11: Diagram of bonding mechanism of the stir casted composite samples during.

Figure 13 :
Figure 13: Diagram of the subsurface zones under the wear surface [53].

Figure 14 :
Figure 14: Drawings of the sliding wear of APB composite samples vs different steps of deformation and recrystallization mechanism.

Figure 15 :
Figure 15: Micro morphology of debris after (a) first step and (b) eighth step.

Figure 16 :
Figure 16: The rapture morphology of composites with (a) one and (b) eight steps APB process.

Table 1 :
Chemical components of AA1060 Diagram of the alternate process to make cast Al/TiO 2 composites.

Table 2 :
Cumulative steps of the APB process to fabricate Al/ TiO 2 composite ** Step #9 refers to step #8 + 1 step of cold pressing.Figure 4: Orientation of the tensile test specimens.