Mechanical behaviour of stir cast TiO2/Redmud/Al6061 metal matrix composites

The present work deals with mechanical characterization of titanium oxide (TiO2)/Redmud/Aluminium alloy 6061 (Al6061) hybrid metal matrix composites (HMMC’s) processed using stir casting method. The weight fractions of TiO2 are varied (2%,4% and 6%) and amount of redmud is kept constant (2%) in Al6061. The optimised parameters for stir casting are employed for fabrication the samples of all the configurations and cut according to ASTM standards. The presence of the elements of the constituents is confirmed by XRD and EDS. Effect of TiO2 and redmud content on Tensile, Compression and Hardness are studied in detail. The results reveal that the tensile, compressive and hardness of the prepared composites enhance with TiO2 content. The yield and ultimate strength increase in the range of 29.77%–72.36% and 29.96%–63.90% respectively than the Al6061. The compressive strength enhanced in the range of 9.28%–33.20% than the Al6061. Enhancement of hardness in the range of 9.41%–41.17% is observed as compared to Al6061. The fractography studies confirm uniform mixing of the constituents and load transfer mechanism.


Introduction
Aluminium based composites are developed and used in response to the ever-increasing need for lightweight materials with high specific strength suitable for aerospace and automotive applications [1]. Metal matrix composites (MMC's) are replacing aluminium alloys in aerospace and automobile structural applications due to their superior strength, low density and energy-saving potential. The reinforcements in these composites are in 2. Materials and methods 2.1. Constituent materials Al6061 in the form of plates are procured from Omshree Alloys, Mumbai, India and used as matrix material. The composition of Al 6061 is represented in table 1. Titanium oxide (TiO 2 ) in powdered form (figure 1(a)) is purchased from Mincometals, Bangalore, India. The SEM image ( figure 1(a)) depicts the size of the particles   Zn 0.05-0. 10 8 Ti 0.07-0. 12 9 Al Balance range from 1-10μm. Redmud in powdered form (figure 1(b)) supplied by Nangal Enterprise, Jamnagar, Gujarat, India. TiO 2 and redmud are used reinforcements to form hybrid Al6061 matrix composites. Three different compositions as depicted in table 2 are fabricated using stir casting. The Quantity of redmud is maintained constant (2% weight fraction) and TiO 2 content is varied in the step of 2% weight fraction up to 6% (table 2).

Fabrication procedure
The Hybrid Al6061 matrix composites in the present study are produced using stir casting setup having tilt table electric resistance melting furnace which operates at 220 V and 50 Hz with a stirrer supplied by Tecnovia Engineering, Ahmedabad, India Stirrer has a speed range from 200 r min −1 to 450 r min −1 The step-by-step procedure is represented in figure 2. 2 kg of Al6061 is melted at 750°C (figure 2(a)) and stirred by an electric motor is driven ceramic coated mechanical stirrer which forms the vortex. crucible is made of graphite and cylindrical die (250 mm length and 25 mm diameter) is made of high chrome high carbon steel. Once the molten pool attains the uniform temperature of 750°C the degassing agent Hexachloroethane (0.3% by weight) is added (figure 2(b)) [39][40][41][42]. Magnesium chips (1% by weight) (figure 2(c)) is added to enhance the wettability of the reinforcements [43]. The Redmud and TiO 2 particles of known proportions are weighed and preheated to a temperature of 200°C for 30 min in an electric muffle furnace (figure 2(d)) and then added in the molten Al6061. The molten Al6061 is further agitated with both the reinforcements in 10 min with the constant stirrer speed of 200 rpm (figure 2(e)). Later the mixture of all the constituents is poured into a preheated cast iron mould and left for solidification (figure 2(f)). The solidified HMMC's as shown in figure 2(g) are machined according to ASTM Standard and tested. Samples of Al6061 are also produced in the same manner for comparison purpose. The stir casting process parameters for the present work are selected according to the initial pilot experimental trials and literature survey carried out. Further, the stir casting process parameters were optimized to obtain the homogeneous distribution of the reinforcements in the matrix phase.

Compression testing
The compressive behaviour of stir casted TiO 2 /Redmud/Al6061 metal matrix composites is studied using the same machine which is used to study tensile properties. The six specimens of each configuration having 25±1.0 mm length and 13±0.2 mm are prepared according to ASTM E9-09 (figure 4).

Hardness test
A Brinell hardness test (BATLI BOI Ltd) carried out using a 5 mm ball indenter with a load application of 250 kgf and a dwell time of 30 seconds for each sample at different locations. The hardness test is performed according to ASTM E10-18.

Morphological analysis using scanning electron microscope (SEM)
In the current work, the SEM images are taken on a VEGA 3 TESCAN machine for different magnifications on a fractured surface.

Results and discussion
After the fabrication of samples, SEM analysis is carried out to understand the uniform mixing of the constituents as represented in figure 5. From the SEM images, it is evident that the TiO 2 and Redmud are being distributed uniformly. Which states the process parameters employed for stir casting have resulted in better distribution. Further, it is also noted that the good adhesion between the constituents is evident from the SEM images. Agglomeration of the TiO 2 and Redmud particulates is not observed in the micrographs (figure 5). As the weight fraction of TiO 2 is increased the greater number of particles are witnessed in the SEM images (figure 5) and the same amount of Redmud particles are observed in all the images. The x-ray diffractograms (XRD) of the fabricated configurations, Al6061 (figure 6(a)), Al6061+2% Redmud +2% TiO 2 (figure 6(b)), Al6061+2% Redmud +4% TiO 2 (figure 6(c)) and Al6061+2% Redmud +6% TiO 2 composites (figure 6(d)) are represented. The XRD is performed by a h-2 h diffractometer  composite with 2 weight percentage of redmud and 6 weight percentage of TiO 2 exhibits the maximum tensile strength due to strong coherent bonding between the matrix and reinforcement phase in the composites developed. The interfacial bonding strength is further accelerated by the addition of magnesium, which predominantly helps in enhancing the wettability of TiO 2 and Redmud with the aluminium 6061 matrix phase. Strong interfacial bonding between the constituents is built up due the addition of the reinforcements. And due to additions of TiO 2 and Redmud particles the pores in the microstructure are occupied leading to enhancement of tensile strength. Addition of both these reinforcements and the formation of precipitates are limiting the mobility of the dislocations, leading to increased yield and ultimate tensile strength [44]. The mobility of the dislocations is limited due the better bonding the constituents [45,46]. Similar effect is observed when the TiO 2 is added into Al matrix composites [22,34,37]. Further, addition of Redmud in MMC have shown increase in tensile strength [28][29][30]47]. The processing temperature and uniform mixing of the constituents in the matrix medium might have led to unstable exothermic reactions at the interface of the constituents and helped in increasing the strength of the composites [48,49].
The ultimate tensile strength of the HMMC's increased with an increase in the weight percentage of redmud and TiO 2 mainly due to the higher strength of TiO 2 and better bonding between the constituents with the aluminium matrix as depicted in figure 7(b) and table 3. However, the addition of TiO 2 beyond the 6 weight percentage leads to severe embrittlement that reduces the tensile strength [12]. It is seen from figure 7(c) and table 3 the tensile strength of TiO 2 (2%)/Redmud (2%)/Al6061 composites shows enhancement of 29.96% and TiO 2 (6%)/Redmud (2%) /Al6061 shows 63.90% this is attributed to strain hardening effect of the composites added and properties rendered due to the addition of constituents. The failure is predominantly due to elongation in the matrix phase than the debonding between the constituents.

Hardness test
The prepared HMMC's are tested to measure the resistance to indentation (Brinell hardness) by determining the depth of indentation as per ASTM E10 ( figure 11). The indentations are made six different locations as shown in figure 10 and average values are reported in figure 1. It is observed from the bar graph (figure 1) that adding Redmud and TiO 2 particles to an aluminium alloy increases its hardness to the tune of 85 BHN to 120 BHN. Previous articles have shown additions of TiO 2 [34] and Redmud [29,30,47] individually have enhanced the hardness. This enhancement may be due to reduction in porosity that has led into reduced plastic deformations of the composites. Further the hardness has been in increased due to the change in microstructure due to  processing temperature of the composites. Apart from the strong interfacial bonding between the constituents, the Hall-Pitch strengthening [34,50,51] mechanism may also be contributed for enhancement of the hardness in the fabricated composites.   presence of Calcium (Ca), Oxygen (O), Iron (Fe), Sodium (Na), Titanium (Ti) etc inside the matrix. The selected area for the EDS spectrum has been shown on the right side of each image(fugure13). Therefore, these SEM structures are evidence of the successful incorporation of hybrid Al/TiO 2 redmud composites.

Conclusions
In this work, three different samples are fabricated and the following inferences are made.
• TiO 2 and Red-mud based composites have been successfully fabricated using the stir casting method.
• The presence of the elements in prepared composites are confirmed using XRD analysis • The results reveal that the tensile, compressive and hardness of the prepared composites enhance with TiO 2 content.
• The yield and ultimate strength increase in the range of 29.77%-72.36% and 29.96%-63.90% respectively than the Al6061.
• The compressive strength enhanced in the range of 9.28%-33.20% than the Al6061.
• Enhancement of hardness in the range of 9.41%-41.17% is observed as compared to Al6061, • SEM images show that a fairly uniform distribution of TiO 2 and red-mud particles have been obtained in the aluminium 6061 matrix.
• The property map is prepared in comparison with studies available, indicated the prepared composites are inline with the other studies and probably can be future replacements.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).