OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 3 2024 research work, a hybrid MMC is prepared using industrial waste red mud and bio-waste coconut shell ash. Previously many researchers utilized coconut shell ash as an adsorbent to remove heavy metals and dyes from aqueous solutions [2], in the development of building materials such as brick tiles [3], cementitious and polymeric composites [4,5] and in the production of activated carbon [6]. Similarly, red mud is used as a coating material [7], as a mortar, aggregate tiles [8], a mineral cementitious material [9], a ceramic material [10] and for heavy metal leaching and wastewater treatment in general [11]. Some of the researchers have used the combination of bio-waste with ceramic material to create and evaluate a hybrid Al MMC. In [12] coconut shell ash and graphene are used to evaluate abrasive wear properties. In [13] and [14] a mixture of agricultural waste in the form of coconut shell ash and baggas ash with aluminum oxide was used to evaluate the mechanical properties of the developed hybrid Al MMC. In [15] and [16], a mixture of rice husk ash with red mud and alumina was used, respectively, to evaluate the tribomechanical behavior of hybrid Al MMC materials. Hardness Hardness is an important parameter to check mechanical strength of composite materials. The hardness of composite material depends on various parameters, such as the particle size, heat treatment, the weight ratio of the reinforcingmaterial and the interatomic bonds between the reinforcingmaterial and the parent matrix. Previously, many researchers optimized this hardness parameter and concluded that the hardness of composite increases with decreasing particle size and the heat treatment. Also, the increases to an optimal weight percentage, which varies from reinforcing material to the composite as there is good inter-atomic bonding between the reinforcing material and the matrix but at higher weight percentage the hardness value decreases due to agglomeration of the reinforcing material in the matrix layer, which leads to the formation of pits and cavities. Cavities lead to the propagation of cracks and a decrease in tensile strength and hardness [17]. The hardness values are dependent on many parameters such as reinforcing material weight fraction, indentation load, treatment behavior, inter-atomic bonding etc. For this reason, a large number of composite specimens are required for experiments and determining its characteristics becomes expensive and labor intensive. Therefore, experimental design and Taguchi analysis are suitable approaches for optimizing input and output parameters. In this paper, the aluminum composite material obtained through stir casting route and its Brinell hardness value are optimized using ANOVA and regression analysis. The selected orthogonal array L27 of the experimental plan and the effect of the signal-to-noise ratio, the graph of the normal probability of the remainder, response characteristics, contour diagrams are tabulated for various composite samples. L27 orthogonal array of design of experiment are chosen and the effect of the signal-to-noise ratio, normal probability plot of residual, response characteristics, contour plots are tabulated for various composite specimens. Method and Materials The 5051 series aluminumalloywas chosen as parent material due to high stiffness and strength-to-weight ratio, high corrosion resistance and optimum thermal properties and widespread use in the development of building materials, in the automotive and aerospace industries. Red mud and coconut shell ash were used as reinforcing material to develop a hybrid composite material. Here, red mud is bauxite residue and is used as the primary reinforcing material, the content of which varies from 5 to 12.5 % and is purchased from the Balco aluminum refinery. Around 100 coconuts were purchased from various temples in Jaipur to obtain ash by burning and sieving. The proportion of coconut shell ash used as a secondary supportive reinforcing material varied from 5 to 12.5 wt. %. Both red mud and coconut shell ash were properly sieved to obtain particles of about 50 μm in size as hardness increases with decreasing particle size. Composite preparation Nine aluminum specimens were prepared via stir casting process. Ceramic crucibles were used for casting of aluminum metal. Red mud and coconut shells were preheated to 200 °C to remove moisture
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