Date of Award

12-2023

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical and Civil Engineering

First Advisor

Shengyuan Yang

Second Advisor

Veton Kepuska

Third Advisor

Albert Bleakley

Fourth Advisor

Ilya Mingareev

Abstract

Today, significant research efforts in the field of manufacturing engineering are focused on developing new lightweight, fuel-efficient and high-performance materials. The evolution of composite materials and related technologies has brought great advantages to material development throughout history. Composites are versatile materials with unique mechanical and physical properties tailored to specific application needs. Composites allow engineers to make design choices that traditional monolithic (unreinforced) materials cannot offer. Particle Reinforced Metal Matrix Composites (MMCs) offer a variety of advantages, including increased stiffness, improved fatigue resistance, increased wear resistance, and a low coefficient of thermal expansion. Aluminum MMC (AMMC) material systems outperform monolithic materials, so they can be widely used in structural and non-structural applications in various engineering domains. However, due to the presence of reinforcement, these composites tend to be brittle in nature, thus affecting their surface characteristics and reducing their lifetime. Engineering applications demand materials that provide both strength and ductility, and it is a challenge to find a single material that offers both simultaneously. Therefore, the development of composites with enhanced strength and ductility is essential. Many researchers are currently exploring the production of surface composites. Methods such as plasma spraying, high-energy electron beam irradiation, high-energy laser melting treatment, casting, and Friction Stir Processing (FSP) are used to fabricate surface aluminum matrix nanocomposites. However, except for FSP, all other techniques involve processing the liquid phase at higher temperatures, which causes undesirable interactions between the reinforcements and the matrix, resulting in the formation of unwanted phases. FSP is an environmentally friendly and solid-state method based on friction stir welding, working below the melting point of the matrix, which has gained much attention from researchers. FSP is the only technique that can provide nanosized reinforcement and uniform surface distribution, surpassing traditional sur- face treatment methods. This study aims to explore and select optimal parameters for friction stir processing (FSP) to fabricate surface composites with Aluminum 7075 alloy. These composites were reinforced with nano-sized particles of titanium dioxide (TiO2), Boron Carbide (B4C), and graphene (Gr) to improve tribological performance. FSP proved advantageous in achieving excellent levels of surface dispersion and nanoscale reinforcement. This study utilized a range of processing parameters, comparing moderate parameters from 800 to 2000 rpm and 25 to 45 mm/min, with a unique configuration utilizing a high rotational speed (2000 rpm) and 45 mm/min feed rate for sample fabrication. The coefficient of friction and wear performance of the surface composite were evaluated using a pin-on-disc/tribo-tester under limited lubrication conditions. In addition, scanning electron microscopy (SEM) was used to examine the surface composites under various FSP conditions. The microhardness of the AA7075/B4C surface composite produced at a rotational speed of 1400 rpm exceeded that of the other two surface composites. Compared to the parent metal, the AA7075/B4C surface composite recorded the lowest wear rate, followed by the AA7075/Gr and AA7075/TiO2surface composites. Optimization was carried out to determine the main parameters affecting tribological properties (surface roughness, wear rate, and coefficient of friction) and microhardness using response surface methodology. The results showed that the surface composites manufactured at a high rotational speed of 1400 rpm and a feed rate of 45 mm/min showed a significant decrease in the coefficient of friction and wear rate, with an increase of 39% and 73%, respectively, compared to the base material. Notably, the AA7075/B4C surface composite manufactured at 1400 rpm showed superior tribological and corrosion resistance compared to the other two surface composites.

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