PROCESS OPTIMIZATION AND MECHANICAL PROPERTY OF FRICTION STIR PROCESSED ALUMINUM FOAM

Abstract

Metallic foams are a vital engineering material that can be used in various engineering fields due to their unique physical, mechanical, thermal, and acoustic properties. A remarkable combination of properties allows for the simultaneous optimization of stiffness, strength, overall weight, thermal conductivity, surface area, and gas permeability. These distinct combinations of properties suggest numerous possible applications. The high porosity of metal foam allows it to absorb a substantial amount of mechanical energy under deformation. Therefore, foams can function as impact energy absorbers, limiting intensity in crash situations. Several conventional methods have been successfully used to fabricate metal foams. Methods like the ARB process route, the powder metallurgy method, direct gas foaming, and the melt route are well-established processing routes for foam fabrication. However, some of the limitations associated with these fabrication methods, so it is important to explore new foam fabrication methods. Additionally, in many industrial applications, especially in the aerospace and transportation industries, one of the most crucial problems is fabricating foam in a specific or selected area. Also, controlling the porosity and pore morphology is still challenging because of the dependency of porosity on a number of the process parameters. Therefore, exploring the fabrication routes by which foam can be fabricated with the desired properties in a specific location with controlled porosity and pore morphology is necessary. A new foam fabrication method, friction stir processing (FSP), has recently been developed and can potentially fabricate the pores in a desired or selected area. The present study uses FSP to fabricate AA6061 closed-cell aluminum foam. TiH2 and Al2O3 are used as blowing and stabilizing agents, respectively. This study uses a statistical approach to control porosity and the influence of major process parameters on porosity in a friction-stir-processed aluminum plate. A second-order Response Surface Methodology (RSM) model has been developed, and relationships among porosity and major process parameters like weight%TiH2, weight% Al2O3, number of passes, tool rpm, and foaming temperature have been established. The influence of each parameter on porosity is also investigated. After that, the effect of varying the foaming temperature from 708 ̊C to 740 ̊C and fixing other process parameters on porosity and pore distribution has been studied.Further, the combined effect of density and pore distribution on quasi-static compression deformation was investigated with a fixed strain rate of 0.001s-1. Experiments have established a relationship between the stress-strain curve and the cell deformation mechanism. The combined effect of density and pore size on different mechanical properties has been studied. Further, the effects of Al2O3 addition from 4 to 10 wt. % on the density and morphology of the pores have been investigated. Quasi-static compression and Vickers hardness testing have been done to investigate the effect of alumina addition on compression and microhardness properties. Experimental results validate the model, optimizing 80–85% porosity for multifunctional applications. The optimal process parameters are 1.2 wt% TiH2, 2.3 wt% Al2O3, three passes, 3000 tool rpm, and 738 °C foaming temperature. It was observed from the ANOVA that the most influencing process parameter is tool rpm, followed by the number of passes and foaming temperature. The scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analyses of the fabricated foam at its optimized condition confirm the uniform distribution of elements and pores. This statistical optimization model reduces experimental time and resource consumption, so this study has significant technical and industrial relevance. The EDS mapping was also done to illustrate the elemental Ti, O, and Al distributions before and after compression. The compression testing identified four types of cell failure, including hinge formation, shearing, bending, and crack initiation with tearing, showing similar results to cell failure of fabricated foam by the conventional method. Quasi-static compression studies confirm that plateau stress and energy absorption also increased in the studied region with increased density and pore size. Energy absorption per unit volume was reported to be highest at 1.21 g/cm3 density. Closed-cell aluminum foam with approximately 0.64 to 1.08 g/cm3 was successfully fabricated at different wt% of alumina by 4-pass FSP at 730 ̊C foaming temperature. A relationship between density and mechanical properties has been established. An increase in Al2O3 wt.% plays a constructive role in increasing the homogeneous distribution of pores and mechanical properties in the studied range. The complete research work is presented in the seven chapters in the proposed thesis. Chapter 1: This chapter of the thesis briefly introduces the background of metal foams fabricated by conventional methods, and in the later section, a new foam fabrication method, friction stir processing (FSP), is explained in detail.Chapter 2: This chapter provides an extensive literature review on foam fabrication by conventional methods and foam characterization, process techniques, modeling, and simulation. In the later section, an update on the research done in aluminum foam fabrication and mechanical properties by a new friction stir processing route. In the end, the research gap and objective of the present study have been included. Chapter 3: This chapter includes the detail of manufacturing and design methodologies adopted for the fabrication of closed-cell aluminum foams. A detailed study of experimental work and equipment used for fabrication and characterization has been conducted. It includes the fabrication procedure (friction stir process), the micro structural analysis of the materials (scanning electron microscopy), the modeling tools (response surface methodology, ANOVA), and the mechanical testing (compression test, hardness test) used in the study. Chapter 4 deals with fabricating different densities of closed-cell aluminum foams by friction stir processing. This chapter examines a systematic statistical approach to control the porosity and distribution in a sandwiched aluminum plate developed using the recently advanced friction stir processing manufacturing method. With the help of statistical means and experimental methods, the model’s adequacy is checked and well-established. The influence of process parameters on porosity has been studied. A second-order model is designed using the Response Surface Method by analyzing the effect of process parameters on porosity. This multiple regression model establishes a relationship between porosity with various process parameters (foaming temperature, number of passes, tool rpm, weight %Al2O3, and weight %TiH2). The Response Surface Method (RSM) is coupled with experimental results to check the validity of the developed model. Porosity is optimized to 80-85% since foams with this porosity can have multiple functions, especially in automobile and naval applications. The SEM and EDS analyses of the fabricated closed-cell Al foam at its optimized condition have been done. At the end of this chapter, the conclusions of the study are included. Chapter 5: This chapter deals with the quasi-static compression behavior of closed-cell aluminum foams at ambient conditions. Aluminum foams with varying densities of 1.21–0.62 g/cm3 and pore sizes of 0.3–5.8 mm were successfully fabricated by friction stir processing (FSP) by varying the foaming temperature and fixing FSP parameters. Further, the combined effect of density and pore distribution on quasi-static compression deformation was investigated with a fixed strain rate of 0.001s-1.Experiments have been done to establish a relationship between the stress-strain curve and the cell deformation mechanism. A separate compressive stress-strain curve is drawn to a minimum density of 0.62 g/cm3, while corresponding deformation points on the curve are analyzed by the SEM micrograph and relate to identifying the cell failure mechanism. The EDS mapping was also done to illustrate the elemental Ti, O, and Al distributions before and after compression. Furthermore, the combined effect of density and pore size on different mechanical properties has been studied. At the end of this chapter, the conclusions of the study are included. Chapter 6: This chapter deals with the investigating effect of alumina addition on mechanical properties and energy absorption of closed-cell aluminum foam fabricated by friction stir processing technique. The effects of alumina addition from 4 to 10 wt. % on the density and morphology of the pores have been investigated. After foam fabrication, quasi-static compression and Vickers hardness testing have been done to investigate the effect of alumina addition on compression and micro hardness properties. A relationship between density and mechanical properties such as plateau stress and absorbed energy has been established, and the alumina addition effect on microhardness was investigated. At the end of this chapter, the conclusions of the study are included. Chapter 7: This chapter summarized the major conclusions from the present study followed by a short discussion on future scope so that consolidated growth of this work can be achieved.