EFFECT OF MICROSTRUCTURAL MODIFICATION ON THE ENERGY ABSORPTION CAPACITY OF ALUMINIUM FOAM

Abstract

With the advancement in technology, there is increasing scientific and industrial interest in the development of material with excellent properties intended for automotive, structural and lightweight applications. Metal foam is a class of material offers a unique combination of mechanical and physical properties such as light weight, good energy absorption and sound absorption capacity. Potential applications of metal foams include light weight cores for sandwich panels, shells and tubes, where the foam can increase the resistance to local buckling, increase the impact resistance and improve the energy absorbing capacity of the structure. Energy absorption capacity offers potential applications in transportation applications where damage due to impact load result in damage, for example, foam-filled hollow sections in automobiles (such as crash box, door assembly and wings) may reduce damage and injuries resulting from impact. For this type of application, closed cell metal foam is more suitable than the bulk alloy, because it deforms plastically under the impact without spring back, preventing further damage. Thus, there is a need to develop the effective and practical ways to improve mechanical properties of closed cell aluminium foam in order to improve energy absorption capability of aluminium foam. From above brief introduction, it is convincible to state that the aluminium foam has the potential to be used in different field of applications mainly related to energy absorption. Therefore, it is necessary to strengthen the closed cell aluminium foam so that it can sustain the compressive loads and absorb maximum energy. In general, load applied on closed cell metal foam is sustained by the cell wall; therefore, it is necessary to strengthen the foam cell wall material. Generally, the strength of the metal foam is refers to plateau stress, which is the extended constant stress domain beyond the elastic limit. Therefore, it is reasonable to state that the strengthening of the metal foam cell wall at microscopic level helps to improve the energy absorption capacity. Strengthening can be achieved through refinement of the cell wall structure. Refined structure can be achieved either by post processing technique or by adding suitable grain refiner which change growth mechanism from dendritic to nucleation and growth. Post processing techniques (mainly related to thermal treatment) and several techniques (i.e. addition of grain refiner, mechanical vibrations, and mechanical stirring) are employed for the modification of the cell wall microstructure. In foaming process due to process limitations, only some of the techniques (stated above) can be used to change the microstructure. Use of the mechanical vibration and stirring is less feasible in the foaming process, whereas addition of grain refinement, reinforcement and post processing technique (i.e. thermal treatment) known to improve microstructure, as well as, mechanical performance of the alloy can be employed. Although the current state of research on the foaming technology and the property improvement is on high, still researchers struggles to find out the way to improve the mechanical property and the stability of the liquid melt (to minimize the drainage problem). Hence, there is an urgent need to establish a serious attention to modify the cell wall microstructure, so that the energy absorption capacity can be improved. Similarly, drainage phenomenon should be addressed property to improve the stability of the foam. Therefore, in the present thesis work a serious effort has been made to deal with the different aspect (i.e. grain refinement, distribution of the intermetallic phases) of cell wall microsturctural modification in order to improve the energy absorption capacity with less drainage. The Chapter 1 presents introduction to the topic and gives a brief idea about the aim and scope of the work. It also provides an insight into the type of foam used in the present work. The Chapter 2 outlines a brief review of literature, discussing the basic description of the foam (Section 2.1-2.2), exist metal foam processing techniques with corresponding overview (section 2.3). Selection of the matrix material (section 2.4), physics of foaming (section 2.5), the factor affecting the stability of the foam structure (section 2.6) and the effect of the cell wall structure on the stability of the foam (section 2.7) are presented in the respective section. Properties and applications of the foam (section 2.8-2.9) followed by various strengthening techniques (section 2.10) are presented in their respective section. On the basis of the literature review, the proposed formulation of the problem is discussed in section 2.11. The Chapter 3, titled experimental details: material and methods, presents materials used in the work (Section 3.1), experimental setup for foam processing, different characterization techniques adopted in the present work are discussed in the subsequent section (Section 3.2-3.5). Chapter 4 describes the modification of the cell wall material of the produced closed cell foam at optimized parameters (i.e. stirring time, stirring speed, amount of blowing agent and holding time) through microsturctural modification using post processing technique (i.e. thermal treatment). Chapter four is divided into four main sections with respective subsections. The chapter begins with the raw material characterization, including compositional analysis, TGA analysis of the TiH2 and master alloy characterization. It is found that the blowing agent decompose in the range of 490- 575 oC. Master alloy characterization mainly shows the importance of Cu addition, it is found that the Cu addition results into proper foaming and this behaviour is attributed to the network like structure distributed evenly throughout the alloy matrix, which helps in lowering the drainage at the time of foaming [1]. Further, to analyze the macrosturctural, microsturctural behaviour and phase present in produced closed cell aluminium foam, a detailed study of produced foam has been carried out in the second section. From the analysis, it has been noticed that the formation of second phase network like structure retard the sever drainage and results in adequate pores structure, however, microstructure evaluation of the cell wall matrix shows the presence of thick dendritic structure. Dendritic structure may results in deterioration of the performance of the metal foam, therefore, to reduce the dendritic structure foam sample further thermally treated at different temperature and duration. The effect of thermal treatment on the cell wall microstructure is discussed in the next section (third) of the chapter 4. The reduction in the dendritic structure has been observed with the thermal treatment. The thermal treatment process is carried out in two steps, first solutionising of the sample followed by water quenching, whereas, in second step sample further aged to evaluate the effect of precipitation hardening on the cell wall microstructure. It is found that solutionising results in homogenized structure with some eutectic phases, whereas, further aging results in precipitation, however, as aging time and temperature increased grain become coarser. The effect of microsturctural modification on the mechanical behaviour of the foam is discussed in the last (fourth) section of the chapter 4. It is observed that after solutionising, microhardness of the foam decreased, which further increased when sample aged, shows that aging results in precipitation hardening. The compressive properties of the foam sample improved after thermal treatment, solutionised sample shows highest property gained, whereas, aging result in moderate improvement in compressive behaviour when compared with sample in as-cast condition. In order to check the grain refinement effect on the energy absorption capacity, a study of the effect of grain refinement has been carried out in the chapter 5. Chapter formation is similar to chapter 4 i.e. master alloy characterization, foam characterization, evolution of the effect of grain refinement and finally compressive behaviour analysis. Scandium is used as grain refiner, the different amount of scandium added (in the form of master alloy) during master alloy formation; using stirring and melting. It has been observed that with the use of grain refinement, the stability of foam improves. Drastic improvement has been observed in the stability of the foam, macrostructure shows negligible, or in other words, no drainage after the scandium addition. Through microsturctural analysis effect of Sc addition in terms of grain refinement is observed, grain size is reduced up-to 27μm, which is approximately 200% lower as compared to the foam without scandium addition (82 μm). Microstructure analysis shows that the grain refinement is related to the distribution of the alloying element and their distribution in the cell wall, which in-turn affects the hardness and improves the compressive behaviour of the foam. Mechanical property analysis of the produced foam shows that the maximum hardness value for Al-Sc foam is observed at 1 wt. % of scandium addition, which is 154 VHN with minimum grain size (27μm). It has been observed that compressive behaviour of the Al-Sc foam also support the microsturctural improvement. Yield strength, plateau stress and energy absorption capacity improves as grain is refined. The improvement in the performance of the foam may be attributed to growth mechanism which changes from dendritic to nucleation and finally growth, results in grain refinement. To check the effect of reinforcement on the energy absorption capacity, reinforcement method with different amount of reinforcement is explored in Chapter 6. Chapter formation is similar to chapter 4 and chapter 5. Short steel fibre (SSF) is used for reinforcement; reinforcement has been done at the time of foaming. SSF is coated using electroless coating technique, which improves wetting behaviour of the SSF. Improvement in the stability of the foam is observed after foam reinforced with CCSSF. Through microsturctural analysis it is found that reinforcement has significant effect on cell wall microstructure. Secondary phases developed after reinforcement is distributed along the plateau boarder and within the alloy matrix. Stability of the foam may be attributed to this distribution, which in-turn affects the hardness and improves the compressive behaviour of the foam. Mechanical property analysis of the reinforced foam shows that the microhardness value improved moderately at small amount of reinforcement (0.5 wt. %) and improves significantly (20-30%) as reinforcement amount is increased. The results of the axial compression test indicates that the compressive properties (i.e., yield strength, plateau stress and energy absorption capacity) improves after the reinforcement. From results it is observed that reinforcement after 1 wt. % result in unreacted fibre surrounded by porosity which in turn reduces the effect of reinforcement.