PROCESSING AND MECHANICAL PROPERTY ANALYSIS OF CLOSED-CELL ALUMINUM FOAM

Abstract

Metal foams are a unique class of engineering material consisting of a rather uniform distribution of deliberately introduced void spaces. Unlike casting defects the void fraction in metal foams is fairly large, often exceeding 70 volume %. The void cavities in closed-cell metal foams are insulated from each other by the cell wall material. The relative density, which is the ratio of the density of foam to the solid material, is generally used to characterize the foam structure. Metal foams have received considerable attention in recent years due to their unique combination of properties which lend them to applications such as impact/blast energy absorption, light weight structures, sound and vibration absorption. In recent years, several new cost effective foaming techniques are available to make metal foams with better structural control. While melt foaming route to produce closed-cell aluminum foam has the potential for commercialization, processing challenges to produce specified foam structure still exists. The mechanical property of the closed-cell aluminum foam formed by degassing titanium hydride (TiH2) in molten aluminum is dependent on the structural parameters, such as, the relative density, pore size, cell wall thickness and aspect ratio. On the other hand, the structural parameters themselves are a function of the process parameters, such as the melt viscosity, amount of TiH2 addition and the duration allowed for degassing. An objective of the present work is to use existing modelling techniques to establish a processing-structure-property correlation which could serve as a design guide for aluminum foam manufacture. Chapter 1 consists of an introduction to metallic foams as a special class of engineering material with unique set of properties which can be tailored to meet specific light weight structural requirements. In chapter 2, an overview of the state of scientific and applied knowledge of metal foams with regard to processing and properties is presented. Three manufacturing methods hold promise for mass production of aluminum foam, namely, (1) powder metallurgy route, (2) melt foaming by gas injection and (3) melt foaming with blowing agents. The present work concerns the aluminum melt foaming by the decomposition of titanium hydride. Foam stability is an important parameter which establishes the time window within which the liquid foam has to be developed and quenched to solidify the structure. An understanding of the mechanism of liquid foaming is essential to improve foam stability and establish control over the foaming process. iii The literature on the mechanical property of aluminum foam, such as compression modulus, plateau stress, tensile property and energy absorption is discussed along with the application potential. Analytical modelling attempts by various researches are briefed of which the Gibson and Ashby model for predicting the Young's modulus and plateau stress is the most popular. The prediction is based on the relative density, cell wall material property and a structural parameter, cp, which is the fraction of solid in the cell edges. Several are those reported difficulties while comparing their results with the Gibson and Ashby model. For this reason statistical modelling approach is taken to correlate the structure to the mechanical property for the first time in this work. Chapter 3, which is titled 'Experimental procedure and the modelling tools' gives the details of the experimental and modelling tools used in this work. A special purpose top opening resistance heating furnace was designed and fabricated to process aluminum foam. Calcium is added to the melted aluminum alloy in the crucible and stirred for a stipulated period. Once the melt viscosity is appropriated, TiH2 is added and stirred at a higher speed. Subsequently, the stirrer is removed and held inside the furnace for different holding time for foaming. Once the foams were made, structural and microstructural characterizations were carried out. The structural analysis included the relative density, average cell size and cell aspect ratio of the foam. The microstructural analysis was carried out using scanning electron microscope (SEM) along with energy dispersive spectroscopy (EDS) to look for elemental distribution within the cell walls. Subsequently, quasi-static compressing test and dynamic impact tests were carried out from which the Young's modulus, plateau stress, energy absorption and strain rate sensitivity data were collected. The --later part of the chapter also contains a brief description of the modelling tools such as design of experiments (DOE), multi-linear regression analysis and artificial neural networks (ANN) which were used in this work. Chapter 4 consists of the results and discussion of the foam process parameters, the mechanical property behaviour, and the modelling of foam processing and properties. By varying the process variables wide range of structural properties of foam are assessed. The relative density of foam obtained ranges from 0.062 to 0.40 with average pore diameter variation from 2.116 to 4.495 mm and having cell aspect ratio between 1.007 and 1.364. Design of experiments (DOE) is used to identify the major influencing parameters and also to determine and control the quality of the finished product. Based on the literature published and by conducting many pilot experiments, the process parameters which are not significant and to bring a focus to the present study, parameters iv such as: composition of base alloy, stirrer design, melt temperature, stirrer speed after calcium and TiH2 addition, holding temperature and solidification rate are maintained the same in all experiments. The process parameters selected for analysis in the first phase of the experimentation with half-fractional factorial design are: (1) amount of calcium metal addition (2) the stirring time after calcium addition (3) amount of TiH2 addition and (4) the holding time after TiH2 addition. An analysis of variance (ANOVA) is then carried out separately for each of the response (structural parameters) in order to test the model signification and suitability. From those observations, it is observed that the amount of TiH2 addition in the melt and the holding time after dispersing the TiH2 are the prominent process parameters influencing the foam structure. Therefore, in the second phase of experiments the response surface methodology (RSM) is employed to evaluate the relevance of these two process parameters on the final cell structure in a wider range by central composite design (CCD). It is observed that both the process variables: the amount of TiH2 added to the melt and the holding time are inversely proportional to the relative density. However, the influence of TiH2 addition is very high in comparison with holding time. The amount of TiH2 addition and holding time are directly proportional and have equal influence on the average pore diameter of the final foam structure. Moreover, holding time had more direct influence on the cell aspect ratio than the amount of TiH2 addition. It is understood that the stable foam structure with less cell aspect ratio and cell wall defects is processed when the amount of TiH2 addition is near 1.0 wt.% and the holding time around 100 s. Particle identification of the cell wall material were done by correlating the Energy Dispersive X-ray Spectroscopy (EDS) results with X-ray powder diffraction results obtained in the 2-0 range of 20-90° using Cu Ka radiation. It is found that aluminum oxide is only confined to the surface of the pores and the intermetallic particles are present within the cell wall matrix. The foam made using the graphite stirrer had A120CaTi2 and Al2Cu intermetallic particles, whereas the samples prepared with the stainless steel stirrer had Al13Fe4 intermetallic along with A120CaTi2 and Al2Cu particles dispersed within the cell wall. Al2Cu lamella is present as fine lamella whereas A120CaTi2 is present as blocky precipitates within the cell wall. A113Fe4 intermetallic was formed within the matrix by the diffusion of iron from the stainless steel stirrer in spite of the alumina coating of the stirrer, is present as elongated platelets of rather high aspect ratio. Al2Cu forms only on solidification so it has no influence on foam stability. On the other hand, A120CaTi2 and A113Fe4 can be present as solids at 730°C in the cell walls and will provide stability to the liquid foam. Foam specimens are subjected to quasi-static compression testing at the strain rate of 1x10'3 s'i. Dynamic impact tests at the strain rate range of about 7.5x102 s'l were carried out on the split Hopkinson pressure bar (SHPB). The test results were analysed for Young's modulus, plateau stress and energy absorption capacity of the foam. It is observed that the mechanical property of aluminum foam depends mainly on their relative density. However, for the same relative density the plateau stress increases with pore diameter which is attributed to the increase in cell wall thickness. Experiments were also being carried out at higher strain rates, commensurate with automobile crash and the results showed strain rate sensitivity, especially at higher relative densities. This result is particularly encouraging for high impact absorption applications as the material displays strain rate hardening. ANN models are developed for the prediction of compressive properties of closed-cell aluminum foam using the large experimental database from the quasi-static compression test. The input parameters chosen for development of ANN models are the relative density, average pore diameter and cell aspect ratio. All the models developed are of acceptable accuracy within the experimental data range, considering the complexity of the property correlation of metallic foams. These ANN models could be beneficial to the foam manufactures to build more general and particular property database of aluminum foam. Similarly, multiple linear regression models are developed from the observed data by correlating the structural parameters with the compressive properties and found to be satisfactory. In summary, the closed-cell aluminum foam processing parameters were characterized and the empirical models developed to obtain tailor-made physical and morphological properties. The microstructural analysis of cell wall material has highlighted the role played by the morphology of the intermetallic particle which affect foam stability. The mechanical properties and energy absorption capacity in static and dynamic compression state are evaluated and found to be consistent with requirements for high impact applications. The modelling approach taken to correlate the process parameters to the structural parameter and in turn to the mechanical properties in this work are of acceptable