On the Spray Forming of Metals, the Formation of Porosity and the Heat Evolution during Solidification

This thesis deals with the heat evolution during solidification and its relation to the formation of porosity. It intends to improve the current understanding of the formation of porosity in cast materials with special interest in nodular cast iron and the spray forming process. Two different systems, a Fe-based alloy, Cast iron, and a Ni-based alloy, Inconel 625, are examined. The effect on the heat evolution of the morphology and the processing parameters in spray forming are treated. An evaluation of the microstructural features, segregation behavior and physical properties such as latent heat of fusion is performed by using thermal analysis under cooling rates ranging from 0.1 to 10 K/s. In order to achieve this a modified differential thermal analysis (DTA) equipment, a mirror furnace and levitation casting are used. Results are presented in terms of the fraction of solidified, the cooling rate and the microstructure observed. The measured latent heat of fusion is not constant throughout the solidification process. Variations in morphology and cooling rate affect the release of the latent heat. A thermodynamic model is used to describe the experimental observations and to explain the formation of pores in nodular cast iron by taking into consideration the formation of lattice defects during the liquid/solid transformation. In this case the formation of porosity is regarded as a consequence of changes in the volume fraction ratio graphite/γ during the solidification process. A numerical model of the spray forming process is developed by means of CFD modelling and compared with experimental measurements performed in an industrial facility. Stagnation pressure measurements provided information about the gas flow velocity and an analysis of the overspray powder provided information about the particle thermal history. Evaluation of the deposit was also performed. It is observed that the process conditions in spray forming promote non-equilibrium solidification even though solidification at the deposit occurs at a lower rate. In this case the porosity formed near the interface substrate/deposit depends largely on the substrate temperature. The presence of certain reactive elements, such as titanium, affects the porosity levels in the rest of the deposit. Descriptors: Thermal Analysis, Nodular Cast Iron, Inconel 625, CFD, F low Assesment, Multiphase F low, Spray Deposition, Microporosity, Superalloys This thesis is based on the following supplements: Supplement 1. J. Tinoco and H. Fredriksson, ’On the solidification behavior of nodular cast iron’, Proceedings of the International Conference ”The Science of Casting and Solidification”, Brasov, Rumania, 305-311 (2001) Supplement 2. J. Tinoco, P. Delvasto, O. Quintero and H. Fredriksson, ’Thermal analysis of nodular and lamellar eutectic cast iron under different cooling rates’, International Journal of Cast Metals, Vol. 16 No. 1-3 (2003) Supplement 3. H. Fredriksson, J. Stjerndahl and J. Tinoco, ’On the expansion and contraction of nodular cast iron’, ISRN-KTH: MG-INR-03:05se, TRITA-MG 2003:05, Royal Institute of Technology, Stockholm, Sweden, Oct. (2003) Supplement 4. J. Tinoco and H. Fredriksson, ’Solidification of a modified Inconel 625 under different cooling rates’, accepted for publication in High Temperature Materials and Processes, Sept. (2003) Supplement 5. J. Tinoco, B. Widell, H. Fredriksson and L. Fuchs, ’Modelling the in-flight events during metal spray forming’, in press in Materials Science and Engineering A, Nov. (2003) Supplement 6. J. Tinoco, B. Widell and H. Fredriksson, ’Fluid flow studies in a metal spray deposition chamber. CFD model and validation’, accepted for publication in Materials Science and Engineering A, Sept. (2003) Supplement 7. J. Tinoco and H. Fredriksson, ’Metal spray deposition of cylindrical preforms’, ISRN-KTH: MG-INR-03:06se, TRITA-MG 2003:06, Royal Institute of Technology, Stockholm, Sweden, Oct. (2003)