Thermal and microstructural parameters in the transient solidification of Al-Mg and Al-Mg-Si alloys and correlation with mechanical and corrosion resistances

Author(s)

Crystopher Cardoso de Brito

• Document type: Doctoral Thesis
• Press: Campinas, SP.
• Institution: Universidade Estadual de Campinas (UNICAMP). Faculdade de Engenharia Mecânica
• Defense date: 2016-02-24

Examining board members

Amauri Garcia; Rezende Gomes dos Santos; Cecília Amélia de Carvalho Zavaglia; Eduardo Netto de Souza; Maria Aparecida Pinto

• Advisor: José Eduardo Spinelli; Amauri Garcia

Abstract

The mechanical characteristics (strength under static and dynamic loading; wear resistance) and chemical characteristics (corrosion resistance) of as solidified metallic components depend on the microstructural arrangement, i.e. grain size and cellular, dendritic, interphase spacings; non-homogeneity of chemical composition; size, morphology and distribution of inclusions; porosity, etc. Additionally to the barriers to slip formed by the grain boundaries there are also obstacles located in intercellular and interdendritic regions. The development of correlations between the as-solidified microstructure and the corresponding properties can be a complex task, which depends on careful experimental development with a view to determining accurate solidification thermal parameters. The present study aims to develop a theoretical/ experimental analysis dealing with the effects of transient thermal parameters in the solidification of Al-Mg and Al-Mg-Si alloys on the microstructure development and on macrosegregation profiles. Correlations between microstructural parameters and mechanical and corrosion resistances will be established with a view to permitting solidification operational conditions to be pre-programmed in order to allow the casting to attain a determined level of final properties. Therefore, directional solidification experiments both in transient (vertical upward and downward) and steady-state regimes (Bridgman growth) were carried out. For binary Al-3Mg and Al-6,5Mg and ternary Al-3Mg-1Si and Al-6,5Mg-1Si alloys, no evidence of macrosegregation has been found, not even for cases of downward directional solidification. With the exception of the Al-3Mg-1Si alloy, all the alloys experimentally examined depicted a dendritic Al-rich matrix along the whole range of experimental cooling rates ( dT/dt ).In contrast, the Al-3Mg-1Si alloy showed cellular morphology for 0,005 < dT/dt > 2K/s, and dendritic for 0,03 > dT/dt < 0,8 K/s. These results characterized the occurrence of a reversal cellular/dendritic transition (high cooling rates cells), which is rarely reported in the literature for metallic alloys. Experimental growth laws relating cellular and dendritic spacings (primary, secondary and tertiary) to solidification thermal parameters (growth and cooling rates) have been proposed for any alloy examined. Mechanical properties (hardness and tensile properties) and corrosion resistance have been examined for the Al-3Mg and Al-3Mg-1Si alloys. The Vickers microhardness of the ternary alloy is shown to the highest due to a more complex microstructure and a higher content of Mg2Si and Al-Fe-Si(-Mg) intermetallics. The best combination ultimate tensile strength/elongation to fracture is shown to be associated with the cellular microstructure of the Al-3Mg-Si alloy, despite the higher values of elongation of the Al-3Mg alloy. Hall-Petch type experimental laws are proposed relating these tensile properties and Vickers microhardness to the length scale of the microstructure, more specifically to the cellular and primary dendritic arm spacings. Corrosion tests in a 0.06M NaCl solution were performed and showed that more refined cellular and dendritic microstructures are associated with higher corrosion resistances when compared with the corresponding values of coarse microestructures. The cellular microstructural pattern associated with a more extensive distribution of Fe-rich intermettalics of binary and ternary eutectic mixtures along the intercellular regions, are shown to be responsible for the better corrosion resistance of tests conducted with electrolytes of different NaCl concentrations. A preliminar study on the effect of microstructural features of an Al-3Mg-1Si alloy on the wear and tribocorrosion resistances is conducted (AU)

• FAPESP's process: 12/08494-0 - Thermal and Microstructural Parameters in the Transient Solidification of Al-Mg and Al-Mg-Si Alloys and Correlation with Mechanical and Corrosion Resistances
• Grantee: Crystopher Cardoso de Brito
• Support Opportunities: Scholarships in Brazil - Doctorate