Microsc. Microanal. 21 (Suppl 5), 2015 © Microscopy Society of America 2015
33 doi:10.1017/S1431927615013975
Corrosion Behavior of the Friction Stir Welded AZ31 Magnesium Alloy F. Nascimento1, J.C.S. Fernandes2, P. Vilaça3 and F.M. Andrade Pires4 1.
IDMEC, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal DEQ/CQE, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal 3. Department of Engineering Design and Production, School of Engineering, Aalto University, Aalto, Finland 4. Faculdade de Engenharia da Universidade do Porto, Porto, Portugal 2.
Magnesium alloys are considered to be a valuable engineering material since they offer a significant potential to reduce weight due to their low density, good thermal and fatigue properties, among others. However, problems appear when trying to apply conventional welding techniques to these alloys. The typical low weldability of magnesium alloys with traditional joining processes involving melting is expected to be improved with Friction Stir Welding (FSW) since it is a solid state joining process. Magnesium alloys are also very susceptible to the heating effects being an advantage for the FSW process since it works at lower temperatures than the conventional methods. The FSW process is used in this work with 2mm thick AZ31 plates and the samples were welded at 100mm/min and 800rpm with a constant applied force of 600kg and 0.50 tilt angle. These alloys are known to be very susceptible to corrosion in saline environments [1] and some authors have found that usually, in magnesium alloys, some particles are not dissolved in the magnesium matrix and will promote pitting corrosion [2]. When analyzing the base material the existence of undiluted particles (white articles) aligned with the material matrix was observed. After welding, there was an increase of these particles in the heat affected zone (HAZ) while the nugget that suffers both mechanical and thermal stresses have promoted the decrease of the grain size and also reduced the number of undiluted particles in this area (Figure 1). After performing an Energy Dispersive Spectroscopy (EDS) analysis in one of these particles, it was confirmed to be an AlMn particle which has not been dissolved in the magnesium matrix (Figure 2). After a corrosion test it was observed that there was pitting corrosion promoted by these alloys (Figure 3) where the particles would act as a cathode corroding the magnesium matrix around it. While this was expected for the exposed particles, it was also observed that the interior particles promote corrosion when in close vicinity of the corrosion front. The reduction of the AlMn particles will improve the corrosion properties and therefore, the usage of the FSW process can be effective since the nugget displayed a great reduction of these particles. It was also observed that there is an increase of particles in the heat affected zone increasing the corrosion in this area. Using higher advancing speeds will reduce the heat released in this process and can effectively reduce the heat influence over the existing particles. The FSW process presents itself as a process that can effectively join magnesium alloys and further studies should focus on the reduction of the heat released to improve the corrosion resistance of the magnesium alloys [3]. References: [1] A Samaniego el al., Corrosion. Science 68 (2013) p. 66. [2] R Zeng et al., Trans. Nonferrous Met. Soc. China 16 (2006) p. 763. [3] The authors gratefully acknowledge the financial support provided by Ministério da Ciência e Tecnologia e do Ensino Superior - Fundação para a Ciência e Tecnologia (FCT) under the project "Development of Integrated Systems for Smart Interiors" reference MIT-Pt/EDAM-SI/0025/2008. FN would like to acknowledge FCT, via the PhD scholarship SFRH/BD/33729/2009
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Microsc. Microanal. 21 (Suppl 5), 2015
Figure 1. Microstructure of the FSWed magnesium: HAZ (left) and Nugget (right)
Figure 2. Detail of inclusions on Mg matrix (left) and the EDS results of its composition (right).
Figure 3. Image showing a higher penetration promoted by pitting corrosion (left) and detail of an inside particle acting as a corrosion starter (right).
33 doi:10.1017/S1431927615013975
Corrosion Behavior of the Friction Stir Welded AZ31 Magnesium Alloy F. Nascimento1, J.C.S. Fernandes2, P. Vilaça3 and F.M. Andrade Pires4 1.
IDMEC, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal DEQ/CQE, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal 3. Department of Engineering Design and Production, School of Engineering, Aalto University, Aalto, Finland 4. Faculdade de Engenharia da Universidade do Porto, Porto, Portugal 2.
Magnesium alloys are considered to be a valuable engineering material since they offer a significant potential to reduce weight due to their low density, good thermal and fatigue properties, among others. However, problems appear when trying to apply conventional welding techniques to these alloys. The typical low weldability of magnesium alloys with traditional joining processes involving melting is expected to be improved with Friction Stir Welding (FSW) since it is a solid state joining process. Magnesium alloys are also very susceptible to the heating effects being an advantage for the FSW process since it works at lower temperatures than the conventional methods. The FSW process is used in this work with 2mm thick AZ31 plates and the samples were welded at 100mm/min and 800rpm with a constant applied force of 600kg and 0.50 tilt angle. These alloys are known to be very susceptible to corrosion in saline environments [1] and some authors have found that usually, in magnesium alloys, some particles are not dissolved in the magnesium matrix and will promote pitting corrosion [2]. When analyzing the base material the existence of undiluted particles (white articles) aligned with the material matrix was observed. After welding, there was an increase of these particles in the heat affected zone (HAZ) while the nugget that suffers both mechanical and thermal stresses have promoted the decrease of the grain size and also reduced the number of undiluted particles in this area (Figure 1). After performing an Energy Dispersive Spectroscopy (EDS) analysis in one of these particles, it was confirmed to be an AlMn particle which has not been dissolved in the magnesium matrix (Figure 2). After a corrosion test it was observed that there was pitting corrosion promoted by these alloys (Figure 3) where the particles would act as a cathode corroding the magnesium matrix around it. While this was expected for the exposed particles, it was also observed that the interior particles promote corrosion when in close vicinity of the corrosion front. The reduction of the AlMn particles will improve the corrosion properties and therefore, the usage of the FSW process can be effective since the nugget displayed a great reduction of these particles. It was also observed that there is an increase of particles in the heat affected zone increasing the corrosion in this area. Using higher advancing speeds will reduce the heat released in this process and can effectively reduce the heat influence over the existing particles. The FSW process presents itself as a process that can effectively join magnesium alloys and further studies should focus on the reduction of the heat released to improve the corrosion resistance of the magnesium alloys [3]. References: [1] A Samaniego el al., Corrosion. Science 68 (2013) p. 66. [2] R Zeng et al., Trans. Nonferrous Met. Soc. China 16 (2006) p. 763. [3] The authors gratefully acknowledge the financial support provided by Ministério da Ciência e Tecnologia e do Ensino Superior - Fundação para a Ciência e Tecnologia (FCT) under the project "Development of Integrated Systems for Smart Interiors" reference MIT-Pt/EDAM-SI/0025/2008. FN would like to acknowledge FCT, via the PhD scholarship SFRH/BD/33729/2009
34
Microsc. Microanal. 21 (Suppl 5), 2015
Figure 1. Microstructure of the FSWed magnesium: HAZ (left) and Nugget (right)
Figure 2. Detail of inclusions on Mg matrix (left) and the EDS results of its composition (right).
Figure 3. Image showing a higher penetration promoted by pitting corrosion (left) and detail of an inside particle acting as a corrosion starter (right).
