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Casting, rolling and friction stir welding in magnesium alloys with Mischmetal addition

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Author(s):
Erenilton Pereira da Silva
Total Authors: 1
Document type: Doctoral Thesis
Press: São Carlos.
Institution: Universidade de São Paulo (USP). Escola de Engenharia de São Carlos (EESC/SBD)
Defense date:
Examining board members:
Haroldo Cavalcanti Pinto; Rodrigo Santiago Coelho; Daniel Rodrigo Leiva; Omar Maluf; Guillermo Carlos Requena
Advisor: Haroldo Cavalcanti Pinto
Abstract

The growing scarcity of renewable energy resources, as well as the continued rise in costs has required in recent decades a dramatic reduction in energy used for transportation freight and passenger, which is increasing daily all over the world. An alternative is to weight reduction with the use of light alloys, this concept the use of magnesium alloys is justified by their low density, about 1/3 lower than that of aluminum. Among the magnesium alloy, the matrix is ZK type that has greater mechanical strength and the addition of rare earth elements (RE) to increase corrosion resistance, working temperature and yield strength due to the formation of thin films and density oxides, and intermetallic of the high melting point and higher hardness. The formation of intermetallic of the MgZn type, which has a low melting point, makes alloys with matrix of the ZK type susceptible to formation of hot crack during the welding, making impossible the use of conventional welding processes, an alternative is the friction stir welding (FSW) that the union is made below the melting point. This work shows the studies the addition of 1.5% wt. of mischmetal (Mm) in the ZK60 alloy and effects of the casting process with mechanical mixing in the semi-solid state. Were produced the alloys: ZK60, ZK60-1.5RE with conventional casting and ZK60-1.5RE Tixo with mechanical mixing in the semi-solid state, all were hot-rolled in a symmetrical laminator and welded with friction stir welding (FSW) process. The methods of casting and cooling gave resulted in materials free of defects and chemical homogeneity, and the mechanical mixing provides homogeneous microstructure with globular grains. The alloy with mischmetal addition had an average grain size of about 22% lower than ZK60 alloy, when compared to the casting method the ZK60- 1.5RE Tixo alloy had an average grain size of about 26% lower than ZK60-1.5RE alloy. The rolling process gave rise to a partially recrystallized microstructure with average grain size of between 3.3 and 4.23 μm, the intermetallic network were broken, however, kept continuous for alloys with Mm addition. As for mechanical strength was higher for ZK60 alloy, due smaller amount and intermittent network of the intermetallic. The alloys with Mm addition had better thermal stability during welding and showed better surface quality, being possible to do a welding with rotation of 1200 rpm and advancing speed of 400 mm/min while the ZK60 alloy only was possible the welding with advancing speed of 200 mm/min. The analyzes of residual stresses had similar values and profiles and follow the flow of material as well as the texture of the weld beads. The micro hardness maps in the cross section of the weld bead showed a higher hardness in the mixing zones, and higher and more homogeneous values for ZK60 alloy, and can thus affirm that the intermetallic MgZn type has higher hardness than the MgZnRE type. (AU)