Abstract
The effect of a travel speed of 200 mm/min as well as a tool rotational speed of 1200 rpm on butt joint quality of friction stir welding (FSW) ZK60 magnesium casting alloy containing 1.5 wt.% rare earths (ZK60-1.5RE) was investigated to determine microstructure and residual stresses. FSW results in the generation of heterogeneous metallurgical structures consisting of the base material (BM), stirred zone (SZ) and thermo-mechanical affected zone (TMAZ). The stirring action also produced a non-uniform distribution and segregation of intermetalics. The transversal distribution of the longitudinal welding stresses exhibits a M-like shape with mostly tensile stresses under the shoulder region and only one compressive stress peak in the advancing side of the SZ and TMAZ. It could be demonstrated that FSW of ZK60-1.5RE alloy was successful in the welding conditions applied during the present work.
Highlights
The results show that the stirred zone (SZ) exhibit a finely fragmented intermetallic structure (Figure 2(a))
The thermo-mechanical affected zone (TMAZ) region is present on either side of the stir zone, which is larger in the retreating side than in the advancing side
The TMAZ region is composed of partially recrystallized α-Mg grains, as well as a network of intergranular intermetallic compounds that were partly dissolved (less bright areas in Figure 2(a)) within the α-Mg matrixdue to the temperature and pressure involved in the casting process[11]
Summary
A good combination of strength and ductility has been reported for the ZK60 magnesium alloy that represents an important matrix for the development of innovative light materials to be used in structural applications[1] due to its highest specific strength among the commercial magnesium alloys. The effect of rare earth (RE) additions on the enhancement of strength and ductility has been investigated[2,3]. RE elements in addition to the Mg-Zn-Zr system[4], such as in the ZE41 alloy[5], improved the castability and elevated temperature strength. Mg alloys possess low sublimation temperature, high thermal expansion coefficient and hot cracking susceptibility induced by intermetallic particles[6] with a low melting point, which impose challenges to conventional fusion welding. The addition of RE elements to Mg alloys can contribute to a better weldability by forming higher melting point intermetallic compounds. The present investigation is to study the feasibility of the friction stir welding of cast ZK60 alloy with 1.5 wt.% addition of mischmetal by evaluating the microstructure and the residual stress of butt joints as welded
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