Casting in the Semi-Solid State of ZK60 Magnesium Alloy Modified with Rare Earth Addition

Abstract

In this work, the casting process under mechanical agitation in the semi-solid state was investigated for the production of ZK60 magnesium alloy modified with the addition of 2.5% wt ofmischmetal. The results show that this process enables the production of ingots with homogeneous chemical composition and free of shrinkage, inner defects and internal oxidation. The as-cast microstructure consists of an α-Mg matrix with globular grains reinforced by a grid of distinct intermetallics of Mg-Zn, Mg-Zn-RE and Mg-RE type along the grain boundaries. The yield strength at room temperature undergoes more than 50% increase during direct T5 aging, thus reaching 170 MPa. At 300°C, however, the dispersion of nanometric precipitates does not modify the hot deformation behavior of the aged alloy, which undergoes dynamic recrystallization in a similar manner to the as-cast alloy. DRX at 300°C is fastest for the alloy solution-treated at 500°C.

Microstructure and Residual Stresses in a Friction Stir Welded Butt Joint of as-cast ZK60 Alloy Containing Rare Earths

“…These fragments appear to be partially pulverised within the α-Mg matrix. On the other hand, a network of intermetallic compounds, mainly along the grain boundaries, is observed to form in the BM with a heterogeneous distribution (Figure 2 (b)) due to the mechanical beating during solidification 11 . In such a case, no dendrite growth of intermetallic phases has been observed i.e.…”

“…The (103), (112) and (201) diffraction lines are those used for the stress estimation. It can be seen that longitudinal stresses resulting from FSW are mostly of tensile nature under the shoulder region and comprise the SZ and the TMAZ turning into compression towards the base metals 11,14 . Also, it shows a symmetrical trend, which is similar to an "M" shape and the maximum residual tensile stress occurring within the retreating side was 30 MPa.…”

“…In this case, the strain rate applied to the retreating side within the welded joint is supposed to be lower than that at the advanced side, which certainly promotes such low deformation temperatures. 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 . Also, the α-Mg grains have no dominant preferred orientation along the TMAZ region induced by stirring and likely lead to a less brittle weld, which contrasts with what is observed in the fusion weld.…”

“…al. [11][12][13] , these intermetallic compounds are those of Mg-Zn, Mg-Zn-RE and Mg-RE type with a total volume fraction of around 7.5%. In the present case, the intermetallic compounds exist in all three zones i.e.…”

Solution and ageing heat treatments of ZK60 magnesium alloys with rare earth additions produced by semi-solid casting

“…In addition, RE elements give rise to a dispersion of thermally stable precipitates with high melting point (above 620 °C) 15 , an improvement over precipitates of the Mg-Zn system which melt at approximately 340°C and thus have poor thermal stability and exhibit tendency for hot tearing. For these reasons, alloys of the Mg-Zn-RE-Zr system have been matter of study in a number of recent researches [16][17][18][19][20] . In the present work, ZK60 (Mg-6%Zn-1%Zr) alloys containing different amounts of RE elements were produced by thixocasting, a semi-solid casting process whereby semi-solid billets with a non-dendritic microstructure are injected into a cast die, resulting in a final product with a globular grain structure and superior mechanical properties compared to those obtained from conventional casting processes 21 .…”