Microstructure and mechanical behavior of similar butt-joints of ZK60 and ZK60–1.5RE magnesium alloys produced by linear friction stir welding

The microstructure and mechanical properties of butt-joints produced by linear friction stir welding of similar plates of as-cast ZK60 and modified ZK60 with 1.5 wt.% RE (ZK60–1.5RE) are investigated. The thermomechanical affected zone is investigated in both advancing and retrieving sides, and the microstructure is compared to the base metal and the stirred zone. Electron backscattered diffraction measurements provide the average microstructural features of the transformed microstructure. The mechanical properties are assessed using hardness, tensile testing, and surface residual stress measured using X-ray diffraction. Higher torque and heat input are obtained for the ZK60–1.5RE compared to the ZK60. The thermomechanical affected zone is notably larger for the ZK60 compared to the ZK60–1.5RE. A gradient microstructure is formed in thermomechanical affected zones where deformed grains are progressively more recrystallised towards the stirred zone. There is no visible interface between the thermomechanical affected zone and the stirred zone. A fine and partially recrystallised microstructure is formed in the stirred zone. The retrieving side of the ZK60–1.5RE has a slightly more refined microstructure compared to the other investigated zones. Anisotropy measured with increment in the maximum intensity of the (0001) increases towards to stirred zone. The formed microstructure is correlated with the role of twinning, recovery, static, and dynamic recrystallisation that can occur during friction stir welding. Twinning and grains with large misorientation spread are more pronounced in the thermomechanical affected zone and regions towards the base metal. Sharp low angle grain boundaries are observed towards the stirred zone. From refined recrystallised grains decorating the grain in the thermomechanical affected zone, a mosaic-like of low and high angle grain boundaries are observed in the stirred zone. The joints have comparable surface residual stresses. The friction stir welding improved the ductility and strength of the as-cast ZK60–1.5RE alloy since the tensile samples fractured in the BM

Effect of Ce-base mischmetal addition on the microstructure and mechanical properties of hot-rolled ZK60 alloy

Mg-Zn-Zr (ZK) alloys exhibit notably high mechanical strength amongst all magnesium alloy grades. However, due to the formation of low melting point Mg3Zn7-precipitates, these alloys are susceptible to hot cracking, thus compromising their metallurgical processing. The addition of rare earths to ZK alloys is an alternative to form higher melting point intermetallic compounds, speed up dynamic recrystallization, refine grain size, enhance corrosion resistance and extend the service temperature due to improved creep resistance. This work deals with the effect of Ce-base mischmetal addition on the hot rolling behaviour of as-cast ZK60 alloy. The microstructure investigation conducted using electron microscopy and X-Ray diffraction shows that precipitation of Mg7Zn3 intermetallics occur during hot rolling, whereas no further precipitation is observed for the ZK60-Mm alloys. The fragmentation of the intermetallic compounds occur during hot rolling and finer particles of Mg7Zn3 are observed for the ZK60, whereas Mg7Zn3 and MgZn2Ce intermetallics are formed in the alloy modified with mischmetal addition. A higher fraction of dynamically recrystallized grains is observed for the ZK60-Mm in comparison to the ZK60. Continuous recrystallization takes place in ZK60 with the formation of sub-grains near to the intermetallics and the addition of mischmetal promotes the occurrence of discontinuous recrystallization with the nucleation of new grains close to the precipitates. The mechanical strength and, in particular, the ductility of the hot-rolled alloys are notably improved when compared to the same alloys in the as-cast condition. The mechanical strength is, however, higher for the ZK60 alloy. Less solid solution strengthening, softer MgZn2Ce intermetallics and more extensive recrystallization contribute to reduce the mechanical strength of ZK60-Mm. Failure in both alloys are initiated at coarse intermetallics and propagate through intermetallic-rich regions

As cast microstructures on the mechanical and corrosion behaviour of ZK40 modified with Gd and Nd additions

The microstructure of ZK40, ZK40 with 2 wt% of Nd and Gd (ZK40-2Nd and ZK40-2Gd, respectively) were investigated with optical, scanning and transmission electron microscopy, X-ray diffraction and Scanning Kelvin Probe Force Microscopy. The mechanical properties and the corrosion behaviour were correlated with the microstructure. The 2 wt% Gd addition enhanced the ductility, while the Nd addition resulted in deterioration in mechanical properties. The corrosion behaviour was also enhanced with the addition of Gd.The authors acknowledge the Deutsches Elektronen-Synchrotron (DESY) for the provision of facilities within the framework of proposal I-20130434. RHB acknowledges University of Sao Paulo for granting the fellowship ´Bolsa Empreendedorismo´. MM acknowledges the Alexander von Humboldt foundation for the provision of financial support in the form of post-doctoral fellowship

Thermo-mechanical Processing of EZK Alloys in a Synchrotron Radiation Beam

Nd, a rare earth element with low solid solubility in Mg, is an ideal alloying element to improve elevated temperature yield strength and creep resistance cost effectively. The addition of Zn leads to further improvement in the elevated temperature properties; therefore, Mg–Nd–Zn alloys are prospective materials for structural and medical applications. In situ synchrotron radiation diffraction was performed during compression at 200 and 350 ℃ for Mg3NdxZn (x = 0, 0.5, 1, 2 wt%) alloys up to a deformation of 0.3 with a deformation rate of 10−3 s−1. The compressed samples were subsequently subjected to electron backscattered diffraction. The results show that at 200Â ℃ the addition of Zn increased the ductility. At the beginning of plastic deformation twinning was the dominant deformation mechanism complemented by sub-grain formation at a later stage. At 350Â ℃, the compression strength was increased with the addition of Zn and the microstructure of the samples underwent partial dynamic recrystallization during compression

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

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 of mischmetal. 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.</jats:p

Microstructure and Residual Stress Formation during Friction Stir Welding of Semi Solid ZK60 Magnesium Alloy

In this work, we report on the friction stir weldability of a semi-solid cast ZK60 alloy modified with 1.5 wt% mischmetall in the lap-joint configuration using a 120WV4 steel tool with concave shoulder and conical pin. The coarser solidification microstructure in the semi-solid cast ZK60-1.5%RE alloy requires low strain rates and increased heat input to produce lap-joints without inner defects. This was achieved with 250 rpm tool rotation and 50 mm/min welding speed. Friction stir welding results in a very fine grained microstructure in the stir zone probably due to dynamic recrystallization. In the thermomechanically affected zones dynamic recrystallization seems to occur within the solute enriched intergranular zones. The distribution of longitudinal residual stresses exhibit stress maxima at both thermomechanically affected zones. A compression peak is observed at the retreating side, whereas a tensile stress maximum occurs at the advancing side.</jats:p