Ke Liu

There is limited research on dissimilar joints of RE-containing Mg alloys and Al alloys, and the diffusion of elements is fundamental for the properties of Mg/Al interfaces. In this study, samples were manufactured by magnetic pulse welding (MPW) with plates of the AA1060 aluminum alloy and the as-cast Mg–4.80Gd–1.92Zn (in wt.%) alloy, and the effects of heat treatments at 200 °C and 250 °C, from 1 h to 4 h, on the diffusion of the Mg/Al interface were investigated. The results indicated that diffusion of the Mg and Al elements occurs at 250 °C for no less than 2 h, since Gd and Zn are mainly concentrated in precipitates in the Mg–4.80Gd–1.92Zn alloy. When the heat treatment time at 250 °C is increased from 2 h to 4 h, the width of the Mg/Al interface increases from ~15 μm to ~20 μm. At positions near precipitates in the Mg alloy, the diffusion of Al atoms into the Mg lattice can be hindered by the precipitates, leading to an abnormal decrease in the width of the interface, which is also related to the difficulties of the Mg element diffusing into the Al matrix.

The hot cracking behaviors of Mg-5Zn-xEr (x = 0.83, 1.25, 2.5, 5 wt.%) alloys are investigated by optimized hot cracking experimental apparatus, optical microscope, and scanning electron microscope, such as contraction behaviors, feeding behaviors, and permeability characteristics. It is found that the solid phase fraction at hot crack initiation and within the freezing range both increased with increasing Er contents up to 2.5 wt.% and then decreased at 5 wt.% Er content. The Mg-5Zn-5Er alloy exhibits the lowest solid phase fraction (87.4%) and a reduced freezing range (74.2 °C), which leads to more effective liquid feeding in the latter stages of solidification. Combined with the grain size, the permeability of the mushy zone, and fracture morphology, the overall permeability is optimal in the Mg-5Zn-5Er alloy, which is beneficial for feeding the cavities and micro-pores. Meanwhile, a large amount of W phase precipitated by the eutectic reaction (L→α-Mg + W phase), which facilitates healing of the incurred cracking. Conversely, the Mg-5Zn-2.5Er alloy shows inferior feeding ability due to the lowest solid phase fraction (98.3%), wide freezing range (199.5 °C), and lowest permeability. Therefore, the Mg-5Zn-2.5Er alloy exhibits maximal hot cracking susceptibility, and the Mg-5Zn-5Er alloy exhibits minimal hot cracking susceptibility. This work provides guidance for improving the hot cracking resistance of cast Mg-Zn-Er alloy and enables an understanding of the hot cracking behaviors of Mg-Zn-RE alloys.

The effect of the Ca₂Mg₆Zn₃ phase on the corrosion behavior of biodegradable Mg-4.0Zn-0.2Mn-xCa (ZM-xCa, x = 0.1, 0.3, 0.5 and 1.0 wt.%) alloys in Hank’s solution was investigated with respect to phase spacing, morphology, distribution and volume fraction. With the increase in Ca addition, the volume fraction of the Ca₂Mg₆Zn₃ phase increased from 2.5% to 7.6%, while its spacing declined monotonically from 43 μm to 30 μm. The Volta potentials of secondary phases relative to the Mg matrix were measured by using scanning kelvin probe force microscopy (SKPFM). The results show that the Volta potential of the intragranular spherical Ca₂Mg₆Zn₃ phase (+109 mV) was higher than that of the dendritic Ca₂Mg₆Zn₃ phase (+80 mV). It is suggested that the Ca₂Mg₆Zn₃ acted as a cathode to accelerate the corrosion process due to the micro-galvanic effect. The corrosion preferred to occur around the spherical Ca₂Mg₆Zn₃ phase at the early stage and developed into the intragranular region. The corrosion rate increased slightly with increasing Ca content from 0.1 wt.% to 0.5 wt.% because of the enhanced micro-galvanic corrosion effect. The decrease in the phase spacing and sharp increase in the secondary phase content resulted in a dramatic increase in the corrosion rate of the ZM-1.0Ca alloy.

In this investigation, the high-strength Mg-12Gd-1.0Er-0.5Zr (wt.%) alloy sheets were produced by hot extrusion (HE) and subsequent hot rolling (HR) at different temperatures. The results indicate that microstructures of these final-rolled sheets are inhomogeneous, mainly including coarse deformed grains and dynamic recrystallized (DRXed) grains, and the volume fraction of these coarse deformed grains increases as the rolling temperature increases. Thus, more DRXed grains can be found in R-385 °C sheet, resulting in smaller average grain size and weaker basal texture, while the biggest grains and highest strong basal texture are present in R-450 °C sheet. Amounts of dynamic precipitation of β phase which are mainly determined by the rolling temperature are present in these sheets, and its precipitation will consume the content of Gd solutes in matrix. As a result, the lowest number density of β phase in R-450 °C sheet is beneficial to modify the aging hardening response. Thus, the R-450 °C sheet displays the best aging hardening response because of a severe traditional precipitation of β' (more) and β H / β M (less) phases densely, which resulting in a sharp improvement in strength, i.e. ultimate tensile strength (UTS) of ~ 518 MPa and yield strength (YS) of ~ 438 MPa. However, the elongation (EL) of this sheet reduces greatly, and its value is ~ 2.7%. By contrasting, the EL of the peak-aging R-385 °C sheet keeps better, which decreases from ~ 4.9% to ~ 4.8% due to a dislocation-induced novel chain-like precipitates, which is good for keeping good balance between strength and ductility.