Three dimensional printing (3DP) additive manufacturing(AM) of magnesium(Mg) alloys is difficult to densify by conventional sintering process which limits its application. In this study, a novel 3DP sintering process with full liquid was utilized to improve performance of Mg alloy. Mg alloy powders were shaped at room temperature with water-based binder, and formed the MgO net-like framework at 90 °C in air atmosphere, then sintered over the liquidus temperature under the Ar atmosphere. The results show that a large amount of liquid alloy can effectively improve the density of the sample by filling the pores and mass transferring, the MgO net-like framework can effectively maintain the original shape of the parts. Density, microstructural, mechanical and corrosion properties of the parts were investigated after sintered at 620 °C for 5, 8, 10 and 12 h. The results indicated an improvement of properties with increasing in sintering time. Swelling phenomenon was not observed in this sintering temperature. The net-like framework structure might prevent the swelling caused by liquid phase flow too. The parts after sintering for 12 h at 620 °C show the remarkable relative density (97.8%), ultimate compressive strength (~354 MPa), and mass loss rate in 25 °C is 139 mm / year.

The effects of Fe addition on the mechanical properties and corrosion behaviors of AZ91D alloys prepared by binder jetting additive manufacturing have been investigated. The results show that the constitution of the second phase is large dependence on Fe content. With the Fe content increasing, the second phase can transform Al-Fe phase to Fe and Al-Fe phase. In addition, with increasing Fe content, the content of second phase in-creases gradually, while the content of pores also increases. We believe that with the Fe content increasing, the content of second phase with Fe element increases which the melt temperature is higher than sintering tem-perature, and the content of liquid phase decreases, that leading the high-viscosity liquid alloy cannot fill the pores fully and the more pores will remain and causes the relative density reduces. The compressive strength shows a parabola relationship with Fe content. Furthermore, adding Fe can accelerate the degradation of Mg alloys owing to the galvanic corrosion. The sample with 5 wt% Fe content shows the highest ultimate compressive strength (440 MPa) and fast degradation rate (9091 mm/Y), which meets the soluble Mg alloy with rare earth but cost much lower.

The effects of Ni addition on the mechanical properties, corrosion behaviors, and corrosion mechanism of MgGd1Nix alloys have been investigated by compressive tests, weight loss, hydrogen evolution, and scanning electron microscopy. The results show that the constitution of the second phase is large dependence on Ni/Gd molar ratios, which can transform from LPSO(long-period stacking ordered)+Mg5Gd, LPSO to eutectic phase. In addition, with increasing Ni/Gd molar ratios, the content of second phase increases gradually, while the LPSO phase shows a parabola relationship. Furthermore, the formation of Ni-containing LPSO phase not only can improve the strength but also accelerate the degradation of Mg alloys owing to the galvanic corrosion. The optimal properties with ultimate compressive strength, degradation rate are 340 MPa, 2066 mm/y for MgGd1Ni0.75, respectively, which can meet the engineering application standard of fracturing ball and can be used as candidate materials for fracturing ball.

Difficult to effectively densify in a short time is an important issue that limits the application of magnesium (Mg) alloy fabricated by binder jetting additive manufacturing (AM) process. In this study, two-step sintering (TSS) process was utilized to improve the density of printed Mg alloy samples. Results show that the relative density, mechanical properties and corrosion resistance of the samples after different TSS process were dramatically improved in a short time. A suitable sintering temperature of first sintering step can rapidly increase the density of the samples in a short time. During the second step sintering process, little exceeding the liquidus temperature can further increase the density of the sample, but little lower than the liquidus temperature will lead to a sharp increase in grain size. The sample after sintering at 680 degrees C for 30 min then sintering at 610 degrees C for 6 h shows the highest relative density (0.995), the highest strength (394 MPa), the lowest corrosion rate (101 mm/year) and the smallest corrosion current density (60.4 mu A/cm(2)). Compared with one-step sintering process, TTS process has higher sintering efficiency.

The microstructure, mechanical properties, damping capacities and corrosion properties of Mg-8.5Gd-5Y-xAl (x=0.2, 0.5, 0.8 and 1.1 wt%) alloys were investigated in this study. The results revealed that the alloys were mainly composed by long period stacking ordered(LPSO) phase, Mg-RE phase, Al-RE phase and magnesium(Mg) matrix. With the increase of Al content, the LPSO phase decreases. After extrusion, Mg-8.5Gd-5Y-0.2Al shows the best mechanical properties and damping performance, with the tensile strength of 376 MPa, yield strength of 263 MPa, elongation of 13%, the Q(-1) is 0.0132 when the strain is 1x10(-3). The corrosion properties also show that with the Al content increasing, the corrosion rate increases. Mg-8.5Gd-5Y-0.2Al shows the lowest corrosion rate of 3.80 mm per year and the lowest corrosion current density of 2.81x10(-5) A/cm(2). The LPSO phase can enhance the mechanical properties and corrosion resistance at same time. (C) 2021 Elsevier B.V. All rights reserved.

In this study, the high cycle fatigue (HCF) performance of the extruded Mg-Gd-Y-Zn-Mn alloy reinforced by Longperiod stacking ordered (LPSO) phase was researched. The results show that the Mg alloy enhanced by LPSO phase can have higher HCF strength, because the LPSO phase can effectively hindering the fatigue crack growth by force them to change the growth direction both in the high cycle tensile fatigue tests and rotating bending fatigue tests whether in smooth surfaces and V-notched samples. The fatigue strength of the smooth surface in this study can exceed 220 Mpa, which is close to the fatigue strength of Al alloy.