Quick Overview of 3D Printing Magnesium Alloy High-strength Plasticity Research

The specific strength of magnesium alloy is higher than that of aluminum alloy.In the aerospace field, magnesium alloy can be used to replace aluminum alloy in the manufacture of aircraft seats, floors, hatches and other parts, as well as structural parts of spacecraft, etc., which can ensure structural strength while making aircraft lighter.In our country's Tianwen No. 1 detector, there are related applications of magnesium alloy.Especially for aerospace applications, the application scenarios of materials may span a variety of temperature spaces.This puts forward higher requirements for the room temperature and high temperature mechanical properties of lightweight metals such as magnesium alloys.However, the rapid diffusion of atoms at high temperatures may lead to abnormal growth of reinforced phases and grain boundary slip, thereby reducing the strength of the material at high temperatures.

Adding rare earth (RE) to magnesium alloys is widely regarded as an effective way to improve their high-temperature mechanical properties, but the tensile strength above 250°C is still unsatisfactory.The introduction of a large number of high-melting intermetallic compounds or eutectic phases can further improve the heat resistance, but the high density of the precipitated phase or the second phase in Mg alloy will lead to a significant loss of room temperature ductility, making it difficult to balance between high temperature strength and room temperature ductility.For example, the liquid die-forged Mg-10Gd-3Y-0.5Zr alloy exhibits a significant tensile strength of 282MPa at 300°C, but the elongation at break is only 2.4%.Therefore, it is necessary to explore new methods to guide the design of magnesium alloys with high high temperature strength and good room temperature tensile ductility.

Researchers from Tsinghua University used friction stir additive manufacturing technology (FSAM) to prepare Mg-Li-Al-Zn duplex magnesium alloy.Through the comprehensive effects of grain refinement and solution strengthening, the Mg-Li-Al-Zn alloy prepared by friction stir has refined grains and uniform phase distribution, and exhibits a synergistic improvement in strength, plasticity and corrosion resistance.

There are coarse precipitates in the substrate sample dispersed in the β-Li phase with a larger grain size.The grain size in the stirring zone is refined, and the small spherical precipitate is evenly distributed in the isometric β-Li grains and along the grain boundary. The grain size of the β-Li phase after FSAM is reduced from 15.1±4.3µm to 4.1±1.3µm.

The yield strength of the matrix sample (BM) was 156.8±1.0MPa, and the yield strength of the PD and TD samples in different directions after FSAM increased to 168.9±9.3MPa and 171.1±4.8MPa, respectively.The elongation of the PD sample showed a maximum value of 49.4±7.9%; the elongation of the TD sample was lower than that of the PD and BM samples.The strength after FSAM is mainly caused by the dispersion strengthening and fine crystal strengthening of the uniform precipitated phase.In addition, due to the dissolution of the AlLi and MCGLI2AL phases, the strength of the alloy is increased by solution strengthening.

Researchers from Huazhong University of Science and Technology used laser powder bed melting additive manufacturing technology to successfully prepare WE43 alloy samples with fine tissue, high degree of densification and excellent mechanical properties.Under the combined process with a laser power of 200 W and a scanning speed of 600 mm/s, samples with a density of up to 99.89% were obtained.The deposition direction produces a periodic heterostructure, that is, there are fine grains (~4.1µm) at the boundary of the bath, and coarse grains (~23.6µm) inside the bath.The tensile yield strength, tensile strength and elongation at break were 276±1 MPa, 292±1 MPa and 6.1±0.2%, respectively.Its tensile properties are superior to other magnesium alloys and magnesium alloys prepared by other processes.Solution strengthening (24.5%), grain boundary strengthening (14.4%) and HDI strengthening (32.2%) are the main sources of high yield strength.

Under different laser volume energy densities, the tensile mechanical properties of WE43 alloys printed by LPBF have changed.When the energy density increases from 88 J/mm3 to 138 J/mm3, the yield strength increases from 252±4 MPa to 276±1 MPa, the tensile strength increases from 286±1 MPa to 292±1 MPa, and the fracture elongation increases from 3.5±0.5% to 6.1±0.2%.Among them, the 138 J/mm3 printed samples showed a significantly higher work hardening rate than other samples.In the early stage of deformation, it has a constant work hardening rate of about 2.3 GPa.

Researchers from Taiyuan University of Technology used cold metal transition (CMT)-arc fuse additive manufacturing (WAAM) to prepare Mg2.4Nd-0.3Zn-0.6Zr (ZM 6, wt%) alloy thin-walled material.The forming quality, microstructure evolution and mechanical properties of WAAM ZM 6 alloy thin-walled parts were studied.The ultimate tensile strength on TD and BD is 226 MPa and 214 MPa, respectively, without significant anisotropy, the elongation reaches 15%, and it has good plasticity.

The change in mechanical properties is due to differences in microstructure.Based on WAAM's layer-by-layer stacking technology, the thin-walled components show obvious layered organization.WAAM ZM 6 has a higher UTS and EL than the cast alloy, showing YS similar to the heat-treated ZM 6 alloy, but its EL is significantly improved.Since the “Z” path is used in the CMT-WAAM process to oscillate the bath, this promotes grain refinement.

Researchers from Harbin Institute of Technology used arc additive manufacturing technology to prepare Mg-2.8 Nd-0.5 Zn-0.4Zr (NZ 30 K) thin-walled structural parts, and systematically studied the tissue evolution and mechanical properties during printing deposition and heat treatment.Compared with cast alloys, deposited alloys have finer grains, with an average grain size between 13.2 µm and 20.1µm, and the grains are completely isometric in random orientation.After 12 h of solid solution at 520℃, the eutectic is completely dissolved, and the grain size increases slightly, with an average of 18.2 µm ~ 23.1 µm.After aging at 200℃ for 8 h, a high-density nanoscale β' phase appeared in the alloy, which produced a significant aging hardening effect. The tensile strength and elongation of the peak aging state alloy can reach 334.6 MPa and 14.3%, respectively. It has good application potential in terms of cost and performance.

After the alloy is treated with a solution, the strength decreases and the plasticity increases. After the solution+peak aging treatment, the strength of the alloy increases significantly and the plasticity decreases. The UTS, YS and EL values are 334.6MPa, 169.7MPa and 14.3%, respectively.This high strength allows WAAM NZ 30 K alloy to obtain significant strength enhancement through heat treatment.Compared with the cast NZ 30 K alloy, the mechanical properties of WAAM NZ 30 K alloy have been improved.Compared with other WAAM Mg-RE alloys, WAAM NZ30K exhibits better strength and plasticity synergy in all three states.At the same time, WAAM NZ30K has the lowest rare earth content, and the strength of the alloy reaches a medium level after T6 heat treatment. The low rare earth content can reduce production costs, so it can bring better economic benefits.

Research scholars from Xi'an University of Science and Technology studied the influence of heat treatment process on the mechanical properties and corrosion properties of cold metal transition-arc fuse additive manufacturing (CMT-WAAM) LA 103 Z magnesium-lithium alloy.Both solution treatment and aging treatment have improved the mechanical properties of LA 103 Z magnesium-lithium alloy.The solution treatment (340°C) dissolves the fine needle-like α-Mg phase and the AlLi phase, and the precipitated Li2MgAl phase is diffused and distributed.The tensile strength of the alloy after solution treatment increased from 144±6.2 MPa to 299±1.2 MPa, an increase of nearly 107.6%.The aging treatment at high temperature (225°C) makes the grain distribution of the alloy more uniform, the AlLi phase particles gradually thicken, and the Li 2 mGal phase is transformed into a large number of fine AlLi phase particles.The yield strength increased from 100±5.0 MPa to the maximum value of 182±2.6 MPa, an increase of about 82%, and the maximum elongation reached 24.1%±2.0%.

Overall, magnesium alloys can be widely used in aerospace, automotive industry and other fields because of their lightweight and high specific strength characteristics.The 3D printing method meets the requirements of personalized customization of magnesium alloys and further expands its application.Through different 3D printing processes and heat treatment, the strong plasticity of magnesium alloys can be significantly improved, and at the same time, the mechanical properties of magnesium alloys can be achieved at a low rare earth content. The medium level of high rare earth content reduces the economic cost in actual production.