Magnesium-lithium alloy is called an ultra-light alloy, with a density between 1.35 and 1.65g/cm3, and is currently the lightest metal structural material in engineering applications. The alloy has the characteristics of high specific strength and specific rigidity, strong cold and hot deformation ability, and not obvious anisotropy.
At the same time, it also has many advantages such as good electromagnetic shielding performance and damping performance, so it is ideal for aerospace, electronics and military fields. The lightweight structural material has a wide range of application prospects and huge development potential.
However, the lower absolute strength severely limits the further development and application of magnesium-lithium alloys in industry. So far, scholars at home and abroad have focused on the influence of preparation methods, alloying, heat treatment and deformation processing on the structure and performance of magnesium-lithium alloys. However, the absolute strength of magnesium-lithium alloys is still basically limited to about 250MPa. On the basis of previous studies, further exploration is still needed to develop magnesium-lithium alloys with better performance.
As a free forming method for processing shaft and tube parts, rotary forging has the characteristics of wide processing range, high processing accuracy, good product performance, high material utilization rate and large production flexibility. During the rotary forging process, the 4 forging dies rotate around the billet axis at high speed while performing high frequency forging on the billet, thereby reducing the size or shape of the billet shaft section, thereby improving its performance.
In the past 10 years, rotary forging technology has been continuously improved, and it has been used to prepare multilayer tubular composite materials and non-ferrous metal materials, such as aluminum alloys or copper alloys, and also used to process materials such as magnesium alloys that are difficult to process at room temperature. Therefore, this study uses low-cost rotary forging technology to plastically deform magnesium-lithium alloys at room temperature. The purpose is to obtain large pieces of high-strength magnesium-lithium alloys and to conduct in-depth and systematic research and disclosure of the deformation mechanism.
Researchers from Nanjing University of Science and Technology have successfully prepared an ultra-light bulk high-strength magnesium-lithium alloy with a new strength record through rotary forging deformation with a strain amount of only 0.32. Its room temperature tensile strength can reach 405MPa. The related paper was published on Materials Research Letters with the title “Achieving ultra-strong Magnesium–lithium alloys by low-strain rotary swaging”.
In this paper, a Mg-4Li-3Al-3Zn alloy sample with a diameter of 20.2mm is used, and the diameter is reduced to 17.2mm through multi-pass low-strain room temperature swaging technology, and the equivalent effect variable is 0.32. Studies have shown that the rotary forging technology introduces high-density twins and nano-stacking faults into Mg-4Li-3Al-3Zn, thereby obtaining an ultra-high-strength and ultra-light magnesium-lithium alloy with a tensile strength of more than 400MP at room temperature, which is the current magnesium-lithium alloy The highest strength in the system has obvious advantages.
At the same time, the high strain rate and three-dimensional stress of the swaging process are used to form better metal streamlines, avoiding stress concentration and cracks at the twin boundary, thereby preparing a large block of high-strength magnesium-lithium alloy, which is magnesium The industrial production and application of lithium alloys provide new directions and possibilities.
Fig.1 Schematic diagram of the rotary forging process and the comparison diagram of the microstructure and mechanical properties of the rotary forging sample
The study found that the high-density twins introduced in Mg-4Li-3Al-3Zn by small-strain rotary forging include tensile twins and compression twins. In addition, as shown in Figure 2(c), there is also a multiple Special layered structure produced by twins. At the initial stage of deformation, a large number of primary twins are emitted in parallel from the grain boundary to form a long and narrow twin belt. At the same time, local stress concentration will be generated at the tip of the twin during the twinning process, which makes a large number of multiple twins activated.
In addition, there are a large number of nano-stacking faults emitted from the grain boundary inside the twins. The types of stacking faults include growing stacking fault I1 and deforming stacking fault I2. GPA analysis vividly demonstrated that nano-stacking faults can strongly hinder the slippage of dislocations and bring strengthening effects. Therefore, in addition to twin strengthening, high-density nano-pitch stacking faults are also one of the main strengthening mechanisms.
For the first time in this study, the deformation of magnesium-lithium alloy was observed by rotary forging, and a large, ultra-light and high-strength magnesium-lithium alloy with a new strength record was successfully prepared. Compared with other severe plastic deformation processes, the rotary forging process introduces high-density twins and nano-stacking faults into the bulk magnesium-lithium alloy under the condition of small strain deformation, which effectively prevents the movement of dislocations and maintains strain hardening This is mainly related to the high strain rate and the constantly changing stress state in the RS process. The above results provide new directions and possibilities for the industrialized production and application of high-strength magnesium-lithium alloys.
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