Nature sub-journal: pulse laser-assisted additive manufacturing Ti-6Al-4V alloy grain refinement

The research team of Korea Institute of Science and Technology (KIST) proposed a pulsed laser assisted additive manufacturing (PLAAM) technology to refine the initial stage of Ti-6Al-4V components β Grain. The research results were published on the Scientific Reports, a sub-journal of Nature, with the title of "Pulsed laser-assisted-additive manufacturing of Ti-6Al-4V for in-situ grain refinement".

Metal Additive Manufacturing (AM) is a widely used layer-by-layer process for rapid prototyping and manufacturing complex three-dimensional metal structures. Among various metal AM materials, Ti-6Al-4V has become the most widely studied and applied material due to its good applicability in biomedical and aerospace industries. However, the typical Ti-6Al-4V AM component has a thick columnar initial- β Grains, showing anisotropic tensile properties. In the typical AM process, the thermal gradient formed in the small molten pool is very steep, resulting in strong epitaxial growth of columnar grains along the construction direction. However, the coarse columnar grain structure of additive manufacturing parts has adverse anisotropic tensile and fatigue properties, which hinders the wide application of additive manufacturing in the manufacturing industry. Therefore, improving the equiaxed grains of AM components has become an important research topic to improve their tensile properties.

In this study, researchers proposed a pulsed laser-assisted AM (PLAAM) technology to refine the initial size of Ti-6Al-4V components during laser directed energy deposition (DED) β Grain. The nanosecond pulse laser is integrated into the DED system to transfer high pulse energy to the molten pool during AM. Because PLAAM is an in-situ and non-contact technology that affects the molten pool, it can be applied to the AM of complex objects of any size and shape. Compared with 1297 provided by conventional AM technology μ M compared with the average initial β The grain size is 549.6 μ m。 In addition, when using PLAAM technology, β The maximum value of the multiple of the uniform phase distribution decreased from 16 to 7.7, indicating that the crystal texture was weakened. These changes confirm that the proposed PLAAM technology can promote finer and more equiaxed initial β Grain. Inspired by the contact ultrasonic technology and the established effect of pulsed laser on liquid, this technology uses laser-induced shock wave, cavitation and accelerated Marangoni effect flow in the melt pool as fine equiaxed initial β The formation of grain structure creates a favorable environment.

The experiment shows that the components manufactured with PLAAM have a more refined and equiaxed initial β Grain structure. In addition, since the proposed technology is non-contact, it can be applied to the existing process without adjusting the tool path.

Figure 1: Pulse laser assisted AM (PLAAM). (a) Off-axis configuration of the PLAAM system. (b) Pulse laser induces shock wave, cavitation and accelerates Marangoni effect flow in the molten pool, providing a good environment for grain refinement.

Pulse laser assisted additive manufacturing, PLAAM technology is shown in Figure 1. In order to accurately locate the molten pool and directly transmit the pulsed laser energy during AM, the pulsed laser guides the focus module to focus on the molten pool through the optical fiber connected to the DED nozzle. Fix the focus module on the DED positioning frame so that the focus of the pulse laser and the DED laser coincide at AM. Although the off-axis configuration is used in this study, the pulse laser can be designed coaxially with a dichroic mirror and a DED laser to achieve complete integration.

The pulse laser effect is shown in Fig. 1b. The wavelength of the pulsed laser is 532 nm, and the pulse duration is 10 ns. The focal length and pulse power density are 2.8 respectively × 10 − 3 cm2 and 0.41 GW/cm2, causing shock waves and cavitation in the molten pool. When the given power density is 0.36 GW/cm2 higher than the dielectric breakdown threshold of titanium, the avalanche ionization process occurs in the molten pool, that is, dielectric breakdown, the ablation sound is clearly visible, and the plasma spark is bright and visible. These phenomena are followed by plasma formation, shock wave propagation and cavitation. In addition, the selected pulse laser parameters are sufficient to accelerate the Marangoni effect flow in the molten pool.

In the multi-layer AM process, it is selected to maintain 250 μ M The height of the target layer. Under the selected DED parameters, the calculated energy density is 100 J/mm2, which can ensure that the total volume fraction of pores is less than 0.1%. Under the condition of scanning speed of 300 mm/min and pulse laser repetition rate of 100 Hz, the distance between two continuous pulse excitation of pulse laser in PLAAM process is 50 μ m。 Because the typical Ti-6Al-4V AM sample has a columnar initial β Grain, several millimeters in the construction direction, hundreds of microns in the scanning direction, given vertical (250 μ m) And landscape (50 μ m) Excitation interval ratio initial β The grain size is one order of magnitude smaller. Therefore, pulsed laser can effectively change the initial β Grain structure.

Initial- β Grain refinement

As shown in the optical microscope (OM) image of Fig. 2a and b, the initial- β Compared with conventional AM samples with grains, PLAAM samples show finer and more equiaxed initial values at the whole construction height of 30mm- β Grain. Use ImageJ software to manually track the initial- β The grain boundary is further analyzed. Initial value of PLAAM sample unit area (6.91 mm − 2) β The number of grains is 3.78 times that of the conventional AM sample (1.83 mm − 2), which means that the PLAAM sample has a finer initial size β Grain. In addition, the researchers also showed the initial- β The length and aspect ratio of grains show the change of grain structure in a statistical way (Fig. 2c, d). Using PLAAM technology, initial β The average length of grain is 1297 μ M reduced to 549.6 μ m. The average aspect ratio is reduced from 3.5 to 2.5. In addition, compared with conventional AM, PLAAM has a significant impact on the initial β The change of grain size and shape is small. These results show that the PLAAM sample has a finer equiaxed initial value than the conventional AM sample β Grain.

Figure 2: Initial β Change of grain structure. OM image follows the construction direction of conventional AM (a) and PLAAM (b) samples. Z and x are the construction direction and the transverse direction, respectively. Initial observed in (a) and (b)- β Histogram of particle length (c) and aspect ratio (d). Overlapped histograms are displayed in darker colors.

To observe the initial β The samples were analyzed by electron backscatter diffraction (EBSD), as shown in Figure 3. β The inverse polar diagram of the phase (Fig. 3a, c) shows that it has a columnar initial- β Compared with the construction direction of grain structure, the PLAAM sample has an almost equiaxed initial- β Grain structure. The inverse pole diagram is reconstructed using the open source MATLAB toolbox MTEX. The β Phase contour pole diagram (Fig. 3b, d) to quantitatively display the texture change of multiple of uniform distribution (MUD) value. The maximum MUD value of PLAAM sample is 7.7, less than half (16) of that of conventional AM sample. Compared with the strong crystal texture in the<001>direction in the AM sample, the weak texture was observed in the PLAAM sample. These results confirm that PLAAM samples have a finer initial size than conventional AM samples β The isotropic structure of grains.

Figure 3: EBSD analysis of conventional AM (a, b) and PLAAM (c, d) samples. β The inverse pole diagram along the construction direction (a, c). β Isopole diagram of phase (b, d). Z and (x, y) are the construction direction and transverse plane, respectively.

Select the pulse laser parameters: (1) Accelerate the Marangoni effect flow in the melt pool through instantaneous local heating; (2) Generating shock waves; (3) Cavitation occurs in the molten pool after dielectric breakdown. This paper discusses how these effects enhance the equiaxed nucleation in the molten pool. Increasing tissue supercooling can promote equiaxed nucleation. To increase the undercooling of the structure, either increase the melting temperature or reduce the thermal gradient.

In summary, researchers have demonstrated that the grain refinement of Ti-6Al-4V components using composite AM technology (called PLAAM). This technology uses high power density pulsed laser as the equiaxed initial β The growth of grains creates a good environment. Because this technology is non-contact, it can be applied to any existing AM equipment without adjusting any tool path. The microstructure evaluation shows that the initial β Compared with conventional AM samples with grains, PLAAM samples have smaller and more equiaxed initial β Grain. When using PLAAM technology, β The maximum MUD value of the phase decreases from 16 to 7.7, indicating that the structure is weakened β Grain refinement. Due to equiaxed initial- β The grain structure is famous for its isotropic and high tensile properties. This technology is expected to be widely studied and used to produce high-quality metal AM components.

Article source:
https://www.nature.com/articles/s41598-022-26758-y