Contributor: Wang Lin, Zhang Hang Contributor: State Key Laboratory of Mechanical Manufacturing System Engineering, Xi'an Jiaotong University

Additive manufacturing technology has become one of the most widely used technologies in metal material manufacturing due to its advantages of flexibility and controllability, easy adjustment of raw material composition, and freedom of part size. Especially for the development of high entropy alloys with a large range of material composition changes, its advantages are more obvious. By optimizing various process parameters during the manufacturing process, as well as post-treatments such as annealing heat treatment and hot isostatic pressing, the mechanical properties of manufactured products can be improved.

The mechanical properties of high entropy alloys are determined by their internal microstructure. Therefore, the study of regulating the microstructure of materials is particularly important. The microstructure of CoCrFeNi high entropy alloys is usually composed of columnar grains grown inside and outside the molten pool during laser melting, as well as equiaxed grains at the edge of the molten pool. This is related to factors such as different cooling rates in the molten pool and its face centered cubic (FCC) matrix structure.

Heng Lu et al. from the Institute of Technology and Science at Wuhan University conducted research on the development of CoCrFeNi high entropy alloys and proposed a new method of synchronized laser shock modulation (LSMMP) in the molten pool. This process directly interferes with the recrystallization process by laser shock to enhance the convection in the molten pool during additive manufacturing and suppress columnar crystal growth, ultimately improving the mechanical properties of printed samples by refining the grains.

The structure of the synchronous laser shock modulation system is shown in Figure 1. The experimental equipment consists of a 1kw continuous fiber laser, a Joule level pulse laser, a 304 stainless steel substrate, and a high-speed camera. The experiment was conducted in a high-purity argon atmosphere, with a continuous laser beam used for the manufacturing process perpendicular to the substrate, while a pulsed laser beam deviated by − 10 ° from the vertical direction.

During the manufacturing process, different laser shock energies (1 J, 2 J, 3 J) were used for research purposes, and high-speed cameras were used to continuously capture the evolution of the molten pool. The results are shown in Figures 2 (a) - (c). By analyzing the laser shock process of the molten pool, it can be divided into four stages as shown in Figure 2 (d). Firstly, the convection mechanism mainly consists of recoil pressure (P) and Marangoni convection (M); II: The impact force (S) dominates the oscillation of the molten pool; III: The rebound force (R) dominates the upward arch of the molten pool; IV. Stage: Gravity (G) dominates the downward movement of the molten pool and restores its original shape.

In the first stage, continuous laser completely melts CoCrFeNi powder, and the heat obtained by the powder is quickly released onto the substrate, causing an upward recoil pressure (P) at the high temperature at the bottom, pushing the fluid away from the substrate. The temperature gradient on the surface of the molten pool leads to Marangoni convection (M), which drives the fluid on the surface of the molten pool to move from the high-temperature region to the low-temperature region. In the second stage, the impact force (S) destroys the original state of the molten pool, and the morphology of the molten pool is related to the energy of laser shock. In the third stage, the fluid in the molten pool moves towards the edge and rebounds towards the center when encountering solids, causing the center of the molten pool to arch again. Finally, in the fourth stage, the molten pool regains calm under the action of gravity (G).

Observing the cross-sectional melt pool of CoCrFeNi printed samples without laser shock and using 0J, 1J, and 2J pulse laser shock respectively, it can be seen from the electron backscatter diffraction (EBSD) pattern shown in Figure 3 that during the laser additive manufacturing process, laser shock not only changes the size and shape of the melt pool, but also affects the microstructure and grain size of the material, ultimately changing the mechanical properties of the formed sample.

The actual stress-strain curve obtained by comparing the performance of printed samples without laser shock and with different laser shock energies is shown in Figure 4. The yield strength of the material after laser shock strengthening has been improved, with the maximum strength value and elongation at a pulse laser energy of 1J.

Reference:

Lu H,He Y,Zhao Z.Strengthening CoCrFeNi high entropy alloys via additive manufacturing with laser shock modulation of melt pool.Materials Science And Engineering.2022;11(6):860.