Tsinghua University has made progress in the field of magnetic field and laser composite processing

The National Key Laboratory of Interface Science and Technology for High end Equipment at Tsinghua University has made progress in the field of magnetic field and laser composite processing - magnetic field assisted laser shock strengthening of Ti6Al4V alloy. The relevant research was published as a cover article titled "Magnetic Field Assisted Laser Shock Peening of Ti6Al4V Alloy" in the journal Advanced Engineering Materials.

Laser shock strengthening has a wide range of applications in improving the mechanical properties of metal materials. An important factor is the non-uniformity of mechanical properties and microstructure, such as surface hardness and grain refinement. The National Key Laboratory of Interface Science and Technology for High end Equipment at Tsinghua University has proposed a metal material strengthening method that combines femtosecond laser shock strengthening with pulsed magnetic field strengthening.

Under magnetic field assisted laser shock strengthening (MFLSP), the grain refinement effect was improved, the uniformity of surface hardness distribution of metal materials was improved, and the laser magnetic field synergistic mechanism of adjusting dislocation distribution to promote grain refinement was revealed. This method, combined with laser processing and external energy field, provides a new way to change the microstructure of metal materials and improve their mechanical properties. In aerospace There is potential for application in areas such as rail transit.

In the paper, the research team used magnetic field assisted laser shock strengthening (MFLSP) technology to refine the grain size and uniformly distribute the surface hardness of Ti6Al4V alloy. The surface morphology and crystal phase composition after treatment were analyzed using scanning electron microscopy (SEM), white light interferometer (WLI), and X-ray diffraction (XRD). The evolution of grain size and dislocation distribution was studied using electron backscatter diffraction (EBSD) for quasi in-situ microstructure characterization. In order to compare the surface strengthening effect, a hardness mapping method of alternating indentation with equal spacing and row by row was used for the original and processed samples.

Implementation of Magnetic Field Assisted Laser Shock Strengthening

MFLSP is performed by combining a magnetic field and femtosecond laser pulses, as shown in Figure 1a. Perform pulse magnetic field treatment (PMT) and FLSP treatment on the original sample in sequence. As shown in Figure 1b, the Coulombic interaction between dislocations and obstacles. Due to the orbital spin coupling effect under external magnetic field excitation, as shown in Figure 1c. As shown in Figure 1d, PMT is carried out using a half sine wave pulse magnetic field, with a pulse current of 2 Hz and a total of 50 pulses. The pulse magnetic field intensity is set to 0.8 T. The direction of the magnetic field is perpendicular to the surface of the sample. After PMT, the morphology of the original sample remained unchanged. FLSP is performed using femtosecond laser pulses (800 nm, 35 fs, 50 mW). The energy distribution of laser pulses and the distance between adjacent laser pulses are shown in Figure 1e. To characterize the surface hardening effect of MFLSP, a quasi in-situ measurement method was used, as shown in Figure 1f. Perform quasi in situ hardness characterization on the original sample, PMT, and MFLSP samples.

Figure 1: MFLSP.
The Effect of MFLSP on Grain Refinement
Use EBSD analysis to perform large-scale detection on the original sample, FLSP, and MFLSP samples. The inverse pole plot (IPF) mapping of grain distribution shows an increase in fine grains, as shown in Figures 2a-c. Measure the uniformity of grain refinement distribution through statistical analysis of grain size and area fraction in Figure 2d-f.

Figure 2: Changes in grain distribution, texture, and grain orientation differences.
The Effect of MFLSP on Surface Hardening
In order to characterize the surface hardening effect of PMT and FLSP, quasi in-situ measurements were conducted on the original sample, PMT, and MFLSP samples.

Figure 3: Comparison of surface hardness. a) Surface hardness mapping of the original sample, b) PMT sample, c) MFLSP sample, and d) FLSP sample. e) Histogram of surface hardness at the detection location. f) KDE and g) WD surface hardness analysis. h) Compare the average surface hardness of the original sample, PMT sample, MFLSP sample, and FLSP sample.
In order to investigate the promotion mechanism of grain refinement, dislocation motion characterization was performed on the original, PMT, and MFLSP samples in the same region. As shown in Figure 4.

Figure 4: Mechanism of the influence of MFLSP on microstructure.

Figure 5: Mechanism of MFLSP. a) Schematic diagram of magnetic field induced magnetic dislocations during PMT process. b) Schematic diagram of microstructure evolution of FLSP and c) MFLSP processes.

conclusion
This study proposes the use of MFLSP to refine the grain size and uniformly distribute the surface hardness of Ti6Al4V alloy by introducing magnetic field assistance. The grain refinement, dislocation motion, and surface hardness of the original, PMT, and MFLSP samples were studied. After MFLSP treatment, the proportion of refined grains is 86.63%, which is 159.57% higher than the original sample. Through the EBSD results, it can be observed that the magnetic field induced dislocation density redistributes towards the internal region, which affects the uniform strengthening effect. Under severe plastic deformation, dispersed dislocations can promote dislocation multiplication. The grain size is uniformly distributed in the MFLSP sample, which improves the uniformity of surface hardness values. The average surface hardness of the MFLSP sample increased by 18.21 HV, which is twice the increase of the FLSP sample.

The results indicate that manipulating dislocation motion during laser induced plastic deformation is beneficial for grain refinement. This study provides a new strategy for promoting grain refinement by adjusting the distribution of dislocations, providing a feasible method for adjusting the mechanical properties of metal materials with uniform microstructure distribution.

Related article links:
https://doi.org/10.1002/adem.202201843
http://sklt.tsinghua.edu.cn/info/1083/1849.htm

Source: Sohu - Yangtze River Delta Laser Alliance