Effect of ultrasonic-assisted laser metal deposition on microcracks in deposition layer

In order to study the effect of non-contact ultrasonic-assisted laser metal deposition (LMD) on microcracks, Febased powders were deposited on a 42CrMoA steel using LMD process at different excitation distances (d). The results show that the maximum length of microcracks is 112.3 mu m under the condition without ultrasonicassisted. When d = 50 mm, a crack of less than 10 mu m is generated at the bottom of deposition layer. When d = 110 mm, the microcrack maximum length is 84.4 mu m. The deposition layer with no microcracks was obtained when d = 80 mm. Ultrasonic-assisted application can synergically inhibit the generation of cracks from two aspects. On the one hand, it makes the distribution of B element more uniform in the molten pool, and the accumulation of borides at the bottom of the deposition layer easily generates hard and brittle borides such as M23(C,B)6, which is more likely to induce cracks. After non-contact ultrasonic-assisted, the width of boride is 54.2 % lower than that without ultrasonic-assisted, and the boride morphology is slightly different. The borides changed from long rod-shape and mesh-shape to nanoscale short rod-shape and granular-shape. On the other hand, it can make the microscopic stress distribution at the bottom of deposition layer more uniform, which reduces the crack sensitivity. Compared to the condition without ultrasonic-assisted, when d is 50 mm, 80 mm and 110 mm, the KAM average value decreased by 44.03 %, 40.36 % and 19.27 %, respectively.