In-situ high-dynamic micro hammering enhanced wire-arc direct energy deposition of magnesium alloy

Wire-arc direct energy deposition (WA-DED) has emerged as a suitable solution for the rapid manufacturing of magnesium (Mg) alloys, and the in-situ improvement of the microstructure and properties is essential to promote its further application. In this work, a novel in-situ high-dynamic micro hammering enhanced WA-DED method for Mg alloys based on linear actuator was proposed, and the relevant system was developed. The special terminal hammering head could be sufficiently close to the molten pool to enable high-frequency uniform hammering of the just solidified Mg alloy. Two hammered layers of AZ31B Mg alloy with different levels of deformation (20% and 35%) were fabricated to compare with the as-deposited layer. The results showed that the temperature of the hammered area reached 450 ℃ and that the hammering flattened the surface of the deposited layer while the induced strain was sufficiently transmitted to the bottom of the layer. The formation of numerous {10−12} extension twins were involved in strain coordination in all regions of the moderately deformed layer (#H-20). Continuous dynamic recrystallisation (CDRX), twin-CDRX and twinning together contribute to grain refinement, reducing the average grain size from 161.5 µm to 32.5 µm. For severely deformed layers (#H-35), a greater number of proliferating dislocations led to a high degree of CDRX, resulting in the reduction of stress concentrations thereby inhibiting twinning, and thus CDRX was the dominant mechanism driving the microstructure evolution, especially in the top region. The microhardness of in-situ hammered samples H-20 and H-35 increased by 17.7% and 31.7%, respectively, and the grain boundary strengthening and dislocation strengthening both contributed to the improvement of mechanical properties. These promising findings will facilitate the application of in-situ hammering technology in additive manufacturing using WA-DED.