Abstract
A quasi-continuous-wave direct metal deposition (QCW-DMD), one of directed energy deposition (DED), with different pulse frequencies and duty cycles was performed on an inclined NV E690 steel plate. Comprehensive effects including the humping effect, the melt pool evolution process, the coalescence phenomenon, the deposition morphology, and the thermal behavior were investigated. The results show that during the continuous-wave direct metal deposition (CW-DMD) and the QCW-DMD with the pulse frequency of 1 Hz, the gravity-induced humping effect and the coalescence phenomenon contribute to the increase and decrease of the melt pool numbers, respectively. A small duty cycle leads to a decrease in the average melt pool numbers. Besides, the coalescence of melt pools is formed by establishing a liquid bridge which is first mainly driven by the surface tension and then by gravity. The humping effect is overcome when the pulse frequency ranges from 5 Hz to 50 Hz, presenting a small height variation. Furthermore, the QCW mode laser can modulate the powder catchment efficiency and produce a higher cooling rate that resulted in a higher microhardness.
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