Precise control of variable-height laser metal deposition using a height memory strategy

In respect of laser metal deposition (LMD) 3-D additive manufacturing technology, the method of non-parallel slicing is applied for the deposition of the overhanging and unequal-height parts and is capable of improving the deposition accuracy, stability and efficiency. Nevertheless, the produced uneven top surface derived from variable height deposition process has a possibility to result in instability and even failure of the deposition. In this study, the novel control strategies for the precise control of the unequal-height part were proposed. An inside-beam powder feeding nozzle was applied for the deposition with large overhanging angle. Based on a developed deposition height control system, multiple PI controllers were designed for varying desired deposition heights in an unequal-height layer. The actual heights of the uneven top surface in different positions were memorized by the deposition of a layer, and the process parameters for the next layer could be planned in advance to smooth this unevenness. Under the proposed control strategies, a geometrical stable and convergent process was made achievable when depositing a fan-shaped part and a bent-pipe-shaped part as the typical overhanging and unequal-height structures. The microstructures were observed to be uniform and compact, and the grain sizes and microhardness in different height positions were found to vary slightly.