Objective Selective laser melting (SLM) is widely used to manufacture complex parts in many fields, such as the biomedical and aerospace industries. However, due to the limited range of a single optical scanner, SLM cannot be used to manufacture large parts that exceed its building size. Therefore, SLM with parallel multilaser beams is attracting the attention of researchers. For the partition scanning strategy required in multilaser beam SLM, there are inevitably overlap areas, and the quality control of these areas has an important influence on the mechanical properties and surface quality of the part. Currently, the stripe scanning strategy with the overlap in parallel lines extension and chessboard scanning strategy with the overlap in vertical and horizontal lines are commonly used. The chessboard scanning strategy has a good controlling defect, alleviating the phenomenon of micropores and bulging in the overlap zones. However, the stripe scanning strategy has better heat dissipation and a smaller temperature gradient, resulting in better mechanical properties. Therefore, a new scanning strategy named " wave stripe" is proposed to improve the quality of the overlap zones by taking advantage of the existing scanning strategies. Methods The scanning path generation principle of the wave_stripe partition scanning strategy proposed in this paper is as follows. First, we use the triangular wave as partition boundary of the bounding box of the sliced contour; it is divided into n regions (Fig. 1). Second, these regions experience merge processing into wave stripe partitions (Fig. 2). Third, the intersection operation is performed between slice contours and wave stripe partitions to get zones filled with scanning lines within its bounding box (Fig. 3). Finally, the scanning lines in each partition are arranged and connected into scanning paths. Moreover, the Dijkstra algorithm is used to optimize the beginning and end points of the scanning paths to reduce frequent path jumps and laser switching times ( Fig. 4). In the experiment, the wave stripe scanning strategy with the period T= 4 mm, amplitude h(1)=1 mm, the length of the scanning line D-s = 5 mm, and threshold value D-f = 0. 5 mm, stripe scanning strategy, and chessboard scanning strategy with a partition width of 5 mm is used to manufacture 10 mm x 10 mm x 10 mm cube specimens, tensile specimens (Fig. 6) , and cantilever (Fig. 7) with the condition of scanning speed of 700 mm/s, scanning hatch of 50 mu m, preheat temperature of 80 degrees C of the substrate, layer thickness of 20 mu m, 99. 99% N-2 as protective gas, and oxygen under 10(-4) in the chamber. The surfaces of overlap and pores of subsurfaces are examined using a metallurgical microscope. The residual stress is evaluated through the deformation amplitude of the cantilever. The density of specimens is measured using the Archimedes method. The tensile test is performed at the speed of 1 mm/min. The hardness of specimens is analyzed by the average value of the hardness at five different positions using the HVS-100 Vickers hardness machine under 9.8 N pressure for 7 s. Results and Discussions The specimens fabricated by the proposed scanning strategy have fewer spatters on the surface and sufficient energy phenomenon (Fig. 8), showing better overlap quality and metallic color compared with stripe and chessboard scanning strategies. Owing to the large heat-effect zone in the overlap zone, partially melted and unmelted powder particles were stuck on the solidified single track, leading to quality issues, such as discontinuous single tracks, spheroidization, and pores in the subsurface (Fig. 9). Based on the test results, it proves that the proposed scanning strategy performs better in terms of the density, reaching 99.74% (Fig. 11), and has similar deformation as that in stripe scanning strategy, but does better than chessboard scanning strategy, and has higher tensile strength (Fig. 12) due to better quality in the overlap areas, and slightly improves Vickers hardness due to fewer pores (Fig. 13). Conclusions In this study, the wave stripe scanning strategy is showed to improve the quality of the junction zone, inheriting the advantages of a small number of partitions of the stripe scanning strategy and vertical and horizontal lines in the overlapping zone of the chessboard scanning strategy. Based on experiment results of the surface morphology of overlapping zone, residual stress, efficiency of space filling, tensile stress, and hardness, the wave stripe scanning strategy achieves the quality of overlapping zone with fewer spatters and reduces the porosity of specimens, considerably improving the tensile strength, efficiency of space filling, and Vickers hardness when compared with chessboard and stripe scanning strategies. In the future, to discover a path planning method to optimize print quality of large parts in SLM using parallel multilaser beams, further study will be performed to optimize the parameters of the partition scanning strategy and to analyze the temperature field, microstructure, and its formation mechanism in the overlap zones.
