Objective In the fields of aerospace, machinery, ships, etc., there are many multivariant twisted structures, such as fan blades in turbofan engine intakes, ternary blades in centrifugal compressors, and ship propellers. These parts have common structural features including large inclination and twisting, which cause great difficulties in processing. Currently, multivariant twisted structural parts are mainly based on computer numerical control milling, casting, electrochemical machining, etc. However, these machining methods have their own problems, such as low material utilization and long production cycles, and in some cases, it is difficult to meet performance requirements. Laser-cladding forming technology is a new type of rapid prototyping technology for metal parts that was proposed in the 1990s. It can be used for rapid and mold-free manufacturing of high-performance and complex parts. Therefore, research on laser-cladding forming of multivariant twisted structures has broad applications. At present, the laser-cladding forming of multivariant twisted structures at home and abroad is mostly based on uniform cross-section and single-direction twisting, while there are few reports on the structural parts that are twisted in multiple directions in space, especially the formation of the gradual cross-section in this type of structural parts. Based on the self-developed optical internal powder feeding technology, this paper adopts the method of conformal discrete layering to obtain the movement trajectory of the laser-cladding nozzle, and realizes the accumulation and forming of the multivariant twisted structure.
Methods The multivariant twisted structure described in this paper presents a three-dimensional twisted shape in space, with complex twisting and tilting characteristics. To realize the laser-cladding forming of the twisted structural part, based on the principle of normal delamination, this paper proposes a method of discrete layering following the shape to layer the multivariant twisted structural part. First, the structure is divided according to the shape characteristics of the structural part. The structure are divided into different parts for layering and the center line of each part is extracted; then, normal slices are made along the center line of each part. Finally, according to the characteristics of each slice layer, the slices are discretized twice to produce a different discrete cladding unit with geometric characteristics. The movement trajectory of the light spot is determined by the position and direction information of each part of the discrete unit. The cladding nozzle moves according to the position and direction information in the discrete unit to accumulate and form the multivariant twisted structure.
Results and Discussions The conformal discrete layering method is proposed to layer the multivariant twisted structure to obtain discrete cladding units with different geometric characteristics [Fig. 3(d)]. The trajectory of the light spot is determined by the position and direction information of each part of the discrete unit. The formation of the cladding layer can be regarded as the translation and rotation of the tool coordinate system, where the light spot is located relative to the base coordinate system in which the substrate is located [Fig. 4(a)]. Through translation and rotation operations, the homogeneous transformation matrix of each discrete unit relative to the base coordinate system is obtained, and the changes in position and direction change of each discrete unit relative to the base coordinate system are obtained, thereby yielding the laser-cladding nozzle's movement track. This study uses the method of robot trajectory approximation, where, through the control of program commands, the robot does not stop at the dislocation position, and realizes the gradual cladding formation of the multivariant twisted structure [Fig. 4(b)]. The experiment uses a self-developed inside-laser powder-feed nozzle, which has good powder-beam bundling, and realizes the laser-cladding forming of multivariant twisted structural parts (Fig. 7).
Conclusions To obtain the forming trajectory of multivariant twisted structural parts, a method of discrete layering according to shape is proposed: a normal discrete slice of the entire structural part is made, and then each layer of the slices is discretized twice to obtain a discrete cladding unit with different geometric characteristics. The conformal discrete layering method is used to solve the layering problem of the gradual structure of the cross section in the multivariant twisted structure, obtain the spatial movement track information of the laser-cladding nozzle, and complete the accumulation of the multivariant twisted structure. The inspection results of the formed parts are as follows: the surface of the formed parts is smooth with a surface roughness value within 5.579 μm; the average thickness of the formed parts is 6.03 mm, and the thickness of each part is slightly increased; the forming accuracy of the formed parts is higher, with a shape and size error from -3.45%--3.09%; the hardness of the different formed parts differed slightly but were basically stable at 271.6--284.5 HV; there is no obvious difference in the overall structure of the formed part; and the structure of each part is dense and uniform, without obvious pores or cracks.
