弧面扭曲结构件激光内送粉熔覆成形研究

Inside-Laser Powder Feeding Cladding Forming of Arc-Shaped Twisted Structural Parts

通过研究弧面扭曲结构件激光熔覆成形轨迹规划难点,解决了以弧线为渐变路径的扭曲结构分层问题。基于中空激光内送粉熔覆成形技术并结合结构件自身几何特征,提出离散渐变分层法。首先在基体法平面内纵向分层,随后将各层二次离散成独立单元,最终按等圆心角将离散单元拼接成熔覆路径,成功实现了弧面扭曲结构件激光熔覆成形。所得成形件尺寸精度较高,表面平均粗糙度低至1.323μm,抗拉强度为765.81 MPa;不同区域的微观组织和显微硬度分布均匀,无明显气孔或夹杂缺陷。

Objective Many components in the aerospace,ocean machinery,biomedical,and other fields have curved twisted structures,such as the axial flow blades of turbojet engines and rotor blades of marine propellers.The majority of these components have geometric features,such as a large-angle twist/overhanging structure and a spatiallygradually changing cross section.When using traditional techniques to process such parts,the processing costs are high,the production cycle is long,and the components may not meet the user requirements under certain working conditions.Because laser cladding technology can achieve near-net shaping of complex structural parts without the use of a mold and with high precision and high efficiency,it has broad prospects in the field of direct shaping of complex twisted structures.The curved twisted structure is circular in cross section and tapered by arcs,with a large-angle overhanging structure at its end.A few reports on path planning for this type of structure were published in China and abroad.To accomplish tool-path planning and generation,a discrete gradient slicing(DGS)method is proposed in this study.Based on the self-developed annular laser beam based on axially fed powder cladding,a twisted structural component with an arc curve spine line was successfully built.Methods Because the structural components studied in this paper had spatially-gradually horizontal and longitudinal section characteristics,as well as end faces with large-angle overhanging structures,the traditional horizontal slicing method did not meet the forming requirements.A DGS method based on the normal slicing principle was proposed to slice the curved twisted structural parts for realizing their laser cladding formation.First,the structural component’s central curve was extracted.Because the structural part’s longitudinal section was a circular arc with different radii and center angles at both ends,its change path was curved.There was no asymptotic growth axis of the longitudinal section in the base plate normal plane.The actual travel path of a cladding nozzle layer was used as the central arc.Subsequently,the structural component was sliced along the central curve in a normal direction,and the longitudinal sections at the start and end of the structural part acted as shape constraints for the slices.Then,the sliced layers were divided along the central curve with equal centric angle,and finally,the sliced layers were further discretized within the layers to obtain discrete cladding elements with different positions and directions.The direction of growth of elements within the same layer was tangential to the longitudinal cross sectional curve,and the direction of interlayer cladding was consistent with the central curve’s tangential direction.Finally,the cladding path was generated by slicing the discrete elements according to equal center angles.Due to the structure’s“inverted growth”characteristic,the arc length corresponding to the unit central angle of the central arc became larger layer by layer,and the number of cladding units on the splicing path increased accordingly.The vector formed by the centerlines of each discrete unit’s upper and lower surfaces was used to characterize the unit’s direction and height.The beam axis was kept consistent with the unit direction vector during the cladding process to ensure that the cladding layer does not accumulate dislocations.Results and Discussions The discrete gradual layering method is proposed to complete the layering of arc-shaped twisted structural parts,and discrete cladding units with different geometric characteristics are obtained[Fig.3(d)].By treating each discrete unit as a point in the base coordinate system and assigning it a tool coordinate system having independent motion,the homogeneous transformation matrix of each discrete unit relative to the base coordinate system is obtained via translational and rotational operations,and its position and posture information are determined[Fig.6(a)].Based on the translation and rotation of the tool coordinate system,where each unit is located relative to the base coordinate system with the substrate,the discrete cladding units with different spatial orientation information are spliced horizontally[Fig.6(b)]to obtain the actual Cladding track.A self-developed optical internal powder-feeding nozzle combined with a six-axis robot line segment fitting technology was used in the experiment to achieve the cladding forming of arc-shaped twisted structural parts with“inverted growth”and cross sectional gradual characteristics[Fig.9(a)].Conclusions Based on the optical internal powder-feeding cladding forming technology and the principle of normal layering,a discrete gradual layering method is proposed:the structure is sliced in a normal direction,and the sliced layer is constrained by the longitudinal sections at both ends.In addition,the sliced layer is longitudinally discrete in the layer to obtain discrete cladding units with different geometric characteristics,and the units are horizontally spliced with equal central angle curves to form actual melting channels.The twisted structure-layering problem with the arc acting as the gradual path is solved,and the laser cladding accumulation of the curved twisted structure is realized.The inspection results of the formed parts are as follows:the surface of the formed parts is bright and smooth;the surface roughness is between 1.323 and 9.638μm;the average thickness of the formed parts is 6.49 mm;the thickness of each part is relatively uniform;and the shape and size error is between-2.1%and 3.05%.The forming accuracy is high;the microhardness of different areas of the formed part fluctuates in the range from 264.1 HV to 277.2 HV;the overall hardness difference is small;the tensile strength and elongation after formed part’s fracture are 765.81 MPa and 11.2%,respectively.The fracture mechanism is a mixed fracture of cleavage fracture and local quasi-cleavage fracture.Besides,the microstructure of the formed part is dominated by dendrites,and the overall structure is dense and uniform without obvious pores and inclusion defects.