Study On Forming Method And Mechanism Of Three-Beam Internal Coaxial Wire Feeding Laser Cladding

The wire-feeding laser cladding method has the advantages of high material utilization,low environmental pollution,low material cost,and less dependent on the space environment.It has broad development prospects and has been widely used in surface treatment and additive manufacturing in recent years.However,the laser cladding method of "laser outside wire feeding"has defects,such as poor coupling accuracy of wire and laser and scanning directionality.The"multi-laser source internal coaxial wire feeding" laser cladding method is composed of multiple laser sources.It has some problems such as large space occupied by the forming system,high use and maintenance cost.The laser cladding method of "single laser source internal coaxial wire feeding" has laser-wire interference and complex optical path conversion system,which brings low energy utilization efficiency.Due to the early research on wire-feeding laser cladding technology abroad and a lot of technology accumulation,some of the wire-feeding laser cladding technologies have been industrialized.However,domestic research on this technology started late and is in the research and exploration stage.There is still a certain gap with foreign countries,which greatly restricts rapid development in this field.There is an urgent need to propose a new method that can solve the problems of the existing wire feeding laser cladding technology,breaking the foreign technology monopoly,mastering the core technology of wire-feeding laser cladding forming,and improving the competitiveness of metal additive manufacturing industry.This study proposes a new method of three-beam internal coaxial wire feeding laser cladding.A prism and focusing mirrors were utilized to separate and focus the laser beam with internal coaxial wire feeding.The ray tracing method was used to optimize and determine the structural parameters of the laser path conversion system inside the nozzle.The core component cladding nozzle was independently developed.The laser cladding system of "three-beam internal coaxial wire feeding" was constructed.It can achieve the precise coupling of laser and wire.It can also avoid scanning directionality,low energy utilization and other problems caused by laser-wire interference and complex optical path conversion system.The optical software Tracepro was used to establish the heat source model of the laser spot energy distribution.The finite element numerical simulation model of the interaction between the laser spot and the substrate was established through ANSYS Workbench.The shape and energy distribution characteristics of the laser spot with different defocus distances and the law of molten pool change were studied.Research shows that:with increase of the defocus distance,the spot size R1、R2 increases,whereas the energy density qm decreases.The different morphology of the molten pool is formed at different defocus distances.A single circular trough-like molten pool is formed by a single Gaussian-like spot with defocus distance 0 mm.A concave circular triangle left offset and the flat trough-shaped molten pool is formed by a Gauss-like spot with partial overlap of the three spots at defocus distance-1 mm.When the defocus distance is between-2 mm and-3 mm three circular-shaped trough-like molten pools are formed by three independent and uniformly distributed Gauss-like laser spots.The laser-wire coupling model was established to analyze the influence of process parameters on the surface morphology of the cladding layer.Research shows that the energy distribution of spot with the negative defocus distance is relatively uniform,and it has higher tolerance and larger process window than positive defocus distance,and it is easier to obtain a cladding layer with higher surface quality.Selecting the defocus distance as-2.5 mm,the process window of influence of process parameters on the surface morphology of the cladding layer is:E1/E2<1,the surface of the cladding layer is mainly "incomplete",and part area with "Sticking wire" phenomenon or "sag"on the surface of the cladding layer.1.1≥E1/E2≥1there is a "sag" in the local area of the cladding layer.1.5≥E1/E2>1.1,the surface of the cladding layer is continuous and smooth.E1/E2>1.6,the surface of the cladding layer is mainly "droplet" or "depressed".A finite element numerical simulation model of a single-pass cladding layer was established through ANSYS Workbench to study the influences of process parameters on the temperature field of the molten pool,the geometric characteristics of the cladding layer and the microstructure.Defocus distance influence results are:at the defocus distance-2.5 mm,the temperature field curve is mountain-shaped,the energy distribution in the dilution zone is relatively uniform,and the typical dilution zone morphology is flat-bottomed and double-valley-shaped.At the defocus distance-1.5 mm,the temperature field curve is sharp,the energy distribution in the dilution zone is relatively concentrated,and the morphology of the dilution zone is a single wave trough.The typical grain morphology with defocus of-2.5mm and-1.5mm are columnar crystals,cellular crystals and dendrites.The average hardness at defocus distance-1.5mm is 20HV0.5 higher than2.5mm.The influence of process parameters on the geometric characteristics of the cladding layer is as follows:defocus distance,laser power and wire feed speed are positively correlated with width,and scanning speed is negatively correlated with width.Defocus distance,laser power and scanning speed are negatively correlated with height,and wire feeding speed is positively correlated with height.Defocus distance,scanning speed and wire feed speed are negatively correlated with melting depth,and laser power is positively correlated with melting depth.By studying the energy distribution relationship between the wire and the laser spot,an optimization model between the process parameters and the geometric characteristics of the cladding layer is established.Compared with the theoretical model,the prediction accuracy of the optimization model is significantly improved.The average error of the cladding width is reduced from 17.2%of the theoretical model to 4.09%of the optimization model;the average error of the cladding height is reduced from 13.74%of the theoretical model to 3.76%of the optimization model.The laser wire coupling model,the cladding layer geometric feature prediction model and the straight wall numerical simulation model are used to determine the process parameters of the straight wall forming part.The straight wall forming and stacking process is stable.The overlap surface among layers is smooth and complete.The size error is small,the internal structure is generally uniform,and there is no obvious defect.The hardness fluctuation is small,and the mechanical properties of the formed part are significantly better than the substrate material.The fracture is distributed like dimples and has excellent tensile properties.At the same time,the cladding system was used to successfully obtain the forming and stacking of circular ring parts,twisted parts and solid parts,laying the foundation for subsequent engineering applications.In this paper,the research on the forming method and mechanism of three-beam laser internal coaxial wire feeding cladding enriches the theoretical method system of wire feeding laser cladding,and it is of great significance to promote the technological progress in the field of laser additive manufacturing.