High-frequency pulse current hot wire assisted titanium alloy laser coaxial fuse wire additive manufacturing method

Abstract

The invention belongs to the technical field of laser additive manufacturing, and discloses a high-frequency pulse current hot wire assisted titanium alloy laser coaxial fuse wire additive manufacturing method. Comprising the following steps: 1, preprocessing; 2, constructing a dual-module pulse hot wire system and setting a process window: constructing a laser coaxial fuse system, and introducing a high-frequency pulse hot wire power supply of a parallel dual-module topological structure into a wire feeding path; 3, metallographic phase preparation and structure evaluation; 4, testing mechanical properties; and step 5, parameter optimization based on'defect-structure-performance 'three-stage screening. Controlled high-frequency pulse current is applied in the wire feeding process, dendritic crystal growth is actively broken through the skin effect, transient thermal shock and high-frequency Lorentz force generated by the high-frequency pulse current, the nucleation rate in a molten pool is increased, and therefore columnar crystal orientation equiaxed crystal transformation (CET) is promoted, and a titanium alloy component with excellent isotropy is obtained.

Description

Technical Field

[0001] This invention relates to the field of laser additive manufacturing technology, and more specifically to an additive manufacturing method that uses a high-frequency pulsed current hot filament to achieve grain refinement during laser coaxial filament additive manufacturing. Background Technology

[0002] Titanium alloys are widely used in aerospace, marine engineering, and other fields due to their high specific strength, strong corrosion resistance, and high temperature resistance. Laser coaxial wire additive manufacturing technology offers advantages such as high deposition efficiency, high material utilization, high forming accuracy, and suitability for manufacturing large structural components. However, due to the low thermal conductivity of titanium alloys and the significant temperature gradient and rapid cooling characteristics during laser additive manufacturing, the deposited microstructure typically exhibits coarse primary β-columnar crystals, which readily undergo epitaxial growth along the deposition height direction. This coarse columnar crystal structure leads to significant differences in the mechanical properties of components in different directions (i.e., significant anisotropy), severely limiting its application in load-bearing structural components.

[0003] To address the aforementioned issues, existing technologies typically employ external field assistance (such as ultrasound or interlaminar plastic deformation) or the addition of grain refiners. However, external field equipment is complex and difficult to integrate, and grain refiners alter the material composition, making them unsuitable for aerospace-grade titanium alloys. While hot-filament laser additive manufacturing exists, its current is mostly constant, primarily aimed at improving deposition efficiency using Joule heating, without actively controlling the solidification behavior of the molten pool using the current waveform. The limitation of existing technologies lies in their inability to disrupt the stable thermal convection pattern within the molten pool, making it difficult to suppress columnar crystal growth from a molten pool dynamics perspective. Therefore, there is an urgent need for a method that can actively control the thermodynamic behavior of the molten pool through power supply parameters, utilizing high-frequency electromagnetic force and transient thermal shock to break dendrites, thereby achieving grain refinement and isotropic enhancement without altering the composition.

[0004] To address the problem of coarse grains, existing technologies utilize electrical pulses to process metallic materials. For example, existing technology (such as CN113444871A) discloses the use of high-frequency pulsed current to perform solid-state heating treatment on ferritic stainless steel sheets to achieve strengthening and toughening. However, such methods primarily utilize the electroplastic effect of current in solid metals or promote recrystallization. This processing logic for "solid-formed parts" cannot be directly applied to additive manufacturing processes dominated by "molten pools." The reason is:

[0005] 1. Different physical field environments: Additive manufacturing involves a rapid solidification process with extremely high temperature gradients, which requires grain control to be completed within a very short time window (millisecond level) at the liquid-solid interface front. The long aging mechanism of solid-state processing fails here.

[0006] 2. Inconsistent process stability: If a high-current, high-frequency pulse is directly applied to a filament with a small diameter, the filament is very likely to melt before entering the molten pool, thus disrupting the printing process.

[0007] 3. Lack of mechanism: Existing power supply devices (such as CN 117144094 A) only provide current output and lack energy control strategies for the "wire-laser-molten pool" coupled system, and cannot produce the magnetohydrodynamic stirring effect for liquid metal.

[0008] Therefore, there is an urgent need for a process method specifically designed for the liquid solidification process in laser filament additive manufacturing, which can balance the preheating stability of the filament with the dynamic disturbance of the molten pool. Summary of the Invention

[0009] In view of this, the present invention provides a method for high-frequency pulsed current hot-wire assisted laser coaxial filament additive manufacturing of titanium alloys. The present invention applies a controlled high-frequency pulsed current during the wire feeding process, utilizing the skin effect, transient thermal shock, and high-frequency Lorentz force generated by the pulsed current to actively interrupt dendrite growth, increase the nucleation rate inside the molten pool, thereby promoting the transformation of columnar crystals to equiaxed crystals (CET), and obtaining titanium alloy components with excellent isotropic properties.

[0010] To achieve the above objectives, this invention provides a laser coaxial fused wire additive manufacturing method for high-frequency pulsed current hot wire-assisted titanium alloy grain refinement, the specific steps of which are as follows: Step 1: Preprocessing Clean the titanium alloy wire; Step 2: Construct a dual-module pulsed hotwire system and set the process window based on the fusing current reference method: A pulsed hot filament system was constructed and a process window was set. A laser coaxial filament additive manufacturing system was built. A high-frequency pulsed hot filament power supply with a parallel dual-module topology was introduced into the wire feeding path. The first module outputs a base current I. b The second module outputs a high-frequency pulse current I. p Preset the process parameters and hot wire parameters to obtain the printed sample; Step 3: Microstructure metallographic preparation and quantitative evaluation of grain refinement effectiveness: The printed samples were subjected to wire cutting, graded grinding, polishing and etching, and their microstructure was evaluated. Step 4: Multidirectional mechanical property testing and anisotropy evaluation: National standard tensile specimens were prepared along the horizontal X / Y direction and the deposition direction Z direction, and room temperature tensile tests were conducted to obtain tensile strength UTS, yield strength YS and elongation EL, and anisotropy coefficient was calculated. Step 5: Parameter optimization based on a three-tiered screening process of "defect-organization-performance". Based on the microscopic analysis in step 3 and the mechanical feedback in step 4, the following screening logic is established to determine the final parameters: Primary screening (forming quality): Eliminating parameter combinations that result in severe splashing or deposition discontinuities; Secondary screening (microstructure): Among well-formed samples, the parameter with the highest proportion of equiaxed crystals is preferred; Level 3 screening (mechanical properties): In the fine grain group, the combination with the anisotropy coefficient %IPAs closest to 1.0 is selected as the final manufacturing process parameter.

[0011] Preferably, in step 2, the process parameters are: laser power 1200W, and movement speed 15mm / s.

[0012] Preferably, in step 2, the standard and method for setting the hot wire parameters are as follows: First, a pre-experiment of wire resistance heating is conducted. Under conditions of no laser and only wire feeding, the instantaneous fusing current value I of the wire at a specific wire feeding speed is measured. m Therefore I m Set the process window for the baseline: Average current I of the hot wire: set to 0.3~0.8I m It is used to preheat the filament to its plastic softening temperature; Hot wire base current I b Set to 0.3~0.5I; Hot wire high-frequency pulse current frequency f p Set to 20~100 kHz; Hot wire high-frequency pulse duty cycle D: set to 30%~60%; Hot wire high-frequency pulse current I p : Set to (II) b ) / D.

[0013] Preferably, the specific wire feeding speed is 35 mm / s.

[0014] Preferably, in step 3, the graded grinding is performed by sequentially using SiC sandpaper of 240#, 600#, 1000#, and 2000# for wet grinding; the polishing is performed by mechanical and chemical polishing using a SiO2 suspension with a particle size of 0.05μm; and the etching treatment is performed by etching in a modified Kroll reagent solution composed of HF, HNO3, and H2O for 10-15 seconds.

[0015] Preferably, the volume ratio of HF, HNO3 and H2O is 1:1:18.

[0016] Preferably, in step 3, the standard for evaluating the structure is as follows: using an optical microscope, grains with a ratio of major axis to minor axis AR < 2.0 are defined as equiaxed grains. The area ratio of equiaxed grains in the field of view is counted. If the ratio is > 60%, then the parameter set is determined to be effective for grain refinement.

[0017] Preferably, in step 4, the anisotropy evaluation criterion is: Define intensity anisotropy coefficient ;in, X max To determine the maximum tensile strength, X min To obtain the minimum value of tensile strength; If %IPAs≥0.95 and |EL Z -EL XY If |≤2%, then the component is determined to be isotropic.

[0018] The essential difference between this invention and existing solid-state electrical pulse processing technology lies in the fact that this invention does not use current to post-process the already formed solid structure, but rather uses a current with a specific waveform to excite high-frequency Lorentz force oscillations inside the liquid molten pool. Through a composite strategy of "base value current preheating + high-frequency pulse perturbation", under the premise of preventing premature vaporization of the filament, the primary dendrites at the solidification front are actively broken by the magnetohydrodynamic effect (MHD), thereby achieving in-situ grain refinement.

[0019] As can be seen from the above technical solution, compared with the prior art, the technical effects achieved by the present invention are as follows: By controlling the in-situ dynamics of the solidification process in laser-fused filament additive manufacturing, this invention fundamentally solves the industry-wide problem of coarse columnar crystal growth and property anisotropy without altering the composition of the titanium alloy. Firstly, regarding microstructure evolution, this invention applies a high-frequency pulsed current of a specific waveform, utilizing its magnetohydrodynamic effect within the molten pool to generate strong transient thermal shock and high-frequency Lorentz force. This high-frequency oscillating energy effectively acts on the liquid-solid interface front, actively interrupting the growth of primary dendrites and significantly increasing the non-spontaneous nucleation rate within the molten pool, thereby forcibly triggering the transformation from coarse columnar crystals to fine equiaxed crystals. Secondly, in terms of improved mechanical properties and isotropic control, this invention achieves significant strengthening effects. By achieving in-situ grain refinement and breaking the epitaxial growth characteristics along the deposition height direction, the difference in mechanical properties between the component in the horizontal direction (X / Y direction) and the deposition direction (Z direction) is largely eliminated. Experimental verification shows that the strength anisotropy coefficient (%IPAs) of the components manufactured by this invention can stably reach above 0.95, with minimal fluctuations in elongation in different directions. This enables the titanium alloy additive parts to maintain high tensile strength and yield strength while possessing excellent isotropy, meeting the stringent requirements for performance uniformity in main load-bearing structural components in aerospace and marine engineering. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0021] Figure 1 This is a schematic diagram of the process flow of the method of the present invention.

[0022] Figure 2 This is a schematic diagram of the high-frequency pulse current waveform applied in Example 2. a is the hot wire current waveform of the control group, and b is the hot wire current waveform of the experimental group.

[0023] Figure 3 These are comparative metallographic images of the cross-section of the titanium alloy deposited layer in Example 3. a is conventional laser coaxial filament additive manufacturing, and b is high-frequency pulsed hot filament assisted additive manufacturing.

[0024] Figure 4 These are the tensile stress-strain curves of the specimens under the optimal parameters in Example 3 along different directions (horizontal and vertical). a is the tensile performance curve of the control group, and b is the tensile performance curve of the experimental group. Detailed Implementation

[0025] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0026] Example 1: The process flow of the present invention is as follows Figure 1 As shown, the method provided by this invention includes the following details in actual operation: Step 1: Pretreatment and Parameter Setting: Using 1.2mm diameter standard TC17 wire, the 10mm rolled TC4 substrate is polished, wiped with alcohol, and dried to remove oil stains from the substrate surface. Parameter setting operation: At a wire feed speed of 35mm / s, the instantaneous fusing current I of the wire is measured. m The current is approximately 70A, so the average current I of the hot filament is set to 50A. Process parameters are set as follows: laser power 1200W, movement speed 15mm / s. Hot filament parameters are set as follows: base current I. b =20A, high-frequency pulse current I p =60A, frequency 40kHz, duty cycle 50%.

[0027] Step 2: Metallographic Preparation and Microstructure Evaluation. The sample is cut along the deposition direction. Preparation: Grind with SiC sandpaper to 2000#, then polish to a mirror finish using SiO2 suspension. Etching: Immerse in an etching solution of 1 ml HF + 1 ml HNO3 + 18 ml H2O for 12 seconds, then immediately rinse with water and dry. Observation: Observe under a 50x microscope. If columnar crystals traversing multiple deposition layers disappear, and fine equiaxed crystals (AR < 2.0) dominate, it indicates that the high-frequency pulse successfully interrupted the epitaxial growth.

[0028] Step 3: Mechanical property testing. Cut tensile specimens along horizontal and vertical lines respectively. Stretch using an electronic universal tensile testing machine at a rate of 1.0 mm / min.

[0029] Step 4: Parameter Optimization Logic. In the actual parameter optimization process, the following criteria are followed: 1. Grain refinement criterion: If the microstructure is still coarse columnar crystals (AR>5.0), increase the pulse frequency or current peak value to enhance the disturbance capability to the molten pool; 2. Defect control criteria: If pores or spatter appear, it indicates that the current is too high, causing the wire to melt prematurely. The hot wire current level or laser power needs to be reduced. 3. Isotropic criterion: The optimal parameters should satisfy: tensile strength %IPAs ≥ 0.95, and the absolute value of the difference between the Z-direction elongation and the horizontal direction elongation is less than 2%.

[0030] Example 2: Grain Refinement Mechanism of High-Frequency Pulse Current. This embodiment explains the mechanism by which the present invention achieves grain refinement, which is also the key difference between the present invention and traditional constant current hot wires. The dual-module parallel power supply used in the present invention generates a severe transient thermal effect at the end of the wire.

[0031] 1. High-frequency electromagnetic oscillation of the molten pool: When a high-frequency changing current flows through the molten metal, it generates a high-frequency Lorentz force, which drives forced convection inside the molten pool. This stirring effect can break up the dendrite arms at the solidification front.

[0032] 2. Transient thermal shock nucleation: The pulsed current causes the temperature of the droplets entering the molten pool to fluctuate periodically and violently. This thermal disturbance increases the fluctuation of supercooling inside the molten pool, greatly increasing the heterogeneous nucleation rate, thereby promoting the transformation of columnar crystals into equiaxed crystals.

[0033] Example 3: To verify the effectiveness of the above method and mechanism, this example determined a set of optimal process parameters based on the optimization logic described in Example 1, and conducted a comparative experiment with the traditional process.

[0034] 1. Experimental conditions setup.

[0035] Materials: TC17 titanium alloy wire with a diameter of 1.2mm and TC4 substrate with a thickness of 10mm.

[0036] Control group (existing technology): laser power 1200W, movement speed 15mm / s, wire feeding speed 35mm / s, interlayer temperature control 170℃, hot wire current 50A.

[0037] Experimental group (optimized parameters of this invention): The optimal parameter combination selected according to the method of this invention. Laser power 1200W, movement speed 15mm / s, wire feeding speed 35mm / s, interlayer temperature control 170℃, base current 20A, high-frequency pulse current 60A, pulse frequency set to 40kHz, duty cycle 50%. To ensure consistent average heat input to the hot wire, the average current in the experimental group under high-frequency pulse mode is kept consistent with that of the control group, both being 50A.

[0038] 2. Analysis of Experimental Results Microscopic organizational comparison: Figure 3 The metallographic structures of the control and experimental groups are shown. In the control group, the grains exhibit a typical coarse columnar morphology, growing continuously through multiple deposition layers with a length exceeding 3 mm, which is the main cause of the anisotropy. In the experimental group, the introduction of high-frequency pulsed thermal shock disrupted the growth of the originally coarse columnar grains, transforming the structure into fine mixed grains with a significantly reduced average grain size. This verifies the significant regulatory effect of high-frequency pulsed current on the solidification process of the molten pool.

[0039] Mechanical property comparison: Tensile tests were performed on the two groups of samples. The control group showed a horizontal tensile strength of 1223 MPa, while the tensile strength in the deposition direction (Z-direction) was only 1148 MPa. The Z-direction strength was significantly lower than the horizontal strength, exhibiting severe anisotropy and failing to meet the requirements for high-performance components. The experimental group: (See attached image). Figure 4 As shown, after optimization by the method of this invention, the tensile strength in the horizontal direction is 1177 MPa, and the tensile strength in the deposition direction (Z direction) is 1162 MPa. The strength difference between the Z direction and the horizontal direction is reduced to less than 1.5% (i.e., the isotropic coefficient is close to 1), and the elongation after fracture in both directions exceeds 7%.

[0040] 3. Conclusion Experimental results show that the high-frequency pulsed current hot wire additive manufacturing method provided by this invention can effectively break dendrites and refine grains, thereby fundamentally solving the problem of anisotropy of mechanical properties of titanium alloy deposits without changing the material composition.

[0041] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for high-frequency pulsed current hot-wire assisted laser coaxial filament additive manufacturing of titanium alloys, characterized in that, The specific steps are as follows: Step 1: Preprocessing Clean the titanium alloy wire; Step 2: Construct a dual-module pulsed filament system and set the process window: A laser coaxial filament additive manufacturing system was constructed, and a high-frequency pulsed hot filament power supply with a parallel dual-module topology was introduced into the filament feeding path. The first module outputs a base current I. b The second module outputs a high-frequency pulse current I. p Preset the process parameters and hot wire parameters to obtain the printed sample; Step 3: Microstructure metallographic preparation and quantitative evaluation of grain refinement effectiveness: The printed samples were subjected to wire cutting, graded grinding, polishing and etching, and their microstructure was evaluated. Step 4: Multidirectional mechanical property testing and anisotropy evaluation: National standard tensile specimens were prepared along the horizontal X / Y direction and the deposition direction Z direction, and room temperature tensile tests were conducted to obtain tensile strength UTS, yield strength YS and elongation EL, and anisotropy coefficient was calculated. Step 5: Parameter optimization based on a three-tiered screening process of "defect-organization-performance". Based on the microscopic analysis in step 3 and the mechanical feedback in step 4, the following screening logic is established to determine the final parameters: Primary screening: Eliminate parameter combinations that lead to severe splashing or discontinuous deposition; Secondary screening: Among those with good formation, the parameter with the highest proportion of equiaxed crystals is preferred; Three-level screening: In the fine grain group, the combination with the anisotropy coefficient %IPAs closest to 1.0 is selected as the final manufacturing process parameter.

2. The method for high-frequency pulsed current hot-wire assisted laser coaxial filament additive manufacturing of titanium alloys according to claim 1, characterized in that, In step 2, the process parameters are: laser power 1200W, movement speed 15mm / s.

3. The method for high-frequency pulsed current hot-wire assisted laser coaxial filament additive manufacturing of titanium alloys according to claim 1, characterized in that, In step 2, the standard and method for setting the hot wire parameters are as follows: First, a pre-experiment of wire resistance heating is conducted. Under conditions of no laser and only wire feeding, the instantaneous fusing current value I of the wire at a specific wire feeding speed is measured. m , with this I m Set the process window for the baseline: Average current I of the hot wire: set to 0.3~0.8I m ; Hot wire base current I b Set to 0.3~0.5I; Hot wire high-frequency pulse current frequency f p Set to 20~100 kHz; Hot wire high-frequency pulse duty cycle D: set to 30%~60%; Hot wire high-frequency pulse current I p Set to (II) b ) / D.

4. The method for high-frequency pulsed current hot-wire assisted laser coaxial filament additive manufacturing of titanium alloys according to claim 1, characterized in that, The specific wire feeding speed is 35 mm / s.

5. The method for high-frequency pulsed current hot-wire assisted laser coaxial filament additive manufacturing of titanium alloys according to claim 1, characterized in that, In step 3, the graded grinding is performed by water grinding with SiC sandpaper of 240#, 600#, 1000#, and 2000# in sequence; the polishing is performed by mechanical and chemical polishing with SiO2 suspension with a particle size of 0.05μm; and the etching treatment is performed by immersing in an etching solution composed of HF, HNO3, and H2O for 10-15 seconds.

6. The method for high-frequency pulsed current hot-wire assisted laser coaxial filament additive manufacturing of titanium alloys according to claim 5, characterized in that, The volume ratio of HF, HNO3, and H2O is 1:1:

18.

7. The method for high-frequency pulsed current hot-wire assisted laser coaxial filament additive manufacturing of titanium alloys according to claim 1, characterized in that, In step 3, the criteria for evaluating the structure are as follows: using an optical microscope, grains with a ratio of major axis to minor axis AR < 2.0 are defined as equiaxed grains. The area ratio of equiaxed grains in the field of view is counted. If the ratio is > 60%, the parameter set is determined to be effective for grain refinement.

8. The method for high-frequency pulsed current hot-wire assisted laser coaxial filament additive manufacturing of titanium alloys according to claim 1, characterized in that, In step 4, the anisotropy evaluation criterion is: Define intensity anisotropy coefficient ;in, X max To determine the maximum tensile strength, X min To obtain the minimum value of tensile strength; If %IPAs≥0.95 and |EL Z -EL XY If |≤2%, then the component is determined to be isotropic.

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