Novel metal powder mixed fluid preparation and semi-melting forming process for SLM 3D printing

Abstract

The invention discloses a novel metal powder mixed fluid preparation and semi-fusion forming process for SLM 3D printing. The process is suitable for low-cost and high-performance batch manufacturing of precision parts such as aerospace and medical implants and high-end molds. Aiming at the pain points of high cost of pre-alloyed powder, high patent risk of elemental mixed powder, low density, large equipment investment and the like, an asymmetric three-peak particle size design avoidance patent is innovatively adopted, and a core-shell structure mixed fluid is prepared in combination with a multi-objective weighted optimization model and a spray coating method; single-laser dynamic power three-section control is adopted in the forming process, and optimized forming environment parameters are matched. According to the method, existing equipment does not need to be modified, the compactness of a formed part is larger than or equal to 99.3%, the tensile strength is improved by 50% or above, the raw material cost is reduced by 35%, the equipment investment is reduced by 40%, and the patent compliance and industrialization adaptability are high.

Description

Technical Field

[0001] This invention belongs to the field of selective laser melting (SLM) additive manufacturing technology, specifically involving a novel metal powder mixed fluid preparation and semi-molten forming process for SLM 3D printing, which is suitable for low-cost, high-performance mass production of precision parts such as aerospace structural components, medical device implants, and high-end molds. Background Technology

[0002] Selective laser melting (SLM) additive manufacturing technology has been widely used in aerospace, medical, and high-end manufacturing fields due to its advantages of not requiring molds, high degree of freedom in forming, and ability to manufacture complex structural parts. Currently, in the SLM 3D printing industry, the metal powders used for forming are mainly divided into two categories: pre-alloyed powders and mixtures of single-element metal powders.

[0003] Pre-alloyed powders, due to their good compositional uniformity, ensure the stability of molded parts and dominate the mainstream market. However, they have significant drawbacks: high raw material costs, long customization cycles (typically ≥15 days), and poor flexibility in compositional adjustment, making it difficult to quickly adapt to the manufacturing of parts with different performance requirements. While the mixed printing route using single-element metal powders can reduce raw material costs by more than 30%, it faces three major technical bottlenecks: First, existing powder particle size distribution schemes (such as patent CN114012085B) protect symmetrical bimodal particle size distributions, and directly adopting this distribution can easily lead to infringement risks; second, the mixed powders have poor flowability and low molding density (typically ≤95%), resulting in large fluctuations in the mechanical properties of the parts, failing to meet the requirements of precision parts; third, the traditional dual-laser preheating-melting process requires special equipment modifications, resulting in high equipment investment costs that are unaffordable for small and medium-sized manufacturing enterprises.

[0004] Furthermore, the fluxes used in existing SLM printing are mostly single-component solvents, which have limited effect on improving the wettability of metal powders. This leads to defects such as porosity and cracks during printing, further affecting the quality stability of the formed parts. In summary, the industry urgently needs a metal powder mixed fluid preparation and forming process that balances patent compliance, forming stability, and cost-effectiveness to overcome existing technological limitations and promote the industrialization and widespread adoption of SLM technology. Summary of the Invention

[0005] I. Purpose of the Invention The purpose of this invention is to overcome the aforementioned deficiencies of the prior art and provide a novel metal powder mixed fluid preparation and semi-molten forming process for SLM 3D printing. Through the synergistic innovation of asymmetric three-peak particle size design, multi-objective optimized proportioning model, and single-laser dynamic power control, the invention achieves a dual improvement in the uniformity of the mixed powder and the performance of the formed parts, while effectively avoiding core patent risks, reducing equipment investment and raw material costs, and meeting the needs of mass production of precision parts. Technical solution

[0006] The technical solution of this invention comprises two core parts: a method for preparing a metal powder mixed fluid and a semi-melt forming process. The specific technical solutions for each step are as follows: (I) Preparation method of metal powder mixed fluid

[0007] This patent-circumventing asymmetric tri-peak particle size design overcomes the limitations of existing patents on bi-peak particle size distribution by dividing the metal powder into three particle size ranges: coarse powder (35-45μm), medium powder (15-25μm), and ultrafine powder (5-10μm), with a mass ratio of 50-60:30-40:5-15. By controlling the proportion of ultrafine powder to fill the gaps between coarse and medium powder, the powder bulk density can be increased to over 65%. Simultaneously, the coarse-to-fine particle size ratio is limited to 3-4:1, avoiding the 5-7:1 protection range of existing patents, thus achieving patent circumvention.

[0008] The multi-objective optimization ratio calculation model is constructed and solved with bulk density (D), wettability (W), mechanical properties (M), and raw material cost (C) as the core objective functions. A multi-objective weighted optimization model is constructed, and the model expression is: Y = α・D + β・W + γ・M - δ・C. In the formula, α, β, γ, and δ are weight coefficients, and their values ​​range from 0.2 to 0.35. They can be flexibly adjusted according to the application scenario of the parts: for example, for aerospace parts, mechanical properties need to be guaranteed first, so the value of γ can be increased; for mold parts, cost control needs to be controlled first, so the value of δ can be increased.

[0009] The constraints of the model include: ① The melting point difference between high and low melting point metals is ≤250℃ to ensure that the metal powder can achieve effective metallurgical bonding during the molding process; ② The content of low melting point metal is ≤30% to avoid excessive low melting point metal affecting the overall mechanical properties of the molded parts; ③ The amount of fluxing agent is 1-2.5% to balance the wettability improvement effect and cost control.

[0010] A genetic algorithm was used to solve the above multi-objective weighted optimization model, and the precise proportions of each metal powder and flux were output, providing a basis for the subsequent preparation of mixed fluids.

[0011] The preparation of the core-shell structured mixed fluid involves the following steps: First, high-melting-point coarse and medium-sized metal powders are placed in a three-dimensional mixer at a speed of 15 r / min and dry-mixed for 1.5 hours to ensure uniform mixing and form a basic powder system. Second, low-melting-point ultrafine metal powders are mixed with a ternary fluxing agent composed of KCl, NaCl, and KBF4 in a mass ratio of 6:3:1 to form a uniform coating slurry. Third, a spray coating method is used to uniformly coat the prepared coating slurry onto the surface of the basic powder, forming a core-shell structure of "high-melting-point core - low-melting-point shell - fluxing agent coating". The coated powder is then dried in a vacuum drying oven at 80°C for 2 hours to remove moisture and impurities, ultimately yielding the metal powder mixed fluid. (ii) Semi-melt molding process

[0012] The single-laser dynamic power three-stage control uses a commercial single-fiber laser (500W). Dynamic switching of the laser power is achieved through a programmable controller. The scanning process is divided into three stages, with parameters for each stage as follows: Preheating stage: Laser power is set to 120-180W, scanning speed is 100-180mm / s, and the powder bed temperature is controlled at 50-80℃ below the high-melting-point solidus line of the metal. The purpose of this stage is to eliminate the internal stress generated in the metal powder during storage and mixing, and to improve the wettability of the powder, preparing it for subsequent... The melting and forming process lays the foundation; in the semi-melting stage, the laser power is increased to 220-300W and the scanning speed is adjusted to 250-400mm / s. This combination of power and speed allows the low-melting-point metal shell to melt completely and the surface of the high-melting-point metal core to melt slightly, achieving metallurgical bonding between particles and ensuring the structural integrity of the formed part; in the heat preservation stage, the laser power is adjusted to 150-200W and the scanning speed is 150-200mm / s. By slowly cooling down, the thermal stress generated during the forming process is reduced, avoiding defects such as cracks in the formed part.

[0013] To ensure molding quality, the molding environment parameters for SLM printing were optimized: the oxygen content in the molding chamber was controlled to be ≤300ppm, and an argon atmosphere was used for protection to prevent oxidation of metal powder and molded parts at high temperatures; the substrate preheating temperature was set to 180-300℃, and the specific temperature could be adjusted according to different metal systems; the layer thickness was selected to be 50-150μm, balancing molding efficiency and molding accuracy to meet the manufacturing needs of different precision parts. Beneficial effects

[0014] Through synergistic innovation of technical solutions, this invention has achieved the following significant beneficial effects: Strong patent compliance: The asymmetric three-peak particle size distribution design limits the coarse-to-fine particle size ratio to 3-4:1, avoiding the protection scope of existing patents for bi-peak particle size distribution and particle size ratio of 5-7:1, effectively reducing the risk of infringement and ensuring the legal promotion and application of the process.

[0015] Excellent molding performance: The combination of core-shell structure design and single-laser dynamic power control process enables the molded parts to achieve a density of over 99.3%, and the tensile strength is increased by more than 50% compared with traditional single-element mixed powder printing parts. The mechanical property fluctuation range is ≤3%, and the molded parts are free from defects such as obvious pores and cracks, which can meet the high-performance requirements of precision parts in aerospace, medical and other fields.

[0016] Significant cost advantages: Compared with pre-alloyed powder, this invention uses a mixture of single metal powders for preparation, reducing raw material costs by 35%; at the same time, the use of a single laser process eliminates the need for retrofitting dual laser equipment, reducing equipment investment by 40%, and significantly reducing the production costs of manufacturing enterprises.

[0017] High industrial adaptability: The process of this invention does not require modification of existing commercial SLM equipment, can be directly connected to mass production lines, and the scrap rate is controlled within 2%, resulting in high production efficiency and good stability, making it suitable for large-scale industrial application. Detailed implementation methods The present invention will be further described in detail below with reference to specific embodiments, but the scope of protection of the present invention is not limited to the following embodiments.

[0018] I. Experimental Materials and Equipment Experimental materials: 316L stainless steel coarse powder (particle size 35-45μm), 316L stainless steel medium powder (particle size 15-25μm), 316L stainless steel ultrafine powder (particle size 5-10μm); Ni powder (ultrafine powder, particle size 5-10μm); ternary flux (KCl, NaCl, KBF4, analytical grade).

[0019] Experimental equipment: SLM-280 3D printer, 3D mixer, spray coating machine, vacuum drying oven, universal testing machine, metallographic microscope. Detailed Implementation

[0020] Powder ratio calculation: Based on the performance requirements of aerospace parts, the weight coefficients of the multi-objective weighted optimization model are set as α=0.25, β=0.25, γ=0.3, δ=0.2. After substituting into the model, the genetic algorithm is used to solve the optimal ratio scheme: 55% coarse powder of 316L stainless steel, 30% medium powder of 316L stainless steel, 5% ultrafine powder of 316L stainless steel, 8% Ni powder, and 2% ternary fluxing agent.

[0021] Preparation of core-shell structured mixed fluid: 316L stainless steel coarse powder and medium powder were placed in a three-dimensional mixer and dry-mixed at 15 r / min for 1.5 hours to form a basic powder system; Ni powder and ternary flux (KCl:NaCl:KBF4=6:3:1) were mixed evenly to prepare a coating slurry; the slurry was uniformly coated on the surface of the basic powder using a spray coating method, and then the coated powder was placed in an 80℃ vacuum drying oven for 2 hours to obtain a metal powder mixed fluid.

[0022] Semi-molten forming: The metal powder mixture is loaded into the powder supply chamber of the SLM-280 3D printer. The forming parameters are set as follows: the substrate is preheated to 200°C, and the oxygen content in the forming chamber is controlled at 250ppm. The laser power parameters are set to 150W for the preheating stage, 260W for the semi-molten stage, and 180W for the holding stage. The corresponding scanning speeds are 150mm / s, 350mm / s, and 180mm / s, respectively, with a layer thickness of 100μm. A tensile specimen of 316L stainless steel is printed.

[0023] Post-processing: The printed tensile specimens are placed in a heat treatment furnace and solution treated at 1050℃ for 2 hours, followed by water cooling to room temperature to remove residual stress generated during the molding process.

Claims

1. A novel metal powder mixed fluid preparation and semi-molten forming process for SLM 3D printing, characterized in that, The preparation method of metal powder mixed fluid and the semi-melt molding process based on the mixed fluid are as follows: (1) Preparation method of metal powder mixed fluid: ① The particle size range of metal powder is designed by using an asymmetric three-peak particle size distribution, which is divided into coarse powder 35-45μm, medium powder 15-25μm and ultrafine powder 5-10μm. The mass ratio of the three is 50-60:30-40:5-15, and the particle size ratio of coarse powder to fine powder is 3-4:1; ② A multi-objective weighted optimization model is constructed and the optimal ratio is solved. The model takes bulk density (D), wettability (W), mechanical properties (M) and raw material cost (C) as the core objective functions. The expression is Y = α・D + β・W + γ・M -δ・C, where α, β, γ, and δ are the core objective functions of the model. The weighting coefficient ranges from 0.2 to 0.

35. Constraints include: melting point difference between high and low melting point metals ≤ 250℃, low melting point metal content ≤ 30%, and fluxing agent content 1-2.5%. A genetic algorithm is used to solve for the optimal solution of the model. ③ Core-shell structured mixed fluid is prepared by spray coating. The steps are as follows: high melting point metal coarse powder and medium powder are dry-mixed in a three-dimensional mixer to form a basic powder system; low melting point metal ultrafine powder is mixed with a ternary fluxing agent to prepare a coating slurry, wherein the ternary fluxing agent consists of KCl, NaCl, and KBF4 in a mass ratio of 6:3:1; the slurry is uniformly coated on the surface of the basic powder and dried under vacuum at 80℃ for 2 hours to obtain a "high melting point core - low melting point shell - fluxing agent coating". (2) Semi-melting molding process: The single laser dynamic power three-stage control is adopted. The scanning process is divided into preheating stage, semi-melting stage and heat preservation stage. At the same time, the molding environment parameters are optimized. The laser power in the preheating stage is 120-180W, the scanning speed is 100-180mm / s, and the temperature is controlled at 50-80℃ below the solid phase line of high melting point metal. The laser power in the semi-melting stage is 220-300W, the scanning speed is 250-400mm / s. The laser power in the heat preservation stage is 150-200W, the scanning speed is 150-200mm / s. The oxygen content in the molding chamber is ≤300ppm, and argon atmosphere protection is adopted. The substrate preheating temperature is 180-300℃, and the layer thickness is selected as 50-150μm.

2. The process according to claim 1, characterized in that, In step (1) ③, the dry mixing parameters for high melting point metal coarse powder and medium powder are: three-dimensional mixer speed 15 r / min, dry mixing time 1.5 hours.

3. The process according to claim 1, characterized in that, In step (1) ②, the weighting coefficients α, β, γ, and δ can be adjusted according to the application scenario of the parts. For aerospace parts, the γ value is increased first, and for mold parts, the δ value is increased first.

4. The process according to claim 1, characterized in that, In step (1) ③, the total amount of the ternary fluxing agent is 1-2.5% of the total mass of the metal powder mixed fluid.

5. The process according to claim 1, characterized in that, In step (2), the single laser used is a commercial single fiber laser with a power of 500W, and the power is dynamically switched through a programmable controller.

6. The process according to claim 1, characterized in that, In step (1), the metal powder includes 316L stainless steel powder and Ni powder, wherein the Ni powder is an ultrafine powder with a particle size range of 5-10 μm.

7. The process according to claim 1, characterized in that, In step (2), the process effect of the semi-melting stage is that the low-melting-point metal shell is completely melted and the surface of the high-melting-point metal core layer is slightly melted, thus achieving metallurgical bonding between particles.

8. The process according to claim 1, characterized in that, In step (2), the purpose of the preheating stage is to eliminate powder stress and improve wettability; the purpose of the heat preservation stage is to slowly cool down to reduce thermal stress and avoid crack formation.