Additive manufacturing of titanium steel dissimilar metal material and method of making same

Abstract

The application belongs to the technical field of heterogeneous metal additive manufacturing, and particularly relates to an additive manufacturing titanium-steel heterogeneous metal material and a preparation method thereof. Ni is used as a main element of a matrix, with a mass fraction of 87-93%; Cr is used as a solid solution strengthening element, with a mass fraction of 5-9%; and Co is used as a grain refinement and plasticity control element, with a mass fraction of 2-4%. A Ni-Cr-Co gradient solid solution transition layer formed by the above elements combines a titanium layer and a steel layer. The metal material is suitable for defect-free and high-performance forming of titanium / steel composite structures in the fields of aerospace and high-end equipment manufacturing.

Description

Technical Field

[0001] This invention belongs to the field of heterogeneous metal additive manufacturing technology, specifically relating to an additive manufacturing method for titanium-steel heterogeneous metal materials and its preparation method. Background Technology

[0002] Steel, with its advantages of low cost, high structural strength, excellent weld toughness, and industrial-scale production, has become a core structural material for civilian industries, rail transportation, shipbuilding, and conventional defense equipment. However, inherent defects in steel, such as low specific strength, weak high-temperature oxidation resistance, and limited corrosion resistance, make it difficult to meet the stringent service requirements of high-end equipment such as hypersonic vehicles, aero engines, and deep-sea pressure-resistant components, which demand lightweight, high-temperature resistance, and long service life. Titanium and titanium alloys combine high specific strength, excellent corrosion resistance, low-temperature toughness, and high-temperature structural stability, making them ideal lightweight structural materials for key fields such as aerospace, high-end rail transportation, and deep-sea engineering. However, the high cost of titanium alloy raw materials, the difficulty in processing and forming, and the limitations on large-scale application restrict its ability to completely replace steel in bearing heavy loads.

[0003] Titanium / steel dissimilar metal composite structures can synergistically combine the lightweight, high-strength, and corrosion-resistant advantages of titanium alloys with the low-cost, heavy-duty characteristics of steel, achieving complementary material properties and optimal cost configuration. This makes them highly valuable and economically beneficial for lightweight and integrated manufacturing of high-end equipment. However, titanium and steel face the challenge of mismatched intrinsic physicochemical properties: their specific heat capacity, coefficient of thermal expansion, melting point, and thermal conductivity differ significantly, easily leading to severe thermal stress and residual stress concentration during metallurgical solidification, inducing interfacial hot cracking, deformation, and even fracture. Simultaneously, titanium and iron differ significantly in lattice structure, atomic radius, and electron configuration, resulting in extremely low solid solubility in the molten metallurgical state. This makes the interface prone to forming a series of brittle intermetallic compounds such as Ti-Fe, Ti-Cr, and Ti-Ni. These hard and brittle phases become crack initiators under service stress, directly causing a sharp drop in weld joint plasticity, deterioration of high-temperature performance, and interfacial fracture failure, severely restricting the engineering application of titanium / steel dissimilar metal composite structures.

[0004] Traditional titanium / steel dissimilar joints often employ mechanical joining, fusion welding, brazing, and multi-layer transition welding processes. Mechanical joining cannot guarantee the airtightness and metallurgical bonding strength of the interface, making it difficult to meet the requirements of sealed pressure-bearing components. Fusion welding processes have high heat input and wide heat-affected zones, leading to uncontrollable formation of brittle intermetallic compounds and a high joint defect rate. Brazing relies on filler metal wetting and filling, resulting in low interface bonding strength and poor high-temperature service stability. Multi-layer transition welding requires multiple adjustments to process parameters, leading to a surge in the number of interfaces, stress superposition, cumbersome preparation processes, and difficulty in guaranteeing forming accuracy.

[0005] Laser additive manufacturing technology boasts core advantages such as high energy density, low heat-affected zone, rapid solidification, and controllable composition and structure. Among these, laser-directed energy deposition (LDED) technology offers high degree of freedom in forming, enables precise control of gradient composition, and is suitable for fabricating large and complex integrated structures, providing a novel technological path for defect-free joining of titanium / steel dissimilar metals. However, existing laser additive manufacturing technologies for joining titanium / steel have not systematically optimized the evolution of thermal stress at the titanium-steel interface, the mechanism for suppressing brittle phases, the matching characteristics of photo-powder coupling, and the composition-process co-design of the gradient transition layer. Consequently, metallurgical defects such as interface cracks, porosity, lack of fusion, and enrichment of brittle phases persist, making it difficult for the joint's mechanical properties to meet the requirements of high-end equipment.

[0006] Therefore, there is an urgent need to develop an additive manufacturing method for joining dissimilar titanium and steel metals, which involves comprehensive coordination of composition design, process control, and stress suppression. This method would enable the integrated molding of titanium / steel dissimilar metal composite structures that are free from metallurgical defects, brittle phase enrichment, low residual stress, and high strength and ductility, thus breaking through the technical bottleneck of titanium / steel dissimilar joining. Summary of the Invention

[0007] This invention provides an additive manufacturing method for titanium / steel dissimilar metals based on laser-directed energy deposition (EDA), overcoming the problems of uncontrollable brittle phases, thermal stress concentration, numerous metallurgical defects, and poor joint performance at the titanium / steel dissimilar metal interface in existing technologies. The technical solution is as follows: An additive manufacturing titanium-steel dissimilar metal material includes a titanium layer, a steel layer, and a transition layer connecting the titanium layer and the steel layer; the transition layer is a nickel alloy with added chromium and cobalt.

[0008] Ni, along with Ti and Fe, has high solid solubility. Furthermore, Ni-Ti intermetallic compounds exhibit better plasticity than traditional Ti-Fe brittle phases, effectively buffering interfacial stress and inhibiting the formation of hard and brittle phases. Cr enhances the strength and high-temperature oxidation resistance of the transition layer through solid solution strengthening. Co refines grains, improves grain boundary bonding, and enhances the plasticity and toughness of the transition layer.

[0009] Furthermore, by mass percentage, the nickel alloy comprises 5-9% chromium, 2-4% cobalt, and the balance being nickel.

[0010] Furthermore, the thickness of the steel layer is not less than 20mm. This avoids safety and forming problems such as heat accumulation overload, substrate deformation, and melt-through cracking caused by high heat input during laser deposition.

[0011] A method for preparing the above-mentioned additive manufacturing titanium-steel dissimilar metal material includes the following steps: a. Take a steel substrate and place a heating rod every 100-150mm around the perimeter of the steel substrate; heat the steel substrate with the heating rods to bring the substrate temperature to 300-400℃; b. Under a protective atmosphere, a laser with a spot diameter of 8-15 mm is used, and the laser focal distance is controlled to be 7-10 mm from the surface of the steel substrate to deposit nickel alloy powder onto the steel substrate. 4-8 layers are deposited to form a transition layer. If the number of layers is too small, the subsequent titanium alloy molten pool will melt through the transition layer, causing the titanium steel to react directly and generate a brittle phase. If the number of layers is too large, the interface bonding efficiency and mechanical property matching will be reduced.

[0012] c. Within 10 minutes of the completion of deposition, deposit the titanium powder required for the target thickness of the titanium layer onto the transition layer; the first 5 to 8 layers are deposited using the same defocusing method as the transition layer in step b; control the depth of the molten pool, reduce the dilution rate, and prevent the titanium alloy from melting through the transition layer and reacting directly with the steel substrate.

[0013] Furthermore, the heating rod described in step a has a height of 60-100mm and a diameter of 20-30mm.

[0014] Furthermore, the average particle size of the copper alloy powder in step b is 60~80μm.

[0015] Furthermore, in step b, the laser power of the first layer of deposition is 2000~2500W, the laser power of the second layer is 90% of that of the first layer, and the laser power of the third layer is 80% of that of the first layer. Subsequent depositions all use the laser power of the third layer. The scanning rate of the deposition is 800~1000mm / min, and the powder feed rate is 60~80g / min. The overlap rate of the deposited molten pool is 5~10%. A low overlap interval scanning strategy is adopted, first depositing single passes at intervals, and then filling in intermediate passes to minimize the accumulation of heat input and suppress the cracking of the transition layer.

[0016] Furthermore, in step c, the laser power for the first 5 to 8 layers deposited is 1800 to 2200 W, the powder feeding rate is 60 to 80 g / min, and the scanning rate is 1000 to 1200 mm / min.

[0017] Furthermore, after depositing 5 to 8 layers, the laser power is increased to 2500W to 3000W, the powder feed rate is 80 to 100g / min, the overlap rate is 30 to 40%, and the scanning rate is 1000 to 1200mm / min.

[0018] Furthermore, after the steel layer is deposited, the material is annealed at 600~700℃ for 6~10 hours.

[0019] By adopting the above scheme, the method of the present invention has the following advantages: 1. This invention achieves high-performance metallurgical bonding of titanium alloy and steel dissimilar metals without cracks, pores, or obvious brittle intermetallic compounds through a comprehensive technical solution including precise design of gradient transition layer composition, optimization of photo-powder coupling process, full-domain constant temperature preheating of substrate, layered variable parameter deposition, and residual stress relief annealing. The prepared composite structure combines the advantages of lightweight and corrosion resistance of titanium alloy with the high strength and low cost characteristics of steel, making it suitable for the large-scale, integrated, and lightweight manufacturing needs of high-end equipment.

[0020] 2. The present invention preheats the substrate, which can significantly reduce the temperature gradient and thermal stress caused by the thermal cycle of laser deposition, and suppress the initiation of interfacial thermal cracks from the source; precise temperature control avoids grain coarsening and performance degradation caused by overheating of the substrate, while preventing poor interfacial bonding and cold crack formation caused by overcooling of the substrate, thus ensuring thermal stability throughout the deposition process.

[0021] 3. This invention combines a large spot size with a defocusing design to reduce heat input to the substrate surface and decrease the depth of the molten pool, thereby achieving the formation of a large-size molten pool with a small depth. This significantly reduces the dilution rate of the transition layer on the substrate and decreases the probability of direct contact between titanium / steel matrix elements to form brittle intermetallic compounds. At the same time, it reduces heat accumulation and further weakens residual stress and the tendency for hot cracking. The appropriate defocusing amount ensures continuous and stable dripping of the molten droplets, achieving rapid solidification and dense forming, and suppressing the precipitation of brittle phases to the greatest extent.

[0022] 4. This invention utilizes the high solid solution properties of Ni with Ti and Fe, combined with the solid solution strengthening effect of Cr and the grain refinement effect of Co, to completely suppress the formation of brittle intermetallic compounds such as Ti-Fe and Ti-Cr. There is no enrichment of hard and brittle phases at the interface. The resulting metal material combines the advantages of lightweight and corrosion resistance of titanium alloys with the high strength and low cost of steel, meeting the requirements of lightweight and long service life of high-end equipment. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the sedimentary structure; Figure 2 This is a schematic diagram of the heating rod and coil structure; Figure 3 This is a diagram illustrating the deposition process mechanism; Figure 4 This is a schematic diagram of the scanning strategy for the "bow" shaped step. Figure 5 This is a cross-sectional view of titanium steel, a dissimilar metal material. Figure 6 These are microstructure images of the titanium layer, transition layer, and steel layer; Figure 7 It is a microstructure of the stainless steel layer.

[0024] In this diagram, 1 is the substrate, 2 is the heating rod, 3 is the powder feeding tube, 4 is the laser head, 5 is the laser, 6 is the powder, 7 is the molten droplet, 8 is the induction coil, 9 is the first deposition pass, 10 is the second deposition pass that fills the gap between the two passes, L1 is the defocus distance, L2 is the overlap ratio, and L3 is the molten pool width. Detailed Implementation

[0025] The technical solutions in the embodiments of the present invention will be clearly and completely described below. 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: (1) Nickel alloy powder was prepared according to the following components: Cr: 7wt.%; Co: 3wt.%; Ni: Bal, with an average particle size of 70μm; 25mm thick DT1900 high-strength steel was used as the substrate. A cylindrical heating rod 2 with a height of 80mm and a diameter of 25mm was processed on the outer surface of the substrate 1 every 120mm to heat the substrate. The substrate surface needs to be polished before the experiment. The surface was polished smooth with 2000# sandpaper, and then cleaned with water, anhydrous ethanol, anhydrous acetone and water in turn to ensure that the substrate surface is smooth and free of oil and impurities. (2) Fix the substrate 1, perform XY positioning, and then use a coil induction heating device to press... Figure 2 Induction coil 8 is placed on each heating rod, and the power is turned on to heat each heating rod at the same time to achieve substrate preheating; an infrared thermometer is used to monitor the substrate temperature in real time. (3) Using argon as the protective gas, when the substrate temperature rises to 300℃, a large laser spot of 12mm is selected, and the processing plane (substrate surface) is defocused with a defocus distance L1 of 8mm to obtain a larger molten pool width L3; the angle of the surrounding powder feeding tubes 3 is adjusted to concentrate all the powder 7 at the core laser head 4, and the powder 6 is concentrated at the focal point of the laser 5 in terms of height to achieve photo-powder coupling, so that the powder 6 melts at the focal point and forms droplets 7 that drip into the molten pool; the laser power is 2400W for the first layer, 2200W for the second layer, and 2000W for the third layer, and the third layer laser power is used to deposit the fourth to eighth layers; the scanning rate is 1000mm / min, the powder feeding rate is 80g / min, and the overlap rate L2 is 5%; during the process, the following methods are used. Figure 4 The scanning strategy shown in the "bow" pattern involves depositing every other pass first, and then filling in the deposit passes between every two passes to minimize heat input and reduce the tendency of the transition layer to crack. (4) Rapidly deposit TA15 titanium alloy powder on the surface of the transition layer; the first 6 layers of deposited titanium alloy powder use the same defocusing method as the transition layer, the laser power is 2000W, the powder feeding rate is 70g / min, the scanning rate is 1100mm / min, and the scanning strategy is the same as that of copper alloy to prevent melting through the copper alloy and forming a large number of intermetallic compounds, and to minimize the dilution rate with the copper alloy; after depositing 6 layers, in order to improve the preparation efficiency, the processing plane (the surface of the highest layer of the deposited layer) is moved to the focal point, the laser power is increased to 2800W, the powder feeding rate is 90g / min, the overlap rate L2 is 35%, and the scanning rate is 1100mm / min; at this time, the thickness of the high-strength steel deposited layer below is greater than the depth of the subsequent molten pool, which improves the deposition efficiency while preventing cracking; (5) After deposition, the sample is quickly placed in a heat treatment furnace at 650°C for annealing for 8 hours to eliminate internal stress, avoid cracking of the transition layer, and improve mechanical properties.

[0027] The sample prepared in Example 1 has an average tensile strength of 670 MPa and an average elongation after fracture of 4%, combining the advantages of lightweight and corrosion-resistant titanium alloys with the high strength and low cost of steel.

[0028] Depend on Figure 5 and Figure 6 It can be seen that the bonding interface of the metal material prepared in Example 1 is smooth and complete, without the enrichment of brittle intermetallic compounds, and without obvious metallurgical defects and hot cracks. This indicates that the method of the present invention can achieve a good bond between titanium and steel and avoid poor interface bonding.

[0029] Figure 7 It can be seen that the matrix material has a uniform structure and there is no grain coarsening due to excessive heat input.

[0030] For those skilled in the art, various other corresponding changes and modifications can be made based on the technical solutions and concepts described above, and all such changes and modifications should fall within the protection scope of the claims of this invention.

Claims

1. An additive manufacturing method for titanium-steel dissimilar metal materials, characterized in that, It includes a titanium layer, a steel layer, and a transition layer connecting the titanium layer and the steel layer; the transition layer is a nickel alloy with added chromium and cobalt.

2. The additive manufacturing titanium-steel dissimilar metal material according to claim 1, characterized in that, The nickel alloy comprises 5-9% chromium, 2-4% cobalt, and the balance being nickel, based on mass percentage.

3. The additive manufacturing titanium-steel dissimilar metal material according to claim 1, characterized in that, The thickness of the steel layer is not less than 20 mm.

4. A method for preparing additively manufactured titanium-steel dissimilar metal material as described in claim 1 or 2, characterized in that, Includes the following steps: a. Take a steel substrate and place a heating rod every 100-150mm around the perimeter of the steel substrate; heat the steel substrate with the heating rods to bring the substrate temperature to 300-400℃; b. Under a protective atmosphere, a laser with a spot diameter of 8-15 mm is used, and the focal distance of the laser is controlled to be 7-10 mm from the surface of the steel substrate to deposit nickel alloy powder onto the steel substrate, depositing 4-8 layers to form a transition layer; c. Within 10 minutes of the completion of deposition, deposit the titanium powder required for the target thickness titanium layer onto the transition layer; the first 5 to 8 layers are deposited using the same defocusing method as the transition layer in step b.

5. The method for preparing additively manufactured titanium-steel dissimilar metal materials according to claim 4, characterized in that, The heating rod described in step a has a height of 60-100mm and a diameter of 20-30mm.

6. The method for preparing additively manufactured titanium-steel dissimilar metal material according to claim 4, characterized in that, The average particle size of the copper alloy powder in step b is 60~80μm.

7. The method for preparing additively manufactured titanium-steel dissimilar metal material according to claim 4, characterized in that, In step b, the laser power of the first layer of deposition is 2000~2500W, the laser power of the second layer is 90% of that of the first layer, and the laser power of the third layer is 80% of that of the first layer. Subsequent depositions all use the laser power of the third layer. The scanning rate of the deposition is 800~1000mm / min, and the powder feeding rate is 60~80g / min. The overlap rate of the deposited molten pool is 5~10%.

8. The method for preparing additively manufactured titanium-steel dissimilar metal materials according to claim 4, characterized in that, In step c, the laser power for the first 5 to 8 layers of deposition is 1800 to 2200 W, the powder feeding rate is 60 to 80 g / min, and the scanning rate is 1000 to 1200 mm / min.

9. The method for preparing additively manufactured titanium-steel dissimilar metal material according to claim 4, characterized in that, After depositing 5 to 8 layers, the laser power is increased to 2500W to 3000W, the powder feed rate is 80 to 100g / min, the overlap rate is 30 to 40%, and the scanning rate is 1000 to 1200mm / min.

10. The method for preparing additively manufactured titanium-steel dissimilar metal material according to claim 4, characterized in that, After the steel layer is deposited, the material is annealed at 600~700℃ for 6~10h.

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