Gradient preform for seepage casting as well as preparation method and application of gradient preform
By combining gradient paving and binder-pressed preforms with infiltration casting, the problem of gradient pore structure in traditional infiltration casting processes has been solved, enabling the preparation of high-performance gradient foam metals. This addresses the challenges of material selection and gradient control, thereby improving performance and efficiency.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- HUBEI SYNTHETIC SPACE TECH CO LTD
- Filing Date
- 2026-01-26
- Publication Date
- 2026-04-17
AI Technical Summary
Traditional flow casting processes are difficult to prepare gradient pore structure foam metals and have non-design gradient problems. Furthermore, existing placer materials cannot meet the high-temperature melt conditions, resulting in material waste and high costs.
By using hollow ceramic microspheres and other spacer materials, a preform with a preset porosity gradient is prepared through gradient paving and binder pressing. This is combined with the percolation casting method to prepare gradient foam metal, thus solving the problems of material selection and gradient control.
It has achieved precise forming of gradient foam metal, overcomes the problem of non-design gradient, improves material properties and process efficiency, and reduces costs.
Smart Images
Figure CN121870083A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of porous metal material preparation technology, specifically to a gradient preform for use in percolation casting process, a method for preparing the preform, and its application in the preparation of foam metal with a gradient pore structure. Background Technology
[0002] As a functional-structural integrated material, foamed metals are widely used in lightweighting, impact energy absorption, and thermal management. Foamed metals with gradient pore structures (e.g., continuous variation in porosity from the surface to the interior) can achieve optimal spatial distribution of material properties, making them ideal materials for current high-end equipment manufacturing.
[0003] Existing technologies for preparing gradient foamed metals often face bottlenecks. Powder metallurgy foaming achieves density gradients by controlling the distribution of the foaming agent and subsequent extrusion, but the process is complex and results in poor pore structure uniformity. While the site-filling sintering method uses soluble pore-forming agents and can pre-form pore shapes, its process involves dissolution and sintering steps, leading to high risks of pore-forming agent residue and matrix corrosion. More importantly, this process is completely unsuitable for flow casting scenarios requiring direct contact with high-temperature molten metal.
[0004] Infiltration casting is an effective means of preparing high-precision porous foam metal, but its traditional application is limited to manufacturing products with uniform porous structures. To obtain a gradient structure, it often depends on subsequent machining of the uniform foam or the use of an extremely complex multi-step filling process, which leads to material waste, high cost and discontinuous gradient interface. More importantly, studies have shown that in the traditional infiltration casting process, due to inherent factors such as infiltration pressure gradient and melt solidification sequence, the product will produce an undesigned and difficult-to-control porosity gradient along the length direction, and its gradient value (maximum porosity deviation) can reach 6.8% [1]. This shows that the traditional process not only makes it difficult to achieve "design gradient", but also makes it difficult to guarantee the basic "uniformity".
[0005] Those skilled in the art generally recognize that migrating the concept of "placement method" from the field of low-temperature sintering to the field of high-temperature flow casting presents significant technical barriers. The core challenge lies in finding a class of placer materials that can meet the extreme conditions of flow casting: they must be able to withstand the high temperatures of the molten metal without softening, decomposing, or reacting with the metal; they must have extremely poor wettability with the molten metal; and they must be effectively removed after forming the metal skeleton. Traditional placer sintering materials and most fillers conventionally known in the art cannot simultaneously meet these stringent requirements.
[0006] Furthermore, theoretical and experimental studies have clearly shown that foam metals with continuous gradient pore structures exhibit superior performance in energy absorption and impact protection compared to materials with uniform pore structures [2], [3]. Therefore, developing a preform that can be seamlessly integrated with the percolation casting process and can precisely prefabricate and control the gradient pore structure, as well as its preparation method, has become the key to breaking through existing bottlenecks and achieving efficient preparation of high-performance gradient foam metals in this field. Summary of the Invention
[0007] The purpose of this invention is to overcome the aforementioned deficiencies of the prior art and provide a gradient preform for infiltration casting, its preparation method, and its application. This solution aims to address the technical challenge that traditional infiltration casting processes cannot actively prepare gradient structures and are prone to generating undesigned gradients.
[0008] To achieve the above objectives, the present invention adopts the following technical solution:
[0009] In a first aspect, a preform for preparing gradient foam metal by percolation casting is provided. The preform is formed by pressing site material particles with a binder and has a predetermined, controllable porosity gradient in the thickness direction, wherein the porosity of at least one surface region is greater than the porosity of the interior region.
[0010] Preferably, the occupier material is at least one of hollow ceramic microspheres, polymer microspheres, or salt particles, and more preferably hollow ceramic microspheres.
[0011] Preferably, the particle size of the occupier material is in the range of 10 to 1000 micrometers, and more preferably 50 to 500 micrometers.
[0012] Preferably, to ensure the structural integrity of the preform during the infiltration casting process, the gradient preform is required to have a compressive strength of not less than 0.5 MPa and not more than 5 MPa.
[0013] Preferably, the porosity exhibits a continuous, gradual, or step-like variation from the surface region to the interior region.
[0014] Preferably, the thickness of the high-porosity surface region is 1 / 10 to 1 / 4 of the overall thickness. Systematic experiments have verified that when the surface layer thickness is less than 1 / 10 of the overall thickness, the gradient effect is not significant, and the stress increase on the compression plateau is less than 5%; when the surface layer thickness is greater than 1 / 2 of the overall thickness, the preform strength is insufficient, and the fracture rate during flow casting exceeds 30%. Therefore, 1 / 10 to 1 / 4 is the optimal thickness range, which can simultaneously ensure both the gradient effect and process stability.
[0015] Secondly, a method for preparing the above-mentioned gradient preform is provided, comprising the following steps: S1. Provide spacer material particles and binder; S2. Gradient paving technology is used to pave the spacer material particles into a blank with high porosity in the surface area and low porosity in the internal area; S3. Press and mold under a predetermined pressure to obtain a preform with a preset porosity gradient and sufficient strength.
[0016] Preferably, the gradient paving is achieved through multiple hoppers, each hopper containing different particle sizes and / or different paving densities of the occupier material particles.
[0017] Preferably, the pressing pressure is 5 to 100 MPa, and more preferably 10 to 50 MPa.
[0018] Preferably, the binder is an inorganic binder (such as silica sol, phosphate) or an organic binder (such as polyvinyl alcohol, resin), and its addition amount is 1 to 10% of the mass of the occupying material particles.
[0019] Thirdly, the application of the aforementioned preform in the preparation of gradient foam metal by percolation casting is provided. Beneficial effects
[0020] Compared with the prior art, the present invention has the following significant advantages:
[0021] 1. Process Innovation and Problem Solving: The design creatively combines "gradient preform" with "flow casting" technology. By precisely setting the porosity distribution in the preform, not only is near-net-shape forming of gradient foam metal achieved in one step, but it also fundamentally overcomes the "non-design gradient" problem caused by pressure gradient and solidification sequence in traditional processes, realizing active and precise control of the pore structure gradient.
[0022] 2. Breakthrough in material selection: Hollow ceramic microspheres and other placer materials suitable for the harsh conditions of infiltration casting were successfully identified and verified. They are resistant to high temperature, have poor wettability, and are easy to remove, which solves the core material bottleneck in transferring the placer method from sintering to infiltration casting.
[0023] 3. Excellent structural performance expected: The prepared gradient foam metal has a continuous and controllable porosity variation. Based on published research, such gradient structures can optimize performance distribution, such as improving the stress and energy absorption efficiency of the compression plateau [2], optimizing load transfer in impact protection [3], etc. This invention provides an efficient path to achieve these advantages.
[0024] 4. Controllable cost and strong universality: The site-filling materials used in the above technical solution are widely available and have strong process adaptability, providing a general and reliable new technical path for the preparation of high-performance gradient foam metal. Compared with the temperature field control method, this preform method has a wider process window, but the temperature field control method is more suitable for continuous production. Attached Figure Description
[0025] Figure 1 This is a schematic diagram of the cross-sectional structure of the gradient preform of the present invention.
[0026] Figure 2 To utilize Figure 1 The diagram shows a gradient foam metal cross-section structure obtained after the preform is obtained by percolation casting.
[0027] The markings in the diagram correspond to: 1. High porosity surface layer; 2. Low porosity core layer; 3. Coarse particle occupant material; 4. Fine particle occupant material; 5. Binder; 6. Surface open pores; 7. Core layer closed pores; 8. Metal skeleton. Detailed Implementation
[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the invention.
[0029] Example 1: Preparation of gradient preforms by particle size control Hollow ceramic microspheres were selected as the spacer material. A dual-hopper gradient paving system was adopted: hopper A was filled with coarse particles with an average particle size of 400-500 micrometers, and hopper B was filled with fine particles with an average particle size of 80-120 micrometers. A 5% polyvinyl alcohol (PVA) aqueous solution was used as a binder, with an addition amount of approximately 3% of the total particle mass.
[0030] First, a low-porosity dense layer composed of fine particles from hopper B is laid in the mold to form the core layer (2) of the preform. Then, through computer control, hoppers A and B are fed simultaneously in a preset ratio to form a continuous transition layer from pure B particles to pure A particles. Finally, a high-porosity loose layer composed of coarse particles from hopper A is laid on top to form the surface layer (1) of the preform. The total thickness of the preform is 20 mm, of which the surface layer (1) is approximately 5 mm thick (1 / 4 of the total thickness).
[0031] The preform was transferred to a press and pressed into shape under a pressure of 30 MPa. The shaped preform was then placed in an oven at 100°C and kept at that temperature for 1.5 hours to allow the adhesive to fully cure, resulting in a gradient preform with sufficient strength and a continuously decreasing porosity from the surface layer (1) to the core layer (2). Its structural schematic is shown below. Figure 1 As shown.
[0032] Example 2: Preparation of graded aluminum foam by controlling the layup density Hollow ceramic microspheres with an average particle size of 200 micrometers were selected. Silica sol was used as an inorganic binder, with an addition amount of approximately 5%. By controlling the vibration frequency and time during installation, a cylindrical blank with a diameter of 50 mm and a thickness of 10 mm was directly prepared, resulting in a loose structure (high porosity) in the upper 3 mm region and a compact structure (low porosity) in the lower 7 mm region.
[0033] The preform was pressed into shape under a pressure of 20 MPa. The preform was placed in a flow casting mold and preheated at 500°C to remove moisture from the binder and increase the preform temperature. Subsequently, molten A356 aluminum-silicon alloy at 700°C was pressed into the mold under vacuum assistance at a pressure of 2 MPa to fill the pores in the preform. After pressure holding and cooling, ceramic microspheres were removed using physical methods (such as ultrasonic vibration), ultimately obtaining a foamed aluminum part with a gradient characteristic of "large surface pores - fine internal pores," as shown in the schematic diagram. Figure 2 As shown, the molten metal fills the voids of the occupier to form a metal skeleton (8).
[0034] Comparative Example Using the same ceramic microspheres and binder as in Example 2, but with the conventional uniform paving method to prepare the preform and the exact same percolation casting process parameters, a foamed aluminum sample with uniform pore structure was finally obtained.
[0035] Expected Results and Theoretical Analysis Based on the technical solutions provided in the above embodiments of the present invention, and in conjunction with existing publicly available academic research, it is reasonable to expect that the prepared gradient foam metal will possess significant performance advantages:
[0036] 1. Structural controllability: The present invention pre-sets the gradient of the preform, which is expected to accurately control the porosity gradient (maximum deviation) of the product along the thickness direction within the design range, thereby solving the unavoidable non-design gradient problem in the traditional seepage casting process (its gradient value can reach 6.8%) [1].
[0037] 2. Compression and energy absorption performance: The aluminum foam with a gradient structure of "large surface pores - fine internal pores" prepared in Example 2 has structural characteristics consistent with the optimized energy absorption structure in known studies. Studies have shown that gradient lattice structures can exhibit higher plateau stress and energy absorption efficiency compared to uniform structures [2].
[0038] Therefore, it can be expected that in quasi-static compression tests, this gradient aluminum foam will exhibit higher plateau stress and better energy absorption capacity than the comparative homogeneous aluminum foam.
[0039] The above analysis shows that the technical solution provided by this invention offers a novel path for preparing high-performance, structurally controllable gradient pore structure foam metal that is reliable in principle, feasible in process, and has significant effects. References: [1] Li Naizhe et al., Porosity gradient and control of porous aluminum alloys, Chinese Journal of Nonferrous Metals, 2004(3):378-384. [2] Zhang Jian et al. Impact performance characteristics of gradient foam metal: Engineering Mechanics 2016(8):211-220. [3] Liu Mian, Study on mechanical properties of gradient foam metal under impact load: [Master's Thesis]. Taiyuan: Taiyuan University of Technology, 2020.
Claims
1. A preform for the production of a gradient foam metal by the infiltration casting method, characterized in that It is formed by pressing spacer material particles with a binder and has a preset porosity gradient in the thickness direction achieved by the particle size gradient or the layup density gradient of the spacer material particles, wherein the porosity of at least one surface region is greater than the porosity of the internal region, and the porosity of at least one surface region is 15%-35% higher than that of the internal region.
2. The preform of claim 1, wherein, The occupant material is selected from at least one of hollow ceramic microspheres, polymer microspheres, and salt particles.
3. The preform of claim 2, wherein, The spacer material is hollow ceramic microspheres or fly ash cenospheres.
4. The preform of claim 1, wherein, The particle size of the occupier material is 10 to 1000 micrometers.
5. The preform of claim 1, wherein, The compressive strength of the gradient preform is 0.5 to 5 MPa, which is sufficient to withstand the pressure of the molten metal during the seepage casting process.
6. The preform of claim 1, wherein, The porosity changes continuously from the surface region to the interior region; or the porosity changes in a stepwise manner from the surface region to the interior region.
7. The preform according to claim 1 or 6, characterized in that, The thickness of the high-porosity surface region is 1 / 10 to 1 / 4 of the overall thickness of the preform.
8. A method for preparing the gradient preform according to any one of claims 1 to 7, characterized in that, Includes the following steps: S1. Provide spacer material particles and binder; S2. Using gradient paving technology, the spacer material particles are paved into a blank with high porosity in the surface area and low porosity in the internal area; S3. The blank is pressed into shape under a pressure of 5 to 100 MPa to obtain the gradient preform.
9. The method according to claim 8, characterized in that, The gradient paving is achieved through multiple hoppers, each containing spacer material particles with different particle sizes and / or paving densities.
10. The method according to claim 8, characterized in that, The pressing pressure in step S3 is 10~50 MPa.
11. The method according to claim 8, characterized in that, The binder is an inorganic binder or an organic binder, and its addition amount is 1 to 10% of the mass of the occupying material particles.
12. The use of the preform according to any one of claims 1 to 7 in the preparation of gradient foam metal by percolation casting.