Calcium hexaboride enhanced hypereutectic aluminum-silicon alloy and preparation method thereof

Abstract

The invention belongs to the technical field of metal materials, and mainly relates to a calcium hexaboride reinforced hypereutectic aluminum-silicon alloy and a preparation method thereof.The alloy is composed of, by mass, 14.00-18.00% of Si, 3.00-6.00% of B, 2.00-10.00% of Ca and the balance Al. According to the preparation method, an Al-5B intermediate alloy serves as the source of the boron element, an Si-30Ca intermediate alloy serves as the source of the calcium element, and the aluminum-silicon alloy reinforced by calcium hexaboride serves as the source of the calcium element; pure silicon and an Si-30Ca intermediate alloy are used as sources of a silicon element. The preparation method comprises the following steps: heating to 950 DEG C, melting the Al-5B intermediate alloy, adding the Si-30Ca intermediate alloy, melting, finally adding the pure silicon, mixing, melting, uniformly stirring, casting, and carrying out T6 heat treatment. The calcium hexaboride reinforced hypereutectic aluminum-silicon alloy prepared by the invention is a high-strength heat-resistant high-silicon aluminum alloy, is high in strength and hardness and good in wear resistance, and has good mechanical properties at room temperature and high temperature. The preparation method provided by the invention is low in cost and simple in process, meets the requirements of automobile safety parts on high-temperature mechanical properties, and is suitable for large-scale popularization and application in parts such as automobile engine cylinders, pistons, brake pads and the like.

Description

Technical Field

[0001] The invention belongs to the technical field of metal materials, and in particular relates to a calcium hexaboride-reinforced hypereutectic aluminum-silicon alloy and a preparation method thereof. Background Art

[0002] In recent years, the rapid development of automobiles, ships, aerospace and other fields has put forward higher and higher demands on engine technology. The working environment of high speed and high torque has greatly increased the mechanical and thermal loads of the core components of the engine. The requirements for the high temperature performance of materials of automobile engine cylinders, pistons and other parts are also constantly increasing. How to improve the high temperature mechanical properties of safety materials such as piston brake pads has become a bottleneck problem restricting the development of my country's automobile manufacturing industry.

[0003] Cast hypereutectic aluminum-silicon alloy is widely used in the production of automobile engine cylinders, pistons, brake pads and other parts due to its high specific strength, high temperature resistance, corrosion resistance and good wear resistance, making the car lighter and more powerful. However, there are a large number of coarse plate-like or star-shaped primary silicon and long needle-shaped eutectic silicon in the hypereutectic aluminum-silicon alloy, which will produce stress concentration under the action of external force, seriously split the aluminum alloy matrix, make the material toughness poor, lack of high temperature stable strengthening phase, and significantly reduce the high temperature performance of the material. With the continuous increase in the service temperature of the material, the high temperature performance requirements of safety materials such as automobile engine pistons and brake pads are more stringent. Therefore, it is crucial to modify the silicon phase in the hypereutectic aluminum-silicon alloy to obtain a fine and evenly distributed structure, which is crucial to improve the mechanical properties of the hypereutectic aluminum-silicon alloy, especially the high temperature mechanical properties, and expand its scope of use.

[0004] At present, there are many studies on the modification and refinement of primary silicon and eutectic silicon in hypereutectic aluminum-silicon alloys. There are two main methods. One is single modification, that is, adding P salt can effectively refine the structure of primary silicon and make it evenly distributed, but the use of P-containing modifiers only has a refinement effect on primary silicon. The absorption rate of P is limited by process conditions, the modification effect is unstable, and it is easy to produce harmful gases that are toxic to the human body. The other is a composite modification, that is, adding rare earth elements and elements such as P and B. Patent CN107326228A discloses a hypereutectic aluminum-silicon alloy and a preparation method thereof using aluminum-titanium-boron master alloy and rare earth elements for composite modification, which reduces the average size of primary Si by 69.80% and improves the roundness of eutectic silicon by 81.60%, which plays a good role in improving the mechanical properties of hypereutectic Al-Si alloys, but does not test its hardness and high-temperature mechanical properties. Patent CN201610989353 discloses a modified aluminum-silicon alloy piston, which uses a P-RE-Sr composite modifier to modify the hypereutectic aluminum-silicon alloy, so that the size of the primary silicon of the alloy is reduced to 8-12μm, and the tensile strength at high temperature reaches more than 160MPa, but the wear resistance is not tested. 6 There are few reports on the refinement of silicon phase in hypereutectic Al-Si alloy, which deserves vigorous research and promotion. Summary of the invention

[0005] The technical problem to be solved by the present invention is to prepare a new type of high-strength and heat-resistant hypereutectic aluminum-silicon alloy for safety components such as engines and brake discs, in view of the relatively few studies on the high-temperature mechanical properties of hypereutectic aluminum-silicon alloys, the unstable modification effect, complex modification operation methods, and difficulties in popularization and application in the prior art. 6 The invention also provides a preparation method thereof, which has low cost, simple process and high production efficiency.

[0006] A preparation method of a hypereutectic aluminum-silicon alloy reinforced with calcium hexaboride comprises the following steps: firstly melting pure aluminum and Al-5B master alloy, then adding Si-30Ca master alloy, adding pure silicon after mixing and melting, and then sequentially performing casting and T6 heat treatment to obtain a high-strength and heat-resistant hypereutectic aluminum-silicon alloy.

[0007] The present invention is achieved in the following ways: A calcium hexaboride reinforced hypereutectic aluminum-silicon alloy, the preparation method of which comprises the following steps: (1) Material preparation: Prepare Si-30Ca master alloy and Al-5B master alloy according to the atomic percentage of Ca:B=1:6, and prepare pure silicon according to the Si content of 14.00-18.00% by mass; (2) Melting: Pure aluminum and Al-5B master alloy are heated to 950°C in a resistance furnace until they are completely melted and then kept warm for 10 minutes. Then, Si-30Ca master alloy and pure silicon are added to the melt and mechanically stirred to make them evenly mixed. The mixture is kept warm at 950°C for 30 minutes. During the warming process, the melt is fully stirred to prevent component segregation. After the warming is completed, a melt is obtained. (3) Casting: After removing the slag on the surface of the melt, casting is performed to obtain a hypereutectic aluminum-silicon alloy casting rod with a diameter of 20 mm and a length of 150 mm; (4) Heat treatment: The hypereutectic aluminum-silicon alloy cast rod is subjected to T6 heat treatment, with a solution temperature of 490±5°C for 2 h and an aging temperature of 160±5°C for 12 h. After the solution aging treatment, a high-strength, heat-resistant aluminum-silicon alloy is obtained.

[0008] The beneficial effects of the present invention are: The source of Ca element in the present invention is Si-30Ca master alloy, AlB in Al-B alloy 2 Reacts with Ca to form CaB 6 , so that Ca in the aluminum matrix changes from a solid solution state to a precipitation state, which not only refines the primary silicon phase and the aluminum matrix, but also has a metamorphic effect on the eutectic silicon phase. 6 The lattice constant is similar to that of primary silicon, which meets the requirements of the heterogeneous nucleation core of primary silicon and refines the primary silicon. The Ca atoms enriched at the front of the solid-liquid interface cause supercooling of the components, which reduces the growth rate of the α-Al phase, thereby refining the α-Al phase grains. The modification of Ca on eutectic silicon is reflected in its influence on the growth of eutectic silicon. The presence of Ca changes the original growth mode of eutectic silicon, induces a large number of twins, and promotes the transformation of the lamellar eutectic Si phase into fine fibers. The addition of B element refines the grains of the alloy, increases the grain boundary area, and improves the mechanical properties of the aluminum-silicon alloy. CaB 6 It has low density, high melting point, high wear resistance and corrosion resistance. As a high-temperature stable strengthening phase, it reduces the wear rate of the alloy while ensuring the light weight of the alloy. A high-strength, heat-resistant hypereutectic aluminum-silicon alloy is obtained. The room temperature tensile strength of the alloy reaches 326 MPa, the hardness reaches 132 HBW, and the elongation is 4.50%. At 250°C, the tensile strength of the alloy reaches 235 MPa, the hardness reaches 104 HBW, and the elongation is 3.97%.

[0009] The present invention proposes a method for preparing a calcium hexaboride-reinforced hypereutectic aluminum-silicon alloy, adjusts the size and morphology of primary silicon and eutectic silicon in the hypereutectic aluminum-silicon alloy, refines the aluminum matrix grains, and improves the room temperature and high temperature comprehensive mechanical properties of the alloy, meeting the high temperature use requirements of engine pistons and brake pads made of current hypereutectic aluminum-silicon alloys. The preparation method proposed in the present invention has low cost, high economic benefit and simple operation, and is suitable for large-scale promotion and use. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0011] Figure 1 is Al-Si-CaB according to Example 6 of the present invention 6 Microstructure diagram of the alloy. The microstructure of the alloy is mainly composed of primary silicon, eutectic silicon, α-Al and CaB 6 In terms of particle composition, the size of the primary silicon of the Al-18Si alloy is reduced and its shape becomes more rounded, presenting a small polygonal shape, and the eutectic silicon is fibrous, which achieves the refinement of the primary silicon and the transformation of the eutectic silicon at the same time. The CaB6 particles are distributed around the eutectic silicon, showing enrichment and agglomeration, thereby optimizing the alloy structure.

Claims

1. A calcium hexaboride reinforced hypereutectic aluminum-silicon alloy and a preparation method thereof, characterized in that Its element composition and mass percentage are: Si: 14.00-18.00%, CaB6: 3.00-8.00%, and the rest is Al. The room temperature ultimate tensile strength of the alloy is ≥320MPa, elongation is ≥6.00%, and hardness is ≥130HBW. The ultimate tensile strength at 250℃ is ≥230MPa, elongation is ≥4.00%, and hardness is ≥100HBW.

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