Method for producing aluminum alloy casting

Abstract

To provide a method for producing an aluminum alloy casting that prevents a reduction in ductility and achieves superior mechanical properties.SOLUTION: A method for producing an aluminum alloy casting includes: a casting step of providing a cast material by solidifying molten Al-Si-Mg alloy material within a mold, a crystallization step of reducing the casting temperature of the cast material to 400°C or less after the completion of solidification of the molten alloy, a solution treatment step of holding the casting temperature of the cast material at 450-550°C for 10 minutes or more after the crystallization step, and a cooling step of cooling the cast material after the solution treatment step.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for producing an aluminum alloy casting. [Background technology]

[0002] In order to reduce the weight of automobiles, the body parts of automobiles are being replaced from iron-based materials to aluminum materials. Body parts using aluminum materials can be manufactured by various manufacturing methods, such as pressing, forging, and casting. These manufacturing methods are used according to the required product characteristics and product shapes. From the viewpoints of safety and productivity, body parts are required to have high mechanical properties (e.g., strength and ductility). In order to obtain high mechanical properties, for example, aluminum alloy castings (Al alloy castings) using Al-Si-Mg alloys have been proposed.

[0003] Patent Document 1 discloses an aluminum alloy (Al alloy) for casting and an aluminum alloy casting (Al alloy casting) that contains, based on the total of 100 mass% (simply referred to as "%), Al: remainder, Si: 5-8.5%, Mg: 0.35-0.7%, Ti: 0.05-0.3%, Zr: 0.05-0.4%, Ti and Zr: 0.2% or more in total, and further contains at least one of Sr: 0.003-0.05%, Na: 0.001-0.03%, and Sb: 0.001-0.2%. Patent Document 1 also discloses that by using this Al alloy, a casting having a cast structure in which primary crystal Al and eutectic Si are finely dispersed can be obtained by gravity casting or low pressure casting, and that this casting is excellent not only in strength but also in ductility and toughness, and is suitable for, for example, vehicle tire wheels, etc. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2017-171960 A Summary of the Invention [Problem to be solved by the invention]

[0005] When manufacturing an Al alloy casting, in order to achieve both strength and ductility, it is preferable to perform a solution treatment after casting an Al-Si-Mg alloy, for example. However, depending on the treatment conditions of the solution treatment, compounds containing alloying elements may crystallize at the grain boundaries in the cast structure. As a result of repeated studies on the relationship between the compounds containing alloying elements crystallized at the grain boundaries and the ductility of the Al alloy casting, the present inventors found that by reducing the Mg-based compounds crystallized at the grain boundaries, the decrease in ductility is suppressed and an Al alloy casting with excellent mechanical properties can be obtained.

[0006] The present invention has been made to solve such problems, and has an object to provide a method for producing an aluminum alloy casting in which the decrease in ductility is suppressed and which has excellent mechanical properties. [Means for solving the problem]

[0007] A method for manufacturing an aluminum alloy casting according to one embodiment includes a casting process for obtaining a casting material by solidifying a molten metal of an Al-Si-Mg alloy material in a mold, a crystallization process for lowering the temperature of the casting material to 400°C or lower after the solidification of the molten metal is complete, a solution treatment process for maintaining the temperature of the casting material after the crystallization process at 450 to 550°C for 10 minutes or more, and a cooling process for cooling the casting material after the solution treatment process. Effect of the Invention

[0008] According to the present invention, a method for producing an aluminum alloy casting having excellent mechanical properties and in which a decrease in ductility is suppressed can be provided. [Brief description of the drawings]

[0009] [Figure 1] 1 is a flowchart showing a method for manufacturing an aluminum alloy casting according to a first embodiment. [Diagram 2] 2 is a graph showing the change over time in the temperature of the casting in the method for producing the aluminum alloy casting shown in FIG. 1. [Diagram 3] These are optical microscope images of the cast structure and EPMA images of Mg elements. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] First embodiment Hereinafter, an embodiment of the present invention will be described with reference to the drawings. However, the present disclosure is not limited to the following embodiment. In addition, in order to clarify the description, the following description and drawings are appropriately simplified. In the following description, the same or equivalent elements are given the same reference numerals, and duplicate descriptions are omitted.

[0011] A method for producing an aluminum alloy casting (Al alloy casting) according to the present embodiment will be described with reference to Fig. 1 and Fig. 2. The method for producing an Al alloy casting according to the present embodiment is a method for producing an Al alloy casting from an Al-Si-Mg alloy material. Fig. 1 is a flow chart showing the method for producing an aluminum alloy casting according to the first embodiment. Fig. 2 is a graph showing the change in casting temperature over time in the method for producing an aluminum alloy casting shown in Fig. 1.

[0012] As shown in FIG. 1, the method for producing an Al alloy casting according to this embodiment includes a casting step (step S1), a crystallization step (step S2), a solution treatment step (step S3), and a cooling step (step S4).

[0013] The casting process (step S1) is a casting process in which a molten metal of an Al-Si-Mg alloy material is solidified in a mold to obtain a casting material. The casting process includes, for example, a filling process in which the molten metal of an Al-Si-Mg alloy material is filled into a mold, and a solidification process in which the molten metal is solidified in the mold. As shown in Fig. 2, in the filling process, at time T0, the molten metal heated to, for example, about 680°C is filled into the mold. In the solidification process, the solidification of the molten metal is completed at time T1 when the casting temperature reaches, for example, about 560°C.

[0014] In this embodiment, the alloy composition of the Al-Si-Mg alloy is the mass ratio of each element when the entire Al-Si-Mg alloy is 100 mass%. The Al-Si-Mg alloy material may contain at least one element selected from, for example, Cu, Zn, Fe, and Mn, in addition to Al, Si, and Mg. The Al-Si-Mg alloy material is preferably AC4CH (JIS) or ADC3 (JIS) because it has excellent mechanical properties such as strength and ductility (elongation), corrosion resistance, heat resistance, and the like. AC4CH is an Al-Si-Mg alloy containing Cu: ≦0.1 mass%, Si: 6.5 to 7.5 mass%, Mg: 0.25 to 0.45 mass%, Zn: ≦0.1 mass%, Fe: ≦0.2 mass%, Mn: ≦0.1 mass%, and the balance being Al and unavoidable impurities. ADC3 is an Al-Si-Mg alloy containing Cu: ≦0.6 mass%, Si: 9.0 to 11.0 mass%, Mg: 0.4 to 0.6 mass%, Zn: ≦0.5 mass%, Fe: ≦1.3 mass%, Mn ≦0.3 mass%, and the remainder being Al and unavoidable impurities.

[0015] The casting method is not particularly limited, and examples thereof include gravity casting, sand casting, and die casting. When AC4CH is used as the Al-Si-Mg alloy material, the Al alloy casting is preferably produced by gravity casting or sand casting. When ADC3 is used as the Al-Si-Mg alloy material, the Al alloy casting is preferably produced by die casting.

[0016] Si can improve the flowability (running property) of molten metal during casting. Mg can form Mg-Si compounds together with Si to increase mechanical strength and fatigue strength. Fe can improve mold releasability, preventing the material from sticking to the mold. Optional elements such as Cu, Zn, Fe, and Mn can contribute to the refinement of crystal grains and the improvement of strength.

[0017] Here, during cooling after casting, the alloying elements contained in the Al alloy, such as Si, Mg, and Fe, are mainly crystallized as compounds at the grain boundaries in the cast structure. The distribution of the compounds containing the alloying elements crystallized at the grain boundaries can be confirmed, for example, by the element distribution (EPMA image) obtained by analyzing the cast structure with an EPMA (Electron Probe Micro Analyzer). However, the compounds containing the alloying elements crystallized at the grain boundaries may cause defects such as breakage and corrosion at the grain boundaries.

[0018] Therefore, the present inventors have repeatedly studied the relationship between the compounds containing alloy elements crystallized at the grain boundaries and the ductility of Al alloy castings, and have found that compounds containing Mg (Mg-based compounds) crystallized at the grain boundaries extremely reduce ductility. Specifically, by casting AC4CH as an Al-Si-Mg-based alloy material using gravity casting, two types of casting samples were produced: one in which compounds containing Si (Si-based compounds) were crystallized at the grain boundaries, and the other in which compounds containing Mg together with Si (Mg-Si-based compounds) were crystallized at the grain boundaries. When the fracture elongations of these two types of casting samples were compared, the fracture elongation of the casting sample in which the Si-based compounds were crystallized was 27%, and the fracture elongation of the casting sample in which the Mg-Si-based compounds were crystallized was 6%.

[0019] Therefore, in the manufacturing method of the Al alloy casting according to the present embodiment, the Mg-based compounds such as Mg-Si-based compounds in the Al alloy casting can be reduced by the crystallization step and the solution treatment step after casting. In the crystallization step (step S2), the casting temperature of the casting material is lowered to 400°C or less after the solidification of the molten metal is completed. As shown in FIG. 2, in the crystallization step, the casting temperature of the casting material is lowered to 400°C or less, so that compounds containing at least Mg among the alloy elements contained in the casting material are crystallized at the grain boundaries. In the crystallization step, the time (times T2 to T3) during which the casting temperature is kept at 400°C or less is not particularly limited as long as the entire casting material is kept at 400°C or less after the solidification is completed (time T1). From the viewpoint of productivity, the shorter the time during which the casting temperature is kept at 400°C or less, the more preferable. In the crystallization step, if the casting temperature does not reach 400°C or less after the solidification is completed, the crystallization of the Mg-based compounds may be insufficient. If the crystallization of Mg-based compounds is insufficient, the ductility of the Al alloy casting may decrease.

[0020] Next, in the solution treatment step (step S3), the casting temperature of the casting material after the crystallization step is held at 450 to 550°C for 10 minutes or more. As shown in FIG. 2, in the solution treatment step, the holding time (times T4 to T5) for holding the casting temperature at 450 to 550°C is not particularly limited as long as it is 10 minutes or more. From the viewpoint of productivity, the shorter the holding time is within the range of 10 minutes or more, the more preferable it is. By such solution treatment, the Mg-based compounds crystallized in the crystallization step can be suitably dissolved in the crystal grains. In the solution treatment step, if the casting temperature is less than 450°C, the solid solution of the Mg-based compounds may be insufficient. In addition, if the holding time is less than 10 minutes, the solid solution of the Mg-based compounds may be insufficient. If the solid solution of the Mg-based compounds is insufficient, the ductility of the Al alloy casting may be reduced. On the other hand, in the solution treatment step, if the casting temperature exceeds 550°C, a part of the material may be dissolved.

[0021] Next, in the cooling step (step S4), the casting material after the solution treatment step is cooled. In the cooling step, the cooling of the casting material is completed at time T6 when the casting temperature becomes about 100°C, for example. Here, the fracture elongation was measured for a total of four types of casting materials, including the casting material immediately after casting (as cast) and three types of casting materials cooled at a cooling rate of 2°C / min (ultra-slow cooling), 20°C / min (natural cooling), and 20000°C / min (water cooling). The fracture elongation was higher after cooling than immediately after casting, but there was no change due to the cooling rate. Specifically, the fracture elongation of the casting material immediately after casting was 7%, and the fracture elongation of each of the three types of casting materials cooled at any of the above cooling rates was 16%. Therefore, in the cooling step, the cooling rate for cooling the casting material is not particularly limited. However, from the viewpoint of material strength, the faster the cooling rate, the more preferable.

[0022] An Al alloy casting can be obtained by the above-mentioned casting step, crystallization step, solution treatment step, and cooling step. Furthermore, it is preferable to perform an aging treatment or the like on the Al alloy casting after the cooling step. This strengthens the primary crystal Al, which is the first phase of the Al alloy casting, by aging precipitation.

[0023] Next, Fig. 3 shows an optical microscope image of the cast structure and an EPMA image of Mg element. The example in Fig. 3 shows the cast structure of an Al alloy cast produced from ADC3 as an Al-Si-Mg alloy material by the die casting method according to the manufacturing method of the Al alloy cast according to the present embodiment. The comparative example in Fig. 3 shows the cast structure of an Al alloy cast that is not subjected to solution treatment after the casting process, unlike the example.

[0024] As is clear from Fig. 3, the Al alloy castings of the comparative examples have a cast structure in which Mg-based compounds are crystallized at the grain boundaries, whereas the Al alloy castings of the examples have a cast structure in which Mg-based compounds are suitably dissolved in the grains, as compared with the Al alloy castings of the comparative examples.

[0025] In addition, the behavior of the Si-based compounds and Fe-containing compounds (Fe-based compounds) crystallized at the grain boundaries was confirmed using EPMA images of the Si element and the Fe element (not shown). Specifically, in the examples, EPMA images (Si element and Fe element) of the cast structure were obtained for the cast material before and after the solution treatment process, and the presence of the Si-based compounds and Fe-based compounds was confirmed using the obtained EPMA images. As a result, the Si-based compounds and Fe-based compounds crystallized at the grain boundaries were present in the same distribution both before and after the solution treatment process.

[0026] In this way, the manufacturing method of the Al alloy casting according to the present embodiment allows Mg-based compounds to be sufficiently crystallized at the grain boundaries in the crystallization step after casting, and then allows the Mg-based compounds to be suitably dissolved in the grains in the solution treatment step, thereby reducing the Mg-based compounds that may reduce the ductility of the Al alloy casting. Therefore, the manufacturing method of the Al alloy casting according to the present embodiment allows the manufacturing of an Al alloy casting with excellent mechanical properties in which the decrease in ductility is suppressed. The Al alloy casting manufactured by the manufacturing method of the Al alloy casting according to the present embodiment can be suitably used for, for example, automobile body parts that require high strength and high ductility.

[0027] The present invention is not limited to the above-described embodiment, and can be modified as appropriate without departing from the spirit and scope of the present invention.

Claims

[Claim 1] A casting process in which a molten Al-Si-Mg alloy material is solidified in a mold to obtain a casting material; a crystallization step of lowering the casting temperature of the casting material to 400° C. or less after the solidification of the molten metal is completed; a solution treatment step of maintaining the casting temperature of the casting material after the crystallization step at 450 to 550°C for 10 minutes or more; a cooling step of cooling the casting material after the solution treatment step; The present invention relates to a method for producing an aluminum alloy casting having the above structure.

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