JP5204968B2 - FUEL TANK, TRANSPORT EQUIPMENT, Saddle-type Vehicle, Cast Part Manufacturing Method - Google Patents

Description

  The present invention relates to a cast part made of an aluminum alloy, and more particularly to a fuel tank made of an aluminum alloy.

  In order to improve the fuel consumption and running performance of motorcycles and automobiles, weight reduction of the vehicle body is an important factor. In order to reduce the weight of a motorcycle or a four-wheeled vehicle, it is conceivable to form constituent members using a lightweight material. Conventionally, many components such as a frame and a chassis are made of iron or a steel material containing iron. On the other hand, in recent years, it has been proposed to use aluminum having a specific gravity smaller than that of iron as a structural material. Since the density of aluminum is about 1/3 of that of iron, it is considered that the weight of a motorcycle or a four-wheeled vehicle can be greatly reduced when aluminum or an aluminum alloy is used as a material for a constituent member.

Patent Document 1 discloses an automobile fuel tank made of aluminum or an aluminum alloy. This fuel tank includes an upper box-shaped body and a lower box-shaped body formed by drawing, and a box-shaped closed space is formed by brazing them.
Japanese Patent Laid-Open No. 8-232789 JP-A-9-3610

  When manufacturing such an aluminum or aluminum alloy part, it is conceivable to produce a part having a desired shape by press forming and welding a plate material of aluminum or aluminum alloy as necessary. However, when a plate material is used, it is necessary to prepare a plurality of plate materials constituting the component, and the number of plate materials increases. Moreover, although it is necessary to join a board | plate material in many places, generally welding of aluminum is difficult compared with steel. For this reason, when manufacturing the components of aluminum or aluminum alloy from a board | plate material, the problem that manufacturing cost becomes high arises.

In order to solve such problems, it is conceivable to form parts by casting.
However, in order to perform casting, it is necessary that the molten metal flow is good. This limits the composition of the aluminum that makes up the part. Specifically, in order to form a part by casting using an aluminum alloy, it is necessary to add a predetermined amount of silicon to aluminum in order to improve the flow of molten metal. On the other hand, the addition of silicon reduces the ductility and toughness of the parts obtained by casting. This is a disadvantage especially for fuel tanks for motorcycles. This is because motorcycle fuel tanks are often exposed so as to appear in the appearance of the motorcycle, and are easily subjected to a large impact by falling. For this reason, it is necessary for the fuel tank for motorcycles to have excellent ductility and toughness so that even if a large impact is applied, cracks and cracks occur and the internal fuel does not leak.

  As a method for adjusting the hardness without changing the composition of the metal member, heat treatment such as quenching and annealing has been known for a long time. For example, Patent Document 2 discloses that ductility is improved by performing a heat treatment on an aluminum die-cast product containing silicon. However, according to the study by the present inventor, it has been found that when such heat treatment is performed, age hardening has an important effect on the physical properties of the heat-treated die cast product. In view of such problems, an object of the present invention is to provide a cast part made of an aluminum alloy excellent in ductility and toughness, particularly a fuel tank for a vehicle.

  The cast part of the present invention is made of an aluminum alloy and has a Vickers hardness of 70 HV or less.

  In a preferred embodiment, the aluminum alloy contains silicon in a proportion of 5 mass% to 13 mass%.

  In a preferred embodiment, the aluminum alloy contains silicon in a proportion of 7.5% by mass to 13% by mass.

  In a preferred embodiment, the cast part is heat-treated at a temperature of 350 ° C. or higher and 390 ° C. or lower.

  The fuel tank of the present invention is a fuel tank made of a cast aluminum alloy, and the cast aluminum alloy has a Vickers hardness of 70 HV or less.

  In a preferred embodiment, the cast aluminum alloy contains silicon in a proportion of 5 mass% to 13 mass%.

  In a preferred embodiment, the cast aluminum alloy contains silicon in a proportion of 7.5% by mass to 13% by mass.

  In a preferred embodiment, the fuel tank includes a first member and a second member, and each of the first member and the second member is made of the cast aluminum alloy.

  In a preferred embodiment, the first member and the second member are joined to define a space for holding fuel.

  In a preferred embodiment, each of the first member and the second member is made of an aluminum alloy cast by a high vacuum die casting method, and is joined to each other by welding.

  In a preferred embodiment, at least a part of the fuel tank includes a portion having a thickness of 2.4 mm or less.

  In a preferred embodiment, the fuel tank is heat-treated at a temperature of 350 ° C. or higher and 390 ° C. or lower.

  In a preferred embodiment, a fuel tank includes a member made of the cast aluminum alloy and a coating film that covers at least a part of the surface of the member and is cured at a temperature of 145 ° C. or higher and 200 ° C. or lower.

  The transportation device of the present invention includes any one of the above fuel tanks.

  The method for producing a cast part according to the present invention includes a step of forming a cast part by casting an aluminum alloy and a step of heat-treating the cast part so that the Vickers hardness of the cast part is 70 HV or less. Include.

  In a preferred embodiment, the heat treatment step holds the cast part at a temperature of 350 ° C. or higher and 390 ° C. or lower for 1 hour or longer.

  In a preferred embodiment, the aluminum alloy contains silicon in a proportion of 5 mass% to 13 mass%.

  The method for manufacturing a fuel tank according to the present invention includes a step of forming a first member and a second member that define a space for holding fuel by casting an aluminum alloy, and the first member and the second member. And heat-treating the first member and the second member so that each member has a Vickers hardness of 70 HV or less.

  In a preferred embodiment, in the heat treatment step, the first member and the second member are held at a temperature of 350 ° C. or higher and 390 ° C. or lower for 1 hour or longer.

  In a preferred embodiment, the method for manufacturing a fuel tank further includes a step of joining the first member and the second member before the heat treatment step.

  In a preferred embodiment, the method for manufacturing a fuel tank further includes a step of joining the first member and the second member after the heat treatment step.

  In a preferred embodiment, the first member and the second member are cast by a high vacuum die casting method, and the first member and the second member are joined by welding.

  In a preferred embodiment, the aluminum alloy contains silicon in a proportion of 5 mass% to 13 mass%.

  According to the fuel tank of the present invention, since it is formed from an aluminum alloy by casting, the number of parts constituting the fuel tank can be reduced, and the length for joining the parts can be shortened. Furthermore, since the hardness is 70 HV or less, it is excellent in deformability, hardly cracks or cracks even when an impact is applied, and does not easily age harden. For this reason, sufficient deformability can be maintained over a long period of time. Therefore, it can be suitably used as a fuel tank for a saddle-ride type vehicle such as a motorcycle that is particularly susceptible to a large impact during a fall.

  The inventor of the present application examined the composition and physical properties of an aluminum alloy necessary for a cast part made of an aluminum alloy having excellent toughness and ductility, particularly a fuel tank that does not crack even when subjected to an impact. In order that a member does not crack even when subjected to an impact, the member is excellent in toughness and ductility and needs to be deformed without causing cracks or cracks. Hereinafter, the superior toughness and ductility may be referred to as excellent deformability. According to the present inventor, it was found that by using hardness (Vickers hardness) as an index for evaluating deformability, it is possible to measure relatively easily and to correctly evaluate the degree of deformability.

  In addition, it has been found that when heat treatment is performed on an aluminum die-cast product containing silicon to improve ductility, age hardening has an important influence on the physical properties of the heat-treated die cast product. The inventor of the present application has found a cast part having optimum characteristics for the fuel tank in consideration of age hardening.

  Hereinafter, a fuel tank of a motorcycle will be described as a cast part according to the present invention. FIG. 1 is a side view of a motorcycle 100 having a fuel tank according to the present invention. The motorcycle 100 includes a front wheel 101, a rear wheel 102, an engine 103, a seat 104, a handle 105, a front fork 106, a cover 107, and a fuel tank 110. The front wheel 101 is rotatably supported at one end of the front fork 106. A handle 105 is fixed to the other end of the front fork 106. The front fork 106 is fixed to a main frame (not shown) so as to be rotatable. The engine 103 is supported by the main frame, and the rotational driving force of the engine 103 is transmitted to the rear wheel 102 via a chain (not shown). A fuel tank 110 is also supported on the main frame in the vicinity of the engine 103. The seat 104 is also supported by the main frame. A cover 107 is provided so as to cover the main frame, the fuel tank 110 and the engine 103.

  FIG. 2 is a perspective view of the fuel tank 110, and FIG. 3 is a cross-sectional view of the fuel tank. As shown in FIGS. 2 and 3, the fuel tank 110 includes a first member 110a and a second member 110b. In the present embodiment, the first member 110a has a shape that defines the upper side of the inner space 110s of the fuel tank 110, and the second member 110b has a shape that defines the lower side of the inner space 110s. . Fuel is held in the inner space 110 s of the fuel tank 110. There is no restriction | limiting in particular in the shape and magnitude | size of internal space 110s, and it determines arbitrarily by the use of the motorcycle 100, etc. For example, the capacity of the internal space 110s is about 20L. The first member 110a and the second member 110b do not need to constitute the upper part and the lower part of the fuel tank 110, and may constitute the left part and the right part, for example.

  The first member 110a and the second member 110b are each integrally formed by casting. Therefore, the first member 110a and the second member 110b need to have shapes that can be removed from the mold during casting. Depending on the overall shape of the fuel tank 110, it may be difficult to make each member a shape that can be cast only by dividing it into two members, the first member 110a and the second member 110b. In this case, the fuel tank 110 may be formed by three or more members. However, as the number of members increases, the number of locations where the members are joined also increases, leading to an increase in manufacturing cost. For this reason, it is preferable that the number of members is small.

  In the present embodiment, the first member 110a is joined to a portion having a substantially rectangular parallelepiped shape provided with a flange 110c for connecting an inlet for injecting into the fuel tank 110 and a somewhat large rectangular parallelepiped portion having an opening. Has the shape. The second member 110b has a rectangular parallelepiped shape having an opening. The first member 110a and the flange 110c may be integrally formed.

  The first member 110a and the second member 110b can be formed by various casting methods used when manufacturing a cast part of an aluminum alloy. When high-precision molding is required, it is preferable to cast by a die casting method.

  With the first member 110a and the second member 110b facing each other's openings, the side surfaces of the respective members that define the openings are joined to each other at the joint portion 111. Brazing or welding can be used for joining. For welding, arc welding such as MIG and TIG, laser welding, electron beam welding, and the like can be used. In order to obtain sufficient mechanical strength, it is preferable to join the first member 110a and the second member 110b by welding. When joining by welding, it is preferable to cast the first member 110a and the second member 110b by a high vacuum die casting method in order to suppress the occurrence of blisters (surface swelling) during welding. In this case, the amount of gas contained in the first member 110a and the second member 110b is preferably 5 cc / 100 g or less.

  The first member 110a and the second member 110b are preferably thinner in order to reduce the weight. Specifically, since the thickness of the conventional steel fuel tank is 0.8 mm and the specific gravity of the aluminum alloy is about 1/3 of that of steel, the first member 110a and The thickness of at least a part of the second member 110b needs to be 2.4 mm or less. However, if it becomes too thin, sufficient strength cannot be secured. For this reason, it is preferable that the thickness of the 1st member 110a and the 2nd member 110b is 1.2 mm or more and 2.4 mm or less.

The first member 110a and the second member 110b are made of an aluminum alloy having a composition capable of forming each member by casting. Since the thickness of the first member 110a and the second member 110b is small, the first member 110a and the second member 110b are preferably made of an aluminum alloy having a composition with a low viscosity when melted so that the molten metal flow is good. Specifically, the first member 110a and the second member 110b are preferably made of an aluminum alloy containing silicon at a ratio of 5 mass% to 13 mass%. When the ratio of silicon is less than 5% by mass, the hot water flow is deteriorated and the castability is deteriorated. In particular, it becomes difficult to form a cast member having the thickness described above. On the other hand, when the proportion of silicon exceeds 13% by mass, primary silicon is precipitated in the cast aluminum alloy, and the ductility is significantly reduced. When the first member 110a and the second member 110b are formed by the die casting method, it is preferable that the hot water flow is better, and the ratio of silicon is 7.5 % by mass or more and 13% by mass or less. preferable. As such an aluminum alloy, an Al—Si alloy, an Al—Si—Mg alloy, or an Al—Si—Cu alloy can be preferably used.

  The first member 110a and the second member 110b have a Vickers hardness (HV0.3) of 70 HV or less. By setting the Vickers hardness to 70 HV or less, the first member 110a and the second member 110b have high deformability and can prevent cracks and cracks from being generated even when subjected to an impact. Here, the impact means an impact in a strength test according to the impact test of SAE (Society of Automotive Engineers) J1241.

  The aluminum alloy containing silicon in the above-described proportion usually has a Vickers hardness of 90 HV or higher. For this reason, the deformability of the first member 110a and the second member 110b immediately after casting is small because the hardness is too high. In the present invention, in order to improve the deformability, the first member 110a and the second member 110b are heat-treated and annealed after casting. The annealing is performed by holding the first member 110a and the second member 110b at a temperature of 350 ° C. or higher and 390 ° C. or lower for 1 hour or longer. The cooling rate after the heat treatment is not particularly limited, and the first member 110a and the second member 110b may be gradually cooled or rapidly cooled. However, in order to suppress a dimensional change, it is preferable to cool gradually by air cooling. Thereby, eutectic silicon is spheroidized and the ductility of the member is improved, so that the deformability is increased. After cooling, the hardness of the first member 110a and the second member 110b is 70HV or less.

  When the holding temperature during heat treatment is lower than 350 ° C., the effect of annealing is not sufficient, and it is difficult to reduce the hardness of the first member 110a and the second member 110b to 70 HV or less in a practical treatment time. It becomes. Moreover, when processing temperature exceeds 390 degreeC, the hardness immediately after heat processing will be 70 HV or less. However, after that, age hardening occurs and the hardness exceeds 70 HV. As a result, the first member 110a and the second member 110b have sufficient deformability immediately after the heat treatment, but curing progresses with time and the deformability becomes small.

As will be described in the following experimental example, when the heat treatment temperature after casting is 390 ° C. or less, the first member 110a and the second member 110b are then heated at a temperature of 180 ° C. for about 30 minutes, Even when heated at 90 ° C. for 96 hours, age hardening does not occur in the first member 110a and the second member 110b. For this reason, even if it exposes to the heat | fever for about 30 minutes at 180 degreeC, the 1st member 110a and the 2nd member 110b are maintaining the hardness of 70 HV or less. Therefore, the first member 110a and the second member 110b of the fuel tank 110 are maintained not only immediately after the heat treatment but also after at least about 7 days at room temperature after the heat treatment or at a temperature of 180 ° C. for about 30 minutes after the heat treatment. After or after holding at 90 ° C. for 96 hours, it has a hardness of 70 HV or less.

  In addition, by setting the heat treatment temperature after casting to 390 ° C. or lower, it is possible to avoid the problem that blisters are generated by heat treatment even if gas is taken into the member during casting. Therefore, in order to prevent the generation of blisters due to the heat treatment after casting, it is not necessary to reduce the amount of gas taken into the member by using a high vacuum die casting method. However, as described above, when the first member 110a and the second member 110b are joined by welding, the first member 110a and the second member 110a and the second member 110b are formed by a high vacuum die casting method in order to prevent blisters from being generated during welding. It is preferable to cast the second member 110b.

  In the motorcycle 100 of the present embodiment, the fuel tank 110 is disposed close to the engine 103. For this reason, during traveling, the fuel tank 110 is heated to about 90 ° C. by heat from the engine. However, according to the fuel tank 110, since age hardening does not occur even in such an environment, the fuel tank 110 maintains sufficient deformability and suppresses the occurrence of cracks and cracks even when subjected to an impact. be able to.

  As apparent from the above description, in the fuel tank 110, the first member 110a and the second member 110b are formed by casting using the aluminum alloy having the above-described composition, and the first member 110a and the second member 110b are formed. It is manufactured by joining the members 110b. The heat treatment for annealing may be performed before joining the two members, or may be performed after joining.

  According to the fuel tank of the present invention, since it is formed from an aluminum alloy by casting, the number of parts constituting the fuel tank can be reduced, and the number of parts to be joined can be reduced. . Thereby, a fuel tank can be manufactured at low cost. Moreover, since the hardness is 70 HV or less, it is excellent in deformability and is not easily cracked or cracked even when an impact is applied. Furthermore, since age hardening does not occur, sufficient deformability can be maintained over a long period of time. For this reason, in particular, it can be suitably used as a fuel tank of a saddle-ride type vehicle such as a motorcycle that is easily subjected to a large impact when it falls. Here, the saddle-ride type vehicle refers to a motorcycle, an ATV such as a buggy, a water vehicle, or the like on which a rider rides.

  Therefore, according to the present invention, the possibility of fuel leakage is low even when subjected to an impact, and a motorcycle equipped with a lightweight fuel tank can be realized at low cost. Further, since the fuel tank is lightweight, the motorcycle of the present invention is excellent in fuel efficiency and running performance.

  In the above-described embodiment, the motorcycle in which the fuel tank does not appear on the outer appearance has been described. However, the present invention can be applied to other types of motorcycles. 4 is a side view of another embodiment of the motorcycle 150, and FIG. 5 is a top view of the motorcycle 150. As shown in FIG. As is apparent from FIGS. 4 and 5, the motorcycle 150 is a sports type motorcycle. The motorcycle 150 includes a fuel tank 160. The fuel tank 160 is located in front of the seat 161 and is provided so as to constitute a part of the appearance of the motorcycle 150. For this reason, a coating film is provided on the surface of the portion of the fuel tank 160 that appears on the exterior of the motorcycle 150. The fuel tank 160 has the structure described in detail so far.

  The coating film provided on the portion constituting the external appearance of the fuel tank 160 is formed by baking painting. The coating is baked at 140 ° C. or higher and 200 ° C. or lower. As described above, according to the fuel tank of the present invention, since age hardening does not occur even when heated at 180 ° C. for about 30 minutes, it has excellent deformability. Further, in the motorcycle 150, since the fuel tank 160 appears on the exterior, the fuel tank 160 is likely to receive a large impact when the motorcycle 150 falls. Even in this case, since the fuel tank 160 has sufficient deformability, the fuel tank 160 can be deformed without being cracked or broken by an impact. For this reason, the driver sitting on the seat 161 is not greatly damaged, and fuel leakage can be prevented.

  As mentioned above, the fuel tank for motorcycles has been described as an embodiment of the cast part according to the present invention. However, the present invention is suitable for use in fuel tanks for transportation equipment including a drive source driven by fuel, such as a four-wheeled vehicle, and cast parts used for various lightweight and thin applications that require excellent deformability. Can be done. In particular, the present invention is suitably used for cast parts that are used in an environment exposed to heat and are required to have sufficient deformability over a long period of time.

  Hereinafter, experimental examples in which the heat treatment temperature, hardness, and age hardening of cast parts according to the present invention were examined and an impact test was performed will be described.

  First, an aluminum alloy (A365 alloy) comprising 10% by mass of Si, 0.5% by mass of Mn, 0.3% by mass of Mg, 100 ppm of Sr, the balance of Al and inevitable contaminants is prepared. Using an aluminum alloy of this composition, a plurality of boxes having a wall thickness of 1.5 mm, open on one side of 300 mm in length, 200 mm in width and 150 mm in height were prepared by die casting. Next, two above-mentioned boxes were prepared, and a plurality of sample tanks having an inner space were created by MIG welding the boxes with the open surfaces facing each other. The sample tank was heated at 300 ° C., 330 ° C., 350 ° C., 375 ° C., 390 ° C., 450 ° C. and 500 ° C. for 1 hour, and then cooled (air-cooled) as it was except for those heated to 450 ° C. The sample tank heated to 450 ° C. was cooled (water cooled) by immersing it in water.

  Then, the process which hold | maintained as aging conditions for 0.5 hours (aging process 1) at 180 degreeC and 96 hours (aging process 2) at 90 degreeC was performed with respect to each sample tank.

  At each stage, the hardness of the sample tank was measured using a micro Vickers hardness tester. The hardness was taken out from the sample tank, the cut surface of the test piece was polished smoothly, the hardness of the cut surface was measured at 7 points, and the average of 5 points excluding the maximum value and the minimum value was obtained.

  The impact test was conducted using the sample tank after the heat treatment of the aging treatment 2. In the impact test, the sample tank was filled with water, and a drop test was performed in accordance with the impact test of SAE J1241. The degree of deformation of the sample tank and the occurrence of cracks after the drop test were comprehensively observed and evaluated.

  Table 1 shows the hardness and impact test results of each material tank. Moreover, the change of hardness is shown in FIG.

  As shown in FIG. 6 and Table 1, the hardness of the sample tank after casting is 95 HV. By performing the heat treatment at a predetermined temperature, the aluminum alloy is annealed and the hardness is reduced. Further, the hardness decreases as the temperature of the heat treatment increases. It can be seen that the heat treatment temperature should be 350 ° C. or higher in order to set the hardness of the sample tank to 70 HV or less immediately after casting.

  Thereafter, when heating is performed under the conditions of aging treatment 1 and aging treatment 2, it can be seen that the sample tanks heat-treated at 450 ° C. and 500 ° C. undergo significant age hardening. The sample tank that has been heat-treated in the range of 330 ° C. to 390 ° C. does not generally harden, and the change in hardness is small. In addition, the hardness of the sample tank that was heat-treated at 300 ° C. decreased after the aging treatment 1. This means that annealing is not sufficient when heat treatment is performed at 300 ° C. for 1 hour.

  The sample tank that has been heat-treated in the range of 350 ° C. to 390 ° C. does not generate cracks in the impact test and satisfies the criteria of the impact test. These sample tanks have a hardness of approximately 70 HV or less after being heated under the conditions of aging treatment 2.

  Therefore, it is considered that if the hardness is 70 HV or less, the deformability is sufficient and the criteria of the impact test are satisfied. In order to confirm this, when an impact test was performed immediately after heat-treating the sample tank at 500 ° C., no crack was generated in the tank. Therefore, sample tanks that have been heat-treated at 450 ° C. and 500 ° C. have small hardness and can meet the impact test criteria immediately after the heat treatment, but then age hardening occurs and the impact test criteria are met. It is thought that it will disappear.

  Based on the above results, the sample tank that has been heat-treated in the range of 350 ° C. to 390 ° C. has a hardness of 70 HV or less, and even when it is exposed to heat of about 180 ° C., it does not age harden and has the same hardness. It is thought that this can be maintained. As a result, it can be seen that stable deformability can be maintained over a long period of time.

  The cast component of the present invention is required to have excellent deformability, and is suitably used for a cast component used for various applications with light weight and thin wall, and particularly suitable for a fuel tank for a vehicle.

1 is a side view showing a motorcycle equipped with a fuel tank according to the present invention. 1 is a perspective view showing an embodiment of a fuel tank according to the present invention. It is sectional drawing which shows embodiment of the fuel tank by this invention. FIG. 6 is a side view showing another motorcycle equipped with a fuel tank according to the present invention. FIG. 5 is a top view showing another motorcycle equipped with a fuel tank according to the present invention. It is a graph which shows the hardness at the time of heat-processing a cast aluminum alloy on various conditions.

Explanation of symbols

100, 150 Motorcycle 101 Front wheel 102 Rear wheel 103 Engine 104, 161 Seat 105 Handle 110, 160 Fuel tank 110a First member 110b Second member 111 Joint portion

Claims (16)

A fuel tank made of a cast aluminum alloy, wherein the cast aluminum alloy has a Vickers hardness of 70 HV or less and is maintained at 90 ° C. for 96 hours even when held at a temperature of 180 ° C. for 30 minutes But after holding it has a Vickers hardness of 70HV or less,
The cast aluminum alloy contains silicon in a proportion of 7.5% by mass to 13% by mass,
The cast aluminum alloy is an Al-Si-Mg alloy.
A fuel tank cast by the die casting method.   The fuel tank according to claim 1, wherein the fuel tank is heat-treated at a temperature of 350 ° C. or higher and 390 ° C. or lower.   The fuel tank according to claim 1, comprising: a member made of the cast aluminum alloy; and a coating film that covers at least a part of the surface of the member and is cured at a temperature of 145 ° C. or more and 200 ° C. or less.   The fuel tank according to claim 1, further comprising a first member and a second member, wherein each of the first member and the second member is made of the cast aluminum alloy.   The fuel tank according to claim 4, wherein the first member and the second member are joined and define a space for holding fuel.   The fuel tank according to claim 5, wherein the first member and the second member are each made of an aluminum alloy cast by a high vacuum die casting method, and are joined to each other by welding.   The fuel tank according to claim 1, wherein at least a part of the fuel tank has a thickness of 2.4 mm or less.   8. A transport device comprising a fuel tank as defined in any one of claims 1 to 7, wherein the fuel tank is disposed close to an engine.   A saddle-ride type vehicle provided with a fuel tank as defined in any one of claims 1 to 7. Forming a cast part by casting an aluminum alloy which is an Al-Si-Mg alloy;
Heat treating the cast part such that the Vickers hardness of the cast part is 70 HV or less;
It encompasses,
The heat treatment step holds the cast part at a temperature of 350 ° C. or higher and 390 ° C. or lower for 1 hour or longer,
The said aluminum alloy is a manufacturing method of casting components containing silicon in the ratio of 7.5 mass% or more and 13 mass% or less .   The manufacturing method of the casting components of Claim 10 cast by the die-casting method. Forming a first member and a second member that define a space for holding fuel by casting an aluminum alloy that is an Al-Si-Mg alloy;
Heat treating the first member and the second member such that the first member and the second member have Vickers hardnesses of 70 HV or less,
It encompasses,
The heat treatment step holds the first member and the second member at a temperature of 350 ° C. or higher and 390 ° C. or lower for 1 hour or longer,
The said aluminum alloy is a manufacturing method of a fuel tank containing silicon in the ratio of 7.5 mass% or more and 13 mass% or less . The method for manufacturing a fuel tank according to claim 12 , further comprising a step of joining the first member and the second member before the heat treatment step. The method for manufacturing a fuel tank according to claim 12 , further comprising a step of joining the first member and the second member after the heat treatment step. The fuel tank according to claim 13 or 14 , wherein the first member and the second member are cast by a high vacuum die casting method, and the first member and the second member are joined by welding. Manufacturing method. The method for producing a fuel tank according to claim 12 , which is cast by a die casting method.

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