Aluminum alloy for cast node of vehicle space frames and method of manufacturing cast node using the same

Abstract

The aluminum alloy of a vehicle space frame includes aluminum as a main ingredient, 0.4 to 0.5 wt % of copper, 2.5 to 3.0 wt % of silicon, 0.1 wt % or less of magnesium, 0.1 wt % or less of iron and 0.2 wt % or less titanium. The method of manufacturing the vehicle space frame includes casting the above material, subjecting the cast aluminum alloy to a solution treatment, and heat treating the resulting material at 150° C.-160° C. for 4-5 hours.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on, and claims priority to Korean Application No. 2004-0035908, filed on May 20, 2004, the disclosure of which is hereby incorporated by reference.

BACKGROUND

1. Field

The present invention relates to an aluminum alloy for a cast node of a vehicle space frame and a method of manufacturing the cast node using the same. More particularly, the present invention relates to how the range of adding each of the ingredients of the aluminum alloy is appropriately adjusted and an optimal heat-treating condition according to the above range of adjustment is presented, so that manufactured products have a high elongation equivalent to that of products manufactured through a high-vacuum die-casting process, even through a sand mold casting process is used.

2. Background of the Related Art

Aluminum has been expanding its uses while primarily being used in the automobile industry where there is a need for improved fuel consumption. Particularly, the development of the design and manufacturing technology of vehicle bodies using aluminum is progressing rapidly because of the lightness of aluminum.

Such developments are exemplified by an aluminum space frame (ASF), which is a vehicle structure used to reduce vehicle weight. The components constituting the space frame of a vehicle body are fabricated from an extended aluminum material, and a cast connection member, i.e., a cast node, is applied for joining the aluminum extrusion components. Then, an inner plate and an outer plate of the vehicle body are made of an aluminum alloy plate material. Accordingly, the aluminum space frame is formed of a structure in which the extrusion material frame and the cast node absorb all the loads applied to the vehicle body.

Ensuring crashworthiness of vehicles is very important. Particularly, in a collision between vehicles, it is required that the vehicle body be deformed at a certain portion while appropriately absorbing impact energy to safely protect passengers in the vehicle. Therefore, the aluminum space frame itself must also have the characteristics in which it is properly deformed without easy fracturing of the frame and the cast node while absorbing impact energy. That is, the material of the frame and the cast node must have high elongation to secure crashworthiness through the impact energy-absorbing capacity.

However, an aluminum casting product generally has low elongation; for example, a sand mold casting product has an elongation of approximately 3% and a metal mold casting product has an elongation of approximately 6 to 7%. Therefore, it is not suitable for components of a vehicle body, such as the cast node, which require high elongation.

Accordingly, a thin-walled high elongation cast node with a thickness of 2 to 3 mm is mass-fabricated using a newly developed alloy (Aural: Al-10%, Si—Mg, Mn, Fe) via a high-vacuum die-casting process and is applied to an aluminum space frame (ASF) vehicle of some automobile makers (Audi A2, A3 of Germany). The cast node made of such a material has physical properties of: tensile strength of 200 MPa, yield strength of 130 Mpa and an elongation of 14 to 18%, and particularly has a sufficient elongation suitable for a component of the vehicle body.

However, in case of using the high-vacuum die-casting process, since its facility itself is expensive and is based on a metal mold casting, the investment expenditure including the equipment cost, metal mold, etc., is very costly. Especially where a small quantity of finished products or test samples are fabricated, there occurs a problem in that an enormous burden of the cost and development cost is placed on a business.

Conversely, since molten metal is poured into a sand mold during casting to produce an object of a desired shape, the investment expenditure for the product using sand mold casting is relatively low; thus being suitable for small batch production. The space frame body structure is mainly applied to a multi-product small batch production model.

However, an aluminum sand casting product has a relatively poor physical property, for example, a low elongation of approximately 3% as compared with the metal casting product, which results in difficulty in its application to manufacturing components of a vehicle body. There is therefore an urgent need for manufacturing a high elongation cast node by applying the sand casting process to the vehicle body components.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide an aluminum alloy for a cast node of a vehicle space frame and a method of manufacturing the cast node using the same, in which the range for adding each of the ingredients of the aluminum alloy is appropriately adjusted and an optimal thermal treatment condition according to the adjustment of the addition range is presented, so that it enables to manufacture products having a high elongation property equivalent to that of products manufactured through a high-vacuum die-casting process, even through a sand mold casting process.

To accomplish the above object, according to one aspect of the present invention, there is provided an aluminum alloy for a cast node of a vehicle space frame, which comprises aluminum as a main ingredient, 0.4 to 0.5 wt % of copper, 2.5 to 3.0 wt % of silicon, 0.1 wt % or less of magnesium, 0.1 wt % or less by weight of iron and 0.20 wt % or less of titanium.

According to another aspect of the present invention, there is also provided a method of manufacturing a cast node of a vehicle space frame, which comprises the steps of: casting an aluminum alloy which comprises aluminum as a main ingredient, comprises 0.4 to 0.5 wt % of copper, 2.5 to 3.0 wt % of silicon, 0.1 wt % or less of magnesium, 0.1 wt % or less of iron and 0.20 wt % or less of titanium; subjecting the cast aluminum alloy to a solution treatment; and heat-treating the resulting material at a temperature of from 150 to 160° C. for 4 to 5 hours.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments of the invention in conjunction with the accompanying drawing, in which:

FIG. 1 is a graph of a thermal treatment curve showing the relationship between time and temperature in accordance with the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENT

Reference will be now made in detail to the preferred embodiment of the present invention with reference to the attached drawing.

In order to fabricate a cast node used for joining between frame extrusion materials as components of an aluminum space frame, it is necessary that a high elongation property be ensured for the case node. Accordingly, the present invention provides a sand casting alloy having good castability and a high elongation property to secure crashworthiness through the impact energy-absorbing capacity, and a manufacturing method of a cast node using the sand casting alloy, especially an optimal heat-treating condition to secure sufficient physical property in the manufacturing process, so that the sand casting alloy can be fabricated for use as a complex vehicle body component.

Since the aluminum alloy proposed in the present invention has a thin-walled and multi-rib structure in terms of the vehicle body component characteristics and is used for fabricating a large-sized component, it must have good castability and high elongation.

To this end, an aluminum alloy for a cast node of a vehicle space frame according to the present invention is characterized in that it is an aluminum-copper-silicon alloy, which comprises aluminum as a main ingredient, 0.4 to 0.5 wt % of copper, 2.5 to 30 wt % of silicon, 0.1 wt % or less of magnesium, 0.1 wt % or less of iron and 0.20 wt % or less of titanium.

At present, the alloy for use in sand casting or gravity casting is mainly AC4C (A35)-series alloy, which comprises about 6.5 to 7.5 wt % of silicon based on 100% by weight of the alloy (KS D 6008). The contents for respective ingredients of the high elongation sand casting aluminum alloy of the present invention and the AC4C aluminum alloy are given in the following Table 1.

	TABLE 1
	Composition and Content (wt %)
	Cu	Mg	Si	Fe	Ti	Al
AC4C	Less than	0.2-0.45	6.5-7.5	Less	Less	Residue
	0.2			than	than
				0.35	0.20
Aluminum	0.4-0.5	Less than	2.5-3.0	Less	Less	Residue
Alloy		0.1		than	than
				0.1	0.20

In the composition and content range of the present invention given in the above Table 1, copper has a solid solution strengthening effect and a precipitation strengthening effect. However, if the solid solution and precipitation strengthening effects are very strong, there is a risk of degrading corrosion resistance. For this reason, the content of copper was limited to a range of 0.4 to 0.5 wt %. In addition, if silicon is added to the aluminum alloy, fluidity is improved. That is, silicon is an element that is added in a large amount to increase castability. However, the content of silicon was limited to a range of 2.5 to 3.0 wt % in the present invention.

Also, magnesium has the same precipitation strengthening effect as that of silicon, but if excessive precipitation occurs during the thermal aging process, this adversely affects elongation of the aluminum alloy. Accordingly, the content of the magnesium (Mg) was limited to a range of less than 0.1 wt %. Moreover, since iron forms a compound together with aluminum and silicon to thereby degrade elongation of the aluminum alloy, the content of iron was limited to a range of less than 0.1 wt %.

In the meantime, the inventive method of manufacturing a case node of a vehicle space frame using the aluminum alloy having the above composition, is characterized in that after casting the aluminum alloy, the cast aluminum alloy is subjected to a solution treatment, followed by artificial thermal aging at a temperature of from 150 to 160° C. for 4 to 5 hours.

FIG. 2 is a graph of a thermal treatment curve showing the relationship between time and temperature in the manufacturing method of the case node in accordance with the present invention. Referring to FIG. 2, a preferred embodiment of the present invention is described, in which after the aluminum alloy has been cast, it experiences a solution treatment at a temperature of 538° C. for 8 hours, followed by artificial thermal aging at a temperature of 155° C. for 4 hours.

In the manufacturing method, if the thermal aging temperature is set to a temperature range of less than about 150° C., an appropriate aging does not occur. Accordingly, a necessary physical strength is not achieved and thus this condition is not desirable. If, on the other hand, the thermal aging temperature is set to a temperature range of above about 160° C., elongation property is deteriorated due to an excessive aging. Consequently, this condition is also not desirable.

Further, if the thermal aging time is set to less than about 4 hours at the above-mentioned temperature range, i.e., at a temperature of from about 150 to 160° C., an appropriate aging does not occur. Accordingly, since a necessary physical strength is not achieved, this condition is not desirable. And if the thermal aging is conducted for more than about 5 hours, an elongation property is deteriorated due to excessive aging. Consequently, this condition is also not desirable.

In this way, a cast node is cast using the aluminum alloy with the above composition and content through a sand mold casting process, and undergoes a solution treatment at a predetermined temperature for a certain time period, followed by artificial thermal aging under the thermal treating condition proposed by the present invention, so that a sufficient physical property required by a cast node of a vehicle space frame can be ensured.

The preferred embodiment of the present invention will now be described in more detail hereinafter.

EXAMPLES AND COMPARATIVE EXAMPLES

In this embodiment, after the aluminum alloy, after casting, was subjected to a solution treatment at a temperature of from 538° C. for 8 hours, followed by thermal aging at a temperature of 155° C. for 4 hours by using the aluminum alloy with the composition and content suggested by the present invention through a sand mold casting process.

Tensile strength, yield strength an elongation were measured using a sample manufactured through the above method. The result of the measurement was given in the following Table 2 along with the physical properties of AC4C alloy widely used in sand mold casting.

	TABLE 2
	Thermal	Tensile	Yield
	treatment	strength	strength
	condition	(Mpa)	(Mpa)	Elongation (%)
AC4C	After solution	230	190	2.8
	treatment,
	thermal aging at
	a temperature of
	160° C. for 6
	hours
Example 1	After solution	242	132	18.3
	treatment,
	thermal aging at
	a temperature of
	155° C. for 4
	hours

It is shown from the Table 2 that the yield strength of the aluminum alloy (Example 1) is relatively low as compared with that of a conventional alloy material (AC4C), but the inventive aluminum alloy exhibits a relatively high elongation of 18.3% that cannot be achieved by a general cast product and can ensure a similar physical property to that obtained by a high-vacuum die-casting process. For example, the alloy product applied to Audi A2, A3 vehicles of Germany exhibits a tensile strength of 200 MPa, an yield strength of 130 MPa and an elongation of from 14 to 18%.

In the meantime, the aluminum alloy having the same composition and content as suggested by the present invention was subjected to a sand casting and a solution treatment under the same condition as in manufacturing the inventive cast product, followed by a thermal aging process under different temperature and time conditions to obtain fabricate respective samples. The evaluation results for the physical properties of the respective samples are given in the following Tables 3 and 4.

Table 3 shows the result of comparison between the physical properties of the respective samples according to a variation in thermal aging temperature, and Table 4 shows the result of comparison between the physical properties of the respective samples according to a variation in thermal aging time.

	TABLE 3
		Tensile	Yield
	Thermal aging	strength	strength
	temperature (° C.)	(MPa)	(MPa)	Elongation (%)
Example 1	155	242	132	18.3
Comparative	145	210	121	18.7
Example 1
Comparative	160	249	143	16.2
Example 2
Comparative	170	260	154	12.3
Example 3

	TABLE 4
		Tensile	Yield
	Thermal aging	strength	strength
	time (hr)	(MPa)	(MPa)	Elongation (%)
Example 1	4	242	132	18.3
Example 2	5	248	140	17.8
Comparative	3	185	121	16.1
Example 4
Comparative	6	252	141	12.1
Example 5
Comparative	7	262	158	8.2
Example 6

The thermal aging time in Example 1 and Comparative Examples 1 to 3 of the above Table 3 was set to be identical to one another i.e., 4 hours, and the thermal aging temperature in Examples 1 and 2 and Comparative Examples 4 to 6 of the above Table 4 was set to be identical to one another, i.e., 155° C.

The measurement result for the physical properties of the respective samples shows that Examples 1 and 2 can obtain a desired physical property suitable for a cast node of a vehicle space frame, for example, a high elongation. It can be seen from the result that the thermal treatment condition depending on the temperature and time suggested by the present invention is optimal.

As described above, according to an aluminum alloy for a cast node of a vehicle space frame and a method of manufacturing the cast node using the same, the range for adding each of the ingredients (copper, silicon, magnesium and iron) of the aluminum alloy is appropriately adjusted and an optimum heat-treatment condition according to the adjustment of the addition range is presented, so that it enables manufactured products to have the physical properties, such as tensile strength, yield strength and elongation, equivalent to those of products manufactured through a high-vacuum die-casting process, even through a sand mold casting process is used.

That is, although the high-vacuum die-casting process is not applied to the manufacturing process of aluminum vehicle body components, it enables one to fabricate the aluminum vehicle body components having the physical properties equivalent to those of products manufactured through the expensive and complex high-vacuum die-casting process, by using a low-priced sand mold casting process, thereby greatly reducing the product development expenditure, cost and complexity.

While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by the embodiments but only by the appended claims. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present invention.

Claims

1. An aluminum alloy for a cast node of a vehicle space frame, which comprises aluminum as a main ingredient, 0.4 to 0.5 wt % of copper, 2.5 to 3.0 wt % of silicon, 0.1 wt % or less of magnesium, 0.1 wt % or less of iron and 0.2 wt % or less of titanium.

2. A method of manufacturing a cast node of a vehicle space frame, which comprises:

casting an aluminum alloy which comprises aluminum as a main ingredient, 0.4 to 0.5 wt % of copper, 2.5 to 3.0 wt % of silicon, 0.1 wt % or less of magnesium, 0.1 wt % or less of iron and 0.20 wt % or less of titanium;

subjecting the cast aluminum alloy to a solution treatment; and

heat-treating the resulting material at a temperature of from 150 to 160° C. for 4 to 5 hours.

3. An aluminum alloy for a vehicle space frame, said aluminum alloy comprising aluminum; about 0.4 to 0.5 wt % copper; about 2.5 to 3.0 wt % silicon; at most about 0.1 wt % magnesium; at most about 0.1 wt % of iron; and t most about 0.2 wt % titanium.

4. A method of manufacturing a vehicle space frame, which comprises:

casting an aluminum alloy which comprises aluminum; about 0.4 to 0.5 wt % copper; about 2.5 to 3.0 wt % silicon; at most about 0.1 wt % magnesium; at most about 0.1 wt % of iron; and t most about 0.2 wt % titanium;

subjecting the cast aluminum alloy to a solution treatment; and

heat-treating the resulting material at a temperature of from about 150 to about 160° C. for about 4 to about 5 hours.