JP4359211B2 - Aluminum alloy foam - Google Patents

Description

  The present invention relates to an aluminum alloy foam used as an impact energy absorbing member that deforms and absorbs impact energy when subjected to a compressive impact load at the time of a collision, such as an automobile structural member.

  As the impact energy absorbing member (crash box) as described above, a steel hollow member having a closed cross section is generally used as a structural member of an automobile. The steel hollow member is crushed and deformed to absorb the impact energy when it receives a compression impact input in the axial direction or the cross-sectional direction. At this time, in order to be able to absorb a larger amount of energy with a limited amount of deformation, it is effective to increase the size and thickness of the member. However, this leads to an increase in the volume and weight of the steel hollow member, which is not preferable because fuel consumption deteriorates and damage to the opponent vehicle at the time of collision between vehicles increases. Moreover, in place of a mild steel plate, a high-strength steel plate (HITEN) is used to suppress an increase in the volume and weight of the steel hollow member, but the high-strength steel plate is inferior in formability. Therefore, there are inconveniences such that the member shape is restricted and the molding process is increased.

  On the other hand, in recent years, foam metal such as foam aluminum having good recyclability has attracted attention as these impact energy absorbing members. This crash box is made of foamed aluminum in a prismatic shape. And this prismatic axis direction is arranged to coincide with the collision direction, and it absorbs the collision energy by receiving the compressive stress and collapsing at the time of collision, and reduces the impact on the occupant and the structure. is there.

  As an application example to such a crash box using aluminum foam, as a structural member such as a side member of an automobile body, foaming is performed in a hollow portion of a steel tube having a substantially circular or polygonal cross-sectional shape. What filled aluminum is known (refer patent documents 1, 2, 3, 4, 5).

  This utilizes the characteristic of foamed aluminum that compresses and deforms while exhibiting a constant reaction force. By controlling the compressive deformation of the tubular body, it is possible to increase the ability to absorb impact energy.

Furthermore, in order to increase the impact energy absorption capacity of the foamed aluminum itself, the aluminum composition is, by weight, Cu: 0.1-7%, Ca: 0.2-5%, Zn: 0.1-10%, An aluminum alloy containing one or more of the group consisting of Mg: 0.1 to 20% and Ti: 0.1 to 5%, with the balance being aluminum and inevitable impurities, has a relative density of 0.20 or less The average cell diameter is 3.7 mm or less (see Patent Documents 6 and 7).
JP-A-8-164869 (Claims, FIG. 1) Japanese Patent Laid-Open No. 11-59298 (Claims, FIG. 1) JP 2003-19977 A (Claims, FIG. 1) Japanese Patent Laying-Open No. 2003-28224 (Claims, FIG. 1) JP 2004-108541 A (Claims, FIG. 1) Japanese Patent Laid-Open No. 11-302765 (Claims, FIG. 1) JP 2000-328155 A (Claims, FIG. 1)

  However, the crush box of the type in which foamed aluminum is filled in the hollow portion of the steel pipe body or hollow member as described above has an initial instantaneous stress, that is, by the steel pipe body or hollow member as its skin material, that is, There is a problem that the maximum load in the load-displacement relationship (characteristic) becomes high and the stability of the plateau stress (compressive stress at the time of compressive deformation) is lacking. For this reason, as a practical problem, the impact energy absorption property of the foamed aluminum itself cannot be utilized.

  Further, when a crush box as a foam aluminum simple substance is assumed, the plateau stress is insufficient in each foam aluminum of the related art as described above. For example, even in the case of foamed aluminum composed of a specific aluminum alloy composition, fine bubbles and specific relative density as in Patent Documents 6 and 7, the plateau stress in the compression test is only about 2 MPa as shown in the drawings. Absent.

  That is, the conventional foamed aluminum as described above cannot cope with the required energy absorption amount as an impact energy absorbing member, which has been increasing in recent years. For this reason, even if the structural member made of aluminum foam has the advantage of weight reduction, it cannot be replaced by a structural member made of high-tensile steel plate such as an automobile.

  For example, in recent years, automobile safety standards have been demanded for vehicle body front structures that can cope with medium-to-high-speed collisions such as 16km / h and 64km / h from conventional low-speed collisions of about 5mile / h. That is, even in such a medium-high speed collision, as in the case of a low-speed collision, it is necessary to design the structural members such as the left and right side members of the automobile body so as to absorb collision energy due to axial crushing deformation.

  On the other hand, a crush box made of a 440 MPa class high-tensile steel sheet that is currently used generally has an energy absorption amount of about 6.0 kJ / kg before the crush box is deformed by 50%. For this reason, in order for aluminum foam to be able to replace such a high tensile steel plate crash box, it is assumed that the aluminum foam crash box has the same volume as the high tensile steel plate crash box. An energy absorption amount equivalent to or higher than that of a high tensile steel plate crash box is required. In addition, if it does not have a volume equivalent to the high tensile steel plate crash box, the advantage of weight reduction that substitutes foam aluminum for the high tensile steel plate crash box does not occur.

  When the performance of energy absorption amount of about 6.0 kJ / kg up to 50% deformation of the crash box made of the above-mentioned 440 MPa class high-tensile steel plate is seen as compression stress (plateau stress) in the compression test, The stress needs to be 4 MPa or more.

  The present invention has been made to solve such problems, and it is an object of the present invention to provide an aluminum alloy foam that can be substituted for an impact energy absorbing member made of a high-strength steel plate.

  In order to achieve this object, the gist of the aluminum alloy foam of the present invention is an aluminum alloy foam used as an energy absorbing member, and is in mass%, Zn: 1.0 to 20.0%, Ca: 0.1% to 5.0%, Ti: 0.1% to 5.0%, Mg: 0.1% to 5.0%, respectively, and a foamed aluminum alloy composed of the balance aluminum and inevitable impurities The average particle diameter of the foam is 5 mm or less, the relative density is 0.1 or more, and the average hardness of the cell wall of the foam is 60 Hv or more.

  The present inventors have found that the plateau stress of the above-described conventional aluminum alloy foam (hereinafter also referred to as foamed aluminum) is low because the average hardness of the foamed cell walls is low. Since the average hardness of the foamed cell wall is low, the plateau stress of the aluminum alloy foam composed of the specific aluminum alloy composition, fine bubbles and specific relative density as in Patent Documents 6 and 7 is as low as about 2 MPa. .

  Thus, the average hardness of the foam cell wall and the compressive stress of the aluminum alloy foam are correlated, and the plateau stress of the aluminum alloy foam (compression in the compression test) increases as the average hardness of the foam cell wall increases. Stress). That is, as the average hardness of the foamed cell wall of the aluminum alloy foam is increased, the energy absorption amount and the initial maximum instantaneous stress of the crash box made of the aluminum alloy foam alone can be increased.

  By the way, in the usual process for producing foamed aluminum, the cooling after foaming is allowed to cool, and even if the foamed particle size is reduced to the average particle size in order to cool to room temperature, foaming is inevitably performed. The average hardness of the cell walls must be low.

  For this reason, in order to further increase the average hardness of the foamed cell wall after making the foamed particle diameter of the foamed aluminum comprising the specific alloy composition into fine bubbles as the average particle diameter as in the present invention, As will be described later, a special manufacturing method is required.

  In the present invention, the relationship between the average hardness of the foamed cell wall and the compression stress (plateau stress) in the compression test of foamed aluminum was quantitatively grasped. And in order to make plateau stress of foaming aluminum into 4 Mpa or more, it recognized that the average hardness of the cell wall of foaming needs to be 60 Hv or more.

  Thus, according to the present invention, it is possible to provide an aluminum alloy foam that can achieve a high impact energy absorption that can be substituted for a structural member made of a high-tensile steel plate.

(Average particle size of foam)
In the present invention, as a prerequisite, the foam diameter (bubble diameter or bubble particle diameter) of the aluminum alloy foam is controlled so that the average particle diameter is 5 mm or less. By setting the average particle diameter of the foam to 5 mm or less, the uniformity of the foam particle diameter is ensured. When the average particle diameter of the foam of the aluminum alloy foam exceeds 5 mm, it becomes difficult to set the average hardness of the foamed cell wall to 60 Hv or more, and the compressive strength and impact absorption characteristics are lowered.

  The average particle diameter of the foam can be measured by a transmission particle diameter measurement method using a foaming high-intensity X-ray source in an aluminum alloy foam, and is obtained by averaging the particle diameters in the visual field. As a high-intensity X-ray source for transmission particle size measurement, an industry beam line BL16B2 of SPring8 or the like is used. And since the imaging | photography visual field area for a particle size measurement is required, it is preferable to perform panoramic imaging. If the average particle diameter of foaming is 5 mm or less, this panoramic photography can secure a field of view of about 10 × 30 mm and a sufficient number of measurement foamed cells. For this panoramic image, a foaming particle diameter in terms of a circle is determined using a tracing method and image analysis software, and a standard deviation of the foaming particle diameter is obtained. These measurements are performed at three locations per sample and the average is taken. FIG. 1 shows a panoramic image of a foamed aluminum alloy foam (viewing field of about 10 × 30 mm) actually taken as a drawing substitute photograph.

(Average hardness of foamed cell wall)
In the present invention, the average hardness of the foamed cell wall is defined as 60 Hv or more. The cell wall of foaming in the aluminum alloy foam is defined as a wall surrounding each foam (bubble) or a wall partitioning each foam. When the average hardness of the foamed cell wall is less than 60 Hv, the plateau stress of the aluminum alloy foam cannot be made 4 MPa or more even if the foamed particle size is reduced.
The hardness of the foamed cell wall of the aluminum alloy foam is obtained as an average value by applying a load of 50 g to the foamed cell wall measurement portion with a micro Vickers hardness tester, for example, at a plurality of locations such as 5 locations. .

(Relative density of foam)
Further, as a condition for obtaining the plateau stress of the aluminum alloy foam, the relative density of the foam (foam) is set to 0.1 or more together with the refinement of the foam particle size and the uniformity of the foam particle size. preferable. If the relative density of the foam is less than 0.1, the plateau stress of the aluminum alloy foam may not be obtained even if the foamed particle size is reduced or the foamed particle size is uniform. The upper limit of the relative density of the foam is not particularly defined, but the higher the relative density, the larger the weight and the smaller the contribution to weight reduction of automobiles and the like. Depending on the application, a higher deformation stress may be required than the weight reduction effect, so 1.0 or less is preferable. The relative density of the foam is controlled by adjusting the amount of foaming agent (TiH ₂ ) added according to the alloy composition, production conditions, equipment conditions, and the like. The relative density is obtained by cutting a 50 × 50 × 50 mm (125 cm ³ ) sample from the foam, measuring the weight of the sample, and dividing by a corresponding volume of water 125 cm ³ = 125 g.

(Plateau stress)
In the present invention, the plateau stress (compressive stress in the compression test) of the aluminum alloy foam is defined as 4 MPa or more. If the plateau stress is less than 4 MPa, an energy absorption amount equal to or higher than that of a high-strength steel plate crash box cannot be secured on the assumption that the plateau has a volume equivalent to that of a high-strength steel plate crash box. Specifically, an energy absorption amount of about 6.0 kJ / kg cannot be secured until the crash box is deformed by 50%.

(Aluminum alloy composition for foaming)
The aluminum alloy composition for foaming, which satisfies the characteristics of the aluminum alloy foam, such as required strength and energy absorbing ability as an energy absorbing member, and also relates to the uniformity of foaming, will be described below.

  In the present invention, the composition of the aluminum alloy for foaming is such that Zn: 1.0 to 20.0%, Ca: 0.1 to 5 in mass% in order to satisfy the necessary properties as a foam, such as the plateau stress. 0.0%, Ti: 0.1 to 5.0%, Mg: 0.1 to 5.0%, respectively, and the balance being aluminum and inevitable impurities.

(Zn)
Zn is an element effective for improving the strength when coexisting with Mg, but has the effect of coagulating and shrinking, and has the effect of increasing the compressive deformability by forming a thin part of the cell wall. In order to exert this effect, the content of 1.0% or more is necessary. However, if it exceeds 20.0% and it contains excessively, stabilization of the bubble particle diameter of foaming aluminum will be inhibited, a bubble will become coarse, and compressive strength will be reduced. Therefore, the Zn content is in the range of 1.0 to 20.0%.

(Ca)
Ca increases the viscosity of the molten aluminum alloy at the time of producing foamed aluminum and stabilizes the bubbles to make the foam homogeneous and to achieve foam refinement and uniformity. Have. In order to obtain the effect, the content of at least 0.1% is necessary. On the other hand, if the content exceeds 5.0% excessively, the viscosity of the molten metal is excessively increased, the fluidity of the molten metal is remarkably lowered, and it becomes difficult to disperse the foaming agent. And reduce the compressive strength. Therefore, the Ca content is in the range of 0.1 to 5.0%.

(Mg)
Mg is an element effective for improving the strength, and as described above, during the production of foamed aluminum in cooperation with Zn, the viscosity of the molten metal is increased and the bubbles are stabilized to make the foam homogeneous. Has an effect. In order to obtain the effect, it is necessary to contain at least 0.1% of Mg. On the other hand, if the content exceeds 5.0% excessively, the viscosity of the molten metal is excessively increased, the fluidity of the molten metal is remarkably lowered, and it becomes difficult to disperse the foaming agent. And reduce the compressive strength. Therefore, the Mg content is in the range of 0.1 to 5.0%.

(Ti)
Ti is an element effective for improving the strength of foamed aluminum. In order to bring out the effect, it is necessary to contain at least 0.1% or more. On the other hand, when it contains excessively exceeding 5.0%, the fluidity | liquidity of a molten metal will be reduced and it will crystallize and it will make aluminum brittle. Therefore, the Ti content is in the range of 0.1 to 5.0%.

  In addition, Cu may inhibit the uniformity of the foamed particle diameter in the foaming process. For this reason, in the present invention, Cu is an impurity, and the Cu content is preferably as low as possible. Further, other elements are also impurities, and it is preferable that the content is as small as possible. However, there are also problems in the production of foamed aluminum, such as melting and refining to reduce the content of these other elements, and it does not degrade the characteristics of foamed aluminum. Inclusion is allowed.

(Production conditions)
Next, preferable production conditions for producing the foamed aluminum of the present invention will be described below. In the present invention, the manufacturing process itself of the foamed aluminum is the same as the conventional one. However, as described above, in order to make the foamed particle diameter of the foamed aluminum having the specific alloy composition into fine bubbles as the average particle diameter and to make the average hardness of the foamed cell wall 60Hv or more, as described later. In particular, after the mold is taken out of the furnace (after completion of foaming), it is necessary to cool it once to a range of about 100 to 200 ° C. and then perform a heat treatment for holding at this temperature range for 10 minutes or more.

  First, in a melting furnace, alloy component elements such as Zn: 1.0 to 20.0% and Mg: 0.1 to 5.0% and calcium 0.1 to 5. 0% is added, and the molten metal is stirred in the atmosphere for about 5 minutes to increase the viscosity.

  And after pouring the molten metal after this thickening into the casting_mold | template in a 600-700 degreeC atmospheric melting furnace, a predetermined amount of titanium hydride is added. Then, for example, after stirring for 1 to 10 minutes, the stirrer is removed, and the mold is held in the atmospheric melting furnace in the temperature range for about 1 to 10 minutes to complete foaming.

  Conventionally, after this holding, the mold is taken out of the furnace and allowed to cool to room temperature. Therefore, the average hardness of the foamed cell walls inevitably decreases.

  On the other hand, in order to increase the average hardness of the foamed cell wall as in the present invention, at the time of cooling, it is not cooled to room temperature as in the conventional case, but is about 100 to 200 ° C. It is necessary to perform a heat treatment for 10 minutes or more in this temperature range after the temperature is once cooled to this range. By performing this heat treatment, it is possible to make the average hardness of the foamed cell wall as hard as 60 Hv or more than the cooling to room temperature.

  After such cooling, the foam is taken out from the mold and machined to obtain a product aluminum alloy foam having a desired shape such as a prism or square.

  Examples of the present invention will be described below. By changing the cooling (cooling) stop temperature and holding time (heat treatment conditions) after foaming shown in Table 2, aluminum alloy foams having the chemical composition shown in Table 1 were produced, The diameter, relative density, cell wall hardness, and compressive strength properties were evaluated.

  Specifically, first, alloy component elements such as Zn, Mg, and Ca were added to industrial pure aluminum in a melting furnace, and the molten metal was stirred in the atmosphere for about 5 minutes to increase the viscosity. And after pouring the molten metal after this thickening into the casting_mold | template in an about 700 degreeC air melting furnace, about 0.1 to 5.0% of titanium hydride was added as Ti. Then, after stirring for about 2 minutes, the stirrer was removed, and the mold was held in the atmospheric melting furnace at about 700 ° C. for about 4 minutes to complete foaming.

  After this holding, the mold was taken out of the furnace and allowed to cool to each cooling stop temperature shown in Table 2. In the inventive examples, heat treatment was performed at these cooling stop temperatures for a predetermined time shown in Table 2. After such cooling, the foam was removed from the mold.

A test piece having a height of 50 mm, a width of 50 mm, and a length of 10 mm was cut out from these aluminum alloy foams by machining, and the average particle size of the foam was determined by the transmission particle size measurement method using the high-intensity X-ray source described above. Each was measured. Under the present circumstances, the average particle diameter of the foaming by the cross-section measuring method which observes the cross section of normal foaming aluminum and measures the particle size of each foaming was also calculated | required for reference.
Moreover, the relative density of these aluminum alloy foams was determined by the method described above. And the hardness of the foaming cell wall of these aluminum alloy foams was also measured with a micro Vickers hardness tester by applying a load of 50 g, and calculating the average value thereof.

  Further, a test piece having a height of 50 mm, a width of 50 mm and a length of 50 mm was cut out from the aluminum alloy foam by machining, and the plateau stress when compressed in the longitudinal direction using a compression tester was determined. These results are also shown in Table 2.

  As is clear from Tables 1 and 2, Invention Examples 1 to 4, which are aluminum alloys A to D within the composition of the present invention, are subjected to heat treatment that is held for a predetermined time at the cooling stop temperature shown in Table 2 after being allowed to cool. As a result, the average particle diameter of the foam is 5 mm or less, the relative density of the foam is 0.1 or more, and the average hardness of the cell wall of the foam is 60 Hv or more. As a result, in Examples 1 to 4, the plateau stress of the aluminum alloy foam is 4 MPa or more. Incidentally, the plateau stress-displacement characteristic of Invention Example 1 is shown in FIG.

On the other hand, Comparative Examples 5 and 7 are the aluminum alloys A and B within the composition of the present invention, but the foam is allowed to cool to room temperature as in the prior art, and is predetermined at the cooling stop temperature as in the inventive example. No heat treatment for holding time. For this reason, the average hardness of the foamed cell wall is less than 60 Hv, and the compressive stress is as low as less than 4 MPa.
In Comparative Example 6, the aluminum alloy A within the composition of the present invention was used and heat treatment was performed for a predetermined time at the cooling stop temperature as in the inventive example, but the time was too short and the heat treatment effect was not achieved. For this reason, the average hardness of the foamed cell wall is less than 60 Hv, and the compressive stress is as low as less than 4 MPa.

  Comparative Examples 8 to 15 use aluminum alloys E to L in Table 1 in which the contents of Zn, Ca, Ti, and Mg deviate from the lower limit and the upper limit of the composition of the present invention, respectively. For this reason, although the heat processing hold | maintained for the predetermined time at the cooling stop temperature like the example of an invention is performed, the average hardness of the cell wall of foaming is less than 60 Hv, and compressive stress is as low as less than 4 MPa.

  In addition, in the average particle diameter of the foam by the usual cross-sectional measurement method, a clear correlation corresponding to the compressive stress was recognized between the inventive example and the comparative example as compared with the standard deviation of the foamed particle diameter of the present invention. I understand that there is no.

  From the above results, the significance of each requirement in the aluminum alloy foam of the present invention and the significance of preferred production conditions are supported.

  As described above, according to the present invention, it is possible to provide an aluminum alloy foam that can replace an impact energy absorbing member made of a high-tensile steel plate. As a result, the present invention can be applied to an impact energy absorbing member that deforms and absorbs impact energy when subjected to a compressive impact load at the time of collision, such as a structural member of an automobile.

It is a drawing substitute photograph which shows the image | video of foaming of this invention aluminum alloy foam. It is explanatory drawing which shows the stress-displacement characteristic of this invention aluminum alloy foam.

Claims (2)

  An aluminum alloy foam used as an energy absorbing member, and in mass%, Zn: 1.0-20.0%, Ca: 0.1-5.0%, Ti: 0.1-5.0% Mg: 0.1 to 5.0% of each, foamed aluminum alloy consisting of the balance aluminum and inevitable impurities, the average particle size of foaming is 5 mm or less, the relative density is 0.1 or more And the average hardness of the cell wall of foaming is 60 Hv or more, The aluminum alloy foam characterized by the above-mentioned. The aluminum alloy foam according to claim 1, wherein the aluminum alloy foam is used as a simple substance for an energy absorbing member.

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