Aluminum extruded door beam material

An aluminum extruded door beam includes an outer flange, an inner flange, and at least one web for connecting the outer flange and the inner flange. The outer corners at the extended ends of the outer flange have a radius R of 2.5 mm or less. The outward corners at the connections between the web and the inner flange and between the web and the outer flange have a radius R of 2 mm to 4 mm. The radius of the outward corners at the connections between the web and the inner flange and between the web and the outer flange is 1.5 to 2 times the width of the web. The length of the extended ends of the outer flange is 1 to 2 times the radius R of the outward corner at the connections between the web and the outer and inner flanges. The aluminum alloy extruded door beam material contains 0.8 to 1.5% by weight (hereinafter the same) of Mg and 4 to 7% of Zn, and the recrystallization surface layer has a thickness of 50 .mu.m or less.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to aluminum door beams used in reinforcing
 members for doors of vehicles, such as automobiles and trucks. The door
 beam is arranged in a door to absorb the shock from a collision in the
 side direction and to secure safety of passengers.
 2. Description of the Related Art
 Recently, the global environment has been regarded as being of worldwide
 importance. For example, regulations for reducing gas emissions including
 carbon dioxide from automobiles have been strengthened in many countries
 in order to suppress global warming. Accordingly, lightweight automobiles
 have been in rapid development.
 A door beam for an automobile is attached to the interior of a door in
 order to absorb the shock from a collision. A typical conventional
 material used is steel, for example, high-tensile steel of 150
 kgf/mm.sup.2 grade. In recent years, however, the use of aluminum
 extrusions has been investigated in view of achievement of a lightweight
 automobile.
 Door beams for automobiles (also referred to as impact beams, impact bars,
 guard bars, or door side beams) are required to have high energy
 absorbability to soften the shock from a collision. For example, Federal
 Motor Vehicle Safety Standard (FMVSS) defines criteria of the bending
 strength and absorbed energy to a load applied from the side of a vehicle.
 At laboratory tests, these bending properties are evaluated by a
 three-point bending strength test simulating side collision of a vehicle
 as shown in FIG. 2A, in which a door beam is supported at the two ends and
 a load is applied to the center.
 FIG. 2B is a typical schematic load (P) vs. displacement (.delta.) curve in
 the three-point bending test shown in FIG. 2A. FIG. 2B shows that the load
 reaches a maximum value as the displacement increases, and then it
 decreases at a further displacement because of overload buckling of the
 aluminum beam. In general, it is preferred that the maximum load be larger
 and the displacement when the buckling occurs be larger, that is, the
 energy absorption be larger, as shown by a solid line in FIG. 3. The
 energy absorption corresponds to the area represented by hatched lines in
 the load (P) vs. displacement (.delta.) curve of FIG. 2B.
 Stricter properties have been required for door beams being highly
 conscious of safety, that is, improvements in maximum load and energy
 absorption without an increase in the weight have been required. For
 example, in a three-point bending test under a specified condition for
 door beams, a current required level of the maximum load is 1,300 kg,
 which is considerably higher than the conventional level 1,100 kg.
 Recently, door beams have been applied to compact cars having short doors.
 Since the distance (L) between the two ends in FIG. 2A is short, in
 collision of compact cars, a small displacement (.delta.) causes a larger
 bending curvature. Thus, rupture will occur more readily with a small
 displacement.
 SUMMARY OF THE INVENTION
 The present inventors have actively investigated a technology for achieving
 an aluminum door beam without an increase in weight, which has a large
 maximum load, a large displacement before buckling (hereinafter referred
 to as buckling displacement), a large displacement without rupture, and a
 large energy absorption in view of a cross-section and dependence of the
 surface texture on the composition of the door beam material.
 The investigation was performed in view of the following two aspects.
 First, the rupture of the door beam causes decreased absorption energy,
 and the ruptured portion is harmful for the passenger. Thus, the rupture
 must be absolutely avoided. A target of the present invention is to
 provide a configuration in which buckling proceeds predominantly before
 the inner flange at the extension side breaks by the limit of
 stress-strain characteristics.
 Second, another possible method to prevent the rupture of the door beam is
 increased thicknesses of the flange and the web; however, this method
 caused an increase in weight. Thus, another target of the present
 invention is to control the composition and the surface texture of the
 door beam material for simultaneously achieving lightweight and high
 performance.
 As a result, the present inventors have made the following finding. In the
 cross-section of an aluminum door beam, the radius R of the outer corner
 at the extended ends of the outer flange (hereinafter referred to as
 R.sub.FO) and the radius R of the outward corner at the connections
 between the web and the outer and inner flanges (hereinafter referred to
 as R.sub.WO) significantly affect the buckling displacement and energy
 absorption in the load (P) vs. displacement (.delta.) curve. In the
 dependence of the surface texture on the composition of the door beam
 material, when the thickness of the recrystallization layer on the outer
 surface of the door beam is reduced or the layer is eliminated, the stress
 concentration during bending deformation is prevented and the energy
 absorption is improved. This is prominent in a door beam having a large
 maximum load.
 The present invention is achieved based on the finding.
 Accordingly, it is an object of the present invention to provide an
 aluminum extruded door beam comprising an outer flange, an inner flange,
 and at least one web for connecting the outer flange and the inner flange,
 the outer corners at the extended ends of the outer flange having a radius
 R of 2.5 mm or less.
 It is another object of the present invention to provide an aluminum
 extruded door beam material comprising an outer flange, an inner flange,
 and at least one web for connecting the outer flange and the inner flange,
 the outward corners at the connections between the web and the inner
 flange and between the web and the outer flange having a radius R of 2 mm
 to 4 mm.
 It is a further object of the present invention to provide an aluminum
 extruded door beam material comprising an outer flange, an inner flange,
 and at least one web for connecting the outer flange and the inner flange,
 the radius of the outward corners at the connections between the web and
 the inner flange and between the web and the outer flange being 1.5 to 2
 times the width of the web.
 It is a still further object of the present invention to provide an
 aluminum extruded door beam material comprising an outer flange, an inner
 flange, and at least one web for connecting the outer flange and the inner
 flange, the length of the extended ends of the outer flange being 1 to 2
 times the radius R of the outward corner at the connections between the
 web and the flanges.
 It is still another object of the present invention to provide an aluminum
 alloy extruded door beam material comprising 0.8 to 1.5% by weight
 (hereinafter the same) of Mg; 4 to 7% of Zn; 0.005 to 0.3% of Ti; at least
 one element selected from the group consisting of 0.05 to 0.6% of Cu, 0.2
 to 0.7% of Mn, 0.05 to 0.3% of Cr, and 0.05 to 0.25% of Zr; and the
 balance being Al and incidental impurities, the thickness of the
 recrystallization surface layer being 50 .mu.m or less.
 It is a still further object of the present invention to provide an
 aluminum alloy extruded door beam material comprising 0.8 to 1.5% by
 weight (hereinafter the same) of Mg and 4 to 7% of Zn, the
 recrystallization surface layer having a thickness of 50 .mu.m or less.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 An aluminum extruded door beam in accordance with the present invention
 includes an outer flange, an inner flange, and at least one web for
 connecting the outer flange and the inner flange. The outer corners at the
 extended ends of the outer flange have a radius R.sub.FO of 2.5 mm or
 less.
 The corners of extended ends of the flange of a conventional door beam are
 rounded as shown in FIG. 4 in view of extrudability, in contrast, the
 corners in the present invention are angular. According to the finding by
 the present inventors, the angular corner is resistive to buckling, and
 thus buckling displacement and energy absorption are improved. That is,
 the angular corner of the extended end of the flange causes a larger width
 of the extended end of the flange compared with the rounded corner, hence
 the angular corner is resistive to buckling. Further, a load is applied to
 a larger area in the angular corner, hence the load is more dispersed and
 the angular corner is resistive to buckling. A radius R.sub.FO larger than
 2.5 mm will not cause such an improvement. A smaller radius R.sub.FO
 causes further improved buckling displacement and energy absorption,
 therefore, it is preferred that the radius R.sub.FO be 2 mm or less, and
 more preferably 1 mm or less. It is preferable that the radius R.sub.FO be
 0.5 mm or more in view of extrudability.
 An aluminum extruded door beam material in accordance with the present
 invention includes an outer flange, an inner flange, and at least one web
 for connecting the outer flange and the inner flange, and the outward
 corners at the connections between the web and the inner flange and
 between the web and the outer flange have a radius R.sub.WO of 2 mm to 4
 mm.
 In conventional door beams, the R.sub.WO is determined in view of
 extrudability. The present inventors discovered that the radius R.sub.WO
 significantly affects the buckling displacement and that the buckling
 displacement is significantly improved when the radius R.sub.WO ranges
 from 2 mm to 4 mm. The buckling at the extended ends of the outer flange
 is not substantially prevented when the radius R.sub.WO is less than 2 mm,
 and thus the buckling displacement and energy absorption of the door beam
 are not improved. Even when the radius R.sub.WO is larger than 4 mm, the
 buckling is not further improved and the weight is unintentionally
 increased.
 Thus, it is presumed that when the radius R.sub.WO is larger than the
 desired size the extended end of the flange is protected from the load
 applied to the extended end. When the radius R.sub.WO has an unnecessary
 large size, the weight is increased whereas the protective effects of the
 flange does not further increase.
 In another embodiment, an aluminum extruded door beam material includes an
 outer flange, an inner flange, and at least one web for connecting the
 outer flange and the inner flange, and the radius R.sub.WO of the outward
 corners at the connections between the web and the inner flange and
 between the web and the outer flange is 1.5 to 2 times the width t.sub.W
 of the web.
 When the radius R.sub.WO is 1.5 to two times the width t.sub.W of the web,
 the buckling displacement and energy absorption are more effectively
 improved. A radius R.sub.WO of less than 1.5 times the width t.sub.W does
 not cause such an improvement, whereas a radius R.sub.WO of larger than 2
 times does not cause a further improvement in prevention of buckling but
 causes an undesired increase in the weight.
 In still another embodiment in accordance with the present invention, an
 aluminum extruded door beam material includes an outer flange, an inner
 flange, and at least one web for connecting the outer flange and the inner
 flange, and the length L.sub.F of the extended ends of the outer flange is
 1 to 2 times the radius R.sub.WO of the outward corner at the connections
 between the web and the flanges.
 A cross-section satisfying both the length L.sub.F and the radius R.sub.WO
 contributes to significant improvement in buckling displacement and energy
 absorption. When the length L.sub.F is smaller than the radius R.sub.WO,
 the buckling displacement is not substantially improved, whereas a length
 L.sub.F which is 2 times or more the radius R.sub.WO does not cause
 further improvement in the buckling displacement, considering undesirable
 increase in the weight.
 In still another embodiment in accordance with the present invention, an
 aluminum alloy extruded door beam material comprises 0.8 to 1.5% by weight
 (hereinafter the same) of Mg and 4 to 7% of Zn, and the recrystallization
 surface layer has a thickness of 50 .mu.m or less.
 It is preferable to control the texture of the door beam material so that a
 fibrous texture is present below the recrystallization layer. The
 recrystallization layer may be not present. In such a case, the fibrous
 texture is present on the surface of the material.
 Preferably, the fibrous texture has an aspect ratio of 1:20 or more. A
 thick recrystallization layer on the surface causes a rough surface in the
 bending deformation process, and the rough surface functions as a notch
 causing stress concentration. Thus, the door beam will be rapidly
 ruptured.
 Preferably, a door beam has two or more among the above-mentioned features.
 In the present invention, the term "aluminum" means both "aluminum" and
 "aluminum alloys".
 The preferred embodiments of the present invention will now be described
 with reference to the attached drawings.
 FIG. 1 is a cross-sectional view of a door beam in accordance with the
 present invention. The door beam includes an inner flange F.sub.I, an
 outer flange F.sub.O, and webs W with a width t.sub.W, which connect the
 inner flange F.sub.I, and the outer flange F.sub.O. The inner flange
 F.sub.I is arranged toward the inner side of a vehicle when the door beam
 is assembled onto a door, and the outer flange F.sub.O is arranged toward
 the outer side of the vehicle. The outer flange F.sub.O has extended ends
 with a length L.sub.F, and the outer corners of the extended ends have a
 curvature radius of R.sub.FO. The outward corners of the connections
 between the outer flange and the webs have a curvature radius of R.sub.WO.
 The shape of the door beam in accordance with the present invention is not
 limited to that shown in FIG. 1. For example, a door beam having only one
 web, that is, an I-shaped door beam is included in the scope of the
 present invention.
 The buckling displacement in the present invention is defined as a
 displacement (.delta.) when the load becomes half the maximum load (P) in
 the deformation region after the maximum load is applied, as shown in FIG.
 5.
 First Embodiment
 Aluminum door beams A and B having the cross-sectional sizes shown in FIGS.
 6A and 6B, respectively, were formed by extrusion of an Al--Mg--Zn alloy
 composed of 1.4% by weight (hereinafter the same) of Mg, 6.5% of Zn, 0.2%
 of Cu, 0.15% of Zr, 0.02% of Ti, and 0.3% of Cr, as follows. The alloy was
 melted by a conventional process and cast to form an ingot with a diameter
 of 200 mm. The ingot was subjected to homogenizing heat treatment at
 470.degree. C. for 8 hours and then extrusion at a temperature of
 470.degree. C. and an extrusion rate of 4 m/min to form the door beams A
 and B. The extruded door beams A and B were subjected to artificial aging
 at 130.degree. C. for 12 hours. The outer flange of the door beam A has a
 length of 38 mm and a width of 4.4 mm, the inner flange has a length of 48
 mm and a width of 4.6 mm, and the web has a length of 28 mm and a width of
 2.1 mm. In the door beam A, the length L.sub.F of the extended ends of the
 outer flange F.sub.O and the curvature radius R.sub.FO of the outer
 corners of the extended ends are different from those of door beam B, and
 other portions have the same size.
 A cut piece was prepared from each of the door beams A and B, and subjected
 to the three-point bending test shown in FIG. 2A at a bending span L of
 1,200 mm. A load was applied before the displacement (.delta.) reached 350
 mm. FIG. 7 is a load (P) vs. displacement (.delta.) curve in the
 three-point bending test. Table 1 shows the maximum load, buckling
 displacement, energy absorption, and the unit weight of the door beam.
 TABLE 1
 Maximum Buckling Energy Unit
 R.sub.FO load displacement absorption weight
 Door Beam (mm) (kgf/mm.sup.2) (mm) (kgf .multidot. mm)
 (kg/m) Judgement
 A 3.0 1,289 214 247,805 1.40 No
 (For comparison) (1.00) (1.00) (1.00) (1.00) good
 B 0.5 1,278 250 272,634 1.38 Good
 (Example) (0.99) (1.17) (1.10) (0.99)
 Remarks:
 values in parentheses represent the relative values to those of the door
 beam A (1.00).
 R.sub.FO represents the curvature radius R of the outer corners at the
 extended ends of the outer flange.
 As shown in Table 1, the door beam B having an R.sub.FO in accordance with
 the present invention shows a similar maximum load, a buckling
 displacement higher by 17%, and an energy absorption higher by 10%
 regardless of a slightly smaller unit weight compared to those of the door
 beam A for comparison having an R.sub.FO out of the scope of the present
 invention. Such advantages can also be achieved with JIS 7N01, 6061, 6063
 and 6N01 alloys, and Alloys 6000 and 7000 series in a list published by
 Aluminum Association, such as Alloy 6082. 7000 series alloys containing
 0.8% to 1.5% of Mg and 4% to 7% of Zn, by weight respectively, are
 preferred in view of strength and extrudability, as described below in
 detail.
 Second Embodiment
 Aluminum door beams C, D and E having the cross-sections shown in FIGS. 8C,
 8D and 8E, respectively, were formed using the Al--Mg--Zn alloy having the
 same composition as the First Embodiment. The details of the
 cross-sections of these door beams C, D and E are shown in Table 2. The
 lengths and the thicknesses of the outer flange and the inner flange, the
 length of the webs, and the distance between the webs are the same in the
 door beams C, D and E.
 TABLE 2
 Door beam R.sub.WO (mm) R.sub.WO /t.sub.W L.sub.F /R.sub.WO
 R.sub.FO (mm)
 C (For 1 0.53 6.85 3
 comparison)
 D (Example) 4* 2.11 1.71* 1.8*
 E (Example) 4* 1.82* 1.64* 1.8*
 Remarks:
 Asterisk* indicates that it is within the scope of the present invention.
 R.sub.WO : Curvature radius of the outward corners of the connections
 between the outer flange and the webs
 t.sub.W : Web width
 L.sub.F : Length of the extended ends of the outer flange
 R.sub.FO :Curvature radius of the outer corners of the outer flange
 A cut piece was prepared from each of the door beams C, D and E, and
 subjected to the three-point bending test shown in FIG. 2A at a bending
 span L of 950 mm. A load was applied before the displacement (.delta.)
 reached 300 mm. FIG. 9 is a load (P) vs. displacement (.delta.) curve in
 the three-point bending test. Table 3 shows the ratios of the energy
 absorption and the unit weight of the door beam.
 TABLE 3
 Ratio of absorption
 Door Beam Weight ratio energy
 C (For comparison) 1.00 1.00
 D (Example) 1.05 1.29
 E (Example) 1.09 1.73
 As shown in Table 3, the door beam D in accordance with the present
 invention, which satisfies the R.sub.WO, LF/R.sub.WO and R.sub.FO ratios,
 shows an increase by 29% in energy absorption to the door beam C for
 comparison, regardless of a slight increase by 5% in weight to the door
 beam C. The door beam E in accordance with the present invention, which
 also satisfies the R.sub.WO /t.sub.W ratio, as well as the R.sub.WO,
 LF/R.sub.WO and R.sub.FO ratios, shows a significant increase by 73% in
 energy absorption to the door beam C for comparison, regardless of a
 slight increase by 9% in weight to the door beam C.
 In the configurations in Second Embodiment, such advantages can also be
 achieved with JIS 7N01, 6061, 6063 and 6N01 alloys, and Alloys 6000 and
 7000 series registered in a list published by Aluminum Association, such
 as Alloy 6082. Open-type 7000 series alloys containing 0.8% to 1.5% by
 weight of Mg and 4% to 7% by weight of Zn are preferred in view of
 strength and extrudability, as described below in detail.
 As described above, there are the following four design requirements for
 aluminum door beams:
 (A) An R.sub.FO of 2.5 mm or less.
 (B) An R.sub.WO ranging from 2 mm to 4 mm.
 (C) An R.sub.WO /t.sub.W ratio ranging from 1.5 to 2.
 (D) An L.sub.F /R.sub.WO ratio ranging from 1 to 2.
 Any combination of these requirements causes further improvement in the
 buckling displacement and energy absorption. Preferred combinations of the
 requirements include (A) and (B); (A) and (C); (A) and (D); (B) and (C);
 (B) and (D); (C) and (D); (A), (B) and (C); (A), (B) and (D); (A), (C) and
 (D); (B), (C) and (D); and (A), (B), (C) and (D).
 The curvature R.sub.FI of the inner corners at the extended ends of the
 outer flange F.sub.O affects the mechanical properties compared less than
 that of the R.sub.FO of the outer corner, and it is not necessary that
 both are equal to each other; however, it is preferable that the R.sub.FI
 be 2.5 mm or less, more preferably 2 mm or less, and most preferably 1 mm
 or less, as in the R.sub.FO.
 The curvature of the corners at the extended ends of the inner flange
 F.sub.I can be determined without restriction based on the practical
 design of the door beam. For example, when the extended ends of the inner
 flange F.sub.I are used for attaching the door beam to the vehicle door
 and a flat surface is required, it is preferable that the corner has a
 smaller curvature. On the contrary, it is preferable that the curvature be
 larger in view of extrudability and surface characteristics.
 Although the curvature of the inward corners (at the hollow section in FIG.
 1) of the connections between the webs and the inner and outer flanges is
 not limited, it is preferable that the curvature ranges from 2 mm to 4 mm
 and that it be 1.5 to 2 times the web width.
 An inner flange F.sub.I longer than the outer flange F.sub.O or an extended
 end of the inner flange F.sub.I longer than the extended end of the outer
 flange F.sub.O causes a shift of the neutral axis towards the inner side
 (passenger side) of the vehicle. Such a shift causes increased energy
 absorption and delayed rupture of the door beam at the inner side by a
 collision load.
 In the present invention, the door beam comprises an outer flange which
 lies in the outer side of the vehicle and is loaded with an impact load in
 the vertical direction, an inner flange which lies substantially parallel
 to the outer flange and lies in the passenger side, and at least one web
 connecting these flanges, and the inner flange or the outer flange
 preferably has a cross-section having extended ends which extend from the
 connecting section with the web.
 In the present invention, another flange may be provided between the inner
 flange and the outer flange.
 Third Embodiment
 An aluminum alloy of Composition 1 shown in Table 4 was melted by a
 conventional process and cast to form an ingot with a diameter of 200 mm.
 The ingot was subjected to homogenizing heat treatment at 470.degree. C.
 for 8 hours and then extrusion at a temperature of 470.degree. C., an
 extrusion rate of 4 m/min and an extrusion ratio of 42 to form two door
 beams F having a cross-section shown in FIG. 10A. The extruded door beams
 F were immediately cooled by blowing liquid nitrogen and cooled nitrogen
 gas and subjected to artificial aging at 130.degree. C. for 12 hours.
 The same aluminum alloy ingot was subjected to homogenizing heat treatment
 at 470.degree. C. for 8 hours and then extrusion at a temperature of
 500.degree. C., an extrusion rate of 12 m/min and an extrusion ratio of 83
 to form a door beam G having the same cross-section shown in FIG. 10A. The
 extruded door beam G was subjected to artificial aging at 130.degree. C.
 for 12 hours without cooling by liquid nitrogen and cooled nitrogen gas.
 TABLE 4
 Chemical component (wt %)
 Compound Mg Zn Ti Cu Mn Cr Zn
 1 1.3 6.7 0.03 0.2 0.2 0.06 0.14
 2 0.72 5.5 0.04 0.07 0.02 0.02 0.18
 Table 5 shows the results of the thickness of the recrystallization surface
 layer, the aspect ratio of the fibrous texture, and the three-point
 bending test at a bending distance of 950 mm of the door beams F and G. As
 shown in Table 5, the door beams F, which were within the scope of the
 present invention in terms of the thickness of the recrystallization
 surface layer and the aspect ratio of the fibrous texture, had a larger
 rupture displacement compared with that of the door beam G having a larger
 thickness and a lower aspect ratio.
 TABLE 5
 Thickness of
 recrystallization Aspect Maximum
 Rupture
 surface layer ratio of bending load
 displacement
 Door beam Compound (.mu.m) fibrous texture (Kgf) (mm)
 Judgement
 G 1 250 1:2 1,000 180
 No good
 (For Comparison)
 F 1 20 1:25 1,020 300
 Good
 (Example)
 Fourth Embodiment
 An aluminum alloy of Composition 1 shown in Table 4 was melted by a
 conventional process and cast to form an ingot with a diameter of 200 mm.
 The ingot was subjected to homogenizing heat treatment at 470.degree. C.
 for 8 hours and then extrusion at a temperature of 460.degree. C., an
 extrusion rate of 5 m/min and an extrusion ratio of 35 to form two door
 beams H having a cross-section shown in FIG. 10B. The extruded door beams
 H were immediately cooled by blowing liquid nitrogen and cooled nitrogen
 gas and subjected to aging at 130.degree. C. for 12 hours.
 A door beam I for comparison having the same cross-section was prepared
 from the aluminum alloy of Compound 2 shown in Table 4 by the same
 process.
 Table 6 shows the results of the thickness of the recrystallization surface
 layer, the aspect ratio of the fibrous texture, and the three-point
 bending test at a bending distance of 700 mm of the door beams I and H. As
 shown in Table 6, although both door beams I and H satisfy the scope of
 the present invention in terms of the thickness of the recrystallization
 surface layer and the aspect ratio of the fibrous texture, the door beam
 I, which is out of the scope of the present invention in terms of the
 composition has a smaller maximum bending load and a smaller energy
 absorption compared with the door beam H.
 TABLE 6
 Thickness of
 recrystallization Aspect Maximum
 Energy
 surface layer ratio of bending load
 absorption
 Door beam Compound (.mu.m) fibrous texture (Kgf) (kgf
 .multidot. mm) Judgement
 I 2 30 1:20 1,310 183,300
 No good
 (For Comparison)
 H 1 20 1:20 1,840 265,100
 Good
 (Example)
 The composition and the texture of the door beam in accordance with the
 present invention will now be described in more detail.
 Mg and Zn
 Magnesium and zinc are essential for the aluminum alloy in accordance with
 the present invention in order to achieve excellent mechanical properties.
 At a magnesium content of less than 0.8% by weight or a zinc content of
 less than 4% by weight, the aluminum alloy does not have the desired
 strength. At a magnesium content of more than 1.5% by weight or a zinc
 content of more than 7% by weight, the extrudability and elongation of the
 aluminum alloy decrease, and the required strength is not achieved. Thus,
 in the aluminum alloy in accordance with the present invention, the
 magnesium content is set to a range from 0.8 to 1.5% by weight and the
 zinc content is set to a range from 4 to 7% by weight.
 Ti
 Titanium is an essential element to form a fine texture in the ingot. A
 titanium content of less than 0.005% by weight does not cause satisfactory
 formation of the fine texture, whereas a titanium content of more than
 0.3% by weight causes the formation of huge nuclei because of saturation
 of titanium in the aluminum alloy. Thus, the titanium content is set to a
 range from 0.005 to 0.3% by weight.
 Cu, Mn, Cr and Zr
 These elements cause increased strength of the aluminum alloy. Further,
 copper improves stress corrosion crack resistance of the aluminum alloy.
 Manganese, chromium or zirconium forms a fibrous texture to reinforce the
 alloy. At least one of these elements is added according to demand.
 Preferred ranges for these elements are as follows: 0.05 to 0.6% by weight
 for Cu, 0.2 to 0.7% by weight for Mn, 0.05 to 0.2% by weight for Cr, and
 0.05 to 0.25% by weight for Zr. If these elements are added in an amount
 of less than their lower limits, these elements will not effectively
 contribute to the strength of the aluminum alloy. If a content of one of
 the elements is higher than its upper limit, the extrudability will
 deteriorate. In particular, copper over the upper limit will cause
 deterioration of general corrosion resistance.
 Incidental Impurities
 The aluminum alloy contains iron as the main component of the incidental
 impurities in a relatively large amount. If the aluminum alloy contains
 more than 0.35% by weight of iron, coarse intermetallic crystals form in
 the casting process, mechanical strength of the alloy decreases. Thus, the
 iron content is controlled to be 0.35% by weight or less.
 Various impurities, derived from the ground metal and the mediate alloy for
 the essential elements, are included in the aluminum alloy. Types of the
 impurities vary with the used ground metal and the used mediate alloy.
 When the sole content of each impurity other than iron is less than 0.05%
 by weight and the total content of individual impurities other than iron
 is less than 0.15% by weight, the aluminum alloy has the desired
 mechanical properties. Thus, the sole content and the total content of the
 impurity are set to 0.05% or less and 0.15%, respectively, by weight.
 Texture in Extruded Material
 When a thick recrystallization layer is formed on the surface of the door
 beam, a rough surface forms in the bending deformation process. The rough
 surface functions as a notch and causes stress concentration. Thus, the
 rupture of the door beam will be prompted, and energy absorption is
 decreased. Since the aluminum alloy in accordance with the present
 invention has a thin recrystallization layer of 50 .mu.m or less, no rough
 surface forms and stress concentration is avoidable. Preferably, the
 recrystallization layer is not present.
 It is preferable that the crystallites in the fibrous texture on the
 surface and inside the alloy have an aspect ratio of 1:20 or more.
 Although granular crystallites or low-aspect-ratio crystallites will
 readily form a rough surface by bending deformation, crystallites having
 such a high aspect ratio do not form a rough surface under a bending
 deformation condition for the door beam. Thus, stress concentration is
 avoided.
 The aspect ratio of the fibrous texture in the present invention is defined
 as the ratio of the crystal grain size in the extruding direction to the
 crystal grain size in a direction in which the smallest crystal grain size
 is observed, in the plane perpendicular to the extruding direction, and is
 determined by a cutting method according to JIS-H0501. That is, a cut
 sample was prepared from the center of the loaded section in the inner
 flange subjected to the three-point bending test as shown in FIG. 2(A).
 The recrystallization layer on the surface of the extruded member is formed
 by the heat, which is generated by large deformation of the surface in the
 extrusion process. Thus, the formation and propagation of the
 recrystallization layer can be prevented by decreasing the extrusion
 temperature, the extrusion speed, and the extrusion ratio by means of
 multinozzle extrusion. Further, the formation and propagation of the
 recrystallization layer can be prevented by rapidly cooling only the
 surface layer of the extruded member near downstream of the outlet of the
 extrusion die.
 Exemplary conditions for producing the aluminum door beam having the
 above-mentioned texture are as follows: a homogenizing heat treatment
 temperature of 450.degree. C. to 500.degree. C., an extruding temperature
 of 400.degree. C. to 500.degree. C., an extruding rate of 6 to 10 m/min.,
 an extrusion rate of 35 to 70, an aging temperature of 130.degree. C. to
 170.degree. C., and an aging time of 6 to 12 hours. The temperature rise
 on the surface of the extruded member is suppressed by liquid nitrogen and
 cooled nitrogen gas blow near the outlet of the extrusion die.
 The cross-section, the composition and the texture in accordance with the
 present invention is described above. Buckling displacement, energy
 absorption and a displacement without rupture can be further improved by
 combining these parameters.