2-series aluminum alloy profile and preparation method thereof

Abstract

The invention provides a 2-series aluminum alloy profile and a preparation method thereof, and belongs to the technical field of aluminum alloys. The profile comprises the following alloy components: less than or equal to 0.05% of Si, less than or equal to 0.06% of Fe, 3.7%-4.1% of Cu, 1.2%-1.4% of Mg, less than or equal to 0.4% of Mn, less than or equal to 0.02% of Cr, 0.02%-0.06% of Ti, 0.08%-0.12% of Zr and the balance of Al. The preparation method comprises the following steps: (1) burdening according to alloy components and proportions, and smelting and casting to obtain a cast ingot; (2) carrying out multi-stage homogenization treatment on the cast ingot, and keeping the temperature of the cast ingot at 380-400 DEG C after homogenization; (3) the cast ingot in the heat preservation state after homogenization treatment is subjected to reverse extrusion, and an extruded profile is obtained; (4) pre-stretching is conducted, and an aluminum alloy profile primary product is obtained; (5) carrying out solid solution heat treatment on the aluminum alloy profile primary product, and carrying out spray quenching; (6) the aluminum alloy profile is stretched and straightened; and (7) performing natural aging to obtain an aluminum alloy profile finished product. By controlling the alloy components, optimizing the preparation process and chamfering the die hole of the die, the effective control on the coarse grain layer is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of aluminum alloy preparation, and in particular to a 2-series aluminum alloy profile and a preparation method thereof. Background Art

[0002] As a lightweight and high-strength metal material, 2026 aluminum alloy profiles have broad application prospects in the fields of aerospace, rail transportation, etc. However, 2 series aluminum alloy profiles are aluminum alloys that are prone to coarse grain layer defects. The presence of coarse grains not only reduces the mechanical properties of the material, but also increases the difficulty of machining and the rate of waste generation.

[0003] Taking the existing civil aircraft as an example, the 2026 aluminum alloy profile is used as a long stringer structure under the wing of the aircraft. The thickness of the coarse grain layer at the corner of the profile is usually greater than 3mm. In order to avoid the coarse grain position, the method of increasing the machining allowance is generally adopted, that is, increasing the cross-sectional area of ​​the profile, and then cutting off the part with the coarse grain area according to the profile size requirements during the machining process. This will cause a large amount of waste during the machining process, which not only increases the production cost, but also affects the production efficiency and product quality; at the same time, it also causes the meter weight of the 2026 aluminum alloy profile to increase, increasing the material cost and transportation cost. In the context of energy shortages and increasingly stringent environmental protection requirements, reducing the weight of materials and improving material utilization has become an urgent need. Summary of the invention

[0004] In order to solve the above technical problems, the present invention provides a 2 series aluminum alloy profile and a preparation method thereof. By controlling the alloy composition, optimizing the preparation process, and adjusting the mold hole layout of the mold, effective control of the coarse grain layer is achieved, so that the profile has a thin coarse grain layer and low weight per unit area.

[0005] The technical solution of the present invention is as follows:

[0006] The invention provides a 2-series aluminum alloy profile, comprising the following alloy components in percentage by mass: Si≤0.05%, Fe≤0.06%, Cu 3.7%-4.1%, Mg 1.2%-1.4%, Mn≤0.4%, Cr≤0.02%, Ti 0.02%-0.06%, Zr0.08%-0.12%, the remainder being Al and unavoidable impurities, and the total impurity content being less than 0.15%.

[0007] The present invention also provides a method for preparing a 2-series aluminum alloy profile, comprising the steps of:

[0008] (1) Melting and casting: the materials are prepared according to the alloy composition and proportion, and aluminum alloy ingots are obtained by melting and casting, and then the car body is made;

[0009] (2) homogenization treatment, subjecting the aluminum alloy ingot to multi-stage homogenization treatment, and keeping the aluminum alloy ingot at 380-400° C. after homogenization;

[0010] (3) extrusion, wherein the aluminum alloy ingot which has been homogenized and is in a heat-insulating state is reversely extruded to obtain an extruded profile;

[0011] (4) Pre-stretching: After the extrusion is completed, pre-stretching is performed to obtain a primary aluminum alloy profile;

[0012] (5) Solution heat treatment: the aluminum alloy profile is subjected to solution heat treatment and spray quenching after heat preservation;

[0013] (6) Stretching and straightening: stretching and straightening the aluminum alloy profile after solution treatment;

[0014] (7) Aging: natural aging is used to obtain a 2 series aluminum alloy profile product.

[0015] Furthermore, the multi-stage homogenization treatment process in step (2) is specifically as follows: first, the temperature is increased to 300±10°C at a heating rate of 15-50°C / h, then the temperature is increased to 420±10°C at a heating rate of 20-40°C / h, and the temperature is kept at that temperature for 6-10h; then the temperature is increased to 495±10°C at a heating rate of 40-50°C / h, and the temperature is kept at that temperature for 20-30h; finally, the furnace is cooled to 380-400°C and the temperature is kept at that temperature.

[0016] Furthermore, the extrusion process in step (3) is specifically as follows: the extrusion barrel setting temperature is 400-420° C., the mold heating temperature is 360-410° C., and the extrusion speed is 0.15-0.60 mm / s.

[0017] Furthermore, the corners of the die used for extrusion in step (3) are chamfered.

[0018] Furthermore, the pre-stretching deformation in step (4) is 0.6% to 1.2%.

[0019] Furthermore, the solution heat treatment process in step (5) is specifically as follows: the solution treatment temperature is 489-499° C., and the holding time is 90-150 min.

[0020] Furthermore, the deformation amount of the stretching and straightening in the step (6) is 1% to 2%.

[0021] Furthermore, the natural aging time in step (7) is at least 96 hours.

[0022] The beneficial effects of the present invention are:

[0023] (1) The present invention provides a 2-series aluminum alloy profile and a preparation method thereof, which adopts a multi-stage homogenization treatment, specifically a three-stage high-temperature heating and a one-stage furnace cooling and heat preservation, and extrusion is directly performed after heat preservation; the three-stage heating and heat preservation can reduce the amount of coarse second phases, and when the homogenization temperature is 495°C, the coarse undissolved second phases can be basically eliminated; in addition, currently the ingot is usually air-cooled to room temperature after homogenization, and then heated and extruded. The coarse second phase is easily precipitated from the ingot during the air cooling process, which is not conducive to subsequent extrusion. The present invention directly extrudes the ingot after homogenization and heat preservation, which can reduce the precipitation of the second phase, and can also promote the precipitation of dispersed and fine Al3Zr particles in the alloy during the furnace cooling stage and the subsequent extrusion process. The Al3Zr particles can pin the grain boundaries in the subsequent hot processing link of the profile, hinder the abnormal recrystallization behavior of the grains inside the profile, and thus prevent the profile from producing an excessively heavy coarse grain layer;

[0024] (2) The present invention also optimizes the extrusion die, specifically, chamfering the corners of the die; the coarse grains at the corners of the 2 series aluminum alloy profiles extruded by the optimized die are basically eliminated, which reduces the reserved processing allowance, that is, reduces the cross-sectional area of ​​the profile, reduces the weight of the profile, and greatly reduces the waste in the machining process; and the microstructure of the profile after using the die is more slender than the microstructure of the original die, the extruded metal streamline is more obvious, and the comprehensive performance is significantly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0026] Figure 1 It is a process flow chart of the present invention.

[0027] Figure 2 This is a schematic cross-sectional view of an aluminum alloy profile prepared in Example 1 of the present invention;

[0028] Figure 3 A schematic cross-sectional view of an aluminum alloy profile prepared in Comparative Example 1 of the present invention;

[0029] Figure 4 This is a metallographic structure photograph of the aluminum alloy profile prepared in Example 1 of the present invention;

[0030] Figure 5 This is a metallographic structure photograph of the aluminum alloy profile prepared in Comparative Example 3 of the present invention. DETAILED DESCRIPTION

[0031] In order to enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0032] The invention provides a 2-series aluminum alloy profile, comprising the following alloy components in percentage by mass: Si≤0.05%, Fe≤0.06%, Cu 3.7%-4.1%, Mg 1.2%-1.4%, Mn≤0.4%, Cr≤0.02%, Ti 0.02%-0.06%, Zr0.08%-0.12%, the remainder being Al and unavoidable impurities, and the total impurity content being less than 0.15%.

[0033] The present invention mainly adds Zr element on the basis of the alloy composition of the existing 2 series aluminum alloy. A part of the Zr element can be dissolved in the matrix, and the other part of the Zr element is mainly used to form Al3Zr particles. The Al3Zr particles can pin the grain boundaries in the subsequent hot working link of the profile, hinder the abnormal recrystallization behavior inside the grains of the profile, and thus prevent the profile from producing an excessively heavy coarse-grained layer.

[0034] The present invention also provides a method for preparing a 2-series aluminum alloy profile, comprising the steps of:

[0035] (1) Melting and casting: the materials are prepared according to the alloy composition and proportion, and aluminum alloy ingots are obtained by melting and casting, and then the car body is made;

[0036] (2) homogenization treatment, the aluminum alloy ingot is subjected to multi-stage homogenization treatment, and the multi-stage homogenization treatment process is specifically as follows: first, the temperature is raised to 300±10°C at a heating rate of 15-50°C / h, then the temperature is raised to 420±10°C at a heating rate of 20-40°C / h, and the temperature is kept at that temperature for 6-10h; then the temperature is raised to 495±10°C at a heating rate of 40-50°C / h, and the temperature is kept at that temperature for 20-30h; finally, the furnace is cooled to 380-400°C and the temperature is kept at that temperature;

[0037] (3) extrusion, wherein the aluminum alloy ingot in a heat-insulating state after homogenization is subjected to reverse extrusion, and the corners of the die used for extrusion are chamfered, the extrusion barrel is set at a temperature of 400 to 420° C., the die heating temperature is 360 to 410° C., and the extrusion speed is 0.15 to 0.60 mm / s to obtain an extruded profile;

[0038] (4) Pre-stretching: after extrusion is completed, pre-stretching with a deformation amount of 0.6% to 1.2% is performed to obtain a primary aluminum alloy profile;

[0039] (5) Solution heat treatment: the aluminum alloy profile is subjected to solution heat treatment at a temperature of 489 to 499°C for a holding time of 90 to 150 minutes, and spray quenching is performed after the holding is completed;

[0040] (6) Tensile straightening: after solution treatment, the aluminum alloy profile is subjected to tensile straightening with a deformation of 1% to 2%;

[0041] (7) Aging: Aging is carried out by natural aging, and the natural aging time is at least 96 hours to obtain a 2 series aluminum alloy profile product.

[0042] In the above-mentioned homogenization treatment, the present invention adopts a multi-stage homogenization treatment, specifically three-stage high-temperature heating and one-stage furnace cooling and heat preservation, and extrusion is directly performed after heat preservation. First, the three-stage heating and heat preservation can reduce the number of coarse second phases. When the homogenization temperature is 495°C, the coarse undissolved second phase can be basically eliminated; secondly, the ingot is usually air-cooled to room temperature after homogenization, and then heated and extruded. The ingot is prone to precipitate coarse second phases during air cooling, which is not conducive to subsequent extrusion. The present invention directly extrudes after the ingot is homogenized and heat-insulated, which can reduce the precipitation of the second phase, and can also promote the precipitation of dispersed and fine Al3Zr particles inside the alloy during the furnace cooling stage and the subsequent extrusion process. Al3Zr particles can pin the grain boundaries in the subsequent extrusion processing of the profile, hinder the abnormal recrystallization behavior of the grains inside the profile, and thus prevent the profile from producing an excessively heavy coarse grain layer.

[0043] In the above extrusion process, the present invention also optimizes the extrusion die in advance, specifically, the corners of the die are chamfered. The coarse grains at the corners of the 2-series aluminum alloy profiles extruded by the optimized die are basically eliminated, which reduces the reserved processing allowance, that is, reduces the cross-sectional area of ​​the profile, reduces the weight of the profile per meter, and greatly reduces the waste in the machining process; and the microstructure of the profile after using the die is more slender than the microstructure of the original die, the extruded metal streamline is more obvious, and the comprehensive performance is significantly improved.

[0044] Example 1

[0045] Reference Figure 1 This embodiment provides a 2-series aluminum alloy profile and a preparation method thereof, comprising the steps of:

[0046] (1) Melting and casting: the alloy composition and proportion are adjusted according to the following: Si 0.029%, Fe 0.038%, Cu 3.91%, Mg 1.31%, Mn 0.30%, Cr 0.021%, Ti 0.046%, Zr 0.109%, and an ingot with a diameter of 600 mm is produced by semi-continuous casting, and then the ingot is rolled;

[0047] (2) Homogenization treatment: The aluminum alloy ingot is subjected to multi-stage homogenization treatment, firstly, the temperature is raised to 300°C at a heating rate of 40°C / h, then the temperature is slowly raised to 420°C at a heating rate of 20°C / h, and the temperature is kept at this temperature for 8 hours; then the temperature is raised to 495°C at a heating rate of 40°C / h, and the temperature is kept at this temperature for 24 hours, and finally the temperature is cooled to 390°C with the furnace and the temperature is kept at this temperature;

[0048] (3) extrusion, the aluminum alloy ingot in a heat-insulating state after homogenization is reversely extruded, and the corners of the die used for extrusion are chamfered, the extrusion barrel is set at 415° C., the die heating temperature is 370° C., and the extrusion speed is 0.3 mm / s to obtain an extruded profile;

[0049] (4) Pre-stretching: after extrusion is completed, pre-stretching with a deformation amount of 1.0% is performed to obtain a primary aluminum alloy profile;

[0050] (5) Solution heat treatment: the aluminum alloy profile is subjected to solution heat treatment at a temperature of 495°C for a holding time of 90 min, and spray quenching is performed after the holding is completed;

[0051] (6) Tensile straightening: after solution treatment, the aluminum alloy profile is subjected to tensile straightening with a deformation of 1%;

[0052] (7) Aging: natural aging is used for aging, and the natural aging time is 96 hours to obtain a 2 series aluminum alloy profile product, and its state is T3511.

[0053] Example 2

[0054] This embodiment provides a 2-series aluminum alloy profile and a preparation method thereof, comprising the steps of:

[0055] (1) Melting and casting: the alloy composition and proportion are prepared according to the following: Si 0.042%, Fe 0.044%, Cu 3.85%, Mg 1.26%, Mn 0.34%, Cr 0.009%, Ti 0.034%, Zr 0.09%, and an ingot with a diameter of 600 mm is produced by semi-continuous casting, and then the ingot is rolled;

[0056] (2) Homogenization treatment: The aluminum alloy ingot is subjected to multi-stage homogenization treatment. The temperature is first increased to 300°C at a heating rate of 30°C / h, and then slowly increased to 420°C at a heating rate of 30°C / h, and kept at this temperature for 8 hours; then the temperature is increased to 495°C at a heating rate of 45°C / h, and kept at this temperature for 24 hours. Finally, the aluminum alloy ingot is cooled to 380°C with the furnace and kept at this temperature;

[0057] (3) extrusion, the aluminum alloy ingot in a heat-insulating state after homogenization is reversely extruded, and the corners of the die used for extrusion are chamfered, the extrusion barrel is set at 410° C., the die heating temperature is 380° C., and the extrusion speed is 0.2 mm / s to obtain an extruded profile;

[0058] (4) Pre-stretching: after extrusion is completed, pre-stretching with a deformation amount of 1.3% is performed to obtain a primary aluminum alloy profile;

[0059] (5) Solution heat treatment: the aluminum alloy profile is subjected to solution heat treatment at a temperature of 495°C for a holding time of 100 min, and spray quenching is performed after the holding is completed;

[0060] (6) Tensile straightening: after solution treatment, the aluminum alloy profile is subjected to tensile straightening with a deformation of 1.3%;

[0061] (7) Aging: natural aging is used for aging, and the natural aging time is 96 hours to obtain a 2 series aluminum alloy profile product, and its state is T3511.

[0062] Example 3

[0063] This embodiment provides a 2-series aluminum alloy profile and a preparation method thereof, comprising the steps of:

[0064] (1) Melting and casting: the ingredients are prepared according to the alloy composition and proportion: Si 0.033%, Fe 0.043%, Cu 4.01%, Mg 1.33%, Mn 0.33%, Cr 0.009%, Ti 0.051%, Zr 0.11%, and the ingot with a diameter of 600 mm is produced by semi-continuous casting, and then the ingot is rolled;

[0065] (2) Homogenization treatment: The aluminum alloy ingot is subjected to multi-stage homogenization treatment. The temperature is first increased to 300°C at a heating rate of 20°C / h, then slowly increased to 420°C at a heating rate of 25°C / h, and kept at this temperature for 8 hours; then the temperature is increased to 495°C at a heating rate of 50°C / h, and kept at this temperature for 24 hours. Finally, the aluminum alloy ingot is cooled to 385°C with the furnace and kept at this temperature;

[0066] (3) extrusion, the aluminum alloy ingot in a heat-insulating state after homogenization is reversely extruded, and the corners of the die used for extrusion are chamfered, the extrusion barrel is set at a temperature of 420° C., the die heating temperature is 380° C., and the extrusion speed is 0.4 mm / s to obtain an extruded profile;

[0067] (4) Pre-stretching: after extrusion is completed, pre-stretching with a deformation amount of 1.5% is performed to obtain a primary aluminum alloy profile;

[0068] (5) Solution heat treatment: the aluminum alloy profile is subjected to solution heat treatment at a temperature of 494°C for a holding time of 120 min, and spray quenching is performed after the holding is completed;

[0069] (6) Tensile straightening: after solution treatment, the aluminum alloy profile is subjected to tensile straightening with a deformation of 1.5%;

[0070] (7) Aging: natural aging is used for aging, and the natural aging time is 96 hours to obtain a 2 series aluminum alloy profile product, and its state is T3511.

[0071] Comparative Example 1

[0072] The difference between this comparative example and Example 1 is that the homogenization process in this comparative example is: 420℃*8h+493℃*24h, and after the homogenization treatment, it is cooled to room temperature, and then the ingot is heated to 390℃ for extrusion; and the corners of the mold used in this comparative example are not chamfered, and the rest is the same as Example 1.

[0073] Comparative Example 2

[0074] The difference between this comparative example and Example 1 is that in this comparative example, the ingot is cooled to room temperature after multi-stage homogenization treatment, and then heated to 390° C. for extrusion. The rest is the same as Example 1.

[0075] Comparative Example 3

[0076] The difference between this comparative example and Example 1 is that the corners of the mold used in this comparative example are not chamfered, and the rest are the same as Example 1.

[0077] The 2 series aluminum alloy profiles prepared in the above-mentioned Examples 1-3 and Comparative Examples 1-3 were sampled and tested, and the coarse grain layer thickness and meter weight thereof were shown in Table 1 below.

[0078] Table 1

[0079]

[0080] It can be seen from Table 1 above that the aluminum alloy profiles prepared in Examples 1-3 have a thin coarse-grained layer and a low weight per meter, while compared with Example 1, the aluminum alloy profiles prepared in Comparative Examples 1-3 have a thick coarse-grained layer and a heavy weight per meter.

[0081] In addition, it should be noted that the cross-section of the profiles produced in the above-mentioned embodiment 1 and comparative examples 1-3 is the same, and the customer requirements are the same. The difference in weight per meter between embodiment 1 and comparative examples 1-3 is large because the thickness of the coarse crystal layer is different; the cross-section of the profiles produced in the above-mentioned embodiments 1-3 is the same, but the customer requirements are different. Therefore, in embodiments 1-3, the thickness of the coarse crystal layer is basically the same, but the weight per meter is very different. This is because the customer requirements are different, and the processing allowances designed for the molds used in the extrusion process are different. The larger the processing allowance, the greater the weight per meter.

[0082] The cross-sectional schematic diagrams of the profiles of the above-mentioned embodiment 1 and comparative example 1 are respectively as follows: Figure 2 , Figure 3 As shown in the figure, the section line area is the actual required profile size, and the area outside the section line is the machining allowance area, which needs to be removed during the machining stage; the coarse grain layer is mainly located at the corner position, and the thickness of the coarse grain layer detected in Table 1 is Figure 2 , Figure 3 The thickness of the coarse-grained layer at the corner is shown. Compared with Example 1, the coarse-grained layer of Comparative Example 1 is thick, and a larger machining allowance needs to be designed, which increases the waste in the machining process.

[0083] In addition, the profiles prepared in Example 1 and Comparative Example 3 were sampled and their metallographic structures were observed. Figure 4 , Figure 5 As shown. Figure 4 , Figure 5 It can be seen that the microstructure of the profile produced by the improved mold in Example 1 is more slender than that of the original mold.

[0084] Although the present invention has been described in detail by reference to the preferred embodiments, the present invention is not limited thereto. Without departing from the spirit and essence of the present invention, a person of ordinary skill in the art may make various equivalent modifications or substitutions to the embodiments of the present invention, and these modifications or substitutions shall be within the scope of the present invention. Any person of ordinary skill in the art may easily think of changes or substitutions within the technical scope disclosed by the present invention, and these shall be within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be subject to the scope of protection of the claims.

Claims

1. A 2 series aluminum alloy profile, characterized in that: The alloy comprises the following alloy components in percentage by mass: Si≤0.05%, Fe≤0.06%, Cu 3.7%-4.1%, Mg 1.2%-1.4%, Mn≤0.4%, Cr≤0.02%, Ti 0.02%-0.06%, Zr 0.08%-0.12%, and the remainder is Al and unavoidable impurities, and the total impurity content is less than 0.15%.

2. The method for preparing a 2-series aluminum alloy profile according to claim 1, characterized in that: Includes steps: (1) Melting and casting: the materials are prepared according to the alloy composition and proportion, and aluminum alloy ingots are obtained by melting and casting, and then the car body is made; (2) homogenization treatment, subjecting the aluminum alloy ingot to multi-stage homogenization treatment, and keeping the aluminum alloy ingot at 380-400° C. after homogenization; (3) extrusion, reversely extruding the aluminum alloy ingot in a heat-insulating state after homogenization treatment to obtain an extruded profile; (4) Pre-stretching: After the extrusion is completed, pre-stretching is performed to obtain a preliminary aluminum alloy profile; (5) Solution heat treatment: the aluminum alloy profile is subjected to solution heat treatment and spray quenching after heat preservation; (6) Tensile straightening: stretching and straightening the aluminum alloy profile after solution treatment; (7) Aging: natural aging is used to obtain a 2 series aluminum alloy profile product.

3. The method for preparing a 2-series aluminum alloy profile according to claim 2, characterized in that: The multi-stage homogenization treatment process in step (2) is specifically as follows: first, the temperature is raised to 300±10°C at a heating rate of 15-50°C / h, then the temperature is raised to 420±10°C at a heating rate of 20-40°C / h, and the temperature is kept at that temperature for 6-10h; then the temperature is raised to 495±10°C at a heating rate of 40-50°C / h, and the temperature is kept at that temperature for 20-30h; finally, the furnace is cooled to 380-400°C, and the temperature is kept at that temperature.

4. The method for preparing a 2-series aluminum alloy profile according to claim 2, characterized in that: The extrusion process in step (3) is specifically as follows: the extrusion barrel temperature is set at 400-420° C., the mold heating temperature is 360-410° C., and the extrusion speed is 0.15-0.60 mm / s.

5. The method for preparing a 2-series aluminum alloy profile according to claim 2, characterized in that: The corners of the die used for extrusion in step (3) are chamfered.

6. The method for preparing a 2-series aluminum alloy profile according to claim 2, characterized in that: The pre-stretching deformation in step (4) is 0.6% to 1.2%.

7. The method for preparing a 2-series aluminum alloy profile according to claim 2, characterized in that: The specific solution heat treatment process in step (5) is as follows: the solution treatment temperature is 489-499° C., and the insulation time is 90-150 min.

8. The method for preparing a 2-series aluminum alloy profile according to claim 2, characterized in that: The deformation amount of the stretching and straightening in the step (6) is 1% to 2%.

9. The method for preparing a 2-series aluminum alloy profile according to claim 2, characterized in that: The natural aging time in step (7) is at least 96 hours.

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