# Patent

## Title
Methods of making a bonded sheath cable

## Abstract

This is a division of application Ser. No. 173,529, filed July 30, 1980 Pat. No. 4,410,370.

## Background
This invention relates to an aircraft stringer material and a method for producing the same.

Conventionally, AA7075 alloy is well known as a typical raw material for an aircraft stringer and has had widespread use in the aircraft field. Generally, the alloy is fabricated into the aircraft stringer by the following process.

The, AA7075 alloy ingot is homogenized by heating at about 460.degree. C. for 16 to 24 hours and hot rolled at 400.degree. C. to provide a plate approximately 4 to 8 mm thick. This plate is then intermediate annealed at around 410.degree. C. for 1 hour, cooled at a cooling rate not exceeding 30.degree. C./hr. and cold rolled to a plate of 2 to 4 mm in thickness. The cold rolled plate is annealed by heating to a temperature of 410.degree. C. for 8-12 hours and holding the temperature for about one to two hours. Further, the annealed plate is cooled to a temperature of approximately 230.degree. C. at a cooling rate of 30.degree. C./hr. or less, maintained at the temperature for 6 hours and air cooled to produce the primary stringer material.

Further the primary stringer material is subjected to a stepped cold working at various cold reductions ranging from 0 to 90%, and subsequently to a solution heat treatment, providing the secondary stringer material. The solution heat treatment may also be carried out without the stepped cold working.

Generally, the aforesaid primary material is known as O-material, while the secondary material as W-material. Thus, hereinafter the former is referred to as "O-material" and the later to as "W-material". W-material is formed into the desired shape, such as hat-shape, Z-shape or J-shape, by section roll forming and the treated W-material is subjected to a T6 temper treatment and, if required, further chemical milling to provide the aircraft stringer and stringer frame.

However, the conventional stringer materials have, for example, the following disadvantages:

The O-material produced from AA7075 alloy according to the above conventional method has a large grain size of 150-250 .mu.m and if the O-material is subjected to cold working with a relatively small amount of cold reduction, such as cold reduction of 10-30%, and then to the solution heat treatment, its grain size further grows, resulting in the reduction of mechanical properties and presenting many serious problems in the subsequent processes. Particularly, cold reduction of 20% is known to cause the marked grain growth.

Such considerable grain growth in the parts which are cold worked with a small amount of cold reduction causes reduction of elongation and fracture toughness, and leads to an undesirable orange peel appearance and occurence of cracks during section roll forming. Further, when material having a large grain size exceeding 100 .mu.m is subjected to a conventional chemical milling, which is practiced in the production of the aircraft stringer, the surface roughness increases remarkably and the fatigue strength is lowered. Hence, in case of the small amount of cold reduction, the production of the stringer is not only very difficult, but also the properties of the product are not satisfactory. In case a relatively large amount of cold reduction of more than 50% is followed by the solution heat treatment, it is possible to make the grain size approximately 50 .mu.m in the material. However, in practice, cold rolling reduction of a wide range of 0 to 90% is conducted on one piece of O-material of about 10 m in length so that it is extremely difficult to achieve a grain size not exceeding 100 .mu.m over the entire length.

Therefore, an O-material which can develop a fine grain size not exceeding 100 .mu.m in the W-material over the above wide range of cold reduction has been requested.

Still further, stringer materials prepared from a conventional Al-Zn-Mg-Cu alloy, such as AA7075 alloy, are generally inferior in corrosion-resistance, and thus highly improved corrosion-resistance is desired.

The primary object of the present invention is to provide an improved unclad or clad material useful for an aircraft stringer or stringer frame and a method for producing the same wherein the above-mentioned disadvantages encountered in the conventional practice are eliminated.

Another object of the present invention is to provide the light aircraft stringer or stringer frame having highly improved mechanical properties and corrosion-resistance.

According to the present invention, because the grain growth in O-material occurring during the solution treatment after cold working is very slight, even in the case of a relatively small amount of cold reduction such as approximately 30% reduction or less, the stringer and the stringer frame material (hereinafter stringer, and stringer material mean stringer and stringer frame, and stringer and stringer frame materials respectively, unless otherwise stated,) having grain sizes of 100 .mu.m and less over their entire length can be readily obtained after solution heat treatment. This imparts a high degree of workability strength and fracture toughness to the stringer material.

The unclad O- and W-materials contemplated by the present invention are 100 .mu.m and less in grain size and are made of an aluminum base alloy consisting essentially of 5.1 to 8.1 wt.% Zn, 1.8 to 3.4 wt.% Mg, 1.2 to 2.6 wt.% Cu, up to 0.20 wt.% Ti and at least one of 0.18 to 0.35 wt.% Cr and 0.05 to 0.25 wt.% Zr, the balance being aluminum and incidental impurities.

In order to produce the O-material, the above aluminum base alloy is homogenized, hot rolled and cold rolled to a given thickness as in the conventional method. The cold rolled alloy is then annealed by rapid heating to a temperature of 320.degree. to 500.degree. C. at a heating rate of more than 11.degree. C./min. and then cooled. In this annealing step, if the succeeding cooling is performed at a cooling rate of 30.degree. C./hour and upward, the material may be further reheated to 200.degree. to 500.degree. C. and cooled at the following cooling rate. The case of a reheating temperature of 200.degree. to less than 350.degree. C., the cooling is performed at a cooling rate of 30.degree. C./hour and less, or by air-cooling, and in the case of a reheating temperature of 350.degree. to 500.degree. C., it is done at a rate not exceeding 30.degree. C./hour. Such reheating procedure is particularly effective to make O-material highly workable.

The thus obtained O-material can to receive a stepped cold working to a 0 to 90% cold reduction in thickness without degradation of mechanical properties and a W-material having a fine grain size not exceeding 100 .mu.m can be obtained from this O-material after solution heat treatment.

According to the present invention, a clad stringer material further having excellent corrosion-resistance in addition to the above advantages can be also prepared in a process similar to the above described process using the above specified aluminum base alloy as a core material and AA7072 alloy as a cladding material. In the clad O- and W-material, the grain sizes of the cores do not exceed 100 .mu.m throughout the cores. AA7072 alloy has a high corrosion-resistance and serves to prevent the core alloy material from corrosion, resulting in an aircraft stringer having a remarkably improved corrosion-resistance. In order to produce the clad material, AA7072 alloy is clad onto the one or both sides of the above aluminum alloy core layer in the manner described hereinafter at the hot rolling step and the production proceeds as with unclad material described above to provide clad O-materials and further W-materials. The thus resulting materials, O- and W-materials, have an extremely high degree of corrosion-resistance, since the cladding layer of 7072 alloy serves as a protective layer and prevents corrosion of the core layer.

The above unclad and clad materials can also receive a normal chemical milling without reduction of fatigue strength.

The above and further objects, features and advantages of the present invention will be more apparent from the following description and the accompanying drawings.

FIG. 1 is a partial perspective view of the inside of an aircraft body.

FIGS. 2, 3 and 4 are sectional views which exemplify the shapes of aircraft stringers.

FIG. 5 is a perspective view showing the state of cold working of stringer material.

FIG. 6 is an enlarged schematic view illustrating the relationship between cold reduction by cold working and grain size after solution treatment for conventional stringer material.

FIG. 7 is a graph showing the relationship between tensile strength of unclad O-material or grain size of unclad W-material and reheating temperature.

FIG. 8 is a graph showing the relationship between tensile strength of clad O-material or grain size of the core of clad W-material and reheating temperature.

In practicing the present invention, the composition limit of the aluminum base alloy described in the preceding paragrahs must be closely followed in order to achieve the objects contemplated by the invention. The reason for the limitation of each component of the alloy according to the present invention is as follows: (hereinafter referred to as "T6-material") after the T6 type heat treatment does not reach the required level. On the other hand, when the content exceeds 8.1 net%, fracture resistance of the T6-material decreases and stress corrosion cracking is apt to occur. after the T6 type heat treatment is low, and, if the content exceeds 3.4 wt.%, cold-workability of the annealed material does not reach the required level. Further the fracture resistance of the T6-material decreases. and a content of more than 2.6 wt.% lowers the fracture resistance of the T6-material. cracking of the ingot during grain refinement of cast structures. However, the addition of more than 0.20 wt.% leads to formation of giant intermetallic compounds. the other hand, a content of more than 0.35 wt.% leads to formation of giant intermetallic compounds and decreased fracture resistance of the T6-material. prevent stress corrosion cracking and to refine the grain size. If the content is less than 0.05 wt.%, the above effect is insufficient an if it exceeds 0.25 wt.%, giant intermetallic compounds are formed. Formation of giant intermetallic compounds should be avoided. As incidental impurities, Fe, Si and Mn must be restricted as follows: content exceeds 0.50 wt.%, the amount of insoluble compounds increases in the alloy, lowering the fracture resistance of the T6-material. more than 0.40 wt.% increases the amount of insoluble compounds in the alloy, leading to lowering of the fracture resistance of the T6-material. material. It its content exceeds 0.70 wt.%, sufficient quench sensitivity and fracture toughness can not be attained.

In order to prepare unclad aircraft stringer materials, such as unclad O-material and W-material, using the aluminum alloy according to the present invention, the alloy ingot is homogenized at a temperature of 400.degree. to 490.degree. C. for 2 to 48 hours so that the main additional elements like Zn, Mg and Cu may fully dissolve and, at the same time, Cr and Zr may precipitate as a fine intermetallic compound. If homogenization is insufficient, due to an inadequate heating temperature or insufficient heating time, hot-workability of the aluminum base alloy ingot and resistance to stress corrosion cracks will decrease and, further, grain growth will occur. On the other hand, when the heating temperature for the homogenizing treatment exceeds 490.degree. C., eutectic melting occurs. Hence, the heating temperature for homogenizing is preferably limited to a temperature not exceeding 490.degree. C.

Hot rolling after the homogenizing treatment is preferably initiated from a starting temperature of 350.degree. to 470.degree. C. A starting temperature of less than 350.degree. C. makes the deformation resistance of the material high and hot rolling is very difficult, while a starting temperature of more than 470.degree. C. reduces the hot-workability of the alloy. Thus, it is preferable to set the initial temperature within the above range.

Following the above hot rolling, an annealing treatment may, if desired, be performed. This treatment is performed by holding the hot rolled material at a temperature of 300.degree. to 460.degree. C. and then cooling it to a temperature of approximately 260.degree. C. at a cooling rate not exceeding 30.degree. C./hr. This annealing step is particularly needed when the degree of the cold rolling reduction in the subsequent cold working is high.

The cold rolling reduction in the cold rolling operation is preferably 20% or more, since, when the rolling reduction is low, the grain size of the resultant stringer O-material and W-material grows to more than 100 .mu.m.

Cold rolled material is further subjected to annealing characterized by rapid heating to a temperature of from 320.degree. C. to 500.degree. C. at a heating rate of more than 11.degree. C./min., and this process is especially useful in producing a high quality and coilformed stringer material.

Conventional annealing of an aluminum base alloy, such as the AA 7075 alloy, has been accomplished by heating to a temperature of 413.degree. to 454.degree. C., holding this temperature for two hours, air cooling, reheating to a temperature of 232.degree. C., holding this temperature for six hours and finally cooling to room temperature. This annealing procedure is proposed in MIL Spec. H6088E. item 5.2.7.2 and has been well known as the most normal annealing method for the 7075 alloy in the aircraft field. Thus, the above annealing process according to the present invention will be found to exceed the above common knowledge.

If the heating temperature exceeds 500.degree. C., the material melts and marked grain growth occurs. On the other hand, when the heating temperature is below 320.degree. C., annealing and recrystallization of the material are insufficient. In producing the aircraft stringer, since such phenomenon causes cracks on the stepped cold working or taper rolling work, such phenomenon should be avoided. It was found that only the above range of heating temperatures, 320.degree. to 500.degree. C., enables the production of a stringer material having fine grain sizes not exceeding 100 .mu.m.

With regard to a heating rate to achieve the above high temperature, 320.degree. C. to 500.degree. C., when rapidly heating at a heating rate of more than 11.degree. C./min., precipitation of Mg-Zn type compounds is very slight and dislocation structure introduced by the cold rolling will be changed to a uniformly fine cell structure by the above annealing treatment including the rapid heating step. When the thus obtained O-material is subjected to the stepped cold working or the taper rolling work with a comparatively small rolling reduction (10 to 30%) and then to the solution heat treatment, such cell structure serves as a nucleation site of recrystallization and develops a uniformly fine recrystallized grain structure. On the other hand, if, in the annealing process, the heating rate is 11.degree. C./min. or less, Mg-Zn type compounds precipitate unevenly during heating to a given annealing temperature, and the dislocation structure formed during the preceding cold rolling step will disappear completely or remain a coarse and nonuniform cell structure. If the thus annealed material receives the cold working with the above comparatively small reduction and then the solution heat treatment, the recrystallized grain becomes coarse so that a uniform and fine recrystallized grain structure can not be obtained.

Referring to a cooling rate after the above rapid heating, a cooling rate below 30.degree. C./hour can achieve a complete O-material and impart a high degree of cold workability. Thus such cooling makes possible a taper rolling reduction of wide range, up to 90% at a time.

On the other hand, if the cooling rate is 30.degree. C./hour or more, the material is hardened, that is, age-hardened, and so an O-material having a higher strength relative to that of usual O-material is obtained. Thus, such rapid cooling does not matter when the O-materials are to be used to make stringer materials which are cold worked to a comparatively small amount of cold reduction. However, the rapid cooling is undesirable for O-materials which are to be subjected to a large amount of cold reduction. For this, further study was conducted and the following thermal treatment was found to overcome the above problem.

In practicing the annealing, when the cooling rate is 30.degree. C./hr. or more, the annealing process is performed by a two-stage thermal treatment. The first stage of thermal treatment is performed by rapidly heating the cold rolled material to 320.degree. to 500.degree. C. at the rate exceeding 11.degree. C./min., as described above, and cooling at a rate of 30.degree. C./hour or more. Following the first state of thermal treatment, the material is subjected to the second stage of thermal treatment.

The second stage of thermal treatment is performed by reheating to a temperature within the range of 200.degree. to 500.degree. C. and subsequently cooling at a rate depending on the reheating temperature. When the reheating temperature range is 200.degree. C. to less than 350.degree. C., cooling is achieved at a cooling rate not exceeding 30.degree. C./hour or air cooling. On the other hand, when the reheating temperature ranges between 350.degree. C. and 500.degree. C., the cooling rate is 30.degree. C./hr. and less. In case the reheating temperature of the second stage of heat treatment is the above comparatively low temperature of from 200.degree. to less than 350.degree. C., air cooling or cooling at the rate not exceeding 30.degree. C./hr. is applicable so that the material may not again age-harden, whereas, in case of a comparatively high reheating temperature between 350.degree. and 500.degree. C., the cooling rate is 30.degree. C./hr. and less. By adding the above reheating step to the first rapid heating step, high workability can be attained, even if the cooling rate after the first rapid heating is 30.degree. C./hr. or more.

By using the above composition and process, unclad O-material having a very high degree of workability can be readily obtained and unclad W-material in which the grain size does not exceed 100 .mu.m over the entire length can be prepared from this unclad O-material after the solution heat treatment, regardless of degree of the cold reduction of the stepped cold working. Grain size not exceeding 100 .mu.m eliminates orange peels and cracks during section roll forming and make possible an improved stringer having superior mechanical properties, such as elongation, fracture resistance and excellent chemical milling capability.

In a further embodiment of the present invention, the clad aircraft stringer materials are made of the core material of the above aluminum base alloy consisting essentially of 5.1 to 8.1 wt.% Zn, 1.8 to 3.4 wt.% Mg, 1.2 to 2.6 wt.% Cu, up to 0.20 wt.% Ti and at least one of 0.18 to 0.35 wt.% Cr and 0.05 to 0.25 wt.% Zr, the balance being aluminum and impurities, and the cladding material of AA7072 alloy is made in a way similar to that described for the unclad material.

The cladding raw material of AA7072 alloy ingot is preferably homogenized at a temperature of 400.degree. to 560.degree. C. for 2 to 48 hours so that major alloying elements such as Zn may sufficiently dissolve in the alloy to provide a uniform solid solution and thereafter the treated ingot is rolled to a desired thickness.

Prior to cladding, the cladding material and the core material which has been homogenized in the same manner as in the case of unclad materials are degreased. The degreased cladding material is bonded to at least one side of the degreased core material by welding and hot rolling. A preferable clad rate is 0.05 to 10% per one side of the core.

A clad rate below 0.05% renders hot clad-rolling difficult and the cladding layer is liable to be damaged. Further, satisfacory corrosion-resistance cannot be achieved. On the other hand, a clad rate exceeding 100% is liable to lower the strength of the clad composite material after heat treatment and presents a problem.

Following the above hot clad-rolling, the clad composite is cold rolled, annealed and further stepped cold worked in the same way as with the unclad material to provide the clad O-material and W-material in which the grain sizes of cores do not exceed 100 .mu.m. The thus produced clad stringer materials attain a highly improved corrosion-resistance as well as the advantages described for the unclad stringer materials.

Aircraft stringers are for reinforcement and, as shown with reference numerals 1 and 3 in FIG. 1, are used in the inside of aircraft body 1. FIG. 2, FIG. 3 and FIG. 4 are, respectively, sectional views of the reinforcements, that is, stringers used in the longitudinal direction and stringer frames used in the circumferential direction of the aircraft body 1.

As shown in the respective figures, FIG. 2 shows a hat-shaped stringer, FIG. 3 a Z-shaped stringer and FIG. 4 a J-shaped stringer. These stringers 2 and 3 are prepared by section roll forming W-material into the hat-, Z- or J-shape and subsequently executing the T6 temper treatment. W-material used in the production of stringers is prepared by stepped cold working of the O-material in the longitudinal direction and solution heat treatment. FIG. 5 is to exemplify a state of the stepped cold working. In FIG. 5, A shows a part which has not been cold worked, and B, C and D show parts cold worked to relatively low degree, intermediate degree and relatively high degree, respectively.

Such stepped cold working is required in order to vary the thickness of each part according to the strength required in each part and to reduce the total weight of the aircraft.

As mentioned-above, the O-material according to the present invention can produce the desired unclad W-material having a fine grain size not exceeding 100 .mu.m and the clad O-material can provide the high corrosion-resistant W-materials having fine core grain size not exceeding 100 .mu.m. However, the O-material prepared according to conventional practice can not produce the clad and unclad W-materials contemplated by the present invention. FIG. 6 illustrates a relationship between the reduction by cold working and grain size after solution heat treatment for the conventional W-material. As can be seen in FIG. 6, in parts D, F and G, cold worked with a large amount of cold reduction, the grain size is small, while in parts A, B, C and E, cold worked with a small amount of cold reduction, the grain size is extremely large.

The experiments proved that when the above annealing process is performed by the two-stage thermal treatment, the reheating temperature at the second stage have much has a significant effect on the tensile strength of the unclad and clad O-materials and grain sizes of the unclad W-material and the core of the clad W-material after receiving stepped cold working and solution heat treatment. This effect is demonstrated in FIGS. 7 and 8, which are graphs plotting the tensile strengths (Curves I and III) of unclad and clad O-materials annealed by rapid heating and subsequently reheated to various temperatures, grain size (Curve II) of unclad W-material and grain size (Curve IV) of the core of clad W-material obtained after cold working the respective O-materials to 20% cold reduction, solution heat treating at 470.degree. C. for 40 minutes and then water quenching against reheating temperature in the annealing process. In this measurement, the first stage of thermal treatment in the annealing process was accomplished by rapid heating, air cooling and holding at room temperature. Thus, this treatment give a hardening effect to the material, increasing the tensile strength of the material thus treated. As can be seen from FIGS. 7 and 8, the increased tensile strength decreased with an increase in reheating temperature. The grain sizes or core grain sizes of W-materials which received the above cold working with a 20% reduction, solution heat treatment and water quenching were dependent on the reheating temperature. A reheating temperature of 200.degree. to 350.degree. C. gave comparatively small grain sizes of 25-35 .mu.m, reheating temperature of 350.degree. to 440.degree. C. increased considerably the grain sizes to the range of 35 and 50 .mu.m and a reheating temperature of 440.degree. to 500.degree. C. again decreased grain size to the range of 30-35 .mu.m.

In order to further understand the present invention and the advantages derived therefrom, the following examples are presented. In these example, ingots of the aluminum base alloy having the compositions shown in the following Table 1 were used. ______________________________________ Al- loy  Chemical Composition (wt. %) No.  Si     Fe     Cu  Mn   Mg  Cr   Zn  Ti   Zr   Al ______________________________________ 1    0.16   0.22   1.6 0.03 2.5 0.22 5.7 0.03 --   Re- minder 2    0.09   0.18   1.7 0.01 2.4 0.24 5.8 0.04 --   Re- minder 3    0.14   0.25   1.8 0.03 2.4 0.20 5.8 0.02 --   Re- minder 4    0.10   0.18   1.7 0.02 2.2 0.25 7.0 0.05 0.10 Re- minder 5    0.16   0.24   2.2 0.01 2.8 0.20 6.8 0.04 0.09 Re- minder 6    0.11   0.19   1.8 0.01 2.4 0.21 5.9 0.04 --   Re- minder 7    0.15   0.23   2.2 0.01 2.7 0.01 6.7 0.05 0.15 Re- minder 8    0.11   0.22   1.0 0.02 1.5 0.19 4.5 0.03 --   Re- minder 9    0.13   0.20   2.9 0.03 3.5 0.23 8.5 0.04 --   Re- minder ______________________________________ Note: Nos. 1-7: Alloys according to the present invention Nos. 8-9: Alloys for comparison

O-materials 3 mm thick having grain sizes not exceeding 100 .mu.m according to the present invention and O-materials 3 mm thick having grain sizes of more than 100 .mu.m according to the conventional method were resepectively prepared using ingots, 300 mm in thickness, of alloy Nos. 1 and 4 in with the following procedures.

Procedure according to the present invention:

Homogenization treatment (at 460.degree. C. for 16 hours).fwdarw.Hot rolling (from 300 mm to 6 mm in thickness at 400.degree. C.).fwdarw.Cold rolling (from 6 mm to 3 mm in thickness).fwdarw.Annealing (rapid heating to a temperature of 450.degree. C. at a heating rate of 215.degree. C./min. and holding for 3 minutes at the temperature.fwdarw.cooling at a cooling rate of 5.degree. C./min..fwdarw.reheating at 300.degree. C. for 1 hour.fwdarw.cooling to 200.degree. C. at a cooling rate of 20.degree. C./min.).fwdarw.O-materials according to the present invention.

Procedure according to the conventional method:

Homogenization treatment (at 460.degree. C. for 16 hours).fwdarw.Hot rolling (from 300 mm to 6 mm in thickness at 400.degree. C.).fwdarw.Intermediate annealing (heating at 420.degree. C. for 1 hour.fwdarw.cooling at a rate of 30.degree. C./hr).fwdarw.Cold rolling (from 6 mm to 3 mm in thickness).fwdarw.Annealing (heating to 420.degree. C. at a rate of 0.5.degree.-1.degree. C./min. and holding at 420.degree. C. for 2 hours.fwdarw.cooling at a rate of 25.degree. C./hr.fwdarw.heating at 235.degree. C. for 6 hours.fwdarw.air cooling).fwdarw.O-materials according to the conventional method.

Following the procedures described in Example 1, the respective O-materials were further treated to produce W-materials in the following procedures.

Procedure according to the present invention:

.fwdarw.Cold working (cold reduction of 0-90%, as shown in Table 2).fwdarw.Solution heat treatment (at 470.degree. C. for 40 minutes, using a salt bath).fwdarw.Water quenching.fwdarw.W-materials according to the present invention.

Procedure according to the conventional method:

.fwdarw.Cold working (cold reduction of 0-90%, as shown in Table 2).fwdarw.Solution heat treatment (at 470.degree. C. for 40 minutes, using a salt bath).fwdarw.Water quenching.fwdarw.W-materials according to the conventional method.

Properties of O-materials and W-materials prepared in Examples 1 and 2 were tested and are given in Table 2. __________________________________________________________________________ Surface Grain       Grain                                   Rough- Fatigue Size     Cold Size              Mechanical Properties Fracture ness Strength of       Re- of   Result of    of T6-Material   Resis- after after O-   duc- W-   Bending Test *.sup.1 Yield Tensile Elonga- tance Chemical Chemical Alloy Material tion Material External Occurrence Strength Strength tion (MN .multidot. Milling Milling No. (.mu.m) (%) (.mu.m) Appearance of Crack (Kg/mm.sup.2) (Kg/mm.sup.2) (%)  m.sup.- 3/2) (.mu.m) (Kg/mm.sup.2) __________________________________________________________________________ 1*.sup.2 30   0 30   Good   None  51.1  57.0  16   120  15   18 10 35   "      "     52.0  57.6  14   120  17   18 20 40   "      "     52.6  57.3  14   122  20   17 30 35   "      "     51.4  57.3  14   122  17   18 60 30   "      "     50.6  57.0  16   122  15   18 90 25   "      "     50.7  56.9  16   125  12   18 1*.sup.3 200   0 200  Orange Consider- 50.0  56.3  10    88  100  7.6 peel   able Crack 10 220  Orange Consider- 51.0  56.9   9    80  110  7.2 peel   able Crack 20 280  Orange Consider- 51.2  56.7   8    80  140  7.0 peel   able Crack 30 190  Orange Slight 50.4  56.2  11    87  143  7.0 peel   Crack 60 50   Good   None  49.7  56.5  14   114  25   16 4*.sup.2 25   0 25   "      "     53.1  60.0  17   115  12   18 10 30   "      "     54.0  60.6  15   115  15   18 20 35   "      "     54.6  60.3  14   117  17   18 30 35   "      "     53.4  60.3  15   117  17   18 60 25   "      "     52.6  60.0  16   117  17   18 90 25   "      "     52.7  60.3  16   120  12   18.2 4*.sup.3 180   0 190  Orange Slight 52.2  59.3  10    84  95    8 peel   Crack 10 210  Orange Consider- 52.9  60.0  10    79  100  7.8 peel   able Crack 20 250  Orange Consider- 53.2  59.7   9    79  126  7.4 peel   able Crack 30 180  Orange Slight 52.7  59.7  10    82  90   8.2 peel   Crack 60 50   Good   None  52.0  59.5  14   110  25   15.6 __________________________________________________________________________ Note: *.sup.1 Bending of 90.degree., Bending Radius = 1.5t (t = Thickness of Plate) The test was carried out 4 hours after the water quenching. *.sup.2 Present Invention *.sup.3 Conventional Method

In comparing the present invention and the conventional method, it becomes clear from Table 2 that grain size of the O-material according to the present invention is very fine and can provide a W-material having a fine grain size below 100 .mu.m over a wide range of cold reduction, that is, 0-90%. Such effects eliminated orange peel and the occurrence of cracks during section roll forming and markedly improved the bending properties.

Further, W-materials were subjected to T6 type treatment to produce T6-materials and the mechanical properties were examined. From Table 2, the T6-materials of the present invention proved to be superior to those of the conventional method.

Still further, the respective T6-materials were chemical milled to a 20% reduction in thickness in 10% NaOH aqueous solution and the surface roughness was measured. After chemical milling, a fatigue test was carried out on each T6-material and the result is summarized in Table 2. As can be seen from Table 2, T6-materials according to the present invention are superior to those of the conventional method in surface roughness and fatigue strength, owing to the fine grain size.

Ingots 350 mm thick of alloy No. 1 were homogenized at 470.degree. C. for 24 hours, hot rolled between a starting temperature of 440.degree. C. and a final temperature of 340.degree. C. to provide plates 6 mm thick. Subsequently, the hot rolled plates were cold rolled to provide plates 3 mm thick, and received the following annealing treatment to provide O-materials 3 mm thick. Annealing was accomplished by heating to a temperature of 450.degree. C. at the various heating rates shown in Table 3, holding the temperature for three minutes and cooling at a cooling rate of 25.degree. C./hr.

The O-materials obtained in Example 3 were further cold worked to various cold reductions shown in Table 3, solution heat treated at 470.degree. C. for 40 minutes using the salt bath and water quenched to provide W-materials.

The relation between grain sizes of O- and W-materials and the heating rate in the annealing step is given in Table 3. ______________________________________ Heat-    Grain Size of W-Material Grain Size ing Rate (.mu.m)              of to 450.degree. C. Cold Reduction (%)   O-Material (.degree.C./min) 0      10      20   30     60  (.mu.m) ______________________________________ 1200     30     30      33   30     25  30 400      30     30      35   30     28  30 86       30     30      35   35     30  30 43       30     35      40   40     30  30 21       35     35      45   40     30  35 14       75     85      85   75     35  75 11*     110    130     180  160    40  110 7*      120    150     200  170    45  110 2.4*    200    230     280  220    45  180 0.9*    200    240     300  210    45  180 ______________________________________ Note: Heating rate according to the conventional practice.

As can be seen from Table 3, when the heating rate to 450.degree. C. exceeds 11.degree. C./min., the grain sizes of O-material and W-material are less than 100 .mu.m and fine. On the other hand, when the heating rate is 11.degree. C./min. or less, marked grain growth occurs.

The O-materials which were heated to 450.degree. C. at rates of 86.degree. C./min., 21.degree. C./min., 14.degree. C./min., 11.degree. C./min. and 0.9.degree. C./min. in the annealing step were further tested. The respective O-materials were cold worked to a cold reduction of 0 to 80%, which is practiced in the usual production of aircraft stringers, solution heat treated at 470.degree. C. for 40 minutes using the salt bath and water quenched to produce W-materials.

Following water quenching, the respective W-materials were aged at 120.degree. C. for 24 hours to provide T6-materials. Properties of the W-materials and the T6-materials are given in Table 4. It will be clear from this Table that a heating rate exceeding 11.degree. C./min. is extremely effective in producing desired stringer materials. __________________________________________________________________________ Mechanical Properties Average    Grain Result of Bending Test of T6-Material Heating Cold  Size of of W-Material* Yield Tensile Elonga- Rate Reduction W-Material External Occurrence Strength Strength tion (.degree.C./min) (%)   (.mu.m) Appearance of Crack (Kg/mm.sup.2) (Kg/mm.sup.2) (%) __________________________________________________________________________ 86    0    30    Good   None   51.0  57.1  16 10    30    "      "      52.0  57.6  13 20    35    "      "      52.4  57.9  13 33    35    "      "      50.8  56.8  16 50    30    "      "      50.4  57.3  16 80    25    "      "      50.2  57.7  16 21    0    35    "      "      51.0  57.0  15 10    35    "      "      52.6  57.3  12 20    45    "      "      53.0  57.0  13 33    40    "      "      50.9  56.8  15 50    30    "      "      49.9  57.6  17 80    30    "      "      50.3  57.3  16 14    0    75    "      "      50.9  56.9  15 10    85    "      "      52.0  57.0  13 20    85    "      "      52.5  56.9  12 33    75    "      "      50.6  56.6  15 50    35    "      "      50.0  57.8  16 80    35    "      "      50.1  57.4  16 11    0    110   Orange peel Slight Crack 50.3  56.5  14 10    130   "      "      51.6  56.8  13 20    180   "      "      51.5  57.0  13 33    160   "      "      50.7  56.7  13 50    40    Good   None   50.3  56.5  15 80    40    "      "      50.0  56.0  15 0.9   0    200   Orange peel Considerable 50.0  56.5  10 Crack 10    240   Orange peel Considerable 51.1  57.0   9 Crack 20    300   Orange peel Considerable 51.2  56.8   9 Crack 33    210   Orange peel Considerable 50.8  56.8  10 Crack 50    45    Good   None   50.0  56.9  10 80    40    "      "      49.8  56.0  10 __________________________________________________________________________ Note: *90.degree. Bending, Bending Radius = 1.5t (t = Thickness of Plate) The Test was carried out 4 hours after water quenching.

Cold rolled plates 3 mm thick were prepared using ingots of alloy No. 2 in the same procedure as in the case of Example 3. Following cold rolling, the plates were subjected to the following two stage annealing treatment. In the first stage, the plates were heated to various heating temperatures of 320.degree. to 520.degree. C. at various rates, shown in Table 5, held at the temperatures for times shown in the same Table and cooled at a rate of 5.degree. C./min. After the first heating treatment, the plates were reheated at 300.degree. C. for one hour and cooled at a rate of 20.degree. C./hr, providing O-materials 3 mm thick.

The O-materials obtained in Example 5 were cold worked to various cold reductions, solution heat treated at 470.degree. C. for 40 minutes using the salt bath and water quenched, providing W-materials.

The relation between the grain sizes of O-materials and W-materials and the first stage heating temperature is given in Table 5. It can be seen from Table 5 that only the O-material which has received annealing treatment characterized by rapid heating to 320.degree. to 500.degree. C. can be converted to W-material having fine grain size even after cold working with a small amount of cold reduction and subsequent solution heat treatment. If the heating temperature is beyond the above range, W-material of fine grain size can not be obtained after cold working with a small amount of cold reduction and solution heat treatment. That is, the grain size of the W-material becomes larger than 100 .mu.m. __________________________________________________________________________ Heat- ing                      Grain Average Tem- Hold- Grain Size of   Size of Heating pera- ing  W-Material (.mu.m) O- Rate  ture Time Cold Reduction (%) Material (.degree.C./min) (.degree.C.) (min.) 0  10 20 30 60 80 (.mu.m) __________________________________________________________________________ 230   480  1   30 30 35 35 30 25 30 220   460  3   30 30 35 35 30 25 30 205   430  5   30 35 35 35 30 30 30 198   415 12   35 40 40 40 35 30 35 188   395 10   40 40 45 45 35 30 40 175   370 20   40 45 55 50 35 30 40 150   320 60   40 50 60 60 35 35 40 250    520* 2   100 120 150 130 50 40 90 __________________________________________________________________________ Note: *Eutectic melting occurred.

Three O-materials selected from the O-materials of Example 5 were further examined. The three O-materials were cold worked up to a maximum reduction of 80%, solution heat treated at 470.degree. C. for 40 minutes using the salt bath and water quenched to provide W-materials. The W-materials were further aged at 122.degree. C. for 25 hours to produce T6-materials. Properties of the above W-materials and T6-materials are shown in Table 6. From this table it is apparent that all materials have sufficient properties to be useful as stringer material. __________________________________________________________________________ Mechanical Properties Average Heating    Grain Size Result of Bending Test of T6-Material Heating Tempera- Cold  of    of W-Material* Yield Tensile Rate ture Reduction W-Material External Occurrence Strength Strength Elongation (.degree.C./min) (.degree.C.) (%)   (.mu.m) Appearance of Crack (Kg/mm.sup.2) (Kg/mm.sup.2) (%) __________________________________________________________________________ 230  480   0    30    Good   None  51.3  57.0  16 10    30    "      "     52.3  57.8  14 20    35    "      "     52.8  56.9  14 30    35    "      "     51.6  57.3  16 60    30    "      "     50.3  57.0  17 80    25    "      "     50.6  57.1  17 205  430   0    30    "      "     51.2  56.9  15 11    35    "      "     53.0  57.3  13 20    35    "      "     52.9  57.3  14 28    33    "      "     50.8  57.0  16 53    30    "      "     49.8  56.9  15 75    30    "      "     50.3  57.2  15 188  395   0    40    "      "     52.0  57.3  16 9    40    "      "     53.1  57.9  14 22    45    "      "     52.8  57.6  13 30    45    "      "     50.6  57.6  16 45    35    "      "     50.1  57.0  17 80    30    "      "     50.7  57.0  17 __________________________________________________________________________ Note: *90.degree. Bending, Bending Radius = 1.5t (t = Thickness of Plate) The test was carried out 4 hours after water quenching.

Cold rolled plates 3 mm thick were prepared from ingots of alloy No. 3 according to the practice described in Example 3. The plates were heated to temperatures of 480.degree. C. to 385.degree. C. at the respective rates shown in Table 7, held at the heating temperature for various times and cooled at a rate of 5.degree. C./min. Following cooling the plates were reheated at 300.degree. C. for one hour and air cooled to produce O-material plates 3 mm thick.

The O-materials produced in Example 7 were cold worked to a 20% cold reduction which causes the marked grain growth, solution heat treated at 470.degree. C. for 40 minutes using the salt bath and water quenched to provide W-materials.

The relation between the grain sizes of materials and the rapid heating condition is shown in Table 7. ______________________________________ Average                     Grain Size Grain Size Heating Heating              of      of Rate   Temperature Holding  W-Material O-Material (.degree.C./min) (.degree.C.) Time     (.mu.m) (.mu.m) ______________________________________ 230    480         20    sec. 30      30 1     min. 35      30 5     min. 35      30 20    min. 40      35 40    min. 45      35 218    455         30    sec. 25      25 2     min. 30      30 10    min. 35      30 30    min. 40      35 50    min. 55      45 195    410         5     min. 35      30 10    min. 35      30 35    min. 40      35 60    min. 55      45 183    385         10    min. 35      30 25    min. 45      35 40    min. 55      45 60    min. 65      50 ______________________________________

In Table 7 it is shown that very fine grain sizes are achieved in both the O- and W-materials over various holding times.

Further, the O-materials in Example 7 were cold worked to a cold reduction of 0 to 90%, solution heat treated at 470.degree. C. for 40 minutes using the salt bath and water quenched. The thus obtained W-materials were less than 100 .mu.m in grain size. A bending test (bending angle 90.degree., bending radius=1.5t, t=thickness of plate) was carried out on the W-material 4 hours after water quenching. As a result of the bending test, cracks and orange peels were not observed. The W-materials proved to be excellent as aircraft stringer material.

O-materials of 2 to 5 mm in thickness were prepared from 400 mm thick ingots of alloy No. 1 shown in Table 1 under the conditions shown in Table 8. __________________________________________________________________________ Hot Rolling       Cold Rolling Annealing Conditions        Conditions Rapid Heating Conditions Thick-    Cold Thick- Av. ness      Re- ness Heat- Init. Final of  Inter- duc- of  ing Temp. Temp. Plate mediate tion Plate Rate          Cooling Re- No. Homegenization (.degree.C.) (.degree.C.) (mm) Annealing* (%) (mm) (.degree.C./min) Heating** Rate  heating*** __________________________________________________________________________ 1  470.degree. C. .times. 24 hr. 430 330 6   not done 33 4   140  450.degree. C. .times.  1 5.degree. C./min. 300.degree. C. .times. 1 hr. 2  470.degree. C. .times. 24 hr. 400 300 6   370.degree. C. .times. 33 4    21  450.degree. C. .times.  2 5.degree. C./min. 300.degree. C. .times. 1 hr.                            1 hr. 3  465.degree. C. .times. 16 hr. 425 310 6   not done 50 3   225  470.degree. C. .times.  3 5.degree. C./min. 300.degree. C. .times. 1 hr. 4  475.degree. C. .times. 16 hr. 425 310 6   390.degree. C. .times. 50 3    17  480.degree. C. .times. 30 5.degree. C./min. 300.degree. C. .times. 1 hr.                            1 hr. 5  475.degree. C. .times. 16 hr. 425 310 6   400.degree. C. .times. 66 2   450  470.degree. C. .times.  2 10.degree. C./min. 470.degree. C. .times. 1 hr.                            1 hr. 6  475.degree. C. .times. 16 hr. 425 310 6   400.degree. C. .times. 66 2    38  400.degree. C. .times. 35 10.degree. C./min. 470.degree. C. .times. 1 hr.                            1 hr. 7  470.degree. C. .times. 16 hr. 440 280 8   not done 50 4   230  480.degree. C. .times.  1 10.degree. C./min. 470.degree. C. .times. 1 hr. 8  470.degree. C. .times. 16 hr. 400 260 8   "     30 4    24  380.degree. C. .times. 40 10.degree. C./min. 470.degree. C. .times. 1 hr. 9  470.degree. C. .times. 16 hr. 440 380 5   "     40 3    57  430.degree. C. .times. 20 20.degree. C./hr. not done 10 470.degree. C. .times. 16 hr. 440 360 5   "     40 3   140  455.degree. C. .times.  2 20.degree. C./hr. " 11 470.degree. C. .times. 12 hr. 435 325 5   "     50 2.5 1300 470.degree. C. .times.  3 20.degree. C./hr. " 12 475.degree. C. .times. 24 hr. 415 345 5   350.degree. C. .times. 50 2.5  38  400.degree. C. .times. 50 25.degree. C./hr. " 1 hr. 13 475.degree. C. .times. 24 hr. 415 290 9   400.degree. C. .times. 66 3   900  460.degree. C. .times.  4 25.degree. C./hr. " 1 hr. 14 475.degree. C. .times. 24 hr. 415 320 8   400.degree. C. .times. 75 2   440  460.degree. C. .times. 15 25.degree. C./hr. " 1 hr. 15 470.degree. C. .times. 16 hr. 420 320 10  not done 50 5   680  470.degree. C. .times.  3 25.degree. C./hr. " 16 470.degree. C. .times. 16 hr. 420 335 8   400.degree. C. .times. 63 5   140  440.degree. C. .times. 10 25.degree. C./hr. " 1 hr. 17 470.degree. C. .times. 16 hr. 420 335 15  400.degree. C. .times. 80 3   215  450.degree. C. .times.  2 30.degree. C./hr. " 1 hr. __________________________________________________________________________ Note: *Cooling rate after heating: 25.degree. C./hr. **First stage heating temperature .times. Holding time ***Second stage heating temperature .times. Holding time Cooling rate after reheating is 25.degree. C./hr.

O-materials produced under the conditions of Nos. 1 to 17 shown in Table 8 were further cold worked to a 20% cold reduction, solution heat treated at 470.degree. C. for 40 minutes using the salt bath and water quenched to provide W-materials.

Table 9 shows properties of the O-materials and W-materials. The W-materials obtained in Example 10 were aged at 120.degree. C. for 24 hours to provide T6-materials. Properties of T6-materials are given in Table 9. __________________________________________________________________________ Result of Grain  Bending Test of Grain Size   W-Material* Size Mechanical Properties of O-       Occur- of W- of T6-Materials Mate- External rence Mate- Yield Tensile Elonga- rial Appear- of  rial Strength Strength tion No (.mu.m) ance Crack (.mu.m) (Kg/mm.sup.2) (Kg/mm.sup.2) (%) __________________________________________________________________________ 1  35  Good None 40  52.5  57.9  13 2  35  "    "   40  52.5  57.3  13 3  35  "    "   40  52.9  57.3  13 4  30  "    "   35  52.9  57.3  14 5  30  "    "   35  52.9  57.3  14 6  35  "    "   45  51.9  56.9  14 7  30  "    "   30  52.3  57.8  14 8  35  "    "   35  52.7  57.8  13 9  35  "    "   40  51.8  57.8  13 10 30  "    "   35  51.8  57.8  13 11 30  "    "   35  52.6  57.5  12 12 40  "    "   50  52.6  57.5  13 13 30  "    "   35  52.6  57.4  14 14 30  "    "   35  52.4  57.4  14 15 30  "    "   35  52.4  57.4  13 16 35  "    "   40  52.8  57.4  13 17 35  "    "   35  52.6  57.5  14 __________________________________________________________________________ Note: *Bending of 90.degree., Bending Radius = 1.5t (t = Thickness of Plate) The Test was carried out 4 hours after the water quenching.

As can be seen from the above Table 9, all grain sizes of O-materials and W-materials were below 100 .mu.m and grain growth was hardly detected in each material. Further, both the W-materials and T6-materials proved to have excellent properties as aircraft stringer material. In Table 9, the result of the case of 20% cold reduction is given, but also, in the cases of cold reduction of 0 to 80%, grain sizes of less than 100 .mu.m can be developed in the produced W-materials and every material exhibited sufficiently improved properties as aircraft stringer material.

Ingots of alloy Nos. 3 to 7 400 mm in thickness were homogenized by heating at 470.degree. C. for 25 hours, hot rolled to provide plates 6 m thick between an initial temperature of 400.degree. C. and a final temperature of 300.degree. C. Following hot rolling, the plates were cold rolled to plates 3 mm thick and annealed by heating to a temperature of 460.degree. C. at an average heating rate of 220.degree. C./min., holding that temperature for five minutes, cooling at a cooling rate of 10.degree. C./min., reheating at 300.degree. C. for one hour and finally air cooling, providing O-materials 3 mm thick.

Comparative O-materials were prepared from ingots of alloy Nos. 8 and 9 400 mm thick according to the procedure described in case of alloy Nos. 3 to 7.

The O-materials prepared in Example 11 were cold worked to a cold reduction of 0 to 75%, solution heat treated at 470.degree. C. for 40 minutes using the salt bath and water quenched to produce W-materials. Grain sizes of the thus obtained W-materials are given in Table 10. ______________________________________ Grain Size of W-Material (.mu.m) Cold Reduction Alloy No. 0%     10%     20%  30%    60%  75% ______________________________________ 3         30     35      40   35     30   30 4         30     30      35   35     30   30 5         30     35      35   35     30   30 6         32     35      35   35     25   30 7         25     30      35   35     25   30 8         30     35      40   35     30   30 9         35     35      40   35     30   30 ______________________________________

From Table 10 it can be seen that grain sizes of all materials are less than 100 .mu.m over the wide range of cold reductions.

Further, O-materials prepared in Example 11 were cold worked to a 20% cold reduction, solution heat treated at 470.degree. C. for 40 minutes using the salt bath and water quenched to provide W-materials. Properties of the O-materials and W-materials are shown in Table 11 below. In addition to these properties, T6-materials which were produced by aging the W-materials receiving the 20% cold reduction at 120.degree. C. for 24 hours were examined. Properties of the T6-materials also are shown in Table 11.

Upper limits of cold reduction practicable in the cold working process were measured and the results are given in Table 11. __________________________________________________________________________ Grain    Upper Grain Result of    Stress Mechanical Properties Size of  Limit Size of Bending Test of Corrosion of T6-Material O-       of Cold W-   W-Material*  Cracking Yield Tensile Elonga- Alloy Material Reduction Material External Occurrence Life of T6- Strength Strength tion No. (.mu.m) (%)   (.mu.m) Appearance of Crack Material** (Kg/mm.sup.2) (Kg/mm.sup.2) (%) __________________________________________________________________________ 3   35   92    40   Good   None  More than 52.3  57.1  14 30 days 4   30   90    35   "      "     More than 55.3  62.4  13 30 days 5   30   90    35   "      "     More than 56.7  63.8  13 30 days 6   35   92    35   "      "     More than 53.1  57.9  14 30 days 7   30   90    35   "      "     More than 57.9  64.0  13 30 days 8   35   95    40   "      "     More than 42.3  50.4  14 30 days 9   35   55    40   "      "     9 days 60.9  67.8  12 __________________________________________________________________________ Note: *Bending of 90.degree., Bending Radius = 1.5t (t = Thickness of Plate) The test was carried out 4 hours after water quenching. **Life to fracture when loading stress of 75% of yield strength to T6materials in 3.5% NaCl aqueous solution.

From the above Table 11, it will be clear that in case of alloy Nos. 3-7 according to the present invention, very good properties can be attained, but in case of alloys Nos. 8 and 9, such good properties can not be attained. Alloy No. 8 was inferior in strength and alloy No. 9 was apt to exhibit stress corrosion cracking. Both alloys of Nos. 8 and 9 presented problems in applications such as aircraft stringers. Further, referring to the upper limit of cold reduction in the case of a composition according to the present invention, an extremely high degree of cold reduction can be achieved easily, while in the case of a comparative composition of alloy No. 9 such high degree of cold reduction is not practicable.

Further, corrosion-resistant clad O-materials and W-materials were produced in the following examples and their properties were examined.

In the following examples, the alloys listed in the above Table 1 were used as core materials and AA7072 alloy sheet having the following composition was used as cladding material.

The cladding alloy sheet was prepared by homogenizing AA7072 alloy ingot at 530.degree. C. for 24 hours and rolling to the required thickness. Cladding Alloy (AA7072 alloy) Chemical Composition (wt. %) Si    Fe     Cu      Mn   Mg     Cr  Zn   Ti  Al ______________________________________ 0.09  0.18   0.01    0.01 0.01   --  1.1  --  Re- minder ______________________________________

Clad O-materials 3 mm thick according to the present invention and clad O-materials 3 mm thick according to the conventional method were respectively prepared using ingots of alloy Nos. 1 and 4 and the above cladding alloy sheet in the following procedures.

Procedure according to the present invention:

Homogenization treatment of core alloy ingots (at 460.degree. C. for 16 hours).fwdarw.Hot Clad-rolling (from 300 mm to 6 mm in thickness at 400.degree. C.).fwdarw.Cold rolling (from 6 mm to 3 mm in thickness).fwdarw.Annealing (rapid heating to a temperature of 450.degree. C. at a heating rate of 200.degree. C./min. and holding for 3 minutes at this temperature.fwdarw.cooling at a cooling rate of 5.degree. C./min..fwdarw.reheating at 300.degree. C. for 1 hour.fwdarw.cooling to 200.degree. C. at a cooling rate of 20.degree. C./min.).fwdarw.Clad O-materials according to the present invention.

Procedure according to the conventional method:

Homogenization treatment of core alloy ingots (at 460.degree. C. for 16 hours).fwdarw.Hot clad-rolling (from 300 mm to 6 mm in thickness at 400.degree. C.).fwdarw.Annealing (heating at 420.degree. C. for 1 hour.fwdarw.cooling at a rate of 30.degree. C./hr).fwdarw.Cold rolling (from 6 mm to 3 mm in thickness).fwdarw.Annealing (heating to 420.degree. C. at a rate of 0.5.degree.-1.degree. C./min. and holding at 420.degree. C. for 2 hours.fwdarw.cooling at a rate of 25.degree. C./hr.fwdarw.heating at 235.degree. C. for 6 hours.fwdarw.air cooling).fwdarw.Clad O-materials according to the conventional method.

The thickness of the cladding in both the present and conventional method is 3% of the total thickness on each side.

Following the procedures described in Example 13, the respective clad O-materials were further treated to produce clad W-materials in with the following procedures.

Procedure according to the present invention:

.fwdarw.Cold working (to a cold reduction of 0-90%, as shown in Table 12).fwdarw.Solution heat treatment (at 470.degree. C. for 40 minutes, using a salt bath).fwdarw.Water quenching.fwdarw.Clad W-materials according to the present invention.

Procedure according to the conventional method:

.fwdarw.Cold working (to a cold reduction of 0-90%, as shown in Table 12).fwdarw.Solution heat treatment (at 470.degree. C. for 40 minutes, using a salt bath).fwdarw.Water quenching.fwdarw.W-materials according to the conventional method.

Properties of the O-materials and W-materials prepared in Examples 13 and 14 were tested and are given in Table 12.

In comparing the present invention and the conventional method, it becomes clear from Table 12 that the grain size of O-material according to the present invention is very fine and can provide W-material having a fine grain size below 100 .mu.m over the wide range of cold reduction, that is, 0-90%. Such effects eliminated orange peel and the occurrence of cracks during section roll forming and markedly improved the bending property.

Further, W-materials were subjected to T6 type treatment to produce T6-material and the mechanical properties were examined. From Table 12, T6-materials of the present invention proved to be superior to those of the conventional method.

Still further, the respective T6-materials were chemical milled to a 20% reduction in thickness in 10% NaOH aqueous solution and surface roughness was measured. After chemical milling, fatigue testing was carried out on each T6-material and the result is summarized in Table 12. As can be seen from Table 12, clad T6-materials according to the present invention are superior to those of the conventional method in surface roughness and fatigue strength owing to the fine grain size. __________________________________________________________________________ Surface Core        Core                                    Rough- Fatigue Grain    Cold Grain             Mechanical Properties Fracture ness Strength Size of  Re- Size of Result of    of T-6 Material  Resis- after after Core O-   duc- W-   Bending Test *.sup.1 Yield Tensile Elonga- tance Chemical Chemical Alloy Material tion Material External Occurrence Strength Strength tion (MN .multidot. Milling Milling No. (.mu.m) (%) (.mu.m) Appearance of Crack (Kg/mm.sup.2) (Kg/mm.sup.2) (%)  m.sup.-3/2) (.mu.m) (Kg/mm.sup.2) __________________________________________________________________________ 1*.sup.2 30   0 30   Good   None  49.1  55.0  15   122  14   18 10 35   "      "     50.0  55.6  13   122  16   18 20 40   "      "     50.6  55.3  13   124  20   17 30 35   "      "     49.4  55.3  13   124  17   18 60 30   "      "     49.6  55.5  15   124  15   18 90 25   "      "     49.0  55.4  15   127  12   18 1*.sup.3 200   0 200  Orange Consider- 48.0  54.3   9    88  100  7.2 Peel   able Crack 10 220  Orange Consider- 49.0  54.9   9    80  100  7.6 Peel   able Crack 20 280  Orange Consider- 49.2  54.7   8    80  130  7.0 Peel   able Crack 30 190  Orange Slight 48.4  54.2  10    87  90   8.4 Peel   Crack 60 50   Good   None  47.7  54.5  13   114  25   16 4*.sup.2 25   0 25   "      "     51.1  58.0  17   117  12   17 10 30   "      "     52.0  58.6  15   117  14   17 20 35   "      "     52.6  58.3  14   121  18   16.3 30 35   "      "     51.4  58.3  15   121  18   16.1 60 25   "      "     50.6  58.0  16   121  12   17.4 90 25   "      "     50.7  58.3  16   122  12   18.1 4*.sup.3 180   0 190  Orange Slight 50.2  57.3  10    84  93    8 peel   Crack 10 210  Orange Considerable- 50.9  58.0   9    79  100   8 peel   able Crack 20 250  Orange Considerable- 51.2  57.7   9    79  130  7.4 peel   able Crack 30 180  Orange Slight 50.7  57.7   9    82  100  7.9 peel   Crack 60 50   Good   None  50.0  57.5  13   110  25   15.4 __________________________________________________________________________ Note: *.sup.1 Bending of 90.degree., Bending Radius = 1.5t (t = Thickness of Plate) The test was conducted 4 hours after the water quenching. *.sup.2 Present Invention *.sup.3 Conventional Method

Core ingots 350 mm thick of alloy No. 1 were homogenized at 470.degree. C. for 24 hours, hot clad-rolled with the cladding alloy on both sides between a starting temperature of 420.degree. C. and a final temperature of 340.degree. C. to provide clad plates 6 mm thick. Subsequently, the hot clad-rolled plates were cold rolled to provide clad plates 3 mm thick, and received the following annealing treatment to provide clad O-materials 3 mm thick. Annealing was accomplished by heating to a temperature of 450.degree. C. at various heating rates shown in Table 13, holding the temperature for two minutes and cooling at a cooling rate of 25.degree. C./hr. The clad rate of the clad O-material was 2.5% on each side.

The clad O-materials obtained in Example 15 were further cold worked to various cold reductions shown in Table 13, solution heat treated at 470.degree. C. for 40 minutes using the salt bath and water quenched to provide W-materials.

The relation between the core grain sizes of O- and W-materials and the heating rate in the annealing step is given in Table 13. ______________________________________ Heat-    Core Grain Size of   Core Grain ing Rate W-Material (.mu.m)   Size of to 450.degree. C. Cold Reduction (%)   O-Material (.degree.C./min) 0      10      20   33     60  (.mu.m) ______________________________________ 1400     25     25      33   30     25  30 380      30     30      35   30     28  30 80       30     30      35   35     30  30 40       30     35      40   40     30  30 20       35     35      45   40     30  35 14       75     85      85   75     35  75 11*     110    130     180  160    40  110 7*      130    150     200  170    45  110 2.2*    200    230     280  220    45  180 0.9*    200    250     300  220    45  180 ______________________________________ Note: *Heating rate according to the conventional practice.

As can be seen from Table 13, when the heating rate to 450.degree. C. exceeds 11.degree. C./min., the core grain sizes of the O-material and W-material are less than 100 .mu.m and fine. On the contrary, when the heating rate is 11.degree. C./min. or less, marked grain growth occurs and the core grain size of the O- and W-materials exceeds 100 .mu.m.

The clad O-materials which were heated to 450.degree. C. at rates of 80.degree. C./min., 20.degree. C./min., 14.degree. C./min., 11.degree. C./min. and 0.9.degree. C./min. in the annealing step were further tested. The respective O-materials were cold worked to a cold reduction of 0 to 80%, which is practiced in the usual production of aircraft stringers and stringer frames, solution heat treated at 470.degree. C. for 40 minutes using the salt bath and water quenched to produce clad W-materials.

Following water quenching, the respective W-materials were aged at 120.degree. C. for 24 hours to provide clad T6-materials. Properties of the W-materials and the T6-materials are given in Table 14. It will be clear from this Table that a heating rate exceeding 11.degree. C./min. is extremely effective for producing the desired stringer materials. Also, highly improved corrosion resistance was achieved since these materials were clad with the AA7072 alloy. __________________________________________________________________________ Mechanical Properties Average    Core Grain Result of Bending Test of T6-Material Heating Cold  Size of of W-Material* Yield Tensile Elonga- Rate Reduction W-Material External Occurrence Strength Strength tion (.degree.C./min) (%)   (.mu.m) Appearance of Crack (Kg/mm.sup.2) (Kg/mm.sup.2) (%) __________________________________________________________________________ 80    0    30    Good   None  49.0  55.1  15 10    30    "      "     50.0  55.6  12 20    35    "      "     50.4  55.9  12 33    35    "      "     48.8  54.8  15 50    30    "      "     48.4  55.3  15 80    25    "      "     48.2  55.7  15 20    0    35    "      "     49.0  55.0  14 10    35    "      "     50.6  55.3  11 20    45    "      "     51.0  55.0  12 33    40    "      "     48.9  54.8  14 50    30    "      "     47.9  55.6  16 80    30    "      "     48.3  55.3  15 14    0    75    "      "     49.0  55.0  16 10    85    "      "     50.2  55.1  13 20    85    "      "     50.4  54.7  12 33    75    "      "     48.5  54.6  14 50    35    "      "     48.3  55.6  14 80    35    "      "     48.2  55.5  15 11    0    110   Orange Slight 48.3  54.5  13 Peel   Crack 10    130   Orange Slight 49.6  54.3  12 Peel   Crack 20    180   Orange Slight 49.5  55.0  12 Peel   Crack 33    160   Orange Slight 48.7  54.7  12 Peel   Crack 50    40    Good   None  48.3  54.5  14 80    40    "      "     48.0  54.0  14 0.9   0    200   Orange Consider- 48.0  54.5  10 Peel   able Crack 10    250   Orange Consider- 49.1  55.0   9 Peel   able Crack 20    300   Orange Consider- 49.2  54.8   9 Peel   able Crack 33    220   Orange Consider- 48.8  54.8   9 Peel   able Crack 50    45    Good   None  48.0  54.9   9 80    40    "      "     47.8  54.6  10 __________________________________________________________________________ Note: *90.degree., Bending, Bending Radius = 1.5t (t = Thickness of Plate) The Test was conducted 4 hours after water quenching.

Cold rolled plates 3 mm thick were prepared using ingots of alloy No. 2 as cores in the same procedure as in the case of the procedure described in Example 15, except that the core ingots were clad with cladding alloy on one side. Following cold rolling, the plates were subjected to the following two stage annealing treatment. In the first stage, the plates were heated to various heating temperatures of 320.degree. to 520.degree. C. at various heating rates, shown in Table 15, held at the temperatures for times shown in the same Table and cooled at a rate of 5.degree. C./min. After the first heating treatment, the plates were reheated at 300.degree. C. for one hour and cooled at a rate of 20.degree. C./hr., providing one-side clad O-materials 3 mm thick. The clad rate of the product was 2.5%.

The O-materials obtained in Example 17 were cold worked to various cold reductions, solution heat treated at 470.degree. C. for 40 minutes using the salt bath and water quenched, providing one-side clad W-materials.

The relation between the core grain sizes of the O-materials and W-materials and the first stage heating temperature is given in Table 15. It can be seen from the above Table 15 that only O-material which has received the annealing treatment characterized by rapid heating to 320.degree. to 500.degree. C. can be converted to W-material having fine core grain sizes even after cold working with a small amount of cold reduction and subsequent solution heat treatment. If the heating temperature is beyond the above range, W-material of fine core grain size cannot be obtained after cold working with a small amount of cold reduction and solution heat treatment. That is, the core grain size of W-material becomes larger than 100 .mu.m. __________________________________________________________________________ Heat- ing Average Tem- Hold- Core Grain Size of Core Grain Heating pera- ing W-Material (.mu.m) Size of Rate ture Time Cold Reduction (%) O-Material (.degree.C./min) (.degree.C.) (min.) 0  10 20 30 60 80 (.mu.m) __________________________________________________________________________ 230  475   1  30 30 35 35 30 25 30 218  455   3  30 30 35 35 30 25 30 210  440   6  30 35 35 35 30 30 30 200  420  12  35 40 40 40 35 30 35 190  400  10  40 40 45 45 35 30 40 175  370  18  40 45 55 50 35 30 40 150  320  55  40 50 60 60 35 35 40 250   520* 2  100 120 150 130 50 40 90 __________________________________________________________________________ Note: *Eutectic melting occurred.

Three materials selected from the one-side clad O-materials of Example 17 were further examined. The three O-material were cold worked up to a maximum reduction of 80%, solution heat treated at 470.degree. C. for 40 minutes using the salt bath and water quenched to provide one-side clad W-materials. W-materials were further aged at 122.degree. C. for 25 hours to produce one-side clad T6-materials. Properties of the above W-materials and T6-materials are shown in Table 16. From this table it is apparent that all the materials have sufficient properties as stringer material. __________________________________________________________________________ Mechanical Properties Average Heating Cold Core Grain Result of Bending Test of T6-Material Heating Tempera- Re- Size of of W-Material* Yield Tensile Rate ture duction W-Material External Occurence Strength Strength Elongation (.degree.C./min) (.degree.C.) (%) (.mu.m) Appearance of Crack (Kg/mm.sup.2) (Kg/mm.sup.2) (%) __________________________________________________________________________ 230  475   0  30    Good   None  50.3  56.0  16 10  30    "      "     51.3  56.8  14 20  35    "      "     51.8  55.9  14 30  25    "      "     50.6  56.3  16 60  30    "      "     49.3  56.0  17 80  25    "      "     49.6  56.1  17 210  440   0  30    "      "     50.2  55.9  15 11  35    "      "     52.0  56.3  13 20  35    "      "     51.9  56.3  14 28  33    "      "     49.8  56.0  16 53  30    "      "     48.8  55.9  15 75  30    "      "     49.3  56.2  15 190  390   0  40    "      "     51.0  56.3  16 9  40    "      "     52.1  56.9  14 22  45    "      "     51.8  56.6  13 30  45    "      "     49.6  56.6  16 45  35    "      "     49.1  56.0  17 80  30    "      "     49.7  56.0  17 __________________________________________________________________________ Note: *90.degree. Bending, Bending Radius = 1.5t (t = Thickness of Plate) The test was conducted 4 hours after water quenching.

Cold rolled plates 3 mm thick were prepared from ingots of alloy No. 3 according to the practice described in Example 15. The plates were heated to temperatures of 480.degree. C. to 385.degree. C. at the respective heating rates shown in Table 17, held at the heating temperature for various times and cooled at a rate of 5.degree. C./min. Following cooling, the plates were reheated at 300.degree. C. for one hour and air cooled to produce clad O-material plates 3 mm thick. The clad rate of the O-materials was 2.5% on each side.

The O-materials produced in Example 19 were cold worked to a 20% cold reduction, which is apt to cause marked grain growth, solution heat treated at 470.degree. C. for 40 minutes using the salt bath and water quenched to provide clad W-materials.

The relation between the core grain sizes of the materials and the rapid heating conditions is shown in Table 17. ______________________________________ Average                     Core Grain Core Grain Heating Heating              Size of Size of Rate   Temperature Holding  W-Material O-Material (.degree.C./min) (.degree.C.) Time     (.mu.m) (.mu.m) ______________________________________ 250    475         20    sec. 30      30 1     min. 35      30 5     min. 35      30 20    min. 40      35 40    min. 40      35 220    455         30    sec. 25      25 2     min. 30      30 8     min. 35      30 25    min. 40      35 40    min. 50      45 180    415         6     min. 35      30 9     min. 35      30 30    min. 45      35 60    min. 55      45 175    390         10    min. 35      30 20    min. 45      35 30    min. 55      45 60    min. 60      50 ______________________________________

From the Table 17 it is shown that very fine core grain sizes are achieved in both the O- and W-materials over various holding times.

Further the O-materials in Example 19 were cold worked to a cold reduction of 0 to 90%, solution heat treated at 470.degree. C. for 40 minutes using the salt bath and water quenched. The thus obtained W-materials were less than 100 .mu.m in core grain size. A bending test (bending angle 90.degree., bending radius=1.5t, t=thickness of plate) was conducted on the W-material 4 hours after water quenching. As a result of the bending test, cracks and orange peels were not observed. The W-materials proved to be excellent as aircraft stringer material.

O-materials of 2 to 5 mm in thickness clad with cladding alloy on both sides were prepared from 400 mm thick core ingots of alloy No. 1 shown in Table 1 under the conditions shown in Table 18. The clad rates all were 2.2% on each side. __________________________________________________________________________ Hot Clad-         Cold Rolling Annealing Rolling Conditions Conditions Rapid Heating Conditions Thick-    Cold Thick- Av. Core                ness      Re- ness Heat- Al-         Init. Final of  Inter- duc- of  ing loy Homogenization Temp. Temp. Plate mediate tion Plate Rate          Cooling Re- No. of Core Ingot (.degree.C.) (.degree.C.) (mm) Annealing* (%) (mm) (.degree.C./min) Heating** Rate  heating*** __________________________________________________________________________ 1  470.degree. C. .times. 24 hr. 425 330 6   not done 33 4   200  450.degree. C. .times.  1 5.degree. C./min. 300.degree. C. .times. 1 hr. 2  470.degree. C. .times. 24 hr. 400 300 6   370.degree. C. .times. 33 4    20  450.degree. C. .times.  2 5.degree. C./min. 300.degree. C. .times. 1 hr.                            1 hr. 3  465.degree. C. .times. 16 hr. 425 310 6   not done 50 3   250  470.degree. C. .times.  3 5.degree. C./min. *300.degree. C. .times. 1 hr. 4  475.degree. C. .times. 16 hr. 425 310 6   390.degree. C. .times. 50 3    15  480.degree. C. .times. 30 5.degree. C./min. *300.degree. C. .times. 1 hr.                            1 hr. 5  475.degree. C. .times. 16 hr. 425 310 6   400.degree. C. .times. 66 2   500  470.degree. C. .times.  2 10.degree. C./min. *470.degree. C. .times. 1 hr.                            1 hr. 6  475.degree. C. .times. 16 hr. 425 310 6   400.degree. C. .times. 66 2    35  400.degree. C. .times. 35 10.degree. C./min. *470.degree. C. .times. 1 hr.                            1 hr. 7  470.degree. C. .times. 16 hr. 420 280 8   not done 50 4   230  475.degree. C. .times.  1 10.degree. C./min. *470.degree. C. .times. 1 hr. 8  470.degree. C. .times. 16 hr. 400 260 8   "     50 4    24  380.degree. C. .times. 40 10.degree. C./min. *470.degree. C. .times. 1 hr. 9  470.degree. C. .times. 16 hr. 425 380 5   "     40 3    57  430.degree. C. .times. 20 20.degree. C./hr. not done 10 470.degree. C. .times. 16 hr. 425 315 5   "     40 3   140  455.degree. C. .times.  2 20.degree. C./hr. " 11 470.degree. C. .times. 12 hr. 435 325 5   "     50 2.5 1200 470.degree. C. .times.  3 20.degree. C./hr. " 12 475.degree. C. .times. 24 hr. 415 345 5   350.degree. C. .times. 50 2.5  38  405.degree. C. .times. 50 24.degree. C./hr. " 1 hr. 13 475.degree. C. .times. 24 hr. 415 280 9   400.degree. C. .times. 66 3   800  460.degree. C. .times.  4 24.degree. C./hr. " 1 hr. 14 475.degree. C. .times. 24 hr. 415 320 8   400.degree. C. .times. 75 2   440  460.degree. C. .times. 15 24.degree. C./hr. " 1 hr. 15 470.degree. C. .times. 16 hr. 420 320 10  not done 50 5   780  465.degree. C. .times.  3 24.degree. C./hr. " 16 470.degree. C. .times. 16 hr. 420 320 8   400.degree. C. .times. 63 5   140  440.degree. C. .times. 10 24.degree. C./hr. " 1 hr. 17 470.degree. C. .times. 16 hr. 420 335 15  400.degree. C. .times. 80 3   215  450.degree.  C. .times.  2 28.degree. C./hr. " 1 hr. __________________________________________________________________________ Note: *Cooling rate after heating: 25.degree. C./hr. **First stage heating temperature .times. Holding time ***Second stage heating temperature .times. Holding time

Clad O-materials produced under the conditions of Nos. 1 to 17 shown in Table 18 were further cold worked to a 20% cold reduction, solution heat treated at 470.degree. C. for 40 minutes using the salt bath and water quenched to provide clad W-materials.

Table 19 shows properties of the O-materials and W-materials. The W-materials obtained in Example 22 were aged at 120.degree. C. for 24 hours to provide clad T6-materials. Properties of T6-materials are given in Table 19. __________________________________________________________________________ Core    Result of Core Grain   Bending Test of Grain Size    W-Material* Size Mechanical Properties of O-        Occur- of W- of T6-Materials Core Mate- External rence Mate- Yield Tensile Elonga- Alloy rial Appear- of  rial Strength Strength tion No. (.mu.m) ance Crack (.mu.m) (Kg/mm.sup.2) (Kg/mm.sup.2) (%) __________________________________________________________________________ 1   35  Good None 38  50.5  56.9  12 2   35  "    "   38  50.5  56.0  12 3   35  "    "   38  51.0  56.0  12 4   30  "    "   32  51.0  56.0  13 5   30  "    "   33  51.0  56.0  13 6   35  "    "   40  49.9  55.4  13 7   30  "    "   33  50.3  56.0  13 8   35  "    "   45  50.7  56.0  14 9   35  "    "   40  49.8  56.0  14 10  30  "    "   35  49.8  56.0  14 11  30  "    "   35  50.6  56.0  13 12  40  "    "   55  50.6  56.0  13 13  30  "    "   40  50.6  56.0  13 14  30  "    "   40  50.4  56.0  13 15  30  "    "   40  50.4  56.0  13 16  35  "    "   45  50.8  56.0  13 17  35  "    "   35  50.4  56.6  14 __________________________________________________________________________ Note: *Bending of 90.degree., Bending Radius = 1.5t (t = Thickness of Plate) The Test was conductcd 4 hours after the water quenching.

As can be seen from the above Table 19, all core grain sizes of O-materials and W-materials were below 100 .mu.m and grain growth was hardly detected in each material. Further, both the W-materials and T6-materials proved to have excellent properties as aircraft stringer material. In Table 19, the result of the case of a 20% cold reduction is given. However, in the cases of a cold reduction of 0 to 80%, a grain size of less than 100 .mu.m can also be developed in the produced W-materials and every material exhibited sufficiently improved properties as aircraft stringer material.

Core ingots of alloy Nos. 3 to 7 400 mm in thickness were homogenized by heating at 470.degree. C. for 25 hours and hot clad-rolled with the cladding alloy on one side to provide one-side clad plate 6 mm thick between an initial temperature of 400.degree. C. and a final temperature of 300.degree. C. Following hot clad-rolling, the plates were cold rolled to plates 3 mm thick and annealed by heating to a temperature of 460.degree. C. at an average heating rate of 300.degree. C./min., holding that temperature for five minutes, cooling at a cooling rate of 10.degree. C./min., reheating at 300.degree. C. for one hour and finally air cooling, providing one-side clad O-materials 3 mm thick with a clad rate of 2.6%.

Comparative O-materials were prepared from ingots of alloy Nos. 8 and 9 400 mm thick according to the procedure described in the case of alloy Nos. 3 to 7.

The O-materials prepared in Example 23 were cold worked to a cold reduction of 0 to 75%, solution heat treated at 470.degree. C. for 40 minutes using the salt bath and water quenched to produce one-side clad W-materials. Core grain sizes of the thus obtained W-materials are given in Table 20. ______________________________________ Core Grain Size of W-Material (.mu.m) Core      Cold Reduction Alloy No. 0%     10%     20%  33%    60%  75% ______________________________________ 3         30     35      40   35     30   30 4         30     30      35   35     30   30 5         30     35      35   35     30   30 6         32     35      35   35     25   30 7         25     30      35   35     25   30 8         30     35      40   35     30   30 9         35     35      40   35     30   30 ______________________________________

From Table 20 it can be seen that core grain sizes of all materials are less than 100 .mu.m over the range of cold reductions.

Further, O-materials prepared in Example 23 were cold worked to a 20% cold reduction, solution heat treated at 470.degree. C. for 40 minutes using the salt bath and water quenched to provide W-materials. Properties of O-materials and W-materials are shown in Table 21 below. In addition to these properties, T6-materials, which were produced by aging the W-materials receiving the 20% cold reduction at 120.degree. C. for 24 hours, were examined. Properties of the T6-materials also are given in Table 21.

Upper limits of cold reduction practicable in a cold working process were measured and the result obtained is given in Table 21. __________________________________________________________________________ Upper                    Stress Mechanical Properties Core Grain Limit Core Grain Result of Bending Test Corrosion of T6-Material Core Size of of Cold Size of of W-Material* Cracking Yield Tensile Elonga- Alloy O-Material Reduction W-Material External Occurrence Life of T6- Strength Strength tion No. (.mu.m) (%)   (.mu.m) Appearance of Crack Material** (Kg/mm.sup.2) (Kg/mm.sup.2) (%) __________________________________________________________________________ 3   35    92    40    Good   None  more than 51.3  56.1  13 30 days 4   30    90    90    "      "     more than 54.3  61.0  12 30 days 5   30    90    35    "      "     more than 55.7  62.8  12 30 days 6   35    92    35    "      "     more than 52.1  56.9  14 30 days 7   30    90    35    "      "     more than 56.9  62.5  14 30 days 8   35    95    40    "      "     more than 40.3  48.3  14 30 days 9   35    55    40    "      "     9 days 60.1  67.0  12 __________________________________________________________________________ Note: *Bending of 90.degree., Bending Radius = 1.5t (t = Thickness of Plate) The test conducted 4 hours after water quenching. **Life to fracture when loading stress of 75% of yield strength to T6materials in 3.5% NaCl aqueous solution.

From the above Table 21, it will be clear that in the case of alloy Nos. 3-7 according to the present invention, a very good degree of properties can be attained, but in the case of alloy Nos. 8 and 9, such good properties cannot be attained. Alloy No. 8 was inferior in strength and alloy No. 9 was apt to exhibit stress corrosion cracking. Both alloys of Nos. 8 and 9 presented problems in applications such as aircraft stringer and stringer frame. Further, referring to the upper limit of cold reduction, in the case of a composition according to the present invention, an extremely high degree of cold reduction can be achieved easily, while in the case of a comparative composition of alloy No. 9, such high degree of cold reduction is not practicable.

## Claims

1. An aircraft stringer material which has been subjected to stepped cold working and then solution heat treatment, said aircraft stringer material consisting of a recrystallized aluminum alloy consisting essentially of 5.1 to 8.1 wt. % zinc, 1.8 to 3.4 wt. % magnesium, 1.2 to 2.6 wt. % copper, at least one of (a) 0.18 to 0.35 wt. % chromium or (b) 0.05 to 0.25 wt. % zirconium, up to 0.20 wt. % titanium, up to 0.05 wt. % iron, up to 0.40 wt. % silicon, up to 0.70 wt. % manganese, and the balance being aluminum and incidental impurities, wherein said impurities do not include any of the components named above, said aircraft stringer material having a grain size not exceeding 100 .mu.m over the entire length thereof and having a first portion thereof which has been cold worked to a cold reduction of 30% or less and a second portion thereof which has been cold worked to a cold reduction greater than 30%.

2. An aircraft stringer material as claimed in claim 1, wherein said material has been subjected to said stepped cold working such that said material includes a third portion thereof which has been cold worked to at least a 90% cold reduction.

3. An aircraft stringer material as claimed in claim 2, wherein said first portion has approximately a 0% cold reduction and said third portion has approximately a 90% cold reduction.

4. An aircraft stringer material which has been subjected to stepped cold working and then solution heat treatment, said aircraft stringer material consisting of a recrystallized aluminum alloy consisting essentially of 5.1 to 8.1 wt. % zinc, 1.8 to 3.4 wt. % magnesium, 1.2 to 2.6 wt. % copper, 0.18 to 0.35 wt. % chromium, up to 0.20 wt. % titanium, up to 0.50 wt. % iron, up to 0.40 wt. % silicon, up to 0.70 wt. % manganese, and the balance being aluminum and incidental impurities, wherein said impurities do not include any of the components named above, said aircraft stringer material having a grain size not exceeding 100 .mu.m over the entire length thereof and having a first portion thereof which has been cold worked to a cold reduction of 30% or less and a second portion thereof which has been cold worked to a cold reduction greater than 30%.

5. An aircraft stringer material as claimed in claim 4, wherein said material has been subjected to said stepped cold working such that said material includes a third portion thereof which has been cold worked to at least a 90% cold reduction.

6. An aircraft stringer material as claimed in claim 5, wherein said first portion has approximately a 0% cold reduction and said third portion has approximately a 90% cold reduction.

7. An aircraft stringer material which has been subjected to stepped cold working and then solution heat treatment, said aircraft stringer material comprising an aluminum alloy core and a corrosion-resistant cladding of AA7072 alloy bonded to at least one side of said core, said core consisting of a recrystallized, aluminum alloy consisting essentially of 5.1 to 8.1 wt. % zinc, 1.8 to 3.4 wt. % magnesium, 1.2 to 2.6 wt. % copper, at least one of 0.18 to 0.35 wt. % chromium or 0.05 to 0.25 wt. % zirconium, up to 0.20 wt. % titanium, up to 0.50 wt. % iron, up to 0.40 wt. % silicon, up to 0.70 wt. % manganese, and the balance being aluminum and incidental impurities, wherein said impurities do not include any of the components named above, said aluminum alloy core having a grain size not exceeding 100 .mu.m over the entire length thereof and having a first portion thereof which has been cold worked to a cold reduction of 30% or less and a second portion thereof which has been cold worked to a cold reduction greater than 30%.

8. An aircraft stringer material as claimed in claim 7, wherein said material has been subjected to said stepped cold working such that said material includes a third portion thereof which has been cold worked to at least a 90% cold reduction.

9. An aircraft stringer material as claimed in claim 8, wherein said first portion has approximately a 0% cold reduction and said third portion has approximately a 90% cold reduction.

10. An aircraft stringer material which has been subjected to stepped cold working and then solution heat treatment, said aircraft stringer material comprising an aluminum alloy core and a corrosion-resistant cladding of AA7072 alloy bonded to at least one side of said core, said core consisting of a recrystallized aluminum alloy consisting essentially of 5.1 to 8.1 wt. % zinc, 1.8 to 3.4 wt. % magnesium, 1.2 to 2.6 wt. % copper, 0.18 to 0.35 wt. % chromium, up to 0.20 wt. % titanium, up to 0.50 wt. % iron, up to 0.40 wt. % silicon, up to 0.70 wt. % manganese, and the balance being aluminum and incidental impurities, wherein said impurities do not include any of the components named above, said aluminum alloy core having a grain size not exceeding 100 .mu.m over the entire length thereof and having a first portion thereof which has been cold worked to a cold reduction of 30% or less and a second portion thereof which has been cold worked to a cold reduction greater than 30%.

11. An aircraft stringer material as claimed in claim 10, wherein said material has been subjected to said stepped cold working such that said material includes a third portion thereof which has been cold worked to at least a 90% cold reduction.

12. An aircraft stringer material as claimed in claim 11, wherein said first portion has approximately a 0% cold reduction and said third portion has approximately a 90% cold reduction.