JPH0839102A - Thick wall,fatigue resisting aluminum alloy sheet and manufacture of said sheet - Google Patents

Abstract

PURPOSE: To increase fatigue effective life of a thick aluminum alloy sheet by suppressing hydrogen content and hammering pressure ratio below prescribed values. CONSTITUTION: The thick aluminum alloy structure of which is strengthened to be fatigue-resistant, is degassed so that hydrogen content of liquid metal at the time period of casting is below 0.15 g/cm<3> . To the sheet of 110 to 150 mm final thickness, hammering pressure ratio C is limited below 2.4 and to the sheet of 150 to 250 mm final thickness, if circumferences require, hammering pressure ratio C is limited below 2. Further in a hot rolling process, final two times of rolling passes (thickness of an entrance-thickness of an exit) are respectively made to be larger than 25 mm. Density of pores which are located near a rolling central plane of a product and are above 20 μm is limited to be smaller than 800/cm<3> .

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a thick, fatigue resistant aluminum alloy sheet and a method of making the sheet.

[0002]

BACKGROUND OF THE INVENTION It is known that the micropores and intermetallic phases formed during casting are favorable sites for initiation of fatigue cracks. Presently, its presence is unavoidable, especially in metallurgical products with substantial cutting surfaces produced by conventional vertical casting.

So far, the density of micropores and intermetallic phases and / or
Alternatively, in order to minimize both maximum dimensions, a high reduction rate C = Ei / Ef
(Ei is the initial wall thickness, Ef is the final wall thickness) When hot rolling is performed, most of the micropores that are usually concentrated near the large central plane of the plate are blocked, and Very thick plates have been cast to destroy the intermetallic phase.

It is known that the conventional method for producing thick sheets (thickness> 10 mm) consists of casting the plate, peeling or trimming, homogenizing and carrying out hot rolling. The sheet thus produced is solution treated, quenched, controlled stretched to reduce internal stress levels, and age hardened (T
351 or T4 state) or T651 or T7 ×
Artificial aging to 51 state.

Therefore, the final wall thickness Ef is 100 mm to 150
For a sheet of mm, the thickness after peeling is usually>
Starting from a plate with 3.5 Ef, ie a forging ratio of C> 3.5.

French patent application No. 25295 in the name of the applicant
No. 78 (= US Pat. No. 4,511,409) and US Pat. No. 5,277,719 (ALCOA) describe a method for enhancing the fatigue resistance of thick aluminum alloy sheets by reducing micropores. And the method includes an initial training step. The latter of the above patent specifications describes an example in which the forging pressure (training and rolling) is 2.6. However, if the pre-forging step is introduced, the plate needs to be heated twice and transferred to two different manufacturing locations, which involves a significant increase in transformation costs.

The present invention relates to sheets having a final wall thickness of greater than 110 mm. INDUSTRIAL APPLICABILITY The present invention has a good internal state in a treated state (age-quenched, or quenched and artificially aged, or aged and artificially aged), and has an elastic limit RpO. Waved tensile under maximum stress equal to 50% (and in some cases 60%) of 2 (R = 0.
It relates to a structurally hardened aluminum alloy sheet exhibiting a fatigue useful life of more than 100,000 cycles for 1).

Contrary to the teachings of the prior art, the Applicants have surprisingly found that the forging ratio C is 2 for sheets with a final wall thickness of 110 mm to 150 mm (thus the stripped plate is <360 mm). .4 and optionally for sheets with a final wall thickness of 150 mm to 250 mm (thus <500 mm for peeled plates), by limiting the forging ratio C to less than 2, a clearly improved fatigue property I found that Preferably, the hot forging effect can be obtained only by rolling without pre-forging.

At the time of casting, the liquid metal is suitably degassed by methods known to those skilled in the art so that its hydrogen content is less than 0.15 g / cm ³ (chlorine or chlorine-containing products or It has been found that the characteristics regarding fatigue resistance are further improved by using a degassing ladle using argon). It has been found that the fatigue properties are further improved if the last two rolling passes (inlet thickness-outlet thickness) in the hot rolling process are each greater than 25 mm.

The product of the present invention which is resistant to fatigue is characterized by having a good internal condition as measured by ultrasonic monitoring in the vicinity of the mid-plane of the thick sheet. The monitoring device used was a Synergetics transceiver PR O2 or Panametrics 5052 adjusted as follows: Ultrasound concentration: φ100 μm × 600 μm oblong elliptical focus (−6 dB or half amplitude); : 50 MHz; Broadband probe (Δf / f ₀ = 70%).

It is equipped with numerical value acquisition and signal processing means,
The probe was controlled and displaced by 0.02 mm in the xy direction. The device is calibrated by a calibration block that has the same properties as the processed product and is also heat treated. Conventionally, the calibration block comprises a flat bottom hole of φ50 and 100 μm that exhibits a maximum amplitude of 80% on the monitor screen, which fixes the total gain Go. Maximum wall thickness 20
The volume investigated in a thin section of mm is 20 × 20 × 0.6.
It was mm ³ .

Measurements were taken at multiple locations to obtain reliable statistics. The pores were defined by peaks above an adjustable threshold set just above background noise.

The following are measured: -the number of pores per unit volume (greater than the detection threshold of 20 μm);-maximum amplitudes.
Numerical average of A _max (A _max is the maximum value of the echo of one pore); and - the numerical average of the intermediate amplitude a- (provided that, a- is

[0014]

[Equation 1]

(Where x and y are the image distances of the pores in question) (see FIG. 7).

Under the above conditions, the product of the invention complies with the following regulations: number of pores ≤ 800 / cm ³ ; mean amplitude A ≤ 22%; mean maximum amplitude A _max ≤ 50%.

Hereafter, we use the following abbreviations or symbols: L = longitudinal direction; TL = cross-section long side direction; TC = cross-section short side direction; R = 1 in fatigue test Maximum stress / minimum stress of cycle; Kt = notch coefficient; ΔK = change of stress strength coefficient in fatigue test; and da / dn = propagation velocity of fatigue fracture.

[0018]

The invention will be better understood by the following examples shown in FIGS.

Alloys are designated using the Aluminum Association nomenclature.

Example 1 A format having a thickness of 420 mm after peeling or trimming (outside the invention: format A) and a format having a thickness of 260 mm after peeling or trimming (invention: format B) A plate of alloy 7010 was produced. The thickness of the metal removed during peeling was the same in both cases. The hydrogen content in the liquid metal was <0.15 g / cm ³ in both cases.

After peeling, the plate was treated under the same conditions (470
Homogenize for 30 hours at ℃, then 390 under the same conditions
It was hot rolled at ℃, but the rolling pass was in the following order: Format A: 20-30-40-40-34-34.
mm; format B: 22-23-26-28-32 mm.

The final thickness of the sheet is 118 mm in the format A, and therefore the forging coefficient C of the format A is
Was 3.56 and 125 mm for format B, thus the forging coefficient of format B was 2.08. These values were obtained from a forging pressure coefficient tracing device which reveals the forging pressure coefficient by the shape factor of metal particles in the cross section L / TS (FIGS. 1 and 2). The shape factor of the cast particles was about 4-6 for Format A and about 2-3 for Format B.

The two sheets were then solution heat treated at 480 ° C. for 8 hours, quenched with water, controlled stretched to 2.1% and then artificially at 120 ° C. for 10 hours and then at 170 ° C. for 8 hours. Aged to give the property of T7651.

The two sheets have a TL at a half-thick position.
It has perfect properties in terms of fatigue according to a "stair-case" test in which a stress of R = 0.05 in the direction of 10 ⁵ cycles is applied. The size and number of micropores were determined by image analysis of cross-sectional micrographs using fatigue specimens from the "staircase" test. The measurement by image analysis shows that in the polished cross section in the L-TS plane located at half the thickness,
It was performed by examining 0 areas of 1 mm × 1 mm at a magnification of 100 times. That is, a total surface of 150 mm ² was investigated per test piece. The measured parameter is D _max , which is the maximum diameter in the observation plane, where D _max is 20.
It should be larger than micron. The results of the "staircase" fatigue properties and the measured surface pore density are shown in Table I. Table II shows the results of fracture propagation speed.

[0025]

[Table 1]

[0026]

[Table 2]

From the discussion of the above results, it is clear that the casting format of the present invention (Format B) can significantly improve the acceptable stress level at 10 ⁵ cycles and the crack propagation rate da / dn at ΔK = 10 Mpa√m. Is. This improvement is from 20 μm (detection limit)
This is shown by the reduction of pores with a D _max of 100 μm by about 40% and the disappearance of pores with a D _max of greater than 100 μm.

3 and 4 show the intermetallic phase dimensions and distributions in the two formats. It can be seen that the size and density of the intermetallic phase are reduced in the format B case.

Example 2 A format having a thickness of 420 mm after peeling (outside the invention: Format A) and a thickness of 260 mm after peeling
Alloy 7050 plates were produced in the same format (Invention: Format B). The thickness of the metal removed during peeling was the same in both cases. The hydrogen content in the metal was <0.15 g / cm ³ in both cases.

The two plates were subjected to the same conditions (47
And homogenized at 5 ° C. for 16 hours) and the last two rolling passes were hot-rolled in the following order: Format A: 28 mm / 32 mm Final wall thickness 204
mm; Format B: 34 mm / 34 mm Final wall thickness 204
mm.

As a result, the forging-force coefficient (working-down) C in the format A is 2.05,
It was 1.27 in format B.

The two sheets were then solution heat treated at 478 ° C. for 20 hours, quenched with water, controlled stretched to 1.6% and further artificially first at 120 ° C. for 6 hours and then at 165 ° C. for 21 hours. It is aged and imparts the characteristics of T7451.

Two sheets, 242 MPa; R = 0.
1; Fatigue useful life under the condition of Kt = 1 was analyzed. The logarithmic average of the useful lives of the eight specimens taken in the TL direction at each midpoint of the sheet width and thickness is shown in Table III along with the maximum number of pores per cm ² . Where the elastic limit is
Sheets produced according to Format B (of the invention) have 411 MPa in the TL direction and sheets produced from Format A (not of the invention) have 420 M in the TL direction.
Pa.

[0034]

[Table 3]

From a discussion of the above results, it is clear that use of the casting format of the present invention (Format B) can significantly improve the level of fatigue service life and reduce the micropore density.

Example 3 A format having a thickness of 260 mm after peeling (the present invention:
Four alloy 7010 plates were produced in format B). The hydrogen content in the metal and the value of the last rolling pass are shown in Table IV. Further, the elastic limit and fatigue limit at 10 ⁶ cycles were measured in the state T7651 and shown in Table IV (TL direction).

[0037]

[Table 4]

The influence of the hydrogen content is extremely large, the influence of the blocking path during rolling is not so great, and it can be detected only when the hydrogen level is low.

Example 4 Four alloy 7015 plates were produced in a 260 mm thick format (invention; format B).

Two of these plates were manufactured using a reusable chip-free fused bed, the other two were 10
Produced with a fused bed containing% recycled chip.

These plates were transformed according to the method described in Example 2 to give final wall thicknesses of 200 mm and 150 mm.
And 242M in state T7451 in each case
The logarithmic average of the fatigue useful life under the conditions of Pa, R = 0.1 and Kt = 1 was measured. The results are shown in Table V.

[0042]

[Table 5]

In the above case, the inventors used the method of analyzing the characteristics of the pores by ultrasonic waves of 50 MHz as described above.

The obtained results are shown in FIG. Characteristic range 20
By comparing 1 with 203, it can be seen that in the 200 mm thick sheet the influence of the compositional part of the fused bed is significant, the chip-free product 201 has the lowest number of pores and therefore the highest fatigue level. I understand. In a 150 mm thick sheet (comparison of characteristic ranges 202 and 204),
As a result of higher work hardening, this difference is smaller due to the smaller number of pores. Nevertheless, the fatigue level of product 202 is much higher than that of product 204, demonstrating the importance of fused bed quality.

[Brief description of drawings]

FIG. 1 Thickness 1 produced from Format A of Example 1
The cross-sectional microscope photograph in the L / TC (longitudinal / transverse cross-section short side) plane of the 18 mm sheet is shown.

FIG. 2 Thickness 1 produced from Format B of Example 1
A cross-sectional photomicrograph in the L / TC plane of a 25 mm sheet is shown.

FIG. 3 shows the density and distribution of the intermetallic phase in the L / TS plane of the sheet produced from Format A of Example 1.

FIG. 4 shows the density and distribution of the intermetallic phase in the L / TS plane of the sheet produced from Format B of Example 1.

FIG. 5 is a graph of the number of pores per amplitude −1 cm ³ , showing the results of average amplitude analysis of acoustic micrographs performed on the product of Example 5 with the calibration block having flat bottom holes of φ100 μm.

FIG. 6 is a graph of the number of pores per amplitude −1 cm ³ , showing the maximum amplitude analysis result of the acoustic micrograph of the product of Example 5 in which the calibration block was a flat-bottomed hole of φ100 μm.

FIG. 7 shows a method for determining the parameters A _max and A.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Filipp Lasance France, 63500 Isowar, Le Bour Bodible (no street number) (72) Inventor Jean-Christoph Elstroem France, 38130 Equiroll , Plas de Jiacoban, 5 (72) Inventor Jiang Miyure, France, 38500 Boaron, Liu Voltaire, 4

Claims (9)

[Claims] 1. A thick aluminum alloy sheet structurally hardened in fatigue resistance, wherein the density of pores having a size of more than 20 μm located in the vicinity of the rolling center plane is 800 / c.
A sheet characterized by being smaller than m ³ . 2. Sheet according to claim 1, characterized in that the average maximum amplitude of the defects is less than 50%. 3. Sheet according to claim 1, characterized in that the average maximum amplitude of defects is less than 22%. 4. The elastic limit RpO. Wave tension under maximum stress equal to 50% of 2 (R = 0.
The thick sheet according to any one of claims 1 to 3, which has a fatigue life of more than 100,000 cycles with respect to 1). 5. The fatigue life is RpO. 60 of 2
A thick sheet according to claim 4, characterized in that it has more than 100,000 cycles under stress equal to%. 6. A method for producing a thick sheet having a thickness of more than 110 mm, wherein a hot forging coefficient C is 110 to 15 in thickness.
Less than 2.4 for 0 mm, thickness 150-25
It is less than 2 for 0 mm. 7. The method according to claim 6, wherein the hot forging operation is only rolling. 8. The hydrogen content of the liquid metal is 0.15 g / c.
Method according to claim 6 or 7, characterized in that it is less than m ³ . 9. The last two hot rolling passes are each 2
Method according to any one of claims 6 to 8, characterized in that it is larger than 5 mm.