FR2715409A1 - Heavy plate made of fatigue resistant aluminum alloys and process for obtaining same. - Google Patents

Abstract

The invention relates to Al alloy heavy plates resistant to fatigue and to a process for obtaining the same. These sheets have a fatigue life greater than 100,000 cycles in corrugated traction (R = 0.1) under a maximum stress equal to 50%, and even 60%, of the elastic limit. The method consists of an elaboration leading to an H2 content of less than 0.15 g / cm3, in a hot working greater than 2 or 3.3 depending on the final thickness with the last two rolling passes greater than 15 mm each. . The invention finds its main applications in the aeronautical and space fields.

Description

l

  STRONG SHEETS OF ALLOY RESISTANT ALUMINUM ALLOYS AND PROCESS

OBTAINING

  The invention relates to plates made of Al alloys resistant to

  fatigue and a method of obtaining the same.

  It is known that the microporosities and the intermetallic phases formed during casting are preferential initiation sites for fatigue cracks; however the presence of these is inevitable especially in the metallurgical products of large section obtained by conventional vertical casting. So far, and in order to minimize both the density and / or the maximum size of the microporosities and the intermetallic phases, it has been sought to sink very thick plates, in order to reseal most microporosities and to fragment most of the phases. intermetallic, generally concentrated in the vicinity of the large median plane of the plate, during hot rolling using elevations C to Ei / Ef, Ei

  being the initial thickness and Ef the final thickness.

  It is known that the usual process for obtaining thick plates (thickness> 10 mm) consists of the casting of plates, their scalping, their homogenization, their hot rolling. The sheets thus obtained are dissolved and quenched, optionally undergo controlled traction to reduce the level of the internal stresses and then are walled (states T 351 or T4) or returned to the states T 651 or T 7x51, following

  the nomenclature of the Aluminum Association.

  Thus, for example, for sheets having final thicknesses Ef of between 100 mm and 150 mm, it is generally possible to use plates having

  a thickness after scalping> 3.5 Ef, that is to say corrections C> 3.5.

  The present invention relates to sheets having final thicknesses> 110 mm. It relates to structurally hardened Al alloy sheets which in the treated state (hardened, tempered and tempered, or quenched matured and tempered) have good internal health, as well as a fatigue life greater than 100,000 cycles in undulating tension (R = 0,1) under a maximum stress equal to 50% (and even 60%) of the

elastic limit RpO, 2.

  Contrary to the teaching of the prior art, the Applicant has found, surprisingly, that significantly improved fatigue properties could be obtained by limiting the wrought to values C <3.3 for sheets having final thicknesses between 110 mm and 150 mm (and therefore with thicknesses of scalped plates <360 mm), and even at C <2 values for sheets with final thicknesses between 150 mm and 250 mm (and therefore with plate thicknesses

scalpées <300 mm).

  It has also been found that the fatigue properties are further improved if the liquid metal at the time of casting has been properly degassed by methods known to those skilled in the art (degassing with chlorine or products containing chlorine, argon degassing pockets), so that the hydrogen content in the liquid metal is less than 0.15 g / cm3. It has also been found that the fatigue properties are further improved if during hot rolling the last two rolling passes (thickness

  input-output thickness) are greater than 15 mm each.

  The products according to the invention resistant to fatigue are characterized by good internal health, determined by a U.S control in the vicinity.

  the median plane of the thick plates.

  The control device used is a Synergetics PR 02 transceiver or Panametrics 5052 transceiver with the following settings: Focused US: focal spot (at -6 dB or mid-amplitude) elongate ellipsoid shape / 100 mm x 600 am frequency: 50 MHz Broad band probe (& f = 70%) fo I1 is associated with digital acquisition and processing means

  signal, with controlled displacement of the probe in xy at a pitch of 0.02 mm.

  The apparatus is calibrated using a standard block of the same nature and in the same state of heat treatment as the tested product, having flat bottom holes / 50 and 100 am, to which the maximum amplitude is conventionally attributed. 80% on the control screen, which fixes the gain

Global Go.

  The volume examined, on a slice of a maximum thickness of 20 mm, is

x 20 x 0.6 mm3.

  The measurements are located in several locations to obtain

reliable statistical values.

  The porosities are defined by the peaks exceeding a threshold, adjustable, and

  set slightly above the background noise.

  We determine: - the number of porosities (of equivalent size> 20 am, detection threshold) per unit volume - the arithmetic mean of the maximum amplitudes Amax, Amax being the maximum value of the echo on a porosity, - the arithmetic mean mean amplitudes A, A being equal to _ _1 (2 A dx dy, x and y being the distances on (xl-x2) (yl-y2)} f /

  the image of the porosity considered (see Fig. 7).

  Under the conditions defined above, the products according to the invention respect the following limits: Number of porosities S 800 / cm3 Average of the average amplitudes A! 22% Average of the maximum amplitudes Amax I 50% In the rest of the text we will use the following abbreviations or notations: L = long direction TL = long direction TC = short transverse direction R = maximum stress / minimal stress of a cycle during fatigue tests Kt = notch coefficient & K = variation of stress intensity factor during fatigue tests da / dn = speed of propagation of fatigue cracks The invention will be better understood with the aid of the following examples, illustrated

by Figures 1 to 7.

  FIG. 1 represents a L / TC (long / short-sided) macrographic section of a 118 mm thick sheet of A format

of Example 1.

  Figure 2 shows a macrographic section along the L / TC plane

  a sheet of 125 mm from the format B of Example 1.

  FIGS. 3 and 4 show the density and the distribution of the intermetallic phases of the sheets respectively from A and B format of

Example 1, L / TC plan.

  FIGS. 5 and 6 represent in an amplitude - number of porosities / cm3 diagram, the results of acoustic micrographs made on

  the product of Example 5, the standard being a flat-bottomed hole of 100 μm.

  FIG. 7 represents the principle of determination of the Amax and A magnitudes. The alloys will be designated according to the nomenclature of the Aluminum Association.

EXAMPLE 1

  7010 alloy plates were developed firstly in 420 mm thickness after scalping (outside the invention: A format) and secondly in 280 mm thickness after scalping (according to the invention: format B format ). The thickness of metal removed on scalping is identical in both cases. The hydrogen content in the liquid metal is in the

two cases <0.15 g / cm3.

  These scalped plates were homogenized under identical conditions (30 hours at 470 C) and then hot-rolled at 390 ° C. under identical conditions, with the following sequence of rolling passes: Format A: 20-30-40-40- 34-34 mm Format B: 22-23-26-28-32 mm The final thickness of the sheets is 118 mm for the A format, which leads to a wrenching factor L of 3.56 for the A format and of 125 mm for the B format, which leads to a wrenching factor L of 2.24 for the B format. A plotter of the regrinding factor revealed by the metallurgical grain form factor in a section L is available. TC (Figures 1 and 2). The shape factor of the pouring grain is of the order of 4 to 6 for the format A and of the order of 2 to 3 for the format B. The two sheets then underwent an 8 hour dissolution treatment at 480 C followed by quenching with water, a controlled pull of 2.1% and a treatment of income of 10 hours at 120 C followed by 8 hours at 170 C, so as to give them the characteristics of state T 7651. The two sheets were subjected to a complete fatigue characterization with "stair-case" tests at 105 cycles in the TL direction at mid-thickness with R = 0.05. The fatigue test pieces of the "staircase" test were used to measure the size and number of microposities by microscopic image analysis. Measurements by image analysis were performed on a polished section in a mid-thickness L-TC plane, under 100 x magnification, with examination of 150 fields of 1 mm x 1 mm, ie a total examined surface of 150 mm 2 per specimen. The parameter measured is Dmax, the largest dimension in the observation plane, for all Dmax> 20 microns. The results of the stair-case fatigue characterizations and measurements of the surface density of the microporosities are summarized in Table I. The results of propagation velocities

  cracks are collected in Table II.

TABLE I

  Thickness. Constraint to Number of RpO, 2 (mm) (MPa) / cm2 having / cm2 having (MPa) <Dmax> 100 mm Dmax (100 amn A-cast 118 212 15 107 3 455 A-casting 118 210 5 110 10 455 B-casting 125 246 5 67 O 453

93193

TABLE II

  Final thickness da / dN to ODM da / dN to 20MPaim Sheet format (mm) (mm / cycle) (mm / cycle) A-casting 92990 118 -4 -3

1.8 10 10

B-casting 93139 125 -4 -3

10 10

  It is clear from the examination of these results that the use of the casting format according to the invention (Format B) makes it possible to very significantly improve the stress level admissible at 105 cycles and the crack propagation speed da / dn. at & K = 10 MPa 4m. This improvement is correlated with a decrease of about 40% of porosities with Dmax between 20 am (limit of detection) and

  rm, and the disappearance of porosities with a Dmax> 100 am-

  Figures 3 and 4 show the size and distribution of intermetallics for both formats. There is reduced intermetallic size and density in the case of the B format.

EXAMPLE 2

  7050 alloy plates were developed on the one hand in 420 mm thick formats after scalping (outside the invention: A format) and on the other hand in 280 mm thickness after scalping (according to the invention: format B ). The thickness of metal removed on scalping is identical in both cases. The hydrogen content in the metal is in both cases

<0.15 g / cm3.

  These plates were homogenized under identical conditions (16 hours at 475 ° C.) and then hot-rolled with the following sequence of the last 2 rolling passes: Format A 28 mm / 32 mm Final thickness 204 mm Format B 34 mm / 34 mm final thickness 204 mm This leads to a hardening factor C of 2.05 for the A format and 1.37 for the B format. The two sheets then underwent a 20-hour resetting treatment at 478 C followed by water quenching, controlled pulling of 1.6% and a 6 hour treatment at 120 C followed by 21 hours at 165 C

  in order to give them the characteristics of the state T 7451.

  Both sheets were characterized in fatigue life under 242 MPa; R = 0.1; Kt = 1. The logarithmic life averages on 8 specimens taken in the TL direction at mid-width and mid-thickness of the sheets are grouped together in Table III, as well as the maximum number of porosities per cm 2. In this state, the elastic limit is 411 MPa in the TL direction for the plate resulting from the B format (according to the invention) and 420 MPa in the TL direction for the plate resulting from the A format.

(except invention).

TABLE III

  Final thickness Average Maximum number RpO, 2 Format of the logarithmic plate of porosity / TL (mm) duration of (cm2) (MPa) life (in cycles) A-cast 96343 204 71000 7 420 B-cast 96381 204 118000 4 411 It is clear from the examination of these results that the use of the casting format according to the invention (format B) makes it possible to very significantly improve the level of fatigue life and to reduce the

density of microporosities.

EXAMPLE 3

  7010 alloy plates were developed on the one hand in 420 mm thick format after scalping (outside the invention: A format) and secondly in 350 mm thick format after scalping (according to the invention: C format ). The thickness of metal removed on scalping is identical in both cases. The hydrogen content in the metal is in both cases

(0.15 g / cm3.

  These plates underwent the transformation range described in Example 1 to a final thickness of 120 mm. The wrought ratio L is therefore 3.5 for the format A and 2.91 for the C.25 format. These sheets have been characterized in fatigue in the T7651 state at 242 MPa.

  with R = 0.1; Kt = 1. The log-average of lifetimes obtained on 8 samples taken in the TL direction in the median plane of the products are shown in Table IV. At this state, the yield point is 470 MPa.

TABLE IV

  Final thickness Average TC elongation (%) Log length (mm) format (mm)

A 120 78599 3.6

C 120 124535 5.3

  The use of the format C according to the invention makes it possible to improve the duration

  of life in fatigue and the value of lengthening Short Travers.

EXAMPLE 4

  Four plates 7010 alloy were developed in 280 mm thickness after scalping (according to the invention: format B). The hydrogen contents in the cast metal and the values of the last rolling passes are reported in Table V. The elastic limits and the fatigue limit at 106 cycles were measured in the T7651 state and are shown in FIG.

Table V (TL direction).

TABLE V

- -

  H2 format 2 last Porosities / cm2 RpO stress, 2 (g / cm3) Passes (mm) (> 20 am) rupture at 106 TL cycles (MPa) (MPa)

B / 1 0.3 7/5 109 115 5.9 446

B / 2 0.3 17/16 90 112 3.8 443

B / 3 <0.15 6/5 16 200 26.1 451

B / 4 <0.15 19/18 7 215 11.3 455

  The effect of the hydrogen content is very significant, the effect of rolling passes to rolling is less pronounced and sensitive only

  on low levels of hydrogen.

EXAMPLE 5

  Four 7050 alloy plates were developed in 280-thick format

mm (according to the invention: format B).

  Two of these plates were developed with a melting bed not containing recycled chips, the other two plates were developed with

  a melting bed comprising 10% recycled chips.

  These plates were transformed according to the range described in Example 2, and conducted to final thicknesses of 200 mm and 150 mm. The logarithmic averages of fatigue life at 242 MPa, R = 0.1, Kt = 1 at T 7451 were measured in each case. The results

are grouped in Table VI.

TABLE VI

  Marked Chip Content Average Thickness RpO, 2 of the final logarithmic TL melting time (MPa) (%) (mm) (cycles)

201 0% 200 118000 411

202 0% 150 145000 436

203 10% 200 93000 413

204 10% 150 112000 447

  In this case we used the technique of characterization of

  Ultrasonic porosities at 50 MHz as described above.

  The results obtained are grouped together in FIG. 5. It is verified by comparing the characteristic domains 201 and 203 a marked effect of the composition of the melting bed for the sheets 200 mm thick, the product 201 without chips having the number of porosity the lowest correlated to the highest fatigue level. This difference is attenuated for the lower number of porosities on sheets of 150 mm thickness (comparison of the characteristic domains 202 and 204) due to the higher work hardening. However, the level of fatigue of the product 202 remains significantly higher than that of the product 204, confirming

  the importance of the quality of the melting bed.

1l

Claims (8)

  1. Thick plates of Al alloys with fatigue-resistant structural hardening, characterized in that the density of the pores of equivalent size greater than 20 mm and located in the vicinity of the median plane of rolling is less than 800 per cm 2. Sheet according to claim 1 characterized in that the average of   maximum amplitudes of defects is less than 50%.   3. Sheets according to claim 1 characterized in that the average of   Mean amplitudes of defects is less than 22%.   4. Thick plates according to one of claims 1 to 3, characterized in   in the treated state, they have a fatigue life greater than 100 000 cycles in undulating traction (R = 0.1) under a   maximum stress equal to 50% of the elastic limit RpO, 2.   5. Thick plates according to claim 4 characterized in that the fatigue life exceeds 100,000 cycles under a constraint equal to 60% of RpO2.   6. Method for obtaining thick sheets greater than 110 mm, characterized in that the hot working C is less than 3.3 for thicknesses between 110 and 150 mm and C is less than 2   for thicknesses between 150 and 250 mm.   7. Method according to claim 6 characterized in that the H2 content   liquid metal is less than 0.15 g / cm3.   8. Method according to one of claims 6 or 7 characterized in that   the last two hot rolling passes are greater than 15 mm each.