IE41631B1 - Aluminium alloys - Google Patents

Abstract

1511702 Alumin alloys CEDEGUR SOC DE TRANSFORMATION DE L'ALUMINIUM PECHINEY and SOC POUR LE FORGEAGE ET L'ESTAMPAGE DES ALLIAGES "FORGEAL" 22 July 1975 [23 July 1974] 30599/75 Heading C7A An aluminium based alloy contains Cu 1À80-3À0 Mg 1À2-2À7 Ti 0À01-0À2 Si 0-0À3 Fe 0À1-0À4 Ni and/or Co 0À1-0À4 Al Bal plus impurities the Al and Co:Fe ratio being 0À9-1À3. Optionally the alloy may contain at least one of Zr, Mn, Cr, V, MO in the amount of 0-0À4%, and further Cd, In, Sn and Be in the range 0-0À2%. Silver at 0-1% and/or Zinc 0-8% may also be present. The alloy may be solution heat-treated at 520-535‹C, followed by quenching in water and optionally stretching by 2% prior to ageing at 150‹-230‹C. An alternative solution treatment at 555‹C may be used. The alloy may be cast-to-shape or cast and plastically deformed by any method, and has improved properties, particularly in the short transverse direction over similar standard alloys i.e. A-U 2 GN in which the Fe and Ni contents are greater. The alloy is suitable for use in aeronautical applications, where good stress-corrosion resistance and high resistance to crash propagation is required.

Description

This invention relates to structurally hardening alloys with excellent mechanical properties and, more especially, to aluminium alloys with excellent high-temperature properties and favourable creep behaviour.

It is well known that the aeronautical industry uses several varieties of this type of alloy whose excellent high-temperature characteristics make them particularly suitable for use in the construction of supersonic aircraft. Among these alloys, A-UgGN, described in particular in French Patent Specification 978,805 has experienced remarkable development in recent years.

It has the following composition: Copper 1.8 to 2.52 Magnesium 1.2 to 2 % Nickel 0.3 to 1.5% ) Iron 0.85 to 1.5% ) Silicon 0 to 0.4% Ti tanium 0.02 to 0.2% with Fe+Ni between 1.52 and 2.75% The proportions of copper and magnesium in this alloy permit structural hardening by submicroscopic precipitation of the phase S'AI-Cu-Mg.

It is known that iron and nickel are added in order to obtain favourable high-temperature characteristics and, in particular, high creep resistance by virtue of the presence of an Al-Fe-Ni-phase which is an obstacle to the propagation of dislocations. On the other hand however, this alloy has strength characteristics, as represented by the stress intensity factor K^C and crack propagation resistence, which without being adequate, do not put it among the best in this field.

The K-jC factor, by which the strength of an alloy may be characterised, is measured in accordance with ASTM - E - 399 - 72.

The present invention relates to a new alloy composition which, by comparison with conventional A-UgGN, has greater strength, especially in the - 2 41631 short transverse direction, a better K^C factor and a greater resistance to the propagation of fatigue cracks whilst, at the same time, retaining the favourable high temperature characteristics and the high resistance to stress corrosion of that alloy.

The alloy according to the invention is a new aluminium alloy which contains copper, magnesium and nickel, in the same way as conventional A-U^GN, but which is distinguished by the fact that the contents by weight of iron and nickel have each been reduced to a level of from 0.10% to 0.40% and by the fact that the ratio by weight of the nickel content to the iron content is from 0.9 to 1.3.

The invention also relates to articles moulded from this liquid alloy, to semi-finished products such as, sheets, panels, and billets cast from this alloy, articles of this alloy such as thick plates for aeronautical construction obtained from such semi-finished products by rolling, extrustion, forging, dropforging, stamping, drawing, roll forminq and, generally by any method of plastic deformation.

We have surprisingly found that the effect of the Al-Fe-Ni-phase and the effect of other intermetallic compounds Al-Cu-Ni and Al-Cu-Fe present in the conventional alloy is by no means beneficial. On the contrary, each of these three compounds has a relatively significant adverse effect upon resistance to the propagation of fatigue cracks and on the stress intensity factor K^C.

We have also found that, contrary to the view generally held upon the effect on high-temperature characteristics of the compound Al-Fe-Ni, a significant reduction in the iron and nickel contents does not have any appreciable effect upon the high-temperature characteristics of the alloy. By contrast, and provided that a ratio by weight of Ni to Fe of approximately 1:1 is maintained, the strength, especially in the short transverse direction the stress intensity factor K-|C and the resistance to the propagation of fatigue cracks are distinctly improved. - 3 5 Accordingly, the composition of the new alloy according to the invention is as follows: Copper Magnesium Nickel Iron 1.80% to 3.00%, preferably 2.10% to 2.70% 1.20% to 2.70%, preferably 1.40% to 2.00% 0.10% to 0.40%, preferably 0.10% to 0.35% 0.10% to 0.40%, preferably 0.10% to 0.35% with = 0.9 to 1.3 Fe titanium 0.01% to 0.20% silicon 0% to 0.30%, preferably 0.15% to 0.25%, and the balance is aluminium, lo The nickel may be completely or partly replaced by cobalt.

Other elements, namely zirconium, manganese chromium, vanadium, and molybdenum, may be added in quantities of from 0 to 0.4% of each.

It is also possible to add cadmium, indium, tin and beryllium in quantities of from 0 to 0.2% Finally, silver and zinc may also be added in quantities of from 0 to 1% and from 0 to 8% respectively.

The heat treatment of this type of alloy is no different from that of conventional A-UgGN. In other words it comprises: solution heat treatment at a temperature in the range from 520°C to 535°C; quenching with cold, tepid or boiling water; and ageing at a temperature in the range of from 150°C to 230°C.

The alloy may also be work-hardened between quenching and ageing. It is of advantage to relax and to render uniform the internal stresses due to hardening, and to increase the mechanical characteristics of the alloy, especially its yield strength.

A heat treatment of the type claimed in French Patent Application No. 7,400,399 filed 7.1.1974 may also be carried out. This heat treatment comprises solution heat-treatment at a temperature above that at which the alloy begins to melt.

This solution heat-treatment should last long enough for the liquid phases, that appear to be almost completely resorbed before hardening. Hardening and ageing are carried out under conditions identical with those mentioned above. For example, solution heat treatment at 555°C (instead of 520 to 535°C) improves the mechanical characteristics of the alloy. The tensile strength is improved by 1 to 2 hbar.

To enable the invention to be better understood, the method of measuring and the significance of the K-jC factor, and the measurement of the propagation rate of fatigue cracks, are briefly recalled in the following.

The stress concentration factor K-jC, is measured by the method described in the Standard ASTM - E - 398 - 72.

Without entering into a calculation of the factor in any detail, it is useful to know the principle on which this method is based so as to be able to understand its significance.

A test specimen of the kind illustrated in Figure 1 of the accompanying drawing is taken from the product. The line joining the centres of the two holes is in the short transverse direction. The edge of the slot produced by machining is in the long transverse direction.

The test specimen is then subjected to a series of undulating traction cycles by fixing it to a fatigue machine by means of pins passing through the holes. The undulating traction cycles are stopped after the initiation of a fatigue crack approximately 1.3mm (l/20th. inch) long, as measured on the surface of the test specimen from the end of the slot. The test specimen is then fixed to a tensile machine, again by means of pins passing through the holes. An increasing force is then applied until the test specimen breaks, the tensile force being recorded as a function of the distance between the lips of the test specimen, as measured by means of a pick-up. A curve of the type illustrated in Figure 2 of the accompanying drawing is obtained.

A graphic method may be used for determining the load whose application caused the propagation of the crack formed by fatigue by a distance equal to 2% of its initial length. The coefficient K^C - 5 41631 is proportional to P , the proportionality factor being calculated in M dependence upon the geometric dimensions of the test specimen and upon the length of the crack previously initiated by fatigue. This coefficient is expressed in hbar Vmm.

The higher the K^C factor is for a given test specimen, the higher is P^ and, hence, the higher is the force required to propagate the crack by 2%.

The propagation rate of the fatigue cracks is measured as follows: A central hole 2rtun in diameter is drilled at the centre of a rectangular test specimen 250mm long, 100mm wide and 1.6mm thick. This test specimen is subjected to fatigue cycles by undulating traction, and the length of the cracks is measured from photographs taken at regular intervals. The curve of the length of the crack as a function of the number of cycles is then drawn, enabling the crack propagationrrate to be measured.

The following Examples are purely illustrative and do not limit the invention in any way. They are based on a series of treatments and property measurements on five alloys with the following compositions: Alloy 0 Cu = 2.65% Alloy 1 Cu = 2.63% Mg = 1.64% Mg = 1.60% Fe = 1.07% Fe = 0.30% Ni = 1.19% Ni = 0.35% Si = 0.22% Si = 0.22% Ti = 0.10% Ti 0.10% Alloy 2 Cu = 2.63% Mg = 1.60% Alloy 3 Cu = 2.65% Mg = 1.64% Alloy 4 Cu = 2.65% Mg = 2.04% Fe = 0.15% Fe = 0.29% Fe = 0.15% Ni = 0.17% Ni = 0.36% Ni = 0.17% Si = 0.22% Si = 0.22% Si = 0.23% Ti = 0.10% Ti = 0.10% Zr = 0.12% Ti = 0.11% The alloy 0 is the control alloy whose composition corresponds to French Patent 978,805. The other alloys have different compositions all falling within the scope of the present invention.

EXAMPLE 1 Alloys 0,1,2 and 3 are cast semi-continuously in the form of panels measuring 120 x 380mm, and then homogenised for 20 hours at 520°C.

After their surfaces have been cut and scalped, the panels are 5 reheated to 480°C, and rolled in 10mm stages to a thickness of 45mm.

The thick plates thus obtained are then subjected to the following treatment: solution heat-treatment for 10 hours at 530°C quenching in water; io stretching between quenching and ageing, corresponding to a permanent elongation of 2%; and final ageing for 20 hours at 190°C.

Test specimen for measuring the tensile properties in the short transverse direction and in the long transverse direction, test specimens for measuring the coefficient K^C, test specimens for measuring the crack propagation rate and test specimens for measuring creep, were taken from the plates obtained from 4 panels of the alloys.

The mechanical characteristics and the K-jC factor are shown in the following Table: 2o (Rp=Yield strength; Rm=Ultimate tensile strength) Alloys Tensile characteristics K]C Short Transverse di recti on Long transverse direction Short transverse direction Rp 0.2 nbar Rm hbar Rp 0.2 nbar Rm hbar E% hbar Vim 0 41.6 45.9 8.5 40.8 46.3 8.7 71.4 1 41 45 8.4 40.2 45.1 7.9 84 2 40.5 44.6 7.9 39.1 44.3 7.1 98.6 3 41.2 45.2 8.9 40.1 45.2 7.8 86.9 - 7 41631 It can be seen that the coefficients KjC of alloys 1,2 and 3 (new composition) are distinctly superior (by 38% in the best case) to that of the control alloy (conventional composition) without any significant reduction in the tensile characteristics.

The rate of propagation of fatigue cracks, expressed in mm per 1000 cycles measured between 5 and 20mm, are as follows: ίο Alloys Mean propagation rate mm/1000 cycles 0 0.87 1 0.69 2 0.64 3 0.79 It can be seen that alloys 1,2 and 3 according to the invention have lower crack propagation rates than alloy 0 used as a control.

The creep stresses o—in the short transverse direction were also determined, o—represents the stress causing a creep elongation of 0.1% when maintained at temperature over a period of 100 hours.

The creep results at temperature of 130°C and 175°C are shown in the following Alloys __100 °~0.1 At 130°C hbar At 175°C hbar 0 27.5 17.6 1 27.0 17.9 2 26.4 17.3 3 27.2 18.0 Accordingly, the creep resistance of alloys 1,2 and 3 according to the invention is entirely comparable with that of alloy 0 used as a control.

EXAMPLE 2 Alloys 0,2 and 4 were cast semi continuously in the form of panels measuring - 8 41631 120 x 380mm and then homogenised for 20 hours at 520 C.

After their surfaces have been scalped, the panels are reheated to 480°C and rolled in 10mm stages to a thickness of 45mm.

The thick plates obtained are then subjected to solution heat-treatment 5 for 10 hours at 530°C, followed by quenching in cold water.

Finally, and on this occasion without “straightening by stretching between hardening and ageing, the plates were aged for 20 hours at 203°C to bring them to a T6 condition which is generally used in the case of forgings and stampings.

The tensile characteristics were measured in the long transverse and short transverse directions.

The results obtained are shown in the following Table: (Rp=Yield strength; Rm=Ultimate tensile strength) Alloys Long Transverse direction Short transverse direction Rp0.2 hbar Rm hbar E% 5.6 V so Rp0.2 hbar Rm hbar E% 5.65 0 37.3 42.8 6.9 38.3 43.5 6.8 2 38.3 42.5 6.6 38.2 42.2 5.3 40,3 44.3 6.2 40.2 44 5 2Q In the T6 condition (ST quench and ageing) alloy 2 has tensile mechanical traction characteristics comparable with those of alloy 0. Alloy 4 has a greater yield strength and breaking load than the control alloy.

The K-jC factor increases from a value of 82hbar V~mm in the case of alloy 0 to 102 hbar V mm in the case of alloy 2 and to 103 hbar Vmm in the case of alloy 4.

Claims (11)

1. An aluminium based alloy containing from 1.80 to 3.00% of copper, from 1.20 to 2.70% of magnesium, from 0.01 to 0.2% of titanium, from 0 to 0.30% of silicon, iron and at least one of nickel and cobalt, wherein the iron content is 5 from 0.10 to 0.40%, the total nickel plus cobalt content is from 0.10 to 0.40% and the ratio of the nickel plus cobalt content to the iron content is from 0.9 to 1.3, together with impurities and incidental constituents, and the balance is aluminium.

2. An aluminium-based alloy containing from 2.10 to 2.70% of copper, from 10 1.40 to 2.00% of magnesium, from 0.01 to 0.2% of titanium, from 0.15 to 0.25% of silicon, iron and at least one of nickel and cobalt, wherein the iron content is from 0.10 to 0.35%, the nickel plus cobalt content is from 0.10 to 0.35% and the ratio of the nickel plus cobalt content to the iron is from 0.9 to 1.

3. , together with impurities and incidental constituents, and the balance is aluminium. 15 3. A modification of an aluminium alloy as claimed in claim 1, or claim 2 wherein the alloy additionally contains at least one of the elements Zr,Mn,Cr,V, Mo, the content of each of these elements in the alloy being from 0 to 0.4%.

4. A modification of an aluminium alloy as claimed in any of claims 1 to 3, Wherein the alloy additionally contains at least one of the elements Cd,In,Sn,Be, 20 the content of each of these elements in the alloy being from 0 to 0.2%.

5. A modification of an aluminium alloy as claimed in any of claims 1 to 4, wherein the alloy traditionally contains at least one of the following elements: silver in a quantity of from 0 to 1%, zinc in a quantity of from 0 to 8%.

6. An alloy as claimed in claim 1 substantially as herein described with 25 reference to either of the specific examples.

7. Slabs and billets composed of an aluminium alloy as claimed in any of claims 1 to 6, and suitable for forging, extrustion, rolling or like methods of plastic deformation.

8. Articles moulded from an aluminium alloy as claimed in any of claims 1 to 6. 30

9. Thick plates intended in particular for aeronautical construction obtained by rolling sheets of an aluminium alloy as claimed in any of claims 1 to 5. - 10 41631

10. Articles produced by forging,stamping,drop-forging,drawing, roll forming or extrustion from the semi-finished products as claimed in claim 7.

11. Thick plates and articles produced by forging,stamping,drop-forging, drawing, roll forming or extrustion from an aluminium alloy as claimed in any 5 of claims 1 to 6, wherein the heat treatment by which structural hardening is obtained comprises solution heat treatment at a temperature above that at which the alloy begins to melt, followed by quenching and ageing.