EP1017867B1 - Aluminium based alloy and method for subjecting it to heat treatment - Google Patents

Description

The invention relates to an aluminum-based alloy, preferably of the system Al-Li-Mg, which contains lithium, magnesium, zinc, zirconium and manganese and is in the field of metallurgy of alloys used as a material for constructions in aerospace engineering, in shipbuilding and mechanical engineering of terrestrial means of transport, including welded constructions.

Known alloys of the system Al-Li-Mg, which have a reduced density and a relatively high strength, but have a low ductility and reduced fracture toughness. The alloy after the U.S. Patent No. 4,584,173 from 22.04.86 has, for example, the following chemical composition, in percent by mass: aluminum Base lithium 2.1 - 2.9 magnesium 3.0 - 5.5 copper 0.2-0.7 and one or more elements of the group containing zirconium, hafnium and niobium: zirconium 0.05-0.25 hafnium 0.10 - 0.50 niobium 0.05-0.30 and zinc 0 - 2.0 titanium 0 - 0.5 manganese 0 - 0.5 nickel 0 - 0.5 chrome 0 - 0.5 germanium 0 - 0.2 When this alloy is quenched at a temperature of 530 ° C and thereafter subjected to stretching with a degree of deformation of 2% and artificial aging at 190 ° C over 4 to 16 hours, the disadvantage arises that the alloy has low plasticity in the heat-treated state (relative elongation 3.1 - 4.5%) and has low corrosion resistance.

The alloy according to the international patent application WO No. 92/03583 has the following chemical composition in percentage by mass: aluminum Base lithium 0.5 - 3.0 magnesium 0.5-10.0 zinc 0.1 - 5.0 silver 0.1 - 2.0

For a maximum total content of these elements of 12% and, if their total content is 7.0 - 10.0%, lithium shall not exceed 2.5%, and zinc shall not exceed 2.0%; In addition, the alloy may contain up to 1.0% zirconium.

This alloy has a strength of 476 - 497 MPa, a yield strength of 368 - 455 MPa, a relative elongation of 7 - 9% and a density of 2.46 - 2.63 g / cm ³ . The alloy is recommended as a structural material for aerospace products. The disadvantages of this alloy are as follows:

• durch einen hohen Lithiumgehalt, doch dabei verringert sich die Verformbarkeit und die Bruchzähigkeit der Legierung, ihre Bearbeitungsfähigkeit durch Kaltverformen, es entstehen Schwierigkeiten bei der Herstellung dünner Bleche, welche für Fluggeräte gebraucht werden;
• durch einen hohen Zinkgehalt; dadurch steigt die Dichte der Legierung auf Werte von 2,60 - 2,63 g/cm³, was den Gewichtseinsparungseffekt des Erzeugnisses wesentlich verringert;
• durch Recken des abgeschreckten Werkstoffes vor einer künstlichen Alterung mit einem Verformungsgrad von 5 - 6%, was zu einer Minderung der Kennwerte für die Bruchzähigkeit führt.

The high strength can be guaranteed:

• by high lithium content, but it reduces the ductility and fracture toughness of the alloy, its workability by cold working, difficulties arise in the production of thin sheets, which are used for aircraft;
• by a high zinc content; as a result, the density of the alloy increases to values of 2.60-2.63 g / cm ³ , which substantially reduces the weight-saving effect of the product;
• by stretching the quenched material prior to artificial aging with a degree of deformation of 5 - 6%, which leads to a reduction in the characteristics of the fracture toughness.

The alloy is alloyed with silver, which increases product costs - from semi-finished products to finished products.

High zinc content alloys and added copper have reduced corrosion resistance, while in fusion welding they show an increased tendency to form defects and significant softening.

A comparable alloy for the entire field of application is from the U.S. Patent No. 4,636,357 known. This alloy has the following chemical composition in percent by mass: aluminum Base lithium 2.0 - 3.0 magnesium 0.5 - 4.0 zinc 2.0 - 5.0 copper 0 - 2.0 zirconium 0 - 0.2 manganese 0 - 0.5 nickel 0 - 0.5 chrome 0 - 0.4

• Abschrecken von einer Temperatur von 460°C, Recken mit einem Reckgrad von 0 - 3% und eine zweistufige Wärmebehandlung:
• Die 1. Stufe bei 90°C, 16 h und die 2. Stufe bei 150°C, 24 h.

The alloy consolidation takes place by a heat treatment:

• Quenching from a temperature of 460 ° C, stretching with a degree of stretching of 0 - 3% and a two-stage heat treatment:
• The 1st stage at 90 ° C, 16 h and the 2nd stage at 150 ° C, 24 h.

This alloy has a sufficiently high strength level 440 - 550 MPa and a yield strength of 350 - 410 MPa.

The disadvantages of this alloy are the low level of relative elongation of the alloy (1.0-7.0%) and the low fracture toughness, insufficient corrosion resistance and limited strength of welded joints compared to the strength of the base material.

The object of the present invention, therefore, is to achieve an increase in the ductility of the alloy in the heat-treated state while maintaining high strength and ensuring high corrosion resistance and good weldability, with sufficiently high fracture toughness and thermal stability values after heating at 85 ° C should be guaranteed in the course of 1000 h.

A method for improving the fracture toughness of an aluminum-lithium alloy is out US 4840682 and US 5076859 known. WO 96/18752 A1 describes a method of heat treating an aluminum-lithium alloy.

This object is achieved according to the invention by an alloy of the system AI-li-Mg having the following chemical composition in percent by mass: lithium 1.5 - 1.9 magnesium 4.1 - 6.0 zinc 0.1 - 1.5 zirconium 0.05 - 0.3 manganese 0.01-0.8 hydrogen 0.9x10 ^-5 -4.5x10 ^-5 and at least one element selected from the following group: beryllium 0.001-0.2 yttrium 0.01 - 0.5 scandium 0.01 - 0.3 aluminum rest

The hydrogen content causes a reduction in the longitudinal shrinkage during solidification to form solid finely divided particles of lithium hydride and avoids the formation of porosity in the material.

The magnesium content ensures the necessary level of strength properties and weldability. When the magnesium content falls below 4.1%, it will reduce the strength and grow the tendency of the alloy to hot cracking during both casting and welding. Increasing the magnesium content of the alloy above 6.0% reduces machinability in casting, hot and cold rolling, and plasticity characteristics of finished semi-finished products and articles thereof.

To ensure the necessary machinability, especially in the production of thin sheets, the necessary level of mechanical and corrosion properties and sufficient fracture toughness and weldability, compliance with the lithium content is essential. When the lithium content was lowered below 1.5%, the density of the alloy increased, the level of the strength properties and the modulus of elasticity decreased, and when the lithium content exceeded 1.9%, workability by cold working, weldability, plasticity characteristics and fracture toughness deteriorated.

Zirconium in the amount of 0.05-0.3% is a modifier in the casting of ingots and, together with the manganese (in the amount of 0.01-0.8%), ensures structural solidification in the semi-finished products as a result of the formation a polygonal or fine-grained structure.

In particular, by adding one or more of the elements beryllium, yttrium, scandium, the formation of a homogeneous fine-grained microstructure in semifinished products made of the alloy according to the invention causes an increase in deformability during cold rolling.

Furthermore, the invention relates to a method for the heat treatment of aluminum-based alloys, preferably of the system Al-Li-Mg.

The object of such a heat treatment process is to increase the ductility of the alloy while maintaining its high strength and at the same time to achieve high values of corrosion resistance and fracture toughness, but in particular the Preservation of these properties upon exposure of the material to increased temperature over a long period of time.

• Die 1. Stufe bei einer Temperatur nicht über 93°C, von einigen Stunden bis zu einigen Monaten; vorzugsweise, 66 - 85°C, mindestens 24 h.
• Die 2. Stufe bei einer Temperatur von maximal 219 °C, von 30 Minuten bis zu einigen Stunden; vorzugsweise, 154 - 199°C, maximal 8 h.

From the U.S. Patent No. 4,861,391 For example, a method of heat treatment is known which comprises quenching with rapid cooling, straightening, and two-stage aging as follows:

• The 1st stage at a temperature not exceeding 93 ° C, from a few hours to a few months; preferably, 66-85 ° C, at least 24 h.
• The 2nd stage at a maximum temperature of 219 ° C, from 30 minutes to a few hours; preferably, 154 - 199 ° C, maximum 8 h.

While the strength characteristics and fracture toughness are increased, this method does not ensure the stability of the properties of lithium-containing aluminum alloys after low temperature heating at 85 ° C over 1000 hours, which simulates solar heating during long-term operation of aircraft. After heating to 85 ° C for 1000 h, the relative elongation and fracture toughness of lithium-containing alloys treated by this method are reduced by 25-30%.

• Erhitzen des Materials auf eine Temperatur von 400 bis 500 °C
• Abschrecken in Wasser oder Luft - Richten mit einem Verformungsgrad bis zu 2 % und
• künstliches Altern, wobei das künstliche Altern in 3 Stufen erfolgt, wovon die dritte Alterungsstufe bei 90 bis 110°C im Verlauf von 8 bis 14 h, stattfindet.

In accordance with the invention, a method for achieving the stated object comprises the method steps

• Heating the material to a temperature of 400 to 500 ° C.
• Quenching in water or air - straightening with a degree of deformation up to 2% and
• artificial aging, wherein the artificial aging takes place in 3 stages, of which the third aging stage takes place at 90 to 110 ° C in the course of 8 to 14 hours.

As an alternative to carrying out the third aging stage at a constant temperature, this invention can also be carried out in such a way that cooling takes place at a cooling rate of 2 to 8 ° C. per hour for 10 to 30 hours.

It has been shown that inventive alloys having the feature of claim 1 have particularly advantageous properties in the sense of the task, if they were treated by the aforementioned method.

These heat treatment processes, due to the use of a third aging step, ensure the thermal stability of the alloys after long low temperature aging due to the additional precipitation of the disperse phase δ '- (Al ₃ Li) which is evenly distributed in the matrix volume. The large volume of the finely divided δ 'phase reduces Li supersaturation of the mixed crystal and prevents δ' precipitation during aging at 85 ° C, 1000 h.

In a preferred embodiment of the method according to claim 2 or claim 3, the first stage of artificial aging takes place at a temperature of 80-90 ° C over 3 - 12 h and a second stage at 110 - 185 ° C over 10 - 48 h.

By adhering to these limits particularly favorable conditions for the implementation of artificial aging are created and obtained the results in the sense of the task with great certainty.

Finally, alternatively, a second stage of artificial aging can be carried out at a temperature of 110 to 125 ° C and a duration of 5 to 12 hours, these method parameters are preferably applied when the third aging stage is carried out according to claim 3.

EXAMPLES

Of the alloys whose chemical composition is listed in Table 1, billets of 70 mm diameter were cast. The metal was melted in the resistance furnace. After homogenization (500 ° C, 10 h) strips were pressed from the billets with a cross section of 15 x 65 mm. The ingots were heated to a temperature of 380-450 ° C before being pressed. Rolled blocks from the strips were heated to 360 - 420 ° C and hot rolled to 4 mm thick sheets, which then up to 2.2 mm Thickness have been cold rolled. The cold-rolled sheets were quenched from a temperature of 400-500 ° C in water or air, with a degree of deformation up to 2%, and subjected to the heat treatments listed in Tab. The properties of the base material and welded joints were determined on samples cut from these sheets. (see Table 3) table Chemical compositions of the investigated compositions Alloy no. Li mg Zn Zr Mn Hx 10 ⁵ Be Y sc Cr Cu Ni 1 2.2 1.2 5.0 - - - - - - 0.4 - - 2 2.4 3.8 3.9 0.18 0.50 - - - - - 0.96 0.2 3 1.5 6.0 0.1 0.15 0.60 0.9 0.2 - - 0.12 - - 4 1.9 5.2 0.8 0.10 0.01 4.5 0.001 - 0.01 - - - 5 1.7 4.1 1.5 0.30 0.05 2.5 - 0.25 - - - - 6 1.6 5.2 0.6 0.05 0.80 2.5 - 0.01 - 0.15 - - 7 1.85 4.8 0.5 0.09 0.20 3.5 - 0.50 - 0.50 - - 8th 1.55 4.2 0.1 0.05 0.10 2.5 - - 0.30 - - - 9 1.9 4.7 0.1 0.15 0.35 2.5 0.1 - - 0.01 - - 10 1.5 4.3 0, 3 0.1 0.40 3.5 0.1 - - - - - Note: Alloys Nos. 1 and 2 are comparative materials
Alloys Nos. 3-10 are materials according to the invention Process for heat treatment of the investigated alloys Alloy no. Heat treatment no. Heat treatment method 3, 5, 9 3 80 ° C, 4 h + 185 ° C, 10 h + 110 ° C, 8 h 8.10 4 90 ° C, 3 h + 110 ° C, 48 h + 90 ° C, 14 h 4, 7 5 85 ° C, 5 h + 145 ° C, 25 h + 110 ° C, 10 h 6 6 85 ° C, 12 h + 120 ° C, 12 h + 90 ° C, 12 h Properties of the investigated alloys Alloy no Heat treatment no. ^σ B, MPa ^σ 0.2, MPa δ,% K _CO , MPa√m (w _o = 200 mm) Before aging at 85 ° C, 1000 h After aging at 85 ° C, 1000 h 1 1 460 345 3.5 59.5 46.0 2 2 470 355 2.5 58.7 44.8 3 3 475 358 9.0 69.5 67.0 4, 7 5 473 347 8.0 68.3 66.8 5 3 458 345 8.5 67.8 66.7 6 6 450 338 10, 0 68.0 67.8 8, 10 4 452 340 9.5 66.8 65.9 9 3 460 345 9.0 65.5 63.3 Notes: Alloys and Methods 1 and 2 are comparative materials from a 2-stage heat treatment process.
Alloys and methods Nos. 3 - 10 correspond to the invention.

Claims (5)

1. Aluminium-based alloy which contains lithium, magnesium, zinc, zirconium and manganese, characterized in that the alloy additionally contains hydrogen and at least one element from the group consisting of beryllium, yttrium and scandium, with the following ratio of the components in per cent by mass: Lithium 1.5-1.9 Magnesium 4.1-6.0 Zinc 0.1-1.5 Zirconium 0.05-0.3 Manganese 0.01-0.8 Hydrogen 0.9 × 10^-5-4.5 × 10^-5 at least one element selected from the following group: Beryllium 0.001-0.2 Yttrium 0.01-0.5 Scandium 0.01-0.3 Aluminium balance.
2. Process for the heat treatment of aluminium-based alloys having lithium constituents according to Claim 1, which comprises the process steps

   - heating of the material to a temperature of 400-500°C

   - quenching in water or air

   - straightening with a degree of deformation of up to 2% and

   - artificial aging,

   characterized in that the artificial aging is carried out in three stages, with the second stage being carried out at a higher temperature than the first stage and the third aging stage being carried out at from 90 to 110°C over a period of from 8 to 14 hours.
3. Process for the heat treatment of aluminium-based alloys according to Claim 1, which comprises the process steps:

   - heating of the material to a temperature of from 400 to 500°C

   - quenching in water or air

   - straightening with a degree of deformation of up to 2% and

   - artificial aging,

   characterized in that the artificial aging is carried out in three stages of which the third aging stage comprises cooling at a cooling rate of from 2 to 8°C per hour for from 10 to 30 hours.
4. Process according to Claim 2 or Claim 3, characterized in that the first stage of the artificial aging is carried out at a temperature of 80-90°C over a period of 3-12 hours and a second stage is carried out at 110-185°C over a period of 10-48 hours.
5. Process according to Claim 2 or Claim 3, characterized in that the first stage of the artificial aging is carried out at a temperature of 80-90°C over a period of 3-12 hours and a second stage is carried out at 110-125°C over a period of 5-12 hours.