WO2010029572A1

WO2010029572A1 - Method for manufacture of aluminium alloy sheets

Info

Publication number: WO2010029572A1
Application number: PCT/IN2009/000436
Authority: WO
Inventors: Vadlamani Subramanya Sarma; Vivek Srivastava; V. L. Niranjani
Original assignee: Aditya Birla Science and Technology Co Ltd; Indian Institute of Technology Madras
Current assignee: Aditya Birla Science and Technology Co Ltd; Indian Institute of Technology Madras
Priority date: 2008-07-31
Filing date: 2009-07-31
Publication date: 2010-03-18
Anticipated expiration: 2011-01-31

Abstract

The present invention relates to a method for manufacture of a high-strength aluminium alloy, the method comprising heating billets of cast alloy containing aluminium and alloying elements at a temperature in the range of 450 °C to 550 °C to homogenize the alloy, deforming the billets in the homogenized state to form a hot worked alloy, first allowing the hot worked alloy to cool to room temperature and then heating the hot worked alloy from room temperature to a temperature at which the alloy forms a solid solution, quenching the solid solution in water, deforming the quenched alloy at room temperature to form a cold worked alloy and aging the cold worked alloy at a temperature in the range of 50 to 150 °C. The invention also relates to high strength aluminium alloy manufactured by the process and to the articles and products shape formed from the alloy.

Description

METHOD FOR MANUFACTURE OF ALUMINIUM ALLOY SHEETS

Field of invention

The invention relates to a method for manufacture of a high-strength aluminium alloy and to the alloy manufactured thereof. The invention also relates to articles and products shape formed from the alloy.

Background

Aluminium alloys are widely used in various engineering structures. Owing to the advantageous features such as light weight and corrosion resistance, they are extensively used in the manufacture of automotive and aircraft bodies. They are also used in ship building and in the manufacture of roofing sheets. Aluminium alloys are classified respectively into cast alloys and wrought alloys depending on whether they are molded or deformed into the desired shape. Rolling, forging and extrusion are the various methods conventionally used to deform aluminium alloys. Deformation introduces strain and improves strength and, therefore, wrought alloys generally have higher strength.

Depending on the alloy composition, wrought aluminium alloys are identified by a four- digit series. 2000 series and 7000 series wrought aluminium alloys have the highest attainable specific strength, but they are usually expensive or their manufacture require sophisticated machinery. 6000 series aluminium are relatively less expensive and are easy to manufacture. However, their strength, attainable by conventional methods, is relatively less.

High strength aluminum alloys could be obtained by severe plastic deformation and/or by cryogenic rolling. However, such methods are either not scalable and/or are very expensive.

US 6562155 discloses a method for producing aluminium alloy where working strain is introduced into aluminium alloy by flask-assisted cold forging. US 4186034 discloses a method for manufacture of aluminium alloy sheets by following a sequence of steps including casting of aluminium alloy melt, hot rolling the alloy, cold rolling of hot rolled alloy and annealing the cold rolled alloy above recrystallization temperature. However, the improvement in strengths by conventional methods are limited. There is scope for further improvement in aluminium alloy strengths.

Objects of the invention

An object of the invention is to provide a method of manufacture of high strength aluminium alloys.

Another object of the invention is to provide aluminium alloys having high hardness and strength

A further object of the invention is to provide articles and products prepared from high strength aluminium alloys Summary of the invention

The present invention realizes the above mentioned objects through a method for manufacture of high strength aluminium alloys. The method of the invention enables manufacture of high strength alloys from billets of aluminium alloys.

Detailed description

Accordingly, the invention provides a method for manufacture of a high-strength aluminium alloy from cast alloy billets.

In one embodiment the invention provides a method for manufacture of a high-strength aluminium alloy, the method comprising heating billets of cast alloy containing aluminium and alloying elements at a temperature in the range of 450⁰C to 550 C to homogenize the alloy, deforming the billets in the homogenized state to form a hot worked alloy, first allowing the hot worked alloy to cool to room temperature and then heating the hot worked alloy from room temperature to a temperature at which the alloy forms a solid solution, quenching the solid solution in water, deforming the quenched alloy at room temperature to form a cold worked alloy and aging the cold worked alloy at a temperature in the range of 50 to 150⁰C.

In another embodiment, the invention provides a method for manufacture of a high- strength aluminium alloy, the method comprising heating billets of cast alloy containing aluminium and alloying elements at a temperature in the range of 450⁰C to 550⁰C to homogenize the alloy, deforming the billets in the homogenized state to form a hot worked alloy, first allowing the hot worked alloy to cool to room temperature and then heating the hot worked alloy from room temperature to a temperature at which the alloy forms a solid solution, quenching the solid solution in water, deforming the quenched alloy at room temperature to form a cold worked alloy and aging the cold worked alloy at a temperature in the range of 50 to 150⁰C, wherein the alloying elements are selected from a group consisting of silicon, magnesium, copper and zinc.

In another embodiment, the invention provides a method for manufacture of a high- strength aluminium alloy, the method comprising heating billets of cast alloy containing aluminium and alloying elements at a temperature in the range of 450⁰C to 550⁰C to homogenize the alloy, deforming the billets in the homogenized state to form a hot worked alloy, first allowing the hot worked alloy to cool to room temperature and then heating the hot worked alloy from room temperature to a temperature at which the alloy forms a solid solution, quenching the solid solution in water, deforming the quenched alloy at room temperature to form a cold worked alloy and aging the cold worked alloy at a temperature in the range of 50 to 150⁰C, wherein the billets contain 0.2 to 2.0% Mg and 0.2 to 1.0% silicon

In another embodiment, the invention provides a method for manufacture of a high- strength aluminium alloy, the method comprising heating billets of cast alloy containing aluminium and alloying elements at a temperature in the range of 450⁰C to 550⁰C to homogenize the alloy, deforming the billets in the homogenized state to form a hot worked alloy, first allowing the hot worked alloy to cool to room temperature and then heating the hot worked alloy from room temperature to a temperature at which the alloy forms a solid solution, quenching the solid solution in water, deforming the quenched alloy at room temperature to form a cold worked alloy and aging the cold worked alloy at a temperature in the range of 5O⁰C to 150⁰C, wherein the billets are heated at a temperature in the range of 45O⁰C to 55O⁰C ⁰C for 1 to 10 hours.

In another embodiment, the invention provides a method for manufacture of a high- strength aluminium alloy, the method comprising heating billets of cast alloy containing aluminium and alloying elements at a temperature in the range of 450⁰C to 550⁰C to homogenize the alloy, deforming the billets in the homogenized state to form a hot worked alloy, first allowing the hot worked alloy to cool to room temperature and then heating the hot worked alloy from room temperature to a temperature at which the alloy forms a solid solution, quenching the solid solution in water, deforming the quenched alloy at room temperature to form a cold worked alloy and aging the cold worked alloy at a temperature in the range of 50 to 150⁰C, wherein the hot worked alloy is heated from room temperature to a temperature in the range of 45O⁰CtO 55O⁰C

In another embodiment, the invention provides a method for manufacture of a high- strength aluminium alloy, the method comprising heating billets of cast alloy containing aluminium and alloying elements at a temperature in the range of 450⁰C to 550⁰C to homogenize the alloy, deforming the billets in the homogenized state to form a hot worked alloy, first allowing the hot worked alloy to cool to room temperature and then heating the hot worked alloy from room temperature to a temperature at which the alloy forms a solid solution, quenching the solid solution in water, deforming the quenched alloy at room temperature to form a cold worked alloy and aging the cold worked alloy at a temperature in the range of 50 to 150⁰C, wherein the billets are deformed by rolling.

In another embodiment, the invention provides a method for manufacture of a high- strength aluminium alloy, the method comprising heating billets of cast alloy containing aluminium and alloying elements at a temperature in the range of 450⁰C to 550⁰C to homogenize the alloy, deforming the billets in the homogenized state to form a hot worked alloy, first allowing the hot worked alloy to cool to room temperature and then heating the hot worked alloy from room temperature to a temperature at which the alloy forms a solid solution, quenching the solid solution in water, deforming the quenched alloy at room temperature to form a cold worked alloy and aging the cold worked alloy at a temperature in the range of 50 to 150⁰C, wherein the cold worked alloy has a strain value in the range of 0.5 to 2.0

In another embodiment, the invention provides high-strength aluminium alloy prepared by the method comprising heating billets of cast alloy containing aluminium and alloying elements at a temperature in the range of 450⁰C to 550⁰C to homogenize the alloy, deforming the billets in the homogenized state to form a hot worked alloy, first allowing the hot worked alloy to cool to room temperature and then heating the hot worked alloy from room temperature to a temperature at which the alloy forms a solid solution, quenching the solid solution in water, deforming the quenched alloy at room temperature to form a cold worked alloy and aging the cold worked alloy at a temperature in the range of 50 to 150⁰C.

In another embodiment, the invention provides high-strength aluminium alloy having Vicker's hardness in the range of 115 to 160, Yield strength in the range of 270 to 400 MPa and tensile strength in the range of 325 to 475 MPa at an elongation to failure ranging from 8 to 16%

In another embodiment, the invention provides high strength aluminium alloy comprising intermetallic compound particles having particle size less than 20 nm

In a further embodiment, the invention provides high strength aluminium alloy having a submicron structure substantially homogeneously distributed throughout.

In a still further embodiment, the invention provides articles shape formed from high strength aluminium alloy.

The method of the invention involves heating billets of cast alloy containing aluminium and alloying elements. The alloy billets are typically cast from a mixture of liquid aluminium along with alloying elements. The alloying elements are usually those elements commonly used for manufacturing heat treateable wrought alloy such as 6xxx, 2xxx and 7xxx series alloys. Typically, alloying elements are selected from a group consisting of silicon, magnesium, copper and zinc. The cast alloy billets are subjected to homogenization treatment by heating the billets at a temperature in the range of 450 to 550⁰C for 1 to 10 hours. During homogenization treatment, the alloying elements distribute themselves substantially , uniformly throughout the alloy.

After the homogenization treatment, the alloy billets are deformed under pressure at high temperature to form a hot worked alloy. The deformation can be carried out by rolling, forging or extrusion. Advantageously, the deformation is carried out by rolling to form a sheet of aluminium alloy. The hot worked alloy is first allowed to cool to room temperature. After the alloy has attained room temperature, it is further heated to a temperature at which it forms a solid solution. Typically, the alloy is heated at a temperature in the range of 450 to 550⁰C for 1 to 10 hours. Usually, during this step of solution heat treatment, a solid solution of alloying elements in aluminum is formed. Solution heat treatment temperature determines the degree of solutionizing of various alloying elements and is therefore chosen carefully for the method of the invention. The solution heat treated alloy is then quenched in water. The quenched alloy is deformed at room temperature to form a cold worked alloy. Usually, the cold worked alloy has a strain in the range of 0.5 to 2.0. The cold worked alloy is then subjected to aging treatment at a temperature in the range of 50 to 150 ⁰C. Advantageously, the aging is carried out for 12 to 48 hours.

The method of the invention provides a high strength aluminium alloy. The aluminium alloy of the invention has Vicker's hardness in the range of 115 to 160 Hv5, yield strength in the range of 270 to 400 MPa and tensile strength in the range of 325 to 475 MPa at an elongation to failure ranging from 8 to 16%. It is presumed that the high values of strength for the alloys of the invention is due to its unique microstructure obtained by the presence of alloying element in solution during cold deformation, leading to high dislocation density and consequently to high degree of strain hardening.

The alloy of the invention comprise intermetallic compound particles having particle size less than 20 nm. The particles are visible in the TEM (Transmission Electron Micrograph) images of the alloy displayed in Fig 1. Typically, an intermetallic compound is magnesium suicide (Mg₂Si), aluminum copper (Al₂Cu), aluminum manganese (Al₂Mn, Al₆Mn) or derivatives thereof. Further, the alloy of the invention possess a submicron structure substantially homogeneously distributed throughout. The submicron structure of the alloy is evident from the TEM images displayed in Fig 2.

The X-Ray Diffraction pattern of the aluminium alloy of the invention is given in Fig 3. The high strength aluminium alloy of the invention has the peaks corresponding to MG₂Si, Al₆Mn, AlFeSi and CuAl₂ in the XRD pattern obtained by using an X-ray radiation having wavelength 1.5418 Angstron

The invention also provide articles and products shape formed from the high strength alloys of the invention. The articles and products that can be shape formed partly or completely from the alloys of the invention include, but are not limited to, airframe structures, automotive body parts, boats, deckhouses, funnels, masts and other components for boat and ship building The invention is further illustrated by way of the following non limiting examples.

In the Examples that follow, the aluminium alloys having elemental compositions as displayed in table 1 were used.

Table 1 : Elemental composition of the alloys ALl to AL5 used in Examples 1 to 4

Remaining percentage by weight

In the examples and results that follow, the hardness measurements were carried out Vicker's hardness tester with indentation load of 5 kg. Yield strength and tensile strength was measured by using universal testing machine. Samples for tensile strength measurement were prepared according to ASTM standard E6. The TEM images were obtained in Phillips CM20 TEM having an Energy Dispersive Detector.

Example 1: Manufacture of high-strength aluminium alloy

(a) Deformation by rolling

Hot-rolled billets of aluminum alloys (alloys ALl to AL4) having elemental compositions as displayed in table 1 were solution treated at 530 to 55O⁰C for lhour each followed by quenching in water. The billets were then cold rolled to about 2mm thickness imparting a total strain between 1.0 to 1.4. After rolling, small pieces of the resulting sheet were aged at 135⁰C for different time intervals. Hardness and tensile values were measured after final aging treatment.

(b) Deformation by forging

Hot-rolled billets of aluminum alloys (alloys AL2 and AL5) having elemental compositions as displayed in table 1 were solution treated at 53O⁰C for 1 hour each followed by quenching in water. The billets were then cold forged imparting a total strain between 1.18 to 1.32. After forging, small piece of the resulting alloy were aged at 135⁰C for 24 hour each. Hardness and tensile values were measured after final aging treatment.

Example 2: Manufacture of aluminium alloy by conventional method

(a) Deformation by rolling

Hot-rolled billets of aluminum alloys (alloys ALl to AL 4) having elemental compositions as displayed in table 1 were cold rolled to about 2mm thickness imparting a total strain between 1.0 to 1.4. After rolling, the cold rolled sheets were solution treated at 53O⁰C for 1 hour each followed by quenching in water. The small pieces of solution treated sheets were aged at 170⁰C for 6 hours. Hardness and tensile values were measured after final aging treatment.

(b) Deformation by forging Hot-rolled billets of aluminum alloys (alloys AL2 and AL5) having elemental compositions as displayed in table 1 were cold forged imparting a total strain between 1.18 to 1.32. After forging, small piece of the resulting alloy were solution treated at 53O⁰C for 1 hour each followed by quenching in water. The small pieces of solution treated alloy were aged to 170⁰C for 6 hours. Hardness and tensile values were measured after final aging treatment.

Example 3: Study of variation in hardness increment at various temperatures of solution heat treatment, of aluminium alloy, manufactured by following the procedure of example 1

Hot rolled billets of alloy AL2 were solution treated at temperatures in the range 400 and 55O⁰C followed by water quenching. The solution treated billets were cold forged imparting a total strain between 1.21 to 1.36. Hardness values of the alloy were measured before and after the forging treatment. Temperature below 45O⁰C were not found effective in ensuring complete dissolution of alloying elements in this case. The hardness increments during forging (difference of hardness value after and before the forging) for different temperatures of solution heat treatment are displayed in table 3. Table 3 shows that the extent of hardening during cold deformation depends on the solution heat treatment temperature.

—- •

Example 4: Study of variation in hardness increment at various temperatures of solution heat treatment, of aluminium alloy, manufactured by following the procedure of example 2

The hardness increments of AL2 alloy treated by the procedure in example 2 were also studied for comparative purposes. The results are displayed in table 3. Table 2: Comparison of hardness, yield strength and tensile strength of aluminium alloys manufactured by following the procedures of various examples.

^ Not applicable Table 3: Comparison of variation in hardness increment of aluminium alloys at various temperatures of solution heat treatments

From table 2 it is clear that under comparable elongation to failure, the hardness, yield strength and tensile strength, of the alloys manufactured by the method of the invention are significantly higher as compared to the corresponding parameter values for the alloys manufactured by the conventional method. It can be observed that there is 30-45% gain in yield strength and 20-40% increase in tensile strength for the alloys manufactured by the method of the invention. Further, from table 3 it is clear that the alloys prepared by the method of the invention has hardness increments increasing with increase in solution heat treatment temperature while the hardness of alloys conventionally prepared remain low. Technical advantages

High-strength aluminium alloys of the invention enable manufacture of articles and products having high specific strength thereby rendering remarkable savings in weight and space. By enabling attainment of high specific strength in 6000 series aluminium alloys, the method of the invention provide considerable reduction in cost in applications such as manufacture of aircraft bodies and ship building where specific strength of the material is critical. Further, owing to the observed enhancement of hardness with temperature of solution heat treatment, the alloys of the invention could be obtained with hardness values tailored for specific applications. Furthermore, the high-strength aluminium alloys of the invention possess good formability, weldability, machinability, and relatively good corrosion resistance.

While considerable emphasis has been placed herein on the specific steps of the preferred process, it will be appreciated that many steps can be made and that many changes can be made in the preferred steps without departing from the principles of the invention. These and other changes in the preferred steps of the invention will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation.

Claims

Claims:

1. A method for manufacture of a high-strength aluminium alloy, the method comprising heating billets of cast alloy containing aluminium and alloying elements at a temperature in the range of 450⁰C to 550⁰C to homogenize the alloy, deforming the billets in the homogenized state to form a hot worked alloy, first allowing the hot worked alloy to cool to room temperature and then heating the hot worked alloy from room temperature to a temperature at which the alloy forms a solid solution, quenching the solid solution in water, deforming the quenched alloy at room temperature to form a cold worked alloy and aging the cold worked alloy at a temperature in the range of 50 to 150⁰C.

2. The method as claimed in claim 1 wherein the alloying elements are selected from a group consisting of silicon, magnesium, copper and zinc.

3. The method as claimed in claim 1 or 2 wherein the billets contain 0.2 to 2% Mg and

0.2 to 1.0 % silicon

4. The method as claimed in any one of the claims 1 to 3 wherein the billets are heated at a temperature in the range of 450 to 55O⁰C for 1 to 10 hours

5. The method as claimed in any one of the claims 1 to 4 wherein the hot worked alloy is heated from room temperature to a temperature in the range of 45O⁰C to 55O⁰C

6. The method as claimed in any one of the claims 1 to 5 wherein the billets are deformed by rolling.

7. The method as claimed in any one of the claims 1 to 6 wherein the aging is carried out for up to 48 hours

8. The method as claimed in any one of the claims 1 to 7 wherein the cold worked alloy has a strain in the range of 0.5 to 2.0

9. High-strength aluminium alloy prepared by the method as claimed in any one of the claims 1 to 8.

10. High-strength aluminium alloy having Vicker's hardness in the range of 115 to 160 HV5, Yield strength in the range of 270 to 400 MPa and tensile strength in the range of 325 to 475 MPa at an elongation to failure ranging from 8 to 16%.

11. High strength aluminium alloy comprising intermetallic compound particles having particle size less than 20 nm.

12. The alloy as claimed in claim 11 wherein the intermetallic compound is magnesium suicide, aluminum copper, aluminium ferrosilicon, aluminum manganese or any mixture thereof.

13. The alloy as claimed in anyone of claims 9 to 12 having a submicron structure substantially homogeneously distributed throughout.

14. Articles and products shape formed from the alloy as claimed in claims 9 to 13.