NO20211106A1

NO20211106A1 - Heat treatable aluminium alloy with improved mechanical properties and method for producing it

Info

Publication number: NO20211106A1
Application number: NO20211106A
Authority: NO
Inventors: Oddvin Reiso; Ulf Tundal
Original assignee: Norsk Hydro As
Priority date: 2021-09-14
Filing date: 2021-09-14
Publication date: 2023-03-15
Also published as: EP4402297A1; NO347077B1; CA3231178A1; WO2023041557A1; US20250129455A1

Description

Heat treatable aluminium alloy with improved mechanical properties and method for producing it

The present invention relates to AA 6xxx series alloys and a method for producing extruded profiles or extruded solid bars, which are subjected to further processing to obtain products with good mechanical properties at reduced costs.

BACKGROUND

From WO 02/38821 A1 a process is described which involves thermal treatment of a billet or ingot before the extrusion process is initiated, as well as a subsequent thermal and mechanical treatment of the extruded blank. The aim of the thermal treatment before extrusion is to increase the extrusion speed while the subsequent treatment of the extruded blank involving thermal treatment and required forming processes gives a product that sustains good mechanical properties.

A schematic view of the process used is shown in figure 1.

WO 02/38821 A1 specifies an alloy with Mg in the range of 0.5-2.0 (wt) % and Si in the range of 0.5-2.0 (wt)%, Fe in the range of 0-1.0 (wt)%, Cu in the range of 0-1.0 (wt)%, Zn in the range of 0-3.0 (wt)% and other elements not specified below in the range 0-0.2 (wt) %. The alloy can contain dispersoid forming elements Mn in the range of 0-1.5 (wt)%, Cr in the range of 0-1.0 (wt)% and Zr in the range of 0-0.3 (wt)%. In the three examples described there are one AA6061 and two AA6082 alloys. The AA6061 alloy in example 1 contains 0.76(wt)% Mg, 0.57(wt)% Si, 0.15(wt)% Cu, 0.07(wt)% Cr, 0.18 et% Fe and 0.02(wt)% Mn. Alloys according to this patent does not provide the required ductility.

WO 02/38821 A1 recommend a cooling rate of 5-50°C per hour from the homogenizing temperature to the isothermal heat treatment temperature between 300 and 450°C. In example 1 of the prior art the extrusion billets of the 6061 alloy are homogenized at 575°C for 3 hours before they are cooled by 25°C/hour down to 400°C where they are held for 8 hours before further cooling down to room temperature. This treatment gives a microstructure of the extrusion billets with a high number of Mg2Si particles with diameters in the range 3-10 µm.

The slow cooling rate that is recommended in WO 02/38821 A1 is disadvantageous in two ways; a) the total cycle time for the soft annealing process is considerably increased; b) it takes much longer time to dissolve the Mg2Si particles in the solutionizing step that takes place after extrusion.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1. Schematic view of the process for manufacture of extruded Al products according to the invention.

Figure 2a. Microstructure of extrusion billets homogenized at 575°C for 3 hours and then cooled to 350°C using a cooling rate of 25°C per hour. and held there for 8 hours.

Figure 2b. Microstructure of extrusion billets homogenized at 575°C for 3 hours and then cooled to 350°C using a cooling rate of 250°C per hour.

Figure 3. Engineering yield stress, Rp0.2, the engineering ultimate stress, Rm and the true strain, εf, for the 5 alloys in Table 2.

Figure 4. Engineering yield stress, Rp0.2, the engineering ultimate stress, Rm, engineering total elongation, A and the true strain, εf are shown for the 4 alloys in Table 3

SUMMARY OF THE INVENTION

The present invention describes a similar process as described in WO 02/38821 A1, but with reduced cost and with more optimized alloys to increase both the strength and the ductility of the final product.

The present invention provides:

A method for processing extrusion billets produced from an aluminium 6xxx alloy containing Mg in the range 0.85-1.15 (wt) % and Si in the range 0.60-0.75 (wt)%, Fe in the range 0-0.5 (wt)%, Cu in the range 0-0.30 (wt)%, Cr in the range 0-0.10 (wt)%, Mn in the range 0-0.20 (wt)%, Zn in the range 0-0.5 (wt)%, Ti in the range 0-0.15 (wt)%, V in the range 0-0.15 (wt)% and other elements not specified below in the range 0-0.05 (wt)%, with balance Al, comprising the steps:

a. homogenizing the extrusion billet,

b. soft annealing the extrusion ingot or billet,

c. preheating the extrusion billet,

d. extruding the extrusion ingot or billet to form a profile or blank,

e. cooling the profile down to room temperature,

f. exposing the profile or blank to a solutionising operation,

g. optionally stretching the extruded profile or blank,

h. artificially ageing the profile

c h a r a c t e r i s e d in that

the Mg/Sieff ratio of the alloy being above 1.6, where Sieff = Si - 1⁄3 wt%(Fe+Cr+Mn+Zr).

The cooling rate from the homogenizing temperature to the soft annealing temperature lying between 350 and 450°C, preferably 380-420°C, is preferably at least 100°C per hour, most preferably at least 200°C per hour. Between 350 and 450°C the maximum amount of Mg2Si particles are precipitated and thereby the material is made as soft as possible. The most optimum temperature range for precipitation is between 380-420°C. The minimum concentration of Mg is preferably 0.90 (wt)%, or more preferably 0.95 (wt)%. A higher Mg content is positive for ductility. This also allows you to increase the Si content and still have a good Mg/Si ratio.

The concentration of Si is preferably 0.63-0.72, more preferably 0.65-0.70 (wt)%.

The minimum concentration of Ti is preferably 0.05 (wt)%, more preferably 0.07 (wt)% in order to increase the corrosion resistance. The content of Ti is limited to 0.15 (wt)% in order not to have a too negative effect on extrudability.

The maximum concentration of Cu is 0.20 (wt)%, or more preferably 0.15 (wt)%, or even more preferably 0.10 (wt)%. By limiting the copper content a higher extrusion rate can be used since the required extrusion pressure diminishes.

It is preferred that the solutionising temperature is above 555°C, more preferably above 560°C, even more preferably above 565°C. A high solutionizing temperature enables dissolution of as much Mg2Si particles as possible at a short time.

The present invention further provides: An extrusion billet produced with the method according to the invention where the average Mg2Si particle diameter of the microstructure of the ingot is in the range 2-5 µm, or more preferably in the range 2-4 µm, and the diameter is calculated based on the measured area in LOM (Light Optical Microscope) in a plane in a polished cross section converted to a circle.

It also provides a semi-finished or finished product produced with the method according to the invention wherein the yield stress Rp0.2 is at least 300 MPa, preferably above 310 MPa, more preferably above 320 MPa.

Example 1

An alloy with 0.90(wt)% Mg, 0.65(wt)% Si, 0.25(wt)% Fe and 0.05(wt)% Cr and the balance aluminium and impurity elements at levels below 0.02(wt)% was cast to billets. The billets were homogenized at 575°C for 3 hours and then cooled at different rates down to the heat treatment temperature 350°C and held there for 8 hours. The picture to the left (Figure 2a) shows the microstructure of an extrusion ingot cooled at 25°C per hour whereas the right picture (Figure 2b) shows the microstructure of an extrusion ingot cooled at 250°C per hour (Mg2Si particles are the ones with the darkest grey color). The reason for cooling fast to the soft annealing temperature is to nucleate many Mg2Si particles. In this way the particle sizes will become smaller. This is beneficial for the solutionizing process because smaller particles will require shorter solutionizing times for total dissolution.

The Mg2Si particles in the left picture of Figure 2 are typically in the range 5-15 µm whereas the Mg2Si particles in the right picture are typically in the range 2-5 µm. In a dissolution process that is controlled by diffusion of the solute elements, the time to dissolve a particle is proportional with the square of the size of the particle. Thus, a particle with diameter 10 µm takes 11 times longer to dissolve than a particle of diameter 3 µm. The particle size will have a large effect when designing a production line for separate solutionizing of the profiles produced according to the prior art. For the casthouse producing the extrusion billets a higher cooling rate down to the temperature where the isothermal treatment is conducted will reduce the cycle time and the production cost.

Example 2

Extrusion billets with diameter 95 mm of 5 different alloys with the chemical compositions shown in Table 1 were homogenized at 575°C for 2 hours, cooled with a cooling rate of 200°C per hour down to 400°C and held there for 4 hours before cooling down to room temperature at a rate of 200°C per hour. The extrusion billets were then preheated to temperatures between 400 and 420°C and extruded to a hollow rectangular section with outer dimensions 29 mm by 37 mm and with a wall thickness all around the cross section of 2.8 mm., where after the profiles were cooled in air to ambient temperature.

Table 1 Chemical compositions of alloys tested in example 2 in (wt)% including incidental impurities and with balance Al.

The profiles were subsequently separately solutionized at 550°C for about 30 minutes and quenched in water before being stretched by approximately 0.5% plastic strain, stored for 24 hours at room temperature and aged at 185°C for 8 hours.

In Figure 3 the engineering yield stress, Rp0.2 and the ultimate tensile stress, Rm are shown for the 5 alloys in Table 1. For alloys 1-4 there is a marked increase in Ultimate tensile stress with increasing Si content. Alloy 5 has the same Si content as alloy 3 but with 0.13wt% lower Mg content. Alloy 5 shows almost the same Rp0.2 value but slightly lower Rm value than alloy 3. These results indicate that a higher Si content is very important to maximize the strength of the material.

The preferred content of Si is 0.63-0.72, preferably 0.65-0.70 in order to maximize strength while being adapted to the preferred Mg content.

Figure 3 also shows the true strain, for the different alloys,

where A0 is the initial cross sectional area of the tensile sample and Af is the area at fracture. The true strain, εf is highest for the alloys with the lowest Si contents. By comparing alloys 3 and 5 one can see that the true strain, εf, also drops when the Mg/Si ratio is reduced.

A0 is the initial cross sectional area of the tensile sample and Af is the area at fracture. For a constant Mg content (alloys 1-4), the true strain, εf decreases with increasing Si contents. By comparing alloys 3 and 5 it is evident that the true strain, εf, also drops when the Mg/Si ratio is reduced.

Example 3

Example 3 of the present invention aims to increase the strength without too much reduction in the ductility. Extrusion billets were cast and processed in the same way as in example 2.

Table 2 Chemical compositions of alloys tested in example 3 in (wt)%, including incidental impurities and with balance Al.

The extrusion billets were then preheated to temperatures between 400 and 420°C and extruded to a hollow rectangular section with outer dimensions 29 mm by 37 mm and with a wall thickness all around the cross section of 2.8 mm. The profiles were subsequently separately solutionized at 565°C for about 15 minutes and quenched in water before being stretched by approximately 0.5% plastic strain, stored for 10 minutes and aged at 195°C for 4 hours. Figure 4 shows the mechanical properties obtained from tensile testing of samples from profiles of the different alloys listed in Table 2.

Alloys 7 and 8 show slightly higher strength levels than alloys 6 and 9. Regarding ductility, as measured by the true fracture strain εf, especially alloy 8, but also alloy 7, show lower values than alloys 6 and 9. Alloy 8 has the lowest Mg/Si ratio, which is the main reason for the lower true fracture strain value.

It is thus clear that a composition according to claim 1 with a Mg/Si ratio of above 1.6 is a requirement for achieving the ductility required according to the invention.

Claims

1. A method for processing extrusion billet produced from an aluminium 6xxx alloy containing Mg in the range 0.85-1.15 (wt) % and Si in the range 0.60-0.75 (wt)%, Fe in the range 0-0.5 (wt)%, Cu in the range 0-0.30 (wt)%, Cr in the range 0-0.10 (wt)%, Mn in the range 0-0.20 (wt)%, Zn in the range 0-0.5 (wt)%, Ti in the range 0-0.15 (wt)%, V in the range 0-0.15 (wt)% and other elements not specified below in the range 0-0.05 (wt)%, with balance Al, comprising the steps:

a. homogenizing the extrusion billet,

b. soft annealing the extrusion billet,

c. preheating the extrusion billet,

d. extruding the extrusion billet to form a profile,

e. cooling the profile down to room temperature,

f. exposing the profile a solutionising operation,

g. optionally stretching the extruded profile,

h. artificially ageing the profile

characterised in that

the Mg/Sieff ratio of the alloy being above 1.6, where Siett = Si - 1⁄2 wt%(Fe+Cr+Mn+Zr).

2. A method for processing extrusion or billets according to claim 1,

characterised in that

the soft annealing is taking place at an temperature in the interval between 350 and 450X, preferably between 380 and 420°C.

3. A method for processing extrusion or billets according to claim 1 or 2,

characterised in that

the cooling rate from the homogenizing temperature to the soft annealing temperature is at least 100°C per hour, preferably at least 200°C.

4. A method according to any of claims 1-3,

characterised in that

the minimum concentration of Mg is 0.90 (wt)%, preferably 0.95 (wt)%.

5. A method according to any of claims 1-4,

characterised in that

the concentration of Si is 0.63-0.72, preferably 0.65-0.70 (wt)%.

6. A method according to any of claims 1-5,

characterised in that

the minimum concentration of Ti is 0.05 (wt)%, preferably 0.07 (wt)%.

7. A method according to any of claims 1-6,

characterised in that

the maximum concentration of Cu is 0.20 (wt)%, or more preferably 0.15 (wt)%, or even more preferably 0.10 (wt)%.

8. A method according to any of claims 1-7,

characterised in that

the solutionising temperature is above 555°C, more preferably above 560°C, even more preferably above 565°C.

9. An extrusion billet processed by the method of any of claims 1-8,

characterised In that

the average Mg2Si particle diameter of the microstructure of the ingot is in the range 2-5 pm, or more preferably in the range 2-4 μm, where the diameter is calculated based on the measured area in LOM in a plane in a polished cross section of the billet converted to a circle.

10. A semi-finished or finished product produced according to any of claims 1-8,

characterised in that

the yield stress RpQ.2 is at least 300 MPa, preferably above 310 MPa, more preferably above 320 MPa.