GB2318586A

GB2318586A - Aluminium base alloy with high strength

Info

Publication number: GB2318586A
Application number: GB9717937A
Authority: GB
Inventors: Soo Woo Nam; Duck Hee Lee
Original assignee: Korea Advanced Institute of Science and Technology KAIST
Current assignee: Korea Advanced Institute of Science and Technology KAIST
Priority date: 1996-10-23
Filing date: 1997-08-22
Publication date: 1998-04-29
Also published as: GB9717937D0; KR19980028588A; KR100199408B1

Abstract

In order to enhance the mechanical properties of aluminium base alloy without heat treating after extrusion process, a non-heat treatable high strength aluminium base alloy comprising 0.5 - 1.0 wt.% of Mg, 0.6-1.2 wt.% of Si, 0.8 - 2.1 wt.% of Mn, 0.1 - 0.3 wt.% of Zr, 0.3 - 0.8 wt.% of Zn, 0.1 - 0.5 wt.% of Cr, less than 0.11 wt.% of Ti and balance of Al is provided. The alloy is homogenisation heat treated at 560 - 570{C for 8-12 hours, and extruded into a product of designed shape.

Description

ALUMINIUM BASE ALLOY WITH HIGH STRENGTH BACKGROUND OF THE INVENTION The present invention relates to an aluminium base alloy with high strength, particularly such an alloy that does not require ageing by heat treatment after shaping, and a method of making an extruded product from such alloy.

It is well known that medium strength aluminium alloys such as Al 6005, Al 6061 and Al 6NO1 have been widely used in structural units for their good combination of strength, resistance to corrosion and hot extrudability.

In such Al-Mg-Si series alloys, mechanical properties have been improved mainly by the addition of alloying transition elements (e.g. Mn. Cr... etc.) and by thermomechanical processing.

However, since most of the Al-Mg-Si series alloys being recently used including the Al 6N01 alloy are precipitation hardening alloys, a separate ageing heat treatment additionally is required after the shaping process (e.g. extrusion or forging process).

In the Al-Mg-Si series alloys, the major hardening effect is known to be the formation of precipitates with Mg and Si. Therefore, the addition of alloying transition elements, especially amount of Mn+Cr, are limited to below 0.5 w.% to enhance the strength through the precipitation hardening.

Therefore, such medium strength aluminium alloys generally exhibit lower strength in case of no ageing treatment and have some difficulties of heat treatment in their big-long extrudates and thus they are utilised in some limited applications such as in the automobile industry and in the manufacture of gas storage tanks.

As a result, in contrast to the small product, the conventional precipitation hardening Al-Mg-Si series alloys are not available to be used as a high strength big-long extrudates for the high speed train or automobile industries which require necessarily ageing heat treatment after the shaping process.

SUMMARY OF THE INVENTION A technical problem with which the present invention is concerned is to provide an aluminium base alloy which would not require ageing by heat treatment after shaping and which exhibits improved strength similar to the conventional precipitation hardening Al-Mg-Si series alloy.

For the present invention, a non-heat treatable high strength aluminium base alloy is designed to have the basic composition of a conventional precipitation hardening Al-Mg-Si series alloy, Al 6N01,and to promote the production of fine manganese dispersoids within the alloy structure by the addition of appropriate amounts of alloying transition elements such as Mn, Zr and Cr as reinforcing elements. Therefore, the designed alloy, without any ageing by heat treatment after the shaping process, has a strength as high as that of Al 6N01 or Al 6061 alloy which need such ageing heat treatment.

In the method of the present invention, homogenisation heat treatment is only conducted before the shaping process to maximise the effect of manganese dispersoids on the mechanical properties and control the shape and size of fine manganese dispersoids within the alloy structure.

DETAILED DESCRIPTION OF THE INVENTION The aluminium base alloy of the present invention is characterised in that it comprises 0.5-1.0 wt.% of Mg, 0.6-1.2 wt.% of Si, 0.8-2.1 wt.% of Mn, 0.1-0.3 wt.% of Zr, 0.3-0.8 wt.% of Zn, 0.1-0.5 wt.% of Cr and less than 0.11 wt.% of Ti, with the balance wt.% of the alloy being Al.

The method according to the present invention of making extruded products using the aforesaid alloy is characterised in that it comprises steps of preparing an alloy of the above described composition, homogenisation heat treating the alloy at 550-5700C for 8-12 hours, and extruding the alloy into a product of designed shape.

The reasons why the alloying elements and the range of the elements are limited as above are as follows.

Magnesium and silicon are added as main reinforcing elements in commercial Al-Mg-Si series alloys, and the content of Si is more than about 0.4 wt.% under the ratio of Mg to Si of 1.73 to maintain high strength, weldability and extrudability.

Manganese is added to further enhance the strength of the conventional Al-Mg-Si series alloys, which has the strengthening effect by the precipitation of Mg and Si.

Therefore, 0.8-2.1 wt.% of Mn is added to produce manganese dispersoids for improvement of the mechanical properties without deteriorating the good ductility.

In particular, Mn is known to be the most essential element among the alloying elements of the present invention in that it is formed into fine manganese dispersoids of 0.1-0.4 pm in size within the alloy structure to carry out the pinning action of moving dislocations, thereby improving the strength. At the same time, the Mn-dispersoids are observed to enhance uniform dislocation motion or uniform deformation through cross-slip at the dispersoids to homogenise the distribution of slip band, thereby preventing the reduction of ductility of the strengthening alloy. In other words, the addition of manganese results in sufficiently enhanced strength of the alloy without sacrificing the ductility, and consequently the ageing treatment after the shaping process (extrusion) can be excluded.

zirconium is added to inhibit recrystallisation, to maintain grain refinement, and to produce fine Zrdispersoids like manganese does. Preferably, 0.1-0.3 wt.% of Zr is added to enhance strength.

Copper is an element which exhibits a solid solution hardening effect and improves toughness.

In addition to Cu, zinc enhances strength without diminishing resistance to corrosion.

In summary, the high strength aluminium base alloy of the present invention, which needs no ageing treatment after shaping, has levels of strength and ductility similar to those of conventional precipitation hardened aluminium alloys having good extrudability and ductility.

The main advantageous point of this invention is the formation of fine manganese dispersoids which have a strong influence on the enhancement of the mechanical properties of the alloy, such as increased Y.S. and U.T.S. and good ductility.

Hereinafter, the present invention will be described with reference to the following example.

Example Samples having compositions as in Table 1 were manufactured and homogenised before extrusion.

Subsequently, homogenisation treatment was conducted at 560-570"C for 8-12 hours for the formation of manganese dispersoids and homogenisation of the segregations. To compare the mechanical properties in the T6 condition as well as in the as-extruded condition, after extrusion, some parts of each extruded bar were treated in solution at 5300C for 1-2 hours, followed by water quenching.

Subsequently, ageing treatment was carried out at 1701800C for 8-12 hours (T6 condition).

Table 1 - Chemical composition (wt.%)

Composition Chemical ComDosition Sample < Al Mg | Si Cu Zn Mn Zr Ti Cr Piece 1 bal. 0.70 0.86 0.79 0.46 0.87 0.19 0.12 0.

Examples 2 baL / 0.67 0.82 0.66 0.24 0.24 0.16 / 0.09 0.30 3 bal. 1 0.78 0ss9 0.77 0.48 0.48 0.19 0.12 036 Comr Al 6N01 1 bawl. Q60 0.90 024 0.30 0.30 - 0.12 0.21 Examples Al 6N01 2 bal. 0.69 0.79 0.48 0.27 0.27 - 0.11 0.30 The mechanical properties measured from the samples which had been as-extruded and T6 treated, respectively, are shown in Table 2.

Table 2 - Mechanical Properties

f YieEAs-extruded SampIe T6 treated SamDe 1 Composition Tensile ) Elongation i Yield JTensile tion jh-ength Strength 1Sength jStrength 1 1 274~ 17.4 3 & 3 2Q0 Examples J 2 256 295 147 3Q4 363 20.6 3 3 224 284 14.3 249 328 21.5 Comp. j Al 6N01 1 173 1 217 21.2 3 300 326 20.4 Examples Al 6N01 2 185 1 227 14.6 303 324 20.4 As can be seen from Table 2, the samples embodying the present invention exhibit better mechanical properties than the conventional precipitation hardened aluminium alloy, Al 6N01, in both as extruded and 6 treated conditions. In particular, it can be noted that the mechanical properties of the alloys embodying the present invention in as-extruded condition are similar to those of the T6 treated aluminium alloy, Al 6N01.

Claims

WHAT IS CLAIMED IS:

1. An aluminium base alloy comprising: 0.5-1.0 wt.% of Mg;

06.-1.2 wt.% of Si; 0.8-2.1 wt.% of Mn; 0.1-0.3 wt.% of Zi; 0.3-0.8 wt.% of Zn; 0.1-0.5 wt.% of Cr; less than 0.11 wt.% of Ti; and a balance wt.% of Al.

2. A method of making an extruded product from an aluminium base alloy comprising the steps of: preparing the alloy defined in claim 1; homogenisation heat treating the alloy at 550-5700C for 8-12 hours; and extruding the alloy into a product of designed shape.

3. A method of making a forged product from an aluminium base alloy comprising the steps of: preparing the alloy defined in claim 1; homogenisation heat treating the alloy at 550-5700C for 8-12 hours; and forging the alloy into a product of designed shape.