GB2246578A

GB2246578A - High strength aluminum alloy with good weldability

Info

Publication number: GB2246578A
Application number: GB9111128A
Authority: GB
Inventors: Soo Woo Nam; Dong Seok Park; Dong Heon Lee
Original assignee: Korea Advanced Institute of Science and Technology KAIST
Current assignee: Korea Advanced Institute of Science and Technology KAIST
Priority date: 1990-07-31
Filing date: 1991-05-23
Publication date: 1992-02-05
Anticipated expiration: 2011-05-23
Also published as: KR920002811A; JP2709869B2; GB2246578B; KR920007936B1; GB9111128D0; JPH05132733A

Abstract

An aluminum alloy with a good weldability and a high strength for use in high speed trains or automobiles consists of 3.5-5 wt% of Zr, 2-3.5 wt% of Mg, 0.5-1.2 wt% of Mn, 0.1-0.3 wt% of Zr, and less than 0.1 wt% of Fe, Si, Cr and Ti as additives, and the balance being substantially Al as a matrix.

Description

HIGH STRENGTH ALUMINUM ALLOY WITH GOOD WELDABILITY BACKGROUND OF THE INVENTION The present invention relates to an aluminum alloy with a good weldability and a high strength.

In general, high tensile aluminum alloys have been widely used especially for military and aircraft applications which require the lightness of material and high strength because of their high strength to weight ratio.

However, such a high tensile aluminum alloys generally have a poor weldability and thus they are so limited to be utilized in some applications such as automobile industry and manufacturing industry of gas storage tank.

Accordingly, there have been made many attempts to develope an aluminum alloy having both the good weldability and the high strength.

Thereby, aluminum alloys such as Al 7017, Al 7039 and Al 2519 have been developed and widely used in several applications. In such aluminum alloys, the Al 7017 is known to be the best high strength ailoy, but the yield strength is 420 MPa, and the tensile strength is 480 MPa, both of which do not exceed 500 MPa.

Most of the weldable aluminum alloys being recently used including the Al 7017 alloy are of the Al-Zn-Mg precipitation hardening ailo > .

This type of aluminum alloys have a composition having aluminum as a matrix and consist of 4-5% of Zn, 1-3.5% of Mg, and also may contain about 0.1-0.4% of Cr and Mn, as occasion demands. In the above composition, the major hardening effect is known to be resulted from the formation of precipitations by Zn and Mg.

Meanwhile, the high strength aluminum alloys such as Al 7075 or Al 7050 as a non-weldable aluminum alloy exhibit the yield strength and tensile strength about over 500 MPa. But since the weldable aluminum alloy such as Al 7017 exhibits a lower strength, it is generally classified to a medium strength aluminum alloy FI.J. Polmear, The J. of the Australian Inst. of Metals. Vol. 17(1972) P.1].

As a result, in case that the above-mentioned weldable aluminum alloys are used for the high speed train or the automobile industry which requires necessarily the high strength and light weight, it causes the volume and weight thereof to be increased compared to the non-weldable high strength aluminum alloy.

SUMMARY OF THE INVENTION Therefore, it is an object of the present invention to provide a weldable high strength aluminum alloy which exhibits the improved strength similar to that of the conventional non-weldable high strength aluminum alloy.

According to the present invention, the weldable high strength aluminum alloy has a basic composition of Al-Zn-Mg and promotes the production of fine manganese dispersoids and zirconium dispersoids within the alloy structure by the addition of small amount of transition metals such as Mn and Zr as a reinforcing element, thereby improving the strength of alloy as much as that of Al 7075 or Al 7050 alloy, with maintaining the weldability and the quench sensitivity.

The aluminum alloy of the present invention having aluminum as a matrix contains 3.5-5 wt% of Zn, 2-3.5 wt% of Mg, 0.5-1.2 wt% of Mn, 0.1-0.3 wt% of Zr, and also a small amount (less than 0.1 wt%) of Cr, Fe, Si and Ti.

The reason why the alloy elements and the range of elements in the present invention are limited is as follows.

Zirconium and magnesium function as a main reinforcement element in the weldable Al-Zn-Mg alloy, and the ratio of zirconium to magnesium must be about 2 or less than 2 so as to maintain a high strength and also the total amount of Zn + Mg should be less than 8.5 wt% in order to exhibit a good weldability.

Manganese is added to supplement the reinforcement effect of the zirconium and magnesium, which are main reinforcement elements because it is impossible to obtain the high strength property about 500 MPa with the precipitations of Zn and Mg alone. So, 0.5-1.2 wt% of Mn are added to produce manganese dispersoids.

In particular, Mn is the most essential element among the alloy elements of the present invention, and produces fine manganese dispersoids of 0.1-0.4 gI m within the alloy structure to carry out a pinning action, thereby improving the strength, as well as the manganese dispersoids coact with the electrical potential to homogenize the distribution of slip band, thereby preventing the reduction ot elongation.

Zirconium is an element which exhibits the recrystalline suppressing and crystal grain refining effect and produces fine Zrdispersoid like the manganese. Particularly, in case that the amount of Zn + Mn approaches 8.5 wt%, the weldability becomes deteriorated. So, 0.1-0.3 wt% of Zr are added to enhance the weldability.

As described above, because the weldable high strength aluminum alloy of the present invention exhibits the strength similar to that of the conventional non-weldable aluminum alloy with maintaining the weldability as in the conventional weldable aluminum alloy, there are provided effects that In case it is used for the high speed train and automobile industry, the weight and volume can be considerably reduced, and also it can be used for various industrial fields which require a good weldability, a high strength and a light weight.

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

ExamDle Sample pieces having the compositions as in Table 1 were manufactured and two-stage aging heat treated. Conventionally, it was possible to obtain the highest strength by carrying out only one-stage aging heat treatment in case of high strength aluminum alloy. However, in the sample pieces of the present invention, it could not be possible to obtain the highest strength with only one-stage aging heat treatment, so two-stage aging heat treatment was carried out. That is, the sample pieces were solution-treated at 420-5000C for 1-5 hours and rapid cooled in the water and then one-stage aging heat treated at 80-1000C for 8-12 hours, and finally second-stage aging heat treated at 120-1500C for 1018 hours.

Table 1. Chemical Composition (wt%)

om position Composition Sample Composition Al Zn i Mg Cu Mn Zr Cr Fe Si Pieces 1 bal 4.38 2.97 0.08 0.50 0.18 -- -- - 0.68 l bal 4.11 2.9 -- 0.68 013 0.01 -- - 3 bal 4.20 2.97 -- 1.13 10.18 0.02 -- 0Q2-I Comp. A17017 j bal 4.70 2.4 0.2 0.3 10.1 0.2 0.1 10.3 Examples Al7039 l bal 4.3 2.6 10.1 .0.2 -- 10.2 0.1 ',0.2 awl2519 1 | bal 0.01 10.2 '2.67 0.18 0.04 -- 0.05 !0.13 The mechanical properties measured from the sample pieces which have been second-stage aging heat treated are shown in Table 2.

Table 2. Mechanical Properties

l Yield Tensile < Mechanical Yield Tensile properties Strength Strength Elongation Remarks Sample + (MPa) (MPa) (x) Pieces 1 1 ~ 495 520 1 10 Examples 1 2 521 569 9.8 3 532 575 l 9.2 At 7017 420 L 480 l 12 l Comp. Al7039 l 400 460 l 12 Comp.

Examples Al2519 l 360 440 i 10 Awl7075 503 ; 575 11 Non-weldable Alloy As can be seen from Table 2, the sample pieces of the present invention exhibit higher mechanical properties than the conventional weldable aluminum alloy. In addition, it can also be noted that the mechanical properties of the present invention are similar to those of the non-weldable high strength aluminum alloy Al7075.

Meanwhile, to examine the weldability of the sample pieces of the present invention, the mechanical properties of the welding portion were measured in the following welding condition, and the results are shown in Table 3.

The welding conditions of the sample pieces of the comparative examples in Table 3 were the same as those of the sample pieces of the present invention.

Welding Condition Kind of Welding: Gas Metal Arc Welding Current : 220-240A Voltage : 30V Wire Speed : 20-30 mm/sec Welding Rod : Al 5356 Applied Electrode : DCRP Thickness of Base Metal : 10 mm Table 3. Mechanical Properties of the Base Metal and the Welding Portion

Mechanical Properties Base Metal Welding Portion Sample Yield Tensile Elongation Yield Tensile Elongation Pieces Strength Strength (%) Strength Strength (%) (MPa) (MPa) (MPa) (MPa) 1 495 520 10 165 275 13 Examples 2 521 589 9.8 160 278 13 3 532 575 9.2 150 272 12 Comp. A17017 420 480 12 175 255 12 Examples A17020 340 405 14 141 276 12 A17039 400 460 12 170 276 11 As can be seen from Table 3, the sample pieces of the present invention exhibit the higher tensile strength than the conventional weldable aluminum alloy (comparative examples) and similar or slightly higher yield strength and elongation. Accordingly, it can be noted that the alloy of the examples of the present invention is superior to the conventional alloys as a whole in the weldability.

Claims

What is claimed is:

1. A weldable high strengt-h-aluminum alloy consisting essentially of:

3.5 - 5 wt% of Zn; 2 - 3.5 wt% of Mg; 0.5 - 1.2 wt% of Mn; 0.1 - 0.3 wt% of Zr; optionally less than 0.1 wt% of Fe, Si, Cr and Ti; and the balance being substantially Al as a matrix.

2. An alloy according to claim 1 in which chronium and/or silicon are present.

3. An aluminium alloy that is weldable and of high strength substantially as disclosed in any of the examples.