High strength aluminum alloy forgings

Details High strength aluminum alloy forgings High strength aluminum alloy forgings

1999-08-24

2003-10-07

King, Roy (Department: 1742)

Metal treatment

Stock

Age or precipitation hardened or strengthened

C420S534000, C420S535000, C148S691000, C148S552000

Reexamination Certificate

active

06630037

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns an Al—Mg—Si series high strength and high toughness aluminum alloy forgings (aluminum is hereinafter simply referred to as Al) suitable, particularly, to parts for transportation machines such as suspension parts for automobiles.
2. Related Art
As is well-known, Al alloys such as of AA 6XXX series (Al—Mg—Si alloys) excellent in moldability and burn on hardenability have been used as structural materials or suspension parts such as knuckles, lower arms and upper arms for transportation machines of automobiles or vehicles, with an aim of reducing weight. The AA 6XXX series Al alloys are also excellent in other required characteristics such as mechanical properties, for example, formability or corrosion resistance or stress corrosion cracking capability and, in addition, also excellent in view of recycling property capable of re-using scraps as melting materials for AA 6XXX series since they contain less amount of alloying elements such as Mg.
Referring to suspension parts for the automobiles, for example, cast Al alloy materials or Al alloy forgings are used in view of the reduction for the production cost and fabrication into parts of complicate shapes. Among them, Al alloy forgings are used for those parts requiring mechanical properties such as higher strength and higher toughness. The Al alloy forgings are manufactured by soaking a cast alloy material and then applying hot forging such as mechanical forging and tempering such as T6 or aging treatment.
In recent years, it has been demanded for reducing the wall thickness and improving strength for those parts including suspensions for use in automobiles and it has been required also for the Al alloy forgings to improve strength and toughness. However, AA 6XXX series used at present for such application uses, inevitably cause insufficiency in the strength.
In view of the above, it has been proposed to improve strength and toughness of the Al alloy forgings, for example, in Japanese Published Unexamined Patent Application Hei 6-256880 by defining the ingredients of AA 6XXX series (Al—Mg—Si alloys) cast Al alloys for use in forgings used as parts such as suspensions of automobiles, reducing the average grain size as small as to 8 &mgr;m or less and reducing the secondary dendrite arm spacing (DAS) as narrow to 40 &mgr;m or less.
However, as shown in examples of Japanese Published Unexamined Patent Application Hei 6-256880, the secondary dendrite arm distance (DAS) of the Al alloy forgings obtained in this prior art is about 30 &mgr;m at the smallest and the Al alloy forgings has characteristics, as a result of up setting forging test for round bars, for example, a tensile strength (&sgr;
B
) of about 39.2-39.3 kgf/mm
2
(385-394 MPa) and a toughness (L
c
) of from 2.2 to 2.3 kgf/mm
2
(about 22 J/cm
2
as the Charpy impact value) in a case where a forging ratio [(original ingot height d
o
−crack occurring height d
t
)/d
o
] is 75%.
That is, in the upset forging test for round bars as in the prior art, since each of the portions for a round bar is forged uniformly, mechanical properties are uniform for each of the portions of the round bar. However, as shown in
FIG. 2
as an example of Al alloy forgings for use in a suspension part of an automobile, the forging ratio is sometimes lowered depending on the portions of the part even by hot forging such as mechanical forging in an actual Al alloy forgings and mechanical properties are not uniform for each of the portions of the forgings. For example, in a case as shown in
FIG. 2
, even if the forging ratio is 75% for a portion T
1
, the forging ratio for the portion T
2
is only about 50%. Then, the toughness for the portion with the lower forging ratio is inevitably lowered compared with other portions of higher forging ratio since cast structure remains even after forging.
Then, although the strength and the toughness of the Al alloy forgings obtained by this prior art are improved compared with Al alloys such as of AA 6061 or 6151, average toughness is poor in the Al alloy forgings, particularly, in such an Al alloy forgings in which the toughness for the portion is lowered because of the portion of the lowered forging ratio. That is, in the prior art, the level for the toughness is further lowered at a portion with the forging ratio of 75% or less, further, 50% or less and high yield strength and high toughness values requires for the entire part can not be obtained.
As a result, the forgings can not be applied to parts requiring higher strength and higher toughness as a entire portion and, more specifically, to those parts or members requiring a high strength of 315 N/mm
2
or more as &sgr;
0.2
and a Charpy impact value of 20 J/cm
2
or more as the entire part, and this hinders the development of the Al alloy forgings to the application uses for suspension parts for use in automobiles.
The present invention has been accomplished in view of the foregoing situations and it is an object thereof to provide a high strength and high toughness Al alloy forgings excellent in average mechanical properties as an entire forgings even if a portion with low forging ratio is present, and applicable to those parts or members requiring high strength and high toughness as the entire forgings.
SUMMARY OF THE INVENTION
In order to attain the foregoing object, the feature of the Al alloy forgings according to the present invention resides in a high strength and high toughness aluminum alloy forgings containing Mg: 0.6-1.6% (mass % here and hereinafter), Si: 0.6-1.8% and Cu: 0.05-1.0%, Fe: 0.30% or less, one or more of Mn: 0.15-0.6%, Cr: 0.1-0.2% and Zr: 0.05-0.2%, hydrogen: 0.25 cc/100 g Al or less and the balance of Al and inevitable impurities, the Al alloy forgings being prepared by casting a cast Al alloy ingot at a cooling rate of 10° C./sec or higher, subjecting the same to a soaking heat treatment at a temperature of 530-600° C. and then hot forging into a forgings, in which the volume fraction of total constituents phase particles (Mg
2
Si, Al—Fe—Si—(Mn, Cr, Zr) series intermetallic compounds) is 1.5% or less per unit area.
As a result of a study on the relationship between constituents and the toughness of Al alloy forgings, the present inventors have found that the volume fraction of constituents phase particles has a close concern with the toughness of the Al alloy forgings.
That is, the present inventors have found that, among the constituents of the cast Al alloy materials, Mg
2
Si and Al—Fe—Si—Mn, Al—Fe—Si—Cr or Al—Fe—Si—Zr series intermetallic compounds constitute starting points for the rupture (starting points for dimples).
More importantly, the present inventors further found that it is not significant that the constituents present in the Al alloy structure are large or of a long chained shape, but that dispersion of them at a spacing with each other contributes to the improvement of the toughness. That is, the constituents can not simply decreased or eliminated since they contribute to the insurance of required strength. However, it has been found that a necessary strength can be insured and a high average toughness can be insured even if the forging ratio is low or even if there is a portion with a low forging ratio, by controlling the form of the constituents that are present inevitably or present by requirement.
For example, no effective contribution can be obtained for the improvement of the toughness by merely controlling the form of the constituents, namely, by merely reducing the average size of the constituents in the cast material as described in Japanese Published Unexampled Patent Application Hei 6-256880. On the contrary to the idea disclosed in Japanese Published Unexamined Patent Application Hei 6-256880, the present inventors have found that the constituents in the cast material, even if their average size is large, can contribute to the improvement of the toughness so long as they are dispersed being spaced apart from each other (present dispersedly). That is,

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