High strength, chromium-and lithium-free aluminum casting alloy and related vehicular structural components

Abstract

There is claimed a high strength, aluminum alloy composition especially suited for cast vehicular structural components. This chromium- and lithium-free, alloy consists essentially of: about 3-3.6 wt. % zinc, about 1.3-1.6 wt. % magnesium, about 0.2-0.8 wt. % manganese, about 0.08-0.16 wt. % zirconium, up to about 0.3 wt. % copper, up to about 0.15 wt. % silicon, and up to about 0.25 wt. % iron, the balance aluminum, incidental elements and impurities. It exhibit a typical tensile yield strength greater than about 29 ksi without having to undergo T6-type thermal processing.

Description

BACKGROUND OF THE INVENTION

[0002] This invention provides an improved aluminum casting alloy that possesses high strengths without the need for solution heat treatments. Most conventional aluminum casting alloys, like A356 and A357 (Aluminum Association designations) require a T6 type heat treatment to obtain properties with an average ultimate tensile strength (UTS) greater than 40 ksi, an average yield strength (YS) greater than 29 ksi and an average elongation greater than 8%. The T6 heat treatment or tempering process typically involves solution heat treatment (or “solutionizing”) at a high temperature between 530-540° C., water quenching down from that temperature, and artificial aging. It is a costly and time consuming process due to capital and space requirements, operation cost, part distortion and subsequent wastewater treatment. An aluminum casting alloy that doesn't require a T6-type heat treatment, especially the solution heat treatment portion thereof, while achieving comparable properties would be highly desired from a significant cost savings standpoint.

[0003] Numerous aluminum-zinc alloys have been patented over the years. These include: U.S. Pat. Nos. 2,388,563, 2,506,788, 3,287,185, 4,874,578 and 5,620,652, just to name a few.

SUMMARY OF THE INVENTION

[0004] The above objectives of this invention are achieved by way of a chromium- and lithium-free alloy composition that consists essentially of: about 3.0-3.6 wt. % zinc, about 1.3-1.6 wt % magnesium, about 0.2-0.8 wt. % manganese, about 0.08-0.16 wt. % zirconium, up to about 0.3 wt. % copper, up to about 0.15 wt. % silicon, and up to about 0.25 wt. % iron, the balance aluminum, incidental elements and impurities. One preferred aluminum casting composition according to this invention consists essentially of 3.2 wt. % zinc, 1.45 wt. % magnesium, 0.55 wt. % manganese, 0.12 wt % zirconium, 0.08 wt. % silicon and 0.11 wt % iron. This new generation casting alloy composition contains no purposeful additions of such elements as chromium, lithium, nickel, and vanadium, thereby distinguishing it from other known casting alloys. Particularly, it has been observed that when small amounts of such elements as Cr were added to the aformentioned composition, no positive property effects were observed. With the foregoing composition, however, castings made from this alloy are able to achieve a typical T6-type tensile yield strength of greater than about 29 ksi WITHOUT having to undergo typical T6 type thermal processing.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0005] Unless indicated otherwise, all composition percentages set forth herein are by weight. As used herein, the term “substantially free” means that no purposeful additions of that alloying element were made, but that due to the presence of impurities and/or leaching from prolonged contact with manufacturing equipment, trace quantities of such elements may, nevertheless, find their way into the final alloy product so cast according to the present invention.

[0006] The term “minimum” strength (or other measurable property/characteristic) refers to that level which can be guaranteed such that about 99% of the product is expected to conform with 95% confidence at that level using standard statistical methods. “Typical” strengths tend to run a little higher than the minimum guaranteed levels associated with plant production but they are a worthwhile approximation when insufficient commercial values exist with which to perform a standard statistical calculation. Typical strengths also serve to illustrate an invention's relative improvement in strength when comparing same to established strength values of the prior art. This invention, for example, is able to achieve typical T6-aged, tensile yield strengths of greater than about 29 ksi, often about 33 ksi or higher, without having to undergo typical T6 type thermal processing, as that term is generally defined for a given aluminum alloy product by the Aluminum Association.

[0007] When referring to any numerical range of values herein, such ranges are understood to include each and every number and/or fraction between the stated range minimum and maximum. A range of about 3.0-3.6 wt. % zinc, for example, would expressly include all intermediate values of about 3.1, 3.2 and 3.3%, all the way up to and including 3.55, 3.57 and 3.59% Zn. The same applies to each other numerical property and/or elemental range set forth herein.

[0008] The alloy composition of this invention can be used in sand or permanent molds, die cast, squeeze cast, or subject to other comparable casting processes. Parts made with this alloy can be artificially aged for 1 to 24 hours at one or more temperatures between 100-200° C., in a single or multi-step process, but they need not be solution heat treated beforehand. With the foregoing representative composition, a single step aging for 4 hours at 350° F. resulted in an average UTS/YS/elongation of 40 ksi, 30 ksi and 16%, respectively. The same composition subjected to a two step age of 2 hours at 250° F., followed by 3 hours at 350° F., resulted in average UTS/YS/elongation values of 42 ksi, 33 ksi and 16%, or roughly the “equivalent” of maximum values for A356 alloys aged to a T6 temper. With these values, the compositions of this invention are suitable for manufacturing into suspension parts and other vehicular, especially automotive, structural components.

[0009] Distinct differences often exist between cast and wrought aluminum alloy products. Registrations for the respective compositions are separately addressed by the Aluminum Association with cast alloys being designated in three digits as compared to the four digit designations for wrought alloy products. Not all wrought alloy compositions have a cast equivalent either. Sometimes subtle differences enable one alloy product to be made by only one of these principal methods and not the other. Further processing, SHT, quenching, thermo-mechanical aging, etc., does not always translate well between product forms. For example, in U.S. Pat. No. 3,790,219, it was observed that cast Al alloy wheels may not withstand the stresses of their wrought “equivalents”, wheels which have been shaped by pressing or spinning from rolled sheet, or forged from a cast billet.

[0010] Having described the presently preferred embodiments, it is to be understood that this invention may be otherwise embodied in the scope of the claims appended hereto.

Claims

1. A high strength, chromium- and lithium-free aluminum alloy product cast from a composition consisting essentially of: about 3-3.6 wt. % zinc, about 1.3-1.6 wt. % magnesium, about 0.2-0.8 wt. % manganese, about 0.08-0.16 wt. % zirconium, up to about 0.3 wt. % copper, up to about 0.15 wt. % silicon, and up to about 0.25 wt. % iron, the balance aluminum, incidental elements and impurities.

2. The cast alloy product of claim 1 which contains less than about 3.4 wt. %zinc.

3. The cast alloy product of claim 1 which contains greater than about 1.4 wt. % magnesium.

4. The cast alloy product of claim 1 which contains greater than about 0.4 wt. % manganese.

5. The cast alloy product of claim 1 which contains greater than about 0.1 wt. % zirconium.

6. The cast alloy product of claim 1 which contains less than about 0.1 wt. % silicon and less than about 0.15 wt. % iron.

7. The cast alloy product of claim 1 which is capable of achieving a typical tensile yield strength greater than about 29 ksi without having to undergo T6-type thermal processing.

8. A chromium-free, lithium-free aluminum cast product which has an ultimate tensile strength of about 40 ksi or higher without having been solution heat treated, said casting consisting essentially of: about 3-3.6 wt. % zinc, about 1.3-1.6 wt. % magnesium, about 0.2-0.8 wt. % manganese, about 0.08-0.16 wt. % zirconium, up to about 0.3 wt. % copper, up to about 0.15 wt. % silicon, and up to about 0.25 wt. % iron, the balance aluminum, incidental elements and impurities.

9. The cast product of claim 8 which contains less than about 3.4 wt. % zinc.

10. The cast product of claim 8 which contains greater than about 1.4 wt. % magnesium.

11. The cast product of claim 8 which contains greater than about 0.4 wt. % manganese.

12. The cast product of claim 8 which contains greater than about 0.1 wt. % zirconium.

13. The cast product of claim 8 which contains less than about 0.1 wt. % silicon and less than about 0.15 wt. % iron.

14. A vehicular structural component made from a chromium-free, lithium-free, aluminum casting alloy consisting essentially of: about 3-3.6 wt. % zinc, about 1.3-1.6 wt. % magnesium, about 0.2-0.8 wt. % manganese, about 0.08-0.16 wt. % zirconium, up to about 0.3 wt. % copper, up to about 0.15 wt. % silicon, and up to about 0.25 wt. % iron, the balance aluminum, incidental elements and impurities, said structural component exhibiting a typical tensile yield strength greater than about 29 ksi without having to undergo T6-type thermal processing.

15. The cast structural component of claim 14 which contains less than about 3.4 wt. % zinc.

16. The cast structural component of claim 14 which contains greater than about 1.4 wt. % magnesium.

17. The cast structural component of claim 14 which contains greater than about 0.4 wt. % manganese.

18. The cast structural component of claim 14 which contains greater than about 0.1 wt. % zirconium.

19. The cast structural component of claim 14 which contains less than about 0.1 wt. % silicon and less than about 0.15 wt. % iron.

20. The cast structural component of claim 14 which is a suspension part.