Al-Si-Mg-Mn casting alloy and method

Details Al-Si-Mg-Mn casting alloy and method Al-Si-Mg-Mn casting alloy and method

2003-02-27

2004-08-10

Wyszomierski, George (Department: 1742)

Alloys or metallic compositions

Aluminum base

Magnesium containing

C420S548000, C420S544000

Reexamination Certificate

active

06773666

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to aluminum-based alloys. More particularly, this invention relates to improved Al casting alloys. The invention further relates to an Al—Si—Mg—Mn alloy that outperforms 357 aluminum, yet may be cast by various die casting methods, including high pressure vacuum die casting, for making improved aerospace parts therefrom.
BACKGROUND OF THE INVENTION
Sand or low-pressure permanent molds have been traditionally used to produce aerospace castings from 357 aluminum alloys. As registered with the Aluminum Association, alloy 357 includes: 6.5 to 7.5 wt. % silicon, up to 0.15 wt. % iron, up to 0.05 wt. % copper, up to 0.03 wt. % manganese, 0.45 to 0.6 wt. % magnesium, up to 0.05 wt. % zinc, up to 0.2 wt. % titanium and 0.04-0.07 wt. % beryllium, the balance aluminum. Subsequent to this registration of alloy 357, many aluminum producers have been working hard to avoid the addition of beryllium to this casting alloy for a variety of reasons. A family of 357-like alloys has since evolved. Yet, casting certain shaped parts from any of existing 357 alloy family members has proved troublesome.
The limitations of casting 357-like Al alloys via known processes include but are not limited to: maximum wall thicknesses castable, dimensional stability and surface finish. Long solution heat treat (or “SHT”) times, for example, are needed to “spheroidize” the Si particles of a 357-like aluminum to achieve adequate mechanical properties, partially due to the generally slower solidification rate for this alloy/alloy family from traditional casting processes. Although known high-pressure die casting practices may produce thin-walled parts with good dimensional stability and surface finish, such parts cannot be heat-treated due to the high gas contents resulting from these die casting practices.
For some time, Alcoa has been practicing its proprietary vacuum die casting process (or “AVDC”). The process is an optimized outgrowth of the Vacural-Process using Muller-Weingarten casting machines, among other subtleties. After closing the die halves, air is evacuated through the die. The same vacuum is used to draw molten metal into the die's filling chamber. As compared to some other known vacuum die casting processes, Alcoa's AVDC is of very high quality and usually yields an extremely low porosity in the resultant castings.
A serious drive exists to lessen aircraft manufacturing costs. AVDC poses an economical means to reduce aerospace piece counts and decrease assembly costs by making it possible to design, make and use monolithic cast structures. AVDC offers airframe manufacturers excellent dimensional tolerances and consistency, superior surface quality—i.e. no need for chills, very little part-to-part and/or lot-to-lot variations in mechanical properties and a near guarantee of no weld repair.
To date, AVDC has been used to make heat-treatable, low gas content parts for the automotive industry. When high-pressure, die casting processes have been used to make other parts, including aerospace components, from 357 or 357-like aluminum alloys, die soldering and sticking issues have arisen. This invention aims to provide a new casting alloy composition that will reduce or eliminate soldering/sticking problems in AVDC and other high pressure, vacuum die casting practices.
SUMMARY OF THE INVENTION
This invention consists of an improved Al—Si—Mg—Mn casting alloy that consists essentially of: about 6.0-9.0 wt. % silicon, about 0.2-0.8 wt. % magnesium, about 0.1-1.2 wt. % manganese, less than about 0.15 wt. % iron, less than about 0.3 wt. % titanium and less than about 0.04 wt. % strontium, the balance aluminum. On a preferred basis, this invention casting alloy is substantially copper-free, chromium-free and beryllium-free. More preferably, this alloy consists essentially of: about 6.5-8.0 wt. % silicon, about 0.45-0.7 wt. % magnesium, about 0.1-0.5 wt. % manganese, less than about 0.15 wt. % iron and less than about 0.2 wt. % titanium, the balance aluminum.
The aforesaid composition can be subjected to known or subsequently developed practices for making die cast, squeeze cast and/or semi-solid metal formed parts thereform, typically for the aerospace industry. Such castings are preferably solution heat-treated at about 950-1020° F., for about 10-45 minutes, before being cold or warm water quenched (at one or more temperatures between about 70-170° F.), then artificially aged for a preferred 1 to 5 hours or more at about 320-360° F. to achieve adequate properties for aerospace applications.


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