METAL CASTING 
Project Fact Sheet 
NATURAL AGING ALUMINUM CASTING ALLOYS 



 

	

 






 



 



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ENERGY 
Naturally aging Al-Zn-Mg (7xxx) casting alloys have excellent mechanical 
properties. No separate solution heat treatment or elevated temperature aging is 
required and holding 30 days at room temperature produces the desired strength. 
Consequently these alloys can be used to produce high-quality castings and 
realize cost savings of $0.20 per pound; a significant amount in today's highly 
competitive casting market. An energy savings of 0.2 kWh (2,200 Btu) per pound 
is realized by eliminating the requirement for high-temperature solution heat 
treating. 
Despite their advantages, these alloys have been seldom used due to their 
propensity for hot cracking. Moreover, there is a lack of sound data on the 
mechanical properties and corrosion and stress corrosion resistance of these 
alloys. A recent improvement in the grain refinement of these alloys reduces their 
hot cracking susceptibility (see figure below). This research will address barriers 
to the adoption of 7xxx alloys. GKS Engineering Service and industry partners 
will lay the technical foundation needed for commercial application of natural aging 
alloys. Specifically, this research will provide information regarding the corrosion 
and stress corrosion resistance of these alloys. The R&D will also provide more 
current information regarding these naturally aging alloys and demonstrate how an 
improved grain refinement process can be used to produce crack-free castings. 
The potential use of naturally aging alloys to produce castings of equivalent 
strength at reduced cost and energy consumption makes aluminum castings more 
competitive. 

	


 
Hot Crack Index 
3.0 
2.5 
2.0 
1.5 
1.0 
0.5 
0.0 
ELIMINATING ELEVATED TEMPERATURE HEAT TREATING SAVES 
712 
53760 
705 
Recent improvments in grain refinement have reduced hot cracking 
susceptibility 
Grain Refinement 
OFFICE OF INDUSTRIAL TECHNOLOGIES 
ENERGY EFFICIENCY AND RENEWABLE ENERGY • U.S. DEPARTMENT OF ENERGY 
2.0 1.0

	

 
Goals: The goals of this research are as follows: 
• Demonstrate grain refining practices to mitigate hot cracking in 7xx alloys, and 
thereby establish 7xx natural aging alloys are a commercially viable alternative 
to the casting alloys used today, such as A356-T6 castings. 
• To develop mechanical property data for these alloys by measuring casting 
tensile properties and hot cracking resistance as a function of alloy 
composition. 
• Gain more detailed scientific knowledge about the aging process in 7xx series 
aluminum alloys. 
• Develop understanding to optimize alloy composition within specification for a 
particular casting. 







 
This R&D project's planned tasks include: 
• Phase I – A survey of the technical literature on the aging of Al-Zn-Mg alloys 
will be conducted. Aging studies will be made with the test bars poured at 
Stahl Specialty: hardness and tensile strength will be determined versus aging 
time at various controlled temperatures. From these results a set of timetemperature-
transformation curves will be constructed. A second set of bars 
will be allowed to age for 60 days. Other bars will be tested for corrosion 
properties. These bars will be compared with conventional Al-Si (3xx) and Al- 
Si-Cu (2xx) alloys. This information obtained will be used to develop a 
statistical model of alloy properties. From this model, two alloys will be 
selected for Phase II. 
• Phase II – The two alloys selected from Phase I will be melted and a 
significant number of bars, step wedge, and at least one commercial casting 
will be poured. Hot cracking trials will be performed. The castings will be 
subjected to three aging processes. They include: a natural age for at least 
six months, an aging treatment at slightly higher temperature (60-100 °C, to 
be determined based on results of Phase I), and a step aging process. The 
results will be tabulated, summarized and presented at the Annual Congress of 
the American Foundry Society. 


	

 
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