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Bulking up aluminum alloys.

Aluminum has the reputation of being a "softie."

But like all reputations, this one is not entirely true.

Because of their difference in material properties, most design engineers believe that aluminum castings are weaker then steel or iron. However, while several aluminum alloys exist that have comparable mechanical properties at a lighter weight, they also have the reputation of being difficult to cast due to an increased tendency for hot cracking.

One example, 206 aluminum alloy, has mechanical properties approaching some grades of ductile iron. With excellent high temperature tensile and low cycle fatigue strength, this alloy could be used to provide cost savings in automotive applications, reducing vehicle weight. The problem is that 206 alloy has a propensity for hot cracking.

In an effort to reduce the alloy's tendency for hot cracking, an improved method to grain refinement and a new ultrasonic inspection technique to test for cracks have been developed. This article will discuss both developments, and illustrate the viability of this aluminum alloy as a high-strength casting alloy.

Grain Refinement

The casting in Fig. 1 was used to evaluate three different alloy conditions. The casting features a sharp radius where the arms join its central heavy section, purposefully concentrating the stress generated during shrinkage of the arms during solidification into a small area. The use of more generous fillets or tapers at this juncture would be better for the casting, but this was intended as a severe test for hot cracking.

Each hot crack test casting was examined and given a numerical hot cracking number. This numerical value was obtained by examining each arm, and assigning a value between 0 and 1 as follows: 1 point for a broken arm; 0.75 points for a severe crack; 0.5 points for a modest crack; 0.25 points for a crack detectable only under magnifying glass; and 0.0 points when no cracks were present. The number for each arm was summed to establish a total for the casting--0 for no cracks and 6 for all arms broken.

Three alloy conditions with varying levels of grain refinement were studied:

* conventional 206 aluminum alloy without any titanium (Ti) or grain refiner additions;

* conventional 206 alloy with an addition of 30 ppm boron (B) as 5 Ti-1B master alloy;

* experimental 206 alloy with an addition of 30 ppm B as 5 Ti-lB master alloy. The conventional 206 alloy has a specified minimum Ti content of 0.15%. The experimental alloy is a low Ti (<0.05%) version of the 206 alloy, which achieves a smaller grain size.

The results of the hot crack tests in all three alloys are shown in Fig. 2. The level of hot cracking in the experimental 206 alloy is nearly zero in this casting, significantly better than the conventional 206 alloy. The importance of grain refinement is obvious from the high level of cracking that was found when no refiner was added.

Control Arm

While a number of commercial applications exist for 206 aluminum alloy, an automotive suspension component was chosen to demonstrate its potential. Aluminum currently accounts for only 5-6% of this market, with most current parts produced in ductile iron. The experimental 206 alloy has a tensile strength equivalent of some grades of ductile iron.

The control arm (Fig. 3) is a permanent mold casting that was originally cast in 356 aluminum alloy. A number of control arm castings were produced in the improved experimental 206 aluminum alloy and heat-treated to the T41 condition (solution treatment followed by a 5-day natural age). Samples then were cut from a "critical" area of the casting and tested until failure. The results of the tensile and fracture toughness tests can be seen in Table 1.

The tensile properties of these castings are shown in Table 1, together with typical results for A356 alloy in the same control arm. It can be seen that the properties obtained with the experimental 206 alloy are significantly higher than those found with the conventional A356 alloy.

This material could be used in suspension components to save weight in automobiles and improve fuel economy. It also could be used in a number of other applications where high-temperature tensile strength and toughness are desired. The experimental 206 alloy also has superior low cycle fatigue strength, nearly ten times that of A356 alloy in the control arm casting.

Ultrasonic Inspection

While the experimental 206 alloy with the smaller grain size is an improvement over the conventional 206 alloy, it remains susceptible to hot cracking. Therefore, foundries must utilize a fast, real-time inspection process to immediately identify casting defects on the foundry floor.

During the trials with the control arm casting, all castings were subjected to fluorescent liquid penetrant inspection. A number of castings also were tested for cracks using a portable system based on a patented nonlinear vibro-modulation ultrasonic inspection technique. The technique is based on the modulation of ultrasonic signals by low frequency vibration, which takes place only when contact-type interfaces, such as cracks, are present.

Because of the modulation produced by cracks, the resulting ultrasonic signals contain newly generated frequencies, or sideband signals to the original probing frequency. As a result, flaw responses can be effectively discriminated from other factors, such as reflections due to geometry or material inhomogeneities.

The test method also possesses the following unique qualities:

* crack detection is not impaired by a component's complex geometry or material properties;

* a relatively large area can be inspected from a single location;

* it has the ability to detect blind cracks, which would otherwise be invisible with conventional fluorescent liquid penetrant testing techniques.

The final test result is an averaged modulation index, which represents the normalized strength of the sideband signals in db.

The evaluations of the inspection technique found that cracks were detected in all the samples where cracks were visible. Also, the size of the defect correlated well with the modulation index--the greater the crack area, the greater the sideband signal.

The ultrasonic technique is capable of detecting cracks as small as approximately 0.1-0.2 cm squared. With optimal grain refining, the validation of 206 aluminum alloy for high-production is underway.

Table 1

Tensile Properties of Experimental 206-T41 and 356-T6 Alloy Castings

Alloy Temperature UTS(MPa) Yield(MPa) 4D Elong 5D Elong.

206 Experimental 20C 432.3 266.8 20.1% --
206 20C 412.8 267.1 17.5% 17.1%
356 20C 299.9 204.8 12.0% 10.7%
206 Experimental 120C 341.7 220.6 20.8% 19.1%
356 120C 244.8 185.5 15.1% 13.2%

Alloy Reduction
 in Area

206 Experimental --
206 22.7%
356 13.3%
206 Experimental 29.2%
356 20.3%

For More Information

"Recent Developments in the High Strength Aluminum-Copper Casting Alloy A206," G.K. Sigworth and F. DeHart, 2003 AFS Transactions, 03-135.

"Grain Refining of Aluminum Casting Alloys," G.K. Sigworth, 6th International AFS Conference on Melt Treatment of Aluminum, 2001, p. 210-221.

About the Authors

Geoffrey Sigworth is the president of GKS Engineering Services, Dunedin, Florida. Frank DeHart is plant metallurgist at Thyssen Krupp Stahl Co., Kingsville, Missouri. Fred Major is a research scientist at Alcan International, Ltd., Kingston, Ontario, Canada. Dimitri Donskoy is associate professor at Stevens Institute of Technology, Hoboken, New Jersey.
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Article Details
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Author:Donskoi, Dimitri
Publication:Modern Casting
Geographic Code:1USA
Date:May 1, 2003
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