Al alloy development spans six decades.

Al Alloy Development Spans Six Decades

The Aluminum Div's Silver Anniversary paper, "Factors Influencing the Properties of Al-Mg-Si Casting Alloys," was presented by Barri Chamberlain, based on an original paper coauthored by Chamberlain and John Sullivan. In 1964, they presented research on the soldification rate and gas content of three aluminum alloy systems at three gas levels: 0.36, 0.27 and 0.15 ml [H.sub.2]/100 g.

After discussing several major developments in Al-Si-Mg alloy research over the past 60 years, Chamberlain noted: "By 1964 we were well aware of the contributory effects of hydrogen gas pickup on castings, what solidification rate is, what modification was doing, what heat treatment we should be getting to achieve the published properties. But 3.5% elongation was typical for sand-cast A356 alloy. That seemed pretty sad. These were very, very low properties.

"So we spent a lot of time after that in both the 356 and A356 alloy system working on grain refinement. Some treatments in our lab furnace allowed us to reduce the phosphorus level, and under these circumstances we found that phosphorus was really a major actor in the modification of these alloys.

"We realized that at the 12% silicon level, if we could get a phosphorus level of 2.5 ppm or less, that the sodium level would be as low as 5 ppm, and you would get absolutely beautiful modification, and at that level the sodium hardly burns off--there's nothing there to burn off.

"We realized that when we were dealing with a 7% silicon alloy, that we could allow the phosphorus level to go as high as 2.5 ppm, and when you put in something like 20 ppm sodium and allow it to come down to 5 ppm, you still had excellent modification."

Chamberlain said they stayed below 750C to avoid gas pickup, loss of sodium and lowered grain refinement. "We obtained that 356 alloy and A356 properties that we put into a paper in 1973," he said. "Well-modified, well-treated material in normal 356 alloy could achieve A356 properties, and that's what we were aiming for.

"So back in 1973, we were set to come up with some really startling properties of the 356 alloy system. I don't think anyone knew--and the information is there--but we are not really picking it up as well as we should."

Chamberlain also noted: "We've come along way with the 356 alloys and with all the Al-Si-Mg alloys. We can get better--there's no question about it--but to make a quantum jump we have to look at composites."

The typical 15% SiC composite, according to Chamberlain, is not a normal aluminum alloy. "You are going to need help getting the material to fill properly and get the properties we anticipate from the material," he said.

Commenting on the fluidity of composites, Chamberlain said, "One of the biggest changes with this type of alloy is its viscosity. And its viscosity is not necessarily the inverse of fluidity. Foundry fluidity is its ability to fill the mold. Viscosity is how we get the gas bubbles and dirt out. This material is extremely sticky. Any entrapped air caused by turbulence stays suspended; any oxides stay suspended and attract silicon carbide and lower your properties."

Chamberlain then went on to examine the thermal expansion and aging properties of SiC composites. Temperature is critical because at elevated temperatures silicon carbide will react with aluminum to form aluminum carbide, he said.

SiC will also settle, however, "it doesn't settle all the way to the bottom to give you something like 100% silicon carbide. It interferes with itself as it settles out giving you various percentage levels," he said.

In conclusion, Chamberlain predicted that applications of composites will move beyond their current aerospace use, but that the challenge lies with the foundry industry: "There's some interesting information being presented. What I'm looking for is that the industry will really move. You are the people who can do it."