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Professor Sindo Kou

Professor Sindo Kou holds a welded aluminum plate with liquation cracks. (Large image)

Materials Science and Engineering

Eliminating cracks in aluminum welds

Because they are strong, yet lightweight, aluminum alloys are becoming a popular replacement for steel in applications such as automobiles, where reduced weight can translate to increased fuel efficiency. However, these alloys—especially the strongest ones—can be difficult to weld.

Now, Professor Sindo Kou and graduate student Guoping Cao have demonstrated a method to predict and eliminate one of the biggest problems: cracking around the weld area.

Called liquation cracking, these cracks can appear small but will over time propagate and cause the welded area to separate from the base metal (the workpiece).

Liquation cracking in Al-6.3Cu alloy

Liquation cracking in Al-6.3Cu alloy (Large image)

Aluminum alloy plates typically are welded with a melted filler metal. As the liquid metal within the weld cools and solidifies, it contracts, pulling on the softened base metal around it, called the partially melted zone, or PMZ. Based on this knowledge, Kou and Cao developed a simple criterion for determining cracking susceptibility: If the weld metal solidifies earlier than the PMZ, it will pull apart the grains in the PMZ, causing cracks.

Thanks to commercial solidification software developed by Professor Emeritus Austin Chang, Kou can compare the solidification rates of the weld metal and the PMZ, then determine if there is a point at which the weld metal will be more solid than the PMZ. If so, the weld is likely to crack.

Using a database of aluminum alloy properties, welders now can match a filler metal to any aluminum alloy they want to weld. In addition, engineers can find or develop a filler metal to meet the criterion where none is commercially available—even for alloys so prone to cracking, they previously were considered unweldable, says Kou.

Next, Kou hopes to apply the same principles to additional groups of problematic metals, such as stainless steel and nickel-based alloys.

Controlling tiny transistors
with the flip of a light switch

A team of UW-Madison researchers has demonstrated a technique that could produce tiny, light-sensitive transistors. Led by assistant professor Padma Gopalan and Physics Professor Mark Eriksson, along with collaborators at Sandia National Laboratories, the team is studying single-walled carbon nanotubes.

Nanotubes are cylindrical sheets of carbon only billionths of a meter across. They have surprising strength and conductivity, functioning like tiny wires, so nanotubes have great potential in electrical circuits. Nanotubes also absorb light, leading to changes in their properties, which makes them promising for optoelectrics. However, inducing lasting changes in nanotubes with light has been difficult, and the changes that occur are usually small and transient.

The UW-Madison team has demonstrated a method that gives researchers greater control over nanotube properties with the flip of a light switch. The researchers tagged nanotubes with dye molecules that change their shape when exposed to certain colors of light, resulting in a reversible shift in charge attached to the dye. The researchers found that this charge shift modulates the nanotube transistor by creating a shift in the threshold voltage. These dye-tagged nanotubes show no indication of degradation, and they can be reliably switched over long periods of time.

This switching property gives the light-sensitive nanotubes potential for applications in optoelectronic devices, such as light sensors, solar energy cells and flat-panel displays.

In the future, synthetic control of the dye molecule will offer the ability to tune both the absorption wavelength—the color of light to which the nanotube responds—and the electrical sensitivity of the nanotube-dye hybrid transistor.

New electron microscope
opens doors into tiny worlds

The University of Wisconsin-Madison is joining the elite ranks of research institutions housing a state-of-the art scanning transmission electron microscope (STEM). The National Science Foundation awarded a major research instrumentation grant to a multidisciplinary team led by Assistant Professor Paul Voyles, including Geology Assistant Professor Huifang Xu, Animal Sciences Professor Ralph Albrecht, and Professors Oswald Uwakweh and Oscar Perales from the University of Puerto Rico, Mayagüez.

Researchers use STEMs to characterize natural and artificial nanostructures and materials, ranging from novel superconductors to biomacromolecules inside cells. The STEM works by using powerful magnetic lenses to focus a beam of high-energy electrons into a tiny probe, which is scanned across the sample. Scattered electrons are collected and sorted by their scattering direction or energy using various detectors. A high-resolution image is built up point by point, which can reveal the sample atomic structure, composition and bonding.

The UW-Madison STEM will be capable of sub-Angstrom-resolution imaging and microanalysis down to the level of individual atoms and the bonds between them. It will enable new research in the atomic structure of glasses, superconducting materials, naturally occurring and designed nanostructured materials, and labeling of biomacromolecules inside cells.

As part of the Materials Science Center, the new STEM will be available to researchers both inside and outside UW-Madison.

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