Photo courtesy of Virginia Tech

De-icing a plane at the airport prior to takeoff is familiar to winter travelers, but planes also experience plummeting temperatures and rapid ice formation in flight. Once ice forms on the wings, it can inhibit a pilot’s ability to safely operate the aircraft by disrupting airflow over lifting surfaces. Researchers believe equipping planes to remove ice while flying at altitudes from 35,000ft to 42,000ft would provide a better way to maintain safety.

Jonathan Boreyko, associate professor in Virginia Tech’s Department of Mechanical Engineering, is leading a team working with Collins Aerospace to use ice to control ice formation on airplane wings.

In a study published in Physical Review Letters, Boreyko’s team leveraged Cassie’s Law, which shows air can be trapped under water drops if the drops are suspended atop a structure that’s bumpy and water-repellent. With a structure that could trap air underwater, the researchers seek to make ice form in a layer with less surface adhesion.

Making a surface water-repellent typically requires a chemical coating that must be periodically replenished, and the bumpy surface also wears. The team took an approach that doesn’t require fragile chemical coatings or ultra-fine bumps, opting for a simple, durable structure of millimeter-sized aluminum pillars.

Pedestals 1mm tall by 0.5mm wide were machined into a pattern, set 1mm apart. As the temperature dropped, frost preferentially grew on the pillar tops, producing elevated frost tips. As more water was added, it was absorbed into the porous frost layer. When water drops subsequently impacted the surface, they were caught on the frost pedestals.

Lead author Hyunggon Park says the freezing drops created tiny ice bridges that sealed air gaps in valleys between the frost-tipped pillars.

“The water drops were being caught by the frost tips and building ice bridges to trap air pockets underneath,” Park notes. In time, a continuous and air-trapping ice canopy formed over the frost-tipped pillars.

Other de-icing methods may still allow a sheet of ice to adhere more directly to a large surface, but these trapped air gaps suspend the sheet, lowering the adhesion ice has to the surface.

“By using larger pillars in place of nanostructures, and frost tips in place of a hydrophobic coating, we found we can get the same benefit of trapping air underneath the forming ice while avoiding the durability concerns,” Boreyko says. “This should make our approach practical for enhancing de-icing on aircraft or heat exchangers.”

With a weaker bond, it’s possible to use the air pockets to then push ice away. This will be the next step in the researchers’ process, as Boreyko’s team continues to develop their method.

Virginia Tech College of Engineering Dept. of Mechanical Engineering

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1) What are typical temperatures for creep flattening titanium parts?

Almost all the titanium products are creep flattened above 1,000°F. I would say creep flattening is usually performed between 1,250°F – 1,350°F but it’s alloy dependent. Temperatures and hold times will vary with composition. Typically, a two hour hold at temperature using work thermocouples is sufficient.

2) Can correcting or creep flattening titanium only be done in vacuum?

I wish, but no. We also do some jobs in our air atmosphere furnaces, but this is typically reserved for parts that have ample stock on them, such as rough machined parts, plates, blanks. Air furnaces allow customers some savings, but there must be enough stock to remove any surface contamination. For finished parts, it’s a different story. You need to bake out the vacuum furnace at 2,400°F prior to the run and ensure vacuum levels remain in the 10-5 Torr scale. Otherwise, you will see surface oxidation and the dreaded alpha case. Don’t forget to bake out the fixturing at a minimum of 150°F higher than the creep flatting temperature.

3) What type of fixturing should I consider when creep flattening? Can I also creep a shape or detail into the titanium?

I like graphite tooling to flatten or even develop a shape or detail into titanium parts. We know metals expand and contract during the thermal process. Graphite allows these materials to move in the desired direction with little friction. Plus, it prevents movement along dimensional axes that must be held constant. We’ve successfully used metallic fixtures in the past, but graphite is my go-to.

4) My titanium part is moving during machining, can creep flattening help?

Yes! This is very common. We perform in-process stress relieving to minimize movement typically seen during machining. Because creep flattening and stress relieving are often done at the same temperature, we can relieve the stress in a part while forming it back to the required shape for final machining. A lot of work takes place in the background to accomplish this, but it can be done successfully.

5) Will reheating titanium above 1,000°F affect my material certification?

Give your heat treater your material certifications. Many mills will certify to aerospace material specification AMS 2801, AMS 4905, AMS 4911, AMS-H-81200, etc. The material often can be re-annealed while simultaneously creep flattening. Know the specification and/or the original mill anneal parameters beforehand. Then you can work within required temperature ranges while creep flattening or shaping titanium parts. Also keep in mind, when stress relieving or creep flattening in deep vacuum, you’re extracting hydrogen at the same time for free. Stress-relieving, shaping, degassing - there is a great deal of value added with creep flattening in a vacuum furnace.

For more information: https://solaratm.com/vacuum-heat-treating/vacuum-creep-forming

1) 3D Contour Milling (CM) and how it’s used

3D CM is a style of machining used heavily in industries that create complex three-dimensional contour parts such as the aerospace, medical, and mold & die sectors. These contours contain a range of angles, shapes, and radii that would normally be very difficult and time consuming using conventional machining styles. When the correct tools are selected, CM also produces better surface finishes and increases productivity and tool life.

2) Tools used in this style of machining

With such complex geometries, there isn’t one tool to do it all (efficiently). Ball nose tools can be used to produce almost any feature, but that’s not to say there aren’t better/more efficient options available. Barrel/circle segment cutters are commonly used when CM. These tools have a very large radius on the OD of the tool and can be used to minimize cusp height (high spots of uncut material between passes), requiring fewer steps to attain a finish similar to a ball tool. Many times, the radius of a tool can be selected to match a specific radius on the part.

3) Axial chip thinning

Used for wide radial, shallow axial cuts, axial chip thinning occurs when using a tool with a lead angle of less than 90° (not square). This produces a larger chip and redirects cutting forces up the tool, toward the spindle. A simple way to picture this is slicing a vegetable diagonally; it produces a larger slice, and in turn, takes fewer cuts to slice it entirely compared to straight, perpendicular cuts. With cutting forces directed up and not out, the user can increase feed rate far above what a square end mill would run.

4) Advantages of 5-axis machines in CM

While it’s possible to 3D CM on a 3-axis machine, a 5-axis allows greater utilization of the cutting tool and creation of more complex/unique parts in a single setup. This is where barrel/circle-segment cutters can be used. The fifth axis allows the tool to be tilted to better match the part’s contour, minimizing cusp height while maximizing productivity.

3D CM is a style of machining used heavily in industries that create complex three-dimensional contour parts such as the aerospace, medical, and mold & die sectors.

5) Tilt angle advantage when using a 5-axis machine

A tilt angle for a ball nose tool does a few things. It gets the tip of the tool (where the surface footage is zero and shearing the material is most difficult) off the workpiece, increasing the effective diameter. Ball nose tools typically have fewer flutes at the tip of the tool, so using a tilt angle can engage all flutes, generating a better finish and spreading wear evenly on all flutes across the radius, ultimately increasing tool life. A higher flute count tool, such as the new T-Carb® 51B, will add additional flutes to share the load, increasing life and resulting in lower finish measurements.

Kyocera SGS Precision Tools