ACE 2001 highlights

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Automated cold working at Northrop Grumman
In support of the T-38 Wing Life Improvement program, the Materials and Process Product Support group at Northrop Grumman performed a metallurgical examination of a test article containing several cold expanded holes drilled in an aluminum/aluminum and an aluminum/steel assembly. A miniature T-38 wing was developed to determine the feasibility of cold expanding or cold working of drilled holes using automated machinery incorporating a split-mandrel process. The test assembly consisted of a landing gear rib (~0.281-in. thick and made of Hy-Tuf low-alloy steel heat treated to 220-240 ksi), a 7075-T73 aluminum wing skin (0.250-in. thick), and spars and ribs made of 7075-T73 aluminum alloy.

The cold working tool is attached to the five-axis head and the T-38 drilling machine.

The assembly was subsequently mounted on a Man-Tech-designed automated drilling machine that was adapted to incorporate a West Coast Industries hydraulically operated split-mandrel tool. Typical holes representing various nominal sizes of 5/32, 3/16, and 1/4-in. were drilled, cold-expanded, and reamed.

A metallurgical examination was performed on a 1/4-in. hole located in the all-aluminum structure and a 1/4-in. hole in the aluminum/steel combination structure. No cracks or objectionable defects were found on the ID surfaces or in the vicinity of the holes.

The cold working, or expansion, of a hole in a metal creates a compressive stress zone around the hole that causes a slight increase in hardness and improves fatigue resistance of the material. Therefore, to determine the thickness of the compression zone, typical hardness surveys were made on two holes using Knoop micro-hardness with 200-gram and 500-gram loads on the aluminum and the Hy-Tuf steel, respectively. As a result of the hardness surveys, it was determined that the compressive stress zone created by the cold working process around a hole can be as thick as the diameter of the hole to be cold worked.

If properly programmed and set up, it is possible to cold-work holes using the split-mandrel automated process and that the cold-worked holes are completely acceptable. However, it is extremely important to study the entire wing surface contour and substructure prior to automatic drilling and subsequent cold working to avoid misdrilling, damage to holes and/or substructure, and failure to meet the edge distance requirement.

To avoid mandrel breakage, it is not recommended to cold-work holes in steels, which have been heat-treated to a high strength level using this split-mandrel automated process. For those applications, the split-sleeve cold-working process is recommended.

Results from the demonstration and testing were positive enough for Northrop Grumman to invest in the design, acquisition, and application of automated cold working on the T-38 wing. The lower surface of the new wings for the legacy trainer was cold-worked to increase its life.

The West Coast Industries cold-working tool used in the demonstration served as a design base for the end effector on the new automated drilling system. The tool was centered and affixed under the drill spindle of a standard Northrop/AIM GADS five-axis head by ball rail and bearings. The head travels out and retracts along the centerline on the ball rails. The tool mandrel is moved into each hole by a pneumatic cylinder. A computer code activates a hydraulic pack that pulls the mandrel through the hole and expands the metal. The mandrel is then retracted from the hole to its previous position.

Information was provided by George Bullen, Northrop Grumman Corp.

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