Effects of Hole Making Processes and Surface Conditioning on Fatigue Behavior of 6061-T6 Aluminum 2000-01-0783

In order to reduce the cost of manufacturing aluminum structures, the automotive and earth moving industries are looking at lasers, high speed machining, and plasma processes to cut holes and various shapes. Tower Automotive in conjunction with Caterpillar as part of a National Institute of Standards and Technology (NIST) funded program, studied the effects of various hole-making processes on the fatigue life of 6061-T6 aluminum. Another part of this program was to study the effects of various surface treatment procedures to increase the fatigue life by a factor of ten or higher.

SAE keyhole specimens were made from 3mm and 6mm thick 6061-T6 aluminum. The center test hole was cut using punch pierce, O₂ plasma, 4KW CO₂ laser, 3KW YAG laser, and HyDefinition plasma. The fatigue testing indicated punch pierce cut holes had the highest fatigue life, while at almost all stress (load) levels, holes cut with CO₂ laser had the lowest fatigue life. Surface roughness analysis agreed in general with this finding.

In the second phase of this program, the SAE keyhole specimens were made from 6mm thick 6061-T6. The center test hole was cut using punch pierce and 4KW CO₂ laser. These two processes were selected to (1) determine whether the hole conditioning will provide greater improvement in CO₂ cut specimens compared to punch pierce cut, and (2) obtain the range of improvement possible. The center test hole in these specimens was treated with traditional shot peening, strain shot peening, laser shot (shock) peening, flow drill, abrasive flow machining, and hole expansion with and without reaming. Fatigue data indicated that the hole-expansion with or without reaming improved the fatigue life by a factor much greater than ten. Laser shot peening improved fatigue life by a factor greater than 7. All other methods provided marginal improvement.