Low-plasticity burnishing (LPB) has been devel- oped as an affordable means of imparting residual compressive stresses to surface layers of metal parts (especially engine components) in order to increase their fatigue lives. Heretofore, surface compressive stresses to enhance the fatigue lives have been produced, variously, by shot peening or laser shock peening. Unfortunately, thermal relaxation has been found to result in loss of the needed surface-layer compressive stresses, with consequent shortening of component lives and reduction of engine performances. Hence, what is needed is a means of imparting thermally stable surface compression.
In the LPB process, a smooth, freerolling spherical ball is pressed against and rolled along the surface of the workpiece to be burnished. The ball must be hard, and it must have a high modulus of elasticity and a high yield strength. To ensure free rolling, the ball is supported in a spherical-socket fluid bearing (see figure) with sufficient fluid pressure and flow to maintain the ball out of contact with the socket. The force with which the ball is pressed against the surface is made large enough to deform a surface layer of material into a state of compression, taking account of any tensile stress that might exist in the workpiece prior to burnishing.
By use of the positioning capability of a computer numerically controlled (CNC) machine tool, the ball is moved along the surface in a raster or other suitable pattern to cover the surface in a series of passes at a controlled separation chosen to obtain maximum compression with minimum cold working. LPB is not limited to flat workpieces: In the case of a complexly shaped workpiece, the positioning capability of a multiaxis CNC machine tool can be exploited to move the ball on any desired path across the surface, as in a typical multiaxis CNC machining operation.
LPB produces minimal cold work, imparting greater (in comparison with shot peening and laser shock peening when performed with multiple shocking cycles) resistance to thermal relaxation at high temperature. The resulting greater retention of surface compression at engine operating temperatures results in substantial increases in fatigue lives and in retardance of the growth of pre-existing cracks. In addition, LPB increases resistance to damage by impacts of foreign objects.
LPB costs less than does laser shock peening and offers greater depth and stability of the compressive layer, relative to shot peening. Because LPB can be performed easily during manufacturing by use of conventional CNC machine tools, there is no need to ship components to separate facilities for LPB. The process can be readily accommodated in an existing machine shop environment. Both the capital cost of LPB equipment and the unit cost of component processing typically are an order magnitude less than for laser shock peening.
This work was done by Paul S. Prevey III of Lambda Research for Glenn Research Center. Technical assistance was provided by Glenn researchers of the Material Division and Structures Division, working on the ULTRASAFE PROJECT'S Crack Resistant Disk Materials SUB-PROJECT.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Commercial Technology Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-17188.