Low Plasticity Burnishing (LPB™)

Description

Low Plasticity Burnishing (LPB™) differs from conventional ball or roller burnishing, also known as “deep rolling”, in using the minimal amount of plastic deformation (or “cold working”) needed to create the level of residual stress to improve fatigue or stress corrosion performance.  Low cold working provides both thermal and mechanical stability of the beneficial compression.  LPB™ uses a patented constant volume hydrostatic tool design to “float” the burnishing ball continuously during operation, regardless of the force applied.  This provides indefinite tool life and eliminates the possibility of dragging the ball and damaging the surface.

LPB™ can be performed in the direction chosen to most-favorably develop the desired state of residual stress (U.S. Patent 6,415,486).  For blade edges, the tool path is commonly either parallel or perpendicular to the blade edge (span-wise or cord-wise in the terminology of blade designers) to which the pressure is applied and; therefore, the burnishing force generated is varied as a function of position both cord-wise and span-wise to achieve the desired magnitude of compression, typically through-thickness on blade leading edges. LPB™ can produce compression ranging from a few thousandths of an inch (comparable to shot peening) to over a full centimeter for nuclear weld applications. Wheel-type tools are also available for tight radii and restricted geometries, such as splines and fillets.

LPB™ is a patented, mature, proven surface treatment for improving High-Cycle Fatigue (HCF), Stress Corrosion Cracking (SCC) and damage tolerance performance. This metal improvement technique was first used in applications in 1996 by Lambda Technologies and has been in commercial production since 2004.  It is available through Lambda Technologies, industrial OEM's and through select third party providers for commercial and military aircraft maintenance applications.  The LPB™ process can be applied during initial manufacture or during maintenance and repair operations.  LPB™ is a practical, cost-effective, shop floor logistically compatible process that provides reliable performance improvement without altering either the material or design. 

Uses and Applications

LPB™ has been applied to a broad range of materials, including high-strength steels, stainless steels, titanium, nickel, aluminum, and magnesium alloys over the last decade.  Applications have been developed for the mitigation of fretting and improving damage tolerance in turbine engines (see our Technical Papers and Application Notes).  Corrosion pitting, SCC, stress concentrations, and Foreign Object Damage (FOD) mitigation have been addressed in structural aluminum airframes.  SCC mitigation in both high strength steel landing gear and austenitic nuclear welds have been researched thoroughly.  Current production applications range from turbine engine vanes and blades (both airborne and ground based), propellers, propeller hubs, landing gear, to welded nuclear components and medical implants.  LPB™ processing can be easily integrated into the aerospace, military, medical, nuclear, and oil industries. 

Process

LPB™ is unique among surface enhancement processes in that the force applied to the tool is synchronized with the CNC tool positioning, using either CNC machine tools or industrial robots.  The process is a highly repeatable surface treatment, as repeatable as CNC machining.  The burnishing force can be synchronized to the burnishing tool positioning within milliseconds, producing unprecedented definition of the residual stress distribution produced. Combining the CNC control with Lambda’s patented design method allows the creation of the ideal residual stress distribution required for the application.  Patented closed-loop CNC control technology provides immediate conformation of the processing, giving assurance that the desired residual stress was, in fact, achieved in the component, and that a data file was created documenting the processing details by component serial number.  Instrumented “smart” fixturing, also patented at Lambda, provides independent confirmation that the residual stresses were induced in the component.  

LPB™ offers a logistical advantage of incorporation directly into the manufacturing environment.  Machining and LPB™ can be performed in the same machine tool, with only a simple tool change.  The CNC tool positioning file and the associated tool pressure file ensure precise reproduction in the manufacturing environment, minimize operator interaction, and processing accuracy within 0.1% for production process control exceeding six-sigma.

Design

Lambda has created and patented a unique Fatigue Design Diagram (FDD) method of designing the residual stress field appropriate for a given application. Knowing the applied stress state, material properties, and failure locations, Finite Element (FE) models of the applied stress are input into Lambda’s FDD code to determine the amount of residual compression required at each element in the FE model to achieve the desired fatigue performance, or to mitigate damage defined by the stress concentration, k_t.  Lambda provides the optimal residual stress field design and the range of compression allowable in production along with the necessary tooling for LPB™ processing as part of the non-recurring engineering supporting each LPB™ application.

Advantages and Benefits