Surface Enhancement Methods

Surface enhancement is the introduction of a surface layer of compressive residual stress to minimize sensitivity to fatigue or stress corrosion failure mechanisms, resulting in improved performance and increased life of components. Surface enhancement methods include:

Shot Peening

Deep Rolling

Controlled Coverage Peening

Low Plasticity Burnishing (LPB™)

Laser Shock Peening (LSP)

Controlled Plasticity Burnishing

With the exception of simple overload, failures initiate from the surface of a part by some combination of fatigue, stress corrosion cracking, or corrosion fatigue. Failures are often exacerbated by a crack initiating damage mechanism such as fretting, corrosion pitting, intergranular corrosion, or foreign object damage (FOD). The surface of a component is inherently weaker than the interior because the free surface lacks the constraint imposed by fully surrounding material. Therefore, as is generally observed, fatigue cracks will initiate at the surface. Internal fatigue crack initiation requires high internal residual tensile stress and/or a discontinuity such as an inclusion, void, or other internal flaw to act as a “surface” for initiation.

Stress corrosion cracking (SCC) under static load, and its dynamic cousin corrosion fatigue, which combines cyclic crack growth with stress corrosion cracking, also necessarily originate at the surface. Only at the surface do the combination of susceptible material, a corrosive environment, and tension exceeding the threshold stress level for SCC occur.

Surface enhancement is the introduction of a surface layer of compressive residual stress to minimize sensitivity to fatigue or stress corrosion failure mechanisms, resulting in improved performance and increased life of components. The presence of a stable compressive layer with a depth and magnitude of compression and cold work designed for the service stresses and environment can dramatically improve the effective material properties. The improvements in life and performance can far exceed those achieved by alloy substitution. If the compressive layer is of sufficient depth, damage mechanisms such as corrosion pits, FOD, and fretting can be completely mitigated. The effective strength improvement achieved by surface enhancement can allow substitution of less expensive materials, reduction in cross sections and weights, and mitigation of failure mechanisms. Component life and performance can be increased, avoiding the expense of changing either material or design.

Surface enhancement is not a new idea. A classic 1959 example of four-fold improvement in fatigue strength resulting from shot peening with different preloads is shown in Figure 1. In the high cycle fatigue regime, with design lives exceeding a few thousand cycles, the presence of a shallow layer of compression dominates the fatigue performance. The fatigue strength is effectively increased from 50 ksi to over 200 ksi by a layer of residual compression only 0.010 in. deep. The lower linear plot of endurance limit as a function of maximum compressive stress is essentially an empirically determined Goodman diagram confirming the four-fold increase in fatigue strength achieved by introducing a layer of residual compression on the surface. Lambda’s patented fatigue design methods allow surface enhancement to be optimized for the material and application.

The fatigue benefits shown in Figure 1 result from retardation of crack nucleation and microcrack growth. Damage from corrosion pitting, FOD, and fretting can penetrate through the shot peening induced compressive layer. Deeper compression achievable with laser shock peening (LSP) and low plasticity burnishing (LPB™), shown in Figure 2, is sufficient to completely mitigate many of the common damage mechanisms that dramatically reduce fatigue performance. The relative depth and magnitude of both residual compression and cold work produced during surface enhancement are compared in Figure 2.

Shot peening, introduced into the automotive industry in the late 1920’s, has been widely used to ensure the fatigue performance of a wide variety of automotive, aerospace and other mechanical components. Other surface enhancement methods have since been developed and are commercially available. These are briefly described here. Interest in surface enhancement has increased as performance improvement through alloy development has encountered cost and material limitations. Lambda’s unique combination of residual stress and cold work measurement, fatigue design, processing and testing capabilities provide the means to select and design surface enhancement processes for optimal component performance.

Table 1 summarizes the processing speed, depth of compression, amount of cold work produced, and relative cost for the different surface enhancement methods available.

Table 1: Relative processing speed, depth of compression, amount of cold work produced and cost of surface enhancement methods.

Shot peening is the oldest, most widely used surface enhancement method. Metallic or ceramic shot, ranging in size from nominally .030 inches to as large as .125 inches, impact the surface of the component, producing spherical indentations. The compressive layer is formed by a combination of subsurface compression developed by Hertzian loading combined with lateral displacement of the surface material around each of the dimples formed. As the dimples overlap with random impacts, the entire surface is effectively elongated, driving the surface layer of deformed material into compression as its expansion is resisted and supported by the equilibrating tension in the material below.

The shot “media” can be metallic or ceramic. Metallic shot is often either cast steel or cut wire blasted against a carbine pate to form a nearly spherical shape. Cut wire shot can be manufactured from virtually any alloy to avoid elemental contamination. Ceramic shot is typically zirconium oxide or glass bead. The shot will wear and fracture during use leaving broken pieces that can cause damage in the form of sharp notches upon impact with the surface. The media must therefore be constantly screened to remove broken shot and dust.

The shot peening process is defined by parameters that include the size and type of shot used, the Almen intensity achieved, and the coverage. Almen intensity is a measure of the deflection of the Almen A, C or N strip that occurs during the peening cycle, and is a measure of the elastic energy stored in the 1070 steel Almen strip by the formation of a layer of compression. Almen intensity is related to the area under a residual stress-depth curve, and it does not uniquely define a depth or magnitude of compression. Coverage is the percent of the surface impacted by shot, and is a function of time under the shot stream. In order to ensure uniform treatment of the surface, coverage is often specified at 200% or even 400%, implying that each point on the surface was impacted at least two to four times. The definitions and means of determining coverage and intensity are well documented in the SAE literature. The reader is also referred to the Shot Peener web site for a more detailed discussion of shot peening and excellent literature archives.

The shot is accelerated to sufficient velocity to deform the surface on impact either by air blast or wheel machines. In wheel peening machines the shot is thrown from a rapidly spinning radially bladed wheel. Wheel machines are widely used for gearing, springs and other automotive applications where numerous components are shot peened without the need for detailed direction of the shot stream. Large volumes of shot are delivered by wheel machines, often in specially designed peening cabinets used for production processing in an assembly line. Air blast machines, which propel the shot in a stream of compressed air through a hardened carbide nozzle, are widely used for precision shot peening with smaller shot, as for aircraft turbine disks and blades. The nozzles can be directed to reach difficult areas such as disk slots or inside bolt holes, and specialized lance nozzles can be used to project shot at right angles to the shot stream to peen the inside surfaces of cavities. Although hand peening is performed in blast cabinets, modern air blast systems use robot CNC control to ensure repeatability of coverage and intensity.

“Flapper peening” is performed manually without free flying shot. The flapper tool has captive shot embedded, rather like rivets, in rubberized fabric flaps that are attached radially to rotate around a shaft. The shaft is rotated so that the flaps successively impact the surface peening of a local area. Flapper peening is used for repair or limited access work or when free shot cannot be tolerated, as in nuclear reactors or on-wing jet engine rework.

Ultrasonic peening utilizes an ultrasonic horn to vibrate a platen to throw shot contained in a cavity against the work piece. A small quantity of shot is used, but it must be confined in a container that includes the peened surface or component such that the shot can return to the driving platen to be recycled after impact. The shot can be of high quality and a wide range of sizes and materials to provide a range of peening and cold work distributions. The containment for the shot and apparatus is generally designed for the component and application.

By its very nature shot peening relies upon random impacts of the shot. Therefore, in order to achieve the coverage requirements, some regions will receive numerous impacts before adjacent areas are impacted at all. The result is a non-uniform and generally very highly cold-worked surface. Cold work levels range from 40% to even over 100% during creation of the layer of surface compression. Cold working can be an advantage in some applications where the inherent increase in yield and ultimate strengths enhances fatigue performance along with the layer of surface compression. In the work hardening materials, such as titanium and nickel alloys, peening induced cold working can exhaust the ductility of the material, leaving a brittle surface layer. Shot peening damage in the form of “laps and folds” creates stress concentrations that reduce fatigue performance.

Shot peening is a very practical surface enhancement method, provided the components are not exposed to elevated temperature or mechanical overload. Research at Lambda has shown that the highly cold worked shot peened surface will relax more completely and much faster than a low cold worked surface at the same state of compressive stress. The yield strength difference between the surface and subsurface material can also result in a loss of compression or even inversion of the surface to residual tension in the event of even a single momentary overload as may occur in a hard aircraft landing, fan blade impact, or similar event. Creation of a layer of compression with minimal cold work offers advantages in many practical applications.

As noted in the section on shot peening, the amount of cold work produced during shot peening is not directly controlled, but is a consequence of the extensive coverage, shot size, and peening parameters. The high cold work is often a disadvantage because the dislocation density produced in the deformed layers accelerates thermal relaxation and develops yield strength gradients, making the surface subject to both thermal and mechanical relaxation and service.

Research at Lambda into the thermal and mechanical stability of peened surfaces has led to the development of the patented controlled coverage peening technology. Lambda found that the coverage required to achieve a depth and magnitude of compression could be far lower than that typically used in production peening. For both steels and nickel–based alloys, coverage as low as 20% can produce the same depth and magnitude of compression as 100% or greater coverage. Two immediate benefits are achieved: (1) Reduced cold work during peening and (2) Reduced peening time and cost.

Controlled Coverage Peening (CCP) uses just the coverage required to produce the necessary depth and magnitude of compression. The cold working can be a fraction of that produced by peening over 100% coverage. The data in Figure 3 for 4340 steel, 50 HRC shot peened with CW14 shot at an intensity of 9A shows that there is no significant difference in the depth and magnitude of compression produced for coverage ranging from 20% to over 400%. Similar results have been obtained for both steels and Ni-based alloys. The reduced cold work provides greater thermal stability. Effectively, low plasticity surface compression can be achieved by shot peening. Alternately, peening can be performed to produce a given controlled amount of cold work, if work hardening of the surface is desired. CCP with 20% coverage produces the same depth and magnitude of beneficial residual compression as conventional 100% coverage as shown in Figure 3.

Laser Shock Peening (LSP) introduces compression with minimal cold working using shock waves to yield the material. LSP utilizes high speed, high powered lasers to focus a short duration energy pulse on a coating, usually black tape, that is placed on the surface of the work piece to absorb the energy of the laser beam. A transparent layer, usually flowing water, covers the surface over the tape, and acts as a tamp to direct the shock wave energy into the surface of the material. When the laser is fired periodically, the laser beam passes through the water, explodes the tape, and creates a shock wave sufficient to deform the material to depths typically on the order of a millimeter. The shocking process is repeated in a computer-controlled pattern across the surface, creating a series of slight indentations and regions of residual compression.

Subsurface residual stress and cold work distributions produced by LSP are compared to shot peening and LPB™ in Figure 2. LSP has been applied to a variety of alloys used in aircraft engines, airframes, and other engineering applications. LSP is now commercially available from at least two suppliers in the United States and is used in Europe. Although limited by cost, quality control and logistics, LSP has been applied successfully to improve the damage tolerance of several critical compressor blade leading edges and has made major contributions to the field of surface enhancement.

An advantage of LSP is the depth of compression exceeding shot peening produced with low cold work. The subsurface compressive residual stress distribution tends to be maximum at the surface, diminishing nearly linearly with depth. The low cold work provides thermal and mechanical stability similar to LPB™, an advantage in hot section engine applications, or where there may be momentary overload due to impact, etc.

Disadvantages include the high capital cost of the equipment, which may include a clean room environment and million dollar plus installations. Repeated coating with tape is required to produce the depth and magnitude of compression achievable in a single pass with LPB™. Because the explosive laser shocks remove the tape from adjacent treatment sites, two coating cycles are usually required to achieve one layer of treatment. Multiple layers are generally required to achieve the desired depth of compression. A total of three layers (six coatings) were required to achieve the compression shown in Figure 2.

Process quality control has slowed the application of LSP due to the difficulty of controlling the processing variables involved. Variation in the thickness and even turbulence of the water layer tamp, as well as debris in the air and on the mirrors affect the compression achieved, even when the laser power is precisely controlled. LSP cannot provide the closed-loop control available with LPB™. Internal cracking caused by the superposition of shock waves from opposing faces of blades, or echoing waves from the opposite wall of thicker sections has been found to limit the application of LSP both in production and in test specimens.  Logistical difficulties are inherent in the need to ship components to facilities that have the LSP equipment. The cost of LSP is the highest of any of the surface enhancement methods, typically 10 to 100 times that of LPB™ or shot peening.

Deep rolling, a term used widely in Europe, uses roller or ball burnishing of a surface with sufficient force and repetitive deformation to deliberately create both a highly cold worked surface as well as a compressive layer of residual stress. A primary goal of deep rolling cited in the literature is the deliberate creation of a highly cold worked surface layer to increase both the yield and ultimate strength of the material in order to improve fatigue strength. X-ray line broadening and micro hardness reveals that deep rolling produces even more cold work than shot peening.*2 3 4

The subsurface residual stress and cold work distributions created by deep rolling are similar to shot peening, but deeper. The high cold work can be beneficial for some materials and applications where elevated temperature exposure and mechanical overload are not issues. At high temperatures the high dislocation density created by cold working allows the beneficial compression to relax rapidly by a dislocation annihilation mechanism. The yield strength gradient with depth through the cold worked layer makes the part susceptible to loss of compression in the event of even momentary overload.

Deep rolling was derived from conventional surface burnishing intended to achieve a fine surface finish or component sizing, modified to control of the burnishing force rather than the interference. Processing is typically performed in a lathe on axially symmetric components such as axles and shafts. Either roller or ball tools are used to deform the surface with repeated deformation cycles and fine feed per pass to produce high cold work levels.

Low Plasticity Burnishing, developed and patented at Lambda Technologies, utilizes a variety of tools including hydrostatic and roller tools to produce just sufficient plastic deformation to create the desired level of compressive stress. The LPB™ process includes a unique and patented way of analyzing, designing, and testing of surface enhancement of metallic components in order to develop the unique metal treatment necessary to provide improved performance and reduce fatigue, stress corrosion cracking, and corrosion fatigue failures. The form of the residual compressive zone is controlled with CNC machine tools or robotic positioning. Since 1996, LPB™ has been developed to produce compression in a wide array of materials to mitigate surface damage including fretting, corrosion pitting, SCC and FOD. Maximum compression on the order of the component yield strength extending up to12 mm deep has been achieved, far exceeding that of laser shocking. Cold work less than nominally 5% provides thermal and mechanical stability superior to shot peening and deep rolling. LPB™ has the advantage of processing in a machine shop environment using CNC machine tools or industrial robots and relatively inexpensive process control equipment. Logistical problems of shipping components to an outside plant are eliminated, and LPB™ can be easily incorporated into the manufacturing work stream. The force applied to the surface to create the compressive residual stress field is computer controlled and synchronized with the CNC tool positioning in a patented control process using hydraulic servo valves or similar means to produce process control exceeding six-sigma in a production environment.

The LPB™ hydrostatic tools developed at Lambda differ from any previous design in the use of a patented constant volume flow to support the burnishing ball in the socket. Forces applied through the tool instantaneously increase the pressure at the ball to maintain a constant gap between the ball and the hydrostatic bearing seat. Previous hydrostatic tool designs using constant pressure allow the ball to rub continuously against the seat so that the tool rapidly wears and requires oil-based coolants for lubrication. For convenience, Lambda’s constant volume hydrostatic tools generally use conventional water-based cutting fluid, allowing machining and LPB™ processing in the same machine tool and setup. Pure distilled water has been used in critical nuclear applications, such as the closure welds on nuclear waste containers.

LPB™ is the only surface enhancement method applied under continuous closed-loop process control. As a result, the process control of LPB™ exceeds that of any other surface enhancement method. The force applied by the tool is computer controlled using a servo-valve synchronized with the CNC tool position. The constant volume hydrostatic tool pressure is proportional to the force applied to the component surface. The hydraulic control pressure applying force to the tool provides a second separate processing variable. The pressures are compared to the target processing pressures and bounds continuously during processing. The processing pressures as functions of CNC tool position are recorded for each component in a file identified by serial number. Closed-loop feedback during processing provides an accuracy of the force applied to the component surface on the order of 0.1%, far exceeding the precision and robustness of any other surface enhancement method.

The primary limitation of LPB™ is the need to have tooling created and pressure and CNC code files developed for each application. The non-recurring engineering required is similar to developing CNC machining code to machine a component. It may not be possible to create tooling that can produce the burnishing forces required for some geometries.

The same tools and control systems developed for low plasticity burnishing (LPB™) can of course be used to deliberately create higher amounts of cold work when warranted. This process, termed Controlled Plasticity Burnishing (CPB) at Lambda, can provide a precisely controlled amount of cold working to achieve a targeted increase in yield strength or hardness along with a controlled depth of compression. Quality control and process robustness of CPB are comparable to LPB™, differing only in the amount of cold working that is introduced and controlled during processing.

In applications requiring improved yield strength, such as some friction stir welds, or grain size refinement by cold working followed by subsequent heat treatment, CPB can be used to modify and achieve desired material properties during manufacturing. Lambda’s x-ray diffraction laboratory verifies optimized CPB processing by diffraction line broadening and micro-hardness measurement.

*1.Mattson, R.L. and Roberts, J.G., "The Effect of Residual Stresses Induced by Strain Peening upon Fatigue Strength, "Internal Stresses and Fatigue in Metals, Edited by G.M. Rassweiler and W.L. Grube, New York, NY: Elsevier Publishing Co., 1959, pp. 348-349.
*2. W. Zinn and B. Scholtes, “Mechanical Surface Treatments of lightweight Materials – Effects on Fatigue Strength and Near Surface Microstructures,” Journal of Materials Engineering and Performance, Volume 8(2), April 1999, pp. 145-151.
*3. I. Altenberger, et.al., “Cyclic Deformation and Near Surface Microstructures of Shot peened or Deep Rolled Austenitic Stainless Steel AISI 304,” Materials Science and Engineering, A264, 1999, pp. 1-16.
*4. A. Drechsler, et.al., “Mechanical Surface Treatments of Ti-10V-2Fe-3Al for Improved Fatigue Resistance”, Materials Science and Engineering, A243, 1998, pp. 217-220.