Laser peening drives deep plastic strain into a part which creates a high magnitude residual compressive stress from 1 to 10mm below the surface. This enhances the fatigue strength, durability, damage tolerance and resistance to stress corrosion cracking of critical metallic components. The process can form and correctively shape components, especially if the component is too thick to be conducive for shot peening. Curtiss Wright’s finite-element physics-based model of laser peening enables rapid virtual peening and thereby rapid optimization and assessment of expected performance.
Laser peening is an alternative for controlled shot peening where deep penetration, upwards to ½ in (12 mm) and low amounts of cold work are advantageous. Assessment of application identifies if one or both methods should be used.
Laser peening has made an important impact on industry providing a reliable and production qualified technology. It offers designers the ability to surgically engineer residual compressive stress into key areas of components. This retards crack initiation and growth thereby enabling increased fatigue strength and component lifetime.
Benefits of Laser Shock Peening
- Deeper residual compressive stress enabling better resistance to:
- Low cycle, high stress situations (LCF)
- High cycle, low stress situations (HCF) in a deteriorating surface environment
- Loss of compressive stress in high temperature applications
- Prevents failures from:
- Erosion, Foreign object damage (FOD), Fretting, Pitting, Stress corrosion cracking (SCC), Galvanic and Cavitation erosion.
- Clean surface condition enables applications where contamination and/or media staining cannot be tolerated.
- Surface finish and topography are easily maintained and controlled.
Laser Shock peening allows for excellent process and quality control. Since the key process parameters of laser energy and pulse duration are measured and recorded for each impact spot generated. It also has the ability to generate large curvature in thick component sections enabling advanced forming and form correction applications.
Applications for Laser Peening
Laser peening is used to increase fatigue strength, prevent stress corrosion cracking and extend the service life of critical system components such as turbine engine blades and discs, aircraft structures, landing gear and control components. It is widely used for electric power generation’s gas, steam turbines and for preventing stress corrosion cracking of nuclear fuel canisters. The technology is initiating deployments in ship and marine applications.
Laser Peen Forming
Augments shot peen forming by generating greater depth of induced strain, thereby allowing forming of thicker material sections and extending the degree of curvatures possible. It is helping to advance the use of machined stringers and ribs for integrally stiffened panels which reduces the need for fasteners. All this allows for lighter aircraft with more fuel efficient profiles.
Other Applications of Laser Peening
Automotive, medical implants, ship structure and propulsion systems, upstream and downstream energy systems and recreational sports. New developments include fatigue enhancement for components operating at very high temperatures and increasing the fatigue life of additive machined parts.
Laser peening (LP), a mechanical surface treatment generates deep levels of plastic compression thereby enhancing a treated material’s resistance to surface-related failures. Although conventional peening works for low temperature applications, at temperatures greater than half a metal’s melting temperature (T> 0.5Tm) peening, rolling and similar surface treatments degrade through dislocation annihilation, stress relaxation, and grain coarsening. Curtiss-Wright (CW) has developed a novel technique, coined laser peening plus thermal microstructure engineering (LP + TME) that imparts thermally stable microstructural modifications in both conventional and additively manufactured (AM) materials. The process is now being tested in a wide range of high temperature applications with emphasis on improving fuel efficiency and reliability of jet engines and gas turbines.
With partner Michigan State University (MSU), CW was awarded a U.S. Department of Energy Advanced Research Projects Agency-Energy (ARPA-E) program to develop an advanced heat exchanger for supercritical CO2 generators – a more energy efficient, more compact, and lower cost electric turbine that offers the potential to reduce greenhouse gas emissions.
Read the press release
Paper 1: Laser shock peening and its effects on microstructure and properties of additively manufactured metal alloys: a review
Paper 2: Thermal stabilization of additively manufactured superalloys through defect engineering and precipitate interactions
Laser Peening Process
An output beam, roughly 20 Joules at 20 nanoseconds (i.e. 1000,000,000 Watts peak) from a Nd:glass laser is projected onto a work piece creating a shock wave that induces deep plastic compression. The area to be peened can be covered with material to act as an ablative layer and simultaneously as a thermal insulating layer. In some applications peening is done directly onto the base metal generating a thin (few microns) recast which if needed may be polished off without significantly affecting the deep peened layer. Peening is robotically controlled with repeatable pattern placement accuracy in the range of 0.004 inches.
A thin stream of water is flowed over the surface to act as an inertial tamping layer. The laser light transparently passes through the water and the leading temporal edge of the laser pulse reacts with the metal surface or ablative layer rapidly ionizing and vaporizing surface material, forming and heating a plasma.
The pressure of the heated plasma builds to approximately 100kBar (1.5 million pounds per square inch) with the water serving to inertially confine the volume. This rapid rise in pressure effectively creates a shock wave that penetrates into the metal, plastically straining the material as it deeply penetrates. Eventually the water is accelerated off the surface but only after the shock wave has propagated into the metal.
The mechanical response of the peened area to this deep plastic strain, .020 inch to .500 inch (1 mm to 12 mm depth) results in a deep residual compressive stress with characteristics depending on the material, stiffness and the processing parameters. The deep level of compressive stress generated creates a damage tolerant layer and a barrier to crack initiation and growth. This enhances the fatigue lifetime and provides resistance to stress corrosion cracking and fretting fatigue. Multiple firings of the laser in a pre-defined surface pattern will impart a layer of plastic strain resulting in a deterministic deep layer of compressive residual stress. The process can be tailored to suit the product and potential failure mechanism or enable higher potential loads through weight sensitive designs.
Laser Peening of AI 6061-T6 Aluminum
One benefit of an exceptionally deep residual compressive layer is shown above. The S-N curve shows fatigue test results of 6061-T6 aluminum. The testing consisted of un-peened, shot peened and laser peened specimens which clearly shows the lifetime and fatigue benefit of the laser process.
FEA Modeling of Components
Curtiss-Wright Surface Technologies’s FEA modeling capability accurately simulates the response of a customer’s part to laser peening. Multiple iterations of process variables can be performed predicting node-by-node stress and strain profiles, enabling accurate predictions of increased fatigue strength and life. This capability rapidly allows initial assessment of process benefits, reduces costs of testing and has proven to accelerate deployment schedules.
Multiple mobile laser peening systems have been deployed on several continents at customer’s sites providing the most cost effective and timely deployments.