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Laser power density driven residual stress and surface quality of AA7075 alloy under laser shock peening

This study investigates the impact of laser power density and overlap strategies on the residual stress and surface quality of AA7075 alloy under Laser Shock Peening, revealing that cumulative strain from optimal pulse overlaps, rather than peak shock pressure alone, is the primary determinant for maximizing compressive residual stress while avoiding surface damage.

Original authors: Himanshu Kumar, Jan Kaufman, Sunil Pathak, Jaromír Kopeček, Jan Šmaus, Elżbieta Gadalińska, Ondřej Stránský, Filip Čáp, Jan Brajer

Published 2026-07-24

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Himanshu Kumar, Jan Kaufman, Sunil Pathak, Jaromír Kopeček, Jan Šmaus, Elżbieta Gadalińska, Ondřej Stránský, Filip Čáp, Jan Brajer

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you have a super-strong metal airplane wing, but deep down inside, it's secretly tired. Over time, tiny cracks start to form from the constant vibration of flight, much like how a paperclip eventually snaps if you bend it back and forth too many times. Scientists have a clever trick to stop this: they want to squeeze the metal's surface so tightly that it becomes "pre-tensed," making it much harder for those cracks to start. This is called giving the metal a "compressive hug."

One way to do this is by hitting the metal with a laser so powerful it creates a tiny, invisible explosion on the surface. This explosion sends a shockwave deep into the metal, rearranging its internal structure and locking it into that tight, squeezed state. Think of it like a martial artist delivering a precise punch: if the punch is too weak, nothing happens; if it's just right, it strengthens the opponent's stance; but if it's too hard, it might actually break the opponent's bones. The big question for scientists is: exactly how hard should that punch be, and how many times should they hit the same spot, to get the strongest metal without damaging it? This paper dives into that exact puzzle for a specific, high-strength aluminum alloy used in aerospace.

The Punch-Off: Finding the Sweet Spot for Metal

In this study, researchers played a high-stakes game of "Goldilocks" with a tough aluminum alloy called AA7075. They used a special laser to deliver shockwaves to the metal's surface, trying to find the perfect recipe to make it stronger without ruining its skin. The goal was to figure out two things: how hard the laser should hit (the power density), and whether hitting the same spot multiple times with a little overlap (like stamping a pattern) works better than just hitting it once.

The team treated the metal with laser pulses ranging from very gentle taps to massive blasts, creating shock pressures up to 3.36 GPa. They tested two strategies: one where they hit the metal in a grid with no overlapping hits, and another where they overlapped the hits by 50%, like laying down tiles that cover half of the previous one.

The Results: More Overlap, Not Just More Power

Here is the twist they discovered: hitting the metal harder isn't always better.

When they increased the laser power, the metal did get stronger, but only up to a point. They found that the "sweet spot" for the laser pressure was around 2.25 times the material's natural limit for elastic bending (a value known as the Hugoniot elastic limit, or HEL). If they went beyond this limit, the surface actually started to relax, and the beneficial squeezing effect didn't get any stronger, even though the metal was getting more damaged.

However, the overlap strategy was the real hero of the story. The samples treated with 50% overlap showed a massive improvement.

Hardness: The metal treated with the highest power (12.5 GW/cm²) and 50% overlap became 35.31% harder than the original, untreated metal.
Stress: It achieved a maximum compressive residual stress of -302 MPa at a depth of 0.2 mm. This is a deep, strong squeeze that helps prevent cracks.
Surface Quality: Interestingly, while hitting the metal harder usually made the surface rougher, the 50% overlap strategy actually helped smooth things out at the highest power levels. It seems that hitting the same spot repeatedly allowed the metal to "flow" and redistribute itself, preventing the surface from becoming as jagged as it did with single, non-overlapping hits.

What's Happening Inside?

The researchers looked under the hood using powerful microscopes and X-rays. They found that the 50% overlap didn't just hit the metal harder; it hit it smarter. The overlapping shocks created a cumulative effect, stacking up tiny deformations (strain) that built a more uniform and deeper layer of strength. In contrast, just cranking up the power without overlap caused intense, localized damage that didn't translate into better overall stress retention.

The study suggests that for AA7075 aluminum, the secret isn't just about the peak force of a single laser blast. Instead, the magic lies in the accumulation of strain from overlapping shots within a specific pressure window. This approach allows engineers to maximize the metal's strength and crack resistance while keeping the surface smooth and undamaged—a vital balance for keeping airplanes safe in the sky.

Technical Summary: Laser Power Density Driven Residual Stress and Surface Quality of AA7075 Alloy Under Laser Shock Peening

Problem Statement
Laser Shock Peening (LSP) is a recognized surface engineering technique for inducing deep compressive residual stresses in metallic substrates, thereby enhancing fatigue life, wear resistance, and stress corrosion cracking resistance. While AA7075 aluminum alloy is widely used in aerospace applications, the specific relationship between laser shock pressure, the Hugoniot Elastic Limit (HEL), and the resulting residual stress in this alloy remains insufficiently understood. A critical gap exists in identifying the maximum useful compressive residual stress limit and the optimal processing window. Specifically, it is unclear at what point increasing laser power density ceases to provide beneficial stress induction and instead triggers surface damage, stress relaxation, or reverse straining. Most existing studies focus on property enhancement at selected conditions without defining the pressure regime where additional energy becomes detrimental or ineffective.

Methodology
This study employed a systematic experimental approach to investigate the limits of compressive residual stress generation in AA7075.

Material and Setup: AA7075 wrought plates were treated using a nanosecond Yb:YAG laser (Bivoj laser, HiLASE Centre) with a 1030 nm wavelength, 10 ns pulse duration, and 10 Hz repetition rate.
Parameters: Laser pulse energies ranged from 0.1 J to 5 J, generating laser power densities between 0.25 and 12.5 GW/cm². This resulted in calculated shock pressures ranging from 0.47 GPa to 3.36 GPa.
Strategies: Two peening strategies were compared: single-pass (no overlap) and 50% overlap. A black tape sacrificial layer and water confining medium were utilized.
Characterization:
- Residual Stress: Surface stresses were measured via X-ray diffraction (XRD) using the $\sin^2\psi$ method. Depth profiles were obtained using the hole-drilling method with electronic speckle pattern interferometry (ESPI).
- Surface Quality: Areal ( $S_a$ ), line ( $R_a$ ), and waviness ( $W_a$ ) roughness were measured using optical and contact profilometers.
- Mechanical Properties: Microhardness was assessed via Vickers indentation.
- Microstructural Analysis: Phase identification, crystallite size, dislocation density, and microstrain were analyzed using XRD. Grain orientation, Kernel Average Misorientation (KAM), and grain boundary distributions were evaluated using Electron Backscatter Diffraction (EBSD).
Theoretical Framework: The HEL for AA7075 was estimated at approximately 0.99 GPa, establishing an optimal target shock pressure range of 2.0 to 2.5 times the HEL (approx. 2.0–2.5 GPa).

Key Results

Residual Stress Limits: Surface compressive residual stress increased with shock pressure up to a critical threshold. The maximum surface compressive stress (-336.78 MPa) was achieved at a shock pressure of ~2.37 GPa (Sample S11, 50% overlap), which corresponds to approximately 2.4 times the HEL. Beyond this threshold (e.g., Sample S16 at 3.36 GPa), surface residual stress did not increase proportionally and showed signs of relaxation or reverse straining, despite continued increases in plastic deformation.
Depth of Stress: While surface stress saturated or relaxed at high pressures, subsurface stress continued to benefit from higher energies. The maximum subsurface compressive residual stress (-302 MPa) was observed at a depth of 0.2 mm for Sample S16 treated with 50% overlap.
Role of Overlap: The 50% overlap strategy consistently outperformed the no-overlap strategy in retaining compressive residual stress and increasing microhardness. For instance, Sample S16 (50% overlap) exhibited ~35.31% higher microhardness than the base material, compared to ~21.39% for the no-overlap condition.
Surface Quality: Surface roughness ( $S_a$ ) generally increased with laser power density. However, at higher power densities (Samples S12–S16), the 50% overlap strategy resulted in lower surface roughness and waviness compared to the no-overlap strategy, suggesting that overlapping shocks promote material redistribution and surface smoothing.
Microstructural Evolution: XRD and EBSD analyses confirmed that LSP induced grain refinement (crystallite size reduction from ~33 nm to ~19 nm), increased dislocation density, and elevated microstrain. The 50% overlap strategy facilitated the accumulation of plastic strain and the formation of more continuous deformation substructures (higher Low-Angle Grain Boundary fraction) compared to single-pass treatments, which resulted in more localized lattice distortion.

Key Contributions and Significance
The study establishes that the effectiveness of LSP in AA7075 is governed not solely by peak shock pressure, but by cumulative strain accumulation mediated by the overlap strategy within an optimal shock pressure window.

The authors claim that:

There is a distinct limit to beneficial surface residual stress induction, occurring when shock pressure reaches approximately 2.25 times the HEL. Exceeding this limit leads to stress relaxation and surface degradation without proportional gains in surface compressive stress.
While higher shock pressures drive plastic deformation deeper into the material, the retention of compressive stress at the surface is optimized by the 50% overlap strategy, which facilitates strain hardening and uniform deformation.
The research provides a mechanistic foundation for selecting LSP parameters. It suggests that maximizing residual stress and minimizing negative surface effects requires balancing shock pressure (to stay near the optimal HEL multiple) with overlap strategies (to accumulate beneficial strain), rather than simply maximizing laser energy.

This work defines a physically meaningful processing window for AA7075, distinguishing between surface residual stress saturation and continued subsurface plastic deformation, thereby offering a more nuanced approach to LSP parameter selection than previous studies focused only on peak pressure enhancement.

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