Dual-Pulse Laser Processing for Deep Residual Stress Control
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Solution Overview
Problem
Conventional laser processing methods struggle to generate residual stress deep inside a workpiece, with poor controllability of peak stress position and limited mass productivity in fields requiring high safety and efficiency.
Innovation Solution
A laser processing method that generates plasma with a low density on the workpiece surface using a foot pulse followed by a main pulse with higher peak intensity, creating a strong shock wave and residual stress deep inside the workpiece, with the foot pulse intensity set between 2% and 100% of the main pulse and a time interval between pulses ranging from 1 ns to 20 ns, and optionally using a protective or plasma confinement film to enhance plasma generation and prevent surface burning.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stress or pressure
If the fluence of laser light is simply increased in conventional laser processing, then the intensity of residual stress increases, but the controllability of peak stress position deteriorates and surface burning occurs
Solution Approach 1:
The laser pulse is segmented into two distinct components: a foot pulse with lower intensity that generates plasma, and a main pulse with higher intensity that generates shock waves. This segmentation allows independent control of plasma generation and shock wave intensity, resolving the contradiction between achieving high residual stress and maintaining precise peak stress position controllability.
Solution Approach 2:
The foot pulse is applied before the main pulse to pre-generate plasma on the workpiece surface. This preliminary action creates the necessary plasma environment that enhances the shock wave generation from the subsequent main pulse, enabling deep residual stress with controlled peak position without direct surface burning.
2Length of stationary object
If the laser fluence is increased to generate deep residual stress, then the processing depth improves, but surface burning occurs
Solution Approach 1:
Plasma generated by the foot pulse serves as an intermediary medium between the laser beam and the workpiece surface. This plasma layer absorbs and modulates the laser energy, enabling deep stress penetration while protecting the surface from direct high-intensity laser exposure that would cause burning.
Solution Approach 2:
The foot pulse performs preliminary plasma generation before the main pulse arrives. This pre-created plasma layer acts as a protective buffer during the main pulse interaction, allowing deep residual stress generation without surface burning.
3Ease of manufacture
If conventional single-pulse laser processing is used, then the process is simple, but mass productivity is limited
Solution Approach 1:
The laser processing uses periodic pulsed action with optimized timing between foot pulse and main pulse. This periodic structure enables efficient energy delivery and rapid processing cycles, improving mass productivity while maintaining process simplicity through automated pulse sequencing.
4Object-affected harmful factors
If the foot pulse intensity is too low, then surface burning is prevented, but plasma generation is insufficient
Solution Approach 1:
The foot pulse intensity is optimized to a specific parameter range that is sufficient for plasma generation but below the threshold for surface burning. This precise parameter control enables the foot pulse to create the necessary plasma density without causing harmful surface effects.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method effectively generates and controls residual stress deep within the workpiece, achieving high compressive stress distribution and preventing surface burning, thereby improving the processing depth and productivity.
Implementation Method 1
plasma having a sufficiently low density with respect to the workpiece is generated on the surface of the workpiece by irradiating the workpiece with the foot pulse
Implementation Method 2
the plasma acts as a low-density ablator and a strong shock wave increased by a difference in shock impedance can be generated inside the workpiece
Implementation Method 3
the plasma acts as a low-density ablator
Data Source
Figure 1
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AI summary
A laser processing apparatus 1 includes a light source 2 which outputs a laser light L, and a waveform control unit 11 which controls a pulse waveform W of the laser light L irradiating the workpiece P, in which the pulse waveform W of the laser light L controlled by the waveform control unit 11 includes a main pulse Wm and a foot pulse Wp temporally preceding the main pulse Wm, and a peak intensity of the foot pulse Wp is smaller than a peak intensity of the main pulse Wm, and a peak position of the main pulse Wm is positioned within a maintaining time M of plasma K generated due to an incidence of the foot pulse Wp on the workpiece P.