Laser Shock Peening/Forming

Laser bending is a non-contact forming method for sheet metals, in which the sheet metal can be bent, shaped and precision aligned with the use of straight or curved laser scan lines to acquire desirable three dimensional (3D) features.  These features would be difficult or even impossible to manufacture using conventional metal forming techniques. Laser bending is of significant value to aerospace, automotive, ship building and microelectronics industries that previously relied on expensive stamping dies and presses for prototype evaluations. Conventionally, the laser bending process is achieved by introducing thermal stress into the work-piece by irradiation with a focused laser beam.  

Recently, non-thermal forming using a high energy pulsed laser has generated growing interest.  Similar to the mechanical shot peening process, high-pressure and compressive shock waves are induced onto the target material by high-energy laser pulses. This non-thermal, shockwave forming mechanism is often denoted as laser shock forming.  During the process, short laser pulse irradiation duration is required, which is in the range of nanoseconds and several-orders less than the radiation time of the thermal forming mechanism.  The energy of each pulse is in the range of several Joules, released in nanoseconds, results in a high laser power intensity of above 1 GW/cm2.  When the highly intensity, short laser pulse is focused on the surface of the sheet metal, the surface of the sheet is vaporized and ionized immediately.  High pressure plasma is then generated and expanded rapidly. With further enhancement of the confined medium layer (usually water or glass), the plasma then expands and explodes violently against the surfaces of the sheet metal.  The confined layer traps the expanding vapor and plasma, and consequently causes a higher pressure shock impact in the level of over 1 GPa.  When the peak pressure of the shock wave induced by the laser at the surface of the sheet is greater than the dynamic yield strength of the material, the sheet metal will plastically deform.  There will be residual stress on the yield region and the sheet metal should be deformed to balance the stress. 

 

 


 

Sheet Metal Micro-Bending using a Nanosecond Pulsed Laser

 

Laser shock bending is a sheet metal micro-forming process using shock waves induced by a nanosecond pulsed laser. It is developed to accurately bend, shape, precision align or repair micro-components with bending angles less than 10°. Negative bending angle (away from laser beam) can be achieved with the high energy pulsed laser, despite the conventional positive laser bending mechanism. In this research, various experimental and numerical studies on aluminum sheets are conducted to investigate the different deformation mechanism, positive or negative. The experiments are conducted with the sheet thickness varying from 0.25 to 1.75 mm and laser pulse energy of 0.2 to 0.5 J.  A critical thickness threshold of 0.7-0.88 mm is found that the transition of positive-negative bending mechanism occurs. A statistic regression analysis is developed to determine the bending angle as a function of laser process parameters for positive bending cases.

 

Experimental Setup

Q-switched Nd:YAG laser with a wavelength of 1064 nm was used in the experiment, with a laser pulse repetition rate of 10 Hz and pulse duration of 7 ns.  The Aluminum 1060 sheet metals were used as the specimens in this research.  

Scanning Path

In order to create a bulk, uniform bending of the large specimen, the laser scanned over the targeted surface area of the clamped specimen by moving the X-Y stage.

Modeling

To numerically delineate the positive-to-negative bending mechanism, the laser shock micro-bending tests were simulated using ABAQUS.

Bending Mechanisms

Positive bending was clearly observed in the specimen of 0.7 mm thickness, while negative bending appeared dominate for thicker specimens with thickness of 1.07 mm and 1.75 mm.  

   

 

Grain Refinement Modeling of Laser Shock Compression

 

This work is concerned with prediction of the microstructural evolution of metallic components subjected to single or multiple LSP impacts. 

A numerical framework is developed to model the evolution of dislocation density and dislocation cell size using a dislocation density-based material model.  It is shown that the developed model captures the essential features of the material mechanical behaviors and predicts that the total dislocation density reaches the order of 1014 m-2 and a minimum dislocation cell size is below 250 nm for LSP of monocrystalline coppers using the laser energy density on the order of 500 GW/cm2.  It is further shown that the model is cable of predicting the material strengthening mechanism in terms of residual stress and microhardness of the LY2 aluminum alloy due to grain refinement in a LSP process with less laser energy densities on the order of several GW/cm2.

 

Process Modeling

The dislocation density-based material model subroutines defined earlier are incorporated in the FE models to calculate the dislocation fields in the workpiece over single or repetitive LSP impacts.

Dislocation Density

Compared with those experimentally measured by Murr, Kuhlmann-Wilsdorf and Meyers at various pressures, the model predicted dislocation densities agree well with the trend of pressure-dislocation density as shown left.  

Residual Stress

Distribution of the residual stress in the horizontal direction of LY2 aluminum alloy specimen after 5 LSP impacts predicted by the model.  An evident compressive residual stress profile with the maximum compressive stress of -140 MPa was predicted.

Hardness

Surface strengthening in microhardness was predicted to reach 154 HV0.2 after the first LSP impact, and to 166 HV0.2 after 3 LSP impacts, as compared with the average microhardness of 138 HV0.2 of the non-treated specimen.