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Laser forming (LF) is a non-contact technique for the shaping of metallic components. The process works by introducing thermal stresses into a component by irradiating its surface with a laser beam in continuous wave form. This leads to compressive strains or buckling being induced into the component depending upon the laser parameters used. Laser forming is grouped by the driving force , it's either:
Direct forming (consists of the mechanisms which use local thermal expansion caused by some internal or external forces and produces thermal stress which is results in deforming the sheet )
Ex: - Temperature gradient mechanism
Residual stress point mechanism
Indirect Forming ( the driving force is not the thermal expansion but some deformation potential already existed in the material before the process starts)
residual stress relaxation mechanism
Martensite expansion mechanism
Temperature Gradient Mechanism:
This mechanism is the most widely reported, and can be used to bend sheet material out of plane towards the laser. The conditions for the temperature gradient mechanism are energy parameters that lead to a steep temperature gradient across the sheet thickness. This results in a differential thermal expansion through the thickness (fig1.1)
The beam diameter is typically of the same order as the sheet thickness, or slightly less. The path feed rate has to be chosen to be large enough that a steep temperature gradient can be maintained. The feed rate/temperature gradient has to be increased if materials are used which have a high thermal conductivity. The laser path on the sheet surface is typically a straight line across the whole sheet.
This straight line coincides with the bending edge. Initially the sheet bends in the direction away from the laser. This is called counter bending. With continued heating the bending moment of the sheet opposes the counter bending and the mechanical properties of the material are reduced. Once the thermal stress reaches the temperature dependent yield stress any further thermal expansion is converted into plastic compression. During cooling the material contracts again in the upper layers, and because it has been compressed, there is a local shortening of the upper layers of the sheet and the sheet bends towards the laser beam.
The yield stress and Youngââ‚¬â„¢s modulus return to a much higher level during this cooling phase and little plastic re-straining occurs. Bends of approximately one degree per pass are achieved with this mechanism.
Temperature field & bending angle calculations:
The calculations were done by simulating a bending process on sheet model.the model consists of 2 parts : the mechanical calculations and the temperature field calculation.
If the plastic zone is smaller than the sheet thickness, the bending angle can be calculated by:
The time of heating tc is given by the ratio of the laser beam spot diameter on the surface of the sheet d1 and the process speed.
The temperature field (in y and z directions) calculation is given by:
A= absorption coefficient; p1= laser beam power; a= thermal diffusivity,
= heat conductivity
Normalized laser beam radius:
Buckling and upsetting mechanisms:
Both of these mechanisms are activated by the use of laser parameters that do not yield a temperature gradient in the depth of the material. For the case of the buckling mechanism a beam diameter much larger than the sheet thickness and a slow traverse speed is used. This results in a large amount of thermo elastic strain that in turn results in a local thermo elastic plastic buckling of the material.
The buckle is traversed along the length of the sample and once the buckle reaches the exiting edge of the sheet the elastic strain dissipates and the remaining plastic strain causes a deflection. This mechanism can be used for out of plane bending of sheet material; it may be accompanied by some in plane shrinkage as well. The part can be made to bend in either the positive or negative directions. The direction depends on a number factors including the pre-bending orientation of the sheet, preexisting residual stresses and the direction in which any other elastic stresses are applied, (for example a forced air stream acting on the bottom of the sheet.) .The buckling mechanism results typically in bending angles between 1 and 15 degrees.
This is significantly larger than observed for the temperature gradient mechanism. This is not a result of a higher degree of performance but a result of the fact that using the buckling mechanism more energy can be coupled into the work piece in one step. For the upsetting mechanism the geometry of a work piece would prevent buckling due to the increased moment of inertia compared to sheet material. This mechanism is used to shorten or upset a work piece in plane; it may be used in different ways for a wide range of forming results such as the bending of extrusions and tubes.
By the careful selection of the sequence of the sides of the geometry heated, a section can be made to step out of plane. The mechanism can also be used for the shortening of small frames. This is useful for aligning operations in micro parts production. The mechanisms of laser forming can accompany each other to some extent because there is a transition region of processing parameters and geometries where a switch from one mechanism to another takes place. Additionally there is usually a coupling between in plane and out of plane deformation in forming operations.
Non thermal method: Laser shock peening :
This new technique is based on shock waves as a source for the forming energy which is created through laser pulses travelling through laser beams of very high energy densities enough to cause deformation of the sheet so that the velocity of the process depends entirely on the velocity of the emitted wave.
Laser induced shock waves are currently used in shock hardening (by pulsed excimer laser beams), which requires energy density higher than the ablation energy. Ablation is reduced through confined plasma transparent facing like transparent plastics or fluids like water (as shown in figure 1.1)
Fig1.2 Metal surface treatments, (Source:https://eldorado.tu-dortmund.de/bitstream/2003/27039/1/17-schulzeNiehoff_etal-040321a.pdf)
Effect of defocusing:
The Defocusing (Zf) is the distance between the work piece and the focus of the laser beam; so that the maximum energy density and the maximum applied stress on the sheet is at defocusing equal zero. The result shows a significant ablation on the sheet depending on the value of Zf (illustrated in figure 1.2).
Fig 1.3 (Zf is positive if the focus is above the work piece, and negative if the focus is within the work piece)
Effect of the power density
The power density is function of the pulse energy and it's very important in the forming result. Experimental results showed that whenever the power density decreases, the dome height of the deformed sheet decreases (as in figure 1.4). So the decrease in power density decreases the velocity of the shock waves and thus the available forming energy.
Other factors affecting the operation:
- Materials: the yield strength is a major factor.
- Diameter of the die: the highest forming degree of the sheet could be reached with the smallest diameter of the die , this is due to that the shock wave is applied on a smaller surface thus more forming energy per area is obtained.
1.3.4- Experimental study on shock peening a sample of 0.075 mm thickness:
The results from these experiments have also been compared to samples formed using thermal laser forming with the temperature gradient mechanism, to investigate the differences between thermal laser forming and laser peen forming.
The main aim of this was to see if there was a heat affected zone in the laser peen formed samples, and if so to compare it to a thermally formed sample. The results for the thermally formed sample showed the large thermal input into the material and the heat affected zone. This would be disadvantageous in highly heat treated alloys in specialist applications such as aerospace. The laser shock peen formed sample showed no sign of heat input and no change to the materials structure.
A 0.4 inch (10mm) thick piece of Aluminum 7050 formed by laser peening. The laser peening process was able to achieve a radius of 9 inches (230mm).