EP2065477B1 - Method and device for building up residual stress in a metal workpiece - Google Patents

Description

The invention relates to a method and a device for building up residual stresses in a metallic workpiece by so-called laser shock peening (LSP).

Laser Shock Peening (LSP) is a process in which internal stresses can be built up by the action of a pulsed laser beam in a metallic workpiece. The residual stresses in the metallic workpiece are often desirable because they can prevent fatigue or crack propagation when loaded on the workpiece.

Conventionally, the material to be treated is subjected to an ablative surface layer prior to laser shock peening. The surface layer is, for example, a metal coating, a metal foil or is formed of organic materials. When the pulsed laser beam acts on this surface layer, it evaporates and is converted to the plasma state (state with ionization). Simultaneously with the application of the laser pulse, a cover layer is produced over the treatment site, which is formed, for example, by running water. This covering layer contributes to the fact that the transient plasma formed by the action of the laser beam on the surface layer is spatially fixed over a period corresponding approximately to the pulse duration. However, this quasi-equilibrium state is broken by the plasma pressure, so that the cover layer can no longer withstand the plasma pressure and the plasma expands freely. This dynamic impulses are transmitted to the surface of the workpiece, which cause shock waves, which in turn lead to the induction of residual stresses in the workpiece.

The laser pulse with an energy of, for example, 5 to 50 J serves both the vaporization of the surface layer and the plasma formation. The laser beams used conventionally have a pulse width of 10 to 50 nanoseconds (nsec).

As required and depending on the workpiece configuration such a laser pulse is spatially offset, simultaneously or offset in time applied to different locations on the workpiece by z. B. the workpiece and the laser generating device to each other are relatively moved and each act on identical identical laser pulses to different locations on the workpiece.

Thus, a single laser pulse, which is sufficiently energetic (by controlling the laser pulse energy as well as the pulse width), always leads to the generation and propagation of the plasma, and that the induced solid-state shock wave has a sufficiently high impact strength, so that the work piece in the workpiece plastic yield point is exceeded and thus the residual stresses in the metallic workpiece are built. The process is mainly used to build up residual compressive stresses on surface areas, that is to say in depths of up to 10 mm, for protection against stress crack corrosion or else for forming.

On this basis, it is an object of the invention to provide a method for building up residual stresses in a metallic workpiece using laser shock peening (LSP), which makes better use of the energy of the laser beam and thus more targeted and stronger shock waves in the workpiece, which in turn induce the induction of residual stresses leads.

This object is achieved by a method having the features of claim 1. Preferred embodiments are given in the dependent claims. A device is specified in claim 13.

The invention is based on the idea of applying at least two temporally staggered laser pulses to one and the same treatment location of the workpiece, possibly with a short pause without laser pulse application between the staggered laser pulses. As a result, the energy introduced by the laser beam can be deliberately adjusted to the physical processes occurring in the surface layer, so that the evaporation or ablation of the absorbing layer is caused, for example, by the first pulse and a pre-plasma which is weakly ionized is formed. In this example, the second laser pulse may be energetically controlled to cause the formation of a fully-formed, quasi-static plasma which is a non-equilibrium state with high ionization, and also for expansion of the plasma by the cover layer and thus induction of residual stresses in the plasma Workpiece leads.

The time-staggered laser pulses acting on the same treatment site are characterized by different energy. When using two staggered pulses while the second pulse is much higher energy than the first pulse. Thus, the first pulse evaporation, for which a much lower energy is required than for the subsequent formation and expansion of a transient plasma with comparatively high enthalpy, a low-energy laser pulse can be used, while the relatively high energy of the second laser pulse for the subsequent conversion of the plasma energy into predominantly energetic flow enthalpy is required, very well exploited and predominantly for the conversion and

Expansion process of the transient plasma is used without massive radiation losses or strong (unnecessary) temperature increases occur. In any case, even if more than two laser pulses are used, the laser pulse intended for the formation and expansion of the transient plasma should be relatively high-energy. Thus, the evaporation and plasma heating process is decoupled in time, so that the energy utilization of the laser pulses better and the impact on the workpiece is more targeted.

Due to the double or multiple pulse excitation, the weakly ionized primary plasma is additionally generated by an initially relatively moderate use of energy. However, this primary plasma state has a high absorption capacity for the radiation of the secondary laser pulse, so that a much higher proportion of the incident laser radiation can be effectively converted into plasma enthalpy than is possible with conventional single-pulse excitation. The resulting losses due to reflection, the passage of the laser pulse unused to the metal surface via an initially optically thin plasma as well as by convection and radiation can thus be minimized.

The laser pulses have a time interval of 5 to 100 nsec.

In principle, it is also possible for the laser pulses to follow one another directly, that is to say without a time offset. However, it is clearly preferred to provide a period of 5 to 100 nsec between the laser pulses, since then by appropriate tuning of the laser pulses, the time interval and the number of laser pulses targeted acted on the individual in the formation of the plasma and plasma processes and each laser pulse can be tuned to a particular state of the plasma.

Preferably, the pulse shapes of the time-staggered laser pulses are also different. In particular, it is preferred that the laser pulses have different rising edge shapes. Thus, in addition to the control of the energy of the laser pulses by means of their width (time) and energy, which may be different or the same, are also acted upon by selective selection of a suitable rising edge on the energetic processes.

Preferably, the different energetic states of the staggered laser pulses are generated by their time width and / or energy.

According to a preferred embodiment, more than two temporally staggered laser pulses are applied to the same treatment site and act there. In this case, the energetic control is even cheaper.

As in the case of the conventional single-pulse method, it is also possible to use flushing running water as cover layer in double or multiple-pulse methods according to the invention. This is a relatively inexpensive to produce and easy to dispose of cover layer.

The staggered laser pulses can be provided by a single laser or by different laser for each laser pulse. Especially in the preferred use of multiple processing lasers for generating the staggered laser pulses, it is easily possible to represent different pulse shapes, different energies of the pulses, etc. Even if several processing lasers are used, however, they have a staggered effect on the same processing location on the workpiece.

The method, which preferably further includes the step of applying the surface layer to be evaporated prior to application of the first laser pulse, which is generated by, for example, metal plating, metal foil deposition, or organic material deposition, may include internal stresses as needed and depending on the workpiece are to be introduced or at which the deformation is to be generated, repeated at several locations of the workpiece to z. B. to achieve a larger area solidification, but then acts at each site a double or multiple pulse.

• Aufbringen eines ersten Laserpulses an einem Behandlungsort des mit einer Oberflächenschicht bedeckten Werkstücks, welcher zur Verdampfung oder Ablation der Oberflächenschicht ein schwach ionisiertes Vorplasma erzeugt mit einem hohen Absorptionsvermögen für die Strahlung eines zweiten Laserpulses;
• Aufbringen eines zweiten Laserpulses an demselben Behandlungsort des Werkstücks, welcher ein voll ausgebildetes quasi statisches Plasma ausbildet; wobei der zweite Laserpuls zeitlich zu dem ersten Laserpuls gestaffelt ist mit einem Zeitraum von 5 bis 100 nsec zwischen den Laserpulsen;
• wobei der zweite Laserpuls wesentlich höher energetisch ausbildet ist als der erste Laserpuls; und
• Vorsehen einer Deckschicht über dem Behandlungsort während des Aufbringens der Laserpulse.

Another aspect of the present invention relates to a device comprising a workpiece clamping device, at least one laser and a control device for controlling the relative movement between clamped workpiece and laser, wherein the control device is adapted, a method for building up residual stresses in a metallic workpiece by laser shock Peening to control that process comprising the steps:

• Applying a first laser pulse to a treatment site of the surface layer covered workpiece which for evaporation or ablation of the surface layer generates a weakly ionized pre-plasma having a high absorbance for the radiation of a second laser pulse;
• Applying a second laser pulse at the same treatment location of the workpiece, which forms a fully formed quasi-static plasma; wherein the second laser pulse is staggered in time to the first laser pulse with a period of 5 to 100 nsec between the laser pulses;
• wherein the second laser pulse is energetically formed much higher than the first laser pulse; and
• Providing a cover over the treatment site during the application of the laser pulses.

The mode of action of the method will be described below.

With laser shock peening, it is necessary to achieve the highest possible plasma enthalpy for a period during which the plasma is largely stabilized by the covering layer, for example running water. Using a single laser pulse to vaporize the ablation material and generate a high pressure transient plasma (with peak pressures up to over 10 bar) is not energetically optimal, since less energy is needed for evaporation and primary plasma generation than for the plasma following training of a transient plasma with a comparatively high enthalpy.

In fact, in the primary plasma phase, there is already some momentum transfer to the workpiece via a transient pressure gradient. This can already induce primary compaction shocks. However, this process is hardly controllable and not very effective and therefore not suitable to form significant and targeted residual compressive stresses. The actual phase of the induction of residual compressive stresses begins with the expansion of the plasma (the flowing plasma) after the quasi-static initial phase, in the outer layer limits the expansion. When breaking the cover layer, a supersonic plasma flow is generated whose retroactive dynamic impulse triggers the shock wave, which deforms the workpiece, in particular its metallic material, beyond the plastic yield point and thus leaves compressive stresses even after the plasma has dropped and relaxed.

It has been found that this process is the more effective, the higher the original enthalpy (defined by pressure and temperature) of the primary plasma, and how it can be converted into kinetic flow enthalpy, which in the phase of expanding and fast flowing, the Cover layer breaking plasma happens.

• Primäre Verdampfung (Ablation) der absorbierenden Schicht;
• Ausbildung eines Vorplasmas, das schwach ionisiert ist;
• Ausbildung eines voll ausgebildeten, quasi statischen Plasmas, das ein Nichtgleichgewichtszustand mit hoher Ionisation ist; und
• Expansion des Plasmas durch die Deckschicht.

Thus, the individual processes that occur during laser shock peening can be classified as follows:

• Primary evaporation (ablation) of the absorbent layer;
• Forming a pre-plasma that is weakly ionized;
• Forming a fully formed, quasi-static plasma that is a high ionization non-equilibrium state; and
• Expansion of the plasma through the cover layer.

By applying at least two staggered pulses to the same workpiece location, it can be avoided that laser energy is transferred into the plasma state in the expansion phase. This is in fact ineffective, since the processes have different temporal development stages and introduced in the expansion phase in the plasma state energy is not converted into kinetic energy (to form the fully formed quasi static plasma) but for example in temperature or radiant energy and thus represents an energy loss.

Therefore, it is advisable to provide a distance of 5 to 100 nsec between the individual laser pulses, so that the time transfer of the forming plasma in the individual stages without energy at times to introduce, to which it can not be implemented, is possible, and In addition, the energies of the laser pulses can be adjusted to the currently running partial process.

Claims (13)

1. Method for building up residual stresses in a metal workpiece by means of laser shock peening, comprising the steps of:

   - applying a first laser pulse to a treatment location of the workpiece covered with a surface coating which, for the purpose of vaporizing or ablating the surface coating, generates a weakly ionized pre-plasma with high absorptivity for the radiation of a second laser pulse;

   - applying a second laser pulse to the same treatment location of the workpiece, which forms a fully formed, quasi-static plasma;

   - wherein the second laser pulse is temporally staggered with respect to the first laser pulse, with a time interval of 5 to 100 ns between the laser pulses;

   - wherein the second laser pulse is of substantially higher energy than the first laser pulse; and

   - providing a cover layer over the treatment location during application of the laser pulses.
2. Method according to the preceding claim, characterized in that the laser pulses have different pulse forms.
3. Method according to Claim 2, characterized in that the temporally staggered laser pulses have different leading edge forms.
4. Method according to one of the preceding claims, characterized in that the temporally staggered laser pulses have different temporal widths.
5. Method according to one of the preceding claims, characterized in that more than two temporally staggered laser pulses are applied to the treatment location.
6. Method according to one of the preceding claims, characterized in that the cover layer is provided by rinsing the workpiece.
7. Method according to Claim 6, characterized in that flowing water is used for rinsing the workpiece.
8. Method according to one of the preceding claims, characterized in that there is provided a single laser which emits the laser pulses.
9. Method according to one of Claims 1 to 7, characterized in that a separate laser is provided for generating each of the temporally staggered laser pulses.
10. Method according to one of the preceding claims, further comprising the step of applying the surface coating, prior to application of the first laser pulse, by means of metal coating, applying a metal foil or applying an organic material.
11. Method according to one of the preceding claims, characterized in that the energy of the first laser pulse is adapted such that it vaporizes the absorbent surface coating and forms a weakly ionized pre-plasma.
12. Method according to one of the preceding claims, characterized in that the method is repeated at various treatment locations on the workpiece.
13. Apparatus containing a workpiece clamping device, at least one laser and a control device for controlling the relative movement between a clamped workpiece and the laser, wherein the control device is designed to control the method according to one of the preceding claims.