Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D10/00—Modifying the physical properties by methods other than heat treatment or deformation
  • C21D10/005—Modifying the physical properties by methods other than heat treatment or deformation by laser shock processing

Definitions

• 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).
• LSP laser shock peening
• LSP Laser Shock Peening
• 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.
• the pulsed laser beam acts on this surface layer, it evaporates and is converted to the plasma state (state with ionization).
• 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.
• 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).
• such a laser pulse is spatially offset, simultaneously or offset in time applied to different locations on the workpiece by z.
• 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.
• 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.
• LSP laser shock peening
• 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.
• 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.
• 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 of high ionization, and also to the expansion of the plasma through the cap layer and thus to induce residual stresses in the plasma Workpiece leads.
• the time-staggered and acting on the same treatment site laser pulses are preferably characterized by different energy.
• the second pulse is preferably much higher in energy than the first pulse.
• 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.
• the laser pulse intended for the formation and expansion of the transient plasma should be relatively high-energy.
• 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.
• the weakly ionized primary plasma is additionally generated by an initially relatively moderate use of energy.
• 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, for example, a time interval of 5 to 100 nsec.
• the laser pulses 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.
• the pulse shapes of the time-staggered laser pulses are also different.
• the laser pulses have different rising edge shapes.
• 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.
• the different energetic states of the staggered laser pulses are generated by their time width and / or energy.
• 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.
• 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.
• the first laser pulse which is generated by, for example, metal plating, metal foil deposition, or organic material deposition

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• Chemical & Material Sciences (AREA)
• Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Optics & Photonics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Mechanical Engineering (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Laser Beam Processing (AREA)

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Citations (7)

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• 2007
  • 2007-11-22 DE DE200710056502 patent/DE102007056502B4/de active Active
• 2008
  • 2008-11-10 EP EP08019597.7A patent/EP2065477B1/fr active Active
  • 2008-11-21 US US12/275,668 patent/US9096913B2/en active Active

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