A spatial axisymmetric finite element model is established to investigate the distribution characteristics of temperature field that monopulse millisecond laser act on aluminum alloy. The thermal process of laser acting on aluminum alloy (melting, gasification and temperature drop) is simulated. Using the specific quivalent heat capacity method to simulate the solid-liquid, liquid-gas phase transition of aluminum alloy, and considering the differences of thermal physical parameters between different states (solid-liquid, liquid-gas) of aluminum alloy in the process of numerical simulation. The distribution of temperature field of aluminum alloy caused by the change of energy density, pulse width and spot radius of monopulse millisecond laser are investigated systematically by using numerical simulation model. The numerical results show that the temperature of target no longer rises after reaching the target gasification. Given the pulse width and spot radius, the temperature of target rise as the energy density increases, the laser intensity distribution is gaussian, so the temperature distribution of the target surface also shows Gaussian. The energy absorption mechanism of aluminum alloy is surface absorption mechanism, the temperature gradient in axial of the target is much lager than the temperature gradient in radial of the target surface, so the temperature rise in axial only exists a thin layer of target surface. Given the energy density and spot radius, as the pulse width increases, the power density of laser decreases, therefore the temperature of target center point decreases as the pulse width increases, and the temperature difference becomes small. As the pulse width decreases, the heat transfer in axial reduce, the deposition of energy enhances on the surface. Given the energy density and pulse width, the distribution of the temperature is enlarged as the spot radius increases, but the distribution of the temperature in axial is independent of the spot radius.
Based on Von Mises yield criterion and elasto-plastic constitutive equations, an axisymmetric finite element model of a Gaussian laser beam irradiating a metal substrate was established. In the model of finite element, the finite difference hybrid algorithm is used to solve the problem of transient temperature field and stress field. Taking nonlinear thermal and mechanical properties into account, transient distributions of temperature field and stress fields generated by the pulse train of long-pulse laser in a piece of aluminium alloy plate were computed by the model. Moreover,distributions as well as histories of temperature and stress fields were obtained. Finite element analysis software COMSOL is used to simulate the Temperature and thermal stress fields during the pulse train of long-pulse laser irradiating 7A04 aluminium alloy plate. By the analysis of the results, it is found that Mises equivalent stress on the target surface distribute within the scope of the center of a certain radius. However, the stress is becoming smaller where far away from the center. Futhermore, the Mises equivalent stress almost does not effect on stress damage while the Mises equivalent stress is far less than the yield strength of aluminum alloy targets. Because of the good thermal conductivity of 7A04 aluminum alloy, thermal diffusion is extremely quick after laser irradiate. As a result, for the multi-pulsed laser, 7A04 aluminum alloy will not produce obvious temperature accumulation when the laser frequency is less than or equal to 10 Hz. The result of this paper provides theoretical foundation not only for research of theories of 7A04 aluminium alloy and its numerical simulation under laser radiation but also for long-pulse laser technology and widening its application scope.
In this paper, we establish a physical model to simulate the melt ejection induced by millisecond pulsed laser on
aluminum alloy and use the finite element method to simulate the melting and vaporization process of aluminum alloy.
Compared with the conventional model, this model explicitly adds the source terms of gas dynamics in the
thermal-hydrodynamic equations, completes the trace of the gas-liquid interface and improves the traditional level-set
method. All possible effects which can impact the dynamic behavior of the keyhole are taken into account in this
two-dimensional model, containing gravity, recoil pressure of the metallic vapor, surface tension and Marangoni effect.
This simulation is based on the same experiment condition where single pulsed laser with 3ms pulse width, 57J energy
and 1mm spot radius is used. By comparing the theoretical simulation data and the actual test data, we discover that: the
relative error between the theoretical values and the actual values is about 9.8%, the melt ejection model is well
consistent with the actual experiment; from the theoretical model we can see the surrounding air of the aluminum alloy
surface exist the metallic vapor; an increment of the interaction time between millisecond pulsed laser and aluminum
alloy material, the temperature at the center of aluminum alloy surface increases and evaporation happens after the
surface temperature reaches boiling point and later the aluminum alloy material sustains in the status of equilibrium
vaporization; the keyhole depth is linearly increased with the increase of laser energy, respectively; the growth of the
keyhole radius is in the trend to be gentle. This research may provide the theoretical references to the understanding of
the interaction between millisecond pulsed laser and many kinds of materials, as well as be beneficial to the application
of the laser materials processing and military field.