Theoretical Study On Laser Shock Forming Of Metal Sheet

Laser shock forming is a kind of plastic deformation technology that makes use of laser shock waves which are induced from the gasified and ionized absorption material irradiated by laser beam with intense energy and short pulse. Based on massive forming experiments with laser shock waves, the theoretical analyses of the deformation characteristics of the sheet metal with various different thicknesses under the action of the laser induced shock waves as well as some relevant issues in laser shock forming were made. The main researches are as follows:At present, there are a variety of overlays, which can be classified into three types according to their material property, such as rigid overlay, liquid overlay and flexible overlay. According to the process of laser inducing shock waves, the pressure attenuation of gasified materials and the reflection of shock waves by overlay, the mechanisms of the overlay affecting the intensity of laser shock waves and the pulse duration were analyzed systematically in this paper.Based on the description of the propagation of the stress waves in the sheet metal, the plates were divided into metal film, metal sheet, thick metal plate and ultra-thick plate in terms of the relationship between the time of overall plastic deformation of the plate and the pulse duration of the shock waves, and the behavior characteristics of the plates with different thicknesses under the action of shock waves were also analyzed.The overall plastic deformation occurs simultaneously in the film at the beginning of the action of the shock waves, where the coupling relation between the movement of the film and the pressure of the shock waves is strongest. However, the film is very thin, so the distribution of the stress in the film soon becomes uniform after multiple reflections of the stress waves. Therefore, it can be regarded that the stress distributes approximately uniform in the film. The deformation magnitude of the film is far greater than the thickness of the film, and it can be regarded that the deformation results from the stretching of the center part, so the membrane force plays the leading role in the process of deformation. In the paper, the movement model of the film was established according to the membrane force mechanism of the film deformation, and then the relational expression between the deformation magnitude of the film and various parameters was deduced.The overall plastic deformation of the metal sheet takes place in the period of action of shock waves, and there is a certain coupling relationship between the deformation of the metal sheet and the pressure of the shock waves, so the metal sheet moves and deforms under the pressure which also is influenced by its own movement. According to the mechanics model and the diffusion model of the gasified materials, a kinetic equation was established in this paper, and the expression of maximum deformation value of the metal sheet was deduced.The overall plastic deformation of the thick metal plate happens after the action of the shock waves, so the overall plastic deformation of the thick metal plate has no influence on the pressure of the shock waves. In the acting period of the shock waves, the shock waves transfer the impulse to the metal sheet to make the thick metal plate acquire a certain initial velocity, and the plate moves and deforms at this initial velocity under the inertia effect after the action of the shock waves. Based on the characteristics of the inertia motion of the thick metal plate, its inertia motion model was established in this paper, and the relational expression between the maximum deformation value of the thick metal plate and the parameters of the shock waves and materials was obtained.When the stress waves get to the back surface of the ultra-thick plate, the strength of the stress waves has attenuated to a value lower than the dynamic strength of the material, so the overall plastic deformation in the plate doesn't occur, and only a micro-dimple formed in the front surface. Based on the discontinuous relationship of the plastic stress wave front, the velocity of plastic waves and the moving velocity of material, the residual strain expression of ideal elastic-plastic material was deduced, and then the estimation equation of the maximum depth of the dimple in the surface of the plate was obtained.After laser shock forming, the surface roughness of the plate has been notably decreased. This issue had been discussed from two perspectives: one is the propagation behavior of the stress waves in the micro-peak on the back surface of the plate, and the other is the back pressure of the air. When the stress waves propagate in the micro-peak, the stress waves reflect and transmit and the amplitudes of transmission waves increase because of the section area of the micro-peak declines gradually. The amplitudes of reflected tensile waves following the transmission waves also increase due to the increase of amplitudes of the transmission waves, and the micro-peak ruptures and becomes lower when the strength of the reflection waves go beyond the tensile strength of the material. On the other hand, when the back pressure of the air surpasses the dynamic yield strength, it has a pressing and grinding effect on the back surface of the plate and makes the height of the micro-peak gets lower furthermore. Under the co-effect of the two actions, the roughness of the back surface obviously declines.In the experiments of forming titanium alloy with half mould by laser shock waves, the samples deformed in the opposite direction which made the component and the concave mould touch badly and decreased the quality of forming. In this paper, the cause of the reverse deformation was analyzed, and the results shows that owing to the high energy of laser, the plate has comparatively high velocity when it contacts with the concave mould, and then the reverse movement velocity resulted from the high-speed impact with the mould is greater than the threshold velocity leading to reverse plastic deformation, consequently, a reverse deformation which has adverse influence on the laser shock forming quality occurs. By decreasing the energy density of laser to avoid the reverse deformation, a high forming quality can be obtained.The issue that the radius of the concave mould has important influence on the maximum deformation value was studied from the perspective of energy of plastic deformation. When the area of the concave mould is smaller than the plasma-affected zone, with the increase of radius of the concave mould, the impulse transferred to the plate from the plasma increases accordingly, and then the energy used for plastic deformation of the plate increases, so the deformation magnitude of the plate increases. When the area of the concave mould is greater than the plasma-affected zone, the deformation area of the plate increases as the area of the concave mould increases, however, the total energy for plastic deformation is invariable, so on the contrary, the deformation magnitude of the plate decreases. When the area of the concave mould increases to a certain extent, the deformation area of the plate does not increase any more, and the change of the area of the concave mould does not have influence on the maximum deformation value of the plate any more.Laser shock forming technology is a novel kind of plastic forming technology for plate, which has many prominent strong points. It has broad application prospects in manufacturing industry, in addition, it also provide new methods and technologies for studying the behavior characteristics of the material under the condition of high pressure and high strain, therefore, it has significant researching value.