Numerical Analysis On Interface Behavior In Initial Physical Contact Stage Of Solid State Diffusion Bonding

With superior advantages of homogeneous interface microstructure,great bond strength,near-net shape forming,and wide range of applicable materials,diffusion bonding(DB)is widely adapted in high-tech sectors such as aviation,micro electronics and machinery manufacturing,etc.Generally,diffusion bonding could be described into three stages: initial physical contact,void shrink dominated by diffusion and creep mechanisms and disappear of origin interface among which the initial physical contact stage determines the size and shape of initial interface void,which is the basis for determining the bonding time and quality of joint.To further optimize bonding parameters and improve interface quality,regulating the size of initial void formed in initial physical contact stage is of great importance.Bonding pressure,temperature,and surface profile are key parameters influencing the initial physical contact stage.By considering these parameters,classic theories of DB give complex-formed analytic models based on plasticity theory to describe the evolution of initial void in this stage.Nevertheless,ease of engineering use of classic initial physical contact model was impeded as it is required to resort professional mathematical software to obtain the analytical solution of a specific welding system.To this end,this thesis concentrated on the initial physical contact stage of DB,studied the effects of bond pressure,temperature,and surface profile on initial void size evolution by means of finite element method(FEM).Further,by adopting five common metal materials in DB,effect of intrinsic mechanical parameter of materials on interface evolution is studied,and an "ease to engineering use" constitutional model was proposed.Finally,electrical current was applied to bond interface,by means of multi-field FEM.Comparative study of surface contact with/ without electrical current was conducted and the mechanism of current was discussed.Main conclusions are as follows.Based on FEM contact simulation,effect of bonding pressure and temperature on initial void size was studied.Results indicate that the initial void width decreases approximately linearly with the bonding pressure,and the void height decreases non-linearly with the bonding pressure.The initial void shape is close to narrow lens.The effect of temperature on the shrinkage of the initial void shows a dependence on the bonding pressure: the greater the pressure,the more prominent the promotion effect.FEM results agree well with the classical analytical solution,which proves the feasibility and reliability of FEM.Taking both vertical and horizontal surface roughness into account,aspect ratio which is asperity height against wavelength,was defined to describe the flatness of bond surface.Effect of surface roughness parameter on the interface contact behavior was studied.Results indicate that the surface roughness parameters and the bonding pressure synergistically affect the physical contact behavior: the surface profile has negligible effect on the void size at low bonding pressure;as the bonding pressure increases,rougher surface deforms more intensive,thus the width of formed void is smaller.The height of initial void mainly depends on the surface roughness in vertical direction.The higher roughness,the larger the initial void height and the void volume are.Using parameters of 5 commonly used metal materials of copper,TC4,SS304,AA7020,and C22 alloy in DB,multiple sets of contact simulation were conducted.An ease-use physical contact constitutive poly-nominal regression model was proposed,which consisted intrinsic yield strength and surface roughness parameters.By adopting the proposed model,predicted and experimental values of contact area were compared and in good agreement which indicate the feasibility model.Electrical current was applied to the interface by means of multi-field FEM simulation.Thermal and athermal effects of current were decoupled and their effects on interface contact behavior were studied respectively.Results showed that the temperature field is uniform at microscopic interface thus no local amplified thermal softening effect will appear at the interface,and the interface deforms under a uniform temperature field.In contrast,the current density was unevenly distributed at interface and concentrated mainly at void tip.Electro-plastic effect reduced the resistance to deformation of local interface that results the width of void to decrease further while the void height change is insignificant.Interface contacts at different mechanisms with/without current: the void surface folds and approaches each other without current while expands horizontally with current.Based on PAS technique,rapid initial physical contact experiments with/without current effect were performed to verify the effect of current on interface contact under same temperature.Results indicated that the initial void width further decreased with effect of current.