Thermal-Mechanical Coupled Effects And Formation Mechanism Of Residual Stress Of High-Intensity Pulsed Ion Beam Irradiation Of Metal Tungsten

High-intensity pulsed ion beam(HIPIB)irradiation,as a new high-power energic beam processing technology,can induce non-equilibrium thermal and mechanical effects to cause significant changes in surface integrity determining the performance of processed components,for instance,surface residual stress that is widely recognized as a primary surface integrity parameter determining the fatigue performance of components.However,the changes of surface residual stress for components is complicated under the high-power energic beam irradiation due to combined influence of the multiple factors including process parameters,component materials and geometric structures etc.As a consequence,selection of irradiation parameters has to be mainly done by trial and error method,restricting the prediction and improvement of the processed component performance.Therefore,it is necessary to clarify the influence mechanism of coupled thermal-mechanical effects of HIPIB irradiation on the surface residual stress of components,in order to provide theoretical basis for the parameters optimization of high-power energic beams irradiation based on a knowledge-based way.In this dissertation,metal W for fusion reactor component application was selected for the experimental and numerical study of HIPIB irradiation,explored by adopting a Process signature method that aims to establish a quantitative correlation between the material processing loads including temperature and stress quantities and the surface integrity of processed component independent from specific process parameters,as elucidating the physical mechanism of influence of the process parameters on the component surface integrity.The influence of HIPIB process parameters,such as energy density and pulse width,on temperature field and coupled stress field of irradiated metal W were investigated,the formation mechanism of surface residual stress of the irradiated metal W was revealed,and subsequently the different residual stress distributions dependent on HIPIB irradiation parameters were understood based on the mechanism.Finally,deriving of a characteristic process signature for surface residual stress independent of irradiation process parameters was discussed,in order to provide a feasible methodology for process optimization of the high-power energic beams processing technology.The main research results include the following four parts:(1)A infrared thermal imaging method was developed to accurately measure the energy spacial distribution of HIPIB beam spot,for optimization of HIPIB equipment output characteristics that was experimentally carried out and the ion beam energy density fluctuation of less than 12%was achieved,significantly better than that of as high as-50%in conventional HIPIB equipment.Using the HIPIB equipment,the irradiation experiments of metal W under energy density of 2.1 J/cm2 and 4.2 J/cm2 at room temperature,and 4.2 J/cm2 at preheating temperature of 200? and 600?,respectively.In all cases,HIPIB irradiated metal W retains the original body centered cubic phase structure.There are microcracks networks on the surface of irradiated metal W,with a maximal depth of 20-30 ?m for samples at room temperature irradiation,and reduced to 10-15 ?m with significant reduction in the microcracks density for the preheated irradiations.The residual stress states in surface layer at energy densities of 2.1 J/cm2 and 4.2 J/cm2 are all tensile,with a distribution feature of "increasing first and followed by decreasing " along with the depth,i.e.having a lower value in the range of 860-960 MPa at the top surface layer and a peak value of 1300-1400 MPa at the sub-surface of about 3 ?m.For the preheated samples,the surface residual stress is about 550-650 MPa.The experimental results indicate that,the different HIPIB irradiation parameters lead to the noticeable variations of residual stress magnitude and distribution features.(2)The evolutions of temperature and stress fields of different thermal-mechanical effects were calculated and analyzed by establishing the experiment verified thermodynamic model of HIPIB irradiation onto metal W,by intentionally screening a variety of the energy densities of 0.5-4.2 J/cm2 and pulse widths of 70 ns,7 ?s and 70 ?s,respectively.At the fixed pulse width of 70 ns,increasing energy density leads to a more significant thermal effect,obtaining a higher surface temperature and temperature gradient.The maximum surface temperature at energy density of 4.2 J/cm2 is 4988 K,as the only case exceeding the melting point of metal W,i.e.3683 K.The maximum surface temperature at energy density of 0.5 J/cm2 is 1007 K,much lower than the recrystallization temperature of metal W of 1473 K.At the fixed energy density of 4.2 J/cm2,both maximum surface temperature and temperature gradient are lower at the lower HIPIB irradiation power densities for the longer pulses that cause the longer thermal diffusion depths.For energy densities of 1.07-4.2 J/cm2 with short pulse width of 70 ns,surface plastic strain of metal W occurs at both heating and cooling stages,and for lower energy density of 0.5 J/cm2 and longer pulse width of 7 ?s,plastic strain only occurs at heating stage.For the longest pulse width of 70 ?s,only elastic strain occurs and then recovers completely after cooling to room temperature,and subsequently no residual stress forms.There are two kinds of stress-depth profiles features for all HIPIB irradiated metal W at either higher or lower energy/power parameters:the residual stress decreases monotonously along the depth at energy densities of 0.5-1.07 J/cm2 and at longer pules width of 7 ?s,and the residual stress firstly increases and then decreases with a maximal value at the subsurface for the energy densities of 2.1-4.2 J/cm2.(3)The evolutions of elastic and plastic strains and their influence on residual stress of HIPIB irradiated metal W were explored to reveal formation mechanism of the residual stress.The critical stresses and temperatures for onset of plastic deformation at different depths in surface layer of metal W are varied in the range of 810-947 MPa and 580-640 K,respectively.Under different irradiation parameters,residual elastic strains increases linearly with respect to the residual plastic strains for all depth data in plastic deformation zone,and the slopes of the fitting lines are close with a subtle change as the respective intercepts of the lines vary significantly,depending on the irradiation parameters.The identical dependence on the irradiation parameters were also found for the linear residual stress changes with respect to the residual plastic strain.Note that,the monotone increasing linear relations between residual stress and residual elastic strain are independent of irradiation parameters.Therefore,that the non-uniform plastic strain along depth of HIPIB irradiated metal W leads to the corresponding distribution of residual elastic strain,is confirmed as the main formation mechanism of residual stress.Particularly,for the higher energy densities,the direction of plastic strain at the cooling stage is opposite to that at the heating stage due to the HIPIB irradiation.As a result,the larger change of the reverse plastic strain at the cooling stage results in a final residual elastic and plastic strain of top surface lower than that of subsurface,which explains the phenomena of the different distribution features of residual stress at different irradiation parameters,i.e.monotonous decreasing along with depth for the lower energy densities but peaking at subsurface for the higher energy densities.In addition,the correlations are established between the plastic strain depth/gradient and the residual stress zero crossing position.The residual stress formation mechanism well explains that the two process signatures,i.e.correlations between temperature gradient and residual stress,temperature field parameter(?)and residual stress zero crossing position,as proposed for metal cutting and induction heating studies in literature are not applicable for the high energic beam processing with higher temperature gradient.(4)Considering the above-mentioned inconsistent influences of elastic strain and plastic strain on residual stress under different HIPIB irradiation parameters,thermodynamic analysis of the residual stress formation process was carried out.The energy dissipation during plastic deformation were analyzed to derive the process signature describing the residual stress formation with energy conversion and dissipation.The external heat energy input transf-orms into the material internal heat energy and elastic-plastic strain energy.Most of the plastic strain energy EML,p dissipates into secondary heat energy with a portion of?EML,p(>0.9),contributing to the maximum heat energy with heat conduction.The undissipated portion of plastic strain energy transforms into stored elastic strain energy,leading to the formation of residual stress.The energy dissipation paths are significantly different due to the various irradiation parameters including ion beam energy density,pulse width and preheating temperature,accounting for the inconsistent influences of elastic and plastic strain on residual stress.A quantitative sensitivity analysis was proposed based on the thermodynamic mechanism of residual stress formation,and a characteristic process signature of HIPIB processing concerning residual stress formation in the irradiated metal W is established by identifying a characteristic material loading,i.e.stored elastic strain energy,according to the maximum sensitivity of the internal energy component with respect to the surface residual stress.The applicability of the characteristic correlation between stored elastic strain energy and surface residual stress is further verified by numerical simulation and experimental results of irradiated metal W at energy density of 4.2 J/cm2 with preheating temperature of 200-800 ?.The process signature incorporating the effect of irradiation process but independent of specific process parameters is applicable to all high energic beams technologies including ion,electron and laser beams etc.,providing a methodology on a knowledge-based way for process parameters optimization according to component performance requirements.