Research On Water Cavitation Peening Process And Mechanism Of TC4 Titanium Alloy

Water Cavitation peening?WCP?is a new surface strengthening method with the energy released from cavitation clouds collapse,which is generated in cavitation water jet to impact the surface of metal parts.The WCP technology also has many advantages such as good surface quality,environmental protection,high versatility and so on.In the WCP technology,the selections of standoff distance and incident pressure are lack of bases.At the same time,the optimization of multi-process parameters and the strengthening mechanism are less studied.In order to promote the further development and application of this technology,this paper has carried out systematic research on the above problems.The main contents and conclusions of this paper are as follows:Based on the theoretical analysis method,the bubble dynamic model of single cavitation bubble in the submerged cavitation water jet was established and then solved by MATLAB,by which the variation of motion characteristics and pressure pulsation were obtained.Based on FLUENT,the numerical simulation of the collapse of near-wall cavitation bubble was carried out.The effects of the distance from the cavitation bubble to the solid wall and the initial radius of cavitation bubble on the morphology and mechanical actions of the bubble collapse were obtained.Based on FLUENT,the numerical simulation of cavitating water jet under different incident pressures?P?and nozzle diameters?d?were carried out.The results show that the cavitation performance of the jet increases with the increase of the incident pressure,but the further increase of the incident pressure has little effect on the cavitation performance of the jet when P=40MPa or so.The increase in nozzle diameter also promotes the occurrence of cavitation,but the sum of the vapor volume shows a small decrease at d=1.8mm.Combined with the high speed photography experiment,the dynamic evolution process of bubble cloud and the concentrated collapse range of cavitation bubbles were studied.The results show that the cavitation cloud has obvious periodicity including incubation,developing,shedding and collapse phases in one cycle;the ratio of the concentrated collapse range of cavitation bubbles to the nozzle diameter is approximately the same under same incident pressure.Finally,the ranges of the process parameters related to WCP were determined.Taking TC4 titanium alloy as an example,the factors and evaluating indicators involved in optimizing the process parameters of WCP were introduced.The effects of nozzle diameter,incident pressure,dimensionless standoff distance and peening time on surface residual stress,surface microhardness and surface roughness were studied by orthogonal experiment.The method of comprehensive evaluation of fuzzy mathematics was used to transform multiple evaluation indicators into single objective for optimization analysis.The optimum combination of process parameters is A₂B₄C₃D₃,i.e.nozzle diameter is 1.4mm,incident pressure is 40MPa,dimensionless standoff distance is 72.5 and peening time is 27.5 minutes.At this time,the surface residual compressive stress is 846MPa,surface roughness is 0.79?m,surface microhardness is 441HV,and the comprehensive score is 92.9538,which is the highest in the orthogonal experiments scheme.Then,the surface integrity of TC4 titanium alloy by WCP and the strengthening mechanism were studied.The results show that the transition zone of the concave-convex morphology formed by WCP has the characteristics of continuous smoothness,while the surface roughness is only 0.79?m under high peening strength.The plastic deformation depth basically reaches the saturation value 55-60?m under high and over peening strength.Deformation twins are formed in some coarse?-phase grains.With the increase of peening strength,the surface grains are refined and self-nanocrystallized,while the average micro-strain increases gradually.The depth of the microhardness field and the residual stress field of the sample can reach to 130?m.Except for the over peening condition,the maximum values of the residual compressive stress all appear on the sample surfaces.When the sample is under the over peening condition,cavitation erosion occurs on the sample surface,which leads to a sharp increase in surface roughness and average microscopic strain,while the surface microhardness and surface residual stress decrease greatly,this phenomenon should be avoided.