Fundamental Study Of Hybrid Wire Arc Additive-Milling Subtractive Manufacturing

Due to the high conductivity plasma characteristics of arc light,wire arc additive manufacture has higher deposition rate and is more effective to manufacture large components than laser additive manufacturing,so it is one of current researching hot directions,but with rough surface quality,resulting in large amount of secondary machining,because the heating area of arc plasma arc column is large,and the energy is relatively dispersed.The hybrid method integrating wire arc additive manufacture and milling(HWMM)could solve this problem by employing the two processes work alternately.On the other hand,the robot could better meet the requirement of HWMM due to higher flexibility and more excellent spatial expansibility.Currently,the studies of HWMM have been mainly based on the CNC machines,and almost no work about the robot has been reported.This dissertation therefore studied the HWMM of Al5 Si Al alloy and AZ31 B Mg alloy by employing two robots,and focused on the interaction of wire arc additive manufacture and milling,and its effects on deposition morphologies,surface residual stress and mechanical properties of deposited thin-walled wall.The main results are summarized as follows.The effects of the milling at arcing surface on sample morphologies of the HWMM was obtained using Al5 Si Al alloy.The surface accuracy of the HWMM is better than that of wire arc additive manufacture.When the milling thickness is at the range of 0.4-1.2 mm,compared to the wire arc additive manufacture,the surface roughness(Ra)and the machining allowance of the HWMM are reduced 22.9% and 31.6%,respectively.It is found that whether the arc starting surface is processed or not has no effect on the arc shape.Therefore,the forming accuracy of sidewall surface is not directly related to the arc light characteristics,which mainly depends on the melt flow.After the arcing surface is milled to a plane,the melt flow changes from the downward along the camber surface to the outward and backflow on the plane.It reduces the fluctuation at the edges of deposited layers.Besides,a reserved layer was designed to prevent the damage of melted metal to milled surface,and avoid the re-milling.According to this design,the parameters about milling reservation and sidewall milling value were optimized.Within the optimal parameters,the minimum surface roughness(Ra)reaches 3.5±1.1 μ m,which is close to the level 8 of semi smooth surface.In order to solve the question that the chatter easily occurs and results in the instability of the milling due to the dynamic deposition of the HWMM,a modelling was developed by the finite element modal analysis.It suggested that the natural frequency value decreases with the increase of deposition height,showing a relative growth relationship.Then,a stability region dynamic lobe diagram was drawn according to the dynamic model of milling system "rigid tool-flexible workpiece",which can predict the optimal parameters.The prediction results of randomly selected parameters within the stable region are in good agreement with the experimental data.Besides,in order to broaden the stable window by increasing the natural frequency,the effects of shaped stiffeners on natural frequency were discussed.A milling scheme of HWMM is proposed on the basis of the stability region simulation and shaped stiffeners assistance,which can determine quickly,and broaden the milling window,and improve the accuracy and efficiency.The effects of the side milling on the surface residual stress of the HWMM using Al5 Si alloy was studied.It indicates that the milling has no effect on the surface residual stress on the sample bottom,but has a significant effect on the middle and the top,both of which firstly decrease and then increase with the increase of milling width.The minimum residual stresses on the middle and top of the sample are 1 and 22 MPa,93% lower and 44%higher than those of the wire arc additive manufacture,respectively.Moreover,the gradient of surface residual stress can be reduced by the milling,the minimum of which reaches to0.17 MPa/mm along the feeding direction,9.8% lower than that of the wire arc additive manufacture.The reduced mechanism was discussed by the transient evolution of longitudinal stress in wire arc additive manufacture and the generation of cutting-induced residual stress: the initial residual tensile stress of wire arc additive manufacture is offset by the milling-induced compressive stress,and is reduced to reach a new equilibrium.The milling almost has no effect on the microstructures and tensile properties of the Al5 Si HWMM,but shortens the fusion line spacing in vertical direction.It results in more fusion lines,and decreases the tensile properties in vertical direction by increasing the fracture area,which increases the anisotropy in tensile strength and elongation.When the milling thickness is 0.8-1.6 mm,the ultimate tensile strength anisotropy increases from 1.0%of the wire arc additive manufacture to 8.3%,and the elongation anisotropy increases from7.7% to 23.1%.The HWMM of AZ31 B magnesium alloy was developed.The modelling of stable region is in good agreement with the experimental data,demonstrating the portability and universality of the stable region modelling proposed in this dissertation.Therefore,the relevant theories and models are also suitable for hybrid laser additive and subtractive manufacturing.The comparative analysis showed the milling vibration response amplitude of Mg HWMM within the stable region is 43.2% of Al alloy,indicating better milling characteristics.It is attributed to the better damping performance and larger damping coefficient of Mg alloy,which helps absorb the vibration impact and suppresses the unstable nonlinear vibration during milling.