Investigation Into Temperature And Stress Fields Of Ultrasonic Impact-assisted Laser Metal Deposition Process

In recent years,additive manufacturing(AM)has attracted considerable attention across multiple industries due to its high-flexible digitized method.However,some inherent characteristics such as obvious mechanical anisotropy and high residual stress exist in additive manufactured parts.In this paper,a hybrid fabrication method combining laser metal deposition(LMD)and ultrasonic impact treatment(UIT)techniques was further investigated to improve the microstructure of additive manufactured metal parts.Investigation into temperature field and stress field of ultrasonic impact-assisted laser metal deposition process was conducted to establish the relationships between input parameters(processing parameters and material parameters)and forming parameters(such as temperature field,cooling rate,microstructure,impact stress field,plastic deformation field,etc.)The distributions of physical quantities such as temperature field and cooling rate in the LMD process affect the solidification of the molten pool and the subsequent solid-state phase transition behavior,which determines the microstructure and mechanical properties of the deposited component.Firstly,based on the fundamental theory of heat conduction,the calculation models of temperature field,temperature gradient,solidification rate,cooling rate and/of the deposited part during the LMD process was established.Based on the calculation models,numerical calculation and visualization tools by combining Python,Numpy and Matplotlib were used to obtain the distributions and evolutions of physical quantities such as temperature field,cooling rate and/of the as-deposited sample under different processing parameters(laser power,scanning velocity,concentration coefficient of laser beam,etc.)and material parameters(density,thermal conductivity,heat capacity,etc.).The LMD technique was applied to prepare the 304 stainless steel(SS)samples.Combining with the calculated results such as temperature field and cooling rate,the microstructure and its evolution of the as-deposited 304 SS sample during the LMD process were investigated.In the UIT process,the pin impacts the sample at a high velocity in which both strain and strain rate hardening effects were considered.In our study,a split Hopkinson pressure bar(SHPB)technique was used to obtain the stress-strain relationships of the as-deposited stainless steel samples at high strain rate conditions.The strain rate is a very important factor in SHPB tests,and analytical method based on the impact dynamics and stress wave theory was adopted to establish a strain rate controlling model,from which the factors that affect the strain rate in SHPB tests are experimental parameter(initial velocity of the striker),material properties(generalized wave impedance())and reflection times6).Combined with quasi-static compression tests,the Johnson-Cook plastic model of the as-deposited 304 SS sample was obtained,which can be used to describe the material behavior in the next analyses of ultrasonic impact on the as-deposited sample.There are many parameters in the hybrid manufacturing process,including the processing parameters(laser power,scanning velocity,height increment of each deposited layer,ultrasonic frequency and amplitude,etc.)and material parameters(density,elastic modulus,Poisson's ratio,yield strength at high strain rate conditions,etc.),and establishing the relationship between these parameters and forming results(the impact stress field and depth of the plastic deformation zone)is of great significance to guide the hybrid manufacturing process.Therefore,a combined analytical and numerical methods based on the impact dynamics and stress wave theory were applied in our investigation to derive a calculation model to predict the impact stress field and depth of the plastic deformation zone under different processing parameters and material parameters.The calculated model can be used to optimize the processing parameters in the hybrid manufacturing process to ensure that a certain degree of plastic deformation occurs in the entire thickness of each deposited layer under certain processing parameters to eliminate residual stress and improve the microstructure.Considering dynamic and transient impacting characteristics and some non-linear factors in the UIT process,numerical method has also been employed in our study,and a three-dimensional finite element model(FEM)of ultrasonic impact on the as-deposited 304 SS sample was established to analyze the distribution and evolution of physical quantity in the repeated impact-rebound-impact process,such as pin velocity,energy conversion,impact stress field,plastic deformation field and residual stress field,etc.In the UIT process,the pin repeatedly impacts the surface of the as-deposited sample at a high velocity,which enhances the intensity of the impact stress field,and plastic deformation occurs in the sample within a certain depth(2.2 mm).The residual stress state changes from tensile stress to compressive stress.The magnitudes and directions of principal stress and principal shear stress were calculated to further analyze the plastic deformation behavior of the sample in UIT process.High compressive hydrostatic stress exists in the plastic deformation zone in the UIT process,which plays an important role to densify the additive manufactured metal parts and prevent cracks in the process of plastic deformation.