The Monitoring And Control Of The Stress And Microstructure In Laser Metal Deposition Of 316 Stainless Steel

Laser additive manufacturing(such as laser metal deposition,LMD)opens a new era of mechanical production.Based on the philosophy of “built layer by layer”,laser additive manufactured products usually have a complex structure and functional graded mechanical property.It has a great application prospect on aerospace,marine engineering,nuclear industry,and petrochemical engineering.However,during the laser additive manufacturing process,the stress concentration,random formation of microstructure,cracking and porosity are hardly to monitor and control,which greatly prevent the development and the application of the laser additive manufacturing.This thesis aims to realize the online-offline control of the LMD process and reveal the mechanism of stress and microstructure evolution of LMDed 316 austenite steel.The research was carried out from three aspects: online monitoring,process regulation,and post-treatment enhancement.The innovative results are listed below:(1)A newly in-situ stress monitoring system called “displacement method”has been developed.Combined with computer vision,finite element method(FEM)and space coordinate calibration,the monitoring of stress evolution inside of the deposition layer have been realized.The system uses a CCD camera and a line laser beam to measure the height of the melt pool and solidified layer,forms a real-time FEM model,and takes the surface displacement between the two states to calculate the stress development during the solidification process.The results show that the contraction of the molten pool happens in a very short period,there is no obvious shape change after solidification.The shape of the melt pool and its solid-state is similar.The trend of stress evolution obtained through online monitoring is similar to the traditional thermalstress simulation.Each time the molten pool contracts,it drags the material behind it and creates a stress superposition inside of the material.The reasonability of the laser scan route can be judged by calculating the stress superposition times on certain critical points.(2)The monitoring of solidification mode,random defects and height change during the LMD process have been realized through laser-induced plasma signal.First,the spectrum signal has been processed by singular spectrum analysis(SSA)to decompose the noise and the principal component and reconstruct the signal.Base on the Hammar-Svensson equation,the spectra lines of the elements which correlate to the solidification mode of the austenite steel have been extracted and set as features.Two different solidification modes: austenitic-ferritic mode and ferritic-austenitic mode has been successfully predicted through the support vector machine with selected element lines as feature and solidification mode as the response.Second,the interrupt change point of standard deviation,slope and mean value of the reconstructed signal has been calculated and correlated with the position of the defects inside of the deposition layer.The relationship between the position of random defects and spectrum signal has been developed and a good prediction accuracy has been obtained.Third,a spatial distribution of the plasma temperature,electron density and signal intensity inside of the plasma plume have been investigated through Boltzmann plot and Stark broadening,and the relationship between these features and deposition height has been studied.The results show that the electron density and signal intensity have a linear relationship with the deposition height and can be used to detect the abnormal change in deposition height during the manufacturing process.(3)The in-process control of LMDed austenitic steel has been performed through a self-made electromagnetic auxiliary apparatus.The results showed that the Lorentz force,electric field force and the Hartmann effect can affect the cooling rate of the material.The nucleation sequence of the material changes in different fields and thus forms different microstructure and residual stress distribution.The ferriticaustenitic mode is altered into an austenitic-ferritic mode in a rotating electric field,while the austenitic mode appears in the electromagnetic field.By controlling the solidification mode of the austenitic steel,the electromagnetic field can change the microstructure of the material,enhance the microhardness and constrain the evolution of residual tensile stress.(4)The stress superposition theory of LMDed initial stress and Laser peening induced stress has been proposed.A FEM model has been built to predict the stress distribution on the laser deposited sample and its changed state affected by laser shock peening(LSP).The microstructure and mechanical properties before and after LSP have been characterized experimentally.The results show that the thermal-induced tensile residual stress in the laser deposited sample can affect the laser peening results in both horizontal and longitudinal directions.The LSP greatly improves the mechanical properties of the laser additive manufactured sample.The hardness on the surface and 1mm depth have been increased by 7% and 22%,respectively,and the yield strength has been increased by 16%.