Numerical Simulation Of Temperature And Stress Field Of Laser And Alloy Steel Wire Additive Manufacturing

As a rapidly developing digital and intelligent advanced manufacturing technology,metal additive manufacturing has the advantages of its short manufacturing cycle,high material utilization,and ability to form complex parts,which can be directly formed with complex structures and superior performance.Additive manufacturing has great potential on manufacturing small batch of parts which can be used for marine,aerospace,medical and other fields.Compared with traditional powder-bed and powder-feeding metal laser additive manufacturing,the advantages of laser and wire additive manufacturing(LWAM)are:high productivity;wire is cheaper than powder;the porosity and density of formed parts are better than powder feeding additive manufacturing parts;low requirements for the protection atmosphere,which means there is no need to vacuum or fill the processing chamber with a protective atmosphere before processing.It enriches the choice of tools and provides more space of operation.On the one hand,large parts with larger dimensions can be manufactured.,the continuous rapid melting and solidification during LWAM process lead to residual stresses inside the parts,which will cause deformation and cracking of the parts.Ultrasonic microforging treatment is an effective method to eliminate residual stress in mechanical parts.It is used in industry to eliminate residual stress in welded structures and can also be used as a posttreatment process for LAWD.In this thesis,the temperature field and stress field distribution during the LWAM and ultrasonic treatment of the LWAM alloy steel deposition layer are studied by the method of combining numerical simulation and experiment.First,a finite element model of LWAM with direct thermal-mechanical coupling has been established.The direct coupling method can be used to calculate the simulation results that are accurate and efficient.The finite element simulation method has been used to analyze the formation and distribution of the temperature and residual stress field of the LWAM deposition layer.The morphology of the molten pool under different power and welding speed has been analyzed,and the influence of process parameters on the size of the molten pool has been studied.The results show that with the increase of power and the decrease of welding speed,the heat accumulation per unit area increases,and the depth and length of the molten pool increase to varying degrees.When it comes to the fifth layer,since the thermal conductivity of the deposited layer itself is lower than that of the substrate,and the cross-sectional area was also smaller than that of the substrate,the heat dissipation condition of the top layer was poor,the size of the molten pool greatly increased,up to 7 mm around,the previous deposited layer has been remelted.A thermal-mechanical coupling numerical simulation of LWAM of the deposited layer was carried out to obtain the residual stress distribution in the formed deposited layer.In the simulation process,the residual stress was applied to the deposited layer as pre-stress.The results show that there is tensile residual stress at the surface of the untreated deposition layer,and as the depth increases,the tensile residual stress turn into compressive residual stress.The compressive residual stress first increases and then decreases,then finally the internal stress tends to zero.In order to verify the validity of the model results,the robot arm is loaded with a laser to achieve precise deposition,and ultrasonic micro forging is performed on its upper surface.Using X-ray diffraction method to measure the residual stress,the results show that the simulation model error is within 10%.The influence of ultrasonic micro-forging parameters on the stress field of the alloy steel deposited layer formed by LWAM has been studied by numerical simulation.Separate amplitude parameters were selected to analyze the changes in the stress field of the deposition layer before and after the ultrasonic micro-forging,so as to study the influence of the ultrasonic micro-forging parameters on the stress field of the alloy steel deposition layer formed by LWAM.The study revealed that the amplitude distribution of the substrate vibration excited by ultrasonic micro-forging is related to the location.The amplitude at the two ends is considerable,while the amplitude changes in the middle of the substrate are regularly distributed in a wavy shape.The simulation results show that the interaction between the ultrasonic micro-forging and the substrate will introduce additional compressive stress at the surface of the substrate,and the distribution of the introduced compressive stress is similar to that of amplitude distribution.After ultrasonic micro-forging treatment,the surface stress of the parts is redistributed,the harmful tensile residual stress at the surface is transformed into beneficial compressive residual stress,and the improvement range is positively related to the depth of action and the amplitude.