Study On Process Parameters And Residual Stress Of 316L Stainless Steel Formed By Hybrid Additive Manufacturing By Laser Forging

Additive manufacturing has the advantages of high efficiency,flexibility and green manufacturing,and has important application prospects in aerospace,defense industry and biomedical.However,the additive manufacturing technology currently has bottleneck problems such as low part forming precision and insufficient mechanical properties.In order to solve the above technical bottlenecks,several kinds of hybrid additive manufacturing technologies that not only maintain the advantages of additive manufacturing technology but also absorb the advantages of traditional technologies have emerged,providing a new path for solving bottleneck problems.Hybrid additive manufacturing by laser forging is a whole new technology,its assisted process laser forging is derived from laser peening technology.Laser forging can use the pulse laser-induced GPa magnitude shock wave to forge the high temperature metal deposition layer directly and can effectively reconstruct the stress distribution.In this paper,the research work on the process parameters of 316 L stainless steel and the residual stress distribution of formed parts are systematically discussed.The main research contents and results are as follows:The causes and distribution of residual stress in laser additive manufacturing formed parts are revealed.Firstly,through the calculation and analysis of the stress-strain relationship of the materials in the additive manufacturing process,it is concluded that the essential reason for the formation of residual stress is the inconsistent plastic deformation inside the material.Then a numerical model based on the thermal stress balance equation is established to simulate the deposition process of 316 L stainless steel.The temperature field and stress field results verify that the residual stress formation is closely related to the inconsistent shrinkage caused by the steep temperature gradient and the distribution direction of residual stress of the formed part is consistent with the scanning direction and mainly are tensile stresses,which also provides a basis for the research of laser forging process parameters.A laser forging shock wave pressure model in unconstrained mode is proposed.Although laser forging is derived from laser peening,it has a very big difference.One of them is that laser forging uses pulsed laser to directly act on the surface of high temperature metal deposition layer to generate shock wave without any constrained layer.It belongs to unconstrained mode.Based on this,the Fabbro laser shock wave pressure model was modified,and the ideal gas was used to replace the constrained layer in the original model.The physical model of the pulsed laser and material interaction process at high temperature was established.The laser forging shock wave pressure mathematical model in unconstrained mode was proposed.The selection basis for process parameters of hybrid additive manufacturing by laser forging was determined.The forging temperature range selected according to the forging characteristics of the material is the most basic process parameter,which determines the shock wave pressure required for the plastic deformation of the material in the temperature range,and the magnitude of the shock wave pressure determines the pulse laser energy density.The pulsed laser repetition frequency is determined based on the forging time window determined by the forging temperature interval and the temperature field simulation result.The forging temperature interval,pulse energy density and repetition frequency determine the laser forging process parameters.Finally,the forming process parameters matching the laser forging process parameters are selected.The residual stress distribution of the formed parts produced by the hybrid additive manufacturing by laser forging was studied.Three sets of experiments were carried out to manufacture ordinary additive manufacturing,laser peening and hybrid additive manufacturing by laser forging.The common samples,peened samples and laser forged samples were obtained.The results show that the ordinary specimens are mainly residual tensile stress,and the peened specimens are effectively implanted with residual compressive stress,while the laser forged specimens not only exhibit residual compressive stress,but also have deeper depth and higher amplitude than the peened specimens.This proves that the hybrid additive manufacturing by laser forging can effectively reduce the residual tensile stress hazard of the formed parts and reconstruct the stress distribution.In summary,this paper takes 316 L stainless steel as research object.Firstly the cause and distribution law of residual stress in the fabricated parts by numerical simulation method are revealed.Secondly,based on the Fabbro model,the laser forging shock wave pressure model in unconstrained mode is proposed.On the basis of these two steps,the selection basis for the process parameters of hybrid additive manufacturing by laser forging was determined.Finally,the residual stress distribution law of the hybrid additive manufacturing by laser forging was studied.Results are showed that stress reconstruction can be achieved,laying the foundation for further research in the future.