Study On Process And Mechanism Of Laser-arc Hybrid Additive Manufacturing Of Stainless Steel

Additive manufacturing is a new technique to fabricate high performance metal parts based on"discrete-stacking"principle.However,all current additive manufacturing methods cannot balance the efficiency and precision in one time.For example,laser additive manufacturing has high precision but low efficiency,while wire arc additive manufacturing(WAAM)has high efficiency but low precision.Laser-arc hybrid additive manufacturing(LHAM)can stabilize the arc by laser-arc interaction,and solve the problem of melt pool instability of WAAM at big current or high speed,and then improve deposition efficiency and forming accuracy at the same time.However,relevant studies,especially the high-power LHAM have not been addressed on.This dissertation therefore investigated the process characterization,microstructure and properties of high-power LHAM of austenite stainless steel in detail.The main findings are as follows.The influences of parameters on the efficiency and morphology of LHAMed thin walls were studied,the optimized parameters for forming stable and pore-free samples were obtained.The balance between deposition efficiency and forming accuracy was achieved.The single-wire deposition efficiency increased from 377 to 942 cm~3/h,2.5 times of WAAM when the surface roughness was at the same level(～200μm).The surface roughness of LHAMed sample reduced by 74%when the deposition efficiency was at the same level(377cm~3/h).The regression models of average layer height,layer width and surface roughness were established to predict the efficiency and accuracy of deposition layers.On the basis of the droplet transition and the forces acting on melt pool boundary,the influencing factors and stabilizing mechanism of WAAM and LHAM melt pool with big current or high speed but without support were discussed.LHAM can improve the droplet transfer duration because the laser can attract and compress the arc,and change the droplet transition from a disordered of multiple transfer modes coexist for WAAM into a stable cold metal transition mode or rotating projected transfer mode with medium-droplets for LHAM.The LHAM melt pool stability depended on whether the surface tension was in balance with internal hydrostatic pressure at the melt pool edge.The rapid downward flow of liquid metal can be avoided when the above two forces were in balance with each other.The effects of parameters on the microstructure of LHAMed samples were investigated.All samples were composed of ferrite and austenite with epitaxial growth characteristics.The stable deposition layer can be divided into fine grain remelt zone(FGRZ)and coarse grain melt zone(CGMZ)according to the ferrite morphology.The CGMZ ferrite was in the form of continuous or partially continuous network,but it can be transformed into a mixture of granular,worm-like and network under the impact of pulse arc droplets or high-frequency stirring of oscillating laser beam.The texture formed along the building direction,and was governed by laser energy transfer mode.The sample only exhibited{001}<100>cube texture because the crystal growth direction was parallel to the building direction when the laser energy transfer was of conduction mode.And the sample was composed of{001}<100>cube texture and{110}<100>goss texture because the crystal growth direction deviated to the<110>direction when the laser energy transfer was of keyhole mode.The influences of parameters on the tensile properties and corrosion resistance were analyzed.Compared with WAAM,the properties of LHAMed samples were significantly improved under the optimized parameters.The maximum ultimate tensile strength(UTS)can be increased from 530 to over 600 MPa,the UTS anisotropy can be reduced from 10.9%to 0.5%,and the corrosion resistance can be of 2.28 times higher.The contributions of fine grain strengthening(166.6 MPa),solution strengthening(27.7 MPa)and precipitation strengthening(3.3 MPa)were calculated by normalizing the dendritic arm space of FGRZ and CGMZ,proving that the influence of fine grain strengthening was the largest,and two approaches to enhance the LHAM tensile properties were proposed.The effects of deposition strategy on the formation,microstructure and tensile properties of 100 mm×100 mm×70 mm rectangular LHAM entities were discussed.It was found that the deposition strategy did not affect the microstructure of different sections,but the entity fabricated with 90°continuous deposition strategy can obtain a better performance due to a more random heat dissipation.Compared with the thin wall part,the single-wire deposition efficiency was further improved to 1036 cm~3/h,the UTS was slightly reduced by4%,but the elongation was increased by over 10%,meeting the ASTM A 479 standard.