Fabrication Of H13 Steel By Selective Laser Melting And Residual Stress Analysis

Selective laser melting(SLM) is one of the greatest potential techniques in the field of Additive Manufacturing(AM), whose principle is based on discrete and accumulation of the SLM processing. It completes the forming of parts gradually point by point, line by line, by plane under the irradiation of high energy laser beam.SLM has many merits such as short production cycle, high material utilization,low-cost, high relative density of components, fine grain size, homogeneous microstructure and forming process is not restricted by the complex geometry shape of the parts. However, the power melting, solidification and subsequent cooling during the SLM processing is finished within a short time, so the cooling rate is very great, generating a large thermal gradient nearby the molten pool, which will lead to a large residual stress in the final parts and do harm to the utilization of products.According to what have been mentioned above, firstly, in this paper the process parameters of SLM is investiated to achieve the nearly full density parts, the microstructure and mechanical property is analyzed by X-ray radiation(XRD), optical microscopy(OM), scanning electron microscopy(SEM) and transmission electron microscopy(TEM), meanwhile compared with the conventional process. Then the residual stress is measured by high-angle X-ray, and the phenomenon is explained from the aspect of microstructure. Finally, the relieved stress annealing is carry out to eliminate the residual stress, and the the mechnical property is investigated.The following conclusions can be drawn:The nearly full density sample of H13 steel is fabricated by SLM. The relative density will be around 98.2% at a laser power of 225 W, a scanning speed of 320mm/s, a thickness of powder layer of 50 μm, a hatching space of 100 μm. It will incur some defects such as the particle is not melted, large size of balling and other defects if the laser energy density is smaller; it will produce small size of balling, deformation and other defects if the laser energy density is too large.The dominating phases is ferrite, martensitic phase and retained austenite phase, it owns uniform and dense microstructure and well-isotropy. The grain size is around 1~2 μm and the fine carbides is well-distributed. H13 steel has hardness of57.4 HRC, elasticity modulus of219.6 GPa, yield strength of 1180 MPa, tensile strength of 1581 MPa, elongation of6.8%. The yield strength and tensile strength can compared with forged, the elongation is low than forged, but the hardness and elasticity modulus are apparent high than forged.The high-level residual stress(~ 800 MPa) is encountered in the H13 steel which fabricated by SLM. High cooling rate and large thermal gradient nearby the molten pool may be contributed to the high-level residual stress discovered. The metastable phases such as martensitic phase and retained austenite phase also contributed to residual stress. In addition, Cr, Mo and V elements are enriched in the α phase in the microstructure of the SLM-H13, causing enlarged lattice parameter,aincur thermal stress.The residual stress can be eliminated to some extent through annealing. The residual stress can be eliminated around 92% when the annealing temperature of650℃, annealing time of 4 h, and the hardness of 50.7 HRC, tensile strength of 1742 MPa, yield strength of 1463 MPa, elongation of 11.2%, elasticity modulus of 215.8GPa. The red hardness can improve 50℃ than the conventional processes. High mechanical properties are due to the precipitation of Mo and V carbides in the microstructure of annealing sample, the carbides have fine size and well-distributed,improved the tempering stability of H13 steel.