| Selective laser melting(SLM),as one of the important forming methods in additive manufacturing,has the advantages of a high degree of freedom,high efficiency and low loss.It is suitable for manufacturing complex parts that can not be directly produced by traditional methods,especially for complex and fine structures such as engines,heat exchangers,turbines and orthopedic implants.However.the SLM forming process is complicated and has many variable parameters,which usually involve intense interaction between laser and material,which seriously affects the precision and application range of parts.Especially,defects such as pores caused by the SLM process reduce the forming quality and service performance.Therefore,it is necessary to determine the optimal combination of process parameters to improve the density of parts,effectively avoid the possible crack initiation and propagation,and further prolong the service life of parts.In this paper,the inner plate material of a heat exchanger is prepared based on the SLM forming process,and the forming mechanism of different types of pores in SLM forming parts is analyzed using samples formed with different parameters.According to the evolution of pore and microstructure,the deformation behavior and properties of the formed samples were studied.The main results are as follows:The material of 316L stainless steel powder was prepared by gas atomization,which can meet the requirements of the inner plate of a heat exchanger.The qualities and microstructures of 316L stainless steel fabricated by SLM(SLM 316L)with different process parameters were observed and analyzed.When the laser power is lower than 255 W and the scanning speed is lower than 1400 mm/s,the laser scanning trajectories are uniformly arranged and form periodic overlapping melt pools,resulting in low porosity.The instability of liquid metal flow in the melt pool intensifies when the laser power and scanning speed are too high,and uneven track form different types of pores,leading to high porosity and poor forming quality.The effect of initial dislocation density and grain morphology on the yield strength of SLM 316L stainless steel was studied,and the reason for the higher yield strength was found.The high yield strength of SLM 316L stainless steel was attributed to the high initial dislocation density.The dislocation densities in 75 W415 mm/s and 135 W-750 mm/s samples are 1.24×1015/m2 and 1.17×1015/m2,respectively.The ratio of dislocation strengthening to yield strength is 71.6%and 73.2%,respectively.The greatest strengthening contribution comes from dislocation strengthening.In addition,different morphologies of grains also affect the yield strength.The microstructure of 316L stainless steel fabricated by SLM has different morphologies due to the epitaxial growth along the direction of the heat flow gradient(perpendicular to the melt pool boundary).In the 135 W-750 mm/s sample,the melt pool is shallow and wide,and the temperature gradient direction in the melt pool is approximately perpendicular to the grains and grows into coarse columnar grains.In the 75 W-415 mm/s sample,the deep and narrow melt pool makes the grains grow obliquely toward the center of the melt pool,cross the boundary of the next melt pool and change the growth direction to form fine fanshaped grains,which have smaller effective grain size and higher yield strength.The quantitative method studied the microstructure evolution and deformation behavior of SLM 316L stainless steel during plastic deformation.It is mainly the change of twins from the perspective of deformation structure.Deformation twins appear in the initial stage of deformation,and the density of twins increases with the increase of deformation and gradually forms secondary twins.Because the deformation twin accompanies the whole plastic deformation stage,the work hardening rate curve shows multi-stage hardening behavior.In the first stage,the dislocation slips in limited space.In the second stage,the initial high-density dislocations not only hinder the dislocation motion but also makes the dislocation proliferation difficult,and thus twinning occurs.In the third stage,the interaction between deformation twins promotes work hardening.The twin boundary stores dislocations,causing them to slip at the twin boundary and increase the workhardening stage.The insufficiency of the hardening ability of deformation twins in the fourth stage leads to a decreased hardening rate.In order to eliminate the residual stress,the heat treatment of the formed samples was studied.With the increase of heat treatment temperature,the Cr,Mo and Ni enriched at the unit cell boundary diffuses to the matrix,which makes the cellular structure disappear and the dislocation density decreases.The yield strength and hardness of SLM 316L stainless steel decrease.Heat treatment does not change the phase of SLM 316L stainless steel,but recrystallization occurs after solution treatment at 1100℃ to form different types of annealing twins.Hydrogen embrittlement of SLM 316L stainless steel has been studied.The 135 W-750 mm/s process sample has better hydrogen embrittlement resistance.An electrochemical dynamic hydrogen charging test was carried out on SLM 316L stainless steel.The hydrogen embrittlement sensitivity was characterized by elongation loss during slow strain rate tensile.In the hydrogen charging stage,hydrogen atoms diffuse the interior along the cell boundary and grain boundary.Hydrogen atoms enter the pores and recombine to form hydrogen gas,which forms a stress gradient field around the pores.Under the condition of applying an external force,hydrogen-induced cracks appear.Therefore,the hydrogen embrittlement sensitivity increases with the increase of porosity,and the 135 W-750 mm/s process with lower porosity has a smaller hydrogen embrittlement sensitivity.The corrosion resistance of 316L stainless steel fabricated by SLM was studied.The initial sample with lower porosity and more substructures had better corrosion resistance.Pitting corrosion is highly sensitive to pores,and the quantity and density of corrosion pits are increased by plenty of pores.With the increase of porosity,the pitting potential decreases,the passivation interval becomes shorter,the polarization reaction resistance decreases,the stability of the passive film,decreases,and the corrosion rate accelerates.In addition,with the increase of heat treatment temperature,the substructure decreases,the pitting potential decreases,the passivation interval decreases,and the passive film,becomes thinner,which is easy to break down and the self-repairing ability becomes weaker.The corrosion resistance of SLM 316L stainless steel decreases. |
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