| Due to the characteristics of small size,high heat exchange efficiency,and compact structure,micro-channel heat exchangers have been widely used in heat exchange systems including offshore oil platforms,hydrogen fuel cells,ships,and aerospace.At present,the micro-channel heat exchangers are mostly manufactured by electrochemical corrosion grooves and diffusion welding processes.This process has several problems such as low processing efficiency,environmental pollution and simple flow channel structure.Selective laser melting(SLM)belonging to Additive Manufacturing(AM)technology,which can meet the rapid and efficient manufacturing of complex structure inner hole runners.SLM technology becomes a new manufacturing method for micro runner heat exchange devices.Comparing with traditional mechanical and electrochemical machining methods,the surface roughness and dimensional accuracy of the micro-channel produced by SLM are relatively poor,which affects the fluidity of the heat exchange medium,and even turbulence and cavity effects occur,thereby reducing the heat exchange efficiency.In addition,pore defects will restrict its bearing capacity.These key problems severely restrict the application of SLM technology in the field of micro-channel heat exchange device manufacturing.In response to the above-mentioned scientific issues,this paper carried out the research on the overall macro-scale accuracy of SLM additive manufacturing,surface roughness,microstructure and performance analysis of SLM forming parts,precision control of micro-channel inner hole forming and abrasive flow polishing.The following research results were obtained:Dependent on the discrete element method(DEM),a three-dimensional transient mesoscopic scale calculation model of the SLM forming process was established.During the SLM forming process,the volume of fluid method(VOF)was used to dynamically track the shape of the molten pool.The simulation results are compared and analyzed with single-pass molding experiments to verify the model accuracy.The factors of laser power,scanning speed,and scanning distance on the surface quality of SLM molding was studied.Moreover,the optimization process parameter interval was obtained.According to the simulation results,the response surface optimization method was used to analyze the influence of the process parameters on the dimensional errors in the X/Y directions and the surface roughness,and the results were verified according to the optimization results.The epitaxial growth of the crystal grains results in the crystal grains being mostly columnar,and has a special hexagonal cell and strip-like substructure.Solidification theory and temperature field analysis are used to discuss the formation mechanism of substructures.The major reason for the formation of hexagonal cell and banded substructures are the larger temperature gradient and faster solidification rate at the edge of the molten pool.There are two types of pores,one is irregularly shaped metallurgical pores,which are caused by insufficient energy input and unreasonable process plans.The other is spherical pores,which are periodically closed by keyholes and the shielding gas is caught in the molten pool too late to overflow.When the energy density of the input body is between 100J/mm3?120J/mm3,the average density of SLM molded samples is greater than 99.73%.The average tensile strength of SLM molded parts is higher than that of hot-rolled plates.The generation of deformation twins during the tensile deformation process makes the 316 L stainless steel formed by laser selective melting and forming have good plasticity.The existence of pore defects makes the impact strength of SLM formed 316 L stainless steel weaker than that of ordinary hot-rolled plates.The corrosion types of SLM molded 316 L stainless steel specimens with different surface conditions in 3.5%Na Cl solution are different.The corrosion types of unpolished specimens are mainly pitting corrosion around the fusion line and spheroidized metal balls,while the polished specimens are the first pitting corrosion and local corrosion occur in the pores,and then spread to the periphery.In order to improve the forming precision of the inner hole SLM of the microchannel heat exchanger,the influence of the inclination angle on the forming ability of the inner hole of the micro-channel with different wall thicknesses was first studied.It found that the forming quality of the horizontal hole was poor.According to the forming characteristics of the horizontal hole,the forming area is divided into overhang area and step area.The formation mechanism of adhering slag was studied,and the method of reducing energy input was used to improve forming precision.The effects of laser power,scanning distance and scanning speed on the formation of the overhang area is analyzed,and the optimized results are obtained.The factors affecting the surface quality of the step area of the SLM molded sample are divided into three categories:splash,powder adhesion,and spheroidization,and their generation mechanism is analyzed.According to the forming characteristics of the step zone,numerical simulation method is used to study the influence of input of heat input,scanning distance and bottom surface state on the surface quality of the melt channel.According to the simulation optimization results,the entity is generated.Therefore,it was found that the forming accuracy of the horizontal hole is nearly doubled,and there is less slagging on the top.In order to improve the relatively poor surface roughness of the inner microchannel produced by SLM,the abrasive flow machining(AFM)mechanism was studied,and the SLM forming channel was polished by AFM technology.The discrete phase model(DPM)was used to simulate the distribution state of abrasive particles in the flow channel and the state of wall erosion under different inlet conditions.The results show that the area with the most serious erosion occurs at the inlet section of the flow channel and the corner.Due to the inconsistency of material removal in abrasive flow polishing process,the ultrasonic vibration and abrasive flow polishing technology were combined by acoustic field coupling model,and the ultrasonic vibration was simulated on the wall of the flow channel by dynamic mesh technology.The distribution maps of pressure,velocity and turbulent kinetic energy were obtained.The analysis results show that the ultrasonic excitation on the channel wall can cause pressure wave and cavitation effect,so as to improve the turbulent kinetic energy in the channel.The results show that with the increase of polishing time,the roughness of the inner hole wall decreases gradually.However,there are under-polishing and overpolishing phenomena on the inner hole wall of the channel.The ultrasonic excitation applied on the SLM sample surface can achieve consistent material removal rate and increase the polishing quality,which is consistent with the simulation results. |
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