| Additive Manufacturing(AM) technologies, which can be used to manufacture complex structures with certain machnical properties, build 3D objects by adding material layer-upon-layer. Laser Metal Depostion(LMD), which is also called metal 3D printing, is a typical AM process for metal. Unlike conventional manufacturing technologies that based on material removel, with LMD, three-dimensional, complex components out of a wide range of materials can be manufactured consecutively layer-by-layer with advantages such as lower material waste, faster development cycle of new products and more flexible process, and it can even be used to control and design material microstructures. After about 30 years of development, LMD are more and more commonly used in industries. Recently, the desire for large size parts manufacturing using LMD is growing. However, despite the technological advantages of the LMD process, currently achieved deposition-rates of approx. 0.3 kg/h for Inconel 718(IN 718) remain a major concern in regards to processing times and economic feasibility. Thus, IN718 was used as powdery additive, and the focus of the current work is to investigate high deposition-rate LMD for AM application. Firstly, a high deposition-rate LMD experimental setup was developed and characterized. Secondly, high deposition-rate LMD processes with deposition rate of approx.2kg/h and 5kg/h were developed, and methods for process optimization and porosisy reduction were studied. Finally, microstructure and mechanical properties of IN718 formed by high deposition-rate LMD were investigated.The main works of this paper are as follows:1. High deposition-rate LMD experimental setup was developed and built up: based on the assumption that the outline of the deposited track cross-section is part of a circle, ways of increasing deposition rate of LMD were proposed, and furthermore the requirements for high deposition-rate LMD experimental setup were proposed. According to these requirements, a high deposition-rate coaxial powder nozzle was developed, a zoom optic, a high power laser source and a rapid powder switch were integrated, and the high deposition-rate LMD experiental setup with high powder efficiency was built up. This setup can work with various laser spot in the range of 3mm-9mm, and it works stably in high powder mass flow working condition.2. The experimental setup was characterized and the powdery additive was analysied: in order to quantify the experimental evirionment and ensure the reproductility of experiments, the experimental setup and powdery additive were characterized. The caustic of laser, laser energy distribution and powder intensity distribution around working plane were analysed. The IN718 powdery additive was characterized in terms of chemical composition, porosity, morphology and so on.3. High deposition-rate LMD processes were developed: based on the characterized high deposition-rate experimental setup, the process windows of high deposition-rate LMD processes with deposition rate of approx. 2kg/h and approx. 5kg/h were developed. In comparison with conventional LMD processes of same kind, deposition rate was increased more than 10 times. In order to find the basic process paramers, the relationship among main process parameters of LMD was established. Based on these primary parameters obtained from the established relationship by giving desired deposition rate, mixing process parameters Energy Mass Density and Energy Area Density were defined and process windows for high deposition-rate LMD process with deposition rate of approx. 2kg/h and approx. 5kg/h were found.4. Effects of main process paramters of LMD on forming properties and process properties were investigated: based on the high deposition-rate LMD process, effects of main process parameters, which are laser power, scanning speed, powder mass flow, on porosity, track geometry, deposition rate and powder efficiency were investigated in order to improve the flexibility of the process. Firstly, the effects of process parameters on porosity were investigated, and methods for controlling porosity were proposed. After that, study on how process parameters influence track geometry were carried out, and the geometry controlmap for high deposition-rate LMD has been drawn. Finally, methods for controlling the process properties were proposed based on the research on the effects of process parameters on deposition rate and powder efficiency.5. Approaches for reducing material porosity were investigated: porosity, which is the main material defect of LMD, is detrimental to material properites. Based on the newly developed high deposition-rate LMD process, the effects of powder humidity, nominal powder particle size, powder shape and shielding gas flow rate on the porosity were investigatied. According to experimental results, several approaches of reducing porosity in high deposition-rate LMD are proposed in this paper.6. Microstructures and mechanical properties of IN718 formed by high deposition-rate LMD were investigated: initially, blocks for tensile specimens producing were deposited using the developed high deposition-rate LMD process that has a deposition rate of approx. 2kg/h. Then, the microstructure of the directly-deposited IN718 was analysied. Afterwards, the material was heat-treated and the microstructures were analysied. Finally, the mechanical properties of the directly-deposited and the heat-treated material were tested, and the results were compared to AMS(Aerospace Material Specification) specifications for casted and wrough IN718. The results show that the hardness, tensile properties and ductility of the heat-treated IN718 that deposied by high deposition-rate LMD developed in the current paper is suporier to AMS specifications for casted and wrought material. |
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