Direct Laser Fabrication Of 3-Dimentional Metal Parts Based On Laser Cladding

Direct laser fabrication (DLF) is an advanced manufacturing technology, which is developed from rapid prototyping and laser cladding over the last decade. DLF can automatically fabricate full density arbitrarily complex-shaped metal parts directly from CAD files without using any modules or tools by the utilization of laser cladding technique. In this Ph.D. dissertation, a control software named HUST-RP was developed in order to achieve the maximum flexibility and extensibility for the direct fabrication of metal parts with various shapes and high accuracy by laser cladding. The software was intergrated into the DLF hardware system, which is composed of a 5kW ROFIN TR050 CO2 laser, a CNC system, a powder feeder, a special designed powder nozzle and a powder recycler. The path pattern plug-ins and G-code profile files were introduced into the software, by which the engineers can choose a suitable filling pattern for a certain part and export it in a suitable G-code format. On the basis of the hardware and software system above mentioned, the DLF process of 3D metal parts was studied systematically. The results demonstrated that the fabrication of complex-shaped metal parts by additive-layer laser cladding was feasible under open-loop control. The metal parts with the open-loop control can achieve satisfied shape quality, if the laser processing parameters were chosen in a suitable range. This conclusion proved that the fabrication of metal parts is feasible under open-loop control. The effect of the specific energy on the cross-section shape of nickel alloy cladding was studied systematically by a single-track cladding experiment with different laser processing parameters. The effect of the laser processing parameters such as laser power, scan velocity, powder feed rate and track overlap on the qualities of metal parts of Ni alloy was also studied by an orthogonal experiment, by which the optimized parameter ranges were gotten. The experimental results showed that the specific energy for laser processing is the most important factor that controls the part qualities. There is an appropriate range of the specific energy, in which the samples of nickel alloy can be fabricated layer by layer with a uniform height. If the specific energy is too small or too high, the difference between the contour height and the inner height of a part will become more and more evident, which will result in the process failure finally. In the dissertation, the ratio of the cladding remelting depth to the cladding height can be regarded as the criterion to determine whether the specific energy is suitable or not. The process parameters capable for building up the parts of Ni alloy stably with uniform height are: laser power 600–900W, scan velocity 4–8mm s-1, powder feed rate 3.0–8.0g min-1. The effect of filling path of laser beam on the fabrication efficiency of DLF process, the shape accuracy and mechanical properties of metal parts was studied systematically. The tensile specimens of stainless steel with 5 different filling patterns were fabricated with same laser process parameters. The experimental results showed that the raster filling path and the contour-offset filling path have nearly same fabrication efficiency to the DLF process. However, the raster filling path can achieve much better the shape accuracy of metal parts than the contour-offset filling path because it can utilize the 'random filling'method in an easy way. Although many metallurgical defects such as oxide impurities were found in the specimens, the yield strength and the tensile strength of the specimens fabricated by DLF process is in same level or even a bit better than the traditional forged materials. It is expected that the mechanical properties of the parts fabricated by DLF would be enhanced even greatly if the metallurgical defects were eliminated by choosing the laser process parameters carefully and by improving the deposition environment. The methods to improve the fabrication accuracy of metal parts with thin-walled structures were studied systematically. Several multi-bead stacks and thin-walled samples of Ni alloy were fabricated with variable process parameters. It showed that the height of each cladding layer of multi-bead stack was unstable if the z-increment value was equal to the first cladding height. There is also an appropriate range for the specific energy in which the multi-bead stack of nickel alloy can be fabricated layer by layer with a uniform height. The continuous interpolation of CNC stage, the randomization of start location of laser beam and the slight lower z-increment value to the first cladding height are the most important steps to achieve excellent thin-walled structure qualities.