Research On The Key Technologies For Selective Laser Melting Process

Metal parts could be directly manufactured with the selective laser melting(SLM) technology.They are applied to biomedical field,aerospace field,etc.There are wide gaps in the SLM technology home,compared with abroad.In the SLM processes,some problems,such as bailing effect,crack,curl,distortion,delamination and low processing efficient,might occur.To solve them,were studied the researches on the SLM machines, formabilities of metal powders such as 304L and 316L,optimum processing parameters, new scan strategies,analysis of temperature,residual stresses and residual strain fields, microstructures by XRD,optical micrograph,and SEM,mechanical properties of SLM parts by microhardness,and densification of SLM hollow parts by hot isostatic pressing (HIP) for higher efficient forming processing and mechanical properties.These research results are given as follows.To develop SLM machines,the Nd-YAG laser with 1064nm wavelength and fiber laser with 1090nm wavelength were utilized.To gain the laser power density of more than 10⁶W/cm²,the focal diameter has to be small enough to the order of magnitude of micron. Two kinds of focusing means such as dynamic focusing and static focusing were used.to avoid the oxidization and nitride of metal,the building chambers of the machines have to be airtight or protected by inert gases(argon,or helium) or in high vacuum(10^-5Torr). Thus the developed SLM machines were equipped with vacuum pumps.Powders are one of important factors having great impact on successfully forming fully dense metal parts wih the SLM technology.The single metal element and single pre-alloyed powders are better for the SLM processes.From the studies,the gas-atomized 316L powders have better formability than the waer-atomized 304L ones,and low-carbon powders do better than high-carbon ones.Thereupon the gas-atomized 316L stainless steel powder,whose size distribution of lower 20 microns is over 50%volume,is more suitable for the HRPM-â…¡SLM machine.A special program on the SLM processing was developed,and then optimum parameters were obtained.Firstly,the features of single tracks built on a thin layer powder deposited on a substrate are given in detail.From these features for processing parameters, the powder formability and track attributes can be gained.Secondly,thin layer multi-track builds are tested,and then the optimum scan space and scan strategies can be optimized to get higher surface finish quality for the full density.Thirdly,blocks are formed with parameters,and the parts densities are related to layer distances,scan strategies and other parameters.The scan strategy is one important factor on the full density of metal parts,new scan strategies such as consecutive sub-sector scan strategy with adjustable scan length, refilling scan strategy with bi-directional scan,and spiral scan strategies were developed, based on sub-area scan strategy,contour equidistant offsetting scan strategy.The tests results show that the refilling scan strategy with bi-directional scan is excellent to directly manufacture fully dense parts.To directly form dense parts with the SLM technology,the above presented program could be used to optimize parameters.The obtained optimum parameters are 98W laser power,30-100mm/s scan speed,0.06mm scan space,lower than 0.08mm layer distance.The temperature field,strain field and stress field in the SLM processing were simulated with finite element methods for the ANSYS software.From the simulation results,there exist residual tensile stresses,and the stress concentration on the connections between the baseplate and SLM parts.The simulation results agree with the phenomena such as cracks by tensile stresses in SLM parts.To reduce the residual stresses,some measurements can be made,including preheating powder and baseplate,stress-relieving treatment by annealing postprocessing,and rescanning after solidification of liquid metal in each layer.To increase the efficient of forming SLM parts and improve their mechanical properties,the SLM/HIP technology is presented.Firstly,the in-situ canings or shells of a part are manufactured with the SLM technology.Then they are processed with HIP technology for full density.From the preliminary studies on the SLM/HIP technology,the SLM canings can be densified completely by HIP.But the cannings must be airtight and the inner pressures must be less than 10^-5 Torr,processed with HIP technology.In conclusion,the problems such as balling effect,crack,distortion,and dilamination, have been solved in the selective laser melting processing.The near fully dense metal parts can be manufactured with the SLM technology.