Study On Process Parameters Of Overhanging Structure On Selective Laser Melting

Selective Laser Melting(SLM)is a new additive manufacturing technology which has been rapidly developed in recent years.In SLM process,due to the drastic temperature gradient change,it is easy to generate residual stress,resulting in warping and deformation of the workpiece in the printing process.In particular,when processing the workpiece with overhanging structure,it is usually necessary to add enough temporary support to ensure the overhanging structure forming,but the printing of temporary support increases the manufacturing cost,and the support inside the complex workpiece is difficult to remove,which limits the practicability and applicability of this technology.In this paper,firstly,the relationship between the scanning speed,laser power and scanning spacing on the forming quality of the track was studied,the reasons for the occurrence of Plateau-Rayleigh instability on the track under the parameters of low scanning speed and low power was analyzed,and a hypothesis similar to crystal growth is proposed.Then,according to the hypothesis of crystal growth,a horizontal branch structure directly formed on powder bed was found for the first time,and the forming principle and structural characteristics of this structure were studied.Finally,the processing parameters of the overhanging structure formed on the horizontal branch structure are studied,and the processing technology of the overhanging structure in SLM is optimized.The experimental results show that severe spatter occurs when forming directly on the powder bed.By using the method of defocusing gaussian laser beam to increase the processing spot,the energy density of the spot can be reduced while the laser power remains unchanged,and the spatter in the printing process can be reduced.Under the scanning parameters of laser power of 40 W,scanning speed of 0.1m/s and increase of laser spot to 90-100?m,the horizontal branch structure can be directly printed on the powder bed.This structure is obviously different from that of conventional SLM.The texture of its dendritic structure is approximately perpendicular to the scanning direction and has good longitudinal growth.The horizontal branch structure has good mechanical strength and can be used in the first layer processing of overhanging structure without temporary support,but the residual stress is easy to accumulate during the forming process,which leads to the bending of the workpiece.In the experiment,the machining method of gradually increasing the line power density can be used to process the overhanging structure.With the layer by layer processing,the strength of the overhanging structure gradually increases,and then the power density gradually increases to improve the forming quality of the overhanging structure layer by layer,finally the complete overhanging structure can be formed.Horizontal branch structure provides a new machining method for large span overhanging structure without temporary support.