Study On The Effect Of Molten Pool Temperature Control On The Morphology And Microstructure During Laser Additive Manufacturing

Laser addictive manufacturing technology is an advanced manufactu r in g technology which integrates mechanical,material,numerical control,computer and other disciplines.In this technology,laser is used as heat source,according to 3D solid parts are sliced to obtain two-dimensional cross-section information,point,line and surface are used as basic units to build up layer by layer on the substrate,and fina lly solid parts are obtained.Laser addictive manufacturing can realize the manufacture of parts with complex structure which cannot be achieved by traditional machin ing method or difficult to achieve,and can greatly reduce the processing process and shorten the processing cycle.However,due to the high sensitivity of the process,the quality of the forming parts produced by laser processing depends to a great extent on the processing parameters,process parameters,and other factors.Laser addicti ve manufacturing is a typical rapid solidificatio n non-equilibrium metallurgical process.The stability of molten pool temperature is of great significance to improve the quality of forming parts.At present,most of the researches on the closed-loop control syste m of molten pool temperature in laser addictive manufacturing are mainly focus on the innovation of control algorithm under conventional substrate and the stability of weld pool temperature in machining process to the effects of macro-morphology and microstructure of the final forming part.There are few studies on the shape control and control under the condition of special-shaped substrate and the effect of molten pool temperature on the shape and microstructure of forming parts.Therefore,the purpose of this paper is to research the process and mechanism of the effect of closed-loop control of molten pool temperature on the macro-dimen s io n precision and microstructure of the parts.Firstly,the influence of process parame te r s on the important process state quantity and morphology structure is established by selecting different specifications of substrate design experiments.Then the closed loop control system of molten pool temperature based on PID algorithm is designed and the open-loop and closed-loop experiments are carried out.Complete comparative analy s is of data results.The specific work is as follows:(1)The temperature monitoring system of molten pool is established and combined with the Laser addictive manufacturing system.The closed loop control system of molten pool temperature is built,which provides the hardware and software basis for the design of the temperature control system of molten pool.(2)The single channel deposition experiments of thin substrate and thick substrate and a certain laser power gradient were carried out.The temperature of molten pool,cooling rate,morphology and microstructure were systematically studied under the condition of thin and thick difference of substrate,and the effect of laser power on the temperature of molten pool was investigated.The relationship among cooling rate,morphology and microstructure is established,and the quantitative relation s hip between molten pool temperature and morphology,cooling rate and microstructure is established,which provided a powerful support for the improvement of dimen s io n precision and microstructure uniformity of closed loop forming parts.(3)The linear relationship between laser control voltage and laser power is established by experiment test.The model of laser augmentation manufacturing syste m is established by system identification method,and the closed loop control system of molten pool temperature based on PID algorithm is designed.The off-line simula tio n shows that the PID controller has excellent transient performance and steady state performance.(4)Based on the single-channel multi-layer open-loop and closed-loop deposition experiments of thin and thick-ended special-shaped substrate,the difference of dimension precision and microstructure between the two groups of experime n ta l forming parts is quantitatively analyzed.The results show that the weld pool temperature PID control system can improve the horizontal and longitudinal dimen s io n accuracy of forming parts by improving the uniformity of layer thickness and balanc ing the powder utilization rate by adjusting the laser power in real time to maintain the temperature stability of molten pool.The uniformity of microstructure can be impr o ve d by reducing the difference of cooling rate in each direction,in addition,it can imp r o ve the uniformity of hardness in each direction.