Research On Microstructure,Properties And Thermal Deformation Of Pulsed Laser On Melting Deposition Based On Process Characteristics

Laser melting deposition(LMD)technology can manufacture parts layer by layer through rapid heating,melting,solidification,cooling process.At present,the most commonly used heat source is continuous laser.However,the high energy input is easy to cause thermal damage and increase the cumulative error like residual stress and thermal deformation in the machining process,which restrict the further promotion of LMD.The periodic heat input of pulsed laser reduces heat input,inhibits heat accumulation and reduces thermal damage.But the low laser energy can only generate thin deposited layer,which limits the response of pulsed laser.Although many attempts have been made on the study of pulsed laser in LMD,the analysis of the processing characteristic of pulsed laser with different parameters is not sufficient,and the study of the influence of pulsed laser on the thermal deformation of substrate is lacking.Therefore,it is of great significance to extract the process characteristic of pulsed laser melting deposition and study the influence of pulsed laser on the microstructure,properties and thermal deformation of the formed parts.The contents are as follows:1.Process characteristic extraction and analysis.In order to fully extract key information of processing,the monitoring platform was established with multiple sensor fusion device.Thermocouple,infrared pyrometer and IR camera were used to monitor the temperature information during the machining process,and coaxial CMOS camera was used to monitor the molten pool morphology in real time.In addition,a simulation model of molten pool flow field was established to simulate the transient change process of molten pool.By comparing the different process between two modes of laser,the mechanism of influence of pulsed laser on the process characteristic was analyzed.The extraction algorithm of temperature characteristics,such as temperature gradient,cooling rate and solidification rate,was developed based on Python language,and the influence rules of different pulse parameters on temperature characteristics were studied.Besides,the contour of molten pool was extracted by image processing technology,and the transient changing process of molten pool during pulsed laser and continuous laser processing was analyzed based on the simulation results of the model.2 The influence mechanism of pulsed laser on workpiece performance is analyzed by combining with the processing characteristics.This paper verified the influence of low heat input of pulsed laser on the surface of the deposited layer,and analyzed the mechanism.Meanwhile,the uniformity of the sedimentary layer was improved by increasing the heat input of pulsed laser.The dilution of pulsed laser was different from continuous laser because the pulsed laser needs to melt the deposited layer formed in the previous period,and the penetration into the substrate is limited.Although the dilution rate of pulsed laser is low,it does not have much effect on the bonding strength as the bonding zone mainly exists inside the deposited layer instead of the substrate.Based on the difference of machining characteristics,the mechanism of pulse parameters on microhardness and microstructure of the product was studied and the reasons for the formation of equiaxed grains were analyzed.3 Study on suppression effect of pulsed laser on residual stress and thermal deformation.The mechanism of residual stress in deposited layer was explained,and the influence of different pulse parameters on residual stress was studied,as well as the reason why pulse laser reduced the difference between transverse residual stress and longitudinal residual stress.Based on the heat transfer characteristics of the machining process,the reasons why the thermal deformation of the substrate went through three stages were analyzed,and the inhibitory effect of pulsed laser on the thermal deformation was verified.Besides,the mechanism of the influence of different pulsed laser parameters on the deformation of the matrix was analyzed.Finally,in order to improve the problems of low deposition height and uneven deposited layer in pulsed laser processing,the pulsed laser with the same heat input was used to carry out experiments.The result is the deformation of substrate was smaller than that of continuous laser mode,and the deposition quality was better.