A Study On The Surface Quality Of The SLMed Titanium Alloy Inner Structure Part

Titanium alloy is considered as an ideal material to achieve lightweight due to its low density,high specific strength,high specific stiffness and so on.Titanium alloy Inner structure part can further decrease the weight of the manufactured part to reduce energy consumption.But traditional manufacturing method is hard to process this kind of part due to the fact that inner structure part is hard to demould and defects are quite easily formed in this process which can further lead to the low precision of the manufactured part.To solve this problem,titanium alloy inner structure part was manufactured using selective laser melting method in this work.To increase the quality of the inner structure,the mechanism of the powder melting and the molten pool behavior was studied first in this work and scan strategy was optimized to increase the property of the inner structure part.After that,the relationship between the process parameters and the quality of the inner structure was systematically studied.One thing should be noted that the quality of the inner structure in this work contained both dimension accuracy of the inner structure and the surface quality of the inner structure surfaces.Based on these,the process parameters were optimized using genetic algorithm and laser rescan technology was also explored in this work to check if it was an effective method to further increase the quality of the inner structure surface.The main contents of this work were listed as follows:(1)A research on the mechanism of powder melting and the behavior of the molten pool.On the basis of the study of laser action on titanium alloy powder including a series of changes of powder and energy,the relationship between process parameters and molten pool behavior was analyzed.Warping deformation,molten pool convection and molten pool sag were explained according to the molten pool behavior and the relationship between molten pool behaviour and printing quality of both lower-positive surface,side surface amd upper-positive surface were systematically studied.The relationship between process parameters,molten pool behavior and printing quality of different inner surfaces was established in this work.ANSYS was used as a simulation method to study the variation trend of the temperature field of the molten pool caused by different process parameters.(2)Optimization of scan strategy on the inner structure part.Different scan strategies were used here to print an ordinary square shape sample and the positive surface and side surface were measured first.According to the experimental data,a scan strategy which might be more suitable to print inner structure layer was proposed.An inner structure part was printed using this scan strategy and compared to the other samples printed using different scan strategies.Based on the character of SLM and the inner structure part,a new scan strategy was designed and a series of experiments were done here to verify the advantages of this new composite scan strategy.(3)The influence of the process parameters on the dimension accuracy of the inner structure.The process parameters chosen before were controlled to study its effect on the dimension accuracy of both circle and square shape inner structure on vertical and horizontal direction separately.On the basis of single-factor experiments,the different behaviout of dimensional fluctuation on different directions were systematically studied to explain the impact mechanism of different process paramters on the dimension accuracy of the printed inner structure.Both experimental results and micro-morphologies of the inner surface were used here to optimize the beam theory to meet the actual situation on the printing quality of overhanging surface with the help to Euler-Bernoulli beam theory.(4)The influence of the process parameters on the inner surface quality.Single-factor experiments were used here to study the relationship between the process parameters and inner surface quality of the square and circle shape inner structure part.Based on the analysis of the morphology of the inner structure,flatness and roughness are used here to represent the surface quality of the square-shape inner structure while roughness,concentricity and surface profile are used to represent the surface quality of circle-shape inner structure.The impact of different process parameters on the lower-positive surface,side surface and overhaning surface quality was systematically studied and the fundamental cause was also researched in this section.From the analysis of the experimental data,the inner surface of both square and circle shape inner structure,especially the overhanging surface quality were divided into different periods with a combination of surface morphology of the printed parts.It helps to change different parameters in different periods to increase the inner surface quality.(5)Optimization of the process parameters on the inner structure part.Based on the previous studies,including the influence rule of the process parameters on the dimension accuracy and inner surface,the inner surfaces which showed a relative significant impact on the quality of the inner structure were selected and response surface analysis was used here to establish mathematical model.Multi-objective genetic algorithm was also taken here to solve the mathematical model in a relative optimal range of process parameters and a Pareto optimal solution set was given accordingly.An experiment was given to verify the solution set gained here.(6)An exploratory experiment to further increase the quality of the inner structure part using laser rescanning technology.Due to the fact that the post-process of titanium alloy inner surface was hard and costly,a series of experiment were done here to explore whether laser rescanning technology was able to further increase the property of the inner structure.Compared with the inner structure quality of the sample processed by traditional heat-treatment,laser rescanning technology(under different laser power)and without any treatment,an analysis was given to judge the feasibility of the laser rescanning technology and the mechanism of this technology was studied based on the experimental data and micro-morphology of the inner surface.