Investigation on the Processing and Mechanism of Tial-based Micro-laminate Composite by Selective Laser Melting
Author:Shi Xue Zhi
Supervisor:ma shu yuan
TiAl alloy has broad application prospects in the aerospace and automotive fields because it has low density,high specific strength,high resistance to oxidation and creep resistance.However,its development and application have been strictly restricted by its low room-temperature ductility and difficulty in thermoplastic processing.In order to improve its room-temperature ductility,TiAl-based micro-laminated composites with bionic structure have been developed,the ductility of which can be strengthened by adding ductile alloy,i.e.,Ti and Nb alloy.The restriction of room-temperature brittleness can be solved,thus excellent mechanical strengths can be presented.However,preparation of TiAl-based micro-stack composites is very difficult.Currently the study stays only in material preparation stage;the fabrication of highly complex parts still cannot be realized.Selective laser melting(SLM)is one of the additive manufacturing techniques for directly fabricating metal components from 3-dimension CAD model,in which functional,complex parts are formed by selectively melting successive layers of powder particle using a laser beam.Based on its principle of stacking-layer manufacturing,the SLM provides the possibility to realize the integrated manufacturing of TiAl micro-laminated composites and components.In this work,a new selective laser melting equipment for heterogeneous materials is developed,and TiAl/TC4 micro-laminated composite for toughening design is proposed to overcome plastic processing difficulties of TiAl alloy.This paper presents the basic research about SLM on TiAl/TC4 micro-laminated composites.The main contents are given as follows:(1)The development of selective laser melting equipment for heterogeneous materials.This equipment mainly includes laser system,powder delivery system,forming elevating system,high temperature preheating system,gas protection system,software and control system,et al.The crucial issues of laying heterogeneous powders and high temperature preheating have been solved.Two powder hoppers were designed structurally,where different kinds of powders can be stored respectively,thus quantitative feeding and laying of heterogeneous materials can be realized.The TiAl and TC4 powder with different particle sizes were applied to fabricate TiAl-based micro-laminated composites and solve the problems of recovery and recycling of the mixed powders.The study found that the residual stress is the key factor for crack growth during the process of selective laser melting TiAl.For preventing the cracking,high-temperature preheating system was developed to reduce the temperature gradient and residual stress of the fabricated components,which can be employed to fabricate components with low plasticity materials by SLM as well.(2)Research on the forming characteristics of TiAl alloy by SLM.Firstly,the main process parameters were optimized based on single-track experiments.Meanwhile,the influences of the laser power and scanning speed on geometrical characteristics were analyzed by linear regression analysis,and the relationships between the processing parameters and geometrical characteristics of single tracks were established.Then,the investigation of multi-layer fabrication was employed combining with the loss of Al content in order to fabricate the desired TiAl parts.The experiment results indicated that fully dense samples cannot be obtained just by applying the optimization of the processing parameters due to the existence of cracks.By analyzing the mechanism of cracking formation,it was found that the residual stress is the fundamental cause for the generation of cracks.In order to eliminate the crack defects,the methods such as substrate preheating,laser pre-sintering and remelting process were adopted to reduce temperature gradient and so to weaken the residual stress,and results showed that remarkable improvements have been achieved.Finally the thin-shaped TiAl parts without crack defects were fabricated successfully,which lays the foundation for the design and fabrication of micro-laminated composites.(3)Research on the forming characteristics of TC4 by selective laser melting.Large particle diameter TC4 powders with the size range of 63-106μ m were used,and the SLM experiments with high layer thickness of 200μ m were carried out to investigate the effects of using two different sizes of laser beam diameter on the performance of SLM parts.The results showed that the small laser beam diameter of 50μ m was easy to generate larger penetration and higher evaporation rate which resulted in higher incidence of pore defects.A relatively large laser beam diameter of 200μ m was utilized to successfully avoid process instabilities,therefore the density and properties of SLM samples were improved significantly.The sample with the density of 99.99%,the yield strength of 1000 MPa,the ultimate tensile strength of 1100 MPa,and the maximum elongation of 8.5% was successfully obtained by applying the optimization of the process parameters.Furthermore,the remelting process and lower layer thickness were proposed to solve the problem of low dimensional precision of SLM products.Meanwhile,the yield strength,ultimate tensile strength and elongation of the SLM specimens were improved to more than 1100 MPa,1200MPa and 10%,respectively.(4)Research on the SLM characteristics of TiAl/TC4 micro-laminated composites.The influences of layer thicknesses and layer thickness ratios on the morphology,compositions and properties of the micro-laminated samples were explored.The results indicated that when the layer thickness ratio was constant,the thinner the layer thickness of TiAl was,the less the crack defects occurred in the parts.When the layer thickness of TiAl decreased to 200μ m,the main parameters such as layer thickness ratio and layer thickness of TC4 were optimized,thus fully dense TiAl/TC4 micro-laminated composites without crack can be fabricated.Meanwhile,the smaller the layer thickness ratio of TC4/TiAl was,the better the room-temperature strength and plasticity of micro-laminated composites could be obtained,but the weight of micro-laminated composites would increase accordingly.Finally,high-performance micro-laminated composite can be obtained when the layer thickness of TiAl was 200μ m and the layer thickness of TC4 was 400μ m.