Study On Strengthening And Toughening Mechanism Of In-situ TiB₂/Al-Si Composite Fabricated By Selective Laser Melting

With the changes of the times,there is a growing demand of materials’properties in those key domains which are beneficial to the people’s livelihood such as aviation,aerospace,transportation and national military defense.Meanwhile,there is a trend that the designs of key structural parts in those domains become more and more complex.It brings great challenges to traditional material forming techonology.Therefore,it’s necessary to find out a new material manufacturing method.Selective laser melting（SLM）technology is undoubtedly a good solution for current situation,because it has several merits such as high degree of freedom and out of the constraint by mould during material forming process.However,there are several defects such as oxide inclusions and porosities in SLMed Al alloy which bring bad influence on mechanical properties due to the low laser absorptivity,high heat conductivity,and high oxidative activity of the Al alloy.In addition,SLMed Al alloy has an anisotropy microstructure caused by laser heat flow.The elongations are lower than 6%in most of researches,which is much worse than forging pieces.Hence,it’s difficult to apply that kind of materials in industry.The in-situ TiB₂/Al composites have several merits such as high specific strength,high specific stiffness and high modulus.What’s more,the presence of ceramic particles increases the laser absorptivity of the Al powder due to the high laser absorptivity of TiB₂ particles.And it would be beneficial to SLM processing.As a result,those problems encountered with Al alloy might be solved,and it also provide a new method to fabricate complex structural components for aviation and aerospace industry.For those reasons,researches were carried out on in-situ TiB₂/Al-Si composite fabricated by SLM in order to explore the influence of TiB₂particles on the evolutions of microstructure and the changes of mechanical properties,to find out an appropriate heat treatment process and to reveal its strengthening and toughening mechanism.The results indicate that the solubility of alloy component increases dramatically.The forming material contains TiB₂ particles with inhomogeneous dispersions state and eutectic Al-Si network upon rapid cooling during SLM.There is an ultra-fine grain structure with an average grain size of 1.05μm in SLMed in-situ TiB₂/Al-Si composite.Compared with casted composite（average grain size of 40μm）,the grains are refined drastically.Furthermore,the TiB₂ particles are grain growth inhibitors in Al.So the average size of composite is only half of that of the SLMed Al-Si alloy.And the pinning effect of TiB₂ particles hinders the grains growing tendency towards the heat flow during SLM.The draw ratio of composite grain is 1.83,compared with that of the alloy.The grains become more and more equiaxial.The anisotropy phenomenon is restrained.When discussing about the mechanical properties,the presence of TiB₂particles improve the strength and elongation of SLMed in-situ TiB₂/Al-Si composites remarkablely.Compared with Al-Si alloy,their tensile strengths increase from 424MPa to 475MPa,which increase by 12%.And their enlongations increase from 8.8%to 13.4%,which increase by 52%.The mechanical properties are much better than those of the casted composites.After some researches carried out on heat treatments,the mechanical properties reach the best after the direct artificial aging（150°C/12h）.The tensile strength reaches 523MPa,and the elongation is 8.2%.It not only meets the requirement of mechanical properties of aviation and aerospace industry,but also provides a new option to fabricate complex structural components.According to the microstructure observation and phase analyze,the strengthening and toughening mechanism of SLMed in-situ TiB₂/Al-Si composites could be concluded as a superimposed effect of solution strengthening,fine-grain strengthening,Orowan strengthening and load-bearing strengthening.