Study On Process And Microstructure Of (TiB₂+TiC) Reinforced Aluminum Matrix Composites By Laser Additive Manufacturing

Laser additive manufacturing(LAM)technology for metallic materials is a rapid manufacturing process that uses a high-energy density laser beam as the heat source to form a tiny molten pool by laser melting metal powder(or wire),which is solidified into a complex structural solid part by infinite accumulation in three-dimensional space.In this process,the rapid melting of material and the rapid solidification of melt pool alternate repeatedly under the local action of laser,thus resulting in a non-equilibrium solidification microstructure with essentially none chemical elements segregation and extremely fine grains in the formed component.Recently,the flexible and efficient fabrication of aluminum matrix composites(AMCs)by LAM has become an important means to improve the comprehensive performance of aluminum alloys.In this paper,AMCs were fabricated by LAM combined with in-situ synthesis method in order to provide theoretical and experimental basis for in-situ synthesis of composites by LAM.For AMCs,although the addition of reinforcements can improve some of the performance indexes of ex-situ composites,the poor wettability between the reinforcements and the matrix,as well as the differences in thermal expansion coefficients,will inevitably deteriorate other properties of the composites.In view of this,the microstructural characteristics and preparation mechanism of in-situ synthesized AMCs by LAM are systematically investigated in this paper.The in-situ(Ti B₂+Ti C)/Al Si10Mg matrix composites were prepared by selective laser melting(SLM)process using B₄C-Ti-Al Si10Mg as the in-situ chemical reaction system.The composite powder properties at different composition ratios,as well as the microstructural evolution and mechanical behavior of the fabricated composites in terms of phase content,lattice constant,etc.were discussed.On this basis,the powder composition ratios were further optimized,and the composite powders for reactions were prepared by powder coating process.The microstructures such as phase formation characteristics and in-situ reinforcement characteristics of the composites were compared and analyzed.Based on the thermodynamic theory,the synthesis and preparation mechanism of in-situ composites under laser induced are described.The results indicate that the in-situ(Ti B₂+Ti C)/Al Si10Mg matrix composites can be successfully fabricated via low laser energy input(119 J·mm^-3)and self-generated negative reaction enthalpy(-760.234 k J·mol^-1).The phase quantification results revealed that the content of transitional compound Ti₃Si C₂ present in the composites was positively correlated with the composition ratios,and the highest value was 18wt.%.The lattice constant a of?-Al matrix phase was measured by graphical-extrapolation method from 0.40466 nm to 0.40528 nm,and it was found that the degree of lattice distortion in aluminum matrix was related to the precipitation of solute Si atoms from the solvent Al lattice in the form of substitution and into the in-situ reaction system to form transitional phase.The morphology of molten pool after solidification changes significantly,facilitated by the chemical reaction thermal effect.Compared with the Al Si10Mg alloy,the average melt width and melt depth of composites increased by 104%and 94%,respectively.In terms of mechanical properties,with the increase of ceramic phases in the composites,the microhardness increased significantly,and the tensile fracture morphology showed that the fracture characteristics gradually changed from ductile fracture to brittle fracture.This indicates that the interface between the ceramics and the matrix tends to be a weak region of stress concentration,making fracture failure occur before the necking of composites.Furthermore,the powder coating process can effectively avoid the generation of transition compounds while enhancing the laser absorptivity and in-situ chemical reaction efficiency of the composite powders,and thus optimize the microstructure of composites.The TEM characterization results show that the grain size of in-situ Ti B₂and Ti C phases are about 150 nm,and a transition zone of B and C elements is formed between them and residual B₄C,resulting in few concentrated dislocations inside the Al matrix located in this region.Thermodynamically,the limited Ti source within the Al melt is only capable of forming Ti B₂ rather than Ti B,when the extremely low Gibbs free energy?G_m in the system indicates the best reaction path and facilitates the formation of nanoscale in-situ reinforcements.