Study On Microstructure And Properties Of Laser Solid Forming And Laser Welding Ti40 Burn Resistant Titanium Alloy

Titanium alloys are widely used in the aerospace field because of their small specific gravity and high specific strength.However,conventional titanium alloys are prone to"Titanium fire"accidents in high-temperature environments,which limits the application and development of titanium alloys in the field of aero-engines.Ti40 flame retardant titanium alloy had gradually received more and more attention because of its excellent flame retardant properties and good mechanical properties.The disadvantages of Ti40titanium alloy metal are poor fluidity,easy cracking of grain boundaries,and poor hot workability,which limits its further promotion and application.Corresponding to these problems,Laser Solid Forming(LSF)technology has the advantages of a wide variety of processing materials,high efficiency and low cost,so it has a good prospect in the manufacture of Ti40 titanium alloy.Therefore,this study took Ti40 flame retardant titanium alloy as the research object,and focused on the microstructure evolution,mechanical properties and flame retardancy of Ti40 titanium alloy during the LSF processing.Based on this,the weldability of Ti40 titanium alloy deposited samples and forged samples was studied,the microstructure and mechanical properties of the welded joints of the two samples of"forged+forged"and"forged+LSF"were investigated.The evolution discipline can provide a theoretical basis for the engineering application of LSFed Ti40 in the field of aero-engines.Conclusion as below:(1)Revealed the microstructure and mechanical properties of LSFed Ti40 flame retardant titanium alloy.The bottom region and top region of the laser-deposited single-wall sample were?equiaxed crystals,the middle region was?columnar crystals,and the grain size of central region was the largest,followed by the bottom region,and the top region had the smallest The?-grain distribution of the block-deposited samples was the same as that of the single-walled samples,but the grain size of the block-deposited samples was larger.In the LSFed Ti40 titanium alloy deposition sample,the?phase and the Ti₅Si₃phase were generated in the?grains,and the content of the precipitate phase gradually decreased from the bottom to the top.The microhardness distribution of Ti40alloy single wall samples and block samples along the deposition direction showed a"U"shape,but the overall hardness of the block samples was slightly lower than that of the single wall samples.The effect of fine grain strengthening on the microhardness of LSFed Ti40 deposition samples was greater than that of precipitation relative hardness.The tensile strength and yield strength of the LSFed Ti40 single-wall deposited samples were989 MPa and 978 MPa.The tensile strength and yield strength of the block deposited samples were 965 MPa and 962 MPa,respectively,which were both greater than those of Ti40 forgings.The elongation of the single wall sample(19.8%)was greater than that of the forging(18%),but the elongation of the block sample(16.5%)was slightly lower than that of the forging,reaching 91.7%of the forging.(2)The flame retardant properties of LSFed Ti40 alloy deposited samples were investigated.After laser ablation of Ti40 alloy deposition samples,widening cracks appeared inside the ablation holes,and?columnar crystals formed around the holes.After laser ablation,a dense oxide layer composed of Ti O₂,V₂O₅,and Cr₂O₃ was formed on the surface of the ablation hole,which could effectively prevent the diffusion of Ti and O elements and had a flame retardant function.At the same time,the low melting point and high thermal conductivity of Ti40 deposited samples could take away a lot of heat and decreased the temperature of the samples.The Ti₅Si₃ and?phases precipitated by the deposited sample may easily cause microstructure defects in the sample.These microstructure defects accelerate the diffusion of V and Cr elements and form a denser oxide layer and Cr element enrichment layer on the surface of the deposited sample than forgings.The three factors make the flame retardant of the LSFed Ti40 alloy deposition sample better than that of the forged sample.(3)The evolution of the microstructure and mechanical properties of welded joints of Ti40 flame retardant titanium alloy was investigated.The welded joint part is divided into three areas:substrate zone,heat affected zone and fusion zone.However,the heat affected zone is smaller;the grain size in the weld is smaller than the forged substrate.The grain size from the fusion line to the center of the weld gradually decreased in the"forged+forged"sample,the grains near the fusion line were?columnar grains,and the center grains of the weld were?equiaxed.The microstructure of the weld in the“forged+LSF”sample near the side of the deposited substrate was?columnar crystal with directional epitaxial growth based on the unmelted substrate.The?and Ti₅Si₃precipitated phases were found in the center of the welds of both samples,but the amount of precipitated phases was relatively small.The hardness of the weld in the“forged+LSF”sample was slightly higher than that in the“forged+forged”sample;and it was found that the hardness of the deposited side was slightly higher than that of the forged side in the“forged+LSF”sample.The tensile performance test of Ti40 welded joints found that both tensile samples fractured in the substrate zone,but the"forged+LSF"sample fractured on the side of the deposited substrate.The fracture modes of the two tensile samples both were a mixed of ductile fracture mode and cleavage fracture mode.