Study On Microstructure And Properties Optimization Of TC4 Titanium Alloy Laser Additive Manufacturing Parts

Titanium alloy with high specific strength,good corrosion resistance and thermal stability has become an indispensable material in the modern aviation industry.At present,Ti6Al4V(TC4)titanium alloy is widely used and mature.Coarse columnar grainss often appear in TC4 laser additive manufacturing parts(LAM-parts),which makes its performance anisotropic.By adjusting and optimizing the process parameters of additive is the most convenient way.In this thsis,TC4 LAM parts were prepared by laser melting deposition,and the effects of process parameters on the microstructure and properties of the LAM parts were studied.With the decrease of laser power,scanning speed and powder feeding amount in an equal proportion,the coarse columnar prior-β grains boundary in the middle and lower parts are transformed into a coarse equiaxed prior-β grains boundary.When the laser power was 1200 W,the scanning speed was 8 mm/s,and the powder feeding rate was 8.4 g/min,acicular martensite with different directions appeared in the samples,and the distribution was uniform and intertwined.The tensile strength of the sample with laser power of 1200 W increased by 26.53 % and the fracture elongation increased by 200 % compared with the sample with laser power of 2400 W.The macrostructure of the sample with laser power of 1200 W is equiaxed prior-βgrains boundary,the distribution of α martensite in the product is more uniform,and the self-corrosion current density of the sample is the smallest.It indicates that the corrosion resistance of TC4 LAM parts can be improved by homogenization of the αphase.When the laser power was 1200 W,the scanning speed was 8 mm/s,and the pulse laser duty cycle was 95 %.The macrostructure of the 4 Hz sample is mainly composed of small equiaxed prior-β grains boundary from the bottom to the middle and upper parts and a small amount of large columnar prior-β grains boundary in the upper part.The average size of equiaxed prior-β grains boundary is 0.43 mm.The macrostructure of the 400 Hz sample is mainly composed of fine equiaxed prior-β grains boundary from bottom to middle and upper parts and columnar grains with narrow upper width.The average size of fine equiaxed prior-β grains boundary is 0.14 mm.The effect of low and medium frequency pulse laser on the stirring of molten pool is more intense,resulting in increasing the nucleation rate.The microstructure of the 400 Hz specimen is composed of the basket weaved by lath martensite,and the width of the middle and lower laths is small.Due to the accumulation of heat in the upper part,the cooling rate is slow,and the width of martensite increases.The 400 Hz sample has the best tensile strength and fracture elongation,which is 1091 MPa and 4.09 %,respectively,thanks to its fine equiaxed prior-β grains boundary and uniformly distributed and fine α lath martensite.The decrease in elongation of specimens at 4 Hz,40 Hz,100 Hz and 4000 Hz is due to the existence of acicular martensite.The self-corrosion current density of the sample with a pulse frequency of 400 Hz is the lowest,which is attributed to the finer α martensite structure and smaller equiaxed prior-β grains boundary.Under the condition of substrate water cooling,the macrostructure of the samples prepared by CW laser is mainly composed of equiaxed prior-β grains boundary at the bottom and coarse columnar prior-β grains boundary in the middle and upper parts,of which the width of the upper prior-β grains boundary is about 0.9 mm.The macrostructure of the samples prepared by pulsed laser is mainly composed of coarse equiaxed prior-β grains boundary at the bottom and wide columnar prior-βgrains boundary in the middle and upper parts,and the width of the upper columnar prior-β grains boundary is up to 2.1 mm.The overall structure of the samples prepared by water-cooling-continuous laser is a basket structure woven by lath martensite,and there is a lot of acicular martensite in the samples prepared by water-cooling-pulse laser.The strength of the samples prepared by water-cooled-continuous laser is improved compared with the samples without water cooling under the same conditions,while the strength of the samples prepared by water-cooled pulsed laser is decreased compared with the samples without water cooling under the same conditions.