Microstructure and Properties of Ti-6Al-4V Titanium Alloy by Laser Deposited with Ultrasonic Vibration
Laser deposition manufacturing(LDM)of metals,typically leads to the formation of columnar grain structures along the build direction in most as-built metals and alloys.These long columnar grains can cause property anisotropy,which is usually detri-mental to qualification or applications.Here,without changing alloy chemistry,we demonstrate an LDM solidification-control solution to printing metallic alloys with an equiaxed grain structure and improved mechanical property.Although ultrasonic assisted LDM is a proven technology to refine grain size,in the application process,ultrasonic will inevitably weaken with the propagation distance,and it is still difficult to implement large key main bearing components such as aircraft fuselage reinforcement frame and main bearing beam.In order to enhance the effect of ultrasonic assisted,the ultrasonic application mode in the normal direction of the vertical sub-strate could make the acoustic wave form Chladni standing wave,so as to realize the effective and in-situ regulation of the ultrasonic field on the microstructure and properties of TC4 titanium alloy prepared by LDM.The waveform distribution and ultrasonic amplitude field of substrate surface were obtained by multi-physical simulation,and the actual waveform distribution was obtained by equivalent substitution method.The microstructure and properties of Ti-6Al-4V titanium alloy deposited by laser in the region of low amplitude and high amplitude as well as without ultrasonic vibration were studied.Optical microscope(OM),scanning electron microscope(SEM)and electron backscattering diffraction(EBSD)were used to characterize the tissue.The mechanical properties were tested by microhardness tester and universal mechanical testing machine.When the ultrasonic frequency was 20.2 kHz,clear Chladni pattern appears on the surface of the substrate.The ultrasonic amplitude field is symmetrical and centered,and the maximum amplitude of standing wave was 76 µm.After 20.2 kHz ultrasonic frequency was input into the substrate with the weight of the preset titanium alloy spherical powder of 40 g,the waveforms obtained were in agreement with the simulation results.When the thickness of the specimen was 15～20 mm,the distribution of surface standing waves was basically unchanged.The average width of columnar crystals without ul-trasonic and with low amplitude specimen were 0.95 and 0.58 mm,respectively.The average size of columnar crystals in the region without ultrasonic vibration was obviously different from that in the region with high amplitude.OM results showed that β columnar crystals with a length of a few millimeters and a width of about 0.7 mm were found in the samples without ultrasonic vibration,which crossed multiple layers.There were equiaxed β grains between 100 and 400 µm in the samples with high amplitude,and dense sub-grain boundaries appeared in the microstructure,which significantly improved the microstructure uniformity of β grains.SEM results showed that there were significant differences in the distribution of flake α length between samples without ultrasonic vibration and those in the high amplitude region.The distribution proportion of 0.3～0.7 µm flake α slats increased by 15.5％,and 0.8～1.2,1.3～1.7 and 1.8～2.2 μm flake α slats decreased by 5.4％,4％and 5.2％,respectively.The higher the amplitude,the shorter the length of α la-mellae was formed.The size of LDM titanium alloy was obviously affected by the ultrasonic amplitude.The results of EBSD showed that α lamellae were partially randomly distributed within the original β grains in the samples without ultrasonic vibration,and the diame-ter of the original β grains in the samples with high amplitude was significantly smaller than that without ultrasonic vibration.By con-trast,it could be determined that the presence of standing wave ultrasonic field could refine α lamellae inside β grain very well,not only refine the original β column grain,but also refine α lamellae significantly,and the adjustment effect on the microstructure was obvious.The microhardness test results showed that the microhardness of the specimen without vibration and low amplitude as well as high amplitude were HV 412,HV 433 and HV 459,respectively.The microhardness of the cladding layer in the two amplitudes was significantly increased by applying ultrasonic vibration.The average microhardness of the specimen in the low-amplitude region and high-amplitude region was increased by 5.3％and 11.4％,respectively.The tensile test results showed that the yield strength(σs)of the specimen without vibration and low amplitude as well as high amplitude were 1044,1046 and 1076 Mpa,respectively.The tensile strength(σb)of the specimen without vibration,low amplitude and high amplitude were 1139,1152 and 1187MPa,respectively.The yield strength and tensile strength of specimens with high amplitude increased by 3.1％and 4.2％,respectively,compared without ul-trasonic application.With the increase of the amplitude,the yield strength and tensile strength of the formed parts increased slightly,and the elongation after fracture decreased,and the properties of the formed parts improved.When the ultrasonic wave propagated in the square plate,the surface amplitude not only presented gradient distribution,but also the extreme amplitude was distributed with centrosymmetric feature.After laser deposition of TC4 titanium alloy by in-situ high amplitude ultrasonic processing,equiaxed β zone and dense subgrain boundary were produced.The results of sound field simulation and EBSD analysis showed that the solidification condition could be improved by enhanced nucleation in the region treated by high intensity.The results showed that the in-situ high am-plitude ultrasonic vibration treatment could promote the equiaxed microstructure transformation and grain refinement of the additive,which was beneficial to broaden the application field of ultrasonic vibration in laser additive.
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