Microstructures And Low Cycle Fatigue Properties Of Ti-6Al-4V Alloy Fabricated By High Power Laser Solid Forming

In recent years,laser solid forming(LSF)based on powder feeding laser additive manufacturing technology has been shown advantages in the manufacture of aerospace large and complex titanium parts,especially Ti-6Al-4V alloy parts.Although the tensile properties of LSF Ti-6Al-4V alloy can meet the forging standard,the low cycle fatigue property is lower than the forging standard.In particular,most of the LSF techniques currently used low laser power,making them less efficient in the fabrication of large complex titanium structural parts.Recently,with the improvement of the energy output stability of high-power(?4000 W)laser equipment;high-power lasers are used for LSF to improve deposition efficiency in the manufacture of large-scale complex high-performance titanium alloy parts.However,there are still few studies on the microstructure and mechanical properties of high power LSF Ti-6Al-4V alloy.Meanwhile,the microstructure of high-power LSF part is different from that under low power conditions,so that its low-cycle fatigue performance will exhibit different characteristics.Therefore,this paper focuses on the high-power(7600W)LSF Ti-6Al-4V alloy,clarifies its microstructure and evolution behavior,and deeply investigates its corresponding tensile properties and low cycle fatigue properties,fatigue crack growth rate and fracture toughness.The main findings obtained are as follows:(1)The microstructural evolution of high-power LSF Ti-6Al-4V alloy was revealed.The as-deposited microstructure is similar to that under low-power conditions.It is also composed of columnar prior-?grains grew epitaxially from the substrate and?-laths with Widamast?tten or basket-weave distribution in the grains.The columnar prior-?grains width is significantly wider,and the thickness of the intragranular?-laths is also coarser.Based on the columnar to equiaxed transition theory and cooling rate of Ti-6Al-4V alloy on the morphology of intragranular?-laths,a modified microstructure selection map of LSF Ti-6Al-4V alloy was established.(2)The microstructure and tensile properties of the LSF samples at the scanning velocities of 900 and 1500 mm/min were obtained.After solution treatment and aging(STA),the width of the columnar prior-?grains was basically unchanged,and the microstructure in the prior-?grains consisted of basket-weave?-laths and?-transformed microstructure.It was found that the sample obtained at high scanning velocities(1500 mm/min)exhibited more excellent mechanical properties after STA,for example,the elongation was 17.8±0.5%,yield strength was 839.5±11 MPa and tensile strength is 937.8±11 MPa.During the plastic deformation process,the shear band appears along the?/?interface,and the slip band appears inside the?phase.(3)The low cycle fatigue life equation of the LSF Ti-6Al-4V alloy at 900 and 1500mm/min after STA were obtained.The low-cycle fatigue life of high-power LSF Ti-6Al-4V is higher in the medium strain range(from 0.8%to 1.1%)than the current reported Ti-6Al-4V alloy.It is found that the low-cycle fatigue failure process under the cyclic strain load has a cyclic softening phenomenon,and the fatigue crack mainly originates along the?/?interface in the aligned?-laths with preferential arrangement(?45°to the load axis).Meanwhile,the empirical relationship between the strain amplitude and the defect equivalent diameter of the LSF Ti-6Al-4V alloy subjected to LCF loading was established.It was found that when the surface defect diameter is less than?60?m,it has little effect on the low cycle fatigue life.(4)The fatigue crack growth rate of the LSF Ti-6Al-4V alloy samples at scanning velocities of 900 and 1500 mm/min after STA were lower than that of the currently reported AM Ti-6Al-4V alloy.At the low load ratio(R=0.1),the fatigue crack growth rate curve shows a turning point phenomenon in the Pairs region,while at the high load ratio(R=0.3,0.5 and0.8),there is no obvious turning point phenomenon.The fatigue crack growth rate of the samples at 900 and 1500 mm/min is basically the same in the Paris region.A method for defining the microstructure characteristics of the LSF Ti-6Al-4V alloy with basket-weave microstructure is proposed.The size of the typical feature unit obtained by this definition is the same as the size of the cyclic plastic zone of the crack tip,and the difference is small.(5)The average fracture toughness value of the LSF Ti-6Al-4V alloy at scanning velocities of 900 and 1500 mm/min after STA in the notch direction is perpendicular to and parallel to the deposition direction is 81.3±0.7 MPa m^1/2,which is better than the typical value of forging standard(74.8 MPa m^1/2).Combined with the relationship between the fracture toughness and tensile yield strength of the current Ti-6Al-4V alloy under different AM processes,the general relationship between fracture toughness and tensile yield strength of Ti-6Al-4V alloy was obtained,and the equation was K_Ic=285.25-0.24648?_y....