Study On Microstructure And Non-destructive Testing Of Residual Stress Of Titanium Alloys By Additive Manufacturing

Laser Melting Deposition(LMD)is widely used additive manufacturing technology which has high forming efficiency,high powder utilization rate and can be designed for composition gradient.It is possible to realize the integrated forming of complex structural parts,due to it overcomes the complexity of traditional processing and long production cycle.It already has application for aerospace,biomedical and automotive manufacturing.The interaction between the laser and the material lead to residual stress in the process of LMD that makes deformation or cracking of the molded component hindering the wider application of this technology seriously.In this paper,mechanical model of residual stress characterization under complex stress state based on acoustic elastic theory was established with laser ultrasonic non-destructive testing technology.Combined with the microstructure of as-deposited sample,the macroscopic residual stress state of the laser-melted Ti-6Al-4V titanium alloy sample and the acoustic characteristics of the as-deposited sample was studied.The main conclusions are shown as follows:(1)The macrostructure of Ti-6Al-4V as-deposited is layered structure of ? equiaxed in lower laser power.After stress-relief annealing,the layered structure is significantly reduced.The microstructure of Ti-6Al-4V as-deposited consists of large number of ?-lamellae and relatively small amount of ? phase.The ? grain boundaries consists of continuous primary ?'phase.The orthogonal acicular ?' martensitic phase and secondary phase ?' growthing along the grain boundary exist in grain inside.The powder,as-deposited and annealed sample phase structures did not change significantly.The micro-hardness in as-deposited and annealed states is around 600 HV,which is much higher than substrate.(2)A laser ultrasonic nondestructive testing system was established for the thermoelastic mechanism.The surface acoustic wave with high signal-to-noise ratio is in accordance with the requirements for non-destructive testing which excited by a pulsed laser with 580 V and 20 Hz on material surface.The accurate surface acoustic wave propagation distance was accurately calculated using a high-accuracy motorized translation stage,and the surface acoustic wave propagation time was accurately measured by a contact ultrasonic probe and a digital oscilloscope.Then,the surface acoustic wave velocity of the titanium alloy in the unstressed state is 3334.48m/s that accurately calculated by means of linear fitting.(3)The acoustic elastic constant of the titanium alloy is AR(1)=-7.26359×10-4 MPa-1,which was successfully calibrated by stress loading the sample based on the accurate surface acoustic wave velocity.Combining the surface acoustic wave elastic elastic equation under uniaxial stress,the residual stress distribution of laser-melted as-deposited samples was calculated.