Effect of Single Annealing and Multiple Heat Treatment on Fracture Toughness of Ti-5321 Alloy Prepared by Laser Cladding Forming
In order to satisfy the requirements of aerospace field for rapid preparation of high strength and toughness complex titanium alloy parts,new Ti-5321(Ti-5Al-3Mo-3V-2Zr-2Cr-1Nb-1Fe)alloy with high strength and high toughness was prepared by laser cladding forming,which possesses the superiority of rapid prototyping,high efficiency,and good formability.Through single annealing and multiple heat treatment(β-annealing with slow cooling and aging,BASCA)on Ti-5321 alloy,the microstructure evolution was revealed,and the influence of different microstructures on the fracture toughness was explored.Results show that after single annealing,the alloy morphology presents the basket-weave structure consisting of elongated lamellar α phases.Its ultimate tensile strength is 1102 MPa,and fracture toughness is 68.1 MPa·m1/2.After BASCA heat treatment,the elongated lamellar α phase changes to coarse lamellar α phase and ultrafine needle-like α phase.Thus,the ultimate tensile strength increases to 1309 MPa,whereas the fracture toughness reduces to 45.5 MPa·m1/2.BASCA heat treatment can enhance the strength but degrade the toughness of alloys.This is because the elongated lamellar α phase in basket-weave structure can greatly increase the crack growth resistance and aggravate the tortuous degree of crack growth path,thus improving the alloy toughness.Coarse lamellar α phase after BASCA heat treatment has a certain degree of directionality,and the cracks only deflect when passing through the coarse lamellar α phase of different β grains.Crack propagation mainly occurs in the ultrafine needle-like α phase.However,due to the extremely small size of ultrafine needle-like α phase,it cannot hinder the development of crack path or deflect the cracks.Thus,the toughness of coarse lamellar structure becomes more inferior after BASCA heat treatment.