Research On Microstructures And Mechanical Properties Of Laser Rapid Forming TC21 Titanium Alloy

TC21 alloy, a type of high strength, high toughness and high damage-tolerance α+β two-phase titanium alloy, is one of the candidate materials for the aeroplane frameworks. Traditional processing methods, including precise casting, precise plastic forming, powder metallurgy, etc., have some disadvantages such as long development cycle, poor flexibility, etc. Laser rapid forming (LRF), a new solidification processing technology, has many advantages which would make it competitive for the processing of TC21 alloy.In this thesis, LRF TC21 alloy was studied experimentally. The structures and mechanical properties of as-deposited TC21 alloy were obtained and analyzed, the influences of laser power on the as-deposited structures and mechanical properties were examined, and the changes of the structures and hardnesses after different heat treatments were explored. The main results are as follows:1. Because of epitaxial growth during LRF and the columnar to equiaxed transition (CET) of TC21 alloy, the macrostructures of as-deposited TC21 alloy mostly consist of the bulky original β grains which grow across several deposit layers along the height direction of deposition, and only the grains in the upper side of the last layer are small equiaxed grains. The existence of layer bands in macrocosmic is result of coarsening of a phase in the heat affected zone of the molten pool. The regions, show light in macrocosmic, of the last more than 10 layers at the top and the 1 or 2 layers adjacent to the substrate of the deposit are made up of acicular martensite α' phases, and other deposit regions, which show dark in macrocosmic, consist of basket-weave microstructures. The basket-weaves are transformed from the martensites for the reheat cycle during LRF.2. The hardness of the light region is nearly 100HV higher than that of the dark region. The tensile properties of the dark region revealed that the tensile strength of the as-deposited TC21 alloy is about 1270MPa, the yield strength 1180MPa, but the elongation modulus only 4%. It shows that as-deposited TC21 alloy is high strength, low plasticity and ductile fracture.3. The increase of laser power will cause increase of the temperature of deposit and of the region covered by the same temperature isotherm, but make decrease of the temperature grade, so the height of the light region at the top of the deposit will...