Fatigue Damage Evolution Behavior of SLM TC4 with Different Annealing Temperatures AITranslate

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Basic Information:

DOI：10.13373/j.cnki.cjrm.XY23100034

Chinese Library Classification Number:TG146

Citation Information:

The mechanical properties and fatigue damage evolution behaviour of SLM TC4 alloy at three annealing temperatures were investigated using a MTS fatigue tester，microhardness tester，optical microscope （OM），and field emission scanning electron microscope （SEM）based on the digital image correlation （DIC）method using the TC4 shaped by laser selective melting （SLM）as the research object. The annealing temperatures were noted as AT1，AT2，and AT3，and the heat-treated specimens were ground and polished，then corroded with Kroll reagent （H₂O∶HNO₃∶HF=50∶1∶2，volume ratio）for 30 s. The results showed that the needle-like α′ phase gradually disappeared and transformed into α+β phase with the increase of annealing temperature，and the α phase became coarser with the increase of β phase content. The grain width of α′ phase in the precipitated state was 1.05 μm，and the grain width of α phase in the three groups of annealing modes （AT1，AT2，and AT3）was 1.5，2.8，and 3.6 μm，respectively. The hardness was gradually reduced. The appearance of acicular martensite α′ phase led to the highest hardness in the as-deposited state. As the annealing temperature increased，the α phase changed from finely-striated to lath-like，and the volume fraction of the relatively softer β phase increased with the temperature，resulting in the lowest hardness of the specimens in the AT3 annealing mode. The strengths of the as-deposited state and the three groups of annealing modes were 1121，1026，935，and 810 MPa，respectively，with the strength decreasing and plasticity increasing，but the coarsening of the α phase in the AT3 mode led to the large dislocation stresses，and the plasticity decreased rather than increased. The fatigue properties increased and then decreased. The presence of shallow and sparse tough nests around the disintegration steps in the sedimentary state fracture indicated that the sedimentary state specimen exhibited both brittle fracture and ductile fracture characteristics. After different annealing treatments （AT1 and AT2），the tough nests became deeper and denser，indicating that the fracture mechanisms were both ductile. In the AT3 annealing mode，the elongation of the specimens decreased，the fracture flattened，and the toughness nests became shallower. These ligament foci were formed by the gradual aggregation of microscopic pores within the slipped material during stretching，which led to an improvement in the plasticity of the annealed specimens，in agreement with the results of the mechanical property tests. Fatigue tests were conducted at a single stress level （35% of yield strength），and the changes in surface cloud strain during fatigue were captured using DIC. In the early stages of fatigue，the surface plastic deformation of the specimens was non-uniform and small. In the late stage of fatigue，all the annealed specimens of SLM TC4 alloy showed obvious large deformation zones，and the large deformation zones appeared on the near-side surfaces of the specimens. It was due to the uneven melting of the tissues on the near-side surfaces during the SLM forming process，and the pores and defects on the near-side surfaces of the internal alloy still played a dominant role in the fatigue damage process. In order to quantitatively study the fatigue damage evolution behaviour of SLM TC4 alloy at different annealing temperatures，and to quantify the degree of surface in homogeneity of the alloy specimen，the average strain factor was introduced as a damage parameter，and the average strain factor of the specimen fluctuated slightly up and down with the increase in the number of cycles in the first and middle stages of fatigue，resulting in very small fatigue damage； in the late stage of fatigue, a stage in which the average strain factor increased exponentially, the specimen entered into the rapid fatigue damage stage, and fatigue damage occurred within a very short number of cycles. Damage factor D was derived from the average strain factor，which was used to quantitatively describe the evolution of fatigue damage in the alloy at three annealing temperatures，namely，AT1，AT2，and AT3. The fatigue evolution of the specimens at different annealing temperatures followed the same trend，and the damage factor D increased slowly at the early stage with the increase of the number of cycles，and then started to increase rapidly after reaching the critical value. According to the theory of damage mechanics，the Chaboche damage model was used to fit the damage factor D，and the fatigue damage evolution equation was established to obtain the critical damage factor D_c. As the annealing temperature increased，D entered the rapid damage stage at an early stage，and D_c decreased gradually，and the best fatigue performance was obtained in the AT1 annealing mode.