Study On Mechanical Properties Of Microstructure And Heat Treatment Process Based On TC4 Laser Cladding

TC4 dual-phase titanium alloy is prone toβ-phase transformation and ordering transformation near the phase transition temperature.During the laser cladding process,the complex transformation of the structure will occur due to multiple thermal cycles.The temperature gradient and rapid solidification are prone to the accumulation of thermal stress,which can easily cause cracks in the formed parts.In view of the uneven mechanical properties and stress concentration of the above-mentioned TC4 titanium alloy laser cladding structure,this paper uses Gleeble thermal simulation tester and a vacuum heat treatment muffle furnace to reheat the multi-layer and multi-channel laser cladding samples.The effects of the peak temperature,cooling rate,heating temperature and holding time during the reheating thermal cycle on the microstructure and mechanical properties of the samples were studied in the heat treatment experiment,.After preliminary process exploration,the process parameters of multi-layer multi-channel laser cladding are laser power 1700W,powder feeding rate 3.231g/min,scanning speed 200mm/min,and lap rate45%.The microstructure and morphology of the prepared as-welded cladding samples can be divided into coarse formerβcolumnar crystals in the solidification zone,equiaxed crystals in the fusion zone and fine grains in the heat-affected zone.The basket structure and the residualβphase formed orthogonally by the bulk phase and the finer secondary acicularα′martensite phase,in addition to the widmandelweiss structure in the fusion zone（formed by the differently orientedαclusters）and A small number of alpha bundles.The results of the reheat thermal cycle experiment show that under the condition of rapid heating thermal cycle,when the peak temperature is lower than 960℃,the internal structure of the material is coarsened,and the aspect ratio of the primary phaseα_p increases with the increase of temperature.When the peak temperature reaches 1100°C,the allotropic transformation and ordering transformation of the metastableβphase at the grain boundary occurs,resulting in the formation of Ti₃Al ordered phase,the reduction of the primary phaseα_p aspect ratio,the refinement of the structure,the hardness and the crack sensitivity increased significantly.As the cooling rate increases,the phase transformation type changes from metastableβ-phase allotropy to martensitic type,the primary phaseα_p morphology changes from lamellar to needle-like,and the material hardness gradually tends to be stable.However,when the cooling rate is higher than 12℃/s,the hardness of the material increases slightly due to the formation of Ti3Al,and the brittleness of the material increases which measured by the shear test.The results of heat treatment experiments show that the segregation behavior ofβphase is obviously improved and the structure ofβ-grain basket is coarsened with the increase of holding time.Otherwise,the primary phaseαat the grain boundary is discontinuously distributed.The interlamellar spacing of primary phaseα_p which is in the shape of lath increases and the equiaxed grain size of the fusion zone and theαthickness distributed along the grain boundaries also increased.Besides,the matrix structure changed from deformed grains to dual-state structures.As the heat treatment temperature was increased to 1060°C,the layer-band structure of the cladding layer was completely eliminated,the columnarβ-crystals were transformed into larger equiaxed formerβ-grains,and a large number ofα-clusters of laths with different orientations appeared in the grains（widmandelweiss）.The heat treatment temperature has a significant effect on the residual stress which continues to decrease with the increase of temperature.The influence of holding time on stress is taken as the dividing point at 960°C（transition temperature ofα+βdual-phase region）.When heat treatment temperature is below 960°C,the residual stress decreases as the increase of holding time,but the conclusion is reversed when the temperature is higher than 960°C.At the same conditions of temperature,the hardness value perpendicular to the laser scanning direction decreased to varying degrees with the increase of holding time.Under the same holding time,the hardness value perpendicular to the laser scanning direction increased first and then decreased with the increase of heat treatment temperature,and the hardness value decreased by 15%at 1060℃.Under the same holding time,the hardness of the sample decreased with the increase of the heat treatment temperature,but the hardness of the sample was higher than that of the welded cladding layer when the temperature was 860°C and 960°C for 1h.On the contrary,when the heat preservation time is 3h at 1060°C,it is all lower than the welded cladding layer.The strength of the material is seriously lost when the heat preservation temperature is 860℃and 960℃.As the heat treatment temperature is increased to 1060℃,the strength and plasticity of the material are recovered.