Surface Layer Plastic Deformation And Microstructural Evolution Of The Q235 Steel And Directionally Solidified Titanium Aluminum Alloy Billet

The mechanical property is determined by its microstructure. Fine grains possess good plasticity, toughness and fatigue property. However, large grains possess premium high temperature creep and anti-fracture property. Therefore, the rectangular billet after surface plastic deformation and subsequent heat treatment, which results in fine grains on the surface and coarse column grains in the center, will meet the demands on high temperature creep, fracture toughness and fatigue strength. This thesis is presented on the idea.Based on this idea, four different surface plastic deformation techniques including: compound rolling, ball burnishing, surface milling and controlled shot peening, which will localize the plastic deformation on the surface of the billet and leave little plastic deformation in the center, were presented in this study. Based on the local load theory, equipments for the plastic deformation on the surface of the rectangular billet have been developed. The surface plastic deformation behavior of the Q235 and the directionally solidified (DS) TiAl billet has been studied using these equipments. The metal flow behaviors after four plastic deformation techniques were studied using the commercial finite element method (FEM) and simulated experiments. The stress-strain distribution along the depth of the as-deformed billet was also obtained. What's more, the microstrucutural evolution of the Q235 and the directionally solidified TiAl billet were investigated after cold deformation and subsequent heat treatment.These four surface plastic deformation processes on the directionally solidified TiAl were simulated with the FEM software ABAQUS-6.7. The results indicate that the plastic deformation was localized within the surface of 1mm and the maximum equivalent strains were 0.2,0.22,0.032 and 0.25, respectively. The strain type distribution of four surface plastic deformation techniques by quantitative and qualitative analysis of elasticity and plasticity was obtained. The strain distribution on the surface of the billet after compound rolling is subject to plane strain; strain distribution along the depth of the billet after ball burnishing and surface milling changes from tension to compression; while, strain distribution along the depth of the billet after controlled shot peening changes from compression to tension.The microstructure after the surface plastic deformation was observed, which indicate that the Q235 grains on the surface of the billet turn into fiber-like line after the compound rolling, ball burnishing, surface milling and controlled shot peening. The compound microstructures that are fine grains on the surface and the coarse grains in the center after the subsequent recrystallization annealing are obtained as for the compound rolling, ball burnishing and surface milling. But for the grains on the surface of the shot peened sample with recrystallization annealing grow coarser because of critical deformation zone.The coarse-column grains of the directionally solidified TiAl on the surface of the billet were recrystallized, while the microstrcuture keeps the original state after the compound rolling. The fully lamellar structures occurs strong kinking after ball burnishing, surface milling and controlled shot peening. The cold deformed billets were subject to vacuum heat treatment aboveαtransus line of 10~20oC for several minutes. The results indicate that the microstructures are characterized by microstructual gradient,namely fine recrystallized grains on the surface and the original directionally solidified microstructures in the center were obtained after ball burnishing and surface milling. The grains on the surface of the shot peened sample with subsequent heat treatment grow coarser because of critical deformation zone.