Microstructure And Propertie Of TiBw/TC4 Composite Sheets Joined By Gas Tungsten Arc Welding

In this thesis, gas tungsten arc welding is performed on 3vol.%, 5vol.%, 8vol.%Ti Bw/TC4 composites sheets with a network microstructure fabricated by powder metallurgy to study the weldability, as well as the microstructure and mechanical property evolution of the weldment. Microstructures of the parent composite(PC), heat affected zone(HAZ) and fusion zone(FZ) were observed by optical microscope, scanning electron microscope. Mechanical properties were characterized by Vickers microhardness test and room temperature tensile test. The microstructures and the mechanical properties of the weldments proved the weldability of this composite. Welding parameters were optimized for successfully joining composite sheets with different thickness(2mm, 3mm, 4mm).The macroscopic analysis of the weldments indicates that the increase of the welding current(I) and the decrease of the welding speed(V) both increase the heat input during the welding, which as a result broaden and deepen the surface depression of FZ. With the increase of the sheets thickness, the heat input increased to achieve successful joining. For the sheets with 2mm, 3mm and 4mm thickness, the weldments with fine macroscopic morphology can be acquired when I was in the range of 90A~120A, 150A~180A, 210A~220A, and V was in the range of 150mm/min~110mm/min, 120mm/min~100mm/min, 120mm/min~115mm/min, respectively.The microstructure observation of the weldments shows that the heat affected zone was divided into two parts: non-phase transformed HAZ consisted of coarsened α phase and β phase, and phase transformed HAZ consisted of transformed β microstructure(acicular and lath α′ martensite and residual β phase). FZ was consisted of coarsened columnar grains which were formed by epitaxial crystal growth. Ti B whiskers were refined after solidification and the aspect ratio of them increased. Ti B whiskers tended to distribute on the boundaries of the grains and formed smaller micro networks. With the increase of the heat input, the fraction of coarsened α phase and transformed β microstructure increased in HAZ, and the more grains coarsened in FZ.The room temperature tensile tests showed that the ductility and the tensile strength of the weldments decreased compare with parent composite due to the formation of α′ martensite both in FZ and HAZ. With the increase of the heat input, the tensile strength of the weldments increased. The 3vol.%, 5vol.%, 8vol.% composites sheets weldments could reach the 90%, 91% and 93% tensile strength of that of parent composite.The microstructures and properties characterization of the weldments after different annealing heat treatment shows: the low temperature annealing barely changed the microstructure of the weldments hence could not improve the ductility of the weldments, however could relief the residual stress and improve the tensile strength of the weldments. The microstructures of the matrix all transformed to exquixed α+β phase after high temperature annealing which totally improved the microstructural inhomogeneity of the weldments, however the corsened columnar grains in FZ had a negative influence on the mechanical properties of the weldments. The residual stress was totally relieved after medium temperature annealing, and the α′ martensite in FZ and HAZ transformed to laminated widmanstatten microstructure, which improved the tensile strength of the 3vol.%, 5vol.%, 8vol.% weldments to 92%、93%、95% of that of the PC at most. The ductility of the 3vol.% and 5vol.% weldments could reach the 55% and 68% of that of the PC, therefore the comprehensive mechanical properties of the weldments have been improved.