Study On Laser Penetration Welding-brazing Of Al/ti Dissimilar Alloys

The composite structure of Aluminum-Titanium dissimilar alloys is widely used in the aerospace and automobile industries for its performance advantage which combines the two kinds of materials. However, the base materials, showing the big difference in physical, chemical and metallurgical properties, influence the welding performance between them greatly. Brittle intermetallic compounds formed in the joint is one of the biggest challenges restricting the widely application.In this dissertation, laser penetration brazing (LPB) was used in joining aluminum and titanium. The laser acts on the surface of Ti alloy. Al alloy is melted by heat conduction from the middle solid Ti layer. The melted Al wets the interface of Ti to form a brazing joint. This method has been effectively applied to avoid forming brittle intermetallic compounds in the joint.Aluminum alloy 6061and titanium alloy TC4sheets with the thickness of 3mm, respectively, were welded using YLS-6000 Ytterbium-doped Multimode Fiber Laser in this experiment. The regularity of joint formation, influenced by processing parameters, is studied. The microstructure in the joint and the phase composition in the interfacial are also studied using Optical Microscopy, SEM, EDS and XRD. Joint mechanical property was measured by tensile test.The experimental result shows that LPB method is a reliable process to realize the connection of high-strength Al-Ti dissimilar alloys. The quality of joint is good. The analysis to the joint indicates that the top and bottom of solid Ti layer in the interface are both melted in different degrees, which could be effectively avoided by adjusting the laser beam offset and welding speed. The microstructure of the seam presents the raphides and layer intermetallic. The thickness of the intermetallic interface is bigger at the top and bottom with the maximum of less than 5μm. The welding process parameter greatly affacted the thickness of intermetallic layers, but not the phase formation. The main phase is Al₂Ti, Al₃Ti, AlTi₃ and AlTi. The fracture takes place at the joint interface in the tensile test. The maximum tensile strength of the joint is 199 MPa. The joint can be classified according to the state of middle Ti layer into sweated joint with cleavage fracture, welding-brazing joint with quasi-cleavage fracture. The top and bottom part of the interface with the thick intermetallic layer are the main location for the fracture occurrence.