Study On Fracture Toughness Of Laser- Welded Joints For Aviation Light Alloy Sheets

Main contents of this research are fracture toughness of Al-Li alloy 1420 and Ti alloy BT20 and their laser-welded joints. The tear test of the 1420 was based on the ASTM B 871-01, "Standard test method for tear testing of aluminum alloy products". The test of BT20 was based on the ASTM E1820-01, "Standard Test Method for Measurement of Fracture Toughness"and British Standard Draft BS7448-1997. At the same time, experiment result is analyzed according to ASTM E1152, "Test method for J-R curve confirmation". Meanwhile, hardness distribution and microstructure of the alloy and welded joints were investigated. It is indicated from the tear test of the 1420 that energy at crack initiation was higher than that at crack propagation. Moreover, the energy at both crack initiation and propagation of base metal was higher than that of weld metal and heat affected zone. The energy at both crack initiation and propagation of base metal with L-T direction was higher than that with T-L direction. Fractography showed that there was long and deep tearing grooves on the fractography of the base metal, especially with the T-L direction. However, there was inter-granular or inter-subgranular brittle feature mainly on weld metal and heat affected zone. Transition from the higher toughness of the base metal to the lower toughness of the weld metal and heat affected zone is because of the change of fracture model. The fracture model was changed from ductile void coalescence fracture to brittle inter-granular fracture. Fracture test of the BT20 indicates that brittle unstable fracture occurs after ductile crack initiation and slow propagation for all the compact tension (CT) specimens. Fracture toughness of base metal is obviously higher than that of welded joint, and rolling directions have no obvious effect on fracture toughness of base metal. Fracture Toughness of heat-affected zone is between the base metal and weld metal. Postweld heat treatment not only can not improve the fracture toughness of welded joint, but also has the trend of deterioration. Addition of activating agent has no obvious effect on fracture toughness of weld metal.