Investigation On Laser Penetration Brazing Of Dissimilar Alloys

Dissimilar alloys composite structures have wide applications in many industrial fields, because they can provide each alloy's advantage. However, the great differences in physical, chemical properties of dissimilar alloys and elements interaction in molten pool make joining them become a challenging task. It is important for investigations and comprehensions on the technique, mechanism and elements interaction in molten pool during laser welding of dissimilar alloys for the application of dissimilar alloys composite structuresIn this dissertation, for welding dissimilar alloys which differ considerably in melting point and metallurgical properties, an approach, namely laser penetration brazing (LPB), has been applied and investigated. A focused laser beam acts on the lower melting point base metal, which results in the low melting point base metal melted by means of the key-hole effect. The higher melting point base metal, however, still maintains the solid state. A penetration brazing joint is formed through the interaction of the higher melting point base metal and molten lower melting point baser metal.As a representative of variety dissimilar alloys couples, brass-steel, aluminum-copper and aluminum -titanium were used in laser penetration brazing experiments. The formation characteristic, microstructure and mechanical properties of laser penetration brazing joints of variety dissimilar joints were investigated. The heat-transfer process was analyzed by finite element analysis.The formation of the laser penetration joint is mainly influenced by the type of dissimilar alloys couple and the processing parameters. The formation of brass-steel joint is formed well, because the wettability of copper on steel is well and the interaction between Cu and Fe can form only solid solution. In the laser penetration brazing of copper and aluminum, more laser energy is needed for wetting of aluminum on copper, due to the high thermal conductivity of copper. The melting of part copper doesn't bring about the negative effect on the formation of joints due to the eutectic reaction between Cu and Al. The poor wettability of aluminum on titanium may lead to incompleted fusion part in the interface of aluminum-titanium joint. It can be improved by optimizing the processing parameters. The formation of Al-Ti intermetallic compounds has negtative effect on the joints, the melting rate of titanium, therefore, must be controlled. For dissimilar alloys couple which can form only solid solution, such as brass-steel, there is no obvious transition layer along the interface in the joint. However, for dissimilar alloys couples which form eutectic or intermetallic compounds, there is a transition layer along the interface in the joint. In the aluminum-copper joint, a transition layer composed of Al-Cu intermetallic compounds and a (Al)+Al2Cu eutectic. The melted copper exists as the form of Al-Cu intermeallic compounds and a (Al)+Al2Cu eutectic in the weld metal. The microstructure of aluminum-copper joints is different with the amount of the melted copper. The transition layer along the interface in the aluminum-titanium joint is only composed of Al-Ti intermetallic compounds.The amount of molted copper results in the different microstructure of aluminum-copper joint, which influence the mechanical properties. For the joints with excessive amount of copper melted, the microhardness is higher than that of base metal. The maximum tensile strength of joints is 79MPa, and the failure occurred at the weld metal. For the joint with part melting of copper, the microhardness is higher than that of base metal. The maximum tensile strength for tensile test is 100.6 MPa. The failure occurred at aluminum side. The maximum tensile strength of failure in interface samples is 94.5 MPa. The microhardness in the weld metal of the joint with small amount of copper melting is higher than that of aluminum base metal, however, lower than that of copper base metal. The maximum tensile strength of joints is 52MPa. The failure occurred at the copper-weld interface. The maximum tensile strength of aluminum-titanium joints is 210Mpa. The failure occurs at the interface. The incompleted fusion rate in titanium is the main factor of the strength of aluminum-titanium joints. The tensile strength increased with decreasing the incompleted fusion rate. However, the defects in weld metal have negative effect on the tensile strength, such welding velocity lm/min joints and deflect angle more than 9°joints.A moving integration heat source was used in the simulation of temperature field. The temperature field and heat cycling were analyzed via Ansys software. The method can be used as a reference in the simulation of laser penetration brazing.