Process,Interface Regulation Of Titanium Alloy/aluminum Alloy Dissimilar Joint By Using Laser-MIG Hybrid Welding-brazing

Ti/Al hybrid structures have potential in the aerospace and automobile industries because of their low cost and light weight.However,titanium and aluminum alloys are difficult to weld by conventional fusion welding because of the large differences in their thermal-physical properties and poor metallurgical compatibility.In particular,brittle Ti/Al interfacial intermetallic compound?IMC?layer in the interface deteriorate the joint mechanical properties significantly.In this dissertation,aluminum alloy AA6061and titanium alloy Ti-6Al-4V plates with 3 mm thickness were joined using laser-metal inert gas?MIG?hybrid welding-brazing.The uniformity of the IMC layer and the weld formation were improved by adjusting the process parameters and the space positions of heat source,and the Ti/Al dissimilar joints with excellent mechanical properties were obtained.A numerical simulation was conducted with abaqus software to calculate the temperature fields and stress fields of Ti/Al dissimilar joint during welding-brazing process,the temperature distribution along the interface was obtained.The results indicate that the peak temperature of interface decreased with an increase of offset,but there was still a large temperature gradient inside the weld seam.An uniform temperature field which the temperature isotherm was almost parallel with the butt plane was obtained by using a laser-deviation angle at a 1.2 mm laser-arc offset.The effect of the welding parameters of laser-MIG hybrid method on weld formation was studied using single factor method.The results indicate both excessive offset and inadequate heat input were unfavorable to the improvement in rear formation because of the poor heat condition at the root region of Ti-6Al-4V.When the ratio of wire feeding speed to welding speed was 3.9⁴.5 with constant laser power and offset,welded-brazed joints with excellent wetting and spreading on both sides could be obtained.The IMC layer microstructure was observed by scanning electron microscopy?SEM?,the elemental content was investigated by energy dispersive X-ray spectrometry?EDS?.The results show only a single layer which was mainly composed of TiAl₃ was formed at the butt plane when the titanium alloy was dissolved.Under the condition that the titanium alloy was melted,three different layers with many voids and cracks could be distinguished along the butt plane,which consisted of Ti₃Al,TiAl and TiAl₃.A uniform serrated TiAl₃ 1?m thick layer without obvious defect was obtained by adjusting the process parameters and the space positions of heat source.The formation and growth mechanism of IMC layer was established.The tensile test of 6061-T6/Ti-6Al-4V welded-brazed joints was tested,the results show the tensile strength of the joint without reinforcement was slightly less than that of joints with reinforcement.Both excessive and insufficient interfacial reaction were unfavorable to the improvement in joint strength.Joints without reinforcement achieved a maximum tensile strength of 230 MPa?80%of the 6061-T6 tensile strength?at a laser-arc offset of 1.2 mm and a laser-deviation angle of 5°.Three different fracture phenomena were observed during the tensile test that correspond to different fracture positions at the interface layer,weld seam and heat-affected zone of the aluminum base metal,and the fractured mechanism of joints was analyzed.