Research On Technology Of TIG Arc Welding Brazing And Interfacial Characteristic Of Ti/al Dissimilar Alloys

Titanium reacts with aluminum to form brittle intermetallic compounds (IMCs)due to their incompatible metallurgical weldability. So joining of Ti/Al hybridstructures has always been one of the hot research topics for welding workers.Nowadays, arc welding-brazing, which has advantages on equipment costs andapplied scopes, is gradually concerned. However, two major problems, including thepoor spreadability of filler metal on Ti substrates with TIG heating and formation ofvast IMCs at the interface, retard its research and application on joining of Ti/Alhybrid structures. Therefore, illustration on the filler’s spreadability on the Tisurfaces with arc heating and the growth of interfacial reaction layer is the most keyfactor to obtain a sound Ti/Al welding-brazed joint.This paper has developed systemic research on butt joining of Ti/Al dissimilaralloys with arc welding-brazing technique. Firstly, a novel technique, referred to asthe keyhole arc welding-brazing (K-AWB), was presented based on the analysis oftemperature field by the finite element methods and filler’s spreading experiments.And the temperature distribution of Ti/Al butt joints and spreadability of fillers onTi surfaces were clarified. A good weld forming of joints was achieved by adjustingthe heat input, heating location and filler patterns. Then, microstructures ofwelding-brazed joints were observed and the mechanical properties of the jointswere evaluated by tensile and hardness tests. Based on the analysis ofmicrostructures and fracture characteristics, the failure mechanism of the joints andits impact factors were revealed. At last, the research was focused on the growthregulation of Ti/Al interfacial reaction layer, and thermodynamical analysis, kineticmethod and experimental research were conducted.The results showed that the weld pool exhibited dynamic circular stages,including elliptic pool, breached pool, keyhole pool and elliptic pool, when a lowwelding speed and pulsing feeding of fillers were adopted. The liquid filler spreadwell on all the joint surfaces, single face welding-double face forming of Ti/Al buttjoints were easily achieved by the K-AWB technique. A serrated layer with athickness of4-6μm was observed at the interface with pure Al filler and theinterfacial structure is Ti(Al)+Ti3Al+TiAl3multilayers, which uniformly distributedat the joints’ thickness direction. With adding of Al-Cu-La filler, Ti2Al20La phasewas produced at the interface near the Al side and tensile strength of the jointreached270MPa. Investigation on thermodynamical and kinetic characteristicsconcluded that TiAl3phase first formed, compared to the other Ti-Al IMCs and the reaction to form TiAl3only occurred at the cooling stage of the welding brazingprocess. The growth of layer included spreading and absorption of liquid filler on Tisubstrate, heterogeneous diffusion of Ti and Al, phase transformation to form Ti3Aland nucleation and growth of TiAl3layer. The thicknesses and morphologies ofinterfacial layers varied dramatically with the increase of heat input. Nucleation ofTiAl3was boosted and a relax-shaped TiAl3layer was observed with the help of La,Zr and B elements. The Ti2Al20La phase formed along the TiAl3interface, while the(Ti,Zr)Al3grew up on the base of TiAl3. TiB2and AlB12particles were detected inthe interfacial layer with B element added. With the adding of Si element in thefiller metal, though the thickness of the layer was increased, the interfacial structurewas also changed and a good bonding was obtained.