Research On Technology Of High-frequency Induction Brazing And Interfacial Characteristic Of Ti/Al Dissimilar Alloys

Large amounts of brittle Intermetallic compounds (IMCs) were formed duringwelding of Ti/Al dissimilar alloys due to the large difference among their physical andchemical properties. Excess brittle intermetallic compounds would degrade themechanical properties of the joints. So quality joining of Ti and Al dissimilar metals hasalways attracted the attention of researchers of the welding field. High-frequencyinduction brazing method was proper to join dissimilar alloys because of its effectiveheating character. Due to poor wettability of Al filler on Ti substrate, a method ofpreparation of an aluminized coating on Ti surface was proposed in order to enhance thewettability and spread ability of the filler and control the growth of IMCs.This paper systematically studied on the joining process and interfacialcharacteristics of joints between Ti/Al dissimilar alloys by induction brazing method.Firstly, induction depositing and hot-dip aluminizing methods were compared to getreasonable hot-dip procedures. Metallurgical bonding was achieved between the coatingand Ti substrate with the assistance of flux during hot-dip aluminizing process. Thewetting process of liquid Al on solid Ti during hot-dip aluminizing was analyzed. Theinterfacial characteristic was focused and the reaction layer forming rules was analyzedthrough thermodynamic calculation.Then, we studied the interfacial characteristics and microstructure of the joints ofdifferent brazing parameters to clearly understand the influence of the aluminizedcoating on the microstructure of the joints and growth of interfacial layers.The results showed that uniform aluminized coating was obtained through hot-dipaluminizing method with the assistance of flux. Metallurgical bonding between thecoating and Ti substrate was achieved. The interfacial layer was composed of TiAl3with less than1μm thickness, which was the prior product of the interaction betweensolid Ti and liquid Al. Microstructure of the brazed joints was composed of thepro-eutectic-Al phase, amount of small needle-like Al-Si eutectic phase and lots ofsmall Si particles. The microstructure of pro-eutectic-Al phase and needle-like Al-Sieutectic phase distributed nonuniformly.When joining Al to un-coated Ti, the interfacial layer exhibited thin layer,needle-like, rod-shaped and serrated structure. However, the interfacial layer showed uniform thin layer and small needle-like structure after Ti was pre-aluminized. With theinhibition of lamellar Ti(Al,Si)3and needle-like Ti7Al5Si12dual-phase structure, growthof the interfacial layer was suppressed. As a result, thickness of the interfacial layerdecreased. Fracture of joints without the aluminized coating occurred at both theinterface between the reaction layer and the brazing seam, and within the brazing seam,which exhibited brittle characteristic. While joints with the coating fractured within thebrazing seam, showing that bonding between the reaction layer and the brazing seamwas enhanced. This was attributed to the lamellar Ti(Al,Si)3and needle-like Ti7Al5Si12dual-phase structure, which could inhibit crack propagation.