Improvement of intermetallic compounds of TC4/QP980 joints using a flexible dual-beam laser heat source

Abstract

In this paper, the well-performed QP980/TC4 lap joint without Fe-Ti intermetallic compounds was successfully obtained by using coaxial pulse-continuous dual-beam laser welding heat source and CuSi3 filler wire. The interface microstructure, mechanical properties and the effect of dual-beam laser power density on the joint were systematically studied. The mechanical properties of the joints increased first and then decreased with the increase of laser power density. The maximum tensile peak tensile load value reached 3820 N/cm and the average measured thickness of the reaction layer at the TC4 interface was 129.7 μm under the central power density of 1480 W/mm² and the surrounding power density of 80 W/mm². The joints were all fractured at the TC4 interface, which was cleavage fracture. The microstructure of TC4 interface was (Cu)ss/Ti₅Si₃/TiCu/Ti₂Cu + TiCu/α + β-Ti. On the base of experiments and thermodynamic calculations of TC4 interface, the chemical potential of Ti and Cu is small when the molar concentration of Si is high, so Ti and Cu tend to transfer to the Si-rich region. It can inhibit the thickness of intermetallic compounds layer at the TC4 interface. However, the hardness of Ti₅Si₃ and TiCu were so high that stress concentration occurred to cause large the residual stress in joints. The fracture behavior of TC4 interface was further analyzed, and the hard brittle phase Ti₅Si₃ might become the crack source. Ti₅Si₃ (001)/TiCu (111) and Ti₅Si₃ (001)/Cu (111) possible matching planes were non-coherent, which might become the crack propagation path.