DUT researchers reach 360 MPa shear strength in TC4 and 316L joints in the lab

Researchers from Dalian University of Technology have developed an amorphous ribbon filler doped with hafnium to vacuum-braze Ti-6Al-4V (TC4) titanium alloy directly to SS316L stainless steel, reporting shear strengths up to 360 ± 12 MPa in laboratory tests, according to a study published in China Welding (中国焊接：英文版) sciencedirect.com中国焊接：英文版sciencedirect.comOpen source ↗. Joining titanium to austenitic stainless steel typically suffers from the formation of brittle intermetallic compounds, specifically TiFe and TiFe₂, which promote premature cracking under mechanical stress.

To control phase growth at the interface, the team synthesized a Ti36.27-Zr12.50-Cu37.48-Ni12.50-Hf1.25 (at.%) amorphous alloy ribbon using melt-spinning. The researchers tested a vacuum brazing parameter matrix spanning temperatures from 910 °C to 950 °C and dwell times between 5 and 40 minutes. The peak shear strength of 360 ± 12 MPa was recorded at 910 °C with a 40-minute hold, while a 10-minute hold at the same temperature yielded 353 ± 9 MPa. Increasing the brazing temperature above 930 °C led to the rapid growth of coarse intermetallic layers, cutting joint shear strength by up to 50%.

Interfacial microstructure and kinetics

Microstructural examination revealed that the brazed seam divides into three distinct zones: an (α + β)-Ti diffusion layer containing fine α-Ti lamellae adjacent to the TC4 base, a central eutectic and intermetallic seam, and an Fe-Cr-rich reaction layer contacting the SS316L interface. The authors reported that hafnium and zirconium remained concentrated within the central seam, suppressing grain coarsening, while mutual diffusion of titanium, copper, and iron formed transitional gradients.

Kinetic calculations confirmed that layer growth follows parabolic time dependence and Arrhenius temperature behavior sciencedirect.com中国焊接：英文版sciencedirect.comOpen source ↗. The activation energy for diffusion was 261 kJ/mol on the TC4 side and 149.6 kJ/mol on the SS316L side, showing that titanium diffusion governs the overall interdiffusion process.

Failure modes and phase effects

Microhardness mapping identified Ti₂Cu and TiFe compounds as the primary strengthening constituents in the joint seam, though their intrinsic brittleness simultaneously acted as the main source of embrittlement. Fractographic analysis demonstrated brittle intermetallic cleavage on the SS316L side, contrasting with a mixed ductile-brittle fracture mode on the TC4 side. The findings define a narrow processing window centered near 910 °C to balance atomic interdiffusion against excessive intermetallic growth in dissimilar titanium-steel assemblies.