Microstructure investigation and fracture mechanism of TC4−304L dissimilar joints fabricated by the cold metal transfer arc-brazing method

Abstract

Wire feeding speeds of 3.5, 4.5, and 5.5 m/min are operated to arc-braze TC4 titanium alloy to 304 L stainless steel with the cold metal transfer method and CuNi filler wire. The microstructure and fracture behavior of TC4−304Ldissimilar joints were characterized by transition electron microscopy (TEM), selected area electron diffraction (SAED) patterns and X-ray microscope (XRM). The results show that the TC4/seam transition zone consists of an inner Ti-rich layer and an outer Ti-poor layer based on the line scanning result, which contains complex eutectic microstructures such as (Cu, Ni)Ti3, CuNiTi2, CuNiTi, (CuxNi1-x)Ti, and (Cu) solid solutions. The microstructure of the 304 L/seam interface consists of CuNi2Ti, (Cu, Ni) and (Fe, Ni) solid solutions. With increasing heat input, the thickness of the TC4/seam transition zone and the size of (Fe, Ni) dendrite grains increase, the ultimate tensile strength decreases from 350.8 to 230.5 MPa, and all samples fracture near TC4 side based on tensile samples and XRM results. The crack propagation is due to both the thickness of TC4/seam transition zone and a large amount of brittle CuNiTi equiaxial dendrite grains with a high hardness of 10.4 GPa.