Features of Intermetallic Formation in the Solid Phase on a Steel–Titanium Bimetal Interface under the Conditions of Arc Welding

Abstract

The object of this study is the formation of intermetallic phases (IMPhs) in the heat-affected zone (HAZ) of joints of steel–titanium bimetal plates produced by arc welding. A titanium layer (2 mm) was welded by the plasma method (PAW), a barrier layer of Cusi3Mn1 bronze was deposited on it by the TIG method, the first steel layer was deposited by CMT, and Puls-MAG was used for filling the groove. Here, heating in the solid phase takes place in the HAZ, which may lead to undesirable formation of brittle IMPhs and further welded joint failure. Mathematical modeling was performed and metallurgical features formed during the processes of heating of the HAZ in bimetal steel–titanium plates were studied to identify the risk of IMPh formation. It was found that at a temperature increase from 900 to 1450 °C, a continuous intermetallic layer formed on the steel–titanium interface, which contained FeTi IMPh, and the width of which increased from 1 to 10 μm. In the temperature range 1300…1430 °C, an intermetallic TiFe2-type phase additionally formed from the titanium side. In the temperature range 1430…1450 °C, the TiFe2 phase was replaced by the TiXFe phase, which formed both from the steel side and from the titanium side. This phase consists of intermetallics (73–75% Ti + 27–25% Fe) and (80–85% Ti + 20–15% Fe), and it is close to the Ti2Fe-type phase. The interlayer of intermetallics, formed at temperatures of 900…1300 °C, has a continuous morphology (HV0.01–650…690). At temperatures rising above 1300 °C, the IMPh interlayer became more ramified (HV0.01–590…610) because of the formation of a larger number of pores and microcracks within it. In the temperature range 900…1450 °C, solid-phase diffusion proceeded in the steel–titanium bimetal near the interface of the two metals. A zone of iron diffusion, 5–10 μm to 40–60 μm in width, formed in titanium. In steel, a zone of titanium diffusion 15–20 μm to 120–150 μm in width formed, starting from 1300 °C and higher. It is recommended to perform industrial welding of steel–titanium bimetal in modes, for which the heat input is equal to 200…400 J/mm. Here, during the period 10–12 s, the heating temperature of the HAZ 1.5–3.5 mm in width is equal to 900–1150 °C. It promotes formation of an intermetallic FeTi-type interlayer of up to 1–2 μm width.