Transient Liquid Phase Bonding (TLPB) of seamless carbon steel tubes

DI LUOZZO N.; FONTANA M.; ARCONDO B.

São Carlos

Encuentro; 1st São Carlos Advanced School on Materials Science & Engineering (SanCAS-MSE); 2012

Universidade Federal de São Carlos

The Transient Liquid Phase Bonding (TLPB) is an isothermal bonding method at temperature Tp - below the melting point of the base metal - which is capable of producing joints that have mechanical properties similar to this. This type of bonding involves three main stages: Liquefaction and dissolution of filler and base metal, Isothermal solidification of the liquid phase and Homogenization of the solute. The process uses a foil (e.g. 25 µm thickness) with specific chemical composition and a melting point below Tp - as filler metal which temporarily melts and then solidifies at the bonding temperature Tp, as the filler material diffuses into the base metal. Under certain conditions of temperature and pressure, the liquid filler metal is completely diffused, leaving almost the equivalent of a solid-state diffusion bonding. Using this technique, steel tubes and rods were welded under both inert (Ar) and reducing (Ar +H2) atmospheres employing amorphous foils of the Fe-B and Fe-B-Si systems as filler metal. Different kinds of carbon steels were bonded. The steel parts were aligned with their abutting surfaces in contact with the amorphous foil and the arrangement was placed into the coil of an induction furnace. The temperature was raised at the highest possible rate to temperature Tp and then held constant for different periods of time. Different temperatures Tp and pressures were employed. Welded samples are characterized using optical and Scanning Electron Microscopy (SEM) and their mechanical properties such as microhardness and tensile strength were measured. The influence on the welding of the main variables - time, temperature Tp and pressure - were analyzed. Recently, we have worked on the following aspects: 1) Development of a new device to weld pieces tubular products. The device consists of a frame which includes the heating system, the jaws, the mechanism for pressure application and both the measurement and control system of temperature. The development of this device has resulted in a patent application; 2) Development of a measurement method to determine the temperature field (thermography) using a CCD camera. This allows the simultaneous temperature measurement of the whole joint and the heat affected zone, and also the detection of hot/cold spots during the process; 3) Welded steel samples using Fe-B-Si glasses as filler material were characterized employing Mössbauer spectroscopy and magnetic properties; 4) With view to industrial application, the tubes diameter to be welded is increased. Welds were made with carbon steel tubes with an outside diameter of 73 mm employing amorphous Fe-B-Si ribbons as filler material. We begin to analyze the influence of process variables: temperature, pressure, process time, cooling rate, surface termination. We begin to characterize the solder joints by optical and electronics microscopy techniques; 5) Numerical simulations of the induction heating process were also performed in order to determine the temperature gradients that occur in steel parts, including tubular geometry. For this purpose we have used the finite element method to evaluate the temperature differences between the inner and outer surface of the tube and the extension of the heat affected zone.