TIG Hotwire Surfacing Bochum

TIG Hot Wire Surfacing J. Franz and H. Heuser, Bochum

1. Introduction The TIG process distinguishes itself by high versatility as regards material quality, welding position and wall thickness, allowing to produce seams of utmost quality, provided that they are performed by experienced welders or mechanized welding. However, this is often disregarded for economic reasons, as the deposit efficiency achieved using the TIG process cannot compete with other welding processes. Considerations with view to making the TIG process more efficient lead to the development and use of the TIG hot wire process. It is known that, using the MSG process or submerged-arc welding, the prolongation of the free current-bearing wire end results in an increased deposit efficiency. This is due to the fact that this causes the electric resistance, and also the temperature, to rise. The temperature increase is proportional with the product of squared current and resistance (I2R) (Joule effect). Even in 1956 the Joule effect was used to increase the deposit efficiency /1/. The hot wire method may be combined with most known fusion welding processes /2/. 2. Process description Figure 1 (enclosed page) shows the principle of TIG hot wire welding. A wire feeder continually conveys filler material into the weld pool. A separate power source is used to heat the filler wire by means of electric resistance heating applied to the free wire end between the contact tube of the welding torch and the weld pool. Depending on the welding job and the workpiece, the filler wire may come from the front, the side or the rear into the weld pool. Successful TIG hot wire welding, however, requires a correct position of the torch and of the wire fed, however, a general rule cannot be defined. Like the cold wire torch, also the TIG torch is connected to the negative terminal and the workpiece to the positive terminal. To preclude any deviation and lag of the TIG welding arc the negative pole is connected to the hot wire and the positive pole to the workpiece. The hot wire power source produces a voltage between 1.5 and 5 V to preclude arcing /2/.

Depending on the type of welding unit, the TIG hot wire process allows to considerably reduce the arc blow effect by means of synchronously pulsed weld and hot wire currents and to increase the weld quality (e.g. due to reduced dilution), especially when applied to out-of-position welds. 3. A comparison of TIG cold wire / hot wire 3.1 Deposit efficiency According to /2/, although using the same arc amperage, the hot wire process allows to achieve the triple deposit efficiency as compared to the cold wire process, that is mainly used for mechanized TIG weld jobs. In our own tests we used Ø 1.6 hot wire (nickel master ally type 625) and achieved a deposit efficiency up to 6 kg/h. Using cold wire under the same conditions, the deposit efficiency amounted to 3.5 kg/h (figure 2). The job was surface welding in h position. Apart from the wire diameter, the deposit efficiency achieved by hot wire welding mainly depends on the intensity of the hot wire current and the free current-bearing wire end. We will go into the difficulties encountered in this connection and the limits of the hot wire process more in detail later in this article. The dependence of the deposit efficiency from the intensity of the weld current, as shown in figure 2 (enclosed page), only refers to the intensity of the welding arc current (as is usual in the literature). It is true that the influence of the hot wire current can be seen from these representations, however, it cannot be quantified. The examinations described in the literature /3/ are based on a constant welding arc amperage, with additional current fed to the wire. They, however, do not take into consideration that this results in an increased energy input. As a result, the following considerations start out from a constant wire end energy, so that the hot wire current and its effect on the total energy balance E = ELB + EHD is taken into account (figure 3 enclosed page). Given a wire end energy E = 8.5 kJ/cm, the figure shows welding arc amperage corresponding to a specific hot wire amperage. This was calculated on the basis of a given hot wire amperage and of the characteristic curves of the two power sources. The deposit efficiency was determined in a various test series where the conditions (welding arc amperage ILB,