New plasma torches for PTA-surfacing

Process of Plasma Transferred Arc (PTA) surfacing is noted for unique technological capabilities [1, 2]. Small depth of penetration in the basic metal, high precision, high production standards and an ability of surfacing of the various alloys - all that makes it irreplaceable at surfacing of valves, stop valves, extruder and heat softening device, tools and many other parts. At the same time there are many parts for which PTA-surfacing is not applied because of different reasons - in many cases because of lack of reliable high-efficiency and various design plasma torches.

The Plasma-Master Co., Ltd. develops the collection of plasma torches for the various purposes. That allows to fill specified lack very largely. At that the world experience and results of own researches of the author are used widely. The special attention is given for choice of the rational scheme of feeding of additive powder in an arc, search of optimum ratio of geometrical parameters of distributive unit and nozzle part of plasma torch, and also quality of protection of a welding pool.

The scheme of powder feeding in arc. It is distinguished the plasma torches with internal and external scheme of feeding of an additive powder in an arc. In the first case the powder comes through arc from outside of plasma torch by one or several lateral channels (fig. 1, a), and in the second - inside of it as the distributed stream through a narrow ring slit between plasma forming and focusing nozzles (fig. 1, b).

One more scheme of feeding of powder in arc is possible - axially through the sleeve cathode (fig. 1, c). The experiments executed by experts of Plasma-Master Co., ltd. and described in work [3], testify to its availability, especially in the field of high productivities (> 8 kg/h). However because of the technical complexity plasma torches with such scheme have not found practical application yet, therefore they are not considered.

Plasma torches with external feeding of powder are enough widely applied abroad, they are simpler and are considered as more reliable from the point of view of a contamination of channels and nozzles by sparks of liquid metal, however they yield essentially to plasma torches with internal feeding in efficiency of heating and fusion of powder [4] as the powder is much less time in an arc. In such plasma torches losses of powder and power inputs are much higher. Especially it is shown under surfacing with the high productivity (> 3 kg/h). Besides one-way lateral feeding of powder results in deformation of arc and hence in trouble of surfacing stability and in deterioration of formation of deposited bead. Therefore in the development we have preferred the internal scheme as more effective.

All plasma torches of Plasma-Master Co., Ltd. independently of their design contain typical distributive unit (fig. 2), consisting of the ring chamber and evenly located on a circle of longitudinal grooves along which the powder is blown into an arc on the appointed angle. The sizes, quantity and inclination of grooves are selected in such a way that powder particles get into the most high-temperature area of arc at the output from focusing nozzle.

The coefficient of powder distribution uniformity on a circle at the optimum transport gas flow is provided not below 0,8, and flight speed of particles at the moment of input in arc - no more 1,5...2,0 m/sec.

Thermal and power characteristics of plasma torches. The basic parameters of effective work of torch is its thermal and power characteristics. As they essentially depend on a torch design and parameters of its work, it is necessary to find such typical solution of a units design that would reduce thermal losses to a minimum. Especially it concerns of focusing nozzle, being an additional element in plasma torches with internal powder feeding.

Thermal characteristics of plasma arc are investigated by the method of flow calorimetry on model plasma torch in which cooling of each unit was independent.

Efficiency of part heating estimated by the thermal stream that is perceived by face of the water-cooled anode. The water flow through the anode and torch units was measured by rotameter, the current and the arc voltage - accordingly by ammeter and voltmeter of a class 0,2. The difference of values of water temperature dТ at the input and the output from plasma torch was defined by special thermobatteries consisting of five chromel-copel thermocouples and was registered by multipoint potentiometer KSP-4. The meter-temperature circuit was calibrated by reference mercurial thermometers with scale factor 0,1°C. Use of thermobatteries has allowed to receive without the additional amplifier an initial signal, sufficient for registration and processing. The relative error of measurement dТ of water did not exceed 3 %.

Measurements were carried out with the following combinations of diameters of stabilizing and focusing nozzles (Ds/Df): 2/4; 3/6; 4,0/7,5 and 5/9 mm. Lengths of each channel of stabilizing nozzle were equaled to its diameter, and focusing - 0,2 Df. The deepening of a tungstic electrode in stabilizing nozzle was 0,8 Ds, distance from the end face of torch up to the anode - 8 mm. Some results of researches are submitted on fig. 3, 4.

Influence of arc current on effective thermal capacity Qp and efficiency of part heating Hp for plasma torch with nozzles 4,0/7,5 mm at typical flows of plasma Qpl and transport Qtr gases is shown In Fig. 3 as an example.

Apparently, Hp is rather high, in an interval of currents 50…250 A it makes 80...60 % that is close to efficiency of plasma torches that are used for welding and cutting [5, 6]. Presence of additional small length focusing nozzle practically doesn`t tell on effective efficiency of part heating. With rise of arc currents fixed reduction Hp is bound up with growth of losses in nozzles as a result of increase of diameter of arc [7].

The plasma and transport gas flow influences on effective thermal capacity and efficiency of arc in different ways.

So, when plasma gas flow increases (fig. 4, a) then parameters Qp and Hp grow a little, reaching maximum at Qpl=5...6 l/min, that is explained by increase of energy concentration in arc owing to its greater squeezing, reduction of losses in stabilizing nozzle and intensification of convective heat transfer of plasma with the anode. However in practice it is impossible to use this effect, as it is necessary to limit plasma gas flow because of excessive penetration of the basic metal.

When transport gas flow increases, on the contrary, reduction of effective thermal capacity and efficiency of arc (fig. 4, b) occurs. It is caused by that transport gas has not a compressing effect on arc because of big diameter of focusing nozzle, and only gas bleeds heat of arc. However, as this influence is little then when you select the gas flow, first of all, it is necessary to start with conditions of the best distribution of a powder in plasma torch.

Effective means of increase of thermal capacity of arc on other equal conditions is reduction of diameter of plasma torch nozzles, i.e. increase of its compression ratio.

As results of calorimetry show (fig. 5, a), reduction Ds/Df from 4,0/7,5 or 5/9 up to 2/4 mm raises values Qp more than 1,5 times for the present design of plasma torch. It is necessary to note, that value of effective efficiency of arc practically does not change at the same time; the increase of heat-embedding into the anode is the result of voltage rise in arc (fig 5, b), i.e. as a result of increase of consumed power from source. With reduction of diameter of nozzles energy concentration grows in arc also: the factor of concentration measured by method in work [8] is increased accordingly from 2,0... 2,5 up to 6,0... 6,5 1/sm*sm. This factor is very important during surfacing of small-sized parts as it allows to adjust character of transfer of heat into part rather subtly. However to reduce diameter of nozzles it is possible only up to the certain limit caused by stability of burning of arc [9, 10].

For each combination Ds/Df of nozzles there is a certain range of currents, in which the arc (in the presence of additive powder) burns steadily, i.e. double arc-formation is absent, and dynamic arc head does not prevent from good formation of deposited bead. It is established experimentally, that for Ds/Df =2/4 mm these values are 20...110 A; for 3/6 mm - 20...170 A; 4,0/7,5 mm - 30...220 A; for 5/9 - 40...300 A. The choice of this or that combination of nozzles in each concrete case is dictated by technological reasons (necessary heat-embedding into a part, character of heat distribution over a surface, productivity of surfacing etc.).