The paper summarises results of measurements of remelting area geometry, thermal efficiency and melting efficiency characterising the surface remelting process applied to castings of MAR-M-509 cobalt alloy. The remelting process was carried out with the use of GTAW (Gas Tungsten Arc Welding) method in protective atmosphere of helium, at the electric current intensity in the range from 100 A to 300 A, and the electric arc scanning velocity vs in the range from 200 mm/min to 800 mm/min. The effect of current intensity and electric arc scanning velocity on geometrical parameters of remeltings, thermal efficiency, and melting efficiency characterising the remelting process has been determined. Keywords: MAR-M509 alloy, Surface remelting, Remelting geometry, Thermal efficiency, Melting efficiency.
1. Introduction
In the last ten or fifteen years, intensive development in the area of heat resistant and creep resistant materials can be observed. This is true particularly with the reference to the aircraft industry where range of application for such materials is the widest [1, 2].
Expansion of aviation as a means of public transport and haulage of goods, is a result of a large number of scientific studies concerning establishment of an optimum chemical composition, best manufacturing practices, and methods allowing to improve service properties of cobalt alloy castings [3, 4].
One of the promising methods allowing to improve service properties of castings consists in shaping the fine-grained structure of their surface layer. To this end, the casting surface remelting technique can be used. Using for this purpose such high-energy heat sources as laser beam, electron beam, or electric arc plasma stream, allows to remelt the surface of a casting that is followed by rapid solidification. As a result, a refinement of microstructure occurs and improvement of service properties of the castings [5–9].
Surface improvement of castings with the use of concentrated heat stream requires knowledge on the amount of heat intercepted by a casting in the course of being heated up as the parameter has a decisive effect on the remelting area geometry, solidification rate, andmicrostructure. The objective of the study was to determine the effectof technological parameters characterising the surface remelting process carried out with the use of GTAW method on geometry of remeltings, thermal efficiency and melting efficiency when applied to MAR-M509 cobalt alloy castings.
2. Material and experimental conditions
Plate castings with dimensions of 200 mm ´ 50 mm ´ 16 mm were prepared for the tests. Melts were carried out in the Balzers vacuum furnace. The moulds were prepared with the use of the investment casting method. Chemical composition of the MAR-M-509 alloy included: 0.57% C, 0.001% S, 0.13% Si, 0.04% Mn, 10.31% Ni, 23.10% Cr, 7.10% W, 0.17% Ti, 0.18% Fe, 3.78% Ta, 0.34% Zr, 0.01% B, rest Co.
Plate castings of MAR-M509 alloy were surface-remelted by mean of GTAW method with the use of FALTIG 315 AC/DC welding machine. Electric current intensities I = 100, 150, 200, 250, and 300 A were used. The used electric arc scanning velocities were vs = 200, 400, 600, and 800 mm/min. Remeltings were carried out in the atmosphere of helium. Tungsten electrode with diameter Ø = 3.0 mm was used, at the electric arc length of 3.0 mm. Evaluation of the quantity of heat intercepted by the casting specimen was performed with the use of a set-up for calorimetric tests [10].
Thermal efficiency and melting efficiency Thermal efficiency of the GTAW process η was calculated from the formula:
where Qk is the amount of heat intercepted by the heated material determined experimentally with the use of calorimeter (J), U -electric arc voltage (V), I -electric current intensity (S), t - electric arc scanning time (s). The melting efficiency ηm was calculated from the following formula:
where Vn is the remelting volume (mm3); QH - amount of heat required to heat up a unit volume of the alloy from temperature T0 to a temperature Tl and melt it (J/mm3); Qt -the specific heat of fusion (J/g), cp - specific heat (J/gK), and - alloy density (g/cm3).