Paper presents the results of research on modified surface grain refinement method used in investment casting of hollow, thin-walled parts made of nickel based superalloys. In the current technology, the refining inoculant is applied to the surface of the wax pattern and then, it is transferred to the ceramic mould surface during dewaxing. Because of its chemical activity the inoculant may react with the liquid metal which can cause defects on the external surface of the cast part. The method proposed in the paper aims to reduce the risk of external surface defects by applying the grain refiner only to the ceramic core which shapes the internal surface of the hollow casting. In case of thin-walled parts the grain refinement effect is visible throughout the thickness of the walls. The method is meant to be used when internal surface finish is less important, like for example, aircraft engine turbine blades, where the hollowing of the cast is mainly used to lower the weight and aid in cooling during operation.
This paper presents the results of measurements of liquid metal fluidity and linear shrinkage of nickel alloy IN-713C in vacuum induction melting furnace Balzers VSG-2. Because of limited volume of the furnace chamber special models for technological trials were designed and constructed to fit in the mould of dimensions 170x95x100mm. Two different designs of test models were proposed: horizontal round rods and modified spiral. Preliminary studies were carried out for alloys Al-Si. Horizontal round rods test was useful for evaluation of fluidity of hypoeutectic silumin, however in case of nickel superalloy the mould cavity was completely filled in each test because of high required pouring temperature. Positive results were obtained from the modified spiral test for all alloys used in the research. Relationship between the linear shrinkage for the test rod and a specific indicator of contraction defined on a spiral was observed.
The paper presents the results of studies on the development of correlation of solidification parameters and chemical composition of nickel superalloy IN-713C, which is used i.a. on aircraft engine turbine blades. Previous test results indicate significant differences in solidification parameters of the alloy, especially the temperatures Tliq and Tsol for each batch of ingots supplied by the manufacturer. Knowledge of such a relationship has important practical significance, because of the ability to asses and correct the temperatures of casting and heat treatment of casts on the basis of chemical composition. Using the statistical analysis it was found that the temperature of the solidification beginning Tliq is mostly influenced by the addition of carbon (similar to iron alloys). The additions of Al and Nb have smaller but still significant impact. Other alloying components do not have significant effect on Tliq. The temperature Teut is mostly affected by Ni, Ti and Nb. The temperature Tsol is not in any direct correlation with the chemical composition, which is consistent with previous research. The temperature Tsol depends primarily on the presence of non-metallic inclusions present in feed materials and introduced during the melting and casting processes.
The paper presents the results of research on the determination of the effect of pouring temperature on the macrostructure of the castings subjected to complex (surface and volume) modification and double filtration. Tested castings were made of post-production scrap (gating system parts) of IN-713C superalloy. Tests included the evaluation of the number of grains per 1 mm2 , mean grain surface area, shape factor and tensile strength. Casting temperature below 1470 °C positively influenced the modification effect. The grains were finer and the mechanical properties increased, especially for castings with thicker walls. On the other hand, manufacture of thin walled castings of high quality require pouring temperature above 1480 °C.
In current casting technology of cored, thin walled castings, the modifying coating is applied on the surface of wax pattern and, after the removal of the wax, is transferred to inner mould surface. This way the modification leading to grain refinement occur on the surface of the casting. In thin walled castings the modification effect can also be seen on the other (external) side of the casting. Proper reproduction of details in thin walled castings require high pouring temperature which intensify the chemical reactions on the mould – molten metal interface. This may lead to degradation of the surface of the castings. The core modification process is thought to circumvent this problem. The modifying coating is applied to the surface of the core. The degradation of internal surface of the casting is less relevant. The most important factor in this technology is “trough” modification – obtaining fine grained structure on the surface opposite to the surface reproduced by the core.
The paper presents the results concerning impact of modification (volume and surface techniques), pouring temperature and mould temperature on stereological parameters of macrostructure in IN713C castings made using post-production scrap. The ability to adjust the grain size is one of the main issues in the manufacturing of different nickel superalloy castings used in aircraft engines. By increasing the grain size one can increase the mechanical properties, like diffusion creep resistance, in higher temperatures. The fine grained castings. on the other hand, have higher mechanical properties in lower temperatures and higher resistance to thermal fatigue. The test moulds used in this study, supplied by Pratt and Whitney Rzeszow, are ordinarily used to cast the samples for tensile stress testing. Volume modification was carried out using the patented filter containing cobalt aluminate. The macrostructure was described using the number of grains per mm2 , mean grain surface area and shape index. Obtained results show strong relationship between the modification technique, pouring temperature and grain size. There was no significant impact of mould temperature on macrostructure.
The study consisted in assessing the influence of surface and volume modification on the characteristics of high-temperature creep of castings made of waste products of nickel-based superalloys IN 713C and the MAR-247. The results of high-temperature creep tests performed under conditions of two variants of research were analysed. The characteristics of creep according to variant I were obtained on the basis of earlier studies of these alloys with the parameters T=982o C, σ=150MPa . Variant II included carrying out creep tests of alloy IN713C with the parameters T=760o C, σ =400MPa and alloy MAR247 with the parameters: T=982o C, σ=200MPa.Developed creep characteristics were compared with the results of these alloys with the parameters according to variant I of the study. It was observed that the conditions of experiments carried out depending upon the value of the creep test temperature and stress with the creep stability depends on the size of the macrograin (I variant of the studies) or such influence was not observed (II variant of the studies). Stability of samples with coarse structure in variant I of creep tests was significantly higher than the samples with fragmented grain. It was found that the observed stability conditions are dependent on the dominant deformation mechanisms under creep tests carried out - diffusion mechanism in variant I and a dislocation mechanism in variant II of the study. The conditions for the formation and growth of the cracks in the tested materials, including the morphological characteristics of their macro-and microstructure were tested.