The role of nanostructural superplasticity in blisk manufacturing
Enhancement of the air propulsion units is to a large extent connected with strength improvement of the materials used for manufacturing the compressor’s blades. One of the most prospective ways of hardening the materials is to manufacture the ultrafine grained (UFG) and nanocrystalline structures due to intensive plastic deformation .
Nowadays, compressors of the modern gas turbine engines (GTE) have blisks wherein blades and disk do not have mechanical fasteners but present a single (see Fig.1a), however, the requirement is that blades and the disk are made of different alloys. Thus, the problem is divided into two tasks.
The first task is to manufacture the blades having high characteristics of static and cyclic strength and reasonable level of impact strength which is achieved by creating within them a UFG structure with a grain size of 600...800 nm. This structure is retained in a blade after punching in the low-temperature superplastisity (SP) mode as well.
The obtained results are of a practical interest but leave open the questions concerning microstructure transformation mechanisms during the LFW.
MATERIALS AND METHODS OF STUDY
The obtained research results [2, 4, 6] of welded joints made of VT6 and VT8-1 alloys were analyzed in order to identify the microstructure transforming mechanisms. In so doing, the dynamics of the parameters recorded in the welding process with the use of optical and electronic metallography tools, micro-hardness measurements and results of temperature field and stress-strain behaviour modeling were used [3, 5].
Appearance (see Fig.2a) and macro structure (see Fig.2b) of two-phase alloys welded joints based on titanium reflect the processes of the weld metal structural yielding. The photos vividly show the upsetted burr extruded out of the joint being formed.
It is easy to see that burr is formed as a thin strip which is a continuation of the weld and consists of the material extruded from both parts.
Fig.3 presents a cyclogram of the welding process illustrating such temporary characteristics of the process like general heating time (1.4 s) and an upset time 0.65…0.68 s. During this time the material in the welding area is being strained at a speed of 6.5…6.8 mm/s and the burr is being formed.
An average strain rate of the metal in the direction perpendicular to the weld plane was 5 s–1 with the maximum strain rate and temperature reached in the joint plane, and, as it can be seen from Fig.4, they can be 16 s–1 and 1,500 K, respectively. The accumulated strain level during extrusion may exceed 450%.
The heating time and upset time in case of blades welding in FG and UFG states do not differ practically because the microscopic structure of UFG blade welding joints has a number of features. The boundary of the weld joint and thermo-mechanical affected zone (TMAZ) in the FG state parts is quite clear and may be easily detected by optical metallographic methods. However, in case the parts are welded in the UFG state, the TMAZ length is sufficiently greater and its boundary is blurred (see Fig.5).
In case of the VТ8-1/FG VТ6 pair the welding joint microstructure zone consists of microneedle martensite which is characteristic of rapid cooling of the alloy in the β-area. If the initial blank is of UFG type, the central layer microstructure of the joint is formed by martensite plates with an average length of about 800 nm and thickness of 30…75 nm (see Fig.6). At this magnification the nanostructured martensite is formed aside of the centre line and is practically invisible. In parallel with martensite, the joint structure contains visible rounded grains of residual α-phase of about 100 nm (see Fig.7). Distribution of microhardness (see Fig.5a) corresponds to the distribution of phases in the welded joints. The maximum value of HV corresponds to the joint zone and decreases to the base material values as the distance from the welding joint increases. Distribution of microhardness outside the welding joint on the VТ8-1 side does not practically depend on the second part structure. Microhardness on the VТ6 side in the alloy with initial UFG structure decreases slower than in case of FG materials. Earlier the similar differences in welding joint of samples made of FG and UFG VТ6 alloy have been observed [4, 7].
In the contact zone of friction surfaces the heating rate is about thousands degrees per second, and the high rate and intensity of shearing strain together with a compressive load lead to reduction of grain size and forms the UFG or nanocrystalline structure. Structural yielding of metallic materials takes place as a shear between two basic strain mechanisms (intragranular sliding and grain-boundary slippage). In nanostructured materials an intragranular sliding is hindered by generation and dislocation movement restrictions, while grain-boundary slippage becomes a priority because of growth of the specific area of boundaries. This is a background for appearance of superplasticity. According to modern concepts, superplasticity results due to action of the cooperated shear strips with the predominant role of the grain-boundary slippage [8, 9, 10]. In this case the strain rate is determined by the shear rate of each strip and a number of the involved strips. The shear rate of the strip is determined by temperature, applied stresses and grain size – when temperature is decreased the grain boundary slippage becomes lower, but it increases with a grain growth. The maximum possible number of acting strips depends on the grain size, when a grain becomes less, the number of strips formed between it increases. Thus, the dependence of strain rate in case of superplasticity noted in most works  is satisfied:
where s is a flow stress, d – grain size.
Superplasticity acquires two unique varieties in the case of UFG and nanostructured materials – low temperature superplasticity and high rate superplasticity. Low temperature superplasticity is a combination of low shear rate in each strip and a great number of strips in UFG structure, and has been studied quite well . We mentioned manufacturing of blades using low temperature superplasticity in the introduction. There is practically no information about high rate superplasticity application due to the fact that its realization needs to combine two different conditions: UFG structure and high temperature. It is possible to combine both conditions with LFW when the heating time and strain are fractions of a second. In the high rate superplasticity mode the metal flow during upsetting provides for removal of the heated metal to a burr, and for tight closure of surfaces and stabilization of the temperature field. The shape and size of the flash extruded from the welding zone, strain degree exceeding 450% and absence of cracks, pores and other defects of continuality confirm presence of high rate superplasticity. After heating and upsetting come to an end, re-crystallization and β-phase grain growth take place, then the martensite transformation occurs while cooling at hundreds degrees per second. The initial blank contains the prepared UFG structure with grain size of 600 nm when welding titanium alloys in UFG state. In this case the same mechanism of micro-structure transformation of the structure takes place, however, due to the higher strain capacity of UFG of the material, the zone of intensive strain becomes larger as compared with the FG structure case. As a result, the hot zone becomes less localized and the maximum temperature decreases. The alloy preserves a two-phase state that favours the high rate superplasticity. In this case the weld composition has the residual α-phase and more uniform and smaller structure and blurred border between welding joint and TMAZ as well as a smooth change of microhardness over the welded joint cross section. These factors are positive for improvement of the joint properties: the strength corresponds to the base material both at static and cyclic loading (see Fig.8). Destruction of samples and real parts occurred in the basic material only.
The described mechanisms determine the structural peculiarities of welded joints of titanium alloys and their exceptional mechanical characteristics. High-temperature superplasticity provides for extrusion of pollutions present on the welded surfaces to a flash, absence of overheating in the welding zone and a high level of strain required to form a physical contact of the surfaces and prepare a welding joint. Welded joints of blades and disk have the same strength as the base metal, and the joints of parts in the UFG state demonstrate a significantly lower level of the structural and mechanical heterogeneity. These mechanisms will act not only on a micro- and submicron level, but also when the grain sizes of the welded parts continue to decrease, therefore, it is necessary to consider LFW a promising method for welding bulky nanostructured materials.