Introduction. Critical cast components of power units in aviation and power engineering are manufactured from heat-resistant nickel alloys [1,2,3,4]. A ZhS3DK-VI heat-resistant nickel alloy has gained widespread acceptance in fabricating modern gas turbine engine blades. These blades are produced by equiaxial investment casting [5,6,7,8,9]. As is known, all types of casting are characterized by the presence of such defects as micropores and other structural imperfections, liquation, variable phase composition, which significantly affects the reliability, physicomechanical, heat-resistant, and service properties of finished products [10,11,12,13].

One of the effective approaches to the healing of defects in critical castings, and hence to improving the complex of their properties and service reliability, is hot isostatic pressing (HIP), which provides the all-round compaction of castings in a special liquid or gas environment at high temperatures [14,15,16]. A maximum temperature chosen for casting working may not cause fusion of the material structure.

The material quality improvement as a result of HIP comes from the removal of internal defects, such as porosity, interior shrinkage, and interdendritic cracks formed on metal hardening. In the process of HIP, the closure of cavity walls is taking place by creep and plastic strain mechanisms followed by diffusion welding of the surfaces in contact. With this, barometric treatment does not remove defects that are connected with the casting surface [14, 15].

Motor Sich JSC facilities were used to perform investigations that revealed the stuctural features and properties of turbine blades made of casting heat-resistant high alloys produced by equiaxial and directed crystallization after HIP [15, 16].

Under modern economic conditions, with the accumulation of processing wastes of expensive materials used in aviation engineering, the problem of their reutilization to reduce productioon costs and raise competitive capacity of products in the world market is becoming especially acute [17,18,19,20].

The object of this investigation is to study the quality of the blade material and specimens cast from a ZhS3DKVI heat-resistant nickel alloy with 50% returns in the charge (after preliminary remelting [19, 20]), which underwent hot isostatic pressing and heat treatment under standard conditions (homogenization at 1210±10°C for 4 h in vacuum).

Material and Methods. The UPPF-3M unit was used to perform meltings of charge of ~8 kg in the main crucible to fabricate blades, as well as pin specimens of 12-mm diameter and of square section.

Hot isostatic pressing was carried out under the following conditions:

(i) initial pressure in the chamber of 38 MPa;

(ii) heating from room temperature to 1040±10°C at a rate of 8–10°C/min;

(iii) holding at heat of 1040±10°C for 1.5 h at 120 MPa;

(iv) heating to 1210±10°C at a rate of 4°C/min;

(v) holding at heat of 1210±10°C for 2 h at 160 MPa;

(vi) cooling of castings to 300°C at a rate of 26°C/min.

The chemical composition of the metal was determined. The appearance, macro- and microstructure of the blades and specimens were studied. The macrostructure was etched with a reagent consisting of 80% HCl and 20% H2O2.

Blade bending tests (up to 90°) were performed on a hand power screw press after GOST 14019-80 using special attachments with V-recesses.

Mechanical and heat-resistant properties of the specimens were determined after their heat treatment under standard conditions (homogenization at 1210±10°C for 4 h in vacuum).

Long-term strength tests were conducted at 850°C and a 350 MPa load.

Basic Material of the Study. The chemical composition of the blades was established to meet the requirements of OST 1.90.126-85 (Table 1).

Table 1. Chemical Composition of ZhS3DK-VI Alloy Blades (wt.%) after HIP and Standard Heat Treatment
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The blades were tested in bending. The bending angle was measured after unloading.

The blades were machined in accordance with the design documentation. According to the OST 1.90.126-85 requirements, the bending angle should be ≥20°. The blade bending results meet this requirement (Table 2).

Table 2. Bending Test Results for ZhS3DK-VI Alloy Blades
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The appearance of blades after bending tests is shown in Fig. 1.

Fig. 1.
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Appearance of ZhS3DK-VI alloy blades (HIP + standard heat treatment), after bending tests.

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Macrofractographic analysis of the structure of fractures on crack opening of the blades in bending, as well as of the fractures of failed blades, revealed casting defects, vis microporosity and looseness coming out at the surface of the blades (Fig. 2). It should be noted that micropores at the surface of components and those in the areas of shrinkagelike through looseness and coming out at the surface are not subject to healing. This is consistent with the barothermic mechanism, when the elimination and reduction of microporosity can be effected only to the bulk of the metal [14, 15].

Fig. 2.
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Structure of fractures on crack opening of the blade (a, b) and blade failure (c, d) after bending tests.

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The macrostructure of the blade cross-section and its face surface is presented in Fig. 3.

Fig. 3.
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Macrostructure of the cross-section (a, c) and blade face surface (b, d) after bending tests.

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As metallographic analysss established, the blade microstructure is a γ-solid solution strengthened by the intermetallic γ′-phase, with carbides and carbonitrides, it corresponds to a ZhS3DK-VI alloy in a normally heat-treated state and the accepted scale of microstructures (Fig. 4). Structures responsible for the superheated state were not found in the HIP-processed turbine blade material.

Fig. 4.
figure 4

Microstructure of blades after bending tests.

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Hot isostatic pressing at 1210°C and 160 MPa was established to result in complete dissolution of the eutectic phase (γ–γ′) in the γ-matrix.

Hot isostatic pressing contributes to porosity and loosenesse healing. After HIP, the structure of the blades is practically free of microporosity and looseness (Fig. 4). Single micropores detected after HIP do not exceed ~ 6 μm (Table 3), which are more than 40 times smaller compared to the pores found in the blades before HIP [15, 16].

Table 3. Parameters of Structural Components in ZhS3DK-VI Blades (μm) after HIP and Standard Heat Treatment
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The size of MC-type carbides and distance between the axes of second-order dendrites in the blade face are about 2.0–2.5 times smaller than in the root.

Mechanical and heat-resistant properties were determined on pin specimens (of 12-mm diameter and of square section) cast by equiaxial crystallization from a ZhS3DK-VI alloy after HIP and homogenization at 1210+15°C for 4 h in vacuum (standard conditions).

The results of mechanical and long-term strength tests are summarized in Table 4.

Table 4. Mechanical and Heat-Resistant Properties of ZhS3DK-VI Alloy Blades
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As is seen in Table 4, the mechanical and heat-resistant properties of the specimens are satisfactory and meet the OST 1.90.126-85 requirements.

The microstructure of pin specimens after HIP and standard heat treatment is similar to that of the blades treated after the specified mode.

Conclusions. The blade material as well as specimens cast from a ZhS3DK-VI heat-resistant nickel alloy with 50% returns in the charge (after preliminary remelting) after hot isostatic pressing (HIP) and standard heat treatment meet the quality requirements. The bending tests demonstrated that the bending angle on the blades was 88–126°. According to the OST 1.90.126-85 requirements, this angle should be ≥20°.

Hot isostatic pressing of the blades at 1210°C and 160 MPa provides the healing of microporosity and looseness that do not come out at the surface of the components (located in the bulk of the metal). The structure of the turbine blades after HIP is practically free of micropores, which contributes to the stabilization of the structure and properties of the material. Single micropores detected after HIP are more than 40 times smaller compared to those revealed in the cast blades before HIP.

Thus, the studies demonstrate the feasibility of fabricating the turbine blades with 50% returns in the charge (after preliminary remelting). Hot isostatic pressing before standard heat treatment is shown to exert the positive effect on the complex of physical, mechanical, and service properties of critical cast products from heat-resistant nickel alloys for aviation and power plants.