Investigation of strength characteristics of heat-resistant nickel alloy VV751P welded joints obtained by electron-beam welding

The paper is devoted to the study of the VV751P nickel alloy weld joints mechanical characteristics. The metallographic research for defects detection and microstructure analysis of the specified weld joints was performed. The impact of heat treatment mode parameters on the hardness of the weld seam and heat affected zone is researched. Weld joints tension tests and cyclic tests as well as hardness tests including the aging curves formation were obtained. As a result, the optimum heat treatment mode that provides the highest weld joint strength was found out.


Introduction
Heat-resistant nickel alloys are used today mainly in the production of machines and constructions operating under extreme conditions. The most striking examples are elements of gas turbines used in aircraft engine design, rocket engineering and power engineering. The transition to welded constructions in these industry branches is an extremely urgent technological solution, causing significant reduction in production costs, and in some cases -quality improvement [1][2][3][4][5][6][7][8].
The main difficulties in welding of these alloys are low mechanical properties of welded joints and heat resistance [12]. Due to the complex chemical composition, these alloys are sensitive to thermal influences, such as welding. Powerful thermal effects associated with metal's melting during the process of welded joint formation cause a change in strength and plasticity of welded joint as compared to base material [13].
In order to study the weldability of VV751P alloy, strength characteristics of a welded joint obtained by electron beam welding, were investigated.

Experimental method
The samples in initial state were produced in a form of plates with 140x60x10 mm dimensions. Welding was carried out on electron beam installation AELTK-344-12 (rated current I n = 155 mA, focusing current If = 795 mA, welding speed V = 120 m/h). The process of welding was carried out in the lower position with through penetration and free weld root formation.
The research of microstructure were carried out with using of optical microscope Zeiss Axio Observer Z1m. Method of bright-field microscopy in reflected light with magnification power from 50x to 1500x was used. The image of the microstructure was recorded with a digital camera; the subsequent processing of the image was carried out in the specialized AxioVision program.
The Vickers method was applied to measure hardness. In this method, the hardness value is determined by the size of tetrahedral diamond pyramid (its angle at the vertex is equal to 136°) indent. The indentation loads were equal to 10 kgf (for HV10 hardness) and 1 kgf (for HV1 hardness). The hardness (HV10) was measured on a Wilson 432SVD hardness tester. Vickers HV1 hardness testing was also performed on the automatic hardness tester Wilson Tukon 2500. All measurements were carried out according to the scheme shown in figure 1. To study mechanical properties of welded material and its weld joints, 19 flat samples of base metal and metal with welded joint were made (see figure 2). Cyclic strength tests were carried out with a zero-loading cycle. The value of cycle amplitude was determined from the results of preliminary tension tests and was set in such a way that sample's loading occurred in the elastic region. A muffle furnace with a maximum heating temperature of 1300°C was used for heat treatment of samples.

Results of studies
There are splashes of metal after welding in the root area, which indicates about uneven formation of welded joint. Figure 3 shows macrostructure of facial and root side of welded joint. Shrinkage cavities of 50-100 microns are observed on the facial side of welded joint, herewith cracks and other defects are absent. Macrostructure of the root side is characterized by smaller crystal sizes compared to the facial side that is explained by higher cooling rates in the root part of the weld. The surface of the root side is smooth, liquid metal outflows are observed in some areas. Cracks on the surface of the root side of the weld are also not revealed. Weld metal has a dendritic structure (figure 4). Group and single inclusions are observed in the photograph of microstructures. It is not possible to determine exactly the type of these inclusions by means of optical microscopy. Nevertheless, we may suppose, that single inclusions are carbides (carbonitrides) of titanium; that is confirmed by the presence of titanium in the researched alloy. When studying microstructure in different cross-sections of weld joint, cracks in the weld metal were found in three samples (one of them is shown in figure 5). These cracks were located in the lower part of the welded joint in the transverse to the weld axis direction (in cross-sections No. 7  sections with a crack were near the splashes of metal in the root part of the weld. It can be assumed that the formation of cracks in weld metal is associated with its shrinkage and lack of liquid metal in splash areas. However, in order to establish a reliable correlation between the presence of a splash and crack in weld metal, more experimental data is required. The spread of cracks occurs on interdendritic sections. Also, micro-cracks were found in widening zone of the weld ( figure 6 (a), (b)). These cracks are only near the widening zone; herewith micro-cracks are not observed in the area where the fusion line becomes close to vertical (figure 6 (c)).    It was also observed, that melted weld metal on the fusion line leaks between the grain boundaries due to decreasing in its strength (the Rebinder effect) (figure 9).
In order to determine the aging temperature of metal, the aging curves were constructed (figure 10). Samples were held in the furnace for 100 hours, with intermediate hardness measurements at control points. To do this, the samples were removed from the furnace at a control point and cooled in a calm air. After hardness tests samples were again loaded into the furnace and exposure was continued at defined temperature. Time required to heat the sample to defined temperature was not taken into account in the total aging time. For this purpose, the time required for sample heating was determined for each aging mode. It was 190 seconds for a temperature of 750°C and 210 seconds for a temperature of 800°C.  . Aging curves at different temperatures of hardening and aging: 1 -hardening at 1185°C, 3.5 hours, air cooling + aging at 750°C; 2 -hardening at 1210°C, 3.5 hours, air cooling + aging at 800°C; 3-hardening at 1185°C, 3.5 hours, air cooling + aging at 800°C.
After hardening at a temperature of 1185 °C, hardness of the metal did not decrease, and after hardening at a temperature of 1210 °C hardness decreased by almost 30 kgf/mm 2 . After aging, all the curves in the first control point showed a growth in hardness, in the second -a decrease, and in the third point the growth was resumed. Perhaps this is due to coagulation and dissolution of γ'-phase in the same way as in other high-temperature alloys considered in [14]. Further growth of aging curves was observed up to their maximum. For 800°C aging the hardness maximum was reached after 5 hours, and for 750°C aging it was reached after 14 hours. Then there was a slight decrease in hardness, over time the curves became flat.
Hardness (HV10) in different sections of the weld was measured in samples immediately after welding and after heat treatment consisting of aging at 800°C for 5 hours ( figure 11 (a)). From this hardness distribution in the cross-section of the weld, it can be seen that minimal hardness of the weld is 438 kgf/mm 2 . After the heat treatment hardness of weld seam increases by 78 kgf/mm 2 , but decreases in heat-affected zone. Hardness (HV1) was also measured in the section of the weld; hardness distribution curves are shown in figure 11 (b). The value of hardness in the weld is lower than in the base metal. After aging the hardness value increases, but decreases in heat-affected zone.  Figure 11. Hardness (HV10) (a) and (HV1) (b) distribution in the cross-section of the weld: n -the distance from the weld axis; 1 -after welding; 2 -after heat treatment (aging at 800°C for 5 hours).

Results
where RU is the ultimate tensile stress of the defined sample. Further, cyclic strength tests were performed, the results of which are shown in table 2 and in figure  12. Hardening was carried out at 1185°C, aging at 780°C and holding for 6 hours. The cyclic strength decrease αN was calculated for each tested sample: where NBM is the number of cycles before sample destruction for base metal and N is the number of cycles before a defined sample destruction. Maximum cyclic strength decrease (65.4%) occurs for the sample No. 8 -the welded sample with a crack. Minimum cyclic strength decrease (32.2%) was observed for the sample No. 16 -the welded sample with no defects in the weld. Table 2. Results of cyclic strength tests (cycle peak stress σmax = 750 MPa). 8