Heat Treatment Effect on the Structure and Properties of Workpieces from Heat-Resistant Nickel Alloys Obtained by Additive Technologies

Abstract

Both conventional technologies for obtaining workpieces and the additive technological process of direct energy and material feeding (direct energy deposition, DED) are employed for manufacturing bulky workpieces for gas turbine engine parts from heat-resistant nickel-based alloys. The DED technology allows managing a highly coordinated energy impact on the microvolume of the alloy, which ensures obtaining the material structure with higher working characteristics compared to castings. At present, application of nickel material in additive technologies is limited by the ultrafast crystallization processes that cause accumulation of significant internal stresses, which leads to formation of micro- and macrodefects. Heat treatment is recommended for residual stress reduction in the products after the DED process, but optimal modes of such processing of a workpiece are not specified. On the other hand, heat treatment implies obtaining high mechanical properties. For products fabricated by additive methods of surfacing powders with nonequilibrium structure, similar recommendations are insufficient. The place of heat treatment in the general cycle of manufacturing parts is set depending on the requirements for the properties of the product. In most cases, heat treatment is performed after mechanical post-treatment. This is associated with the requirements for high strength, hardness, and wear resistance of the product material. The article studies the effect of various heat treatment modes on the hardness, microstructure, and residual stresses of the samples made of the KhN50VMTYuB heat-resistant nickel-based alloy obtained by the DED technology. The DED technology of workpiece manufacturing from the KhN50VMTYuB alloy leads to a fairly high hardness of about 190 HB. It is well known that the growth of products from the highly alloyed powder of nonequilibrium structure proceeds by rapid cooling, which causes structural changes similar to the aging while heating by a laser beam. Heat treatment of the grown products may be aimed at increasing the machinability by cutting and reducing the warping of products, as a result of the redistribution of residual stresses. In this case, the decrease in hardness may be the criterion of goal achieving. The results of the presented study demonstrate that the most cost-efficient mode of heat treatment for the residual stress removal is the mode consisting in heating up to 1180°C and holding for 4 h with subsequent air cooling, which allows reducing hardness from 191 ± 1 HВ to 135 ± 1 HВ. The lowest hardness values of HB 128 ± 1 were obtained after heating to 1140°C, holding for 4 h, and cooling with a furnace. Air cooling allows obtaining the hardness of HB 130 ± 18. On one hand, this indicates slightly higher hardness values, but deviations are of a higher level, and the level of residual stresses in the annular samples herewith are of the lowest values, which follows from the results of change in the geometry of samples after cutting. The highest hardness of 311 ± 8 HB was obtained at the end of heat treatment, which includes heating up to 1100°C, holding for 4 h, and air cooling; then heating up to 950°C, holding for 3.5 h, and air cooling; then heating up to 800°C, holding for 7.5 h, and air cooling; then heating up to 700°C, holding for 14 h, and air cooling. The microstructure analysis of the grown samples reveals that, after all types of heat treatment, an inequigranular structure is formed in the samples, and the layered structure characteristic of the deposited particles is lost.