Heat treatment effect on the structure and properties of workpieces from heat-resistant nickel alloys obtained by additive technologies

Metallurgy and Material Science


Аuthors

Balyakin A. V.*, Nosova E. A.**, Oleinik M. A.***

Samara National Research University named after Academician S.P. Korolev, 34, Moskovskoye shosse, Samara, 443086, Russia

*e-mail: balaykinav@ssau.ru
**e-mail: eanosova@mail.ru
***e-mail: oleynik1997@mail.ru

Abstract

Both conventional technologies for workpieces obtaining and additive technological process of direct energy and material feeding (DED) are being employed for manufacturing bulky workpieces for gas turbine engines parts from heat-resistant nickel-based alloys.

The DED technology allows managing a highly coordinated energy impact on the micro-volume of the alloy, which ensures the material structure obtaining with higher working characteristics compared to castings. As of now, nickel materials application in the area of additive technologies is limited by the ultrafast crystallization processes specifics that cause accumulation of significant internal stresses, which leads to micro- and macro-defects forming. Heat treatment is recommended for residual stresses reduction in the products after the DED process, but optimal modes of such kind of the workpieces processing are not clearly specified. On the other hand, heat treatment implies obtaining high mechanical properties. For the products fabricated by additive methods of surfacing powders with non-equilibrium structure, the similar recommendations are of rather small volume.

The place of heat treatment in the general cycle of parts manufacturing is being set depending on the requirements for the product properties. In most cases, heat treatment is being performed after mechanical post-treatment. This is associated with the requirements to 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 HN50VMTUB heat-resistant nickel-based alloy obtained by the DED technology.

The DED technology of workpieces manufacturing from the HN50VMTUB alloy leads to a fairly high hardness of about 190 NB. It is well-known that the products growth from the highly-alloyed powder of non-equilibrium structure proceeds by rapid cooling, which causes structural changes similar to the aging while heating by the laser beam. Heat treatment of the grown products may be aimed at increasing the machinability by cutting and reducing the of products warping herewith, as the result of the residual stresses redistribution. In this case, the decrease in hardness may be the goal achieving criterion.

The results of the presented study demonstrate that the most economical mode of heat treatment for the residual stresses removing is the mode consisting in products heating up to 1180°C, holding for four hours 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 hours and cooling with a furnace. Air cooling allows obtaining hardness of HB 130 ±18. On the one hand, this indicates slightly higher hardness values, but deviations are of a higher level, the level of residual stresses in the annular samples herewith are of the lowest values, which follows from the results of samples geometry changing 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; air cooling, and then heating up to 950°C, 3.5 h holding, air cooling; then heating up to 800°C, exposure 7.5 h, air cooling, then heating up to 700°C, holding time of 14 h, air cooling.

The microstructure analysis of the grown samples reveals that after all types of heat treatment, an inequigranular structure is being formed in the samples, and the layered structure characteristic for the deposited particles is lost.

Keywords:

additive manufacturing, direct metal deposition, heat-resistant alloy heat treatment, microstructure after heat treatment, hardness of material, residual stresses in the material structure

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