Effect of heat treatment on microstructure and functional properties of additively manufactured NiTi shape memory alloys

Additive manufacturing of NiTi shape memory alloys has attracted attention in recent years, due to design flexibility and feasibility to achieve four-dimensional (4D) function response. To obtain customized 4D functional responses in NiTi structures, tailorable phase transformation temperatures and stress windows as well as one-way or two-way shape memory properties are required. To achieve this goal, various heat treatments, including direct aging, annealing and annealing followed by aging, were optimized for the Ti-rich NiTi (Ni49.6Ti (at. %)) fabricated by laser powder bed fusion (L-PBF). Microstructural evolution, phase transformation, precipitation and shape memory behaviour were systematically investigated by multiscale correlative microstructural, differential scanning calorimetry analysis and thermomechanical analysis. Based on optimized heat treatments, ∼25 K phase transformation temperature windows and ∼90 MPa stress windows were achieved for the one-way shape memory effect. Solutionized annealing was found to be the most effective way to improve one-way shape memory degradation resistance, due to the reduction of defects and solid solution strengthening. One of the main findings of this study is that the heterogonous microstructures between hard intergranular Ti2NiOx and soft NiTi matrix, induced by solutionized annealing with subsequent aging, result in strain partitioning and enclosing the internal stress state, which was found to promote a pronounced two-way shape memory effect response. The results of this work provide in-depth knowledge on tailoring and designing functional shape memory characteristics via heat treatments, which contributes to expanding L-PBF NiTi application fields, such as biomedical implants, aerospace components, and other advanced engineering applications.</p

Cracks formation in nickel-based single crystal alloy manufactured by selective laser melting

The aim of this study was to determine the influence of selective laser melting (SLM) process parameters on the formation of cracks in nickel-based single crystal alloy. Several sets of laser scanning parameters, with varied laser power, scanning speed and hatch distance, thus resulting in different volumetric and linear energy densities, were investigated. The relationship between energy density, specimen densification, cracks formation and evolution of directional microstructure was shown. It was found that higher linear energy density results in fewer cracking and defects. Cracking in lower energy samples was attributed to the formation of thermal shrinkage pores in the interdendritic spaces and precipitation of Ta, Mo and Nb carbides along the dendrites boundaries.</p

New materials and technologies in mechanical engineering

International Scientific Conference "New Materials and Technologies in Mechanical Engineering" (NMTME 2019) Selected, peer reviewed papers from the International Scientific Conference "New Materials and Technologies in Mechanical Engineering" (NMTME 2019), March 12 - 15, 2019, St. Petersburg, Russian Federation

Processing, Microstructure, and Mechanical Properties of Laser Additive Manufactured Ti2AlNb-Based Alloy with Carbon, Boron, and Yttrium Microalloying

In this work, Ti-22Al-23Nb-0.8Mo-0.3Si-0.4C-0.1B-0.2Y (at. %) alloy powder was used to fabricate the Ti2AlNb-based alloy samples using Laser powder bed fusion (L-PBF) Additive Manufacturing with a high-temperature substrate preheating. L-PBF process parameters, including laser power, scan speed, hatching distance, and preheating temperature, allowing for obtaining fully dense (99.9% relative density) crack-free samples, were determined. The effects of substrate preheating temperature during the L-PBF process on microstructure, phase composition, and properties of the obtained Ti2AlNb-based alloy were investigated using X-ray diffraction, scanning electron microscopy, electron backscatter diffraction analysis, and microhardness testing. The results obtained for the material with C, B, and Y microalloying were compared to the Ti2AlNb-based alloy fabricated by L-PBF from the powder not alloyed with C, B, and Y. The results revealed that the microalloying reduced the number of solidification cracks; however, no significant microstructural changes were observed, and high-temperature substrate preheating was still necessary to suppress cold cracking of the alloy. The microstructure of the alloy varied from fully-&beta;/B2, B2 + O, to fully-O depending on the preheating temperature. Effects of hot isostatic pressing and heat treatment conditions on microstructure and mechanical properties were investigated

Modification of Inconel 718 Properties by In Situ Y Addition in Selective Laser Melting

The paper presents the results of a study of the possibility of Inconel 718 alloy properties modifying by adding Yttrium in situ in the process of selective laser melting. The single and double laser processing of each layer was used. Yttrium was introduced into the alloy in an amount of 0.1, 0.2, 0.5, 1, and 2 mass %. Studies of the structure of the material showed that undissolved Yttrium particles remain in the material. With an increase in the proportion of yttrium in the alloy, the hardness increases. Tensile test showed that with an increase in the content of yttrium from 0 to 0.1%, the yield strength and tensile strength decrease, but the relative elongation increases. With a further increase in the yttrium content, there is a sharp decrease in the relative elongation and an increase in the yield strength, which is caused by the existence of a large number of undissolved yttrium particles in the sample

Interface Characterization of Bimetallic Ti-6Al-4V/Ti2AlNb Structures Prepared by Selective Laser Melting

Additive Manufacturing (AM) of multimaterial components is a promising way of fabricating parts with improved functional properties. It allows for the combination of materials with different properties into a single component. The Ti2AlNb-based intermetallic alloy provides high temperature strength, while the Ti-6Al-4V (Ti64) alloy has good fracture toughness, ductility, and a relatively low cost. A combination of these alloys into a single component can be used to produce advanced multimaterial parts. In this work, Ti2AlNb/Ti-6Al-4V bimetallic structures were fabricated from pre-alloyed powders using the Selective Laser Melting (SLM) process. The effects of high-temperature substrate preheating, post-processing by annealing, and hot isostatic pressing on defect formation, the microstructural evolution of the interface area, and the mechanical properties of the bimetallic samples were investigated. High-temperature substrate preheating during the SLM process was necessary to prevent reheat cracking of the Ti2AlNb part, while annealing and hot isostatic pressing post-processing improved the chemical and microstructural homogeneity of the transition zone and enhanced the tensile properties of the bimetallic structure

Microstructure and mechanical properties of tial-based alloy produced by selective laser melting

Additive Manufacturing (AM) is an attractive way of producing parts of intermetallic titanium alloys. However, high brittleness of these alloys makes it challenging to produce crack-free intermetallic parts by AM. One way to overcome this problem is to use high-temperature powder-bed preheating. In this paper, Ti-48Al-2Cr-2Nb alloy was obtained by selective laser melting process with high-temperature preheating of 800-900 ºC. Crack-free specimens with a relative density of 99.9% were fabricated using an optimized process parameter set. Microstructure and phase composition were studied using scanning electron microscopy and X-Ray diffraction to reveal a fine microstructure consisting of lamellar a2/? colonies, equiaxed ? grains, and retained ß phase. Compressive tests and microhardness measurements showed that the produced alloy exhibited superior properties compared to the conventionally obtained TiAl-alloy.</p

Effect of Hot Isostatic Pressing on the Microstructure of Directionally Solidified Nickel Alloy After SLM

The paper investigates the effect of hot isostatic pressing of single-crystal nickel-based alloy manufactured by selective laser melting (SLM) with a high-temperature substrate preheating. A study of the structure and phase composition of the material before and after treatment has been carried out. It was found that as a result of such treatment, the ratio and proportion of the strengthening phases change; however, due to slow cooling after treatment, the optimal ratio and shape of the inclusions are not fixed. In addition, the hardening particles are precipitated.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Team Vera Popovic

Investigation of the Possibility of Tailoring the Chemical Com-Position of the NiTi Alloy by Selective Laser Melting

In this work a study of the selective laser melting process of two NiTi alloys of equiatomic, and rich Ni composition were conducted. A study of the influence of the technological parameters on the alloy density was carried out. Values of technological parameters were obtained to ensure production of samples with the lowest number of defects. When using process parameters with the same energy density but different values of the constituent technological parameters, the amount of nickel carried away by evaporation changed insignificantly. An increase in the energy density led to an increase in the amount of nickel carried away, causing final samples with lower Ni content. When using multiple laser processing in the low-energy parameter set, it was possible to achieve a decrease in the nickel content in the alloy, similar to that with single high-energy processing. DSC studies showed a significant increase in transformation temperatures upon repeated laser processing due to the higher evaporation of nickel. The use of double laser treatment gave a decrease in the final density of the sample compared to a single treatment, but its value is still higher than when using a single treatment with a higher energy density

Tailoring the Microstructure of Laser-Additive-Manufactured Titanium Aluminide Alloys via In Situ Alloying and Parameter Variation

Titanium aluminide (TiAl) alloys have emerged as promising materials for high-temperature applications due to their unique combination of high-temperature strength, low density, and excellent oxidation resistance. However, the fabrication of TiAl alloys using conventional methods is challenging due to their high melting points and limited ductility. Selective laser melting (SLM), an additive manufacturing technique, offers a viable solution for producing TiAl alloys with intricate geometries and the potential for tailoring their microstructure. This study investigates the effect of in situ copper alloying and multiple laser scans on the microstructure and mechanical properties of TiAl-based alloys fabricated using SLM. The results demonstrate that copper alloying enhances the formation of the α2-Ti3Al phase, refines the microstructure, and improves the mechanical properties of TiAl alloys. Multiple laser scans allow for the creation of distinct microstructural regions within a single component, enabling the tailoring of properties that are suitable for specific operating conditions. The findings provide valuable insights into the fabrication and optimization of TiAl intermetallic alloys with diverse applications