Nickel-titanium alloy (NiTi) has the characteristics of reversible strain when heated or unloaded, high power-to-weight ratio, good functional stability, and light weight. It is used in various engineering applications such as medical, machinery, aerospace, etc.
The additive manufacturing technology based on the powder bed laser melting process (L-PBF) can not only manufacture complex geometric shapes, but also adjust the performance of 3D printed metal parts. The printing process parameters and scanning strategies show the microstructure of the finished parts. , Performance and dimensional accuracy.
The absorption of impurities in the additive manufacturing process can change the microstructure and thermomechanical properties of the part. According to market observations from 3D Science Valley, in a study, a research team from the University of Toledo and other institutions produced three kinds of nickel-titanium alloy tubes with different thicknesses through selective laser melting 3D printing technology using nickel-rich NiTi powder material. In this way, the torsion performance and microstructure characterization of 3D printed Nitinol tubes are studied.
Research on 3D printing NiTi thin-walled tube
Compared with traditional drive systems, the main advantages of shape memory alloys are high working output, noise-free operation, greatly reducing the weight/size of the execution system, and large deformation. Because of these advantages, shape memory alloys have been applied in the field of rotary drives. Over the past 30 years, scholars have carried out a lot of applied research.
In 1984, Nitinol wire was used for rotary drive for the first time. Compared with other loading modes, Nitinol wire obtained higher work output, which promoted the further exploration of other torsion forms (such as Nitinol tube and rod). Afterwards, some scholars conducted experimental and theoretical studies on the mechanical properties and shape memory effect of NiTi alloy rods with equal atomic ratio, and proposed a rotary actuator for active helicopter rotor blades. In more than 20 years, several work has been devoted to the development of shape memory alloy torque tube applications, such as variant wings and solar collectors. In 1995, a rotary drive based on NiTi torque tubes was developed to manufacture small-scale adaptive wings for fighter jets. Early designs indicated that these tubes can achieve 4.5° rotation with a corresponding torque of 141 Nm.
In more recent research, scholars have studied high-temperature shape memory alloys. High-temperature NiTiHf alloy has become a candidate material for rotary drives. Compared with other high-temperature shape memory alloys, high-temperature NiTiHf alloys have a high phase transition temperature (austenite end temperature, Af) up to 500°C, and have good stability and Low prices. Some scholars have studied the development of high-temperature NiTiHf alloy torque tube as a rotary drive. In short, these shape memory alloys are used to make devices that change the rules of the game in the aerospace industry.
Most of the torsion tubes/rods mentioned above are manufactured through hot working processes such as extrusion and forging and tube forming processes. Additive manufacturing technology is an alternative technology that can not only manufacture complex geometric shapes, such as splines, hexagonal cut end connections and embedded cooling channels, but also customize material properties by changing process parameters.
Selective laser melting (SLM) is the most widely used method for additive manufacturing of shape memory alloys. Fully documented, including laser power, scanning speed, scanning distance and other process parameters play an important role in the density, transition temperature, thermomechanical response and microstructure of 3D printed parts. There are also a large number of documents documenting the influence of SLM 3D printing process parameters on the printability, microstructure, mechanical response, lattice structure and post-processing of shape memory alloys.
So far, most of the research on 3D printing of shape memory alloys is compression testing, and a small amount of research is on tensile properties, but there are few studies on the torsional properties of additively manufactured NiTi alloys. In the related research shared in this issue of 3D Science Valley, the research team studied the microstructure and thermodynamic properties of SLM 3D printed nickel-rich NiTi alloy torsion tubes.
Materials and Methods
The material used by the research team is Ni50.1Ti49.9, which is slightly nickel-rich. The material is manufactured using electrode induction molten gas atomization (EIGA) technology and has a very low impurity content, with a spherical shape ranging from 25 m to 75 m. Cylindrical tubes with an outer diameter of 5 mm, a length of 25 mm, and wall thicknesses of three different sizes (0.45 mm, 0.5 mm and 0.55 mm) are manufactured by SLM3D printer. Use a contourless two-way scanning strategy in which the laser moves in a zigzag shape, and then the next layer is processed after the laser beam is rotated 90°.
Transition temperature (TT)
The DSC results of the 3D printed tube and powder are shown in Figure 2. The measured austenite endpoint temperatures of the powder and 3D printed tube are 92°C and 0°C, respectively. Due to hardware limitations, it is impossible to capture the martensite transformation end point of the 3D printed tube. The layer-by-layer processing and high cooling rate of the SLM 3D printing process result in an uneven thermal history, and an uneven microstructure and composition due to the presence of precipitation, residual stress, and dislocations. All of the above factors will increase the phase transition temperature interval and cause peak broadening.
After the SLM process, there was an unexpected shift, that is, to a lower TT. The composition of NiTi may be one of the main factors affecting TT. When the nickel content decreases, TT transforms to a higher temperature, and when the nickel content increases, TT transforms to a lower temperature.
When it comes to additive manufacturing methods, process parameters that affect the microstructure and chemical composition can significantly change the transition temperature. There are two main mechanisms in the SLM method that may change TT: (i) nickel evaporation (ii) precipitate formation.
Micro structure
The powder is composed of single-phase B19' martensite at room temperature as expected from the DSC results, while the 3D printed tube contains an austenite B2 matrix with a second phase Ti 4 Ni 2 O x oxide. XRD measurement can determine the secondary phase of powder and polished samples. As discussed in the transition temperature section, the formation of titanium-rich oxide not only depletes the main matrix in titanium, but also suppresses the loss of nickel by covering the molten pool. Therefore, the TT transition observed in the DSC results can be attributed to this effect.
Thermodynamic characterization
The research team used a stepwise strain loading curve to evaluate the mechanical properties of the precast tube under pure torsional loading. In each step, the tube is loaded to a shear strain of approximately 0.5%, and then 0.5% loading steps are performed sequentially until failure. Using this method, the smooth curve and cyclic behavior of the material up to the fracture can be captured.
Cycle performance
The cycle test is carried out at room temperature and then heated to 100°C. These tests are mainly used to check strain behavior and subsequent strain recovery. Two thicker 3D printed tubes (t = 0.4 mm and t = 0.45 mm) have been tested for up to 10 loading and unloading cycles from zero to maximum shear stress.
The results show that even though the SLM3D printing process parameters have been optimized so that the main part has no major defects, for thin-walled pipes, the surface roughness is also very important for the quality of the parts. The specific conclusions of the research team are as follows:
The scanning strategy and high-speed scanning will affect the size of the thin-walled SLM tube. In addition, the sharp valleys on the surface of the NiTi SLM tube formed by satellite particles are likely to cause microcracks.
Oxygen absorption during the high-temperature melting process will cause the formation of titanium-rich precipitates on the grain boundaries, moving the main matrix to the nickel-rich area, thereby reducing the phase transition temperature to 90°C.
The strain monitoring of the 3D printed tube showed that the strain localization started at an angle of 30° and continued to propagate until it ruptured.
The cycle performance showed an unrecoverable strain of up to 1.2% in the first cycle, which may be the result of the formation of titanium-rich oxide precipitates. The superelastic behavior became stable after 4 cycles and reached a stable transition strain of 2.3% after 10 cycles.
Surface roughness and surface defects are challenges for SLM 3D printing of NiTi thin-walled tubes. Using contour scanning with a lower scanning speed, using finer powder particles and performing a remelting process on each layer may be a way to improve the surface quality of the manufactured parts. In addition, heat treatment is a method to further improve the thermomechanical properties of 3D printed NiTi alloy thin-walled tubes. Comparing the thermomechanical properties of the 3D printed NiTi tube with the tube made in the traditional way with the same composition and transition temperature will also be an important step in the next step.