Abstract:

Laser powder bed fusion (LPBF) is an appealing additive manufacturing technique suitable for producing intricate products with complex shapes, where traditional subtractive manufacturing methods may be impractical. When dealing with NiTi shape memory alloys, the fabrication process encounters additional challenges due to the material’s high ductility and work hardening characteristics. These properties lead to elevated tool wear and poor workability. However, LPBF overcomes these challenges by eliminating the need for tooling and enabling direct fabrication of complex shapes based on predefined computer-aided designs. Although there has been a growing interest in the LPBF of NiTi in recent years, the fabrication of NiTi structures with architected designs has received limited attention. This study aims to bridge this research gap by investigating the phase transformation behavior of two specific types of triply periodic minimal surface (TPMS) architected NiTi lattices, namely primitive and gyroid, manufactured using LPBF. The study utilized a methodology involving the design of TPMS structures, additive manufacturing, and the characterization of printed samples through electron microscopy, x-ray diffraction, and thermal analysis to measure phase transformation temperatures. The research not only examines the impact of process parameters on the behavior of the fabricated samples but also establishes the influence of cellular geometry on the functional response of TPMS NiTi structures. It was noteworthy to observe that gyroid samples displayed a higher thermal hysteresis and lower reverse transformation enthalpy in comparison with primitive samples.

Abstract

Due to their high ductility and superior strength, the machining of NiTi shape memory alloys using conventional subtractive manufacturing technologies poses significant challenges. However, additive manufacturing (AM) offers a viable solution by circumventing the limitations of machinability through the elimination of tooling, thereby enabling the production of NiTi structures with previously unachievable intricacy. This study focuses on the fabrication of base layers of architected triply periodic minimal surface (TPMS) lattices using laser powder bed fusion (LPBF) and explores primitive and gyroid topologies. The investigation involves a comprehensive analysis and discussion of the influence of geometric properties and process parameters on the microstructural characteristics and the distribution of solid phases within the samples. Notably, the study reveals a substantial impact of process parameters and structural topology on the microstructural features. Additionally, notable observations are made concerning nickel evaporation, as well as the formation of oxide- and titanium-rich phases in relation to their distance from the base plate. Investigating intricate TPMS geometries in NiTi alloys in conjunction with varying laser process parameters represents a relatively new and unexplored area of research. These geometries may offer unique structural and functional properties that can potentially lead to innovative applications and advancements in various fields.

Abstract

In this paper, thin layers of NiTi shape memory alloy (SMA) triply periodic minimal surface lattices (TPMS) are fabricated using laser powder bed fusion (LPBF), considering different laser scanning strategies and relative densities. The obtained architected samples are studied using experimental methods to characterize their microstructural features, including the formation of cracks and balling imperfections. It is observed that balling is not only affected by the parameters of the fabrication process but also by structural characteristics, including the effective densities of the fabricated samples. In particular, it is reported here that higher densities of the TPMS geometries considered are generally associated with increased dimensions of balling imperfections. Moreover, scanning strategies at 45° angle with respect to the principal axes of the samples resulted in increased balling.

Abstract

The ability of shape memory alloys (SMAs) to recover inelastic strains larger than any other metallic alloy has prompted their use in a wide range of applications. However, for the most common SMAs, including NiTi and Cu-based systems, fabrication using conventional means raises important challenges, including poor workability and potentially high tool wear. Additive manufacturing offers a direct answer to these challenges by eliminating the need for tooling and allowing the production of samples of complex geometries directly from computer-aided designs. The present work provides a comprehensive review of additive manufacturing applied to various SMA systems, with focus on the influence of process parameters and heat-treatment on the microstructure, printability, and the structural and functional behavior of additively fabricated samples.

Abstract

Effect of nano-CeO2 doping was studied on self-accommodating Cu–12Al–4Ni based shape memory alloys. Cu–Al–Ni based alloys are known for their high thermal stability and good shape memory properties but mechanical processing of these alloys is very difficult due to their high brittle nature and susceptibility to inter-granular failure. The present work deals with the enhancement of mechanical as well as shape recovery characteristics of these alloys with the addition of nano-CeO2. Significant improvements in ductility (1.8 times), strength and shape recovery were observed with the addition of nano-CeO2 with respect to the base alloy.

Abstract

The advantages of Cu-based shape memory alloys (SMAs) over the more popular Ni-Ti alloys are their low cost and easy processability coupled with good electrical and thermal conductivity. However, these alloys suffer from severe brittleness due to highly ordered structure causing difficulty in cold working; consequently, their applicability in the form of wires and sheets is impeded. In an attempt to improve the ductility of these alloys, varying amounts of Nb and Ag were added to Cu-12Al-4Mn, a known SMA, and the effect of these additives on the shape memory properties as well as the microstructure through grain refinement were analyzed. Detailed microstructural observations indicated that the addition of more than 2 wt% of Ag or Nb was detrimental to the martensitic structure that was formed on quenching. Furthermore, the addition of Nb led to the formation of a more desirable martensitic microstructure, namely the fine β1’ phase, whereas the inclusion of Ag caused the formation of the coarse γ′1 phase. Based on the observed strength and shape memory effect, it was concluded that Nb significantly improved the mechanical as well as the shape memory properties of the base alloy.

Abstract

Ternary Cu-12Al-4Mn and Cu-12Al-4Ni have been chosen for the present study and different alloying elements have been added in an attempt to further improve upon the shape memory properties in terms of the desired microstructure in the quenched condition. It has been observed that all compositions do not exhibit the martensitic structure on quenching and different types of martensitic phases are exhibited depending on the alloying constituents. Attempts have been made to roll the samples into thin sheets through multiple passes. It has been found that although the ternary base alloys can easily be rolled the quaternary alloys with both Mn and Ni make the sample brittle rendering it unsuitable for rolling. Also adding of Zn and Mg has not exhibited the desired microstructure for exhibiting the shape memory properties and also could not be rolled though these samples were not brittle. The ease of roll-ability has been compared between the alloys and correlated with the microstructure which helps in understanding which type of martensitic phase is useful in rolling the samples.

Abstract

Cu–Al–Ni shape memory alloys have good thermal stability and shape memory properties, but mechanical working of these alloys is very difficult due to inter-granular fracture and brittleness. This paper deals with improvement of shape memory properties and ductility of these alloys with the addition of boron. It was observed that with addition of boron in a designed experiment, ductility improved by more than four times than that of the base alloy. In addition, shape memory properties also improved significantly. These results obtained are discussed in this paper systematically.

Abstract

This article deals with the effect of adding nano CeO2 to act as a grain pinner/refiner to a known Cu-Al-Ni shape memory alloy. Elements were taken in a predefined ratio to prepare 300 g alloy per batch and melted in an induction furnace. Casting was followed by homogenization at 1173 K (900 °C) and rolling to make sheets of 0.5-mm thickness. Further, samples were characterized for microstructure using optical and electron microscope, hardness, and different phase studies by X-ray and transformation temperatures by differential scanning calorimetry. X-ray peak broadenings and changes were investigated to estimate the crystallite size, lattice strain, and phase changes due to different processing steps. A nearly uniform distribution of CeO2 and better martensitic structure were observed with increasing CeO2. The addition of CeO2 also shows a visible effect on the transformation temperature and phase formation.

Abstract

In the present study, an attempt has been made to study the effect of the proportion of the main alloying constituents in a Cu–Al–Mn alloy, which is a known shape memory material. Four compositions of the alloy with varying ratios of Al:Mn, varying from 1 to 4 [added to copper], were synthesized using the liquid metallurgy route. After appropriate heat treatment to induce shape memory behaviour, they were studied for microstructure, X-ray diffraction, hardness and transformation temperature in an attempt to understand the effect of the varying ratios of the major alloying constituents on the properties mentioned. With an increase in the Al:Mn ratio, increase in grain size as well as cast hardness were observed. On the other hand, an increase in percentage decrease in hardness was observed with increase in Al:Mn ratio. Increase in Al:Mn ratio also favoured formation of martensitic structure with less amount of retained austenite.

Abstract

In the present study, Cu-12.5 wt. %Al-5 wt. %Mn-80.5 wt. % shape memory alloy is chosen as the base alloy and 2 wt. % of quaternary additions of Fe, Ni, and Ti added to the base alloy. The effects of these additions on in terms of feasibility, microstructure, hardness, and transformation temperature were studied. The findings suggest the possibility to improve martensite formation, attain higher transitions temperatures, and longer retention over the base alloy through such additions.

Abstract

Advantages of Cu based shape memory alloy include amongst other features, high transformation temperature, low cost of production, ease in manufacturing processes and ability to vary the achieved properties through alloying additions. It has been often reported that these alloys are very sensitive to the alloying additions in terms of properties achieved and phase precipitation necessary for development of shape memory properties. This behaviour in Cu based shape memory alloys i.e. being very sensitive to its constituents can be used positively to design alloys with pre set properties if the alloying additions and their percentages are properly controlled. In an attempt to understand the effect of different alloying additions, 2% of different elements [Zn, Si, Mg & Cr] were added to a known Cu-based shape memory alloy [Cu-12.5 wt% of Al-5 wt % of Mn]. The objective was to ascertain changes or improvements achieved due to the additions in terms of microstructural changes, hardness, phase precipitation and transformation temperatures. Attempts have been made to analyze the changes in properties achieved in the base Cu-Al-Mn alloys due to the quaternary additions. Grain structure with α+β phases, which is a pre requisite for martensite formation on quenching is seen in all the alloys indicating that all the alloys have potential to exhibit the shape memory behaviour. The martensite formation with different morphologies is observed in the quenched samples however. XRD results have identified the precipitated phases to be the martensitic phases. The DSC results indicate clear transformation peaks in most of the samples with significantly high transformation temperatures. The findings confirm the variation in properties achieved due to different additions and improvements achieved in terms of higher transformation temperatures and martensite formation due to the alloying additions. An attempt has been made to understand the findings.

Abstract

The effect of alloying seven different elements [Zn, Si, Fe, Ni, Mg, Cr and Ti] on the microstructure, hardness, phase precipitation and transformation temperature in a Cu–12.5Al–5Mn alloy with a view to possible improvements as a result of these additions is the focus of the reported study. The base alloy has been chosen keeping in mind its ability to exhibit shape memory properties and improved ductility over other Cu-based SMAs. The objective was to ascertain changes or improvements attained due to the individual tertiary additions.

The samples were prepared through liquid metallurgy route using pure copper, aluminum, manganese and the respective quaternary alloying elements in right quantities to weigh 1000 g of the alloy in total and were melted together. Samples from the cast alloys were subject to homogenisation treatment at 200 °C for 2 h in a mufflefurnace and furnace cooled. Samples from the homogenised alloys were heated and held for 2 h at 920 °C followed by ice quenching to obtain the desired martensitic structure for shape memory behaviour. The alloys in the cast, homogenised and quenched conditions were metallographically polished to observe the martensitic phase formation mainly in quenched samples which is a pre requisite for exhibiting shape memory properties in these alloys. X-ray Diffraction studies were carried out on the cast and quenched samplesusing Cu Kα target; and the phases identified indicate martensitic phase precipitation; however in some cases the precipitation is incomplete. Differential Scanning Calorimetric [DSC] studies were carried out on quenched samples from room temperature to 600 °C maintaining a constant rate of 10 °C/min. Results indicate clear transformation peaks in all the samples which are significantly high than conventionally reported except with the addition of Mg in which case no distinct peaks have been recorded. The range of martensite retention is the maximum in ternary Cu–Al–Mn alloys; addition of quaternary elements decreases this range significantly. Presence of Ni delays austenite formation and completion [As and Af] significantly as compared to the ternary alloys; whereas with other additions the As and Af temperatures are brought forward. This means that whereas the alloys without quaternary additions would be better suited for its shape memory properties, ternary alloys would be better suited for higher transition temperatures.

The role of different alloying additions has been highlighted in the findings. Variations in properties have been attained due to different additions and improvements attained in terms of higher transformation temperatures and martensite formation due to the alloying additions.

Abstract

The paper discusses the attempt made to improve upon the transition temperatures and amount of martensite formation in Cu–Al alloys with alloying additions of Mn, Ni and Zn in varying proportions and combinations. The alloys have been subjected to heat treatment cycles to improve upon the microstructure and precipitate the required martensite phase. The effect of these ternary and/or quaternary additions has been studied on the phases precipitated through micro-structural analysis and X-ray diffraction and Differential Scanning Calorimetric studies. The findings confirm the possibility of improvement on the shape memory properties like martensite formation, higher transitions temperatures and longer retention in selected alloys based on the properties exhibited through proper alloying additions and heat treatments.