Investigating Elastocaloric Effect of NiTi Shape Memory Alloy Fabricated by Laser Powder Bed Fusion Technique

Abstract It has been shown that about 20% of the global electricity supply is used for refrigeration and cooling. Thus, the substantial global warming impact of refrigerants remains a crucial concern. In an attempt to solve this issue, caloric cooling technology has shown significant potential as an alternative to vapor-compression technology. Caloric cooling technology utilizes the latent heat of a solid-state phase transformation of a caloric material by applying an external trigger. The caloric cooling technologies can be broken down into four categories including magnetocaloric, electrocaloric, barocaloric, and elastocaloric. Among these four materials, the considerable elastocaloric effect and easy actuation of these materials make them an excellent alternative to the usual approach. The elastocaloric effect is defined by the adiabatic temperature change (△ Tad) in elastocaloric materials when uniaxial stress (load) is applied or removed in the adiabatic state. This phenomenon can be found in all shape memory alloy (SMAs) materials. To elaborate further, SMAs undergo a stress-induced transformation between austenite and martensite phases upon adiabatic loading and unloading. These phase transitions result in △ Tad (heating or cooling) within the materials. For example, during unloading, the endothermic reverse transformation from martensite to austenite leads to decreased temperature (cooling) within the sample. It has been well-defined in the literature that NiTi materials, a well- known type of SMA materials, are the best candidate in the elastocaloric industry due to their large latent heat and excellent mechanical properties and result in a large elastocaloric effect. There has been a limited number of studies that have explored the elastocaloric effect of NiTi. Additionally, it has been shown in the literature that the elastocaloric effect of this material can be optimized by adjusting the process parameters, providing a new route to investigate the elastocaloric effect of additively manufactured NiTi. It should be known that any change in the thermomechanical and mechanical behavior of NiTi materials results in a difference of the elastocaloric effect. To elaborate further, any process or building orientation controls that enhance the strength of the martensitic and austenitic phases will reduce slipping and enable recoverable strains to reach higher levels, resulting in a larger △Tad. In this comprehensive study, the effect of building orientation on the elastocaloric response was investigated and followed by an investigation of the effect of porous structures on the elastocaloric response.