Advanced Processing Techniques: Additive, Porous, and Others: Session 4
Program Organizers: Aaron Stebner, Colorado School of Mines

Thursday 4:00 PM
July 13, 2017
Room: Gold Coast
Location: Hyatt Regency Chicago

Session Chair: Markus Chmielus, University of Pittsburgh

4:00 PM  
Microstructure and Superelastic Behavior of Ti-Ni-Ag Scaffolds Prepared by Sintering of Alloy Fibers: Shuanglei Li1; Yeon-wook Kim2; Tae-hyun Nam1; 1Gyeongsang National University; 2Keimyung University
    49Ti-50.3Ni-0.7Ag (at.%) scaffolds were prepared by sintering of rapidly solidified alloy fibers. Microstructures and transformation behaviors of alloy fibers and scaffolds were investigated by using electron probe micro-analyzer (EPMA), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The B2-R-B19' transformation occurs in alloy fibers. The annealing heat-treated alloy fibers have good superelasticity with superelastic recovery ratio of 93 %. The as-sintered Ti-Ni-Ag scaffolds with porosity level of 80 % exhibit three-dimensional and interconnected pores. The Ti-Ni-Ag scaffolds heat treated at 723 K not only have an elastic modulus of 0.67 GPa, which match well with that of cancellous bone, but also show good superelastic behavior at human body temperature. In terms of the mechanical properties, the Ti-Ni-Ag scaffolds in this study can meet the main requirements of bone scaffold for the purpose of bone replacement applications.

4:20 PM  
Dissimilar Laser Welding of Superelastic NiTi and CuAlMn Shape Memory Alloys: Joćo Pedro Oliveira1; 1Ohio State University
    Dissimilar joining of advanced engineering alloys is fundamental for the development of new applications. However, joining two distinct materials poses difficulties owing to the several metallurgical and thermo-physical problems that can arise. This presentation describes the work performed on dissimilar laser welding of NiTi and CuAlMn shape memory alloys, superelastic at room temperature. Detailed microstructural characterization was performed. The complex microstructure of the dissimilar joint is explained based on the characteristics of laser welding, namely material and heat flow, high cooling rates and thermal gradients within the fusion zone. Cycling tensile testing revealed that the joints preserved the superelastic behaviour despite the unfavourable microstructure of the fusion zone which translates into an irrecoverable strain of 2 % when cycled at 5 % strain. These results may open the possibilities for new applications based on this dissimilar combination which can combine superelasticity and higher thermal and electrical conductivity (with the latter two characteristics arising for the CuAlMn shape memory alloy).

4:40 PM  
Effect of Cyclic Martensitic Transformation on Grain Refinement of Additive-Manufactured Maraging Steel: Fuyao Yan1; Gregory Olson1; 1Northwestern University
    Grain refinement is regarded as an effective way to enhance strength and ductility of materials simultaneously. Objects fabricated by additive manufacturing (AM) generally experience high thermal gradient during laser melting, which results in columnar grains with high length-to-width ratio along the build direction. Cyclic martensitic transformation, which utilizes high-density defects arising from martensitic transformation as nucleation sites for reversed austenite, provides a possibility for grain refinement of AM net-shaped martensitic steels, without dramatic deformation to the objects. In this project, cyclic heat treatment is performed on PH48S maraging steel manufactured by laser engineered net shaping (LENS). It is found that 3 cycles of martensitic transformation lead to the highest extension of grain refinement and therefore the grain boundary strengthening. The effects of thermal cycling conditions on grain refinement and related microstructural features are discussed.

4:55 PM  
Martensitic Transformation in Ni-Mn-Ga Net Shape Components Produced by Additive Manufacturing: Constantin Solomon1; Matt Caputo1; 1Youngstown State University
    Net shape components have been produced by binder-jet 3D printing using three different pre-alloyed Ni-Mn-Ga powders. Spherical and hollow Ni-Mn-Ga powders have been synthesized by spark-erosion in liquid argon and liquid nitrogen, respectively. Irregular shaped powder was obtained by ball milling of Ni-Mn-Ga ingots. After 3D printing and post printing processing the morphology, martensitic transformation behavior, chemical composition, and crystallography of printed materials have been investigated using LM, SEM, DSC, XEDS, XRD, and TEM. Net shaped components with varying densities, due to inherent porosity, have been obtained by 3D printing. All printed materials showed reversible martensitic transformation behavior with composition dependent austenitic and martensitic transformation temperatures. The comparison between martensitic transformation behavior in 3D printed materials and martensitic transformation behavior in Ni-Mn-Ga single crystals, bulk polycrystalline, and polycrystalline foams will be discussed in the presentation.

5:15 PM  
On the Mechanical and Thermomechanical Properties of Additively Manufactured NiTi Alloys: Mohammad Elahinia1; Haluk Karaca2; Soheil Saedi2; Narges Shayesteh Moghaddam1; 1University of Toledo; 2University of Kentucky
    The traditional NiTi fabrication methods are challenging and not flexible to produce porous and complex structures. Selective Laser Melting (SLM) is an attractive method in overcoming these problems and process parameters can be used to tailor the microstructure and shape memory properties to achieve desired properties of applications. In this paper, we have summarized our efforts to characterize and tailor the behavior of superelastic (SE) and shape memory (SM) NiTi alloys. This work discusses the microstructure, transformation temperatures, mechanical properties, and functional characteristics of as-fabricated and heat treated SLM NiTi in porous and dense forms. The effects of processing parameters, porous structure and aging time and temperature on the transformation temperatures, strength, shape memory effect and superelasticity behavior of dense and porous Ni-rich SLM NiTi alloys were systematically investigated. It is shown that dense and porous Ni-rich SLM NiTi alloys can show superelasticity at room and body temperatures after selected heat treatments. Thus, these alloys are found to be very promising for biomedical applications.