Functional Bimetallic Joints of Ti6Al4V to SS410.

Bimetallic structures provide a unique solution to achieve site-specific functionalities and enhanced-property capabilities in engineering structures but suffer from bonding compatibility issues. Materials such as titanium alloy (Ti6Al4V) and stainless steel (SS410) have distinct attractive properties but are impossible to reliably weld together using traditional processes. To this end, a laser-based directed energy deposition (DED) system was used to fabricate bimetallic joint of Ti6Al4V and SS410 keeping niobium (Nb) as a diffusion barrier layer. Both shear and compression tests were used to characterize the joint's strength, and compared with the base materials. The bimetallic-joint shear and compressive yield strengths were 419± 3 MPa (~ 114 % of SS410) and 560 ± 4 MPa (~ 169 % of SS410), respectively. The increase in interfacial shear and compressive yield strengths over the base material indicates strong metallurgical bonding between the base materials and the interlayer, Nb. Proof-of-concept part for direct application of the bimetallic joint was demonstrated by welding base metals, end-to-end, to the joint. The interfacial microstructures, elemental diffusion and phases, including failure modes were examined using secondary and backscatter electron imaging, X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS). The bimetallic-joint interfaces were free from brittle intermetallic compounds such as FeTi and Fe2Ti that are generally responsible for weak bond strength.

Bond Strength Measurement for Additively Manufactured Inconel 718- GRCop84 Copper Alloy Bimetallic Joints.

To benefit from the fascinating properties of multi-material structures, the interfacial joint should exhibit good mechanical strength. Evaluating the shear strength of a bimetallic joint via conventional methods is usually complex, and in most cases produces unreliable data due to induced bending stress among others. In this work, a novel single-shear test device was designed and fabricated to measure shear strength of bimetallic joints. The device was first standardized by shearing standard materials, and the results were in good agreement with published data. Subsequently, the shear strength of Inconel 718/copper alloy (GRCop-84) bimetallic joint built via laser engineered net shaping (LENS™) was evaluated. Compression test on the bimetallic joint was carried out as well for mechanical characterization. Both shear and compressive yield strengths of bimetallic joints were compared with the base materials in addition to influence of thermal cycling on the joint strength. Inconel 718/GRCop-84 bimetallic-joint shear strength was 220 ± 18 MPa and 231 ± 27 MPa for as-printed sample and after thermal cycling, respectively. Likewise, the bimetallic yield strength after compression test was 232 ± 3 MPa and 337 ± 15 MPa. No cracking through or along the interface was observed even after thermal cycling, which indicates no thermal degradation at the bimetallic interfacial joint. Increase in compressive yield strength after thermal cycling could be attributed to precipitation of Cr2Nb particles in GRCop-84 matrix along with strengthening due to gamma phases in Inconel 718. Scanning electron microscopy (SEM) and backscatter electron imaging were used to examine the interfacial microstructures and failure modes. EDS was used as well to analyze the interface elemental composition. The development of the single-shear test device can provide an added opportunity to effectively evaluate mechanical behavior, reliability and performance of additively manufactured multi-material structures through bond strength analysis.

Influence of deposition orientation on fatigue response of LENS™ processed Ti6Al4V.

Understanding processing-property relationships for directed-energy-deposition (DED) parts remains a major roadblock to widespread process implementation. Herein we investigate the effect of scanning-strategy and testing-orientation on the fatigue response of as-printed Ti6Al4V components. At ~106 cycles, samples tested in the build-direction exhibited ~ 45% decrease in fatigue strength relative to the horizontally-tested samples, owing to higher overall porosity and the testing orientation relative to residual pores. Samples failing <106 cycles demonstrated tortuous surfaces, whereas samples enduring >106 cycles exhibited smoother-surfaces. Our results indicate that DED-produced parts can exhibit directionally-dependent fatigue performance, and print-strategy must be taken into consideration for dynamic-loading applications.

Starch-Hydroxyapatite Composite Bone Scaffold Fabrication Utilizing a Slurry Extrusion-Based Solid Freeform Fabricator.

Significant efforts have been made to treat bone disorders through the development of composite scaffolds utilizing calcium phosphate (CaP) through additive manufacturing techniques. However, the incorporation of natural polymers with CaP during 3D printing is difficult and remains a formidable challenge in bone and tissue engineering applications. The objective of this study is to understand the use of a natural polymer binder system in ceramic composite scaffolds using a ceramic slurry-based solid freeform fabricator (SFF). This was achieved through the utilization of naturally sourced gelatinized starch with hydroxyapatite (HA) ceramic in order to obtain high mechanical strength and enhanced biological properties of the green part without the need for cross-linking or post processing. The parametric effects of solids loading, polycaprolactone (PCL) polymer addition, and designed porosity on starch-HA composite scaffolds were assessed through mechanical strength, microstructure, and in vitro biocompatibility utilizing human osteoblast cells. It was hypothesized that starch incorporation would improve the mechanical strength of the scaffolds and increase proliferation of osteoblast cells in vitro. Starch loading was shown to improve mechanical strength from 4.07 ± 0.66 MPa to 10.35 ± 1.10 MPa, more closely resembling the mechanical strength of cancellous bone. Based on these results, a reinforcing mechanism of gelatinized starch based on interparticle and apatite crystal interlocking is proposed. Morphological characterization utilizing FESEM and MTT cell viability assay showed enhanced osteoblast cell proliferation in the presence of starch and PCL. Overall, the utilization of starch as a natural binder system in SFF scaffolds was found to improve both green strength and in vitro biocompatibility.